hen a plastic part fails on a food-processing line, or when you’re specifying one for the first time, the temptation is to open a datasheet and compare tensile strengths. Resist it. In food plants, most material failures aren’t mechanical at all. They’re chemical, environmental or regulatory, and they are often caused by someone optimizing for the load case and ignoring everything else. Here is a working method for picking a material or picking a better one.

Filter one: compliance. If the part touches food or a food-contact surface, the material’s specific grade must carry food-contact compliance, FDA 21 CFR in the U.S. and Regulation (EU) 10/2011 in Europe. Note the word grade. Compliance belongs to the formulation, not the polymer. The colorant, filler, mold-release or catalyst can disqualify an otherwise compliant resin, so “it’s nylon and nylon is food-safe” is not a sentence, a careful engineer says. Get a supplier declaration of conformity tied to the actual grade, matched to your conditions of use: temperature, food type (fatty, acidic, aqueous) and contact duration.

Filter two: sanitation chemistry. The cleaning protocol is part of the material spec. Chlorine, quats, caustic, acids, peracetic acid and steam/CIP each attack different polymers, and a part that survives the process can still be destroyed by the wash down. Before you compare any two materials, know exactly what the sanitation crew sprays on them and how hot it gets.

Only after both filters do mechanical properties matter. At that point, the field of common candidates is small enough to know cold.

Sliding surfaces: wear strips, guide rails, chute liners. UHMW-PE is the default. It’s slick, self-lubricating, extremely abrasion-resistant, cheap and available in FDA grades everywhere. Its weakness is structure: it creeps under sustained load and softens around 80 °C. UHMW is a sliding material, never a load-bearing one.

Precision parts: gears, bushings, bearings. Acetal (POM/Delrin) is the machinist’s plastic: stiff, dimensionally stable, low moisture absorption, holds tight tolerances. Its fatal flaw is chlorine. Hypochlorite sanitizers embrittle and crack it over time. Nylon (PA6/PA66) is the alternative: tougher, more heat-tolerant, quieter in gear trains and available as large cast sections. Nylon’s flaw is water; it absorbs moisture, swells, and softens, so a gear sized for dry conditions can bind on a wet-cleaned line.

Resilient wear parts: rollers, scrapers, wheels, guides, bumpers. Cast polyurethane owns this niche. It combines abrasion resistance with resilience. It takes impact and flexes without cracking in a way no rigid plastic can, and it bonds well to metal cores. Its flaw is hydrolysis: ester-based PU degrades in hot, wet environments, turning gummy and disintegrating. For wash-down zones, specify ether-based grades and verify the specific polyol/isocyanate/catalyst system carries food-contact compliance.

Seals, gaskets and temperature extremes. Silicone covers roughly −60 to +230 °C while staying flexible, making it the choice for steam/CIP gaskets, oven applications, tubing and molds. Use platinum-cured grades for food contact. PTFE is the other option: chemically inert against essentially everything, lowest friction of any solid, good to 260 °C. PTFE’s flaw is cold flow. Clamped under load, it slowly deforms and loses preload; use filled grades or design for the creep. Neither is a wear material.

Packaging and containers. HDPE, PP and PET carry the commodity roles. The relevant distinctions: PP handles hot-fill but turns brittle in freezers; standard PET can’t take heat; polycarbonate is clear and nearly unbreakable but carries BPA scrutiny and stress-cracks under aggressive cleaners.

When a part fails repeatedly, the failure mode tells you what to change. Match the symptom to the cause:

Cracking or embrittlement on a chlorine-sanitized line points to acetal degradation. Switch to nylon or a stainless-compatible alternative; don’t just replace the part. A gear or bushing that binds after weeks in service in a wet zone is nylon swell. Either design in clearance for moisture growth or move to acetal, if the sanitizer allows, or UHMW, if the loads allow. A urethane surface turning soft and gummy indicates ester hydrolysis. Respecify it as ether-based PU. A gasket that loses its seal over time with no visible damage indicates PTFE cold flow. Switch to a filled PTFE grade or silicone. A part that slowly loses dimension under load indicates creep in UHMW or polyethylene. The material was never intended for a structural application; move up to acetal or nylon. Cold-room cracking in an otherwise fine part is usually polypropylene’s low-temperature brittleness.

The pattern in every case: The original material wasn’t wrong on the datasheet; it was wrong for the environment. That’s why the diagnosis has to include the sanitation chemistry, moisture and temperature cycle, not just the load.

Every time, in this order: apply the food-contact compliance filter; apply the sanitation-chemistry filter; then optimize mechanical and thermal properties among what survives. When switching materials after a failure, add a step zero: identify the actual failure mechanism first because a material change that doesn’t address the mechanism just buys you a different failure.

One last discipline: document why the material was chosen. Six months from now, when purchasing finds a cheaper “equivalent” grade, the note that says ether-based PU required due to washdown hydrolysis or no acetal due to chlorine sanitation is what keeps the failure from coming back. The right material is rarely the strongest one on paper. It’s the one that survives the plant.