Selecting the proper material is one of the most critical steps in injection molding design. The choice of thermoplastic resin determines not only the mechanical strength and durability of a molded part but also its manufacturability, surface finish, cost, and long-term reliability.
This guide provides engineers and designers with a structured framework for quantitatively evaluating injection molding materials. It focuses on the key mechanical, thermal, and chemical properties that influence performance, helping you select the optimal resin for your specific application.
Introduction
plastic injection molding materials
Choosing a thermoplastic resin for injection molding can be a complex engineering decision. Each polymer family exhibits unique characteristics that respond differently to mechanical loads, temperature variations, and environmental conditions. To make a sound selection, engineers must understand the relationship between material structure and performance, as well as the specific demands of the application.
For safety-critical, high-performance, or reliability-sensitive components, a thorough engineering analysis is mandatory. This includes evaluating stress, strain, and environmental exposure to determine which material meets all operational requirements.
However, not every project requires extensive simulation or testing. Many molded components operate under mild conditions—such as room temperature and low loads—where common resins like ABS, polypropylene (PP), or polycarbonate (PC) are more than adequate. For such cases, simplified guidelines—covered later in this guide under “Don’t Make Me Do the Math”—can help non-engineers make practical and cost-effective material decisions.
Why Material Selection Matters
Material selection often becomes a guessing game when there’s a lack of understanding about:
How internal polymer structure affects properties.
How to clearly define real-world application requirements.
Where to find reliable and comparable data for candidate materials.
Even small variations in resin composition or processing parameters can significantly affect tensile strength, dimensional stability, or impact resistance. Therefore, informed material selection ensures consistent quality, cost efficiency, and predictable performance throughout a product’s lifecycle.
Comparing Common Injection Molding Materials
| Resin Type | Key Benefits | Typical Applications | Design Considerations |
| POM (Acetal) | Tough, stiff, strong, low friction, fatigue-resistant | Gears, pumps, impellers, switches, knobs | Sensitive to shrinkage; difficult to paint or coat |
| PMMA (Acrylic) | Excellent clarity, scratch-resistant, low shrink | Lenses, signs, light pipes | Brittle, poor chemical resistance |
| ABS | Tough, chemical-resistant, dimensionally stable, low-cost | Electronic housings, handheld devices, cosmetic parts | May show knit lines; thick sections can sink |
| HDPE | Tough, impact- and chemical-resistant, lightweight | Containers, toys, furniture | High shrinkage, low surface energy |
| PC (Polycarbonate) | High strength, clarity, and impact resistance | Lenses, lighting, housings, bulletproof glass | Poor chemical resistance; may bubble in thick parts |
| ABS/PC Blend | Good balance of toughness and processability | Automotive, electronics | Lower cost than PC; improved molding properties |
| PP (Polypropylene) | Flexible, fatigue-resistant, chemical-resistant | Hinges, caps, medical tubing | Shrink and warp possible; may void in thick areas |
| PEEK | High-temperature and chemical resistance | Aerospace, pumps, medical devices | Very expensive; Ultem or PPSU are cheaper alternatives |
| PEI (Ultem) | Strong, heat-resistant, dimensionally stable | Medical, lighting, HVAC components | Expensive, but cheaper than PEEK |
| PPSU | High heat and chemical tolerance, sterilizable | Medical trays, aircraft interiors | Thick sections may void; limited color options |
| PA (Nylon) | High strength, abrasion resistance | Gears, bushings, structural parts | Absorbs moisture; may warp |
| PBT/PET | Excellent electrical and dimensional properties | Appliances, automotive, electrical | Prone to warp in glass-filled grades |
| LCP | High flow, chemical resistance, low thermal expansion | Connectors, PCBs | Expensive, anisotropic shrink |
| PPS | High chemical and thermal resistance | Automotive, electrical housings | Requires precise crystallization control |
About the Standard Material Data Sheet
A material data sheet provides essential mechanical and thermal properties—typically measured at room temperature. While these values (e.g., tensile strength, elongation, flexural modulus) are important, they often describe failure conditions rather than real-world performance. Engineers must therefore interpret data sheets carefully, considering environmental conditions, part geometry, and expected loads.
More than 85,000 commercial plastics exist, grouped into around 45 polymer families. Thermoplastics dominate modern manufacturing, accounting for roughly 85% of all plastic use, while thermosets make up the remaining 15%. Supplementary information from suppliers—like design manuals and case studies—can help bridge the data gaps for engineering-grade materials.
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Don’t Make Me Do the Math!
Many material property charts show ratios like strength-to-weight or stiffness-to-cost, but in practical mold design, few engineers have time to crunch these numbers manually. Instead, the best practice is to create a shortlist of viable materials based on mechanical and thermal requirements, then compare specific, real-world properties (e.g., tensile strength at operating temperature, shrink rate, and melt flow).
Online material databases and supplier datasheets make this easier than ever. Simply filtering by your performance range can narrow hundreds of resins down to a handful of candidates. Once that shortlist is ready, your molder or resin distributor can provide detailed test data for validation and part trials.
Pro Tip: Always test with the exact resin grade you plan to use in production — even small differences in additives, moisture content, or supplier formulations can significantly change flow and shrink behavior.
Testing and Validation: Never Skip This Step
Material selection doesn’t end at the data sheet. After you’ve identified one or two promising candidates, prototype testing is essential. Build a small pilot mold, run short shots, and inspect critical features. This reveals how the material flows, fills, and warps in your actual part geometry — insights that simulations can’t always predict.
Typical validation checks include:
Dimensional accuracy — Compare measured dimensions with CAD data.
Surface finish — Confirm texture replication and gloss levels.
Mechanical testing — Evaluate tensile, impact, and fatigue performance.
Environmental resistance — Test against chemicals, UV exposure, or temperature cycles.
By catching material-related issues early, you can prevent costly redesigns or warranty claims later.
Partner with Your Molding Expert
Even the most experienced design engineers benefit from collaborating with an experienced molder during material selection. Mold designers and process engineers can advise on:
Proper mold steel choices to match resin abrasiveness or corrosion tendencies.
The need for mold coatings or hot runner compatibility for difficult materials.
Adjusting gate placement, venting, and draft angles for specific resin flow properties.
Their production insight often leads to better long-term consistency and fewer processing challenges — ultimately saving both time and money.
Sustainability and Recycling Considerations
Today’s manufacturing environment places increasing emphasis on sustainability. When possible, consider recycled or bio-based materials. Many resin suppliers now offer PCR (Post-Consumer Recycled) or bio-resin versions of standard polymers such as PP, PET, and PA.
While these sustainable options can slightly alter mechanical or color properties, they help reduce environmental impact and support greener product initiatives. Always test these materials under real processing conditions to ensure consistent quality.
Final Thoughts
Choosing the right injection molding material is one of the most important decisions in part design. The resin you choose determines not only product strength and performance but also cost, durability, and manufacturability.
Here’s a quick summary to guide your process:
Define your requirements — strength, temperature, chemical exposure, appearance, and cost.
Shortlist materials that meet your property range.
Evaluate process compatibility — shrinkage, moisture sensitivity, moldability.
Prototype and validate before final selection.
Consult your molder to fine-tune production parameters.
A thoughtful, data-driven approach ensures every molded component meets performance targets and budget expectations — the foundation of reliable, repeatable production.
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