Extrusion basics: How to turn scrap into '$crap'

Almost every extruder knows that material is the main cost of manufacturing, with energy way down the list, and direct labor often a minor factor, too. Plastics extrusion is a very efficient operation, if you run a tight ship. That means using scrap and trim in ways that replace new material. It may be enough for the environmentalists to keep it out of the landfill (although plastics are not toxic—none of them), but that shouldn’t be enough for the accounting office. They will see the following alternatives:

Run it right back into the extruder, sometimes with grinders right at the line with controlled-proportion refeeders. You can even re-extrude film trim right back into the melt stream. (Make sure your screening is tight enough to keep out contaminants).

Grind offline, and keep it until you can feed it back into the appropriate products. This needs a scrap feeder next to the primary hopper, with controllable proportions. It also needs enough film-lined containers; storage space for the containers; training, so everyone knows what goes into these scrap bins and what doesn’t; and a careful system of inventory that marks the containers and keeps records of how much of what is where. Keeping these containers covered to minimize contamination is very important.

Sometimes two or more of these containers can be used together, like two colors that can both go into a dark or black product. That needs a big enough blender or several online feeders. Feeders are better than a blender, as different shapes and lubricities may prevent uniform blending.

Reprocessing into pellets is almost as good as direct rework, and may even be better if the cost of the repro extruder can be justified by a cleaner and more-uniform (easier feeding) product. Heat history matters, as all plastics degrade to some extent at processing temperatures, but some (much) more than others, and extra stabilizer/antioxidant can be added, if needed. For HDPE, the heat may promote crosslinking so that the product is actually stronger than if it were made with new resin, although the original color (or lack of it) will not be preserved. Screening is critical, as contaminants can act as stress concentrators and cause early failure in service, even if the resin is not seriously degraded in the reprocessing.

Reprocessing and resale is another option, if you can’t use the repro yourself. That may mean shipping to another division, or another company, or another country, in decreasing order of profitability.

Regrind and sale is often done, without the reprocessing—less work, less gain.

Last and least is resale of the scrap in as-is condition, often overseas.

Actually, sending to landfill is last, but this article is all about avoiding that fate. If avoidance means “send it out to the backyard until we can get to it,” be careful that the piles out back don’t get too big. I remember a case where there were 100,000 lb of mixed scrap product “out back,” which we were able to convert to 100 gaylords of identical material, using some hand sorting to remove the broken pallet pieces and other visible junk before grinding using a 10,000-lb ribbon blender. If you need to do this and can’t figure it out, call me. It took two weeks, but it paid off.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at algriff@griffex.com.

Griff will present two webinars, sponsored by the Society of Plastics Engineers (SPE; Bethel, CT), early in the new year. “TANA: Throw almost Nothing Away” will discuss the economic and technical aspects of recycling, in plant and out, on Jan. 5, 2017. “How to Read a Spec Sheet,” scheduled for Feb. 2, 2016, will explain how to extract the most value from information provided by resin suppliers. For more information and to register for the January webinar, go to https://www.eiseverywhere.com/ehome/217468; for the February webinar, go to https://www.eiseverywhere.com/ehome/217478.

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