Moulding thin walled containers by intrusion and injection - increasing shot volume by fifty to sixty percent

摘要

Articles of high and low density polyethylene, etc., are produced on injection moulders with single or dual plasticising cylinders and nozzles, by a technique incorporating the following steps- 1) a mass of thermoplastic material is introduced into an extended mould cavity having a larger volume than that of the product, 2) a further quantity of thermoplastic material is fed in, 3) either before, at the start of or during the feeding of the second thermoplastic material, the volume of the moulding cavity is reduced to correspond to the dimensions of the finished article, and 4) a further mass of the first thermoplastic is injected so as to fill the mould inlet funnel. In the first step, the material is intruded with the plasticising screw held at a predetermined point. In the second the material is injected into the mould. The first material may form the skin whilst the second forms the core of a composite product. Makes it possible to produce articles with good surface characteristics, with the thinnest possible walls, e.g. 2 to 4mm, and with a flow/wall thickness ratio of >=30. The products have a volume, normally 50-60% more than the nominal size of the shot. With articles of a suitable profile and the right material, the excess may be raised to 100%.