What are the best plastic materials for injection molding? The short answer is that the right material depends on the part’s job. Strength, heat, chemicals, regulations, and budget all shape the choice. This guide compares common resins, their properties, and where each one fits.
We have molded precision plastic parts in Riverside, California, since 1984. Our customers are in medical, aerospace, defense, automotive, electronics, and industrial markets across the U.S. Material selection is one of the first decisions we work through with them.
No single resin works for every part. The best choice balances performance, processing, and cost for one specific application. A resin that suits a consumer housing may fail inside an aircraft cabin or a surgical tool.
No. When customers ask us what are the best plastic materials for injection molding, we first ask what the part must do. Then we look at where it will operate and which rules it must meet. The answers narrow the field quickly.
For example, a gear needs low friction and wear resistance. A lens needs clarity. A medical component may need to survive repeated sterilization.
Most molding resins fall into four groups:
Our plastic injection molding work covers all four groups. Each group processes differently, which affects tooling and cycle time.
The table below compares widely used resins. It covers key properties, common trade-offs, and typical applications.
| Material | Key Properties | Trade-Offs | Common Applications |
| Polypropylene (PP) | Chemical resistant, flexible, low cost | Hard to print or bond without surface treatment | Living hinges, containers, automotive trim |
| ABS | Tough, good surface finish, easy to mold | Limited UV and chemical resistance | Housings, enclosures, consumer parts |
| Polycarbonate (PC) | High impact strength, clear, heat resistant | Can stress crack with some chemicals | Lenses, covers, medical housings |
| Nylon (PA6, PA66) | Strong, tough, wear resistant | Absorbs moisture, which shifts dimensions | Gears, bushings, brackets |
| Acetal (POM) | Low friction, stiff, dimensionally stable | Hard to bond; poor UV resistance | Precision gears, clips, valve parts |
| PBT | Good electrical properties, low moisture uptake | Can warp, especially glass-filled grades | Connectors, sensor housings |
| PEEK | High heat, chemical resistance, strength | High cost; needs high mold temperatures | Aerospace, medical, high-heat parts |
| PEI | High heat, inherent flame resistance | Higher cost; sensitive to sharp notches | Aircraft interiors, electrical parts |
| TPE / TPU | Flexible, rubber-like, soft touch | Lower heat resistance | Grips, seals, overmolds |
Polypropylene is one of the most widely molded resins. It resists chemicals, costs little, and flexes without breaking. That makes it a good fit for living hinges and containers. Polyethylene and polystyrene also serve low-cost, high-volume parts.
ABS offers toughness and a clean surface finish at moderate cost. Polycarbonate adds impact strength, clarity, and higher heat resistance. Many enclosures use a PC/ABS blend to balance both.
Nylon handles loads and wear well, but it absorbs moisture. Acetal holds tight dimensions and runs with low friction, so it suits precision gears. PBT works well for electrical connectors because it absorbs little moisture.
Some parts face continuous high heat, harsh chemicals, or repeated sterilization. PEEK keeps its strength at high temperatures and resists most chemicals. PEI offers high heat resistance and inherent flame resistance. PPSU handles repeated steam sterilization, which suits reusable medical devices.
These resins cost far more per pound. They also need hotter barrels and molds. Many of these medical programs run through our cleanroom medical injection molding operation in an ISO 7 (Class 10,000) cleanroom.
TPE and TPU give parts a rubber-like feel. They work for grips, seals, and soft-touch overmolds. LSR handles a wider temperature range and comes in biocompatible grades. LSR uses a related liquid injection process instead of plastic pellets.
Good material selection starts with requirements, not with a favorite resin. These questions help buyers set clear selection criteria.
Review these properties against your part’s real operating conditions:
Rank these by importance. Few resins excel at everything, so trade-offs are normal.
Glass fibers add stiffness and strength. Mineral fillers reduce warp and cost. UV stabilizers protect outdoor parts. Lubricants such as PTFE lower friction in moving parts.
Additives bring their own trade-offs. Glass fibers can cause uneven shrinkage and visible surface marks. They also wear mold steel faster.
Polypropylene, polyethylene, and ABS often give the most value for general parts. They cost less per pound and mold quickly. Over-specifying a high-performance resin adds cost without adding function. We help customers match the resin to real requirements, not worst-case guesses.
Every resin shrinks as it cools, and each one shrinks at a different rate. Toolmakers size the mold cavity to match that shrink rate. Semi-crystalline resins like nylon and acetal shrink more than amorphous resins like ABS and polycarbonate.
Tolerance stack-up adds another layer. Mating parts made from different resins can drift apart as temperatures change. Design for manufacturability (DFM) review catches these issues before tooling starts.
Applications drive material choice as much as properties do. Each industry adds its own requirements on top of basic performance.
Common medical resins include polycarbonate, polypropylene, PEEK, and PPSU. Each must survive the device’s sterilization method, whether that is steam, gamma, or ethylene oxide. Device makers often evaluate biocompatibility under ISO 10993-1. We hold ISO 13485:2016 certification and mold medical parts in an ISO 7 cleanroom.
Aerospace parts often need high heat resistance, low smoke, and flame resistance. PEEK and PEI appear often in these programs. Traceability matters as much as the resin itself. Suppliers serving this market usually hold AS9100 certification, which requires controlled records from resin lot to finished part.
Automotive programs often use polypropylene for trim and interior parts. Glass-filled nylon handles under-hood heat and loads. PBT appears in connectors and sensor housings. Cost per part matters heavily at automotive volumes.
Electronics often need flame ratings and stable electrical properties. PBT, polycarbonate, and flame-retardant nylon are common choices. Industrial parts often use acetal and nylon for gears, bushings, and wear surfaces.
We have built molds and molded parts for more than 40 years. That experience covers a wide range of resins, fillers, and industries.
Secondary operations deserve early attention. Polypropylene resists ink and adhesives without surface treatment. Amorphous resins often weld ultrasonically more easily than semi-crystalline ones. These details affect total part cost.
What are the best plastic materials for injection molding medical devices?
Common choices include polycarbonate, polypropylene, PEEK, and PPSU. The right grade depends on sterilization method, biocompatibility testing, and chemical exposure. Medical-grade resins also need documented lot traceability.
Does material choice affect tooling cost?
Yes. Glass-filled and abrasive resins wear molds faster and often need hardened steel. High-temperature materials may need special mold heating. These factors raise tooling cost and maintenance.
Can I switch materials after the mold is built?
Sometimes, but not always. Each resin shrinks differently, so a new material can shift part dimensions. Switching usually requires mold trials, and sometimes tooling changes. Test before committing to production.
What documentation should come with aerospace-grade resin?
Aerospace programs typically require material certifications, resin lot numbers, and traceability from raw material to finished part. AS9100 suppliers keep these records as part of their quality system.
Does material choice affect secondary operations?
Yes. Some plastics, like polypropylene, resist ink and adhesives without surface treatment. Amorphous resins usually weld ultrasonically more easily than semi-crystalline resins. Review secondary operations during material selection to avoid added cost.
So, what are the best plastic materials for injection molding for your project? The answer comes from your part’s function, environment, and regulatory needs. Keep these takeaways in mind:
If you are comparing resins for a new part, our team can review your requirements with you. Contact us or visit Hi-Rel Plastics & Molding to learn more.
