TROUBLESHOOTING

Smaller than expected dimensions in a part can be caused by decreased pressures in the mold. The actual injection pressure depends on many variables such as melt temperature, mold temperature, part geometry, wall thickness, flow length, and other considerations associated with the mold and equipment. Pressures in the mold work to increase the contact between the shot material and the mold surface. 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. Verify that the set up conditions are correct using the Start Up Procedures.

Mold Is Under-Packed
Mold under-packing causes poor contact between the shot material and the mold walls, which seriously reduces cooling efficiency. Under-packing of the mold cavity can lead to excessive shrinkage, shrink voids in thicker wall sections, sinks in large surface areas, and poor part appearance. Typically, mold fill time takes between 0.5 and 2 seconds, depending on part volume, runner-gate style and size, cavity locations, and injection pressure. We also recommend reducing the cushion in the cylinder to between 6.0 and 3.2 mm (0.250 and 0.125"), especially with softer grades that can compress more during the injection and packing phase.

If these suggestions do not work, it may be necessary to decrease the viscosity of the material entering the mold cavity. Increasing the injection velocity, 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. Our TPV materials depend mainly on shear to increase flow. However, increasing melt temperature will also lower the viscosity of our materials to a lesser extent, and is an option.

Mold Temperature Is Too High
Ensure that mold temperatures are not too high. Refer to the Quick Processing References on our website for information on specific grades of our TPVs.

Mold Designed Incorrectly
We always recommend using a "steel safe" mold for prototyping prior to building a production tool. If the dimensions are too small, removal of steel in these areas will increase part size.

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