Although gas-assist injection molding does reduce shrinkage and warpage, in general terms, it is important to note that the technology does have its limitations. When the gas bubble is positioned correctly, the formation of sinks and voids are eliminated. However, it is possible for warpage to occur in areas that have dissimilar shrinkage values.

As shown in the following diagram, uniform pressure prevails in the areas where the gas channel is formed. However, in the adjacent areas dissimilar shrinkage values can be displayed. The goal should be to make the wall thickness in the flat region equal to the residual wall thickness in the gas channel, if possible.

Increasing gas pressure reduces the amount of shrinkage for different materials. Similar to increases in pack and hold pressures and times, gas pressure and hold time increases can reduce shrinkage. Under the action of the holding pressure, the material in the cavity is compressed and additional melt flowing into the cavity compensates for volumetric contraction due to cooling. As the gas pressure is increased during gas-assist injection molding, gas compensates for volumetric shrinkage due to cooling. The following graph shows the shrinkage behavior of various grades of Durethan® polyamide.

In areas where material is allowed to accumulate, such as an abrupt change in geometry or at the end of the gas channel, higher shrinkage values and in extreme cases, sink marks are seen. Asymmetrical, or non-uniform wall thicknesses, as shown in the following diagram, can lead to differential shrinkage, and consequently warp the part in a manner that is not always easy to predict. Because material accumulates on the outer region of the bend, higher shrinkage values are predicted. However, there is also a conflicting effect, where higher temperatures prevail on the inside of the bend. This is due to the inadequate cooling for this surface to volume ratio and also leads to higher shrinkage. With a sufficient amount of cooling though, the resultant warpage is most likely going to be towards the outside.

The addition of filler, especially glass fiber, will also affect the shrinkage and warpage of the molded part. The added filler reduces the residual wall thickness thus reducing the shrinkage and warpage. Glass fiber in particular has been seen to noticeably reduce residual wall thickness and, depending on it's orientation in the molded part, will influence the shrinkage and warpage.