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Question from a Molder About Profiling Injection Speed

We recently received the question below about profiling injection speed. We thought other injection molders would be interested in learning more, so we’re publishing our answer here…

Question:
First, how do you decide when or when not to use  profiling of injection speeds vs. single speed injection. Secondly, how do you go about selecting the optimal speed profile setting for smooth fill?

injection molding machine profilingAnswer: Modern injection molding machines all have the ability to change plastic injection speed in several steps during mold filling. Profiling can solve specific injection molded part problems.

For example, “Jetting”, sometimes also called worm tracks because of their appearance on the surface of the injection molded part occur when the initial plastic entering the cavity squirts through the gate before the plastic is able to form a flow front. If you’ve seen the jetting defect, you know exactly what I’m describing. To eliminate jetting you would reduce the initial fill rate so that a plastic flow front forms, then increase the fill rate after you get that flow front.

Another injection molded part problem that is solved by profiling injection speed is burn marks. Burn marks occur in the last area of the mold to fill. They are caused by air not being able to escape fast enough as the mold cavities fill with molten plastic and subsequently getting so compressed that there is auto-ignition, like a diesel engine. If you have a burn mark problem on your injection molded part, you would want to slow down the final fill rate which would give the trapped air more time to vent from the mold cavity.

If you have flash in your injection molded part at the far end of the cavity you would also want to reduce the final plastic fill rate.

Finally, parts that develop cracks in the direction of plastic flow are also affected by the overall plastic flow rate. When plastic flows, the molecules tend to orient in the direction of flow. This orientation causes the plastic part to be strong in the flow direction but weakened in the direction perpendicular to flow. So, the plastic part may fail in the weakened direction.  Slowing the overall fill rate or running a warmer mold will reduce the molecular orientation.

Summary: What is the best fill speed and filling profile? It is the filling profile that allows you to fill the mold  at the fastest speeds that avoid causing the problems discussed above. Profiling your fill in this way will also minimize your cycle time, which maximizes your profit.

– Don Paulson

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