Excessive Injection with Multi-Cavity Molds

You should always transfer from injection to packing with all cavities short-shot.


Let’s say a mold is imbalanced, the injection speed is 4 IPS and the mold is filled to 95% full. The first cavities finish filling at one speed and the last cavity to fill would fill faster than the first cavities. Why would it do that?

My Response:

What was being discussed is that if the screw is moving at a constant speed (such as 4in/s) then there is a constant amount of material being forced into the mold. In the case of a 1″ dia screw, this would be ~3 cu-in/s

In the case of a 4 cavity mold, all 4 cavities are receiving 25% of this flow or ~.75 cu-in/s.

Once the first cavity fills, the remaining 3 cavities are receiving 33% of the flow or ~1 cu-in/s.

Once the second cavity fills, the remaining 2 cavities will split the flow and get ~1.5 cu-in/s

Following this logic, the last cavity to fill will receive the full 3 cu-in/s.

Additional Thoughts:

During this process, the injection pressure is increasing, viscosity is shifting, and the mold cavities are packing with injection pressure for differing amounts of time before transfer. When the screw bounces back at transfer, there will also be a differing degree of back flow from each cavity causing more cavity to cavity variations and shot to shot inconsistencies.

If you transfer when all cavities are short, then they will all complete filling and pack with the same pressure. This gives you the highest potential to get equal dimensions across all cavities. This also reduces your clamp tonnage requirements as well as make the requirements unchanging over time.

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