Material Decision Tree

Choose filament by working in this order: safety/consequences, real environment (heat/UV/chemicals/water), mechanical behavior needed (stiff vs tough vs flexible), then what your printer can reliably run (temps, enclosure, drying, abrasion). This decision tree narrows you to 1–3 sensible materials, then you confirm with a small test print and a simple in-use proof test before committing to a long job.

TL;DR

Pick filament by matching the part’s real heat/sun/water/impact conditions first, then check your printer limits (nozzle/bed temps, enclosure, drying). Once you’ve narrowed to 1–3 options, run a small test print and a quick in-service heat/load check before a long job.

Flowchart of the order to ask questions (risk → environment → mechanics → printer limits) and the material shortlist each path suggests.

What this decision tree is for

Filament choice goes wrong when you optimize for “prints nicely” and ignore where the part will live. Start with consequences (what happens if it fails), then the environment (heat, sun, water, chemicals), then the mechanical behavior you actually need (rigid, impact-tough, flexible), and only then decide how much printing hassle you’re willing to manage (drying, enclosure, bed adhesion). The goal is not to name one “best filament”, but to quickly narrow to a shortlist you can print reliably on your machine.

Decision points (answer in order)

1) Safety / consequences: If failure could injure someone, start a fire, damage equipment, or cause a crash/jam: be conservative. Use larger safety factors, more walls, and real proof testing. Prefer known, consistent materials and avoid “just trust the print” parts for critical loads.
2) Heat exposure (most common failure): If the part will sit in a hot car, near motors/heaters, in hot water, or on a sun-heated surface, PLA can soften/creep. If you can’t confidently keep service temps low, move away from PLA and validate with a simple heat test of a small coupon.
3) Outdoor / UV / weather: Sun + time can embrittle plastics. Outdoor parts also see thermal cycling and rain. Plan for thicker sections, rounded corners, and a material that tolerates weather better than basic PLA; then test a sample outdoors or under UV if the part matters.
4) Flexibility / repeated bending: If it must bend repeatedly (clips, gaskets, bumpers), treat it as a flexure problem: choose a flexible material (often TPU) and design to reduce strain (thin, long bends; no sharp notches). Stiff plastics forced to flex will crack at layer lines or corners.
5) What “strength” you actually need: Stiffness (resists bending), strength (resists breaking under load), and toughness (resists cracking on impact) are different. Decide which failure you’re preventing: sagging, snapping, or shattering. Also consider load direction versus layer lines—orientation can matter more than the filament label.
6) Printer capability checklist: Confirm you can hit the needed nozzle and bed temperatures, and whether you have an enclosure for warp-prone materials. If the filament is abrasive (many filled filaments), plan for a hardened nozzle. If the material is moisture-sensitive, plan to dry it.
7) Printing behavior you’re willing to manage: Choose the easiest material that meets the environment and mechanical needs. “Harder to print” usually means more warp risk, more drying, narrower temperature window, and more failed long jobs if your setup isn’t ready.

Common filament options (quick comparison)

PLA easy

Very easy printing and clean detail
Stiff feel for simple brackets and fixtures
Low warp on most open printers

Poor heat margin (softens/creeps when warm)
Can deform over time under constant load
Outdoor/UV durability varies by blend

PETG medium

Better heat margin than PLA for many functional parts
Good layer adhesion and ductile failure
Resists water and many household chemicals

Stringing and blobs are common if tuned poorly
Less crisp edges than PLA
Can be “springy” when you need rigidity

TPU (flex) harder

Flexible and impact-resistant
Great for bumpers, grips, seals, vibration isolation

Slow printing; retraction is sensitive
Dimensional accuracy and sharp corners are harder
Moisture sensitive; drying often required

Fast routing rules (first pass shortlist)

Low heat + want easiest success and best surface: start with PLA.
Moderate heat/water exposure + general functional parts: PETG is often the next step.
Needs bending, grip, sealing, or impact absorption: TPU.
Any requirement that sounds like “hot, structural, chemical, or safety-critical”: pause and verify printer temps/enclosure/drying and plan a proof test; don’t assume your current setup can do it reliably.

Proof test before a long print (what to test and what it means)

Part warps or lifts at corners during the test print

Likely cause: Shrink/adhesion mismatch for the material, geometry, or ambient drafts

Fix: Run a small footprint/corner test; improve first layer and bed cleanliness; add brim; reduce drafts/use enclosure if available; if the environment requirement allows, switch to a lower-warp material.

Part gets rubbery, sags, or permanently deforms in use

Likely cause: Service temperature too close to the material’s softening range and/or creep under load

Fix: Heat-test a small coupon at the real temperature for 30–60 minutes under light load; if it deforms, choose a higher-heat material and/or redesign with thicker sections and less stress.

Cracks along layer lines under load

Likely cause: Load is pulling layers apart (poor inter-layer strength for that geometry/orientation)

Fix: Reorient so layers run along the load path; add walls; reduce fan/increase temp within safe range for better bonding; re-test with a small bracket that matches the real load direction.

Stringing, zits, or fuzzy surfaces

Likely cause: Material behavior plus too-high temp and/or wet filament

Fix: Dry the filament; run a small stringing/retraction test; lower temperature slightly and tune retraction and travel; keep the spool protected from humidity.