A professional injection molding supplier may process more than 30 common thermoplastic families, from PP, PE, ABS, PC, PA, POM, PBT, and PMMA to PPS, PEI, LCP, and PEEK. Material choice depends on service temperature, chemical exposure, dimensional tolerance, impact strength, moisture absorption, flame rating, and annual production volume. A 30% glass-filled PA66, for example, behaves very differently from unfilled PA66 in shrinkage, stiffness, mold wear, and fiber orientation. PEEK may require melt temperatures near 350–400°C, while PP is often processed below 260°C. Supplier capability therefore depends on equipment, drying control, tooling, material traceability, and process repeatability.
Commodity plastics remain common because they keep part cost low while covering a large share of everyday molded products. Polypropylene has a density of about 0.90–0.91 g/cm³, lower than ABS at roughly 1.04–1.07 g/cm³, so switching an equal-volume part from ABS to PP can reduce part mass by more than 10%. PP also has very low moisture absorption, commonly below 0.1%, which reduces the drying demands seen with hygroscopic engineering plastics.
That low density works well for closures, containers, appliance parts, laboratory products, automotive trim, and living hinges. Homopolymer PP generally provides higher stiffness, while impact copolymer grades trade part of that stiffness for better impact resistance. Mold shrinkage can often fall around 1–2.5%, depending on grade, wall thickness, packing, gate design, and fiber content, so a mold built around one PP grade should not automatically be used for another without dimensional review.
Polyethylene follows a similar cost-focused role but behaves differently during molding. HDPE commonly has a density near 0.94–0.97 g/cm³ and strong resistance to water, many acids, and many bases. Its relatively high shrinkage, frequently around 1.5–3%, matters more than its easy flow when a part contains snap features, mating surfaces, or long flat walls.
ABS becomes more suitable when surface quality, rigidity, and impact performance matter more than minimum density. Many commercial ABS grades are molded at melt temperatures around 200–260°C, and molded shrinkage is often close to 0.4–0.9%. That lower shrinkage makes dimensional control easier than with many polyolefins, although exact values still depend on grade and mold conditions.
ABS is widely used for electronic housings, appliance panels, interior automotive parts, equipment covers, and consumer products because mold texture reproduces well and secondary finishing is practical. Painting, printing, plating, laser marking, and assembly can all be added after molding. Outdoor exposure needs more care because standard ABS can lose color and mechanical performance under long-term UV exposure, so UV-stabilized ABS, ASA, or another weather-resistant material may be better.
Polycarbonate moves the material range toward higher impact strength and heat resistance. Typical PC density is about 1.2 g/cm³, while water absorption after prolonged exposure is higher than PP or PE. Resin manufacturers commonly require drying before molding, often around 110–130°C for several hours, because absorbed moisture can break polymer chains during high-temperature processing.
Moisture control can change the finished part even when the mold, machine, cycle time, and operator remain the same. A resin lot above the supplier's recommended moisture limit may show splay, reduced impact strength, bubbles, or molecular degradation.
PC is frequently used for transparent covers, machine guards, lenses, light housings, electrical components, and structural enclosures. Melt temperatures commonly sit around 260–320°C, depending on grade. PC/ABS blends lower some processing demands while providing a balance of heat resistance, impact performance, flow, and surface quality; flame-retardant grades are also common in electrical housings produced after 2000 as electronic devices became thinner and more thermally concentrated.
Nylon introduces a different manufacturing concern because PA6 and PA66 absorb moisture from air. Depending on grade and environmental exposure, equilibrium moisture content can reach several percent by weight. Dimensions and mechanical properties can therefore change after molding, especially where tolerances are tighter than ±0.1 mm.
Drying also affects processing. PA materials are normally dried before molding according to the resin manufacturer's specification, and uncontrolled exposure after drying can allow moisture to return. A supplier producing thousands of parts per shift needs closed material handling, suitable dryers, hopper control, and documented drying conditions rather than relying only on machine settings.
Glass fiber changes nylon again. A PA66-GF30 compound contains about 30% glass fiber by weight, increasing stiffness, reducing some shrinkage, and improving dimensional stability compared with unfilled material. The same glass fibers make the melt more abrasive and create directional properties because fibers tend to align with material flow.
That alignment influences gate location, weld lines, flatness, and strength. A long molded bracket may shrink differently along the flow direction than across it. Mold-flow analysis can help before steel cutting, while experienced Professional mold design and manufacturing work should account for gate size, venting, cooling balance, steel wear, draft, and the exact reinforced grade rather than only the resin family.
Acetal, or POM, is often chosen when friction and dimensional stability matter more than high impact resistance. Its density is usually around 1.40–1.43 g/cm³, considerably higher than PP, but it offers low friction, good wear resistance, high stiffness, and low moisture absorption compared with nylon.
POM works well for gears, rollers, sliding guides, clips, buckles, valves, and moving mechanical components. Homopolymer and copolymer POM grades have different thermal and chemical behavior. Processing temperature control matters because prolonged overheating can degrade the polymer, so residence time and barrel temperature should remain inside the resin producer's recommended window.
PBT occupies another large part of the engineering-plastic market. It is commonly found in automotive connectors, sensor housings, relays, switches, electrical components, and appliance parts. Glass-filled grades of 15%, 20%, 30%, or higher are widely available, allowing engineers to adjust stiffness and dimensional behavior without changing polymer family.
| Material | Approx. Density | Typical Processing Concern | Common Use |
|---|---|---|---|
| PP | 0.90–0.91 g/cm³ | Shrinkage, warpage | Closures, housings, hinges |
| ABS | 1.04–1.07 g/cm³ | UV exposure, cosmetic control | Enclosures, trim, panels |
| PC | ~1.20 g/cm³ | Moisture, high melt temperature | Guards, lenses, housings |
| PA66-GF30 | ~1.3–1.4 g/cm³ | Drying, fiber orientation, wear | Brackets, structural parts |
| POM | 1.40–1.43 g/cm³ | Thermal degradation | Gears, sliders, precision parts |
| PEEK | ~1.30 g/cm³ | 350–400°C melt range | High-temperature components |
PMMA is more suitable when optical clarity and outdoor weather resistance matter more than impact strength. Light transmission for clear commercial acrylic grades can exceed 90% in favorable thicknesses, which explains its use in light guides, lenses, displays, illuminated controls, and exterior lighting components.
Clear molding puts more pressure on mold polish, material cleanliness, gate position, weld-line control, and handling after molding. A black housing may tolerate a minor flow mark that would make a clear lens unacceptable. Scrap percentage can therefore rise sharply when optical inspection standards are strict, even when dimensional inspection shows 100% conformity.
Flexible materials add another layer. TPE and TPU can be molded alone or overmolded onto rigid substrates for grips, seals, wheels, buttons, protective edges, cable parts, and soft-touch products. Shore hardness can range widely, including grades below Shore A 50 and grades approaching rigid-plastic behavior.
Overmolding success depends on substrate compatibility rather than softness alone. A TPE that bonds strongly to PP may not bond equally well to PC, ABS, PA, or PBT. Where chemical adhesion is weak, designers may use holes, undercuts, ribs, or other mechanical retention. Overmolding trials often compare several material grades because a change of only 10–20 Shore A points can noticeably alter filling, ejection, grip feel, and deformation.
High-temperature applications narrow the resin list. PPS, PEI, LCP, and PEEK require higher material cost, higher processing temperatures, and closer control than commodity plastics. PPS is widely used with 30–40% glass or mineral reinforcement in automotive, electrical, pump, and fluid-handling components because it maintains useful properties at temperatures where ABS or standard nylon would no longer be appropriate.
PEEK sits further up the temperature range. Many grades are processed with melt temperatures around 350–400°C, and mold temperatures may exceed 150°C depending on crystallinity and application requirements. Equipment that molds PP every day is not automatically suitable for PEEK; heaters, barrel condition, screw design, temperature control, tooling, and operator experience all matter.
LCP is different again because it can fill very thin sections. It has been used extensively in electrical connectors and small electronic components since the 1990s, especially where thin walls and dense pin layouts make conventional engineering plastics difficult to fill. Its strong flow orientation requires careful attention to weld strength and direction-dependent mechanical properties.
Material selection also includes additives rather than only polymer names. UV stabilizers, flame retardants, impact modifiers, antistatic agents, lubricants, pigments, mineral fillers, carbon fibers, glass beads, and conductive compounds can change processing and part performance. A resin described only as “PA66” provides far less production information than a full commercial grade designation.
Flame performance provides a good example. UL 94 classifications such as HB, V-2, V-1, and V-0 are tied to tested material grades and specimen thicknesses. A resin rated V-0 at 1.5 mm should not automatically be assumed to have the same classification at every wall thickness or in every color formulation.
Recycled content needs similar control. A part containing 30% post-consumer recycled resin may show different color, odor, flow, contamination level, and mechanical consistency from a 100% virgin grade. PCR can work well for many housings and noncritical components, but specifications should define acceptable recycled percentage, resin source, color range, mechanical requirements, and lot-control method before mass production.
Supplier capability therefore has to be checked against the exact commercial resin grade. Ask whether the factory has molded the specified material, what dryer type is used, how moisture is measured, whether abrasive compounds require hardened screws or mold inserts, and how resin lots are recorded. For a program producing 500,000 parts per year, a 1% scrap difference equals 5,000 parts, making material control commercially significant long before a visible failure reaches the customer.
A useful supplier should also review wall thickness, gate location, cooling, expected shrinkage, tolerance, chemical exposure, operating temperature, UV exposure, and annual volume before the mold design is released. Material choice made before tooling can prevent later changes to gates, cooling lines, cavity dimensions, surface finish, and molding-machine requirements.