TROUBLESHOOTING

Larger than expected dimensions in a part can be caused by increased 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. However, too much pressure during the fill, pack, and hold cycle could lead to an over-packing of material in the mold. Verify that the set up conditions are correct using the Start Up Procedures.

Mold Is Over-Packed
Mold over-packing is a leading cause of oversized parts. Over-packing, however, will do more than cause larger parts; it will increase processing and maintenance costs. The parts will consume an excessive amount of material, increase rejection rates (flashed parts), and cause damage to the parting line.

• First, verify that injection fill time is correct. 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.
• Next, reduce the hold pressure, pack pressure and/or pack time in these instances to help alleviate the problem. We also recommend a small cushion of 3.2 to 6.35 mm (0.125 to 0.250"), especially with softer grades that can compress more during the injection and packing phase.

Mold Designed Incorrectly
We always recommend using a "steel safe" mold for prototyping prior to building a production tool. If the dimensions of the mold are too large, steel addition may be an option.

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