Gate Freeze Time vs Hold Time, what is the difference? This may seem like a simple question but if you do not have a background developing plastic molding processes than you may have accidentally thought that they were interchangeable (they’re not). The main difference between Gate Freeze Time and Hold Time can be less than a second, but it can be a big nightmare. It is recommended that the Hold Time is longer than the Gate Freeze Time and this is why.
Gate Freeze Time is how long the gate connecting your runner to your part stays molten enough to be filled with more plastic. While this gate is still “open” plastic is able to be pushed into the part. This is part of the filling stage of a molding process.
Hold Time however is the amount of time Hold Pressure is held on the press to pack out a part. This time is suggested to be slightly longer than the Gate Freeze Time of the part. If the gate is not frozen when the Hold Time is up there is a chance for backflow of plastic through the gate or at least an inadequately filled part.
Once the gate is closed or “frozen” plastic material can no longer be packed into the part. If the Hold Time is 1 second longer than the Gate Freeze Time, than the hold pressure is not filling the part during this final second of Hold. This means that any time after the gate seal with hold pressure on can be considered wasted time. There is however a balancing act to perform to ensure the parts are good and the gate is sealed.
It is suggested that the Hold Time is longer than the Gate Freeze Time to ensure the part is packed out and the gate is no longer open to plastic flow. This is done by determining the Gate Freeze Time and adding extra time to create the Hold time. A Hold Time is generally put 0.5-2.0 seconds longer than the gate freeze time depending on the size of the part. A smaller part may need a lower safety factor to ensure a gate is frozen when compared to a larger part.
This extended amount of Hold Time past the Gate Freeze Time allows the process to handle cycle to cycle variation as well as job to job and material lot to material lot variation. Through extended runs there are slight changes to the material as well as chances for setup variation and temperature variation. By having a cushion of time on the Gate Freeze Time you ensure your Hold Time can handle the typical variation a process can see. You can always perform some testing to find what Hold Time achieves the most consistent results for your process.
Hopefully this quick comparison of Gate Freeze time and Hold Time can help you understand what the difference is. There can be quite a difference and understanding the difference can ensure good parts in the long run.