BLOW MOLDING

Blow molding is a process to produce hollow objects. It was practiced with glass from ancient times, and the basic techniques used by the plastics industry have been derived from those developed by the glass industry. Currently, a wide range of blow molded bottles and containers are produced for use in food packaging.

In blow molding, a molten tube of thermoplastic (known as a parison) is surrounded by a cooled mold having the desired shape. A gas (usually air but occasionally N₂) is introduced into the tube causing the molten mass to expand against the walls of the mold where it solidifies on cooling. The mold is then opened and the bottle or jar ejected.

Generally, the process for manufacturing plastic bottles and jars consists of three stages: melting the resin, forming the parison and blowing the parison to produce the final shape. The blowing step may take from a few seconds to more than a minute for large shapes; the rate limiting step is the cooling of the molded shapes.

There are two techniques of plasticizing resin (i.e., making the material flow) and forming the parison:

1. Extrusion, which produces a continuous parison that has to be cut; this is the most common method used

2. Injection molding, where the parison is formed in one mold and then transferred into another mold for blowing

5.11.1 EXTRUSION BLOW MOLDING

Extrusion blow molding (EBM) uses many arrangements for making and forming the parison (Irwin, 2009). In the simplest method, a mold is mounted under an annular die and the parison extruded between the open halves of the mold. When the parison reaches the proper length, the extruder is stopped, the mold is closed around the parison, and the bottom of the parison is pinched together by the mold. A blow pin mounted inside the die head allows air to enter and blow the parison, which expands to fill the mold. The shape of the bottle or jar is defined, but the distribution of material (and, thus, wall thickness) is less well controlled. The cycle restarts after the part has cooled and the mold opened, as shown in Figure 5.11.

Since the extrusion of the parison is a continuous process, numerous systems have been devel-oped to use the full capacity of the extruder. One uses more than one mold, moving the filled mold away to cool while another is moved into position to receive the next section of extruded tube. The molds can be reciprocating ones, or can be mounted on a rotary table. In several food packaging applications (e.g., pasteurized milk), the bottles are blow molded and filled online in a continuous operation.

EBM is widely used with the following resins: HDPE, PP, PVC and AN copolymers. The most common blow molding resin is HDPE used to produce containers ranging in size from 30 mL to 200L.

The common grades of PET cannot be extrusion blown.

A related process is the production of coextruded bottles, where two or more extruders, each handling different plastic materials, produce a multilayer parison having the desired properties. For example, a high barrier, high cost material might be sandwiched between layers of a relatively low cost material to give a bottle with the desired barrier properties at an economical price. Coextruded structures with up to seven layers, of which one or more can be a barrier layer, are common today.

5.11.2 INJECTION BLOW MOLDING

Injection blow molding (IBM) is a noncontinuous cyclic process shown in Figure 5.12, and most closely resembles the blowing of glass bottles. The parison is formed in one mold and then, while still molten, is transferred to a second mold where blowing with compressed air forms the final shape. After cooling, the mold is opened and the bottle ejected. Several molds must be available if the injection molding machine is to operate near full capacity.

Extruder head Continuous plastic tube

Cutting blades

Split mold Airflow through

blow pin Finished bottle FIGURE 5.11 Extrusion blow molding of plastic bottles.

Injection mold

Preform

Blowing stick

Blown container Blow

mold Air

FIGURE 5.12 Injection blow molding of plastic bottles.

The major advantage of IBM over EBM is that the process is virtually scrap-free, the finished parts usually requiring no further trimming, reaming or other finishing steps. In addition, the dimensions of the bottle (including the neck finish) show very little variation from bottle to bottle, and with some materials, improved strength and clarity are obtained due to the effect of a limited degree of biaxial orientation (see Section 5.7).

The resins most commonly used for IBM are HDPE, PP, PS, PVC and PET. PET has replaced PVC in many countries, especially in Europe where PVC has a poor image among many consumers.

Coinjection blow molding has been developed using two or three injection units working with one mold to produce a preform that is later blow-formed using compressed air inside a mold to make a bottle or jar. The various component materials are metered into cavities in such an order that the barrier material flows through the main structural material to create a multilayer structure. This process is used to produce five layer retortable containers from three materials, typically PP as a structural layer and EVOH copolymer as a barrier layer with tie layers in between.

5.11.3 STRETCH BLOW MOLDING

Stretch blow molding (SBM) is a process where bottles with appreciable orientation in both longi-tudinal and TDs are produced; it is sometimes known as biaxial orientation blow molding (Irwin, 2009). Orientation in the TD only is produced in normal EBM, while appreciable transverse and some longitudinal orientation are produced in IBM. True biaxial orientation produces bottles with improved properties including increased tensile and impact strength, improved surface gloss, reduced creep, improved gas and water vapor barrier properties and a reduction in haze in transpar-ent bottles. As a result, lighter weight, lower cost bottles can be produced.

To produce a biaxially molded bottle, a preform or parison (produced either by injection molding or extrusion of a continuous tube or parison, which is then cut to the required length and closed at one end) is stretched longitudinally under heat and blown into a bottle with consequent transverse orientation. A metal stretch rod enters the bottle to assist in the stretching process (Figure 5.13).

The process of SBM is particularly important in the field of carbonated beverage packaging using PET. For best results, the resin molecules must be conditioned, stretched and oriented at just above the T_g where the resin can be moved without the risk of crystallization. Although PET is the major stretch blown resin, PVC, PP and AN copolymer resins are also stretch blow molded.

Heated preform

Stretch rod

Blow mold

Bottle cavity

(a) (b)

Blow molded bottle Action of stretch rod and

high pressure air flow

FIGURE 5.13 Stretch blow molding process. (a) Mold closed on preform. (b) Stretching and blowing. (From Irwin, C., Blow molding, in: The Wiley Encyclopedia of Packaging Technology, 3rd edn., Yam K.L. (Ed.), John Wiley & Sons, New York, pp. 137–154, 2009.)

Two different stretch blow techniques are available for SBM. In the one-stage method, parison injection molding, temperature conditioning and blow molding take place in the same machine. This method is commonly used for widemouthed jars and bottles with unusual cross-sectional shapes. In the two-stage method (also known as the “reheat and blow” method), the parisons are first injection molded in a completely separate stage and stored at ambient temperature until required. They are then reheated to between 90°C and 110°C and blown to their final shape. For many PET bottles, the preform length is typically less than half the final bottle height. No stretch occurs at the top and bottom of the bottle. Typical stretch ratios for a 2-L PET bottle are 2.3:1 in the axial (longitudinal) direction and 3.9:1 in the hoop (transverse) direction. Often production of performs and bottles are physically separated in different facilities, with the SBM often being done on-site by the filler or the packaging supplier in a hole-through-the-wall (HTW) operation.

For hot fill applications, heat resistant PET bottles are required, since a normal PET SBM bottle cannot be filled much above 65°C without causing bottle shrinkage. Manufacture of a heat resistant bottle is called the “heat set process” and involves further heating after being stretched so that it undergoes “after crystallization” (known as thermally induced crystallization). This increases the crystallinity, mainly in the sidewalls, to about 30% or more as opposed to 20% in a container pro-duced by the conventional blow molding process (Tekkanat, 2002). Increased crystallinity gives the material significantly enhanced thermal stability; the T_g and rigidity also increase.

Two methods are used. In the first, a single mold method, the preform is stretch blow molded into a hot mold where the mold halves are up to 160°C and the mold base 90°C–95°C, resulting in the preform being heated to between 100°C and 110°C, about 10°C hotter than in the conventional blow molding process. Heat setting takes place as the blown container encounters the surface of the hot mold while being constrained by high pressure against the mold. The container is then cooled down to below its T_g before it can be removed from the mold using compressed air on the inside.

In the second (dual mold) method, the preform is first stretch blow molded into a first mold, fol-lowed by a reheating in an oven to relieve stresses and shrivel the shape, folfol-lowed by reblowing in a second mold to produce the final bottle shape (Irwin, 2009). To avoid unsightly bottle appearance as a result of vacuum formation inside the bottle after hot filling, hot fill bottles are molded with sidewall panels and base designs that move inward as the product cools and ensure that the vacuum-induced forces do not distort the bottle. As a rule of thumb, shrinkage of heat set containers after hot filling should be limited to <10% (Tekkanat, 2002).

Production of multilayer co-injection preforms is the most economical way to achieve enhanced properties in a rigid PET container. Multilayer co-injection is the process by which one or more interior layers of material are totally encapsulated by outer virgin PET layers. It is the only technol-ogy able to provide any combination of clarity, gas barrier, gas scavenger and recycled PET (rPET) in a single process (Swenson, 2002). Multilayer parisons can be produced by either extrusion or injection-molding techniques.

Co-injection SBM was a major breakthrough, enabling longer shelf lives for beverages to be achieved. It has been used to produce three-layer (PET-MXD6-PET) and five-layer (PET-rPET-PET-MXD6-PET) bottles, mainly in smaller sizes where the lack of a barrier layer would severely limit shelf life. Although EVOH copolymer could be used as a barrier layer, MXD6 is preferred as it has similar melt flow characteristics to PET. Although multilayer PET bottles have been com-mercialized, the focus has now turned to coating bottles with oxides of silicon or aluminum or with HCs (see Section 5.5).

Aseptic blow molding is becoming increasingly popular for the packaging of beverages. It is usually based on the extrusion process where the bottle is blow molded in a commercially sterile environment, often with the product filler combined with the blow molder. In one system, the bottle is molded and sealed in the blow molding machine and then stored for minutes or days. At the filler, the outside of the bottle is resterilized and the top seal area cut off; after filling aseptically with sterilized product, the bottle is resealed. In an alternative approach, bottle blow molding, filling and sealing are all carried out in a commercially sterile environment (Irwin, 2009).

5.12 THERMOFORMING

In this relatively old and simple process, a sheet (generally 75–250 μm thick) of thermoplastic mate-rial is heated to its softening temperature, usually by means of an infrared radiant panel heater.

Pigmentation of the sheet aids the heating process. By either pneumatic or mechanical means, the sheet is forced against the mold contours and, after cooling, is removed and trimmed. Typical ther-moplastics used for thermoforming include HIPS, PVC, PP and PA.

There are two dominant means of thermoforming sheet for food packaging containers: the melt phase process and the solid phase pressure forming (SPPF) process (Chougule and Piercy, 2009).

The melt phase process is most applicable to monolayer structures that have relatively high melt strength at thermoforming temperatures, for example, HIPS, PVC and PC. The SPPF process is primarily used to thermoform PP, a crystalline polymer that is difficult to thermoform uniformly in melt phase machines due to the sharp decrease in melt strength (viscosity) at its melting point (Brown, 1992).

Thermally stable PET containers are in common use for dual ovenable (i.e., conventional and microwave) applications for chilled and frozen foods, as well as retortable, shelf stable applications where the food can be reheated in the package using either a microwave or a conventional oven.

These containers are known as crystallized PET (CPET) and are stable at temperatures up to 230°C, compared to amorphous PET (APET), which begins to soften at temperatures over 63°C. The CPET process is based on conventional reheat thermoforming where an extruded PET sheet containing nucleating agents to speed up and maximize crystallization is reheated to around 170°C where it softens. It is then thermoformed into a hot mold and held long enough for the optimum amount of crystallinity to develop, after which it is transferred into a second mold where it is cooled. CPET containers must be crystalline enough to be heat resistant but not so crystalline as to be too brittle for the application, for example, impact resistance at freezer temperatures. The optimum amount of crystallinity is 28%–32%. Foamed CPET is also available.

The SPPF process involves forming at lower temperatures below the crystalline melting point, that is, 5%–8% lower than melt phase forming, depending on the material. For example, PP is melt phase thermoformed at 154°C–157°C and SPPF at 141°C–146°C where it is still virtually a solid with high viscosity, requiring the application of strong forces. In SPPF, the sheet is heated by infrared heaters and stretched into the mold cavity with a heated plug. Cold air at pressures of up to 700 kPa then force the hot sheet against the cooled inner wall of the mold, finishing the form-ing operation at a high speed. This process (developed mainly for PP) improves the strength of the containers as well as their clarity, and because lower temperatures are used, the containers are free from odor and taint, thus making them very suitable for food packaging.

The ability to produce extruded PS foam sheet has provided additional packaging markets for thermoforming. The first of these was meat and produce trays, followed later by egg cartons. Other applications include fast-food carry-out cartons, institutional dinnerware and inserts for rigid boxes.