Blow moulding stretch process

By far the most important plastic is polyethylene terephthalate (PET). Bottles of this material are formed in a two-stage process. So-called pre-forms are made by injection moulding and, in a second process, are then stretch-blow-moulded to produce a bottle. PET has properties surprisingly like those of glass, but it does not have the same disadvantages of weight and brittleness. 

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. 

Fig. 20.4 The injection stretch blow moulding process (ISBM).

Although both processes can produce reasonably good quality large containers, injection stretch blow moulding is more widely used for pharmaceutical containers. The first stage of moulding is basically as injection blow moulding. In the second... 

Blow moulding describes any shaping process in which air is used to stretch and form plastic materials and can be compared to inflating a balloon. A hot thermoplastic tube, usually made by extrusion, can be inflated with compressed gas while inside a cooled split mould. Hot thermoplastic tubes or parisons can also be blown into free shapes without the aid of a mould (Figure 3.33). Plastic bottles, drums, car fuel tanks and other containers are often made using blow moulding (Figure 3.34). 

Clarifying additives enable PP to compete for applications that were previously captive to PET or polycarbonate, both of which cost more. PP supplier Borealis and Milliken have developed a two-stage injection stretch blow moulding process, used for still water bottles and juice containers, that allows the rapid manufacture of clear PP bottles at up to 1500 bottles per hour from each mould cavity, using Milliken s clarifiers. 

Figure 4.3 illustrates the various stages of injection-stretch blow moulding. The biaxial orientation also provides permeation to gas, liquid and odour due to an increased molecular alignment. This process produces bottles or containers which are scrap-free, and have close tolerance, i.e., a complete finish which does not required any secondary operations. 

Blow moulding simulation softwares are effective tools for blow moulders to predict and improve the performance of blow moulding process. Not many such software are available in the market. This paper presents a comparative study of two blow moulding software - B-SIM and Blow View - for specific application of stretch blow moulding process. Features and capabilities are compared based on the simulation study of a plastic bottle. 

In stretch blow moulding process, the preform is stretched in both hoop direction and the axial direction as it is blown into Ae mould. Air blow with stretch rod create the final product by stretching the preform to the mould cavity. Injection stretch blow moulding process has two different blowing stages which are pre-blow and final blow. Stretch rod comes to the end of the bottle cavity with preblow assistance. The time in between stretch rod touches the preform and reaches the end of the cavity is crucial in order to get desired stretch ratio. These two points are named point 0 and point 10. Final blow is applied for the rest of the blowing process right after preblow. 

Preforms are reheated before the blowing process in two stage SBM (stretch blow moulding) process. B-SIM software runs the analysis without preheat stage unlike BlowView. Preform temperature distribution were entered in advance instead. In this case oven set up parameters were omitted. Therefore the temperature profile throughout the preform is set at 95C to 117C. Cavity and stretch rod temperature is assigned as 20C and 30C respectively all the time. 

In this study, two virgin PET grade materials are blended with post consumer recycled PET. The heat-setting technique is used for the manufacturing of injection stretch blow moulded bottles for hot-fill plications. The process parameters (the preform temperature, the heatsetting timing and blow mould temperature) and the blend ratios for PET bottles are optimised based on thermal and mechanical characterisation. 

Biaxial orientation of PET containers by mechanical means - axially by a stretch rod, radially by high-pressine air- during injection stretch blow moulding process introduces strain-induced crystallinity. Strain-induced crystallisation has a direct influence on the level of crystallinity of the final product. Factors, which influence the crystallinity level and the morphology, include the ISBM mould temperature, blow pressure and time to stretch the preform. 

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