Material selection for injection molding is one of those things that seems straightforward until you’re actually doing it. The injection molding market is worth $285.5 billion (and climbing toward $397 billion by 2030), with hundreds of different materials to choose from. Choosing the wrong material can cause cracking in part production, inaccuracies in the dimensions from the design, defects on the surface, increased cycle times, incompatibilities with chemicals, and more.
Since there are hundreds of injection molding material grades out there, each with their own processing properties and property trade-offs, matching numbers on a spec sheet is just the beginning. Real material selection can be more complicated.
Understanding the Injection Molding Material Categories
At the most basic level, you’re picking between thermoplastics and thermosets, which are the most common materials for injection molding.
Thermoplastics make up about 85% of injection molding applications. Their polymer chains are held together by these relatively weak intermolecular forces, so you can melt them, mold them, break them down, and melt them again. Thermoplastics are very forgiving. Your standard injection molding materials: ABS, polycarbonate, nylon, are all thermoplastics.
Thermosets are different entirely. Once those polymers cross-link during cure created an irreversible chemical reaction. The covalent bonds are permanent. Sure, you lose the ability to rework or recycle, but sometimes you absolutely need that thermal stability. Picture an electrical connector sitting next to an engine block at 200°C. ABS would be a puddle, but a thermoset would be totally fine.
Material Selection Parameters
Mechanical Properties That Go Beyond Basic Specs
Many plastic injection molders prioritize tensile strength as the primary material property, as it is clearly presented on the data sheet with a straightforward figure, such as “10,000 PSI tensile strength.” While this number is certainly appealing, it is important to note that it is being pulled from a controlled test using a perfect specimen that is pulled uniformly and without any variations. In contrast, the actual molded part often contains complex features such as gates, knit lines where flow fronts converge, and transitions from thick to thin sections, all of which introduce potential failure points not present in the idealized test specimen.
High impact resistance provides a more accurate reflection of real-world performance. For example, polycarbonate may look impressive on paper with its high notched Izod values at room temperature, but its performance drastically declines when exposed to temperatures as low as -40°C. In contrast, certain impact-modified polypropylene (PP) grades maintain their toughness even in freezing conditions.
Another critical property is flexural modulus, which plays a key role in applications such as snap-fits. To achieve that satisfying “click” when parts come together, it’s essential to target a flexural modulus in the 200,000-300,000 PSI range. A modulus that is too low results in a lack of retention force and heat resistance, while one that is too high risks cracking, potentially destroying prototypes. While geometry certainly plays an important role, material selection provides the foundation for optimal performance.
Thermal Considerations with Potential Chemical Resistance
Heat deflection temperature (HDT) at 264 PSI provides a baseline, but it’s hardly the complete thermal picture. Consider creep behavior at elevated temperatures – a part that survives static 80°C testing might deform unacceptably under constant load at 60°C. Thermal cycling introduces additional complexity through differential expansion and fatigue.
Thermal conductivity and thermal resistance is often overlooked. Most unfilled plastics sit around 0.1-0.3 W/m·K – fine for many applications, problematic for electronics housings requiring heat dissipation. Thermally conductive compounds can reach 1-5 W/m·K but come with trade-offs in cost, processability, and tool wear.
Glass transition temperature (Tg) matters more than many realize. Operating too close to Tg results in dimensional changes and property degradation over time. A good rule of thumb is to keep continuous use temperature at least 20-30°C below Tg for amorphous materials.
Processing Realities
Material properties are nice and all, but they mean exactly nothing if you can’t actually mold the part. Processing windows, shear sensitivity, moisture content; these properties matter.
Take POM (polyoxymethylene, or acetal), great dimensional stability, self-lubricating, holds tight tolerances. The downside is you will likely get above 0.2% moisture content and you’ll see foaming, splay marks, potentially even some formaldehyde generation. Minimum four hours drying at 80°C, and that’s assuming your material was stored properly to begin with. (Spoiler: it usually wasn’t.)
Then you’ve got materials like liquid crystal polymers. They flow like water, which is great for those impossible thin-wall applications. The molecular orientation is so strong that properties along the flow direction can be three times higher than across it. Your FEA software assumes everything’s isotropic. There are plenty of “unexplained” failures that suddenly made sense once someone checked fiber orientation.
Common Materials and Their Real-World Performance
Polypropylene – More Than Just Commodity
Modern polypropylene (PP) grades are nucleated versions that can reduce cycle time by 20-30% through faster crystallization, resulting in significant cost savings during production. Additionally, talc-filled grades (with a 20-40% loading) approach the stiffness of engineering plastics, yet cost only about 70% of the price.
The fatigue resistance of PP is notably high; a well-designed PP living hinge can withstand up to a million cycles without failure. However, temperature limitations remain a notable drawback. Below -20°C, its impact properties sharply decrease, and above 100°C, even with effective stabilizers, heat aging becomes a serious concern. Despite its many advantages, PP has its performance boundaries.
The ABS/PC Decision Matrix
ABS is often the go-to choice – it processes smoothly, provides an excellent surface finish straight from the tool, and is reasonably priced. However, it quickly falls short when UV resistance, chemical resistance, or temperatures above 90°C are required. In these cases, polycarbonate (PC) is typically considered.
Yet, the PC comes with its own set of challenges. Processing temperatures range from 280-320°C, so ensuring that the heater bands are functioning properly is critical. Additionally, moisture is a significant concern, with levels needing to stay below 0.02%. Cycle times are also longer, and the cost is typically 2-3 times higher than that of ABS.
PC/ABS blends attempt to strike a balance between the two materials. Sometimes, this works wonderfully, but other times, the results fall short. Properties don’t always average out as expected: a 50/50 blend might deliver superior impact resistance compared to either material alone, or it could combine the worst aspects of both. The success of the blend depends on factors like compatibility choice and the dispersion of phases, making it crucial to test the specific blend rather than relying on assumptions.
When Engineering Plastics Become Necessary
Glass-filled nylon (typically 30-50% GF) remains the go-to for structural applications requiring stiffness, strength, and moderate temperature resistance. Moisture absorption (2-8% depending on grade) causes dimensional changes and property reduction; critical for tight-tolerance applications.
For extreme environments, specialty materials justify their premium cost. PEEK handles 250°C continuously with exceptional chemical resistance. PPS offers similar temperature capability with better processability and lower cost. LCP enables wall thicknesses below 0.5mm with maintained properties.
Application-Specific Considerations
Medical device manufacturing requires an exceptional level of precision and attention to detail. Biocompatibility is essential (USP Class VI, ISO 10993 – the full range of regulatory standards), but it’s just the beginning. Sterilization compatibility becomes critically important. Gamma radiation can be tolerated by some polymers, while others experience chain scission and become brittle. Ethylene oxide (EtO) sterilization leaves residuals that require weeks to outgas, and autoclaving demands resilience to 135°C steam, repeatedly.
Take polycarbonate (PC) – it seems ideal for medical applications due to its clarity, strength, and sterilizability. However, it turns out that certain lipids can cause environmental stress cracking, which presents a significant issue. Polypropylene (PP), on the other hand, handles gamma radiation well and is cost-effective, but achieving optical clarity remains a challenge. Every material choice involves trade-offs.
Automotive applications present particularly harsh conditions for materials. Under-the-hood components are exposed to temperatures ranging from -40°C to 150°C (sometimes within the same day), a wide variety of fluids, constant vibration, and possibly hot oil splashes. Glass-filled nylons, particularly PA66 with 30-35% glass content, are commonly used. However, moisture absorption (ranging from 2-8%, depending on the grade) can lead to dimensional changes, meaning that a snap-fit designed at 23°C/50% relative humidity might not perform the same after a humid summer.
Electronics are becoming increasingly demanding as well. Previously, materials only needed to offer electrical insulation and possibly some flame retardance. Today, however, thermal management has become essential. Modern processors generate substantial heat, and traditional plastics with a thermal conductivity of 0.2 W/m·K simply trap it. There are modified compounds available that reach thermal conductivities above 10 W/m·K, but processing these materials is challenging. The fillers that enhance conductivity can severely damage tooling, making them difficult to work with.
Practical Selection Methodology
The first step to selecting the right materials is to clearly define your requirements. Simply stating that a material needs to be “chemical resistant” might not be sufficient. Which chemicals will it be exposed to? At what concentration? What are the temperature conditions, and how long will the exposure last? The “occasional” outdoor use mentioned needs to be more precisely defined, as different stakeholders may interpret the term “occasional” in vastly different ways than designers expect.
One step to avoid mistakes is to connect with your injection molding partner early in the process. They have likely encountered a wide range of material-related issues (and probably some you haven’t even considered). For example, they might inform you that while PEEK works well for certain applications, their equipment can only handle temperatures up to 380°C. Or that the super-filled compound you’ve specified could cause severe wear on screws within a few months. Addressing these issues upfront can save significant time and resources compared to discovering them later in the design phase.
Testing is another aspect, but not all testing is equal. Lab conditions are controlled, but real-world conditions are far less predictable. Temperatures don’t stay constant at 85°C. Chemical exposures are rarely isolated and they typically involve a mix of substances. UV exposure often occurs in conjunction with heat and moisture. To get an accurate assessment, test how the material will be used in its actual environment, not under idealized, lab-based conditions.
Finally, consider the material costs. While commodity resins may seem inexpensive at $2 per pound compared to engineering-grade materials at $5 per pound, the more expensive option might allow you to reduce wall thickness, consolidate parts, or achieve faster cycle times, ultimately making it more cost-effective. Additionally, factor in scrap rates (some materials produce less waste), tool maintenance, and any necessary secondary operations. What may initially appear as the “expensive” choice could turn out to be the more economical option in the long run.
Making Informed Decisions
For engineers dealing with these decisions, having an experienced partner makes all the difference. Companies like KS Plastic bring 50+ years of hard-won knowledge to the table. They know which materials cause headaches, which ones run clean, and most importantly: which ones actually make sense for your application and budget. Sometimes you just need someone who’s seen it all and can help you avoid the pitfalls.
Because at the end of the day, good engineering isn’t about perfection. It’s about making smart trade-offs based on real constraints. And material selection? That’s where it all starts.