TROUBLESHOOTING

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Weld marks on a part are caused when two melt flow fronts come together. If the melt fronts have distinctly different temperatures or are too cool, they will form a line on the part where the materials come together. In some cases weld lines can cause part weakness, along with being aesthetically displeasing. Gating correctly to minimize weld lines and promote efficient flow is always suggested. To prevent weak and apparent weld lines, ensure that when the flow fronts reach each other, they have a low enough viscosity and sufficient pressure behind them to allow proper bonding. Hold times may need to be extended to allow for solidification of the bonded flow fronts. Finally, as flow fronts come together they will tend to trap air between them. Sufficient venting is recommended in these areas.

Polymer Cooling Rate Is Too High
When two cool material fronts come together, a weld line will appear. Increasing melt and mold temperatures will help maintain heat and lower viscosity in the flow fronts. It is also important to make sure there is enough pressure behind the flow fronts to ensure a sufficient bond. Increasing injection pressure and injection speed will help increase pressure in the mold cavity. Lengthening cycle time will encourage solidification of the bonded flow fronts.

Mold Design and/or Function Are Improper
The location and size of the gate are important to maintain efficient melt flow. The location of the gate should minimize the number of flow fronts in the mold if possible. The gate should be small enough to ensure proper shear, yet large enough to allow proper flow. We recommend a gate size no smaller than 0.5 mm (0.020") and no larger than 3.0 mm (0.120"). Refer to the Quick Processing References for details on gating using specific grades of our TPVs.

Generous venting is necessary when using our TPVs because of their fast filling times. As molten TPV material enters a mold quickly and under high pressure, enclosed air must exit just as quickly. For large parts or parts with long flow lengths, it may be necessary to reduce the fill speed to allow time for air to escape the part. Increasing the mold temperature will also help material flow by maintaining the melt temperature and reducing the cooling rate of the material.

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