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Brittleness of plastic injection parts

Plastic injection part brittleness is caused most often by a loss of molecular weight in the polymer material. A reduction in molecular weight results in a reduction of the mechanical properties of the material, such as tensile strength, elongation, and impact strength. Here are some causes and solutions below for Brittleness of plastic injection parts.

Brittleness from material degradation can be caused if materials are not dried sufficiently. This applies specifically to hygroscopic materials. In this case, a chemical reaction called hydrolysis takes place when materials are melted with moisture present. When hydrolysis occurs, a loss in molecular weight results in a reduction in physical properties.

Excessive use of regrind can cause part brittleness since regrind has already been exposed to heat and as a result suffers a loss in molecular weight.

Melt temperatures may be higher than recommended process temperatures. In this case, melt temperatures at the middle of the recommended melt temperature range should be used.

The molecular weight reduction is a result of material sitting in the barrel for long periods of time, referred to as residence time. The BSR should fall between 30 and 65%. If this falls well below 30%, this will indicate a residence time problem that can lead to brittleness.

High screw speeds and back pressure can overwork the material, resulting in an increase in melt temperature.

Solutions: 1.The barrel-to-shot ratio (BSR) is measurement, in terms of percentage, used in determining residence time.  2. A reduction in nozzle temperature will reduce melt temperature avoiding brittleness form occurring. 3. Reducing screw speed and lowering back pressure can prevent resin melt temperature. 4.Increasing wall thickness, rib designs, avoidance of sharp corners, and addition of radii can reduce the chance of brittle parts.

 

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