injection molding brittleness

Definition
A brittle molded part has a tendency to break or crack. Brittleness results from shorter molecular chain length (thus lower molecular weight). As a result, the physical integrity of the part is substantially less than the specification. molding brittleness

Causes
Material degradation
This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.

Weld line weaknesses
Non-optimal crystallinity
High residual stress
Incompatible materials blended together
Too much regrind
Improper drying conditions
Excessive drying either drives off volatiles in the plastic, making it more sensitive to processing, or degrades the material by reducing the molecular weight.

Remedies
Set proper drying conditions before molding
Material suppliers can provide optimum drying conditions for the specific materials.
Reduce regrind material
Contact material suppliers to get the recommended levels of regrind to use.
Change material
Optimize the runner system design
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.
Modify screw design
Contact material/machine suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.
Select machine with smaller shot size
Minimizing residence time reduces material degradation.
Reduce residual stress
Strengthen weld lines
Increase melt temperature within limits, not to overheat the material.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

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