How to Choose a Plastic Material for Injection Molded Parts

A part owner picked polypropylene for an underhood bracket rated at a 120 °C peak. The first 50,000 parts passed bench checks at 23 °C. On the vehicle, the bracket sagged during an 80 °C soak and lost its clamp load. The rescue required a new resin, a re-cut cavity, and four lost weeks of launch. The datasheet looked fine; the service condition was not on it.

That bracket failed on creep, not on temperature. The resin met the 120 °C peak rating. It could not hold a load at 80 °C for 10,000 hours. Sustained stress at temperature bends, sags, and relaxes any plastic. Cheap resin choices die on this test.

This guide is for part owners and buyers who must shortlist a resin before the mold is quoted. It starts with the requirements, not the datasheet. It walks the first fork, amorphous versus semicrystalline. Then it maps application families, heat, load, regulation, cost, and mold impact. Each resin has its own deep guide on this site; this page is the selection layer above them.

The Snapshot

• Write down service temperature, load, chemicals, flammability, cosmetics, cost target, and volume before opening a datasheet.
• The first fork is amorphous versus semicrystalline: shrinkage of 0.4-0.7% versus 0.8-2.5% typical by grade.
• Shortlist by application family: ABS, PC, PC/ABS, and PP housings; PBT, LCP, PPA, PPS, and PA66 connectors; PSU, PPSU, PEI, and PP medical; PMMA and PC optical; TPE, TPU, and LSR soft touch.
• HDT is a short-term test number; continuous service temperature and creep decide survival.
• Pick the material first, then set cavity compensation and mold steel; glass fill changes both.

Start with Requirements, Not Datasheets

Every bad resin choice traces back to a missing requirement, not a bad polymer. Service temperature is where buyers slip first. Separate the continuous maximum from the peak. A part that sees 100 °C for 1 hour a day is different from one that runs at 90 °C around the clock.

Load is the second line. Static load asks for creep resistance at temperature. Dynamic load asks for fatigue life, which most datasheets do not show. Impact at cold start matters for outdoor and automotive parts. A bracket load case belongs on the print before any polymer does.

Chemical contact decides whole resin families. Cleaners, fuels, oils, refrigerants, and disinfectants attack some polymers fast. Amorphous resins crack under stress and solvent together; semicrystalline grades resist more. Confirm every fluid the part will touch, including wipes used at the point of care.

Flammability, cosmetics, and cost come next. Flammability class and the wall thickness it applies to are usually specified together. Gloss, texture, color, paint adhesion, and soft-touch feel each remove candidate resins. Cost target and annual volume set the ceiling on resin price per kilogram. A $0.10 per kilogram spread on a 100 g part across 250,000 parts is $2,500.

Write the list before you open a datasheet. Every fork in this guide maps back to that list. A requirement you cannot name yet will surface later as a field failure.

The First Fork: Amorphous or Semicrystalline

Thermoplastics split into two families at the molecular level. Amorphous resins, like ABS, PC, PMMA, PSU, and PEI, have no crystalline melting point. Semicrystalline resins, like PP, PA, POM, PBT, PEEK, PPS, and LCP, do. The family drives shrinkage, chemical resistance, clarity, wear, and how the mold must be built.

Shrinkage is the loudest difference. Amorphous resins shrink 0.4-0.7% typical, evenly in all directions. Semicrystalline resins shrink 0.8-2.5% typical by grade, and the rate differs with flow direction. Cavity compensation lives or dies on which family you pick.

Chemical resistance follows the same fork. Semicrystalline grades resist fuels, oils, and solvents far better in most cases. Amorphous grades crack under the combination of stress and a bad cleaner. If the part lives in an engine bay or a chemical plant, that alone decides the family.

Clarity also splits cleanly. Amorphous resins give true optical transparency in PC, PMMA, and PSU. Semicrystalline resins look milky because crystals scatter light. Only thin-wall PP and clarified grades approach clear, and they stay hazy next to PC.

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Table: Typical published values for injection-molding grades. Confirm shrinkage and heat figures on the grade datasheet before cutting steel.

Read the fork as a filter, not a verdict. Medical sterilization without high heat, optical clarity, and tight flatness push you amorphous. Sustained heat, chemicals, wear, and bearing duty push you semicrystalline. Both families survive into every shortlist that follows; the shortlist picks within the family.

Shortlist by Application Family

Once the family fits, application shortlists narrow the field fast. These racks are a starting point, not a certification. Geometry, wall thickness, and part cost move you within each row. Deeper property detail for every resin listed here sits in its dedicated guide on this site.

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Table: Typical published values for injection-molding grades. HDT and shrinkage vary by grade, fill level, wall thickness, and supplier. Resin cost is a typical market index where PP = 1x and varies by grade, volume, and region. Elastomers are rated by continuous service, not HDT.

Read the row, then step down. Pick one to two candidates from the row, not five. Two candidates share the mold discussion; five freeze the project. Connect the row to the cost column early, because an index gap of 4-10x survives no budget meeting.

Connector and medical rows deserve special notes. Connector resins are almost always glass filled for pin retention and reflow solder heat. Medical rows split into disposable PP and reusable PSU and PEI tiers. Picking PEEK for a PP job wastes the budget; picking PP for a PEEK job scraps a launch.

Heat, Load, and Chemical Reality Checks

HDT is the most misused number in material selection. HDT at 1.82 MPa measures deflection under load in a short test. It is a ranking tool, not a service limit. A PC/ABS at 90-115 °C HDT can still serve years at lower continuous temperatures.

Continuous service temperature and UL RTI ratings describe the long game. Creep, not yield, ends most loaded plastic parts. A bracket that passes a room-temperature pull test fails at 80 °C under sustained load. Specify the load, the temperature, and the hours; then check creep curves for the candidate grade.

Glass reinforcement is the cheapest way to buy heat and stiffness. A 30% glass PA66 moves HDT from the 80-110 °C class toward 250 °C. PBT and PPS compounds follow the same pattern by grade. Glass adds anisotropic shrinkage, weld-line weakness, and mold wear, so it never comes free.

Chemicals come next, and hydrolysis is the silent filter. PC, PA, and PBT degrade in hot, humid service by grade, and wet pellets hydrolyze in the barrel during molding. Semicrystalline PP, POM, PPS, and PEEK shrug off most fluids by grade. Amorphous parts under load and solvent crack in a way that looks like a part defect, not a resin defect. UV exposure, salt fog, and flame retardant migration each deserve a line in the requirement list.

Regulatory Gates and the Real Cost

Regulation filters candidates before cost does. UL 94 V-0 at 1.5-3.2 mm is the common flammability target for housings and electrical parts. V-0 stops a vertical flame within 10 seconds; 5VA is the stricter class for enclosures. FR grades cost more, and some flame retardants need hotter molds or attack steel over long runs.

Food and medical contact add layers. FDA 21 CFR lists acceptable resins and additives for food contact under specific sections. ISO 10993 covers biocompatibility for medical devices, and RoHS and REACH restrict substances for EU sales. The rules apply to the final assembly, so adhesives, inserts, and inks are in scope too. Our medical injection molding guide walks sterilization and documentation in one place.

Now cost. The resin index tells you the tier, not the part cost. PP sits at 1x, ABS at 1.2-1.5x, and PC near 2x. PA66 and PBT run 2-3x, PPS runs 4-6x, LCP runs 6-10x, and PEEK clears 20x. These are typical market indices and move with grade, volume, and region.

Processing cost often dwarfs the resin spread. Hygroscopic resins like PC, PA, PBT, TPU, and PSU need dehumidifying dryers before every run. Wet pellets hydrolyze in the barrel and scrap the first shift of parts. Semicrystalline resins need tighter mold temperature control to hold dimensions. Add drying time, energy, scrap, and reject rate to the resin price before you compare quotes.

Volume decides how much resin economics matter. A $1 per kilogram premium on a 50 g part across 100,000 parts is only $5,000. The same premium across 10 million parts is $500,000. Run the annual volume against the index gap before a premium resin wins on datasheet alone.

Shrinkage, Glass Fill, and the Mold

Material choice comes first; cavity compensation comes second. A cavity cut for 0.4% shrinkage cannot serve a 1.8% semicrystalline resin. Pick the resin, lock the grade, then compensate the steel. Our 5 shrinkage compensation rules walk the cavity math for rates from 0.3% to 2.0%.

Semicrystalline resins keep moving after ejection. Parts pulled hot from the mold relax and shift for days as crystallinity completes. Measure at 23 °C after full conditioning, and compare against the grade shrinkage value. Amorphous parts settle fast; glass-filled semicrystalline parts move most along flow direction.

Glass fill changes everything downstream. At 30% glass, stiffness and HDT climb, shrinkage drops to roughly half, and wear on steel jumps. Anisotropic shrinkage appears: the part shrinks less along flow than across it. Weld lines weaken because glass fibers do not bridge a knit. Mold steel for glass-filled compounds runs hardened, with DieStrike cavity steel at HRC 62.

Additives shift the same numbers. Flame retardants change flow; impact modifiers soften modulus; UV stabilizers protect color. Each compound carries its own shrinkage and flow, so compensate per grade, not per family. Tolerances at ±0.005 mm standard and ±0.002 mm precision only mean something when the shrinkage number is right.

Grade Lock, Second Source, and Supply Reality

A family name is not a purchasable grade. PA66 alone spans 20 to 80 percent glass, heat-stabilized and non-stabilized versions, and a price spread wider than the family itself. The datasheet that matters is the one pinned to a specific grade code, because shrinkage, flow, and drying behavior are locked at that code. Lock the grade before the cavity is compensated.

Branded and generic grades of the same family are not interchangeable on an open mold. Two PA66 grades can differ in shrinkage by 0.1 to 0.3 percent, which on a 100 mm dimension is 0.1 to 0.3 mm of drift against a mold cut for one number. Flame-retardant grades move more again, because the additive package changes flow and packing behavior. If a second supplier must qualify, run a trial with both grades on the same tool and record the dimensional spread before production.

Drying requirements follow the grade into the plant. PC, PBT, and PA must run dry to 0.02 to 0.2 percent moisture depending on family, and a shop without the right dryer temperature for that exact grade will show splay within the first shift. The mold-side version of this question is simpler: which mold temperature class does the grade need, and does the tool have the circuits to deliver it? A PPS or LCP grade that wants a 120 to 180 °C mold changes the heating system, the steel, and the quote.

Supply risk is a design input, not a purchasing footnote. Specialty grades such as LCP, PEEK, and some PPA compounds carry longer lead times and larger minimums than commodity PP or ABS, and single-sourced grades put a production program one supply hiccup from a shutdown. Where volume justifies it, design the part to accept two approved grades from different producers, and compensate the mold to a mid-point shrinkage that keeps both inside tolerance.

Lead time is part of the cost. A grade that ships in 2 to 4 weeks versus 8 to 12 weeks decides how fast a prototype tool can run and when a production tool can qualify. On a 2 to 5 week mold program the resin lead time can exceed the tool lead time, so the material order belongs on the critical path from day one.

Lock the compensation to the approved grade, then protect the program. When a mold is cut to a mid-point shrinkage between two approved grades, the tool must prove both at trial before production starts. The trial that validates the primary grade is the moment to check the backup too, because a mold approved on one grade and run on another is a mold running on an unvalidated cavity.

The 10-Question Selection Checklist

Run this list before you email a molder. Ten answers produce one to two candidates, a shrinkage rate, and a mold temperature class.

• What is the maximum continuous service temperature, and what peak temperature for how long?
• What loads act on the part, at what temperature, for how many hours, and is impact or fatigue involved?
• Which chemicals, cleaners, fuels, or fluids touch the part, at what temperature and stress level?
• Which flammability class applies, and at what wall thickness does the certification hold?
• Is clarity, haze, light transmission, or a specific gloss level required?
• Do FDA food contact, ISO 10993, RoHS, or REACH rules apply to the final assembly?
• Is the surface painted, plated, bonded, overmolded, or wiped with solvents in service?
• What is the part cost target and annual volume, and which resin index can the budget survive?
• Which dimensions are critical, and how much shrinkage directionality can the print tolerate?
• Does the geometry accept the shortlist's draft, wall, and flow requirements, and where may weld lines sit?

Conflicting answers resolve toward the part, not the datasheet. If two candidates survive, quote the mold for the tougher one and validate both at trial. Send the geometry early, because our how to design for moldability rules prune candidates before steel is ordered.

FAQ: How to Choose a Plastic Material

Q1. Why does my part pass lab tests and fail in service?

Lab tests usually run at 23 °C with no sustained load. Service adds temperature, time, chemicals, and UV together. Compare the datasheet property against the worst real combination, not the best single number. The underhood bracket that started this guide is the standard example.

Q2. How do I decide amorphous or semicrystalline quickly?

Ask three questions. Does the part need clarity, flatness, or repeated sterilization without high heat? Go amorphous. Does it face fuels, oils, sustained heat, or bearing wear? Go semicrystalline. Shrinkage of 0.4-0.7% versus 0.8-2.5% is the mold-side cost of that choice.

Q3. Is a glass-filled grade always the upgrade?

No. Glass buys stiffness, HDT, and dimensional stability. It sells back impact strength, weld-line strength, transparency, and easy mold wear. Use glass for structure and heat; stay unfilled for thin walls, optics, and ductile impact.

Q4. What should I budget for resin?

Use the index as a bracket. PP is 1x, ABS is 1.2-1.5x, and PC is near 2x. PA66 and PBT run 2-3x, PPS runs 4-6x, LCP runs 6-10x, and PEEK clears 20x. These are typical market indices and shift with grade, volume, and region, so add drying, scrap, and cycle cost before comparing.

Q5. When do I bring the mold maker into material selection?

Before the cavity is cut, and ideally before the tool is quoted. Shrinkage sets the cavity math, mold temperature sets the heating system, and glass content sets the steel grade. A molder that asks for the service profile instead of a part name is doing this guide's job.

The Bottom Line

Material selection for an injection molded part is a filter, run in order. Requirements first, family second, application shortlist third. Heat, load, chemicals, regulation, and cost then thin the list to one or two candidates.

Lock the grade before the mold quote. Set cavity compensation from that grade's shrinkage, harden the steel for glass fill, and design the tool around mold temperature and flow. Every downstream failure in this guide traces back to a requirement that never made the list.

DieStrike builds production molds under IATF 16949 across 120+ machines, with molds that ship in 2-5 weeks. Standard mold components arrive in 3-7 days, and geometry holds ±0.005 mm standard with ±0.002 mm on precision features. Customers including TE Connectivity, Amphenol, and Luxshare run connector and electronics programs on the same discipline. Send the part, the service profile, and the volume; the material discussion starts there.