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Injection molding bubbles/ voids

You need to be very cautious when evaluating the bubbles/voids when using valve plastic injection molding gated systems. I don't want to come off as a know it all, because I don't, however I have spent more than 30 years studying valve gate systems and have become a bit of an expert in this area.

Voids/ bubbles can appear in two areas and both are cause by the same phenomenon. Plastic flows laminar "period". As the plastic enters the cavity depending on the shape, there will be volume changes. The material as it flows will go changes in velocity which has a direct bearing on the material viscosity. Sharp corners create the biggest changes where the material wants to continue to flow straight, but laminar flow forces a portion of the material to wrap around the corner, thus forcing a separation of the material and a void is formed. At the end of fill you can repack this void if the material remains molten enough to do so. Valve gate sizes, venting, injection speed, moisture, shrinkage, hot runner bore channels, decompression, and whatever else you want to throw at it, has no bearing on this phenomenon. What does correct it is possibly increasing the injection mold temperature, but you may have to go so high that it sacrifices cycle time and part dimensional's. The other is to change the plastic part geometry to minimize sharp corners, or move the gate location so the part is filled differently minimizing the filling voids.

I am a little bit of an expert when it comes to valve gating. But there is a simple way to find out if I am right. If it is voids, it is more than likely caused by the valve pin and it is what called a vortex fill. The material flowing around the valve pin creates a void or bubble and this can be carried into the melt. One simple way to find out if this is the case is to leave the valve pins open and mold parts. If the voids go away then it is the opening of the valve pin causing the problem. Simply delay the injection unit 1-2 seconds after the pins open.

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