Figuring out why some parts occasionally stick can be something of a nightmare. Things are not always logical here. The part could be stuck in the moving side of the mold, but the cause can be in the stationary side. The part can have a problem releasing from the stationary side upon mold opening, and this slight distortion causes it to catch in the moving side upon ejection.
First find which side of the mold is causing the problem. The best way to find out is to record the mold opening and part ejection with a high-speed camera. The cause can easily be missed by the human eye even with slow mold opening and ejection. The cause can also be a slight distortion at the start of ejection. The part does not release evenly and distorts as a corner sticks, then releases, only to have the part catch on the other side.
A recording tells the story more clearly. Not everyone has access to high-speed video, but check out renting one for a half a day. If it’s a no-go for the camera, then close and open the mold as slowly as the press will allow, looking and listening for any telltale signs. Then, as slowly as possible, eject the part, paying close attention to when the ejector pins first start pushing on the part. You might need a vacuum release or air blow to solve the problem.
Another procedure to help define the problem is to remove the second stage by taking the second-stage or pack-and-hold pressure down to near zero. Does the part stick when it is short, or only when it is packed out? If it sticks when short there may be too much shrinkage, or perhaps there is a tool issue. If it only sticks when packed, there might be too little part shrinkage.
What else can cause parts to stick?
1. Scratches can cause material sticking in the sprue, part sticking on the “A” side of the mold upon mold opening, or part sticking to the “B” side of the mold at part ejection. Again, scratches must be polished out, and polishing must be done in the direction of draw or mold opening.
2. Plate-out on the mold surface can cause sticking. Inspect the mold surface for buildup and clean appropriately. Actual process temperatures may be too high, so check actual melt vs. set temperature.
3. Is the polish correct or do you have too deep a texture? You need 1.5° of draft for every 0.001 in. (0.025 mm) depth of texture.
4. Too high a polish can cause parts to stick by creating a vacuum upon ejection. This is especially valid for soft-touch resins. Use a vacuum break or venting to aid release. Texture polish will also help ejection of soft-touch resins.
5. A mechanical source of trouble can be cocking of the ejector plate. Check for even length of the knock out bars, which should be within 0.003 in. of each other and use guided ejection. In addition, you may need more, larger or better-placed ejector pins.
6. Keep your ejection pressure down to a minimum; you will find out if a pin is galling and that could cause cocking of the ejector plate.
7. Check the part for areas that show stress whitening. These are sure signs of trouble; find what is causing this stress. More draft or larger ejector pin may be needed.
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