Common Problems with Injection Molding and How to Avoid Them [PART 2]

by Trevor English17 May 2017

Manufacturing components and parts through injection molding can present certain challenges in the finish quality of the final product. Whether you are molding in house or outsourcing, here are some common problems with the injection molding process and how to avoid them. Read part 1 here.

Warp

Warp is a common problem in injection molding that can occur when the walls of a molded component are too thin causing the plastic to curl up. This problem is very similar to sink, a problem we addressed in our first part of this series. Components that use plastic with glass-filled fibers can be more susceptible to warp due to the uneven nature of cooling internal to the material. If you are looking to avoid or mitigate warping in your finished product, small support structures or ribbing will be needed. By either widening the wall thickness of a given warp zone or by adding thin support walls, your parts will fare much better against the strains of warping.

Swirling

Swirling tends to be more of an aesthetic problem in finished components compared to ones more structural in nature. This problem occurs when resin pellets are mixed with colorants to match a desired color. While the color may match before mixing, depending on the type of plastic polymer used, swirls can occur in color underneath the finished surface. To avoid this, you should use pre-compounded resin that has been color matched so there is no guesswork in the desired color between part runs.

Drag

Drag is a problem that occurs due to improper draft built into surfaces parallel to the direction of mold operation. This more specifically means that vertical walls within your mold may not have sufficient angles or space to allow for proper part ejection. If there is insufficient draft, then part ejection can cause excess stress on the part causing drag or scrape marks on the surface. By designing in a minimum draft angle of .5 degrees up to 2 degrees, you can ensure that proper ejection will occur.

Silver Streaking

Silver streaking is caused by water absorption or material contamination in the molding process. It results in a white color appearing in the direction of material flow. Along with absorption, material contamination can mean too much additive or aeration during mold injection. To counteract this problem, you will want to slow down your injection speed, make sure your molds are clean, adjust the gate size, or increase the temperature. Given the nature of silver streaking, there can be many place contamination occurs. When mitigating this issue, make sure you examine every possible culprit in your molding and injection setup.

Are there any other injection molding problems we haven’t addressed in this series that you’d like us to? Leave us a comment below and we’ll look into them!

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