Gas assisted injection is a more advanced molding technique compared to the conventional injection molding process. In this process, an inert gas is injected under pressure in the molten resin which has been previously introduced in the mold's cavity. Usually, the resin is allowed to fill partially the cavity. In very specific cases, a simultaneous injection of both the resin and the gas is preferred.

The purpose of injecting gas in the cavity is not to cause a fluid mixture of molten resin and gas; but instead to have the gas (usually nitrogen) displace the resin towards the volume of the cavity which has not been filled yet, thus leaving an internal channel without resin formed by the flow of gas. A second important function of the gas action in the injection molding process is to reach the widest cavity's sections so that resins have a good contact with the mold's walls in order to improve the superficial termination of the injected part. This last function is equivalent to improving or increasing a good pressing effect of the resin inside the mold. All the other features that are frequently attributed to the gas assisted injection molding are considered as benefits of the process; however they do not comprise the basic part of the process' mechanical aspects.

When separated injection of the phases is performed in the gas assisted injection molding, an internal pressure differential is created in the mold. In many occasions this fact is not taken into consideration and in such cases undesirable results can be achieved concerning the quality of the product. For example, one of the fluids is highly compressible (nitrogen gas) and the other fluid (molten resin) is relatively incompressible. Both fluids however, can move inside the cavity of the mold. The points of multiple injections (of the gas or of the resin) directly influence the movement of the two fluids, while the resin is still in a molten state. Therefore, the success of the gas assisted injection molding depends on the position of resin injection gates, on the gas and on the flow pattern of the resin that is introduced into the mold.

This kind of molding is a high speed and low pressure process, where a short injection in the cavity of the mold takes place, and where the mold filling is completed by injecting a second fluid; in this case, nitrogen gas is at a much lower pressure than the pressure which is experienced in conventional injection molding. The second phase of the resin filling in the mold is achieved thanks to nitrogen injection, and the flow of the resin in this second phase is facilitated by the displacement caused by such gas, which pushes the resin towards the areas of low pressure of the mold, which are still empty.

It is important to keep in mind that the gas assisted injection molding is only a modification of the conventional injection molding, and that the basic principles will not change simply because a gas is used in the second phase of the mold filling. The gas provides a solution to reduce or to eliminate certain faulty features of the conventional process. Basic considerations are kept in mind in the conventional process, such considerations are: parts geometry, wall thickness, gates localization, flow longitude inside the mold, air vent and the way of carrying out the reinforcement of the mold filling. In gas assisted injection molding, considerations are parallel, but in many cases the operation tolerances are wider in the modified process.