With injection molding there are many variables that go into a good part. These include everything from the resin to the press to the mold and so forth. It also includes processes and drying and water temperature. Everything needs to be dialed in to get that repeatable, cycle after cycle quality product that can be shipped to the customer. One injection molding defect that happens from time to time is part brittleness. Brittleness appears when the part breaks or cracks upon ejection or after it is ejected during handling. We will investigate this defect and the likely causes resulting from either Material, Mold, or Machine causes.
1.) Machine: With the process, the defect can be caused by several means. Brittleness can be caused if the mold filling speed is too slow. In this case increase the injection speed and be sure to maintain a consistent cushion. The melt temperature of the resin could also be too low. There are several things a molder can do to increase the melt temperature including increasing the heat profile to certain barrel zones or increasing the rpm which in turn increases the shear rate and melt temp.
2.) Mold: When molding components, if the surface of the mold is too cold it can often cause brittleness in the part. It is important that the correct water temp is used for quality molding. The design of the mold can work against you as well. If the runners and gates are too small for the application it can create excessive shear rates during filling which in turn can cause the brittleness.
3. )Material: In cases where the material being processed causes the brittleness of the part sometimes too much regrind is being added to the resin. Regrinded resin breaks down significantly after a couple of generations of grinding. In this case, reduce the amount of regrind percentage or mold with virgin resin for a while to see if the defect goes away. Then the regrind can be added back into the process at a lower percentage that works with the process. Contamination by foreign material can also cause brittleness. Inspect the resin and perhaps run some tests to be sure that the resin isn’t contaminated with foreign matter or additives causing this defect.
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