July 14, 2022

Gas injection molding

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This technology can be very effective in eliminating or substantially reducing the stress in flatter molding. In internal gas injection, where the gas is injected into one or more gas channels. For a flattish, panel-type part such as this, this is not always the gas injection process to be recommended. The added gas channels can leave a slight witness mark on a cosmetic ‘A’ surface and depending on the part geometry, the gas packing pressure will probably not be equal across the whole surface area of the part, especially with thinner wall sections and wider surface areas.

For this type of part, another gas injection process called External Gas Molding may be more appropriate. External gas molding applies the gas through a number of gas nozzles onto the ‘B’ side of the injection molding, in the form of a pressure cushion. The gas applies the packing pressure and takes up the material shrinkage across the whole surface area, on the non-visible side of the component. The gas leaves a slight rippling on the under surface, so this must be cosmetically acceptable. With some materials, this process can eliminate altogether the need for packing pressure from the injection molding. It can reduce the clamp force requirement by up to 30% and it can reduce the material content by around 10% and the cycle time by similar amounts. In some cases, it can allow the designer to introduce ribs where they would otherwise produce unacceptable sink and even allow a rib:wall ratio of 1:1, introducing the possibility of reducing the wall thickness of the part.

Water injection would not be appropriate for this type of part, unless there are thick concentrations of material with tube/bar/handle-type forms that are causing the distortion and lengthening the cycle time.

One final consideration: It is possible to adjust many existing tools for external gas molding, but if you have a lot of moving parts, the tooling alterations can get tricky.

If you are starting from scratch and you have the gas injection equipment and you can tolerate the thickened sections that allow the gas to apply holding pressure within the channels. The net result will certainly be less flow-induced residual stresses. Where the gas cannot penetrate, however, the actual pressures acting within the solidifying core of the remainder of the part will be subjected to the same pressure drops that are present in a conventionally molded part during the holding phase. The benefit with the gas- (or indeed, water-) assist is that the extent of those pressure drops will be more localized and less severe. To paraphrase the words of the classic advertisement for a certain lager, the gas pressure “reaches parts other systems cannot reach.” Get the thermal balances badly wrong (heat flow from the melt into and out of every single part of the component/mold entity) and even low flow-induced stresses achieved by gas-assist will be swamped by really severe uneven shrinkage, which is bound to result in warpage as the part cools from time of ejection to reaching ambient.

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