Gas injection profiles are used in gas injection molding to control the velocity of the gas, which in turn controls the velocity of the polymer flow front.
A filling profile is used to control the polymer injection. There is often a delay between the end of the polymer injection and the start of the gas injection to allow time for the material in the mold to cool a little. Gas is injected to complete the filling stage and to maintain pressure during the packing stage.
As the gas displaces the polymer, there is less polymer between the gas front and the polymer flow front; therefore the gas pressure required to push the polymer through the mold to finish filling reduces over time. The image below illustrates how the distance between the gas front and polymer flow front becomes less over time.
Using a constant gas pressure during the gas injection will increase the shear heating of the polymer as it is pushed by the gas to fill the mold. As the polymer injection is typically very fast, this is difficult to profile. Using profiles to taper the gas injection pressure in the remaining filling phase and in the packing phase helps prevent the following problems:
You can specify gas injection profiles to start after a short delay by using a pressure that is less than the polymer injection pressure. The gas pressure is then reduced over the remaining filling and packing phases.
The following graph and table show a relative gas profile where the gas injection is specified by the gas volume controller. The graph shows the change from filling profile to gas injection profile. There is a 0.2 s delay between the end of the polymer injection and the start of the gas injection. The percentage duration of the gas injection is shown above the graph.
(0–0.6 s) Polymer injection profile, (0.8–1.8 s) Gas injection profile by gas volume control
|Gas injection time (%)||Gas volume (%)|