July 14, 2022

In-house scrap for injection molding

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Some suggestions for effectively using in-house scrap, whether for injection molding, extrusion, vacuum forming (which generates a lot of offcuts).

1. For companies producing multiple products, a good strategy would be to use up in-house scrap in components/molding with lesser service severity requirements molded from the same polymer. e.g. scrap from moldings of high aesthetic requirements could be used up in molding where aesthetics are not so important/darker in color. Load bearing molding regrind could be used up in non-load bearing items.
Sometimes it is safer to use up all the regrind in a dark, non-critical item if such an item is in the manufacturing mix in sufficient numbers to be a scrap sink.

2. For processes generating a lot of scrap like vacuum forming, it is of paramount importance to keep the offcuts clean. Plastics attract dust like magnets because of static electricity and dust coated offcut quickly degenerates cash into useless warehouse ballast. So grind up the offcuts and store in sealed bags ASAP. When re-extruding the scrap (which may sometimes require 30-40% blending with prime) be sure to compensate the additives which are likely to be depleted in the primary processing. These will depend on the polymer being processed, and are typically antioxidants, stabilizers, and most important, impact modifiers. If the impact modifier is correctly chosen the sheets produced may have better properties than the parent sheets. Of course all this is best done if vacuum forming shops have their captive sheet extrusion line, but can also be followed by those who vendor out the offcut conversion.

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