When you’re developing a plastic part, injection molding is one of the most widely used manufacturing processes today. However, not all plastics behave the same in the mold or the final application. The resin you select will directly affect not only how your part performs, but also your tooling costs, cycle times, surface finish, and even dimensional stability over time.
With dozens of thermoplastics available, and each with different strengths, limitations, and trade-offs, it’s easy to feel overwhelmed.
So, how do you know which plastic is best suited for your project? That’s what we’ll help you figure out.
What Makes a Plastic ‘Good’ for Injection Molding?
Not all plastics are created equal when it comes to injection molding. A “good” plastic for this process must strike the right balance between performance, processability, and cost. Here are the key factors to consider:
- Moldability: The resin should flow well into complex molds without requiring excessive pressure or heat. Plastics with good flow characteristics reduce the risk of short shots, warping, or weld lines.
- Dimensional Stability: Some parts—like gears or housings—require tight tolerances. A good injection molding plastic resists shrinkage and warping during cooling, maintaining consistent shape and size.
- Cycle Time Compatibility: Materials that cool and solidify quickly lead to faster cycle times, which directly impact production efficiency and cost. High-volume projects often favor resins with shorter cooling profiles.
- Cost-Efficiency: The price of raw material, mold wear, scrap rate, and secondary processing all factor into the total cost. A plastic that’s easy to process and meets performance needs without over-engineering is typically ideal.
Most injection molded parts today use thermoplastics, not thermosets. Thermoplastics soften when heated and can be remelted, making them easier to mold and recycle. Common thermoplastics behave differently during molding—some fill molds easily but shrink more, while others resist warping but require higher injection pressures.
Understanding these trade-offs is the first step to choosing the right resin for your part. A qualified plastic molding service can also help you evaluate options based on moldability, dimensional stability, and your specific production needs.
Common Plastics Used in Injection Molding
Choosing the right plastic is about matching your design needs with how each material behaves during and after molding. Here’s a look at some of the most trusted plastics and what they bring to the table:
1. ABS (Acrylonitrile Butadiene Styrene)
If your product needs to look good, feel solid, and keep costs down, ABS is often the first choice. It molds easily into detailed shapes and provides a clean, matte surface right out of the mold—no extra finishing required.
- Where it’s used: Housing for electronics, appliance panels, and tool casings.
- Why it works: Excellent impact resistance, dimensional stability, and a great price-to-performance balance.
- Watch out for: UV exposure and harsh chemicals—it’s not the best for outdoor or industrial use.
2. Polypropylene (PP)
When flexibility, fatigue resistance, and chemical inertness matter, PP shines. Think of the snap-fit lid that clicks shut hundreds of times—that’s PP at work.
- Where it’s used: Hinged caps, medical containers, automotive clips.
- Why it works: Lightweight, recyclable, and doesn’t crack under repeated stress.
- Watch out for: It’s softer and can be harder to mold with tight tolerances.
3. Polyethylene (PE: HDPE/LDPE)
PE is a go-to for moisture-resistant parts. Whether it’s a milk jug or a surgical tray, PE forms tough, chemically inert parts that hold up well to daily use.
- Where it’s used: Food containers, bottles, trays, and chemical storage.
- Why it works: Excellent toughness, chemical resistance, and low water absorption.
- Watch out for: Lower stiffness—designers may need to reinforce parts.
4. Nylon (PA6, PA66)
Nylon is built for strength and wear resistance. It’s used when mechanical stress is high, and friction is part of the job.
- Where it’s used: Gears, cable ties, bushings, structural components.
- Why it works: High tensile strength and natural lubricity mean it runs smoothly under load.
- Watch out for: It absorbs moisture, which can affect dimensional accuracy.
5. Polycarbonate (PC)
When strength meets transparency, PC delivers. It’s the material of choice for parts that need to survive impacts without cracking—and still look good.
- Where it’s used: Safety goggles, lighting covers, display windows.
- Why it works: High impact resistance and optical clarity.
- Watch out for: Can be prone to stress cracking if overmolded or cleaned with harsh solvents.
6. POM (Acetal/Delrin)
If your part needs to move smoothly, hold tolerances, and endure wear, Acetal is your solution. Its balance of stiffness and low friction is hard to beat.
- Where it’s used: Gears, sliding mechanisms, pump components.
- Why it works: Precision molding with low shrinkage, great wear resistance.
- Watch out for: Less tolerant of acidic environments and UV exposure.
7. TPE / TPU (Thermoplastic Elastomers)
When you need a rubbery grip or flexible hinge, TPE is ideal. It combines the formability of plastic with the feel and bounce of rubber.
- Where it’s used: Tool handles, gaskets, wearable straps.
- Why it works: Soft touch, resilient under repeated deformation.
- Watch out for: Requires careful processing and may have longer cycle times.
How to Choose the Right Plastic for Your Application
Choosing a plastic is about knowing what your part needs to endure. Will it face constant movement? Does it need to hold a tight fit over the years? Will it sit inside a food processor, or under a car hood?
Let’s walk through some common trade-offs you’ll likely encounter:
If your part needs to flex again and again—say, a cap hinge, a snap-fit clip, or a live joint—polypropylene is often your go-to. It bends without breaking, tolerates chemical exposure, and keeps costs down. But it won’t offer the stiffness or dimensional precision that nylon can deliver, especially when structural loads are involved. So if strength and rigidity matter more than flex, nylon may win.
When optical clarity is critical—like for safety lenses, light covers, or display housings—polycarbonate (PC) stands out. It’s strong, transparent, and impact-resistant. But it’s pricier, so not ideal for parts where clarity isn’t essential.
In harsh chemical environments—think fuel systems or cleaning devices—polyethylene (PE) and acetal (POM) both resist solvents and oils well. But acetal also brings dimensional stability and wear resistance, making it a better fit for moving parts like pump housings or gears.
If your design depends on tight tolerances—snapping shut precisely, aligning with other parts—ABS, PC, or POM are the usual suspects. These materials hold shape well during molding and resist warping over time.
And of course, budget matters. When cost is a constraint, PP or ABS offer solid mechanical properties without breaking the bank.
One last tip: Don’t overlook regulatory needs. Food-safe or medical-grade parts often call for certified materials, like FDA-grade acetal or flame-retardant PC. Fitting the spec isn’t optional; it’s the baseline for approval.
In the end, choosing the right plastic is less about picking a “best” material and more about matching the right one to your part’s story.
Material Selection Tips for First-Time Projects
If you’re working on your first injection molding project, picking the right plastic might feel like a data-driven decision—just compare tensile strength, thermal resistance, or chemical compatibility, right? But in reality, those numbers on datasheets only tell part of the story.
In practice, how a material flows inside the mold, how it fills thin walls or complex geometries, and how it cools and shrinks can make or break your design. A resin that looks perfect on paper might warp or flash in a real mold. That’s why it’s smart to consult your injection mold maker early, especially if they can provide mold flow analysis. This simulation helps predict where problems might occur and which material would handle the design best.
Budget-conscious? Try prototyping your part using a lower-cost resin with similar flow and shrinkage characteristics. You’ll get faster feedback without overspending on high-grade engineering materials too soon.
Most importantly, treat material choice as part of your design, not an afterthought. The earlier you loop in your manufacturing partner, the more you’ll avoid costly rework and discover performance gains that might surprise you.
Final Thoughts
A plastic part rarely gets noticed until it breaks, warps, or fails to fit. And by then, the mold is done, the timeline’s ticking, and the costs are mounting.
No plastic is perfect. But one will always fit your part, your mold, your industry better than the rest. The key lies not just in knowing the properties, but in knowing which trade-offs you’re truly willing to make.
Because in the world of injection molding, good design begins with good material judgment.
