Scientific Molding & Valve Gates…

The basic principles apply whether you are using valve gates or not. In theory, you try to use one flow rate through the gates whenever possible to fill the mold because it reduces variation in the process due to a consistent shear on the material.

Question:One of your recommendations on scientific processing is to avoid profiling and perform mold filling in 1st stage.
How this more relevant in today’s scenario as many molders are molding the molds having Hot runner systems with sequence in gating?
My Response: The recommendation of using one injection speed when possible is based on the goal of maintaining one shear rate (flow rate) through the gate as the material fills the mold.  Not every mold make a good part with one flow rate, but you always strive for this since it simplifies the process and reduces variability. The approach to sequential valve gating is based on the gating layout and purpose. I will lay out 2 common scenarios and how this can be dealt with the same theoretical approach.
Scenario 1 – Gates opens after melt front has passed: In this case, the flow front begins when the first gate in the cavity opens and then more gates open sequentially only after the melt front has passed. In this situation, you should maintain the same injection speed when possible. This is because the material flow front will maintain the same flow rate and will result in a more consistent shear rate across the entire part.
Scenario 2 – Gates open at different locations starting new flow fronts: In this case, the first gate in the cavity opens, possibly to fill a larger area, and then a second gate opens in a different location and begins a new flow front. These flow fronts will eventually meet as the cavity fills. Assuming the gates are of similar size, you would want to double the injection speed as the gate opens since you doubled the number of flow fronts being serviced. This will cause the flow rate though each gate to remain the same resulting in an even shear rate across the part.

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