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Injection Molding Troubleshooting Tips

Tip #2: Brittleness

Brittle parts can be a molders nightmare especially if the brittleness occurs once parts are at their customer or in the field. When troubleshooting brittleness problems one should consider potential causes from the material, process and machine (equipment/mold). Brittleness is often caused by improperly dried material or from foreign contamination but can also be caused by a number of other possible things as well.

From the ‘material’ side, is the correct grade of material being used? Very high melt flow grades are often more brittle than lower melt flow grades. Is the material contaminated or was it dried improperly? If regrind is being used, it is properly sized and properly dried? Too large of regrind size may not adequately melt resulting in un-melted material in the molded part which can cause stress-concentrations. These can all potentially cause part brittleness issues.

Is it possible that human error was involved in improperly drying or handling the material? Some materials, like polycarbonate and polyester, will become very brittle if not properly dried, while nylon could become brittle if dried at too high of a temperature or for too long of a time. Did careless handling result in contamination of the material?

From the ‘machine’ side, are the barrel temperatures correct for the material being processed or are they too high causing material degradation? Is the screw back-pressure too high or the residence time too long which could lead to material degradation? Is the machine too large for the part size being molded? Is the screw speed excessively fast? Are the gates of adequate size for the part and material being molded?  Small gates often result in high shear stress and localized degradation of the material which could result in brittleness.

While there are a number of things that can cause part brittleness, a systematic approach to evaluating potential causes should result in determining the root-cause of the brittleness so that corrective measures can be implemented to prevent the problem from occurring in the future.

 

Entec Polymers Technical Service Team