Gas assisted injection moulding (GAM) has been available for commercial moulding for more than fifteen years. However only recently, during the last three or four years, it has become recognised as a viable and necessary process for achieving economies and technical advantages for both the product designer and the moulder.
Originally GAM was considered to be merely a means of making thick section rod-like mouldings with cored out centre sections, resulting in material cost savings of up to 45% and comparable savings in moulding time cycles. Now, by far the bigger volume of applications are based on the use of the gas as a medium for applying pressure to the plastic whilst cooling and solidifying, instead of from the moulding machine as in the case of conventional injection moulding.
The process is based on the principle that the low viscosity gas when injected into molten plastic will seek the path of least resistance, which will be at the centre of the thicker sections within the moulding where the plastic is least viscous. Therefore by including thicker sections, it becomes possible to control the flow of the gas and to achieve continuous hollow sections. This enables pressure to be applied at positions where it is required, and often remote from the plastic feed points.
As a result of applying the pressure more efficiently, less in-mould pressure is required, leading to lower lock forces and reduced energy costs. Reduced moulded in stress often results in eliminating any tendency for mouldings to distort after ejection from the mould.
The ability to apply gas pressure where it is required provides a means of eliminating sink marks opposite thicker ribs, bosses or other functional features, thereby providing a high quality visual surface finish.
This combination of benefits is resulting in a wide range of moulding applications, and recently it is being applied to mouldings of very thin general wall thicknesses, even down to 0.8mm thickness. This enables screw bosses and other fixing and assembly features to be included without creating sink marks. Also the high pressures involved in thin section moulding can be reduced significantly, also leading to dimensionally stable mouldings. Such applications will include laptop and hand-held computer casings, where there is a continuing drive to reduce product weights and overall enclosure thicknesses. Materials such as Polycarbonate and PBT/PET, in some cases with glass fibre reinforcements, are being used.

COMPUTER AND TV CABINETS

Most TV cabinet mouldings of size 25" and above are being designed assuming production with GAM. Over the last three or four years TV cabinets have been reduced in wall thickness, and therefore plastic weight, by 35% or more. Also mouldings which were previously made on lock forces of 1300 tons can now be moulded on presses of 850 tons, substantially reducing the cost of investment for the moulder. The quality of surface finish has enabled

paint finishing to be reduced to a single mist spray coat. There is a growing interest in avoiding paint finishing altogether and this is being achieved in many smaller TV cases and computer monitor fronts. Material thicknesses are being reduced to as little as 2.0mm, but at the same time including clusters of screw bosses for supporting the monitor CRT. Local injection of gas at multiple positions is being favoured. 

Injection Moulding Machinery -Indian Scenario 2000-01