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The factors affect the percentage of regrind materials

The bulk density of the regrind is probably the single biggest factor effecting how much regrind one can use.

If you are using a recycled film scrap that hasn't been densified then you pay a tremendous penalty in your rate. There are densifiers like a Munchy that will compact the film scrap to something close to a virgin pellet. In this case then you can use quite a bit of scrap - 20 - 30%.

Another thing effecting your percentage of regrind is the use of grooved feed sections for extruders. They really are not designed for anything but pelletized material. It is best not to use more than 5% regrind if you have a grooved feed otherwise the grooves will clog and production rate will drop significantly and product quality will also suffer.

In blow-molding, for instance, it is not uncommon for bottlers to use 35% regrind as the moels and tails comprise a considerable amount of material in relationship to the end product. That stuff is usually compressed and put back in the hopper.

Pre-hot runner systems had a lot of regrind as dense material sitting in the runners and gates was recycled. Those rates were quite high. I have seen that as high as 50%.

The other points are quite good such as contamination from streams and other things that need to be considered. In the end I would say there is NO rule of thumb it greatly depends on so many factors.

There is no 1 rule of thumb. It will depend on several elements;
  • What type of Polymer and how they degrade (heat stability and moisture content)
  • whether filled and type of filler. Glass length will degrade making more brittle, lower modulus
  • whether it is a blend, PC:PBT interact with each other (transesterification)
  • can you add heat stabilizers, can increase the level of regrind
  • what kind of shear are you putting into the mix
  • you won't try to color match or all bets are off on color stability
Ultem PEI is great, but high heat, I've seen >50% regrind work fine, unfilled PP of PE can go a long way with addition of phosphate/phenol stabilizers.

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