TROUBLESHOOTING

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There are two reasons for voids appearing in a part:

• Material in the core solidifying slower than material on the surface
• Moisture in the melt flow

Material In Core Sets Up Slower Than Surface Material
Melt flow cools quickly against cool mold surfaces. It takes longer for the center of the part to cool. This is especially true for parts with very thick cross-sections. Melt flow will continue to shrink until it solidifies. The difference in the shrinkage between the center and the surface creates voids in the part. Increasing pressure in the mold will force material towards the mold wall until the material solidifies, reducing the gaps. To increase the pressure in the mold, increase injection speed, hold pressure, and hold time. Make sure that the sprue, runner, and gates are large enough to assure free melt flow. Increasing the mold temperature and cooling time will slow down the solidification of the material against the surface of the mold. Increasing the back pressure and screw RPM will increase shear (lowering viscosity) of the TPV material and ensure a more homogeneous mix of the melt. A lower viscosity, more homogeneous melt will improve packing out of the cavity.

Moisture In The Pellets
Most of our TPVs are slightly hygroscopic. Effective moisture control ensures high quality parts that have an appealing surface appearance. With any TPV, moisture impacts processing, physical properties, product performance, appearance or a combination of these properties, along with silver streaking. When moisture is trapped inside a part, it will create a void. Improper venting can also trap moisture in the part. It is important to use sufficient venting when using our TPVs. See the drying section of our Injection Molding Guide for more details.

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