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Molder Gets “Owned” by His Sink Marks
Don’t let this happen to you!
When you mold parts that have defects and want to know the cause it’s best to talk to your plastic.
But you have to talk in their very limited vocabulary because to the plastic molecules there are only four basic variables, plastic pressure, temperature, flow and cooling.
That’s all the plastic knows or feels.
Here is a conversation a molder recently had with his plastic over a sink mark problem.
Molder: “Why are you causing sink marks in my molded parts?”
Molecule (speaking knowingly): “Because there are not enough of us molecules in here”
Molder: “But I filled and packed the injection mold”
Molecule (speaking condescendingly): “Sure, but at molding temperatures, us molecules move further apart. You packed more of us in but it wasn’t enough. When we cool we moved closer together. Those of us who cooled last pulled on our neighbors trying to get closer. I guess we pulled so hard that we collapsed the wall. You call that a sink mark. To us, well, it’s just what we do”
Molder: “How can I go about fixing sink marks?”
Molecule: (patronizing tone): “Isn’t that obvious? You’re the molder. Take whatever steps you can to put more of us molecules into the mold cavities. Or strengthen the wall so we can’t cave it in. (Ha!)”
Molder: (after a little thought): “OK. So the problem is too few molecules, what can I do? I could…”
- Increase pack and hold pressure to get more molecules in the cavity. (If the pack/hold time is allowing discharge, increase the hold time.)
- Fill a little slower to the keep the gates open longer and let more molecules enter before the gates freeze (I could even increase the gate size to accomplish the same result).
- Reduce the mold temperature so instead of adding more plastic I strengthen the plastic surface.
If you have molding problems ask the molecules. They only know four variables:
- Flow rate
- Cooling rate