Sophisticated moulded products like cell phones, computer monitors or automotive side mouldings as well as large flat surface moulded products like cover panels etc demand excellent surface finish with no blemishes like sink marks that are extremely difficult to eliminate in the conventional injection molding process. In the last decade two different gas assist technology have been offered by several organizations that include Asahi Kasei Corp in Japan, Battenfeld and Intellmold System Division of Textron. The external gas moulding technology essentially uses two different approaches to introduce gas. The first method involves introduction of gas from one exterior side after the mould is filled almost 99% with melt. The gas enters from core and presses the critical surface against the opposite face of the mould. The second method involves gas which is used to pressurise the mould before melt is injected to achieve continous pressure of melt against the mould before the cavity gets filled. Essentially both these systems hold material to mould surface while the melt gets cooled. This almost provides the effect of compression moulding.

 

The external gas moulding requires lower pressures compared to internal gas assist moulding. While it is difficult to judge at this stage whether it would offer an economy over internal gas assist moulding, it is certain that the technology is likely to get more popularity because it provides on opportunity to mould critical products from versatile polymers like PP that unfortunately due to its semi-crystalline structure is more prone to sink marks on account of higher shrinkage characteristics.