TROUBLESHOOTING

Generally, high injection pressures maintain part properties, improve part appearance, and reduce molding cycle times. A reduction of pressure in the mold cavity can be caused by the gate freezing off prematurely, poor mold design, or low pack/hold pressures. Since the gate area is the last place where molten material solidifies, this is the location splay marks or blemishes will show up if pressures are not maintained in the mold. Verify that the set up conditions are correct using the Start Up Procedures.

Low Pressurization At Gate Area
Pressures in the mold work to increase the contact between the shot material and the mold surface. A localized area of low pressure at the gate will cause poor surface detail at the gate. First, increase velocity and pack/hold time to help ensure there is enough pressure to pack out the mold. Next, increase the back pressure and screw RPM to increase shear (lowering viscosity) of the TPV material and ensure a more homogeneous mix of the melt. Increasing the mold temperature will reduce the cooling rate of the material and ensure a good surface finish. Finally, reduce decompression. There may be air being sucked back into the melt flow which will show up in the next shot, affecting surface area and causing voids in the part.

Mold Set Up Is Incorrect
Make sure the sprue and runner system are directed and sized correctly to ensure fast, efficient flow. Runners regulate the melt flow from the sprue to the cavities. For balanced melt flow and cavity filling, design runners for the shortest, straightest flow path consistent with best gate location(s). In order to reduce pressure drop and facilitate heat retention, the ratio of surface area to volume should be kept to a minimum. Runners also function to transfer pressure and regulate the pressure drop between an injection molding machine and a mold cavity. Shortening the land length of the gate will also help regulate pressure and aid in heat retention.

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