Injection Molded Plastic Products Material Selection

Selecting the right material for injection molded plastic products is typically done early in the design phase, as the thermoplastic may impact the moldability of complex designs. Material requirements may vary depending on the industry, the product’s working environment, the products’ expected life span and your budget. In this post, we will guide you in choosing the appropriate material for your product by evaluating the product’s needs and common materials’ properties.

Understanding Types of Thermoplastics

Thermoplastics soften and melt when heated and can be re-melted and re-shaped without impact on its properties. This is the most common plastic used in injection molded products.

Thermoplastics are often categorized by general performance characteristics:

Commodity plastics are widely used for high-volume injection molding. Commodity plastics tend to have lower mechanical strength and lower heat resistance, but they are inexpensive and easy to process.
Engineering plastics provide better mechanical strength, moderate to high heat resistance, good chemical resistance, and dimensional stability, making them a good choice for precision parts.
High-performance plastics tend to be the most expensive but offer exceptional strength that can be maintained at high temperatures, very high chemical resistance, and excellent dimensional stability.

Another classification is based on polymer chain structure, which drives dimensional accuracy, strength, chemical resistance, appearance, and tooling strategy.

Amorphous plastics have polymer chains that are randomly oriented, and they remain that way when cooled. They tend to shrink less and more uniformly, hold tighter tolerances more uniformly, making them better for parts that need precise fits or have complex geometries.
Semi-crystalline plastics have polymer chains that are arranged in a tightly packed orderly manner with some amorphous areas. When the melted plastic is injected, the high shear rates, especially near the cooler mold walls, cause the typically coiled and entangled polymer chains to uncoil, stretch, and align in the direction of flow. Unlike amorphous polymers where chains relax after flow stops if still hot, in semi-crystalline polymers, the aligned chains maintain their orientation and crystallize in place along the direction of the flow. This can lead to increased strength in the direction of flow, but there is also higher shrinkage perpendicular to flow. Faster cooling on the outer layers and core layers can lead to warpage and residual stress in the molded part. Understanding and controlling the orientation is crucial for designing and producing high-quality plastic injection-molded products with consistent performance.

Evaluating What Your Injection Molded Plastic Products Needs

Material selection starts with the part itself. Injection molded plastic products perform best when the chosen polymer aligns with how the part will be used, what it must withstand, and how it must look. Before comparing resins, it helps to get clear on the conditions your product will face. Temperature swings, chemical exposure, load-bearing requirements, and cosmetic expectations all narrow the field.

Think about the forces the part will experience in normal use, but also the extremes. Will it see repetitive impact, vibration, or UV light? Does it need to retain dimensions over time or through sterilization cycles? The more accurately the application is defined, the more effectively you can match the material. Budget also plays a role because higher-performance materials can improve longevity but may increase the cost per part. Balancing these trade-offs early reduces redesigns later.

Critical Questions to Ask During Material Selection

There are several questions that can be asked about the part’s expected performance and use that will keep the decision of material choice grounded in engineering needs rather than trial and error.

What environment will the part operate in, and for how long? Outdoor exposure, heat from nearby components, and contact with chemicals all influence how a material behaves over time.
How precise must the part be? Tight tolerances, thin walls, and complex geometry behave differently in amorphous versus semi-crystalline materials. A material that processes cleanly often prevents dimensional issues downstream.
How much strength and stiffness does the design call for? Loads may be static or cyclical. Some thermoplastics hold strength after repeated stress while others are more prone to creep.
What cosmetic requirements matter? Surface finish, colorability, and clarity vary widely. If the part is customer-facing, aesthetics may be as important as mechanical performance.
How sensitive is the part to weight, cost, or regulatory requirements. Industries such as medical and food processing add constraints that shape the material options early.

Product designers who want to avoid the most frequent pitfalls in resin choice may find our guide to common mistakes in selecting injection-molding materials helpful as a companion to this article.

Looking Closer at Common Materials

Most injection molded plastic products fall into a familiar group of resins, each with trade-offs that make it more or less suitable depending on the demands above. The following overview frames how engineers compare these materials during design.

Acetal offers low friction, excellent wear properties, and strong dimensional stability. This makes it a good candidate for moving components or parts that need predictable tolerances over time.
ABS blends toughness with good cosmetic capability. It molds cleanly, accepts color well, and provides sufficient rigidity for housings and structural parts while keeping material cost in check.
PBT offers higher heat resistance than ABS and strong electrical properties. It performs well in parts that need durability near heat sources or exposure to moisture.
Nylon is valued for strength, impact resistance, and fatigue performance. It absorbs moisture, which must be accounted for, but remains a dependable choice for mechanical components.
Polypropylene is lightweight, fatigue resistant, and chemically resilient. It’s commonly chosen when cost, durability, and flexibility need to be balanced.
Polycarbonate provides excellent clarity and high impact strength. Its toughness makes it suitable for protective covers or high-strength enclosures.
PEEK sits at the top of the performance spectrum. It maintains strength at high temperatures and withstands aggressive chemicals, making it appropriate for demanding environments.
TPU brings elasticity and abrasion resistance. It performs well in parts that need flexibility and repeated motion without cracking.

Selecting material for injection molded plastic products is a technical decision that must blend together product requirements with polymer behavior. When you evaluate the application first, then compare resin options through the lens of performance, cost, and manufacturability, you reduce risk and improve the part’s long-term reliability.

Rely on Experience

Metro Plastics has extensive experience helping clients choose the correct resin. When you bring us into the process early, our engineering team can help you match your performance requirements with the right plastic, flag potential molding issues, and recommend adjustments that improve consistency and reduce cost. Contact is to start the conversation.