Last updated: 2026-09-05 · 5 min read
Resin Selection Checklist for Injection Molding — What 8 Criteria Must You Evaluate Before Choosing a Material Grade, and How Do You Avoid the Most Common Selection Mistakes?
A resin selection checklist covers 8 criteria: (1) mechanical load, (2) use temperature, (3) chemical exposure, (4) UV/outdoor exposure, (5) regulatory requirements, (6) part geometry and wall thickness, (7) dimensional tolerance, (8) annual volume and target cost. Skipping any one criterion leads to field failures — the most common mistake is selecting by tensile strength alone while ignoring impact, HDT or chemical resistance.
- Mechanical criteria: tensile strength (resistance to pulling), flexural modulus (stiffness), impact strength (resistance to sudden load), elongation at break (flexibility). A part requiring high stiffness and impact: PC (tensile 60 MPa, impact 70 kJ/m²). A part requiring flexibility: PP (elongation 100–300%) or TPE. The mistake: selecting ABS for a snap fit based on tensile strength (40 MPa) while ignoring elongation (15%) — the snap fit breaks at 15% extension. For snap fits: use PP (elongation 200%) or POM (elongation 25%).
- Thermal and environmental criteria: HDT (maximum use temperature), UV stability (outdoor exposure), chemical resistance (contact with solvents, oils, cleaning agents). A part at 120°C continuous: PP fails (HDT 100°C), PC works (HDT 130°C). A part in outdoor sunlight: ABS degrades (yellowing in 6 months), ASA works (UV-stable for 5+ years). A part contacting gasoline: ABS dissolves, nylon (PA) resists. The mistake: selecting a resin based on room-temperature properties while ignoring the actual use environment.
- Processing and cost criteria: MFI (flow into thin walls), shrinkage (dimensional tolerance), resin cost ($/kg), annual volume. Thin walls (below 1mm): high MFI resin (above 20 g/10min). Tight tolerance (±0.05mm): low-shrinkage amorphous resin (PC, ABS at 0.5–0.7%). High volume (500k+ parts): resin cost dominates — PP at $1.20/kg vs. PC at $3.00/kg saves $900,000/year at 500k parts × 10g. The mistake: over-specifying an expensive resin when a lower-cost grade meets all functional requirements.
What 8 Items Must the Resin Selection Checklist Cover?
| # | Criterion | Key Property | Common Mistake |
|---|---|---|---|
| 1 | Mechanical load | Tensile, impact, elongation | Selecting by tensile only |
| 2 | Use temperature | HDT at correct load | Using 0.46 MPa for loaded parts |
| 3 | Chemical exposure | Chemical resistance chart | Ignoring contact with oils/solvents |
| 4 | UV/outdoor exposure | UV stability (ASA vs. ABS) | Using ABS outdoors |
| 5 | Regulatory | FDA, flammability, RoHS | Missing food-contact requirement |
| 6 | Part geometry | MFI vs. wall thickness | Low MFI in thin walls = short shots |
| 7 | Dimensional tolerance | Shrinkage rate | Semi-crystalline for tight tolerance |
| 8 | Volume and cost | Resin $/kg × part weight | Over-specifying expensive resin |
Work through all 8 items for every new part. The checklist takes 30 minutes to complete but prevents 6–12 weeks of mold rework and field failures. If any criterion is unclear: request the application environment details from the customer before selecting the resin.
Flexural modulus — a measure of a material’s resistance to bending, reported in MPa. High flexural modulus = stiff (resists bending). Low flexural modulus = flexible. PC: 2,400 MPa (stiff). PP: 1,500 MPa (semi-flexible). TPE: 10–100 MPa (very flexible). The modulus determines whether a part snaps back or deforms under load. The snap-fit design guide shows how elongation and modulus interact for living snap fits.
What Three Steps Ensure the Resin Selection Is Correct Before the Mold Is Cut?
- Complete the 8-item checklist with the customer — do not guess the application environment. Ask: maximum use temperature? Chemical contact? Outdoor use? Regulatory requirements (food contact, flammability)? Annual volume and target cost? The customer’s answers determine the resin. If the customer says “I don’t know”: request an existing part sample or a field-failed part to identify the failure mode. Never select a resin without confirmed application data.
- Request 2–3 resin grades from the supplier and compare datasheets at the same test conditions. For each grade: check MFI (matches wall thickness?), HDT at 1.82 MPa (exceeds use temperature?), impact strength (survives drop test?), shrinkage (meets tolerance?). Request the guaranteed minimum values (not typical values) for design calculations. If two grades meet all criteria: choose the lower-cost grade. If no grade meets all criteria: the design needs modification (change wall thickness, add draft, relax tolerance) or a different resin family.
- Validate with T1 samples before approving the resin — datasheet values are not actual part performance. T1 samples reveal: actual shrinkage (may differ ±0.1% from datasheet), actual impact strength (gate location and weld lines reduce impact 20–40%), and surface quality (flow marks, sink, warpage). If T1 samples fail: adjust molding parameters first (packing pressure, mold temperature). If parameters cannot fix: switch to a different grade or resin family. Resin selection is not final until T1 samples pass all functional tests.
Chemical resistance chart — a matrix showing how a resin reacts to specific chemicals (solvents, oils, acids, bases) after 24–1,000 hours of contact. Ratings: A (no effect), B (minor swelling), C (moderate degradation), D (dissolves). ABS dissolves in acetone (D) but resists dilute acids (A). PC resists gasoline (A) but crazes in isopropanol (C). The resin datasheet guide covers where to find chemical resistance data on material TDS.
Resin selection checklist: 8 criteria — mechanical load, use temp, chemical exposure, UV, regulatory, geometry, tolerance, volume/cost. Most common mistake: selecting by tensile strength alone. Complete checklist with customer. Compare 2–3 grades. Validate with T1 samples.
- All 8 criteria evaluated — mechanical, thermal, chemical, UV, regulatory, geometry, tolerance, cost.
- HDT checked at correct load — 0.46 MPa unloaded; 1.82 MPa load-bearing.
- Chemical contact confirmed — solvents, oils, cleaning agents checked against resistance chart.
- MFI matched to wall thickness — thin wall (<1mm) = high MFI; thick (>3mm) = low MFI.
- T1 samples validated — actual shrinkage, impact, surface checked before final approval.
Frequently Asked Questions
What is the most common resin selection mistake in injection molding?
The most common mistake: selecting by tensile strength alone while ignoring impact, HDT or chemical resistance. Example: ABS has good tensile (40 MPa) but poor chemical resistance (dissolves in acetone, gasoline). If the part contacts gasoline: ABS fails within hours. Correct choice: nylon (PA) or PBT (fuel resistant). Another mistake: using HDT at 0.46 MPa for a loaded part — HDT at 1.82 MPa is 10–15°C lower, the part deforms. Always check HDT at the load matching the application.
How do I select between two resins that both meet the requirements?
If two resins meet all 8 criteria: choose the lower-cost grade for the target volume. At 500,000 parts/year: a $0.50/kg resin savings translates to $2,500–$5,000/year (depending on part weight). Also consider: supply chain reliability (can the supplier deliver consistently?), color matching (does the supplier offer custom color?), and processing window (wider MFI range = easier to mold). If both grades are equivalent on all factors: request samples from each supplier, run T1 trials, and compare actual part quality before final selection.
Can I change the resin after the mold is built?
Yes, but with constraints. Switching within the same family (ABS grade A to B): minimal changes (shrinkage ±0.1%, adjust cavity). Switching to a different family (ABS to PC): significant changes (shrinkage 0.5% vs. 0.7%, mold temp 60°C vs. 100°C, gate size). The mold may need: gate relocation, cooling modification, venting changes. Cost: $2,000–$10,000 minor, $10,000–$30,000 major. Prevention: complete the checklist before mold build — changing resin after is expensive and delays 2–6 weeks.
Key Takeaways
- 8 criteria: mechanical, thermal, chemical, UV, regulatory, geometry, tolerance, cost.
- Most common mistake: selecting by tensile strength alone.
- Complete checklist with customer. Compare 2–3 grades.
- Validate with T1 samples before final approval.
Send the part drawing, application environment and performance requirements — the review completes the 8-item resin selection checklist, recommends 2–3 grades with datasheet comparison, and specifies the processing window for consistent part quality.
Sources: Plastics Technology — Resin Selection Guide for Injection Molding · Plastics Technology Online — Material Selection Checklist · Society of Plastics Engineers (SPE) — Resin Selection Technical Papers · MatWeb — Resin Property Comparison Database · ThomasNet — Injection Molding Material Selection Guide
— Arthur, Tooling Engineer at aCatMould