One of the most frequently discussed themes in conjunction with gas-assist injection molding is the wall thickness that results in the region of the gas bubble. This wall thickness has an influence on material consumption, required cooling time, surface quality, mechanical properties (rigidity and strength) and the cost of the part.

Investigations show that the residual wall thickness is very stable and cannot be easily altered by processing parameters. Neither the melt cushion, mold temperature, melt temperature, external gas pressure, or the gas delay time has a notable influence on the wall thickness. Even the viscosity of the material has only a negligible influence when different resins from the same material class are compared (unreinforced thermoplastics, blends, fiber reinforced grades). The very limited influence of the gas delay time and the mold temperature becomes clear when the thickness of the frozen surface layer is observed as a function of the cooling time. As a general rule, long time lags like this lead to defects such as visible switchover or hesitation marks or major differences in wall thickness over the length of the molded part. Although modifying the process parameters has virtually no effect on the wall thickness, it does have a clear impact on the residual cushion of melt and hence on the amount of melt that will be injected into the mold beforehand. This is because even just small changes in wall thickness are clearly reflected in the amount of material used and hence in the residual cushion of melt at the end of the flow path. When use is made of the blow-out (or spill-over) technique it is possible to selectively exploit this effect of a thicker frozen surface layer. The difficulties outlined above do not occur in this case on account of the process employed, since the molded part surface is formed on a uniform basis during the injection process. If the gas bubble unexpectedly penetrates into thin-walled areas, the gas delay time becomes important for the wall thickness. A longer gas delay time or a lower mold temperature will increase the thickness of the frozen surface layer slightly and will lead to a longer gas bubble for the same amount of melt injected in beforehand.