Injection Molding Materials: ABS, PP, PC, PA, POM, TPE

Every injection-molded part carries two decisions inside its resin name: one about the part and one about the molding floor. The resin sets how the part behaves in service, its strength, its heat limit, its chemical tolerance, its feel, and it sets how the part behaves in the mold, its shrink, its drying demand, its melt and mold temperatures, and how hard it is on the tool. This page compares the six resins that cover most molded work, ABS, polypropylene (PP), polycarbonate (PC), nylon (PA6 and PA66), acetal (POM), and thermoplastic elastomer (TPE), together with the grade variants that appear in real RFQs: PC/ABS blends, glass-filled PA6, and talc-filled PP.

The injection molding hub carries the process story and a one-paragraph resin summary. This spoke carries the selection depth: a quick-fit table, a property grid on consistent test conditions, the processing windows and drying discipline that only matter because the material is being molded, and a decision chain that runs from a specification to a resin. For the generic polymer taxonomy, the three cost-performance tiers, and the high-temperature polymers such as PEEK and PEI, see engineering plastics; this page stays on the process-specific question of which resin to mold, when, and why. Every property value here is a typical published range for the resin class. The specific grade datasheet governs, and grade-to-grade spread inside one family is real. Izod impact values appear in both J/m and kJ/m² as sourced, and the two units are not directly comparable down a column.

The resin choice reaches the molding floor

Material price is the number buyers see, but the resin drives part cost through at least five other channels. The first is drying. PP absorbs essentially no moisture and molds straight from the bag, while PC and the nylons require dryer time and dew-point control before the first shot, which adds equipment, energy, and schedule. The second is melt temperature: PC processes at 260 to 320°C against PP at 220 to 280°C, and hotter melt means more energy and less residence-time tolerance. The third is cycle time, which cooling dominates and which the resin’s thermal behavior feeds directly. The fourth is shrink. The mold steel is cut oversize to compensate for shrink, so a resin that shrinks 0.5 to 0.8% predictably, like PC, puts the accuracy in the steel, while a resin that shrinks 1.5 to 3.0% with direction, like PP, shifts the hard work onto warpage control and process discipline. The fifth is tool wear: glass fiber in filled nylon is abrasive to runners, gates, and cavity steel, so a stiffness decision made at the CAD screen becomes a maintenance line in the tool room. A low resin price can coexist with a high part cost, and a pricier resin can be the economical one once drying, melt energy, scrap, and tooling are counted. The cost tiers used on this page are relative only: PP low; ABS, PA6, and POM medium; PC, PA66, filled grades, and TPE medium-high.

PP and ABS: the volume workhorses

Two resins carry the bulk of molded volume by part count, and they split the work cleanly. PP wins on cost and chemistry, ABS on appearance.

Polypropylene

PP is the low-cost tier and the chemical-resistance resin. It resists acids, bases, oils, solvents, and fuels, which is why packaging, caps and containers, medical disposables, and automotive fluid-system parts are PP. It is also the only common thermoplastic that works as a living hinge, a thin molded web that flexes repeatedly without cracking, so a container and its lid can leave the press as one part. Typical published properties put homopolymer tensile strength at 25 to 40 MPa and heat deflection at 55 to 65°C unfilled, rising into the 130 to 145°C class for 30% glass-filled grades. Two behaviors matter more than the strength numbers. First, PP homopolymer turns brittle cold: published notched impact of 3 to 5 kJ/m² at 23°C falls below 1 kJ/m² at -30°C, so cold-service parts move to impact-copolymer grades, which publish nearer 25 to 35 kJ/m². Second, PP shrinks 1.5 to 3.0%, high and directional, so large flat parts warp and fine dimensional work fights the material. Its processing economy is part of the appeal: melt 220 to 280°C, mold 20 to 80°C, and no drying step at all.

ABS

ABS is the cosmetic workhorse: tough, dimensionally well-behaved, and finished by the mold itself, since it takes a high-gloss polish or an etched texture and plates readily. Consumer electronics housings, keyboard keys, automotive interior trim, appliance panels, and toys are ABS territory. Typical published tensile strength runs 30 to 45 MPa with notched impact of 200 to 400 J/m, and heat deflection about 80 to 100°C, grade-dependent. Shrink is low and manageable at 0.4 to 0.7%, one reason ABS holds cosmetic and fit-critical features well. The limits are environmental. ABS yellows under UV, so sustained outdoor service points to ASA instead, and its resistance to solvents and hydrocarbons is limited. It processes at melt 200 to 280°C with 220 to 260°C typical, mold 25 to 80°C, and moderate drying of 80 to 90°C for 2 to 4 hours. Published moisture limits for ABS conflict, under 0.1% against under 0.02%, so the grade datasheet sets the number. A team prototyping in the same polymer can carry the comparison over from ABS filament.

PC and PC/ABS: heat, impact, and why blends exist

Polycarbonate

PC is the impact and heat resin of the amorphous family. Typical published tensile strength is about 60 to 72 MPa, notched impact 640 to 960 J/m, and heat deflection 125 to 140°C at 1.82 MPa, which is why safety glazing, helmets, machine guards, electrical housings, and automotive lighting are PC. It is also optically clear, so lenses, light pipes, and diffusers use it. Shrink of 0.5 to 0.8%, low and nearly equal in every direction, makes PC the most dimensionally predictable resin in this roster and a natural fit for tight tolerance bands. Its two costs are processing and chemistry. Drying is mandatory, typically 110 to 130°C for 3 to 4 hours down to under 0.02% moisture, because wet PC hydrolyzes: molecular weight drops, the part embrittles, and silver streaks mark the surface. And many solvents, cleaners, and hydrocarbons stress-craze PC, so a chemically exposed housing is usually not PC.

Why PC/ABS exists

A blend is a way to buy the half of each parent you want. PC/ABS keeps much of PC heat and impact and much of ABS flow, surface finish, and cost position, with properties landing between the two rather than above them. Typical published processing runs melt 232 to 266°C, mold 54 to 82°C, and drying 82 to 93°C, with heat deflection around 100 to 125°C. The blend is also a common route to flame-retardant housings: many FR ABS and PC/ABS grades achieve a UL 94 V-0 rating at a defined thickness. That rating is thickness- and color-dependent, so the specification that matters on a drawing is the rating plus the minimum wall and the color, not the polymer name alone.

Nylons: PA6 against PA66, and what filler changes

The two workhorse nylons are close cousins with a real processing split, and both are hygroscopic, which shapes everything below.

PA6 or PA66

PA6 is the tougher, easier, cheaper mold. Typical published dry tensile strength is about 80 MPa, flexural modulus about 2.8 GPa, heat deflection 65 to 75°C at 1.8 MPa, processing melt 250 to 270°C, mold 60 to 90°C. PA66 is the stiffer, hotter-running structural resin: 80 to 90 MPa dry tensile, 2.8 to 3.8 GPa modulus, a higher melting point at 255 to 265°C against PA6 at 215 to 225°C, melt 275 to 295°C, and mold 80 to 100°C, where mold temperature is critical because it sets crystallinity. Continuous service reaches roughly 120°C for PA66 against about 105°C for PA6, which is why under-hood parts such as intake manifolds and charge-air ducts, electrical connectors, and threaded or clipped structural parts tend to PA66, while general tough mechanical parts mold more easily in PA6. Published heat-deflection numbers for nylons vary with test stress and filler; the grid on this page uses 1.82 MPa values, with filled grades reaching the 195 to 210°C class.

The moisture story

Nylon is hygroscopic, and that single word drives most nylon processing pain. PA66 takes up about 2.5 to 3.0% moisture at 65% relative humidity, growing linearly about 0.5 to 0.8% from the dry to the ambient state. PA6 sits near 2.7 to 3.5% at the same ambient humidity and approaches 9% only at full immersion saturation, so both figures are true and the conditioning state must be named when either is quoted. The consequences are practical: a nylon part’s dimensions and stiffness shift with its environment, so a fit dimensioned in the dry state may sit differently after conditioning. Molding wet is worse than using a part wet. Published PA66 guidance ties molding near 0.3% moisture, against a target near 0.08%, to a tensile loss of about 20% and a viscosity change of about 50%, and nylon weld lines retain roughly 30 to 50% of bulk strength even when dry. Hence the drying discipline: about 4 hours at 80°C for PA6, and for PA66, 4 hours at 80°C with a -30°C dew point to a residual moisture under 0.2%, ideally 0.08%.

Glass-filled PA6 and the fiber trade

Adding 30% glass fiber to PA6 changes the resin’s job. Tensile strength rises to a published 170 to 190 MPa and flexural modulus to 8.0 to 9.5 GPa, roughly triple the unfilled resin, and heat deflection reaches 195 to 210°C at 1.82 MPa. That is metal-replacement territory. The trade is directionality. Fibers align with flow, so shrink splits into about 0.25 to 0.40% along flow against 0.80 to 1.10% across it, and that ratio is what warps parts. A PA6-GF30 part is designed with gating and cooling planned around fiber orientation, and it costs more to tool and run: the fiber is abrasive to steel and the melt flows less freely. The grade is also more brittle than its unfilled parent. For example, a load-carrying bracket that needs the stiffness but not the heat can weigh that trade against the filled-PP alternative directly below, since talc-filled PP buys flatness and stiffness at a lower tier without the glass-fiber tooling penalty.

PP-TD20: the filled alternative for flat parts

Talc-filled PP, commonly around 20% talc and written TD20, is the answer when the requirement is stiffness, flatness, and shrink control rather than strength. The talc raises stiffness and dimensional stability, reduces shrink, and improves warpage behavior on large panels, at the cost of higher density. Automotive interior trim, appliance covers, and low-warpage structural panels are the classic uses. It is not a substitute for filled nylon in a load path, but for a flat, appearance-critical panel it is often the better economics.

POM: precision mechanisms on a narrow window

Acetal is the mechanism resin. Typical published tensile strength is 60 to 70 MPa with flexural modulus 2.5 to 3.2 GPa, and its friction against steel runs about 0.20 to 0.35, lower than nylon’s typical 0.30 to 0.45, which is why precision gears, bushings, rollers, wear strips, latches, and snap-fits are standard practice in POM. Its decisive property for mechanisms is moisture. POM absorbs only about 0.20 to 0.25% with negligible dimensional change, and published comparisons cite holding tolerances within about ±0.025 to 0.050mm across humidity swings, so a gear tooth molded to size stays in band through conditions that would swell a nylon tooth out of it. Shrink is high, 1.8 to 2.5% for copolymer and up to about 3% for homopolymer, but consistent, so the mold compensates predictably. The processing window is the care point. Copolymer processes at 190 to 210°C within a short window, homopolymer at 180 to 230°C, and acetal degrades at 280 to 300°C releasing formaldehyde, so residence time and melt discipline are part of molding this resin and an overheated barrel announces itself. Drying is optional and light. UV resistance is poor, so outdoor mechanisms look elsewhere.

TPE: softness as a specified property

TPE is not one polymer but a class: styrenic block compounds, olefin blends, vulcanized rubber-thermoplastic blends, polyurethanes, and copolyesters, all materials that process like thermoplastics and flex like rubber. TPE parts are specified by hardness rather than stiffness. Stock styrenic series span Shore A 30 to 85 with the wider family reaching about Shore D 80, and published tensile strength across series runs 3 to 14 MPa. Processing is mild: melt 180 to 230°C, mold 20 to 50°C, shrink 1.0 to 2.0%, and little or no drying except the TPU class, which wants 100 to 110°C for 2 to 4 hours. Service temperature is class-dependent, so the class is chosen with the heat requirement, not just the hardness: styrenic compounds degrade above about 80°C, while TPU and copolyester classes hold to about 150°C. Typical uses are soft-touch grips, seals and gaskets, medical-device components such as syringe seals, and cable strain reliefs. Where TPE earns its keep is overmolding, and adhesion there is substrate-specific: good to excellent on ABS and PC, good on nylon with a nylon-bonding series, workable on PP only with olefin-based bondable grades, and poor on POM, where the joint is made mechanically instead. The tooling and bond depth of that subject live on overmolding and insert molding, and the printed flexible cousin is TPU filament.

Melt, mold, and the drying discipline

Two temperatures define each resin’s processing window, and the grid below carries both. Melt temperature is the barrel: hot enough for flow, not so hot that the resin degrades. Mold temperature is the cavity surface, and it trades directly against cycle time, since cooling dominates the cycle. Mold temperature also does quiet structural work in the semi-crystalline resins. PA66 at 80 to 100°C develops the crystallinity its published properties assume, and a cold mold in PP or POM leaves a dull surface and under-developed properties. A mold run too cold freezes a skin over a molten core and builds in stress; run too hot, the cycle pays for it.

Drying is where the molding resins sort into three groups. PC and the nylons are hygroscopic and drying is mandatory: PC at 110 to 130°C for 3 to 4 hours to under 0.02%, PA6 at 80°C for about 4 hours, PA66 to a residual under 0.2% and ideally 0.08%. ABS and PC/ABS are moderately hygroscopic, drying at 80 to 90°C and 82 to 93°C respectively for 2 to 4 hours, with the moisture limit set by the grade datasheet. PP, POM, and most TPE classes absorb so little water that drying is unnecessary or a light option. Skip drying on a resin that needs it and the evidence arrives fast. Moisture above roughly 0.2% commonly shows as splay or silver streaks on the surface, PC hydrolyzes into a brittle, streaked part, and wet nylon loses tensile strength and shifts viscosity on the order of the 20% and 50% figures above. Drying is the cheapest defect prevention in the plant.

The architecture question: amorphous or semi-crystalline

Under the trade names, molding resins come in two architectures, and the architecture predicts behavior better than any single datasheet number. Amorphous resins such as ABS, PC, and PC/ABS soften gradually over a range rather than at a point, shrink little and nearly equally in all directions at 0.2 to 0.8%, and can be optically clear. Their weakness is chemistry: the same disordered structure that gives predictable shrink gives modest resistance to solvents and stress-crazing agents. Semi-crystalline resins such as PP, the nylons, and POM melt sharply, shrink more at 1.0 to 3.5% and with direction, and resist chemicals, fatigue, and wear far better, which is where the living hinge, the gear tooth, and the snap-fit live. The architecture usually settles a tie. A cosmetic, tight-tolerance, indoor part points amorphous; a chemically exposed, flexing, or sliding part points semi-crystalline. Shrink appears in this page’s grid as one column on purpose: the grade-level shrink tables, the tolerance grade bands, and the anisotropy data live on injection molding tolerances, and the per-resin wall thickness ranges live on the injection molding design rules page.

Spec first: a decision chain to a resin

Resin selection works as a chain because each link removes candidates, so run it in order and let the survivors narrow.

Temperature exposure comes first. Compare the part’s continuous and peak temperatures against heat deflection at 1.82 MPa: unfilled PP leaves the field at 55 to 65°C, ABS and POM occupy the 80 to 110°C middle, PC reaches 125 to 140°C, and filled nylon grades run 195 to 210°C. Chemical exposure comes second. Sustained contact with solvents, cleaners, or fuels rules out PC and makes ABS a cautious choice, while PP and POM carry broad chemical resistance. Load and deflection come third, and the stiffness ladder is short: PP, then ABS, then PC and POM together, then the unfilled nylons, then glass-filled nylon at roughly triple its unfilled modulus. A deflection-limited part picks its rung and moves on. Appearance comes fourth: gloss, texture, and plating point to ABS, clarity and light management to PC, soft touch to TPE. Tolerances come fifth, since shrink class plus architecture decide what the mold alone can hold, and that is the tolerances page’s subject rather than this one’s. Regulatory touchpoints come last, and only as named frameworks: UL 94 for flammability, specified as a rating together with a minimum wall and color because results are thickness- and color-dependent, and food-contact or medical frameworks such as FDA food-contact compliance, USP Class VI, and ISO 10993. Grades exist within these families under those frameworks, but compliance is grade-specific and is confirmed with the supplier datasheet, never assumed from the polymer name. Cost tier and processing overhead close the chain, because the drying, melt energy, and tool wear described above are part of the real price of the part.

Two resin decisions, worked from the grid

Worked example: a small instrument gear train runs indoors at modest load, and tooth geometry must hold through seasonal humidity. POM takes this decision on the numbers on this page. It absorbs 0.20 to 0.25% moisture against nylon’s ambient class of 2.5 to 3.5%, so tooth size stays put, and its friction against steel of 0.20 to 0.35 suits a quiet, unlubricated mesh. Its 60 to 70 MPa tensile strength and 2.5 to 3.2 GPa modulus are ample for a lightly loaded train, and its shrink, though high at 1.8 to 2.5%, is consistent enough for the mold to compensate. The molder holds melt inside the 190 to 210°C copolymer window and minds residence time, because acetal degrades toward formaldehyde when overheated. Had the same gear carried a structural load or replaced a metal part, the answer flips to PA6-GF30: 170 to 190 MPa tensile, 8.0 to 9.5 GPa modulus, and 195 to 210°C heat deflection, paid for with directional shrink of 0.25 to 0.40% along flow against 0.80 to 1.10% across it, gating planned around fiber orientation, and a tool that wears faster against the glass.

For example, a desktop electronics enclosure in ABS holds its own until the specification adds a flame rating and a warmer interior. Plain ABS gives 30 to 45 MPa tensile, 200 to 400 J/m notched impact, heat deflection about 80 to 100°C, a good gloss or texture surface, moderate drying, and a medium cost tier, which is exactly right for a consumer housing with no flame requirement. Once that enclosure sits over live power electronics and needs a UL 94 V-0 rating, PC/ABS is the standard route: many of its FR grades achieve V-0 at a defined thickness, its heat deflection of about 100 to 125°C buys margin over ABS, and it keeps much of ABS flow and finish, at drying of 82 to 93°C and a cost tier between the two parents. The drawing in either case states the rating with the minimum wall and the color, because the rating is thickness- and color-dependent. Both decisions came off the grid above, and neither needed a price to reach.

The resin decision deserves the grade datasheet, not the family name. Within ABS or PA66 alone, published property ranges spread widely enough that a grade choice can move a part across a requirement line, so treat this page’s values as orientation, confirm the specific grade with the supplier, and settle shrink and drying with the molder before steel is cut. For lookups beyond the molding roster, the material properties database covers the wider material set, and the materials hub frames how these families compare across processes.