Blackwell Plastics partners with OEM manufacturers and sub-tier suppliers in South Texas including Houston, San Antonio, Austin and Corpus Christi to provide plastic injection molding and plastic extrusion of engineering grade resins that satisfy demanding industrial applications. Blackwell Plastics supports plastic injection molding and plastic extrusion projects from design to fulfillment.

Friday, December 2, 2011


At Blackwell Plastics “A challenge to conquer” inspires learning and yields satisfaction and pride in our work when we have accomplished our goal. Today’s challenge was an 8 cavity clear rectangular part that presented a weld line or flow line in the face of the part. Upon analysis each cavity was gated at the corner and vented at end of fill in the opposite corner. We completed a number of shots to progressively fill the cavities and confirm that the cavities were balanced and filling properly. Process Engineering quickly ruled out material rheology or the tool geometry as a cause of the defect. The root cause was determined to be a venting issue. We considered three options to address the problem.

1. Perimeter venting – This strategy would involve a modification to the cavity of the mold to vent the entire perimeter of the cavity.

2. Static vent pins in the cavity – This strategy would involve putting static vent pins in the cavity area where the weld line was generated to allow uninhibited flow to the vent thereby eliminating the visual weld line.

3. Coining Technique - This strategy would involve filling the cavity with a relaxed toggle at low pressure allowing the part to vent along the entire perimeter. Upon 96% fill we would clamp, pack and cool. This would be similar to option 1, perimeter venting, but would not require a modification to the mold.

We tried the coining first to avoid modifications to the mold and after the progressive fill to ensure balanced filling we shot at 96% fill, clamped, packed, cooled and ejected. For today’s challenge the “coining” of the mold solved the problem without modifying the mold. The result was a part with less stress in the part and elimination of the weld line.

Understanding the conditions and events in the cavity from the perspective of the plastics allows our process engineers to solve problems. The idea of getting in to your work has a whole new meaning when solving problems outside the two plates of steel. Doing what you love and loving what you do is a joy that few get to experience and are at the heart of what makes our team go. If we can help you solve your problem, feel free to contact one of our engineers at

-Jeff Applegate