Plastic Resin Selection for Injection Molding: Decision Tree + Table

Choosing the wrong resin for plastic resin selection injection molding costs more than a respray or a field return. It costs tooling restarts, customer chargebacks, and program delays that average 8 to 14 weeks when a material swap forces a gate or cooling redesign. Pick the right resin before steel is cut, and you avoid all of it. This guide gives you a structured decision path and a side-by-side comparison of 12 resins to get there fast.

The Three Tiers: Commodity, Engineering, and High-Performance Resins

Every thermoplastic falls into one of three cost and capability tiers. Where your part lands in that hierarchy sets the floor on tooling design, processing window, and unit economics. Misassigning a part to the wrong tier is the single most common error we see in early-stage programs.

Commodity resins include PP, HDPE, LDPE, PS, and ABS. Raw material cost runs $0.60 to $1.40 per pound as of Q1 2025 pricing from major distributors. They process easily, tolerate wider gate and runner sizing, and need mold steel no harder than P20 for most volumes. You use them when the mechanical and thermal requirements are modest and cost is the lead driver.

Engineering resins include nylon (PA6, PA66), POM (acetal), PC, PBT, and PC/ABS blends. Raw material cost runs $1.50 to $4.50 per pound. They deliver meaningful jumps in tensile strength, heat deflection temperature (HDT), and dimensional stability. Processing is tighter. Melt temperatures often exceed 500 degrees F, and moisture content at the barrel must be controlled below 0.02% for PA66 or you get splay and degraded properties.

High-performance resins include PEEK, PPS, LCP, PSU, and PEI (Ultem). Raw material cost starts at $15 per pound and runs past $80 per pound for medical-grade PEEK. You need hardened tool steel, H13 or better, barrel temperatures above 700 degrees F, and a processing team with documented experience on the material. Structural substitution for a metal component is the common justification.

The Decision Tree: How to Choose a Plastic Material for Injection Molding

Work through these questions in order. Each branch eliminates a tier or a family. You do not need to evaluate all 12 resins if you exit the tree early.

Step 1: Define the Thermal Requirement

What is the maximum continuous service temperature the part will see? If the answer is below 180 degrees F (82 degrees C), commodity resins stay in play. If the part must survive 180 to 300 degrees F (82 to 149 degrees C) continuously, you move to engineering resins. Above 300 degrees F, you are in high-performance territory and the material list shortens to PEEK, PPS, or PEI.

HDT under load (ASTM D648 at 264 psi) is the number that matters, not the generic “service temp” marketing sheets give you. PP homopolymer HDT runs around 200 degrees F (93 degrees C). PA66 30% glass-filled hits 500 degrees F (260 degrees C). PEEK runs to 570 degrees F (298 degrees C). Use the ASTM number in your spec.

Step 2: Define the Mechanical Requirement

Pull your FEA or load case. What tensile strength, flexural modulus, and impact resistance does the part need? ABS delivers tensile strength around 6,500 psi and notched Izod impact around 3.5 ft-lb/in. PC delivers 9,500 psi tensile and 12 to 16 ft-lb/in impact. PA66 GF30 delivers 25,000 psi tensile. If your FEA says you need more than 20,000 psi, you are past commodity territory entirely.

Impact resistance is its own branch. PC and PC/ABS blends lead the commodity-adjacent group on impact, but they are notch-sensitive at weld lines. If the part geometry creates weld lines near stress risers, your real-world impact performance drops sharply from datasheet values. Note that in our project reviews, we typically see a 30 to 50% reduction in weld-line strength versus base material strength for glass-filled grades.

Step 3: Define the Chemical Exposure

If the part contacts fuels, oils, cleaning agents, or aggressive solvents, POM and PA outperform PC and ABS significantly. PC crazes in contact with many organic solvents, including common industrial degreasers. PP has broad chemical resistance but poor UV stability without stabilizer packages. Check the resin supplier’s chemical resistance guide against your specific reagents, not generic categories.

Step 4: Define the Cosmetic Requirement

Class-A visible surfaces push you toward ABS, PC/ABS, or ASA. ABS accepts paint and electroplating well. ASA handles outdoor UV exposure where ABS yellows within one season. PP can achieve good cosmetics but is notoriously difficult to paint without adhesion promoter. If the part is structural and hidden, cosmetics drop off the decision tree entirely, and you optimize for mechanical and cost.

Step 5: Set the Cost Constraint

Once you have narrowed the candidate list by thermal, mechanical, chemical, and cosmetic requirements, price out the remaining options. A 10,000-unit annual run at a part weight of 0.5 lb per shot looks like this: PP at $1.00/lb means $5,000/year in raw material. PA66 GF30 at $3.50/lb means $17,500/year. PEEK at $50/lb means $250,000/year. Those differences justify tooling redesign, wall thickness optimization, or even a metal-to-plastic conversion analysis.

Resin Comparison: ABS, PC, PP, and Nine More

The table below covers the 12 resins we process most often in our offshore tooling programs. All property values are for unfilled or base-grade material unless noted. Source data from resin supplier datasheets and CAMPUS material database.

Resin	Tier	Tensile Strength (psi)	HDT at 264 psi (degrees F)	Notched Izod (ft-lb/in)	Shrinkage (%)	Approx. Raw Material Cost ($/lb)	Typical Melt Temp (degrees F)	Key Strength	Key Limitation
PP Homopolymer	Commodity	4,500	200	0.4 to 0.8	1.5 to 2.0	$0.70	410 to 460	Chemical resistance, low cost	High shrinkage, UV degrades
PP Copolymer	Commodity	3,800	185	1.5 to 5.0	1.5 to 2.0	$0.75	410 to 460	Impact resistance vs homo	Lower stiffness
ABS	Commodity	6,500	195	3.0 to 5.0	0.4 to 0.7	$1.10	420 to 500	Cosmetics, plating	UV, solvent sensitivity
HIPS	Commodity	3,500	180	1.5 to 3.0	0.4 to 0.6	$0.85	380 to 450	Low cost, easy processing	Low strength, UV degrades
PC	Engineering	9,500	270	12 to 16	0.5 to 0.7	$1.80	540 to 600	Impact, optical clarity	Solvent crazing, notch sensitive
PC/ABS Blend	Engineering	8,000	230	10 to 14	0.4 to 0.7	$1.60	480 to 560	Balance of impact and cosmetics	Moisture sensitive
PA6 (Nylon 6)	Engineering	11,500	150	1.0 to 2.0	1.0 to 1.5	$1.90	460 to 510	Wear resistance, fatigue	Moisture absorption, dimensional shift
PA66 (Nylon 66)	Engineering	12,500	185	1.0 to 2.5	1.0 to 1.5	$2.10	500 to 540	Higher HDT vs PA6	Brittle dry, moisture sensitive
POM (Acetal)	Engineering	10,000	230	1.0 to 2.0	1.8 to 2.5	$1.70	380 to 430	Dimensional precision, low friction	Formaldehyde release if overheated
PBT	Engineering	8,500	230	0.8 to 1.5	1.5 to 2.0	$2.20	480 to 530	Electrical, chemical resistance	Low HDT unfilled
ASA	Engineering	6,000	210	2.5 to 4.5	0.4 to 0.7	$1.50	420 to 490	UV resistance, outdoor cosmetics	Lower impact vs PC
PEEK	High-Performance	14,500	570	1.0 to 1.6	1.2 to 1.6	$50 to $80	700 to 760	Thermal, chemical, mechanical peak	Cost, processing complexity

Use this resin comparison across ABS, PC, and PP as a shortlist filter. Do not use it as a substitute for datasheet review on your specific grade from your specific supplier. Color concentrates, regrind ratios, and filler packages shift every number in the table.

How Resin Choice Drives Tool Design

Resin selection is not just a material decision. It is a tooling decision. Shrinkage rate alone changes cavity dimensions across the whole tool. A part dimensioned for ABS at 0.5% shrinkage will be undersized if someone swaps it to PP at 1.8% shrinkage without a cavity rework. That is not a hypothetical. We have seen it cause full cavity replacements on programs where the material change happened after tool approval.

Gate type and location follow the resin. POM requires generous gate sizing and short land lengths to avoid shear-induced degradation. PC needs high injection pressure and well-vented tools to prevent burn marks at weld lines. PA66 GF30 is highly abrasive and accelerates gate wear on soft steel. If you are running PA66 GF30 at volume, specify H13 or 420SS at the gate insert, not P20.

Cooling design also shifts with resin. PA66 and POM crystallize during cooling, and mold temperature matters more than it does for amorphous resins like ABS or PC. Running PA66 at a mold temperature below 140 degrees F produces a part with lower crystallinity, higher dimensional variation, and weaker mechanical properties versus a tool running at 160 to 180 degrees F. Cycle time goes up with higher mold temperatures, but part quality justifies it on structural components.

Draft angle requirements tighten with glass-filled grades. Unfilled PP runs well at 1 degree of draft per side. GF30 grades benefit from 1.5 to 2 degrees on textured surfaces because the glass orientation at the skin creates higher surface friction against the tool wall during ejection. Specify this in your tool design review before the mold goes to steel.

Engineering Plastic Selection for Offshore Tooling Programs

When you are sourcing tooling offshore, engineering plastic selection requires more front-end documentation than a domestic program. Chinese mold shops are capable of processing every resin in the table above. The risk is not shop competence. The risk is ambiguity in the purchase order. If your PO says “Nylon” without specifying PA6 or PA66, unfilled or GF30, and the required predrying protocol, you will get variation between tool trials.

We run every offshore program through a material specification checklist before the purchase order goes out. It covers resin grade and supplier, filler type and loading percentage, predrying temperature and time, melt temperature range, mold temperature range, and the ASTM property values the part must meet. That document travels with the tool through sampling and production. It eliminates the most common root cause of first-shot failures on offshore tools.

According to the Plastics Industry Association’s 2023 industry report, material-related defects account for approximately 22% of injection molding quality escapes in contract manufacturing environments. Most of those defects trace back to a moisture or processing parameter issue, not a bad resin lot. Documentation and process control prevent them.

Cost vs. Performance: Where to Draw the Line

Overspecifying resin is as expensive as underspecifying it. A structural bracket that works fine in PP GF20 at $1.20/lb does not need PA66 GF30 at $3.80/lb. On 50,000 parts per year at 0.3 lb per shot, that difference is $39,000 annually in raw material alone, before you add the processing cost delta from the higher melt temperature and tighter moisture control PA66 requires.

The correct method for this decision is not experience or instinct. It is a structured thermoplastic selection guide applied against your verified load cases, thermal environment, chemical exposure, and annual volume. Run the numbers. If the cost delta is small and the performance margin is large, step up the tier. If the cost delta is large and the performance margin is small, defend the lower-cost resin with test data and hold that position through design review.

For programs where a metal-to-plastic conversion is on the table, the calculation changes. PEEK or PPS at $60/lb can be cost-justified when it replaces a machined aluminum component at $85/piece. The injection molded PEEK part may run $12/piece at volume. That is a 86% unit cost reduction that funds the tooling investment in under 1,000 cycles. Our project managers run this analysis on every metal conversion candidate we evaluate.

Frequently Asked Questions

What is the best all-around resin for a first injection molded part?

There is no single best resin, but ABS is the most forgiving starting point for visible, structural-light, ambient-temperature parts. It processes easily, holds tight cosmetics, and tolerates moderate mechanical loads. If the part sees outdoor UV or contact with oils, move to ASA or PP with a UV stabilizer package instead.

How does moisture content affect nylon processing?

PA66 must be dried to below 0.20% moisture by weight before processing, per Dupont and Ascend Performance Materials processing guides. Above that threshold, the polymer hydrolyzes at barrel temperatures, reducing molecular weight and dropping tensile strength by 15 to 30% in the molded part. Predry at 180 degrees F for 4 to 6 hours in a desiccant dryer. Do not use a hot-air oven.

Can I switch resins after the mold is already cut?

You can switch within the same shrinkage family without cavity rework, for example from ABS to PC/ABS, where shrinkage rates are both in the 0.4 to 0.7% range. Switching from an amorphous resin to a semi-crystalline one, such as from ABS to PP, changes shrinkage by a factor of three or more and requires full cavity remachining. Gate and cooling changes are almost always required as well.

When does a glass-filled grade make sense over an unfilled grade?

Add glass fill when you need to raise HDT, lower shrinkage for tighter dimensional control, or increase tensile and flexural stiffness without stepping up to the next resin tier. A 30% glass-filled PA66 delivers roughly double the tensile strength and three times the flexural modulus of unfilled PA66. The tradeoffs are higher abrasion on tooling, reduced surface cosmetics, and lower impact at the weld line.

What mold steel do I need for engineering and high-performance resins?

P20 is adequate for commodity resins and lower-volume engineering resin tools. For glass-filled engineering resins at production volumes above 500,000 shots, specify H13 at 48 to 52 HRC for cores and cavities. For PEEK and PPS, use H13 or 420SS throughout, with chrome or electroless nickel plating on the cavity surfaces. Running abrasive resins on unhardened steel compresses your tool life by 60 to 80% compared to a properly specified tool.