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Voids can appear in two areas and both are cause by the same phenomenon. Plastic flows laminar "period". As the plastic enters the cavity depending on the shape, there will be volume changes. The material as it flows will go changes in velocity which has a direct bearing on the material viscosity. Sharp corners create the biggest changes where the material wants to continue to flow straight, but laminar flow forces a portion of the material to wrap around the corner, thus forcing a separation of the material and a void is formed. At the end of fill you can repack this void if the material remains molten enough to do so. Valve gate sizes, venting, injection speed, moisture, shrinkage, hot runner bore channels, decompression, and whatever else you want to throw at it, has no bearing on this phenomenon. What does correct it is possibly increasing the injection mold temperature, but you may have to go so high that it sacrifices cycle time and part dimensional's. The other is to change the plastic part geometry to minimize sharp corners, or move the gate location so the part is filled differently minimizing the filling voids.
Increase the venting of the plastic injection mold. From my experience in the connector industry, venting is the number one culprit for variations in plastic parts quality. I have had many injection mold makers tell me, the plastic injection mold can not be vented anymore. Then after another review of the injection mold, we manage to increase the venting. Sometimes the vents have no channels out to the atmosphere, ejector pins/blades are vent channels (is the injection mold maker greasing the ejectors, results in no venting) Sometimes, you may need to split an insert into several inserts as a way of increasing the venting. Vent the runners.

In the case of the beginning of a run, the holding fixtures have acceptable warpage. Then the warpage increases after 2 days. This may be an indicator the vents are plugging and need cleaning. Make sure to vent to channels before going to the atmosphere as a way to help maintain the venting condition. SPC part weight during the production as a measuring tool. You may discover that your processes are not under control to begin with.
The gloss level of glass filled nylon is mostly controlled by the injection mold heat. Because the resin shears away from the fiberglass with the fountain flow effect you get a resin rich surface. Higher mold heats allow the resin to mirror the tool surface best.

Surface interruptions that disrupt the fountain flow will create dull spots. If you shoot past a pin, as the plastic moves past the pin, you will see a duller surface on the opposite side. You can reduce the effect of pins that do not go through the entire wall by generous radii. The same holds true with ribs.

Lastly, as you increase the injection mold temperature, you will also increase the crystallinity of the plastic part. This will change the plastic properties, most notably, the shrink rate. Your plastic part will get smaller.
This is true I have gone through this problem not in one company but in many, from the period when I started carrier from trainee engineer. Here are some techniques I used to troubleshooting this kind of issue through process. (By considering that injection mold design and mold maintenance is ok in all aspects)
  • Decrease the shot wt. Decrease the hold pressure at starting.
  • If cold water circulation required in injection mold, keep it off initially.
  • Increase the cooling time 4-5 secs for startup.
  • Ensure the purging of barrel for degrading material.
  • Dipper cavity mold, optimize injection mold initial opening speed some vacuum pull back component to cavity.
  • After repair of injection mold, ejector plate become much free in movement, (as all the contamination has cleaned, and if ejector rod is not coupling type). When injection mold clamping drops to zero, ejector plate move forward push the component to cavity.
There is a law for warpage but unfortunately it does not take you much further forward. It is that warping is proportional to internal stress in the molded plastic part. There is always going to be a lot of that because conventional plastic injection molding is a forceful process thanks to the very high viscosity of the plastics melt. It involves, inter alia, high pressures on compressible materials, high shear stresses on long tangled molecules, large temperature gradients in conflict with poor thermal conductivity, and movement constraints applied by some but not all parts of the plastic injection mold. It is allied to shrinkage: indeed many authorities have defined warpage as non-uniform shrinkage. So within your particular scenario of product geometry, material choice, injection mold design and process conditions, it will help to think about ways of reducing shrinkage magnitude and variation. Which element will have the major influence depends so much on the scenario, which is why the general law does not help much.
Moldflow is wonderful and is true that it is absolutely useful tool, specially when driven by knowledgeable user. However please ask your team a simple question "How close to best in class is your cycle time after moldflow?"

Moldflow is vastly market leader and every technical director demands moldflow from engineering team. After this is done, you visit shop floor, there is never a case that parts were right as with dimensions and warpage and cycle time. Molding tools are predominantly built without conformal cooling and will have warpage in a part with uniform wall thickness that every trainee engineer will design with?
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