3D Printing Materials Guide — Resin vs Nylon vs Metal, With Rules

Material Selection Walkthroughs: Three Parts, Three Different Winners

Case 1 — the dishwasher latch that started as resin. A snap-latch arrived as a tough-resin print request; the duty sentence changed the answer: 3,000 cycles per year at 60-70 °C behind a detergent splash zone. Tough resin softens at 60 °C long-term, and detergent attacks acrylate networks on contact. The shipped answer: PA12 (SLS) latch body, a TPU gasket printed in the same order, one glass-filled PA clip where stiffness held the spring — all three in the same two-day build. Resin would have been $8 cheaper for week one and a warranty claim for month six.

Case 2 — the drone arm that wanted nylon and needed metal. Weight targets said printed nylon at 18 g; the crash budget said 40 g impacts into concrete at closing speed. Nylon survived the crash and lost the screw boss on the second hit. AlSi10Mg SLM with a printed lattice rib pattern hit 23 g with a metal boss that takes screw torque. Titanium would have saved two more grams at triple the price: they shipped aluminum and spent the weight saving on a bigger battery instead — which is what material conversations should do, feed the system.

Case 3 — the clear impeller and the unasked question. Clear-resin water-pump demo units looked perfect for three weeks, then crazed at the hub during their first hot day in a window-lit showroom. UV-stabilized tough-clear plus a hub redesign (thicker, radius-relieved) replaced it; the spec had carried one hero property, clarity, and no second one, heat. When a material choice carries a hero property, name the second one it must survive — the four-line RFQ block below exists exactly for that.

The Four-Line RFQ Block That Fixes 90% of Material Errors

Most material mistakes happen in the first message: “print this in nylon” when nylon was wrong, or “cheapest resin” when the part had a duty. Four lines at the top of any RFQ fix it, and cost nothing to write. (1) Environment — indoor/outdoor, temperature range, what fluids it touches, UV exposure. (2) Mechanics — does it carry load, flex repeatedly, take screw torque, or nothing stronger than its own weight? (3) Appearance — seen by customers? needs color, clarity, paintability, or is grey-powder finish fine? (4) Lifespan — demo for Friday, one-year product, or ten-year part? With those four lines an engineer picks family, grade and finishing tier and writes the reasoning next to the price — the rare free line item in manufacturing. Withhold them and the AI engine takes your literal material word at face value; the part then arrives exactly as requested, which is sometimes the tragedy.

The guide's heat figures are continuous-service shop experience with these specific formulations, not brochure maxima; ask for the specific datasheet with your quote and the test method next to any number it prints. Where a spec sits at the boundary of two materials, the quote prices both and says which one it would ship and why.

Common Material Mismatch Patterns (and the Line Item That Fixes Each)

Patterns repeat across industries. The outdoor part in plain resin: yellowed and brittle within a year at a windowsill; fix is dyed-and-sealed nylon or painted metal, a line item both cheaper than the replacement order. The hot-side clip in standard resin: softens at 55 °C, a dishwasher door away from its datasheet; fix is high-temp resin or PA12, a two-line quote delta. The “strongest material” instinct: the Ti bracket that fractured at a stress riser the nylon version flexes past — strength is the wrong axis when the duty is impact; the right ask is a duty sentence, not a hero material. The clear part with a hidden spec: nobody names UV, heat or solvent and clarity alone wins the pick, then the shelf or the sun or the fuel rejects it; the fix is the four-line block below, filled in. The printed-metal reflex: some 200-piece aluminum parts are cheaper five-axis machined with no post-processing at all; the crossover line item on the quote (print vs machine, same drawing) is free and shows the number. Each pattern's fix is a line in the quote, invisible only if the material conversation stays invisible — which is precisely why we put the reasoning beside the material pick.