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Injection molding processing can affect shrinkage, but not as much as fillers.
Increase shrinkage: shorter cooling time in injection mold. Higher mold temperature. Greater product thickness. Higher resin melt temperature during injection. More plasticizer in resin. Less injection speed. Lesser packing / holding pressure and volume.

Decrease shrinkage: longer cooling time in the injection mold. Lower mold temperature. Thinner product walls. Lower resin melt temperature during injection. Greater injection speed. Greater packing / holding pressure and volume. Fillers such at talc, glass beads, or glass fibers (more anisotropic shrinkage with fibers).

Injection rates, packing, and mold temperatures can significantly affect molded-in stress or allow voids and sink marks. Molded-in stress can affect warp, solvent sensitivity, dimensional stability, and impact resistance; so, these tertiary effects need to be considered.
Injection mold technologies seminars may help mold companies to educate their people. But I think that each injection mold shrinkage problem you face in real life is caused by too many effects that one rarely can keep track of it.
  1. One thing you always should keep an eye on is the crystallization of materials which is responsible for the biggest part of the shrinkage. And this crystallization is not just influenced by the process (PVT- is one head point).
  2. But also by the limitations made by the machine and the injection mold (cooling time-->crystallization rate).
  3. Also the fillers in a material can have so much influence on crystallization.
  4. Also keep in mind that post process shrinkage also causes crystallization and thus injection mold shrinkage.
  5. Not every part has uniform wall thickness, this will cause ununiformed shrinkage occurs.
  6. Also so-called amorphous thermoplastics are not 100 % amorphous und thus can shrink depending on the overall conditions named above.
Both are highly developed and very capable products. Either one is much, much better than no process calculation at all. In that sense, you can’t go wrong whichever one you choose. That’s just as well, because it is going to be difficult to answer the question in a truly informed manner. There can be few if any people who are equally fluent and experienced in both systems, under industrial conditions.

A niche product such as injection molding process simulation needs support by industry experts who can hold their own on the factory floor as well as at the keyboard. I think Autodesk’s CAD reseller channel struggles to deliver that. I am just an onlooker now, but I get the sense that Moldflow has stalled, and all the momentum is with Moldex. Certainly, over the last few years, I have read and seen quite a lot about Moldex and almost nothing about Moldflow. It is just a niche for Autodesk but it’s core business for Moldex.
Venting can be one of the most important parts of an injection molded product. Many engineering materials like PBT (Polybutylene terephthalate) have a tendency of out-gassing. The best practices that I have to date are:
  • Add 10mm wide vents spaced 30mm from each other all the way around parting line.
  • Always follow the recommendations of the material company for vent depth.
  • If the product has a 50mm or larger core out the injection mold must be vented through the ejector plate, which means that you must drill through the core side (without hitting any water lines etc.) I usually use a 5mm or 1/4" drill bit to do so.
  • Always vent to atmosphere.
  • Polish the vents and vent channels. This makes the vent self-cleaning. It also allows the gases to escape much better.
A really old trick to ascertain whether the injection mold has an inherent venting problem or not, and this trick is somewhat dependent upon injection mold construction so must be evaluated as applicable on a case by case basis (more about this at the end) is to use shipping tape, or masking tape, depending upon the gap desired laid across the injection mold major parting line to artificially "spread" the mold and create a vent gap. Evaluate the fill in the taped v non taped shots without making changes to the injection molding process settings.
Over the years we have been taken advantages of with the low cost of plastic injection mold manufacturing in China. You get what you pay for. I have heard of an injection mold cracking in half just last year from China, water poring out of it in the press. As an injection mold maker back in the 80's and 90's we started to see import steel making its way to our machines. We could tell on the CNC's or polishing we were not getting what we bargained for. After heat treat OMG, warp. We use to say they put old "bed springs" in the alloy mix. When drilling a hole in this steel it would not be constant feed rate by hand, you could feel the drill push easy then hard etc. So we had to be very precise in ordering our plastic injection molds with certification. We ordered 4 large injection molds from a Chinese manufacturer in 2004, the heat treat was horrible, we did not get our certification, it looked fake. Someone was trying to pull a fast one. I hardness tested several blocks from the plastic injection molds, not good. We spent approx $450,000 for those injection molds back in 2004 and we tried to outsource and handle the engineering real close. We tried to do everything real and it back fired. These injection molds were for electrical components and tolerances on my parts were critical, 4 to 8 cavity molds, the quality was just not there. Glass filled nylon wore down these plastic injection mold in 8 months, we had to retool.
Chinese injection molds cut major corners on water, mold steels, unless you really get after them to use good mold steels. I have seen Chinese plastic injection molds that literally warp in the press because their injection molds are the softness of lead. I have seen well-made Chinese molds, but again, the materials used have been sub-par. The delay in communications is also an ongoing problem. That's one reason I use America, UK or Italy over China -- never have to worry about any of the things previously mentioned. The only reason that Chinese injection molds remain more competitively priced is the fact that the country pegs its currency to the dollar, so you never see the true cost. In the near future, the Chinese will not be able to continue pegging their currency to US dollar especially when the dollar is getting trashed by our own government. Because of this weak dollar policy, it makes everyone except China very expensive. Yet, for our European friends, it makes US made stuff cost competitive. However, even with our virtually worthless currency, I still get 25-30% less costly tooling in Portugal.
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