Mould parison

If B 1, there is marked temperature gradient, but if B 1, there is not. For a sheet of polymer of thickness >1 mm cooled by water or by contact with a steel mould, B 3> 1. When B > 10, it is a good approximation to say that the polymer surface temperature is immediately reduced to that of the cooling medium. For a blow-moulded parison (Section 5.5.2) of thickness L = 0.002 mm, cooled in still air on the outside, B = 2. If the lower end of the parison cools for 150 s before the mould closes, it is a reasonable approximation to ignore the temperature gradient through the wall, and calculate the average temperature drop at the end of the parison. 

Parnaby, J. and Worth, R. A., Die variator mandrel forces encountered in blow moulding parison control systems—Computer-aided design, Proc. Instn. Mech. Eng. 7 357-364 (1974). 

This would have been due to the point of melting completion moving further downstream as speed increased. The same effect was found by Boes [1] for blow moulded parisons containing 0.8% masterbatch. Boes also found that the movement of melting completion towards the screw tip also caused a greater colour development in the outer zones of the parison. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it to take up the shape of the mould. This is illustrated in Fig. 4.21(a). Although this process is principally used for the production of bottles (for washing-up liquid, disinfectant, soft drinks, etc.) it is not restricted to small hollow articles. Domestic cold water storage tanks, large storage drums and 2(X)... 

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. 

This expression therefore enables the thickness of the moulded article to be calculated from a knowledge of the die dimensions, the swelling ratio and the mould diameter. The following example illustrates the use of this analysis. A further example on blow moulding may be found towards the end of Chapter 5 where there is also an example to illustrate how the amount of sagging of the parison may be estimated. 

Example 4.4 A blow moulding die has an outside diameter of 30 mm and an inside diameter of 27 mm. The parison is inflated with a pressure of 0.4 MN/m to produce a plastic bottle of diameter 50 mm. If the extrusion rate used causes a thickness swelling ratio of 2, estimate the wall thickness of the bottle. Comment on the suitability of the production conditions if melt fracture occurs at a stress of 6 MN/m. ... 

Example 5.10 During the blow moulding of polythene bottles the parison is 0.3 m long and is left hanging for 5 seconds. Estimate the amount of sagging which occurs. The density of polythene is 760 kg/m ... 

During the blow moulding of polypropylene bottles, the parison is extruded at a temperature of 230°C and the mould temperature is 50°C. If the wall thickness of the bottle is I mm and the bottles can be ejected at a temperature of I20°C estimate the cooling time in the mould. 

The principle (Figure 5.6) is to produce a preform or parison by extrusion or injection, and then to blow it into a cooled mould to obtain the final hollow recipient. There are four steps fabrication of the parison... 

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. 

Reheat and blow moulding in which parisons are bought from an external furnisher and are blown on an in-house machine. 

A multi-layer expansion-moulded article is obtained by moulding a multi-layer parison comprising a foamed resin... 

Hoechst have developed a process that sucks the parison onto the mould walls in blow moulding equipment, thereby producing lightweight hollow parts with good insulation properties. The process was developed for foamed PP air ducts for cars. A description is given in the article. HOECHST AG... 

Hoechst has developed a method moulding PP foam automobile air ducts that optimise air flow and also provide good sound and heat insulation. Machines are adapted to suck the parison onto the mould walls rather than to blow mould it. The process also uses a blowing pin operating in reverse mode to create a partial vacuum inside the moulding in order to roughen its inner surface. HOECHST AG... 

An an alternative to printing the bottles or to affixing labels subsequently it is possible to apply the label at the moulding stage. Paper or thermoplastic film labels are printed in advance and are placed in the moulds before parisons are collected, being held in position by vacuum adhesion of the label takes place when the blown material fills the mould. 

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 ... 

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. 

When the parison has reached a sufficient length a hollow mould is closed around it. The mould mates closely at its bottom edge thus forming a seal. The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould. 

A method of fabrication in which a parison (hollow tube) is forced into the shape of the mould cavity by internal air pressure. 

Injection blow moulding A blow moulding process in which the parison to be blown is formed by injection molding. 

Parison The hollow plastic tube from which a container, toy, etc. is blow moulded. 

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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