Selecting the right plastic resin is one of the most critical decisions in any injection molding project. The material you choose directly impacts part performance, durability, cost, and manufacturability. With hundreds of thermoplastic options available, narrowing down the best fit for your application can feel overwhelming — but it doesn’t have to be.
Here are five essential factors to consider when choosing a plastic resin for your next injection molded part.
1. Mechanical Requirements
Start by defining the mechanical loads your part needs to withstand. Consider tensile strength, impact resistance, flexural modulus, and fatigue life. A structural bracket exposed to continuous stress requires a very different resin than a decorative cover that simply needs to look good.
For high-strength applications, engineering resins like nylon (PA6/PA66), polycarbonate (PC), or glass-filled compounds offer excellent mechanical properties. For general-purpose parts, ABS and polypropylene (PP) provide a great balance of strength and cost.
2. Operating Temperature Range
Every resin has a heat deflection temperature (HDT) — the point at which it begins to deform under load. If your part lives under the hood of a vehicle or inside an electrical enclosure, you need a material that can handle sustained elevated temperatures without losing dimensional stability.
PEEK and PPS handle temperatures above 400°F, while standard ABS starts softening around 200°F. Matching your resin’s thermal properties to your application environment prevents premature part failure.
3. Chemical and Environmental Exposure
Will your part contact solvents, oils, acids, or UV light? Chemical resistance varies dramatically between resins. Polypropylene and HDPE resist most chemicals but lack the strength of engineering plastics. Acetal (POM) handles fuels and solvents well while maintaining excellent dimensional stability.
For outdoor applications, UV-stabilized grades or inherently UV-resistant materials like ASA prevent yellowing and brittleness from sun exposure.
4. Regulatory and Compliance Needs
Medical, food-contact, and aerospace parts must meet specific regulatory standards. FDA-compliant, USP Class VI, and ISO 10993 certifications narrow your resin options significantly. UL flame ratings (V-0, V-1, V-2) are mandatory for electrical and electronic housings.
At Legacy Molding, we work with certified material suppliers and can guide you through the compliance landscape to ensure your parts meet all applicable standards.
5. Cost and Production Volume
Resin cost can range from under $1 per pound for commodity plastics like polypropylene to over $50 per pound for high-performance polymers like PEEK. When you’re running high volumes, even a small price difference per pound translates into significant savings.
Often, a more affordable resin with the right additives (glass fiber reinforcement, flame retardants, UV stabilizers) can match the performance of a premium material at a fraction of the cost. Our engineering team specializes in finding these cost-effective alternatives without compromising part quality.
Let Us Help You Choose
Material selection doesn’t happen in isolation — it’s deeply connected to part design, mold design, and processing parameters. At Legacy Molding, we provide material selection guidance as part of every project, helping you balance performance, cost, and manufacturability from the start.
Have a project in mind? Send us your requirements and we’ll recommend the ideal resin for your application.