Home » Troubleshooting » How is silver streak / splay caused?

How is silver streak / splay caused?

Silver Streaks / splay is usually caused by moisture, but can also be caused by high filling rates and poor vents which can cause entrapping of gas either from the environment, mold water leaks, etc. A single line that appears like a silver streak, I refer to as jetting. There are many causes but some to look at are the following. Silver Streaks
  • The filling process should be such that the speed is not so high until the gate is reached and material will solidify causing less shear heat as the injection speed is ramped up. If you are blasting the material through a small gate, random shear stress can cause burning and jetting.
  • In the case of a nozzle, if small amounts of material solidify on the nozzle tip and then subsequently get injected into the mold/gate, the cooler material will tend to hang at the gate and free flowing material will flow around this point. Where the injected material meets can cause what looks like a silver streak but can actually be a single line that is actually a flow or knit line.
  • Having a cold slug well at the end of the sprue/runner can trap this initial material from the nozzle before it gets to the gate and cavity.
  • Gate thickness variation can cause the same issue if the tooling/ gate dimension is not equal.
Also, keep in mind that during the plasticizing stage, if you have too much screw speed and not enough back pressure coupled with varying pellet dimensions, you will trap air into the melt and cause silver streaks, but they "usually" have more than one line. Also I have experienced many times were central feed system may be feeding another line but vacuum leaks will cause pellets to not drop freely into the feed section of the screw. The mis-feeding can cause the material in the screw/barrel to be overheated and entrap gas as it pushed through the compression zone that will show in the part as silver streak/splay. There are many more possible causes, but need more information and true defect type of actual injection molding.

Few questions regarding troubleshooting silver streak / splay
What type of gate, runner, etc is this? Are you using a single speed injection profile? Is this single cavity and happening random on each shot or multiple cavities and random on each cavity? Are you sure the single line is not jetting or a weld/knit line?

Do you find the short line, in any area say closets to the gate or farthest from the gate? Closets to the gate can indicate what mentioned above, farthest from the gate or material entry can be related to the venting or material fill. Have you done short shots to see if the material if flowing evenly into the part? These lines can be very subtle and you may have a larger issue but are only catching the surface defects. I also have sample pictures of these type defects in optical clear PC that are not visible with the naked eye, but under a polarized strain viewer can be seen easily. Point being some are seen with the eye, but many are not visible. I think the defect itself needs to be clearly understood whether it is a silver streak/splay or another defect such as jetting, weld line, knit line, flow mark, etc.

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