Will This Bale Mold? Screening Recycled Plastic Before It Reaches the Press

The question

Regional molders want to use more recycled plastic. Their customers ask for it, and post-consumer polypropylene from the Porter Valley Recycling Cooperative costs about 30 percent less than new resin. The trouble is consistency. One bale runs beautifully; the next one short-shots, flashes, or produces parts that crack in a cold-drop test. By the time a molder finds out, it has already run a few thousand bad parts.

The cooperative brought the question to the Discovery Center in the fall of 2025: can a small plant tell, before a lot goes into the hopper, whether it will mold? Dr. Colin Nguyen, teaching professor of plastics and packaging engineering, took it on as an undergraduate-led project in the Applied Materials & Polymer Research Group, where students lead and faculty advise.

The approach

The team pulls samples from every bale of recycled polypropylene the cooperative bales over a two-week window, three times a semester, and characterizes each one in the Polymer Characterization Laboratory. The work is deliberately unglamorous:

• Melt flow index on every sample, run in duplicate, to see how much the material’s flow behavior swings bale to bale.
• Differential scanning calorimetry to detect polyethylene contamination, which shows up as a second melting peak and makes polypropylene parts brittle in the cold.
• Ash content to estimate fillers, labels and dirt.
• Molding trials on a test-bar mold, followed by tensile and impact testing at room temperature and at −20 °C.

The goal is not a perfect description of each bale. It is the smallest set of quick, cheap tests that predicts which lots will cause trouble on the press. The molding data feed the department’s recycled-content pilot line, which asks the companion question of how much recycled material a structural part can tolerate.

Students on the team

Six undergraduates are paid on the project, 8–10 hours a week each:

• Marisa Thorsgaard, senior, plastics engineering — student lead; runs the scheduling with the cooperative and owns the dataset.
• Jonah Pfeiffer, junior, plastics engineering — DSC and contamination analysis.
• Keira Abernethy, junior, packaging — melt flow and ash testing.
• Tomas Villanueva, sophomore, plastics engineering — molding trials and test-bar inventory.
• Abby Lindahl, sophomore, manufacturing engineering — mechanical testing.
• Dev Raman, first-year, undeclared — sample logging and chain of custody, and, by his own description, “mostly cutting open bags.”

Most of the team learned the instruments in PLE 3700 Polymer Testing and Characterization; the first-year student learned them on the job. Nguyen meets the group every Thursday afternoon, reviews the week’s data and signs off on anything that goes to the cooperative, but the students set the agenda. Two of them have already presented the work at a regional plastics conference, and the senior lead is listed as first author on the technical report the cooperative now hands to its customers. That arrangement is the point of the research group: a first authorship credit before graduation, earned on a problem a company actually has.

What they found so far

After three sampling rounds and 117 bales:

• Melt flow index varied by as much as 38 percent between bales from the same week, far more than a molder’s process window can absorb.
• Polyethylene contamination ranged from about 4 to 11 percent. Above roughly 8 percent, test bars failed the cold impact test more often than they passed.
• Three tests together — melt flow, a single DSC scan, and ash — correctly flagged 25 of the 31 lots that later caused molding or impact failures, at a cost of about $45 of lab time per lot.
• Blending two bales with high and low melt flow before molding narrowed the variation by more than half, which the cooperative is now trying at its own facility.

“Nobody needs a doctoral thesis on every bale. They need to know which three tests to run on Monday morning before they commit a press for the week.”

— Marisa Thorsgaard, senior, plastics engineering

What’s next and who’s involved

This year the team is adapting the screen for equipment a small plant already owns, replacing DSC with a density test and a simple bend test where possible, and writing a two-page procedure that a molding technician can follow. Spring results will be presented at the undergraduate research symposium, and the sustainability angle is being picked up in PLE 4100 Sustainable Polymers.

Partners: the Porter Valley Recycling Cooperative (bales, sorting data and a staff liaison) and two regional molders that run confirmation trials. Funding comes from a two-year, $186,000 award from the Wesconsin Circular Materials Fund, routed through Research & Sponsored Programs.