Chapter 14: Plastics manufacturing processes for packaging materials – Packaging Technology

14

Plastics manufacturing processes for packaging materials

A. Riley,     Arthur Riley Packaging Consultant International, UK

Abstract:

Polymers are finding new opportunities in packaging every day. The inventiveness of the polymer producer and the converting machinery manufacturer has seen plastics become the most used packaging material on a value basis. Plastic packaging is replacing metal retort cans, glass bottles, paperboard cartons and even corrugated cases. It provides barrier, ensuring food is kept safer for longer, reduces the weight of packaging, provides convenience, transparency where required, and still only uses less than 4% of oil resources. This chapter will provide the understanding of how polymers can be used on their own, or combined together to form packaging for all needs.

Key words

moulding

extrusion

thermoforming

coextrusion

laminating

14.1 Introduction

This chapter will concentrate on the forming of thermoplastic packaging components from oil-based polymers. Polymers, as covered in the last two chapters, are highly versatile. A major advantage of plastics is that they can be combined together to provide packaging which is lightweight yet provides the barriers required, is tough and strong yet can be made easy to open. Most plastics used today are copolymers rather than homopolymers and are increasingly supplied in multiple layers rather than as monolayer. Using special techniques, plastic components can be produced to closely rival the absolute barriers available from metal and glass. This chapter will examine the processes required to make flexible, semi-rigid and rigid packaging formats for a variety of applications and the controls that are required to ensure consistency of supply is achieved.

To convert polymers into useful packaging requires specialised equipment and an understanding of their chemistry and properties. Polymers are converted into films, coatings, trays, bottles, jars, cans, closures and blister packs. They are combined together through coextrusion, coinjection and lamination processes; combined with paper and aluminium foils; coated with other polymers and undergo many chemical and physical treatment processes; all with the aim of changing their properties to suit the needs of the marketplace.

14.1.1 Selection of materials

When developing any new pack, there are some basic considerations with respect to the product and the pack which influence the choice of packaging materials, in this instance the polymer(s) that may be suitable. For example:

What aspects of the product are important?

• What are the barrier requirements – gas, moisture, alcohol, UV, etc.?

• Is the product likely to be aggressive to the packaging – acidic, alkaline, greasy, etc.?

• Is the product hazardous or dangerous – poisonous, high/low pH, etc.?

• What is the expected shelf life of the packed product?

• What are its physical properties – solid, liquid, gas, powder gel, flake, etc.?

• Does it need to be visible through the packaging?

• At what temperature will the product be filled – hot, warm, cold?

• At what temperature will the product be stored – ambient, chilled, frozen?

What aspects of the packaging are important throughout the expected life of the product?

• How many size variants are there going to be?

• How many of each variant do I require per annum?

• How soon do I require the packaging?

• Is it going to be a flexible or rigid container?

• What shape is it going to be?

• Is absolute clarity important?

• What quality of decoration is required?

• What temperatures does it have to withstand?

• What type of closure/closing mechanism will be used?

• What surface finish is required?

• Does it require tamper evidence?

• Does it have to meet child safety requirements?

• Will it meet product contact regulations?

• Is it environmentally responsible?

• What can I afford to pay?

The reason for asking these questions is that polymers, though excellent packaging materials, are not as definitive as paperboard, metal or glass in their absolute properties. Paperboard has little or no barrier to gas and moisture; glass, and metal (over 20 mm in gauge for aluminium) are total barriers and metals provide a total barrier to UV light. Polymers have a wide range of properties, depending on their chemical structure, the materials and the coatings added and how they are converted into packaging materials, and thus there are far more variables to consider. See Chapter 13 for more detail about the range of materials available.

14.2 The plasticating extruder

Excluding regenerated cellulose film (e.g., Cellophane™), rotational moulding and the thermosetting materials, all thermoplastic materials are converted using one or more plasticating extruders (Fig. 14.1). The polymer resin, which must be pre-dried if necessary (e.g., PET, PLA and PA) is fed into the hopper of the plasticating extruder, together with any additives such as colour or process and performance aids. See Chapter 12, Section 12.8.9 for a list of typical additives and their functions.

14.1 Plasticating extruder.

Most resins are in the form of small pellets and are delivered to the converting company either in bulk, in which case they are blown into storage silos, or, for smaller quantities of specialised materials, in 25 kg sacks or larger intermediate bulk containers (IBC). Whatever the delivery format, there must be an incoming quality assurance process to ensure the resins, and the additives, are correct to the relevant specification.

New technologies are being developed to control the particle size of functional fillers, such as carbon black (anti-static) and clay (improved barrier). These particles are nano in size, the filler being reduced to this size within the process, thus reducing any potential health hazards. They are exfoliated onto the polymer surface prior to the polymer being added to the extruder (Fig. 14.2).

14.2 The use of carbon black nanofillers to produce anti-static polyethylene carbon nanocomposites. (reproduced courtesy of Polyfect Solutions Limited; www.polyfectsolutions.com)

The polymer and additives travel along the heated barrel of the extruder. Different temperatures are applied to separate zones down the length of the outside of the extruder. These heater bands ensure that sufficient heat energy is applied to the resin, melting it prior to it reaching the breaker plates and filters. The breaker plates and filters are situated between the end of the screw and the melt thermocouples. The filters are there to ensure unmolten polymer (often high molecular weight polymer, known as gels) and debris are held back and do not contaminate the component.

The resin enters the throat of the extruder and is immediately transferred down the barrel by the screw. The screw is designed so that the core diameter increases along its length. This is to ensure that as the resin melts the decrease in occupied volume is accounted for so that the polymer melt continues to be worked, providing most of the energy to change the polymer from solid to liquid. To keep the feed hopper cool and to ensure the polymer does not overheat and therefore degrade, cooling systems are placed around the barrel alongside the heater bands.

Temperature control of the polymer melt is very important. There are normally three to eight temperature zones, depending on the size of the extruder. The heating is controlled by electric heating bands and the cooling by forced air or chilled water contained in pipes. The die has separate heating zones (no cooling required) to control the temperature of that area independently (Table 14.1).

Table 14.1

Plastic conversion process: process temperatures for some common thermoplastics

Polymer Acronym Processing temperature (°C)
Low density polyethylene LDPE 150–315
High density polyethylene HDPE 200–280
Polypropylene PP 205–300
Linear low density polyethylene LLDPE 190–250
Polyethylene vinyl acetate EVA 150–205
Polyethylene vinyl alcohol EVOH 200–220
Ionomer None 180–230
Polystyrene PS 180–260
Polyvinyl chloride PVC 160–210
Polyethylene terephthalate PET 260–280
Polycarbonate PC 245–310
Polyamide PA 240–290

Polymers melt at different rates. The more crystalline the polymer, the shorter the temperature range from start to completion of melting and therefore the quicker the volume loss. Extrusion screw profiles are designed for specific polymers (Fig. 14.1 shows the difference between a gradual and rapid transition screw). Some polymers, for example PVC (polyvinyl chloride) and PVA (polyvinyl acetate), give off acidic fumes when processed through the extruder, therefore extruders and extruder screw and parts need to have special coatings (e.g., chromium plate) to ensure the steel is not subjected to excessive wear.

Pressure control is also critical. The required pressure depends very much on the melt viscosity of the polymer and the forming process. For example, injection moulding requires high pressure to force the molten material through a small orifice whereas the relatively wide die used in extrusion moulding means less pressure is needed. Pressure can build up inside the extruder and in certain circumstances, if not controlled, be high enough to cause an explosion. Extruders are equipped with safety devices which rupture if the pressure builds to a dangerous level. Pressure fluctuation also leads to inconsistent output leading to inconsistency in the formed components.

The extruder feeds a die, which defines the shape and/or quantity of polymer which is fed from the extruder. The die can be a simple slot or annular die form, used for cast sheet and film manufacture, or more complicated to manufacture a wide variety of solid and hollow profiles. For coextrusion processes there are multiple extruders feeding into one die. Extruders for injection moulding are of a different design and are covered later in this chapter.

14.3 Sheet and film extrusion

Sheet is thicker than film, but there is no numerical value to define this. Sheet plastic as used for thermoforming is mainly, but not exclusively, made by the cast extrusion method. Films can be made by two main methods:

• cast extrusion, using a slot die

• blown extrusion, using an annular (ring) die.

There is little difference in the properties of the film produced by each method, although mono orientation is difficult in blown extrusion and the optical properties of blown film can be less than cast. However, the MFI (melt flow index) and melt strength of a polymer affect its suitability for each process. Melt flow index is a measure of the resistance to flow (viscosity) of the polymer melt at a given temperature under a given force for a predetermined period of time.

The polymer’s molecular weight and molecular weight distribution have a direct influence on the MFI: the higher the molecular weight of the polymer the higher the viscosity; the less the polymer flows over a given time, the lower the MFI for any individual polymer. The wider the molecular weight distribution the less resistance to flow; the more material flows over a given time, the higher the MFI. PET homopolymer and PP homopolymer both have low melt strength which makes it difficult to produce either of them using the conventional blown film process, where the bubble is blown upwards. Thus PET is more commonly made using the cast process, and while PP is made using the blown process, it requires a special adaptation to be successful (see later).

14.3.1 Cast sheet and film extrusion

The cast extrusion process can be used to produce film or sheet as a monolayer or a multilayer coextrusion. Multilayer coextrusion requires one extruder for each different type of polymer used. The number of polymers required is often greater than the number of functional polymers used, as tie layers (adhesive) are required where two incompatible polymers (e.g. EVOH and PP) are adjacent to one another in the multilayer construction. The molten polymer is transported by the extruder(s) into a slot die. It is here where the polymer layers combine as shown in Fig. 14.3.

14.3 Typical PP/EVOH/PP barrier layer coextrusion for a thermoform sheet (including process scrap).

The slot die in Fig. 14.4 has a narrow opening, which is adjustable and controls the flow rate as well as the initial thickness of the emerging film. There is also a reservoir to help prevent polymer surge through the die, which would result in uneven film thickness and thus uneven performance in use. The polymer is extruded through the slot die which is often a series of small slots adjacent to one another rather than one wide slot, and falls onto large chilled metal rolls to form either sheet or film. Providing the film or sheet are not stretched, there is no orientation of the material (see below). It remains the same width and thickness as controlled by the slot die and has no stiffness, stress or other change to its physical and barrier properties (see Fig. 14.5).

14.4 Cross-section of a slot-orifice die.

14.5 Cast-film extrusion line.

For sheet applications the plastic melt is often extruded onto a temperature controlled three-tier calender stack instead of a chill roll. This smoothes out the surface of the sheet and adds a textured finish if required. The calendered sheet is then cooled by passing through a number of chill rolls or a quench tank, before being wound up ready for despatch for further conversion such as thermoforming into pots and tubs. In some cases, especially when using polypropylene homopolymer which has a low melt strength, it is better to extrude the sheet in line immediately before the thermoforming process. This overcomes the difficulties in controlling the re-heated web on a conventional thermoformer, fed with unmolten preformed sheet requiring reheating.

Film properties can be improved by physically orientating the film in one or two directions (as mentioned in chapter 12) (see Fig. 14.6). Mono-orientation can be achieved by pulling the film in the machine direction (MD) at a faster rate than it is being extruded, i.e. the take-off speed is increased. This realigns the polymer molecules in the direction of stretch, rather than leaving them in their ‘natural’ random state. Stretching can occur immediately the melt comes into contact with the chill roll or, more commonly, after the first chill roll, often requiring reheating before it is stretched. Mono-orientated film can also be produced by using the stentering method, which stretches and orientates the film in the cross direction (CD). Mono-oriented film is used for shrink sleeve and roll on shrink on labels and pallet strapping. If both orientation mechanisms (MD and CD stretch) are performed in one process, the film becomes biaxially oriented. This method is commonly used to manufacture polypropylene (BOPP) and polyester films. The film is stretched in the machine direction first; grips then take hold of the edges of the film and gradually stretch it in the cross direction. Films can also be stretched in both directions simultaneously.

14.6 Orienting cast film: clips grasp the film along each edge and stretch it in the cross direction, while acceleration in the machine direction orientates the film in the machine direction.

If the polymer being orientated is crystalline it must be below its melt temperature (Tm) but higher than its glass transition temperature (Tg) to maximise orientation. If the temperature is too high the less the orientation achieved; if the temperature is too low uneven stretching causes thin spots and even rupture. Where the film must remain thermally stable in use, it is annealed at controlled temperature and then cooled to ‘freeze’ the orientation before the tension is released. This helps to overcome the tendency of the polymer to return to its natural, more random molecular arrangement when heat is applied.

Biaxial orientation has the following effects on the properties of a film:

• improved moisture barrier

• improved gas barrier

• improved tear resistance

• improved mechanical properties such as tensile strength

• impaired heat-sealing characteristics.

Most films produced using cast extrusion are coextruded or coated films, especially where coefficient of friction, barrier and heat seal are important. Using a coating or polymer layer with a lower heat seal temperature than the main film means that the resultant multilayer film can be heat sealed without melting the main substrate (Fig. 14.7).

14.7 Coated overwrap film for tobacco packaging (shrink properties can be added to ensure a tight overwrap of the carton).

14.3.2 Blown film

Blown film, for all but polypropylene homopolymer (which is easier to blow downwards – see later), is routinely carried out by forming a bubble vertically upwards, as shown in Fig. 14.8. From the plasticating extruder, the molten polymer enters the annular die and is formed into a tube of material. This tube is taken up to the nip rollers where it is sealed, then air is introduced inside the tube to inflate it, creating a bubble. The inflation of the bubble increases its diameter thus orientating the film in the cross or transverse direction. The greater the ratio of the diameter of the bubble to the diameter of the annular die, the greater the orientation. This is known as the ‘blow up ratio’ or ‘blow ratio’, which is determined by the melt strength of the polymer; the greater the strength the higher the blow ratio that can be used.

14.8 Standard blown-film process.

Chilled air is blown on the outside of the film to cool the polymer bubble below its melting temperature Tm. The frost line is the point at which crystallisation occurs as the melt solidifies; as a result some transparency is lost. It is important therefore that the cooling speed is controlled carefully, to manage the change from liquid to solid state. The slower the film is cooled, the larger the crystals formed, resulting in less transparency and gloss of the resulting film. Some extruders employ internal cooling to increase production rates. The external and internal cooling air is normally refrigerated to allow for better control of the final properties of the film.

To achieve machine direction (MD) orientation, the film is stretched in the longitudinal direction by drawing it through the nip rolls at a faster rate than it is coming out of the die. The final thickness is controlled by the die gap and the amount of orientation imparted to the film. Orientation in both directions takes place while the polymer is still molten (Fig. 14.9).

14.9 Cross-section of an annular blown-film die.

It is important to control the symmetry of the bubble, i.e. the area of film on both sides of the centre line must be equal. If the film is uneven in either thickness or solidification, the bubble will be asymmetrical and the thicker side of the bubble will not expand as much as the thinner side, thus the gauge of the film will be uneven across the web. Also, as the thinner side will expand to a greater extent than the thicker side, it is possible that this will lead to excessive thinning and the bubble will burst. Certain parts of the film have high spots caused by imperfections in the process. These are often in the same place across the web of the film and when it is wound up these high spots can multiply, resulting in a ridge in the finished roll of film. To prevent this happening either the die, or more commonly with mono films the bubble frame and nip, are rotated to and fro (oscillated) to evenly distribute the high ridge, greatly reducing any adverse effects.

Once cooled the film approaches the nip roll and the bubble is gradually flattened into what is known as lay flat tubing. The nip rolls, one metal and one rubber transport the film to the in-line slitters and roll winders at the base of the line (see Fig. 14.8).

The slitting of the film can be carried out off-line if a high level of dimensional accuracy in slit width variation is required. When very wide films are required (e.g. agricultural film) the lay flat tube is slit on one side only, to allow the film to be opened out to its full circumference in use.

The bubble film method is used mainly to produce monolayer or co-extruded polyolefin films, used for stretch and shrink wrap materials. Stretch wrap is used to hold pallet loads of transit packaging in place. It works because when stretched, the polymer molecules want to return to their original formation, providing the elastic limit has not been exceeded. Most stretch films are made from modified LLDPE. EVA blends and plasticised PVC are also used. Coextrusions are produced to add cling features to one or both sides of the film. Ideally stretch film should have:

• a smooth surface to improve cling

• good elasticity (elongation and recovery)

• transparency (some special films are opaque)

• high tensile strength in the machine direction (MD)

• high fatigue resistance (relaxation with time)

• low creep properties

• good puncture resistance

• good tear resistance

• low neck-in properties (reduction in film width when stretched).

Some stretch wrap used for wrapping pallets (especially in the brewing industry) is perforated to allow the free flow of air through the pallet, reducing the effects of condensation.

Shrink wrap also relies on a film’s tendency to want to return to its unstressed state. Shrink wrap relies on the in-built stresses in the film being stable until heat energy is applied. This allows for the shrink wrap to be loosely applied to the pack and heat sealed into a loop. It is only when heat is applied to the film that it shrinks tightly around the pack, holding the contents in place.

Shrink film is made by orientating the bubble in both directions and then freezing the stressed molecules by freezing the film as quickly as possible. When heated the in-built stresses are released and the film shrinks. Lightly crosslinked materials with elastomeric properties are often used to increase the shrink properties of the film. Shrink wrap is made from a variety of polymers, including LDPE, LLDPE, PP copolymer and PVC. The properties required from shrink film are similar to those required for stretch film:

• transparent (some special films are opaque or all over printed)

• controlled shrink ratio is both directions

• good puncture resistance

• good heat-seal characteristics

• appropriate slip characteristics

• low creep properties

• high fatigue resistance

• good tensile strength.

While polypropylene copolymer films can be made using the blown process described so far, as already mentioned polypropylene homopolymer has low melt strength and a better approach is to blow vertically downwards as shown in Fig. 14.10. Molten polymer travels to the annular die as already described, but instead of being immediately blown, it is formed into a cast tube which runs vertically downwards. This tube is then reheated to its softening point and inflated to form a large transparent bubble, which effectively orientates the material equally in both directions. The bubble is then collapsed and two knives are used to slit the film into two webs. These are annealed to reduce their tendency to shrink and may also be surface treated to improve print adhesion (as discussed in Chapter 12). BOPP film produced in this way is very similar in properties to cast BOPP already discussed and competes in the same markets. BOPP PP film has many uses, from overwrap for cigarette packets and chocolate boxes to FFS packaging for fresh produce and in its coated and laminated form, bags for potato chips (crisps).

14.10 The ‘bubble’ process for producing biaxially-oriented polypropylene (BOPP). (courtesy of Innovia Films, www.innoviafilms.com)

14.3.3 Cellulose film

There is one other transparent film type used in packaging which was the forerunner of BOPP. That is cast regenerated cellulose (Cellophane™ and much more recently the biocompostable regenerated cellulose film NatureFlex™). As it is not made from oil, but from cellulose fibre, it is neither thermoplastic nor thermosetting and is not a true plastic. However, it is a useful packaging film. Wood pulp produced by the chemical process (see Chapter 10) is chemically converted into a thick liquid form called viscose, which is extruded through a flat die into a regeneration bath (Fig. 14.11). At this point the viscose converts into a solid thin film form. Many processes incorporate two dies on the same machine, allowing the manufacturer to double output on the same casting machine. The web is carried down the casting machine on rollers, through a series of baths which wash and soften the film in order to produce a kind of ‘transparent paper’. At this stage the film is transparent and glossy but has no heat seal and moisture barrier as one would expect from cellulose. In most cases, an anchor resin is applied in the final bath, prior to drying, to prime the surface to make it receptive to secondary coatings applied off-line. The secondary coatings are tailored to provide the heat seal and barrier properties for the intended use, (e.g., PVdC (polyvinylidene chloride) to provide a heat seal, gas and moisture barrier).

14.11 The casting process for producing regenerated cellulose film. (courtesy of Innovia Films, www.innoviafilms.com)

14.3.4 Coextrusion of cast and blown films

Coextrusion is the process used to combine two or more different polymers during the extrusion process. The use of three layers is common but more than nine is possible to achieve a variety of functional benefits by careful choice of each layer. Aesthetic effects such as coloured layers and layers with coloured stripes can also be achieved. The combining takes place while the polymers are in the molten state, just before the extrusion die in the cast process and just after the extrusion die in the blown film process. This allows the different polymer layers to bond together, without mixing, to form a laminar structure. The purpose of using this technique is to maximise the properties of polymers at optimum cost (see Fig. 14.12). Coextrusion can also be used to produce cast sheet, as explained earlier, with coloured stripes or layers as well as sheet for thermoforming containing barrier layers such as EVOH.

14.12 Coextrusion processes: slot cast coextrusion die (left) and annular blown film coextrusion die (right).

Each polymer type requires its own extruder. All the extruders feed into an adapter, known as the feed box, before entering the die or directly into the multi-manifold coextrusion die (Fig. 14.13). In the cast process the individual extruders connect with the feed box, where the polymers combine in layers. This permits a simpler die design. However, multi-manifold dies (as used for blown film) are used where the flow properties of the polymers are widely different. The multi-manifold system provides a shorter flow path before the polymers solidify and therefore less chance for distortion at the interlayer interface.

14.13 Cast film coextruders feeding into the feed box and die.

When some materials are combined together (e.g., HDPE and PA or PP copolymer and EVOH), their adhesion to each other is very weak, which would result in delamination during subsequent conversion processes such as printing or bag making.

To overcome this, a third component has to be added, to act as a tie or adhesive layer. As mentioned previously, this means the use of an additional extruder for every tie layer. Processing of the coextruded film is the same as for the monofilm, described earlier. Once films are reeled, they often have to be left for 48 h for the molecular structure and slip additives to stabilise before proceeding to the conversion stage.

14.3.5 Performance parameters of polymeric films which require strict control

Packaging films require control of many performance parameters to ensure that there is consistency throughout conversion and use of the film. The main areas which must be specified are listed below:

• Thickness: Control of thickness is very important for ensuring consistency of heat sealing, printing and mechanical strength. Thickness of film is difficult to measure by hand as individual points will vary considerably. It is therefore important to take a number of measurements over a small area using a micrometer with a 25 mm head, and to average the results.

• Basis weight: Basis weight, also known as grammage is an alternative to thickness as a way of controlling film.

• Density: Once thickness and basis weight are known, the yield can be determined, i.e. the number of square metres that can be obtained for 1 kg of polymer.

• Moisture barrier: Where appropriate, moisture vapour transmission rate (MVTR, also known as water vapour transmission rate, WVTR) needs to be accurately measured at a predetermined temperature and humidity.

• Gas barrier: Where appropriate, gas barrier (e.g., oxygen, carbon dioxide, nitrogen) needs to be measured at a known temperature and humidity. Separate measurements need to be taken for each individual gas.

• Grease barrier: Not all polymers have a grease barrier that is required for high fat content products, such as butter and dry pet foods. The type of fat is also important. There are several test methods which usually reflect the actual filling and storage conditions using the product provided. Where this is not possible, tests are carried out with chemicals selected to provide a guide to the barrier properties of the particular film.

• Coefficient of friction (CoF): Coefficient of friction is the reciprocal of slip. The more slippery a surface is, the lower is, the coefficient. There are two types of test. One uses an inclined plane where the angle is altered until the sample slides. The second and far more accurate method uses a moving sled on a flat bed. This method is used to determine both the static (CoF at the point the sled starts to move) and the dynamic (CoF as the sled is moved) at a constant rate under controlled conditions of temperature and pressure. It is very important to have a constant CoF within a film, otherwise it will not move over packaging machinery smoothly, having a negative effect on the line efficiency.

• Heat-seal temperature: Polymer films are complicated, ranging from films made up of one or many monomers having varying properties to films that are coextruded, laminated or coated with other polymers. The ideal film for heat sealing is one where the outer part of the film has a significantly higher sealing temperature than the inner, and the inner part of the film has a very wide sealing range. To ensure the correct film is selected and that the heat-sealing characteristics are uniform throughout the web, tests are carried out where the temperature, dwell time and pressure are varied to determine the ideal sealing conditions.

• Cold-seal strength: Sometimes, for example when packaging a chocolate bar, a cold-seal adhesive is used. The same type of test is carried out as for heat seal to ensure consistency of performance.

• Tensile strength and elongation at break: Tensile strength determines how much force is required to break the film of a given thickness and the elongation determines how much it will stretch before it breaks. Both these parameters are important especially for a printed film. The film has to pass through the tensioning rollers of the packaging machine without breaking or distorting the print.

• Stiffness: Stiffness of films is very important when making bags, sachets and pouches which need to stand up without sagging. It is also important when placing a bag into a carton. The thicker the material the greater the stiffness but the higher the cost; therefore for many applications stiffness at lowest thickness is a very important attribute.

• Puncture resistance: Plastic films are used to pack many items which have sharp edges. If the puncture resistance is too low then the product will place a hole in the film thus significantly reducing any barrier which has been engineered into it.

• Surface energy: Polyolefins have a poor surface energy which results in print and adhesive not bonding to the surface of the film. To overcome this, the surface is activated by corona discharge, flame treatment or application of a special coating. This increases the surface energy to about 42 dynes/cm which is sufficient to ensure the surface print or adhesive keys to it

Other characteristics of the film may also be tested. These would include gloss, haze, optical density, anti-fog, anti-blocking and direct food contact. There are other market specific tests which need to be carried out, such as:

• reel geometry – accuracy of slitting and reeling – edges parallel

• which way wound? – side A or side B on the outside

• how palletised? – care of edges and flattening of the reel – reels of film should be stored and transported suspended between A frames or for less critical films (e.g., stretch and shrink) on their ends with or without edge protection.

14.4 Film treatments after forming

Coextruded, laminated, coated or single material flexible packaging can be found in thousands of different specifications, specially developed to suit the needs of the product, machinery, distribution chain, aesthetic, convenience and environmental considerations. Examples are shown in Figs 14.14, 14.15 and 14.16. Films can, for example, be given special treatments or coated to improve their properties. Examples include corona discharge to improve surface adhesion and vacuum deposition of mineral oxide, metal oxide or metal particles (Fig. 14.17). Films are treated with a vacuum deposition to improve aesthetic and barrier properties. Aluminium is the most common vacuum deposition material used in packaging, providing a very bright silvery surface improving the UV, gas and moisture barrier of the film. Aluminium oxide and silicon oxide are also used where clarity is important, but improvement of gas and moisture barrier is still necessary. The two main films metallised are PET and PP copolymer. The new biocompostable films (e.g., PLA) are now being treated by vacuum deposition in an attempt to improve moisture barrier.

14.14 Example of a typical packaging laminate. (courtesy of Elopak; www.elopak.com)

14.15 Example of a laminate for a carton containing liquid. (courtesy of Tetra Pak; www.tetrapak.com)

14.16 Examples of laminates for specific product applications.

14.17 Film metallising chamber.

14.4.1 Lamination and coating of flexible materials

As already discussed, it is possible to combine polymers using coextrusion, but this is not always practical, for example for short-run lengths of specialised materials. Also, it is not possible to include layers such as aluminium, paper and paperboard in coextrusions. Lamination processes may be required, in which webs of individual materials are combined using adhesives (Fig. 14.18).

14.18 Extrusion coating process. (courtesy of Walki Group; www.walki.com)

The simplest laminate is paper or paperboard extrusion coated with a polymer on one or both sides. Polymer granules are placed in the extruder where they melt and pass through a slot die. The extruded film width and thickness are controlled by the die and the speed of application of the plastic to the substrate. The melt is at such a temperature that it will adhere to the substrate after which time it is passed over a chill roll which cools the melt prior to it being wound up into a reel. If both sides of the substrate are to be coated, the procedure is repeated or another extruder is situated in line with the first. This process addresses some of the inadequacies of the base material and typical end uses include ream wrap, sandwich cartons, frozen food, pet food and soap powder bags, sacks and cartons. The polymers normally used are PE, PP and PET. Hot melt adhesives are also applied in this way or via a roller application (Fig. 14.19).

14.19 Process for coating both sides of a substrate. (courtesy of Elopak; www.elopak.com)

More complicated extrusion coated/laminated structures are used for liquid packaging cartons. Here two or more extrusion heads are used to produce a laminate of plastic/paper/plastic/aluminium/plastic. The paper gives rigidity and a good printing surface, the aluminium provides barrier to UV, oxygen and moisture vapour and the plastic seals the surfaces together. The plastic also provides an external barrier to condensation and an internal barrier to product penetration into the paper.

In addition to extrusion lamination, other laminating techniques which can be used, depending on the combination required and the end use, are dry bond lamination and wet bond lamination. Dry lamination can be achieved by a variety of means. The oldest is probably wax bond lamination (Fig. 14.20). This will bond substrates together but as it is a non-polar material, it relies mainly on mechanical bonds, for example to paper, but can achieve moderate chemical bonds to aluminium and plastic substrates. The wax is applied molten to the substrate via a wheel applicator; whilst the wax is still molten a second substrate is applied to the wax coating on the first and the whole is bonded by passing through a nip roller and cooled via a chill roller immediately prior to being reeled up. One of its most common uses is in lamination of foil to paper in the wadded closure used for jars of coffee. The laminated wad in the closure is induction sealed to the top of the glass jar. The heat generated melts the wax which is absorbed into the paper. When the jar closure is unscrewed the waxbond breaks leaving the paper wad in the closure and the aluminium foil diaphragm sealed to the jar.

14.20 Wax bond laminator.

Dry bond adhesion is very useful when two non-porous materials need to be laminated (Fig. 14.21). Water- or organic solvent-based adhesives are applied to one surface, and the solvent is driven off in a heated oven. The hot tacky adhesive-coated substrate is then laminated to a second substrate, using a nip roller which is often heated to ensure the dried coating is still active. It is important that all solvent is driven off prior to bringing the two substrates together, otherwise it is trapped between the substrates, which can have a negative effect on adhesion and also can contaminate the packed product. It is also necessary to consider the management of the volatile solvents given off in the drying process, to comply with relevant legislation.

14.21 Dry bond laminator.

The final type of dry bond adhesion to be discussed is the use of two-part polyurethane adhesives (Fig. 14.22). These are 100% solid and therefore, as with extrusion lamination, do not require drying. They are applied to the web at ambient conditions and rely on a chemical crosslinking reaction for the bond to be completed. This can take up to 48 h before the laminate can be further converted, e.g. slit into smaller reels, otherwise delamination may occur. Once cured these adhesives have much higher temperature resistance than their thermoplastic counterparts and therefore can be found in the construction of retort pouch laminates.

14.22 Wet bond laminator.

Wet bond lamination is used where one of the substrates is porous (e.g., paper). The two substrates can be brought together before the oven, where the heat dries the adhesive and the substrates. Care needs to be taken during drying to ensure the moisture profile along the length and across the web of the finished laminate is stable.

Coatings can be added to substrates during the lamination process (as with wet bond lamination), as part of the printing process (e.g., cold-seal adhesives for sealing of confectionary bars) or as a separate operation (e.g., water-based functional coatings to replace polymer films and fluorocarbons, for pet food and fresh produce). They can be added via a gravure or anilox roller, or knife or rod metering system (Fig.14.23).

14.23 Gravure coating process (engraved cell shown enlarged on left).

Often the board is preprinted before it is coated with polyethylene. The tie layers ensure good adhesion between incompatible substrates. The barrier layer is usually aluminium but could be ethylene vinyl alcohol (EVOH) coextrusion. In Fig. 14.15 you will notice a difference from the specifications shown in Fig. 14.14. Liquid packaging serves a vast market with products from acidic orange juice to high fat dairy goods, all requiring very different packaging performance.

From lids for single portion jam pots (replacing traditional aluminium) to collapsible tube laminates for toothpaste, film for tobacco overwrap, savoury snacks, shampoo sachets and many, many more end uses, flexible packaging can be specified to replace other more traditional forms of packaging. It uses minimal materials and can be formed into sachet, bag, carton, can and bottle shapes. For example, retort pouches are currently replacing metal cans, sachets and pouches with reclosable pour spouts are replacing traditional bottles, cartons are being replaced with block bottom bags with reclosable zip systems. These flexible packs rely on judicious choice of materials to meet the end use and the requirements of the packaging machinery on which they will be formed. These requirements will be covered in Chapter 20, but at this point it is important to note that all of these processes rely on the sealing properties of the packaging material being compatible with the sealing characteristics of the relevant forming machine.

14.5 Thermoforming process for making plastic packaging

Thermoformed packaging produces a less dimensionally accurate moulding and less complicated shapes are achievable (e.g., undercuts) but is often quicker and less expensive to produce than an injected moulded container. In its simplest form the thermoforming process involves heating a sheet (which can be mono material, a coextrusion or a laminate) of even thickness and drawing it over, or into a mould to form a rigid or semi-rigid shape. The excess material is trimmed off usually, leaving a rim around the finished article. The greater the depth of the object to be formed, the more likely it is that the material will thin, even to the point of breakage, and this is one of the most serious disadvantages of this relatively simple process. This unwanted thinning can be reduced by various means as will be mentioned below.

The thermoforming process is used to make many different packaging articles, for example, tubs, pots, display trays and blister packs, and can be broken down into seven basic steps:

1. Making the sheet – normally by the cast extrusion method.

2. Heating the sheet.

3. Forming the pack by stretching the sheet either into or over a mould.

4. Cooling the formed packaging.

5. Cutting and trimming the multi-unit moulded sheet into individual units. This can be carried out at the same time as the forming process.

6. Printing or decorating as required.

7. Stacking the individual units before packing and labelling for despatch to the customer.

As with all processes, quality control throughout is crucial.

Sheet extrusion is described earlier in the chapter. The sheet can be foamed (e.g. EPS expanded polystyrene or cellular polypropylene or polyethylene) if required. As already noted, if polypropylene is being cast, it is usual to extrude the sheet in line with the thermoforming. Coextruded multilayer sheet is the most popular for thermoforming as it provides the performance and barrier characteristics required. Many packaging suppliers make the sheet and thermoform it into the required pack shape, while some packer-fillers thermoform trays from a pre-supplied reel of sheet material, place the product into it, and heat seal a flexible lid onto the tray, all on one machine.

Once the sheet is heated, the container can be formed using different methods. The traditional method was to drape the heated sheet over a cavity or plug mould, draw a vacuum and form the sheet to the shape of the mould (Fig. 14.24). This method is adequate for shallow thermoformed packaging components of uncomplicated design. If the depth is greater than the diameter then plug assist vacuum forming is the better choice (Fig. 14.25). In conventional vacuum thermoforming, the sheet is formed, the wall thins and there is a risk that the sheet will not conform well to the contour of the mould, especially in the bottom edges. Plug assist overcomes some of these inadequacies by acting as a heat sink and displacing the material in a more even manner, reducing the thinning of the wall section. This is especially useful for deep-drawn items.

14.24 Vacuum forming over cavity and plug moulds.

14.25 Plug assist vacuum forming.

‘Vacuum snap back’ can also be used where overall wall thickness becomes important (Fig. 14.26). This is used for items where the depth of the article is up to 2.5 times greater than its width. The initial stretching of the sheet is free of any contact points and therefore more even than in the direct forming processes already described. As a result, the wall thickness throughout the item being formed is much more uniform. Where the depth of the article is greater than 2.5 times its width, ‘billow’ forming can be used (Fig. 14.27). Heat and/or pressure cause the sheet to billow upwards, a heated plug is introduced and the vacuum is turned on, forming the sheet over the mould.

14.26 Vacuum snap back forming.

14.27 (a) Billow forming over a plug and (b) billow forming over a cavity.

Solid phase pressure forming (SPPF) has been developed to produce thermoformed articles with better definition, especially at higher pressures. The process has improved the quality of thermoformed articles to the extent that it can now compete with injection moulding for some moulding designs. It is also possible to mould two sheets, one over the other, at once. In solid phase pressure forming the sheet is reheated inside the machine until it becomes plastic and easy to form (Fig. 14.28). Using plugs and then compressed air, the sheet is pushed into the shape of the mould. It is here that by using high pressures moulding definition is greatly improved, the plug reducing the variation in wall thickness which occurs with conventional thermoforming. Once frozen, the shape is cut out of the web – this is completed while the article is still held to ensure the accuracy of the cut. The articles are ejected through the front of the machine.

14.28 Solid phase pressure forming.

Before the moulding is removed from the mould, it must be cooled so that deformation is avoided. This is very important when thermoforming polypropylene as it has a very high shrinkage rate and, if not cooled sufficiently, it can continue shrinking for several days after it is removed from the mould. Cooling is carried out using temperature controlled water directed to cooling channels designed into the tooling. Once the article has been formed and cooled, it needs to be cut from the sheet. The cutting process leaves a rim around the article, which can be used for heat sealing and as a lip for an overcap. The forme used for cutting is similar to that used for cutting paperboard cartons. Modern developments have made it possible to minimise the lip on the article.

Thermoformed items can be decorated using the following methods:

• In-mould labelling – this provides the best overall graphics of all the decoration methods; however, the adhesion of the label can be adversely affected if the item is too cold.

• Dry offset (offset letterpress) and screen printing – graphics are not as good as when using in-mould labelling, although quality is improving due to modern developments.

• Colour – can be added to the extruded sheet. Multicolour stripes can be added and placed in such a way to allow one section of the item to be decorated differently from the others.

• Surface finishes – can be designed into the final thermoformed article, but the quality of finish is not as good as with injection moulded items.

14.6 Injection moulding

The injection moulding process is suitable for all materials, only the tool design having to be changed depending on the shrink characteristics of the polymer/polymer combinations. Injection moulding is a process which converts polymer granules into one of the most dimensionally accurate moulded thermoplastic parts possible. It does so using a reciprocating or ram, plasticating extruder (Fig. 14.29). Dry plastic granules are added to the plasticating extruder in the same way as described earlier in the chapter for film and sheet extrusion. Most importantly, the drying time of the polymer and its masterbatch significantly affect the process time, with PET, PA and PLA needing between 6 and 12 hours for complete drying. To overcome this problem, pre-hoppers are used to pre-dry the polymer and liquid colorant is used to negate the need to dry the masterbatch.

14.29 Reciprocating (top) and ram (bottom) type of injection moulding machine.

Once heated in the extruder, the homogeneous, molten mass is then injected into a mould through a gate, known as the injection point. A predetermined mass of polymer, designed to completely fill the mould, is metered out within the injection moulding machine, by controlling the stroke of the reciprocating screw or the ram piston. The polymer (PP has a much higher shrinkage than HDPE, for example) and the colour used significantly affect the shrinkage of the moulding. This can result in a separate die being required or changes made to the cycle time and cooling conditions used, to ensure the moulding once cooled meets its dimensional specification.

The mould consists of two or more steel parts, one with a cavity accurately cut away to form the female section of the moulding, the other with a corresponding profiled section (Fig. 14.30). When the two halves are clamped together, the gap between the male and female sections of the mould corresponds to the shape, finish and thickness of the moulding required after taking shrinkage into consideration. The parts are locked together with a clamping force sufficient to ensure that both singleand multi-cavity moulds stay closed, until the moulding is cool enough to be ejected. Multi-cavity moulds are used to increase the number of units produced over a given time and therefore reduce the unit cost. The number of cavities possible is dependent on the surface area of the cavity and the locking force of the injection moulding machine, the larger the locking force the greater the number of cavities possible.

14.30 A multi-cavity mould.

The mould is cooled with temperature controlled liquid to ensure the mouldings cool as evenly as possible. Injection points are positioned so that the flow of material into the mould is as even and thus stress free as possible, and the whole moulding is free of weak areas where the polymer has flowed together in the mould. Injection points can be visible on the mouldings, therefore consideration must be given to this at the mould design stage. With care, they can be hidden from view (Fig. 14.31).

14.31 Injection points (gates) for an injection mould.

The injection moulding machine must have sufficient clamping force to prevent any of the injected plastic from escaping at the interface of the two halves of the mould. This would cause an unsightly part-line or excess material, known as ‘flash’ which has to be removed or, in severe cases, the moulding scrapped. Excessive shot size or injection pressure can also cause flash to occur, as can the use of an old, worn mould.

The size of the injection moulding machine is specified by its clamping force and injection capacity and the mould must be compatible with the machine (Fig. 14.32). Moulds are clamped together and opened using mechanical (toggle lock), hydraulic (direct lock), or a combination of both (lock and block or hydro-mechanical) methods. Mechanically operated machines use integrated hydraulic systems for the motive power. However, during more recent years, to improve cleanliness and reduce energy, servo-motors are used, hence the ‘all-electric’ machines. As stated, clamping forces have to withstand the internal pressure within the mould cavity, and injection pressures of around 2,000 bar (29,000 psi) are common.

14.32 Simple injection mould.

Normally the female cavity side of the mould is attached to the stationary end of the injection moulding machine (fixed or nozzle platen) and the male cavity side is attached to the moving end (moving or ejector platen). The molten polymer is injected into the mould through the fixed or nozzle platen. To ensure the two halves of the mould are precisely in line (necessary to achieve accurate mouldings), large guide pins are situated on the four corners of one side of the mould with matching locating holes on the other. If moulds are not vented effectively, the injected plastic cannot displace the air inside and therefore an imperfect moulding will result. The consequences can vary from an imperfect finish on the surface of the moulding to an incomplete moulding, due to the pressure build-up in the mould preventing free flow of polymer.

Moulded parts are ejected from the mould usingejector pins, air pressure or stripper rings. The ejector pins leave a tell tale ring in the moulding, stripper rings and air pressure are much kinder in this respect. Some polymers are not tough enough to withstand the high pressures employed when using air to eject, for example polystyrene is usually too brittle. Air pressure ejection lends itself to thin section parts rather than thick.

14.6.1 Mould designs

Mould design is an important part of the development process when considering the production of a new moulding. Some considerations are given below.

• The shape of the moulded part has a direct bearing on the time involved in making the mould, the cycle time of the moulding. For example, undercuts and deep screw threads often require a separate moving part which increases the cycle time of each moulding operation.

• Angles on the side walls are necessary to ensure the moulding can be removed from the mould.

• The surface finish of the moulding needs to be decided before the mould is completed. Embossing, etching and other finishes can be added to the moulding to enhance the aesthetic qualities of the moulding. Any imperfection on the finish of the moulding will transfer to every mould made.

• Weight, surface area and thickness of the moulding directly affect cycle time. Where mouldings have significantly varying thickness, cooling has to be controlled very carefully otherwise depressions form (sink marks) on the outer surface of the moulding caused by excessive shrinkage of the thick section of the moulding.

• The number of cavities in a mould is governed by the number of units per annum required. The number of cavities directly affects the size of machine required for the multi-cavity mould; this in turn affects the cost of the mould and the unit cost of the moulding. Where there are many cavities, each one must be uniform compared to the others and the cooling profile needs to ensure that the outer and inner cavities cool at an equal rate, otherwise moulds of differing dimension will result. Hot runner systems are usually used for multi-cavity systems. This reduces the cycle time and amount of waste material formed but increases the overall cost of moulds (see Section 14.6.2).

• The need for an insert or label to be inserted during the moulding sequence has to be considered at the mould design stage.

• All injection moulds require a point or points at which the molten plastic is introduced into them. If not considered at the design stage, this can leave unsightly surface blemishes on the finished moulding which require a further stage to remove them, incurring extra costs.

Successful removal of the moulding from the mould is another critical consideration in mould design. Some mouldings, such as closures, have undercuts, which complicates the mould design. Sometimes the moulding is flexible enough, and the undercut small enough to pop or blow it off the tooling without damaging the moulding. However, this is not always the case and a more complex mould design may be required (see Fig. 14.33). Other options are the use of a collapsible core or, in the case of threaded mouldings, the incorporation of an unscrewing device.

14.33 Mould design for parts with undercuts (in this case a cup with a pedestal).

14.6.2 Hot runner moulds

Figure 14.34 shows a hot runner mould (the figure shows a co-injection design which is discussed in more detail in Section 14.7.2). Feeding of the plastic material to the mould can be carried out using hot or cold runner systems. In a cold runner system, the sprue feeds polymer to the runner, which in turn supplies polymer to each individual mould within a multi-cavity tool. The polymer is then fed through the gate into the individual mould cavities. This feed system cools and is ejected with the mouldings which then have to be removed from the cold runner as a separate operation. The removed plastic is then sent to waste or reground and fed back into the hopper along with virgin material at a controlled percentage. Hot runner injection moulding tools, though more expensive to design and make, overcome these issues. The runner is either insulated within the mould preventing the polymer solidifying, or it is heated to ensure the plastic is held at the most efficient temperature. These ‘hot runner’ systems reduce waste and increase cycle time. Hot runner mouldings are now commonplace in plastic packaging applications.

14.34 Hot runner mould. (courtesy of Kortec Inc.; www.kortec.com)

14.6.3 Decoration of injection moulded parts

Just like paperboard and plastic films, injection moulded parts can be decorated by vacuum deposition of metals such as aluminium. Other types of decoration are possible and are covered in Chapter 18. Mould surface treatments such as spark erosion is used to give special effects, and mouldings can also be laser etched or coloured in a variety of ways. Colours can also be added together, to give unique multicolour effects, by using masterbatches of dissimilar MFI.

14.7 Multi-injection moulding

Multi-injection systems are used where more than one polymer or colour is required. This can be achieved in two ways:

• Two-stage injection moulding and overmoulding

• Single stage coinjection.

14.7.1 Two-stage injection moulding and overmoulding

The two-stage injection moulding process can be used to produce a component with different coloured areas or with a core and an outer skin (Fig. 14.35). For example, the core could be foamed product of one polymer, the skin could be a completely different polymer. Overmoulding of another object is also possible, e.g. overmoulding of a glass bottle with a clear resilient plastic such as an ionomer or a thermoplastic elastomer (TPE). The first polymer is injected into the smaller mould, the item is removed from the mould and inserted into a second mould adjacent to the first. This second mould is closed and the second polymer injected around the first moulding. The first stage can be injected at the same time as the second stage, once the process has started, therefore reducing cycle time. Overmoulding of glass and metal can be carried out in the second mould.

14.35 Two-stage overmoulding process: (a) step 1; (b) step 2; (c) step 3; (d) step 4, (e) step 5. (courtesy of Husky Injection Moulding Systems; www.husky.ca)

14.7.2 Single-stage coinjection

The single-stage coinjection process is shown in Fig. 14.36. The main polymer, e.g. PET when moulding a preform for injection stretch blow moulded bottles, is delivered from the first extruder. As the molten polymer travels along the hot runner system, a second extruder injects another polymer, for example PA. The more viscous PA travels through the core of the less viscous PET. The melting temperature of the materials should be similar. The main material (PET) is first injected into the neck finish of the mould. The core material is injected into the body portion of the preform. The base, like the neck finish of the preform, contains little or none of the core material to ensure maximum strength is achieved when the bottle is blown. The injection system is carefully sequenced to ensure the core material is delivered to the exact areas required, during the coinjection process, using a hot runner coinjection system.

14.36 Single-stage coinjection process. (adapted from Giles, G. and Bain, D. (eds), Technology of Plastics Packaging for the Consumer Market, Wiley, 2001)

One of the main uses for this technology is PET bottles and jars requiring an improved oxygen barrier and, in the case of carbonated beverages, a carbon dioxide barrier. The two types of barrier materials used are ethylene alcohol-based polymers and special polyamide and polyester polymers. The polyamide and polyester-based polymers actually act as oxygen scavengers, preferentially absorbing the oxygen as it passes through the PET, thus reducing the amount of oxidation of the contents of the container. Polyethylene naphthalate (PEN) is sometimes used as a minority percentage blend with PET, to improve the UV barrier and processability. It is, however, restricted in its use due to its high cost. To allow for easier recyclability, the different material layers in the coinjected preforms are not tied together. Once the neck is cut from the blown bottle and the bottle ground into small particles, the two polymers can be separated by water flotation and other methods. Where preforms are made combining PP and PET, as shown in Fig 14.37, the core barrier layer in the PP preform has to be incorporated all round the preform, including the neck and base areas. This is because PP has less barrier to oxygen than PET.

14.37 Coinjected PP and PET preforms. (courtesy of Kortec Inc.; www.kortec.com)

14.8 Comparing injection moulding and thermoforming

Table 14.2 compares injection moulding and thermoforming. Key issues to bear in mind in choosing between the two include the following:

Table 14.2

Thermoforming versus injection moulding

Thermoforming Injection moulding
Not suitable for all materials Suitable for all materials
Expensive material Less expensive material
Less expensive tooling More expensive tooling
Less time to make tooling More time to make tooling
Multi-barrier less difficult Restriction on barriers
Less accurate dimension Excellent accuracy
Undercuts restricted Undercuts possible
Poor distribution of material Excellent distribution of material
In-mould labelling difficult In-mould labelling possible
Good for short runs Less suitable for short runs
Surface effects limited Many surface effects possible
Lip or rib on packs No lip or rib on packs

• The amount of material used to make thermoformed items is always greater than for injection moulded items. This is because the injection moulded items only require the exact amount of material to fill the mould (thick and thin sections are predetermined at the design stage so as to reduce the amount of material). The thermoforming process relies on a sheet of material of even thickness.

• Thermoforming thins the walls as they stretch resulting in more material being required to compensate for the thinning. Once formed, the items have to be cut from the multimould, creating waste. Hot runner injection moulding creates no waste.

• The tooling costs for injection moulding are in general more expensive than for thermoforming, due in part to the extra accuracy required and the pressures involved in the process. However, the costs as well as the time required to make the tooling for both processes is becoming very competitive.

• Multilayer sheet for the thermoforming process is straightforward to produce, but multilayer injection moulding is still being developed, therefore predetermined barrier requirements are less difficult to produce with thermoforming than with injection moulding.

• Thermoformed mouldings do not have the high level of dimensional accuracy of injection moulded items. The thermoforming process relies on pressure and/or vacuum to form the semi-molten sheet into a mould. Injection moulding forces molten polymer into a precisely formed tool, under high pressure, ensuring the moulding conforms to the design of the tool.

• Undercuts are restricted to very shallow ones, on thermoformed mouldings. This is governed by the ability to remove the moulding from the mould. Injection mould tooling can accommodate any undercut by designing special tooling.

• Accurate screw threads are not possible with the thermoforming process.

• The distribution of material when in sheet form is consistent, therefore the material distribution after thermoforming the article is far less controllable than with the injection moulding process. As a result, mechanical and barrier properties are much more variable in thermoformed articles compared to injection moulded items.

• The temperatures and pressures involved in the manufacture of thermoformed articles are much less than in the injection moulding process. This results in the adhesion between in-mould labels and the moulded item being far superior in injection moulding.

• Injection moulding is comparable in cost to thermoforming on long runs (millions of items), especially when thin wall injection processes are used for such items as tubs for yellow fats.

• Thermoforming leaves a lip on packs, where the moulding has been cut away from the remaining sheet. There is also a tendency for a rib to be formed just under the orifice of the thermoforming where undue pressure has been applied to the moulding while still soft. Neither of these phenomena occur with injection moulded items, though in a moulding where there are thin and thick sections, sink marks can occur due to the extra shrinkage of the thicker section.

14.9 Blow moulding

Blow moulding can be achieved via an injection, extrusion or combination process. It is used where the orifice of the moulded item is smaller than the overall cross section of its body, for example bottles and jars.

14.9.1 Injection blow moulding

Injection blow moulding is a combination process. First, we have to injection mould a preform and then blow it into the shape required, thus two moulds are required: one for the preform and one for the final blown form. The following methods can be used for producing blown mouldings from an injection moulded perform.

The two-stage blow moulding process is used for standard and stretch blow moulded items (Fig. 14.38). The preform is injection moulded as a separate stage, in a separate machine. The preform is designed to have a profile and variable wall thickness to provide the correct mechanical and barrier properties in the final blown moulding. Once moulded, the preform is reheated (different zones of the preform are heated to different temperatures to best suit the final blown form requirement) and placed in the blow mould. Air is introduced via the preform neck and the preform is blown into the shape of the mould. Venting is necessary so that all trapped air is removed from between the moulding and the mould. The mould has cooling ducts incorporated into its design. The coolant is usually water maintained at a constant temperature. To produce bottles suitable for carbonated beverages, the gas barrier and mechanical properties (tensile and burst strength) of the final bottle have to be greater than for a non-carbonated product not requiring a gas barrier.

14.38 Injection blow moulding process.

Stretch blow moulding can be carried out on both injection and extrusion processes (Fig. 14.39). In injection (the most common use) stretch blow moulding, the preheated preform held on the stretch rod is placed in the blow mould. Both the rod and the preform are heated to a controlled constant temperature, usually just above the Tg of the polymer, the bottle finish area being kept cool so that it does not distort. Once in the blow mould, the stretch rod pushes the preform to the bottom of the blow mould, air is introduced through the rod which expands the preform to the shape of the blow mould. In this way the material is orientated in both directions; this improves clarity, mechanical and barrier properties. Very lightweight bottles with good moisture and carbon dioxide barrier and pressure resistance properties can be produced using this method. Improved oxygen barrier can be obtained by using barrier materials such as polyamides and polyvinyl alcohol, the former can be added as a monolayer, but usually the barrier (oxygen scavenger) is added as a separate layer in the centre of the preform.

14.39 Stretch blow moulding process.

Injection moulds for the bottle industry are multi-cavity (over 100 per tool) to ensure the cost is kept to a minimum (Fig. 14.40). The preform is so designed that once the bottle is blown, the neck finish and base are five times thicker than the wall of the final bottle. The incorporation of a barrier layer to the preform structure extends the shelf life of the packed product (e.g., beer) to an acceptable level. Barriers are also used in wine bottle manufacture but often added in the monolayer, rather than coinjected as for beer. As well as providing an excellent oxygen barrier, it is claimed to offer improved clarity compared to coinjected bottles.

14.40 Multilayer injection moulded PET bottles. (courtesy of Kortec Inc., www.kortec.com)

Single-stage injection blow moulding requires only one piece of moulding machinery, but still requires two sets of tooling (one for the preform injection mould, one for the blow mould). The preform is injection moulded, transferred from the injection mould to a reheating station and then transferred to the blow moulding station where the item is formed, cooled and ejected. The benefit of this process is that, for small quantities of items (e.g., cosmetics and toiletries) all mouldings can be carried out by the packer-filler, in one process.

14.9.2 Extrusion blow moulding

Extrusion blow moulding is a less expensive process than injection blow moulding and can provide a wide variety of barriers and features such as handles, but the dimensional accuracy is not as well controlled (Fig. 14.41). Some materials such as PET and standard PP homopolymer are difficult to impossible to extrusion blow mould on a commercial basis due to their low melt viscosities. However, polypropylene and polyester copolymers are available which can be extrusion blow moulded acceptably. The parison (hot hollow plastic tube) is extruded through an annular die. The thickness of the parison is controlled in the die by varying the wall thickness of the parison, whilst leaving the outside diameter the same. This is accomplished by having a conical inner sleeve, which can be moved up and down in the die, resulting in a controlled variation in wall thickness of the parison (Fig. 14.42).

14.41 Extrusion blow moulding process.

14.42 Control of parison thickness in the die.

This control is important, especially when producing bottles of complex shape with widely different cross-sectional areas from base to neck, for example a cylindrical bottle with a heavily waisted section for ease of holding, or figurine-shaped bottles used for children’s products. It is equally important for more regular shapes such as square or rectangular cross sections, where the plastic has to be stretched a long way from the centre at varying distances. The control of wall thickness of the parison is also important where, due to its weight, it flows downwards before it is taken into the blow mould (Fig. 14.43).

14.43 Problems in controlling parison wall thickness: (left) parison ‘neck down’ due to weight and (right) inflation of parallel-walled parisons in containers with variable diameters.

Parison thickness control is also important to ensure the correct weight of polymer is applied to the correct areas, based on the design of the final item, for example integral handles. To produce a handle, material needs to be ‘ stolen’ from the parison. Sufficient material needs to be placed in the area of the parison where the handle is to be formed to allow this. It is also important to control the handle area so that a balanced moulding results. Sometimes the neck is designed to be away from the centre of the moulding. This too requires redistribution of the polymer to ensure good performance of the final moulding. The control of the parison wall thickness ensures a minimum amount of material is used to produce the final moulding without compromising its performance. The parison is extruded to a given length (the length of the blow mould) and the blow mould is closed around it. Once closed, air is introduced to the parison, blowing it to the desired shape. The moulding is ejected once cooled sufficiently so that it will not deform as it comes out of the mould (Fig. 14.44).

14.44 Cooling of moulding ready for ejection.

The moulding process requires the blow mould to seal the parison by pinching it together. This results in excess material being squeezed out when the mould comes together, and this excess has to be removed. This is carried out by transferring the moulding, contained in the mould to a stripping area, where the excess material is cut off with sharp knives. This action leaves a tell-tale scar on the base of the moulding. This is an easy way to differentiate between an item produced by injection blow moulding, which will show an injection point nipple at the bottom, from an extrusion blow moulding. The waste material is, where acceptable, reground and coextruded, as a sandwiched layer within the parison, thus reducing waste and keeping costs to a minimum.

Multilayer extrusion blow mouldings are now the norm. The combination of different polymers provides a vehicle to increase the performance characteristics of the final moulding, whilst at the same time keeping costs competitive (Fig. 14.45). Multilayer preforms can be so designed as to leave the outside layer free of colour at a predetermined point, allowing for a translucent stripe to be incorporated, so that the product level can be seen. This is very convenient where multi-dose bottles are used.

14.45 Multilayer extrusion blow mouldings: layers for a high oxygen-barrier bottle (left) and a bottle with a coextruded transparent viewing stripe to view fill level (right).

The normal machine design for most extrusion blow moulding manufacture uses alternating moulds (either single- or multi-head). However, for very high volume on one design, rotary blow moulding machines are commonly used (Fig. 14.46). Here the air can be introduced via a needle through the side of the parison rather than the more common method of through the centre hole. As the needle is introduced and extracted before the moulding cools the needle hole self seals. This type of rotary system is often used for moulding HDPE milk bottles.

14.46 Alternating and rotary blow moulds.

14.9.3 Design specifications for blow moulded bottles

The standard terminology when describing bottles (and other blow mouldings) should be understood, especially when including in specifications (see Fig. 14.47 and Table 14.3).

Table 14.3

Typical specification for a carbonated soft drinks bottle

Resin intrinsic viscosity 0.78–0.82 g/cm3

Volume

  2,000 ml ± 20 ml (individual)

  2,000 ml ± 10 ml (average)

Fill level drop

  40 mm under 4 bar (function of the bottle shape)

Fill point variation (from bottle to bottle)

  max 2.5 mm (function of the bottle shape)

Bottle creep (24 h filled with 4 vol. CO2)

  diameter increase max 3%

  height increase max 3.5%

Deviation from perpendicularity

  max 9 mm

Top load (empty and peak values)

  > 200 N (function of the design and weight)

Drop test

  no leakage for bottle drop from 2 m height

Thickness

  minimum 0.25 mm

  in the heel 0.20 mm

CO2 loss, shelf life

  14weeks with 15% loss CO2

Stress cracking

  15 min in 0.2% NaOH (weight) at 4 vol.

Burst pressure

  > 8 bar

14.47 Standard bottle terminology.

14.10 Environmental considerations in plastic packaging

As covered in Chapter 5, the environment is a serious consideration when choosing the type of packaging for a product. Plastic materials are often viewed by consumers and ill-informed bodies as environmentally irresponsible. However, the consumer only sees them lying in the street, and the complexities involved when determining if a particular packaging material or design is good or bad for the environment compared to the alternatives are not well understood. Whether polymers are used for single- or multilayer items, the properties required for the end use, cost and environmental issues are the main considerations which determine the type and thickness used. Plastic items are all recoverable in some way or another (re-use, recycle, energy recover, biocompost); their main advantage is in reducing materials used and preserving the products inside, at the minimum total cost. One of the difficulties of film with respect to re-use and recycle is the cost of collection and sorting, as not all polymers are compatible. Incineration for energy recovery is possible and this simplifies the sorting process, although it requires special incinerators to ensure there is no pollution from the gases produced during the burning process.

14.11 Sources of further information and advice

Baner, A.L., Piringer, O. Plastic Packaging Materials for Food. New York: Wiley; 2008.

Baner, A.L., Piringer, O. Plastic Packaging: Interactions with Food and Pharmaceuticals. New York: Wiley; 2008.

Giles G., Bain D., eds. Materials and Development of Plastics Packaging for the Consumer Market. Oxford: Wiley, 2000.

Giles G., Bain D., eds. Technology of Plastics Packaging for the Consumer Market. Oxford: Wiley, 2001.

Hernandez, R., Selke, S., Culter, J. Plastics Packaging. Munich: Hanser; 2000.

Nicholson, J. The Chemistry of Polymers, 4th edn. Cambridge: RSC Publishing; 2011.

In addition to the above texts, the British Plastic Federation (www.bpf.co.uk) is a valuable source of information on all aspects of plastics, including material properties, industry applications and forming methods.

Other useful sources of information include: