Moulding extrusion blow

The parison extrudes down vertically and the process relies on the hot strength of the plastic to hold the parison weight in shape. For this reason blow moulding uses far more viscous materials than would normally be employed for the injection moulding process. A low viscosity material would simply pour out of the die onto the floor or split off before the parison had formed. 

Multi-layer blow mouldings, give the process added versatility by allowing the use of layers of different materials that can be tailored for purpose and economy. Multi-layers also provide a means of incorporating recyclates and masking the inherent colour of the recyclate itself. For example, consider a two-layer bottle structure that used blue virgin material as the outer visible layer and black recyclate as the inner layer. If you actually looked into the bottle you would see the black recyclate material, but all of the outer surface would be blue. 

Any recyclate derived from multi-layer mouldings will consist of a combination of the materials used to make up the individual layers. This does not usually present a problem as the layers are all of suitable properties for blow moulding and compatibilised with each other. 

For thin walled containers, PVC-U gives a combination of high clarity, inherent rigidity and good barrier properties. However, the use of PVC in this area has greatly decreased due to the changeover to polyethylene terephthalate (PET) for cost reasons. 

Extrusion is mainly by a single screw horizontal machine which changes to vertical flow via a swan neck arrangement. Air is fed to the centre of the parison. 

In the stretch blow moulding process, which is the most common, the extruded parison is initially blown undersized, and then stretched and blown to its final shape in a second mould. This produces biaxial molecular orientation of the container walls, which improves impact resistance, rigidity and clarity. However, permeability is reduced. 

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Extrusion blow moulding of bottles has been successfully accomplished in reeent years by attention to the points mentioned above. It is to be noted here that UP VC has a much lower average specific heat between the proeessing temperature and room temperature than polyethylene and, being essentially amorphous, no latent heat of fusion. This leads to much less heat needing to be removed on cooling of mouldings and very short cycle times are possible. 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

Extrusion blow moulding is continually developing to be capable of producing even more complex shapes. These include unsymmetrical geometries and double wall mouldings. In recent years there have also been considerable... 

Since this is less than the melt fracture stress (6 MN/m ) these production conditions would be suitable. These are more worked examples on extrusion blow moulding towards the end of Chapter 5. 

In Section 4.2.7 we considered the process of extrusion blow moulding which is used to produce hollow articles such as bottles. At that time it was mentioned that if molecular orientation can be introduced to the moulding then the properties are significantly improved. In recent years the process of injection blow moulding has been developed to achieve this objective. It is now very widely used for the manufacture of bottles for soft drinks. 

During extrusion blow moulding of 60 nun diameter bottles the extruder output rate is 46 X 10 m /s. If the die diameter is 30 mm and the die gap is 1.5 mm calculate the wall thickness of the bottles which are produced. The flow curves in Fig. 5.3 should be used. 

As a melt viscosity and melt-strength-enhancing additive for modifying PET for film blowing and extrusion blow moulding applications... 

Extrusion blow moulding (the simplest), in which the parison is an extruded tube that is blown with air. Various types of machinery are marketed shuttle, reciprocating and wheel machines. 

Injection blow moulding, in which the parison is injected into a first mould and then blown in a second mould having the shape of the final recipient. This process is more expensive and the cycle time is a longer. The aspect and dimensional quality are better compared to extrusion blow moulding. 

In the method of producing a hollow foamed plastic body from polyethylene or polypropylene by means of an extrusion blow-moulding process using a single-screw... 

Polypropylene (PP) bottles have the advantage that they are inherently hot-fillable (at 85°C) and retortable to 120°C. The bottles are normally extrusion blow-moulded, and can also be made with multi-layering to include barriers against oxygen permeability. This technology is commonly used for sauce bottles and juice products. Since most of the bottles are of an oval shape, product contraction resulting in volume reduction is countered by allowing die bottle to become more oval under the effects of vacuum. 

Since two sets of moulds are required the technique is more costly than extrusion blow moulding but can offer advantages when special shapes or a particularly high degree of uniformity are required. It is useful also when there must be no risk of contamination of the containers—as in products for use in medicine. Articles of high quality, including visual appeal, can be obtained. 

In extrusion blow moulding a parison may sag under its own weight after leaving the die, with unwanted and perhaps random thinning as a result. In part this behaviour may be attributable to an elastic effect (to uncoiling of the chains) but also to viscous flow as the molecules move in relation to each other. It is reasonable to believe that the elastic element will become more important with ... 

Blow moulding is the most common process for making hollow articles such as bottles. There are two main types of blow moulding, injection blow moulding and extrusion blow moulding. 

Injection blow moulding is used for the production of hollow objects in large quantities. The main applications are botdes, jars and other containers. The Injection blow moulding process produces bottles of superior visual and dimensional quality compared to extrusion blow moulding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. 

For extrusion blow moulding, the blow moulding machine is based on a standard extruder barrel and screw assembly to plasticise the polymer. The molten polymer is led through a right angle and through a die to emerge as a hollow (usually circular) pipe section called a parison. 

Alpha Packaging manufactures botdes and jars made from polyethylene terephthalate (PET) and high-density polyethylene (HDPE) for the pharmaceutical, nutritional and personal care markets. Technologies used include injection blow moulding, injection stretch blow moulding, and extrusion blow moulding. Alpha manufactures stock and custom containers in a variety of styles and colours. 

Bomatic, Inc. has been a producer of plastic bottles and containers since 1969. The company serves the personal care, automotive, pharmaceutical, medical, lawn and garden, food, household cleaners, and industrial chemicals markets. Production capabilities include extrusion blow moulding and injection moulding products made from HDPE, PVC, LDPE, PET, PETG, polycarbonate, polystyrene, polypropylene, and polyurethane. 

This monomer (diol) is used to broaden the processing window, increase line speeds, slow the crystallisation rate and reduce stress in biaxially orientated structures. It is used in concentrations ranging from 1% up to about 18%. The higher concentration PET is usually referred to as PETG and is mainly used for extrusion blow moulding and sheet extrusion. Polymers from PTA and CHDM are approved in the US by 21 CFR 177.1315 recently the use of CHDM as comonomer in PTA-EG polymers has been listed in the US FDA regulation for PET (21CFR 177.1630). Moreover, the use of CHDM has been approved in Europe for plastics intended for food contact applications since the first directive 90/128/EC. 

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