July 14, 2022

Eliminate glass fiber marks on injection molded parts

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There is virtually no way to completely eliminate the appearance of glass fibers at the surface of an injection molded part, particularly with this high a loading percentage.

As you probably know, the glass fibers are like logs floating in a stream of water, as long as the channel is wide enough and the flow through the channel is not turbulent, the logs will orient themselves axially, parallel to the stream flow. When the channel changes in depth or width to create turbulence the logs will begin to randomly orient themselves and get bunched into clumps of logs. If two streams of logs come together the collision of the two log flows will randomly orient where they come together jamming and restricting flow.

If you have followed this analogy so far, the trick to minimizing fiber exposure at the surface is to get the least turbulent flow of plastic in the cavities. In your case, increasing the mold temperature helps, increasing the injection speed makes it worse. Increasing holding pressure may force some of the resin to the surface, but if you’ve already log jammed the surface with fibers, there is little probability that you will force resin into the interstices of the fibers. Glass fibers will not bend when they go around corners, they will break into smaller fibers. Also high screw rpms will break fibers. Using a percentage of regrind has the same effect of dicing up the fibers so that can help. Use this knowledge with caution as you could be trading cosmetics for strength.

You may be forced to adding flow leaders to try and change the fill pattern and reduce turbulence in the melt stream. It is quite difficult to say without having a sample in hand to examine.

I can tell you this, that a properly designed part, gated correctly (size and location), vented correctly, with adequate runner channel cross sectional area, should not be difficult to mold with minimal exposed glass at the surface.

If this is a new part or new mold I’d start looking at the runner, sprue, gates and vents first to make sure that they meet material suppliers recommendations. Toolmakers hate to put in vents but there is one thing I’ve learned, you can’t have too many vents in a tool so long as they are properly sized.

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