Injection moulding high-density polyethylene

M.E. Sotomayor, I. Krupa, A. Varez, B. Levenfeld, Thermal and mechanical characterization of injection moulded high density polyethylene/paraffin wax blends as phase change materials. Renewable Energy 68, 140-145 (2014)... 

FeUahi, S., Favis, B.D., and Fisa, B. (1996) Morphology stability in injection-moulded high-density polyethylene/polyamide-6 blends. Polymer, 37, 2615-2626. 

In broad tonnage terms the injection moulding markets for high-density polyethylene and polypropylene are very similar. The main reasons for selecting polypropylene have been given above. In favour of HDPE is the inherently better oxidation and ultraviolet resistance. Whilst these properties may be greatly improved in polypropylene by the use of additives these may increase the cost of polypropylene compounds to beyond that which is considered economically attractive. It is for this reason that HDPE has retained a substantial part of the crate market. 

The Phillips process for the manufacture of high-density polyethylene may be adapted to produce copolymers of ethylene with small amounts of propylene or but-l-ene and copolymers of this type have been available since 1958. These soon found application in blown containers and for injection moulding. Properties of two grades of such copolymers are compared with two grades of Phillips-type homopolymer in Table 11.11. 

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. 

HDPE exists as both a homopolymer and a copolymer with higher ethylene olefins. The latter offers properties which lie between low and high density, with typical densities of 0.945-0.950. Copolymers have found wide usage for extrusion blow, injection blow and injection moulding. They are, however, generally classified under HDPE. Virtually all high density polyethylenes contain an antioxidant. 

Figure 18.42 Effect of temperature on falling weight impact strength of 0.080 inch thick injection moulded specimens of various polymers moulded at 240 °C. KMT 61 and GMT 61 = polypropylene copolymers, 65-045 = high-density polyethylene, GM 61 and KM 61 = polypropylene homopolymer, ABS = acrylonitrile-butadiene-styrene terpolymer. Source Author s own files)...

In 1980s a breakthrough occurred by processing starch at approximately its natural water content (15%) in a closed volume at temperatures above 100°C. Using conventional injection moulding, glassy, amorphous, thermoplastic starch (TPS) polymers (Tg 60 C) were obtained with moduli similar to those of polypropylene and high-density polyethylene. 

Polyethylene can be purchased in a variety of densities and properties dependent upon the formation of the polymer chains. The most common materials encountered in injection moulding are low density polyethylene (LDPE) and high density polyethylene (HDPE). 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

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