Selecting the right plastic is fundamental to the success of any injection-molded part. This guide provides a detailed overview of the most widely used plastic injection molding materials, helping you make an informed choice for your next project.
1. Polypropylene (PP)
Key Traits: Semi-rigid, excellent chemical resistance, good impact strength, and fantastic fatigue resistance (it can be bent repeatedly without breaking). It is also very low cost.
Why It’s Common: Its balance of properties, low cost, and ease of processing make it suitable for a vast range of applications.
Typical Uses: Food containers, bottle caps, automotive interior trim, medical syringes, laboratory equipment.
Watch Out For: Poor UV resistance (can become brittle in sunlight unless stabilized) and low temperature tolerance.
2. Polyethylene (PE)
Key Traits: Waxy surface, chemically inert, excellent electrical insulator, and low moisture absorption. It comes in two main grades:
- HDPE (High-Density PE): Stiffer and stronger.
- LDPE (Low-Density PE): More flexible and tougher.
Why It’s Common: It’s the most produced plastic in the world, making it extremely cost-effective for high volume, disposable, low-strength items.
Typical Use: HDPE: milk jugs, shampoo bottles, cutting boards. LDPE: Squeeze bottles, plastic bags, lids.
Watch Out For: Low strength and stiffness compared to engineering plastics; can be prone to stress cracking.
3. ABS (Acrylonitrile Butadiene Styrene)
Key Traits: Excellent impact resistance, good rigidity, and superior surface finish for painting and plating. It offers a great balance of mechanical properties.
Why It’s Common: It’s easy to mold, readily available, and delivers a high-quality, durable feel at a reasonable cost.
Typical Uses: LEGO bricks, automotive dashboards and trim, electronic enclosures (computer keyboards, power tool housings), consumer appliances.
Watch Out For: Poor resistance to sunlight (UV) and weathering; turns yellow and brittle if not protected. Low chemical resistance to solvents.
4. Polyethylene Terephthalate (PET)
Key Traits: Brilliant clarity, high gas barrier (keeps carbonation in), strong, and hard. It is also highly recyclable.
Why It’s Common: It is the ideal material for transparent, rigid packaging that needs to contain pressure and resist chemicals.
Typical Uses: Water and soda bottles, food containers, synthetic fibers (polyester).
Watch Out For: Absorbs moisture from the air and must be thoroughly dried before molding. The common “preform” for bottles is injection molded before being blow-molded into its final shape.
5. Polycarbonate (PC)
Key Traits: Exceptional impact strength and toughness, high heat resistance, and outstanding transparency.
Why It’s Common: When you need see-through clarity and extreme durability, PC is the go-to material.
Typical Uses: Bullet-resistant windows, safety glasses, face shields, electronic display covers, automotive headlamp lenses, medical devices.
Watch Out For: Prone to scratching, susceptible to UV degradation (often requires a protective coating), and can be more expensive. It must be dried meticulously.
6. Polyamide (Nylon)
Key Traits: High mechanical strength, excellent wear and abrasion resistance, good temperature resistance, and low friction.
Why It’s Common: Its combination of strength, toughness, and “lubricity” makes it perfect for moving parts and mechanical components.
Typical Uses: Gears, bearings, bushings, electrical connectors, zip ties, sports equipment.
Watch Out For: Hygroscopic—it absorbs water from the air, which causes it to swell and lose dimensional stability. Parts must be conditioned or designed to account for this.
7. Acetal (POM)
Key Traits: High stiffness, excellent dimensional stability, very low friction, and high fatigue endurance. It has a slippery, smooth feel.
Why It’s Common: It’s the ideal material for high-precision, rigid parts that require minimal friction and long-term performance under load.
Typical Uses: Precision gears, bearings, fasteners, lock systems, aerosol valves.
Watch Out For: Poor resistance to strong acids and UV light. It is difficult to bond due to its low surface energy.
8. Polymethyl Methacrylate (PMMA / Acrylic)
Key Traits: Crystal-clear transparency (better than PC or PS), excellent UV resistance, and high gloss. It is stiff but has low impact strength.
Why It’s Common: When optical clarity and weatherability are the top priorities, PMMA is the best choice.
Typical Uses: Car light covers (tail lights), lenses, aquariums, windows, signage, display shelves.
Watch Out For: Brittle and prone to cracking under impact.
9. Polyvinyl Chloride (PVC)
Key Traits: Inherently flame retardant, good chemical resistance, and high rigidity. It can be made flexible by adding plasticizers.
Why It’s Common: Its low cost, flame resistance, and ability to be formulated as either rigid or flexible give it a wide range of uses.
Typical Uses: Rigid: Pipes, window frames. Flexible: Medical tubing, wire insulation, shower curtains.
Watch Out For: Can degrade at high processing temperatures, releasing hydrochloric acid. Stabilizers must be used.
10. TPE/TPU (Thermoplastic Elastomers / Urethanes)
Key Traits: Flexible and rubber-like, with good tear strength and elasticity. They can be overmolded onto rigid plastics like ABS or PP to create soft-touch grips.
Why It’s Common: They bridge the gap between rigid plastics and thermoset rubbers, offering flexibility with the fast processing of thermoplastics.
Typical Uses: Tool handles, seals, gaskets, watch bands, phone cases, automotive weatherstripping.
Watch Out For: Generally lower structural strength and can be more expensive than rigid plastics. Can have a sticky surface feel.