How to fix injection molding defect: Brittleness

Nowadays most of our daily products are made from plastic materials.However, many plastic product manufacturers encounter one injection molding defect that happens from time to time and that is part brittleness.  Brittleness appears when the part breaks or cracks upon ejection or after it is ejected during handling.  According to our years of experience,  we found that the the likely causes could be contributed to either Material, Mold, or Machine causes.


Injection molding Process 

Cause: (a) the melt temperature is too low

(b) The plastic degrades in the shot cylinder, causing cracking of the molecular structure of the plastic.

(c)  Mold filling speed is too slow.


Remedy: (a) increase the mold temperature. Keep sufficient supplementary material on the barrel of                              injection molding machine.

(b) Reduce the temperature of the shooting tank in all areas.Reduce back pressure.Use a shooting tank with vented holes to ensure proper operation of the venting holes and set the correct temperature for each hole.

(C) increasing the heat profile to certain barrel zones or increasing the rpm which in turn increases the shear rate and melt temp.



Cause: (d) The mold surface is too cold.

(e) The runners and gates are too small and create excessive shear rates during filling which in turn can cause the brittleness

Remedy: (d) increase the mold temperature.Limit the speed at which the cooling water flows through the mold.

(e) Use a full circular flow path and increase the flow path and gate size to provide an acceptable shear rate during the mold filling phase.




(f) Added too much recycled material/regrind

(g) The plastic material is contaminated by foreign material..



(f) reduce the amount of regrind percentage or mold with virgin resin

(g) 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.

At as a injection molder with over 20 years of experience, HS Mold knows how to help you avoid defects in the injection molding process!  For example study, please click

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