Processing polyethylene terephthalate blow

Polyethylene terephthalate is most often extruded into films or fibers, or blow molded into bottles. Polybutylene terephthalate is primarily found in injection molded parts. Such parts are highly crystalline, which makes them opaque. Polybutylene terephthalate is often modified with glass fibers or impact modifiers. Table 24.1 contains applications by processing method and resin. 

Polyesters exhibit excellent high temperature strength and electrical properties making them a good choice for many demanding applications. They also are physiologically inert allowing them to be used in food contact applications. The two common polyesters, polyethylene terephthalate and polybutylene terephthalate, are both used in injection molded products. Polyethylene terephthalate is often used in both extrusion and blow molded processes also. 

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. 

Extrusion blow molding process (a) single-screw extruder, (b) die with a gas supplier, and (c) mold. The blow molding condition of high-density polyethylene (HOPE) was set as typically used in industry. The maximum extrusion temperature of a single-screw extruder was set at 220°C for HDPE and 250°C for HDPE/polyethylene terephthalate (PET) blend. 

Multilayer injection blow-molded containers have been available for a much shorter time than multilayer extrusion blow-molded containers, as development of the process was more difficult. However, these too are now available. A prominent example is multilayer polyethylene terephthalate (PET)-based ketchup bottles, which contain three layers of PET, and two layers of ethylene vinyl alcohol (EVOH) as an oxygen barrier. 

Applicable materials for the injection blow molding process include high-density polyethylene (HOPE), low-density polyethylene (LDPE), polystyrene (PS), styrene acrylonitrile (SAN), ethylene vinyl acetate (EVA), polypropylene (PP), polycarbonate (PC), and polyurethane (PU). Polyethylene terephthalate (PET) and polyacrylonitrile (PAN) are less commonly used in the injection blow molding processes. 

Common thermoplastic materials suited to the blow-molding process include high-density polyethylene, polyvinyl chloride, polypropylene, and polyethylene terephthalate. 

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

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