Pressure blow moulding

A blow-moulded container, cylindrical in shape but with one spherical end, is prepared from the polysulphone whose creep curves at 20°C are illustrated in Figure 9.9. The cylindrical part of the container has an outside diameter of 200 mm and is required to withstand a constant internal pressure of 7 MPa at 20°C. It is estimated that the required service lifetime of the part will be one year and the maximum allowable strain is 2%. What will be the minimum wall thickness for satisfactory operation ... 

High molecular weight HDPE (HMW-HDPE) is used for blown film and for demanding moulding and structural uses. Examples are blow moulded drums for packing dangerous chemicals and pressure piping. 

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. ... 

The process is attractive for a number of reasons. Firstly, since it is a low pressure process the moulds are generally simple and relatively inexpensive. Also the moulded articles can have a very uniform thickness, can contain reinforcement, are virtually strain free and their surface can be textured if desired. The use of this moulding method is growing steadily because although the cycle times are slow compared with injection or blow moulding, it can produce very large, thick walled articles which could not be produced economically by any other technique. Wall thicknesses of 10 mm are not a problem for rotationally moulded articles. 

Using the manufacture of PP battery cases as an example, the authors show how to produce thin-walled (2-5mm), foamed articles with good surface properties by means of low pressure injection moulding. Neopor 35 was used as the blowing agent. 5 refs. 

The volumetric filler is also electronically controlled, ensuring an accurate dose of product to each container. For PET bottles, which creep under car-bonation pressure, there is the drawback that they will have differing fill levels over a filling cycle due to the fact that no two bottles are ever exactly identically blow-moulded. With light weighting, this problem is exacerbated. 

Blowing Once the preform is within the blow mould area the moulds close, a stretch rod is introduced to stretch the preform longitudinally and using two levels of air pressure, the preform is blown circumferentially. 

Drop impact resistance of fluid-filled plastic containers is of considerable concern to containers manufacturers as well as distribution industries using the containers for transportation of various liquids. This is due to potential failure of the containers following the drop impact and subsequent spillage of the transported liquid, and consequent safety and economical Issues. In this work, a series of drop impact experiments is conducted on water filled bottles made of blow moulded high-density polyethylene (HDPE). During experiments, pressure and strain histories are recorded at various positions. The experiments are then simulated numerically. 

In a number of processes, air pressure is applied to a bubble of polymer melt to change its shape. We will examine one continuous process—blown film production—and two cyclic processes—blow moulding and thermoforming. All of them involve some melt extensional flow with a resultant thinning of the bubble, and at least one side of the polymer solidifies without the constraint of contact with a mould. 

Stages in the expansion of a PET preform into a bottle, in the stretch blow moulding of PET. The stretch rod extends the length before the Internal pressure expands the diameter (Pham X.T et ai, Polym. Eng. Sd., 44, 1460, 2004) John Wiley and Sons Inc. reprinted with permission. 

Plastic bottles are manufactured by extrusion followed by blowing (extrusion blow moulding), as shown in Fig. 7.33. A certain length of tube is extruded (the parison) and a mould applied. The latter obturates the bottom end of the parison. Air pressure is supplied inside the parison, forcing the melt onto the cold mould and hence giving it its final shape. In this process, the orien-... 

E/CO copolymers of appropriate melt index can be injection moulded and blow moulded similarly to LDPE. Mould release, shrinkage, cycle times, clamping pressures, and other factors in the moulding of E/CO should be much like those encountered with LDPE of comparable MI and MWD. 

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