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How pellet shape and size influence molded products

Regarding the pellet shape: you have been using cylindrical before and now switched to spherical. The pellet shape and size has an influence in the processability. In comparison to cylindrical pellets the spherical pellet has a different volume to surface ratio and a different the shape has an influence on the friction between the pellets. It's easy to imagine the influence this has on the conversion from solid to melt in your machine.

In my previous company we changed for some products the production method and made the same switch. Only in at very low percentage of customers we saw that issues developed in their process. The issue seen in most cases was the occurrence of splay (streaking, silver streaks), I'm not aware of any impact issues although one can not rule that out.

Sorting out this type of issue can be quite complicated and although the advices given above are all very helpful and sound, one would require a full picture of the issue (part, machine, material, processing conditions etc.) to resolve it. My first suggestion would be to hire someone who is experienced in resolving this type of issues. Here are four tips:
  1. Change in pellet shape (was stranded, now underwater) implies a different compounding line, so maybe mixing/uniformity is less, degradation more, etc.
  2. Grind both the parts made from old (good) compound and the new (suspect) compound, and test them the same way (can you tell the bad ones from some quick test like tensile or impact)? Record screw amps or torque to compare viscosities.
  3. Infrared on such a mix may be too complicated to read clearly. DSC is better. Test solid density, melt index (dry samples), other rheometry.
  4. Once you prove that the materials are substantially different, it's the material supplier's problem to figure out why. Look for another supplier, at least until the supplier pays for more testing, replaces the bad lot, or any other solution that you agree to. If it's a "bargain" supplier who is charging less than the average elsewhere, you are getting what you have paid for, and still should look elsewhere.
The material is from another compounding line. I have been in a material project where it was documented beyond any doubt, that material from an underwater line had a larger content of mono- and dimers and thus was more susceptible to degradation as material from another line. Also the size and shape of pellets influence the plasticizing. The pellet transport from hopper to barrel and the frictional behavior in the first zone are affected.

So I suggest investigating the process to detect differences in the material behavior, like measured injection pressure and other factors that can tell you how the material has changed and supplement this with some kind of measurement of degradation and degradation sensitivity (monomers, unknown radicals, break force in the molded part).
why does some pellets are in cube shape ?
- - - -> by: sha

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