These physical differences lead to the following essential differences in the two processes. As soon as the gas arrives at a specific point in the cavity, the skin component will no longer continue to flow. This means that the wall thickness of the skin component will not undergo any further change at this point as the filling phase progresses. This leads to a virtually constant thickness in the skin component, which is determined almost exclusively by the geometry of the part and the material characteristics. The gas volume flow cannot be controlled easily by an external control system due to the compressibility of the gas, contrary to the case with standard co-injection molding. The gas expands inside the melt, which has already been injected into the mold. This means that neither the gas volume flow nor the melt propagation can be controlled.

The two processes are comparable in the following ways:

Although co-injection can be used for thin-wall molding, the figure below shows the typical skin thickness profile in a thick-walled molded part for both sandwich injection molding and the gas-assist injection technique. In sandwich injection molding, there is a parabolic reduction in the thickness of the core component up to the end of the flow path, whereas, with the gas-assist injection technique, the thickness of the core component remains almost constant. The flow front in the gas-assist injection molding is approximately parabolic in shape because of the viscosity of the gas, which is negligible by comparison to that of the melt. Once the gas has started to flow into the cavity, only a slight influence can be exerted on the molded part formation process. In tests with particularly thick-walled molded parts, it has been seen that a more uniform wall thickness distribution can be achieved if the gas flows into the cavity at a lower pressure, i.e. at a lower velocity, to begin with.