Have you ever had a scenario where you are trying to troubleshoot a problem where a part periodically goes short, or a surface randomly seems shiny? Chances are your hold time is to short, or right on the edge of the gate freeze time. If the gate of a part is not frozen before the end of the second stage, the part will not be packed out consistently shot to shot.
Gate Freeze Time, or Gate Seal Time can be a critical study to perform whether working with a cold or hot runner. A gate freeze study can be a simple process to complete but can also be one of the most important parts of a molding process. If the gate is not frozen for a plastic part then there is no guarantee that the part is fully filled out. It is even possible for the plastic to flow out of the cavity if it has not settled yet.
Performing the Gate Seal Study will help ensure consistent part weight, surface finish, strength and dimensional stability from shot to shot. Performing a Gate Seal or Gate Freeze study is a simple task that should not be overlooked when developing an injection molding process.
A Gate Seal Study should be performed after determining an optimum Injection Speed for a 95% full part. It is ideal for multi cavity tools to have a part weight difference of less than 5% before adding hold pressure. This can be determined using a Cavity Balance Study. Once the first stage of filling the part is set, a Gate Seal Study can be performed to begin the second stage of filling.
Begin by determining a hold pressure for the study slightly above when the part is fully filled out. Then begin from a low hold time and incrementally raise it. This could be by 0.5, 1.0 or 2.0 seconds each time depending on the size of your part. As you raise the hold time it is good practice to also lower the cooling time by the same amount. Lowering the cooling time inverse to raising the hold time will help maintain a consistent cycle time throughout the study. At each increment, collect a shot and weight the parts. Plot the part weight versus time. As you collect the data you will see the part weight plateau. This plateau shows where the gate is frozen and the part can no longer be filled out.
Once the gate freeze time has been determined, the hold time can be set. Depending on the part there should be 1 to 2 seconds more hold time than when the part weight plateaus. This helps to ensures that the gate is frozen each shot if there is any variation in the cycle.
This describes an ideal process which should normally work with a cold runner. This process also works with hot runners and valve gates but the resulting graph will vary. Since a hot runner never solidifies it may be possible to pack more material around the gate area. You may notice small incremental increases in the part weight after the gate appears it should be frozen.
In conclusion a Gate Freeze Study should be performed whether you are using a hot runner, cold runner, or valves gates. This study provides the data used to back the decision on setting Hold Time. By determining a data driven Hold Time, there will be better success avoiding future problems.