Common Mistakes In Selecting Injection Molding Materials

Choosing The Right Thermoplastic Goes Beyond The Spec Sheet 

If you are like many product engineers who outsource component manufacturing, injection molding is not likely your core focus. And while you have a comprehensive understanding of your product, you might not be deeply familiar with the nuances of injection molding materials selection. Understanding common pitfalls in thermoplastic selection helps engineers select materials that deliver the optimal balance of form, fit, and function.

Assuming General Material Specs Translate to Molded Performance

Engineers accustomed to working with raw material specs rather than molded part behavior may find that relying solely on data sheets for product performance can lead to detrimental results.  A datasheet provides a baseline of a material’s properties under standardized, ideal laboratory conditions, using standardized test bars, not the real-world performance of a finished, molded part, so actual strength or impact resistance may not align with published values.

The actual performance of a molded component is significantly influenced by a variety of factors related to the manufacturing process and part design.  For example:

• A resin’s datasheet may report tensile strength and impact resistance at ideal moisture levels, but molded parts might be processed with higher moisture content, leading to brittleness or unexpected failures.
• Material specs are often based on the neat resin, but once colorants, flame retardants, or UV stabilizers are added during molding, the actual performance (like strength, shrinkage, or weldability) can shift significantly. For example, fiber-reinforced injection molding materials can have regions of varying strength and stiffness because the fibers tend to align with the plastic’s flow path. 
• The datasheet assumes uniform, relaxed samples, whereas molded parts can have residual stresses from cooling or flow orientation, which can reduce durability or cause warpage.

Not Accounting for Mold Design Constraints

Designing a product for injection molding is much different from designing for machining or even 3-D printing. The way the material enters (gate placement), how it flows through the part, and the complexity of the design can all impact the final part.  A few things to consider include:

• Gates – Ideally, gates should be placed in the center of the part in a non-functional, non-appearance area so that the melt flow has the same flow length to each end. In some instances, several gates may be used. The type of gate will also impact the part. If aesthetics is critical, place the gate along the part’s edge or another non-visible area.  
• Wall thickness – Datasheet properties assume a uniform test specimen, but molded parts may not have uniform sections. While the nominal wall thickness is determined by the functional performance requirements of the part, thickness consistency is important to avoid uneven cooling and defects that impact performance.  When varying wall thickness is necessary, smooth thickness transitions should be used to reduce stress.  
• Undercuts – Although undercuts do not directly change the material’s properties, we are including them because they can add to the cost. Core pulls or cams will be required to ensure the part can be removed from the mold.
• Ribs/gussets – Ribs and gussets must be properly designed to avoid sinkage, warpage, or part failure. A fillet can be added where the rib meets the wall to reduce stress. However, if it’s too small, it won’t reduce enough stress, and if it’s too large, it may have sink marks.  
• Bosses – Bosses (used for mating parts with screws or other fasteners) that are not isolated from a corner can cause sinkage in the nominal wall. They can be strengthened by connecting them with ribs to the walls or gussets to the base. 
• Radii – Sharp corners weaken parts, so corners should be radiused. Internal radii (fillets) are used to smooth inside corners.   
• Tolerance sensitive features – Snap fits, press fits, and mating features rely on precise shrinkage. If the material shrinks differently than expected (due to geometry), the part won’t assemble properly, even though the datasheet shrinkage looked acceptable.

Overlooking Operations Compatibility With Injection Molding Materials

Some processes may not be compatible with a specific material choice.  For example, materials in an overmolding process can act differently than their individual data sheets suggest. The interaction between the substrate and the overmold material introduces complex variables that are not present when each material is molded on its own. The final part’s performance depends on the combined properties, bonding characteristics, and thermal interactions of both materials. 

Secondary operations, such as ultrasonic welding, spin welding, and even some light assembly, can cause the product to react differently than might be expected if relying solely on the data sheet, where, as mentioned previously, properties are measured on ideal molded specimens. 

When ultrasonic welding, for example, fillers (such as glass and carbon) scatter ultrasonic energy, which reduces weld quality, and flame retardants or lubricants in the resin can interfere with bond strength. 

For spin welding, it isn’t enough to assume that if the resin has a high melting temperature and toughness, it must weld well. Good frictional heating doesn’t guarantee the weldability of plastic. While frictional heating is the mechanism that melts the plastic, spin welding performance depends on melt layer formation and surface friction behavior, which specs don’t capture.

The performance of snap fits, press fits, and fasteners during assembly may not be as expected with some materials. For example, glass-filled grades can crack during snap fits, even if the datasheet suggests adequate elongation. Moisture-sensitive resins, like Nylon, change dimensionally after molding, resulting in assemblies loosening or jamming.

Make Better Injection Molding Material Choices With Metro Plastics

Avoid costly missteps by partnering with experts who understand the real-world behavior of molded parts. At Metro Plastics, our engineering team doesn’t just read datasheets; backed by years of experience, they help you interpret them in the context of your design, mold constraints, and performance goals. With access to over 110 engineering-grade and commodity resins, plus the ability to compound custom materials, we tailor solutions that fit your unique application. From early design assistance to full process validation, we guide you through every step to ensure your thermoplastic selection delivers the form, fit, and function your product demands. Let Metro Plastics be your trusted advisor in injection molding because choosing the right material starts with choosing the right partner.