Plastic Material Selection Mistakes that Affect Product Performance
Choosing a plastic material can look straightforward on a specification sheet. Strength, temperature resistance, chemical resistance, flexibility and cost can all be compared.
But selecting the right resin for an injection-molded product requires another important question:
How will this material actually behave when we try to mold the part?
A material may appear to meet every product requirement and still create challenges during manufacturing. Flow characteristics, shrinkage, cooling, moisture sensitivity, wall thickness and part geometry can all influence whether a design that works on paper can be manufactured consistently in production.
That’s why material selection shouldn’t happen in isolation.
At PRECISION, we believe the best results come from looking at the relationship between part design, material, tooling and the molding process before production begins.
Here are some of the material selection mistakes that can affect manufacturability and product performance.
1. Selecting Material Based Only on the Datasheet
Material datasheets provide valuable information, but they don’t tell the entire manufacturing story. Two materials with similar published properties can behave very differently inside an injection mold.
Viscosity and flow characteristics, for example, influence how easily material can fill thin sections, complex geometries or areas far from the gate. Shrinkage characteristics can affect dimensional accuracy. Cooling requirements can influence cycle time and part consistency.
The question shouldn’t simply be, “Does this material have the properties we need?”. Engineers and product designers need to consider asking, “Can we reliably mold the part we designed using this material?”
Considering both questions early can prevent costly surprises later.
2. Ignoring Material Flow
Every injection-molded part requires molten material to travel from the gate throughout the mold cavity before it begins to solidify. The farther it has to travel, and the thinner or more complex the geometry, the more challenging that can become.
Choosing a resin without considering its flow characteristics can contribute to problems such as incomplete filling, weld lines, inconsistent features or difficulty maintaining dimensional requirements. This becomes particularly important with complex parts, thin-wall designs and products with demanding cosmetic or structural requirements.
Material selection and part geometry need to work together.
3. Overlooking Shrinkage
Plastic materials shrink as they cool, but they don’t all shrink at the same rate or necessarily in the same way. If material shrinkage isn’t properly considered during product and tooling design, the finished component may not match the dimensions anticipated in the CAD model.
The consequences can include:
- Parts that don’t fit properly with mating components.
- Dimensional inconsistencies.
- Warpage.
- Difficulty maintaining tight tolerances.
- Additional tooling modifications.
For products with demanding dimensional requirements, understanding how a particular material behaves during cooling can be just as important as its mechanical properties after molding.
4. Choosing a Material that Doesn’t Match the Part Geometry
Sometimes the problem isn’t the material or the design. It’s the combination of the two.
A resin that performs extremely well in one application may be difficult to process in another because of wall thickness, ribs, bosses, undercuts, flow length or other design features.
This is where Design for Manufacturability (DFM) becomes especially valuable.
Rather than treating material selection, part design and mold design as separate decisions, an experienced injection molding partner can evaluate how they interact. A relatively small design adjustment early in development may allow a preferred material to be used successfully without compromising the function of the product.
5. Failing to Consider Warpage and Internal Stress
A molded part doesn’t always cool uniformly. Different wall thicknesses, part geometry, fiber orientation, cooling rates and material characteristics can create internal stresses that cause a component to warp or change dimension after molding. Also, certain reinforced materials can introduce additional considerations because fibers may orient themselves in the direction of material flow.
The challenge with plastic injection molding is that a part can look perfectly acceptable when it leaves the mold but behave differently as it cools or when it is assembled into the final product.
Understanding these relationships before tooling is finalized can help engineers anticipate potential problems instead of trying to correct them after production begins.
6. Specifying Material Too Early
There is often pressure during product development to make decisions quickly. Selecting a material early can feel like progress. But, locking in a resin before the design has been evaluated for manufacturability can unnecessarily restrict the options available later.
Material selection should ideally be part of a collaborative engineering process that considers these important elements:
- Product requirements
- Part geometry
- Material
- Tooling
- Manufacturing process.
When these elements are considered together, there is a much better opportunity to optimize the product for both performance and manufacturability.
7. Overlooking the Economics of Material Selection
A material can be technically appropriate for a part and still create an unexpected production challenge if its availability and overall processing economics are not considered.
Availability and minimum order quantity (MOQ) are important considerations, particularly for smaller parts or lower-volume production runs. Some materials may only be available in Gaylord quantities. If a project requires only a fraction of that amount, purchasing substantially more resin than the production run requires can have a significant impact on the economics of the program.
Material density also matters. Comparing resin only by its cost per kilogram or pound can be misleading because materials with different densities can produce a different number of parts from the same weight of resin. The more useful question is: “What is the actual material cost per finished part?”
The material can also affect molding cycle time. A resin that requires longer cooling, drying or processing time may reduce the number of parts that can be produced in a shift. Across a large production run, even a relatively small increase in cycle time can have a meaningful effect on manufacturing cost.
That is why material economics should be evaluated as a complete picture: availability, MOQ, density, cost per kg/lb, processing requirements and cycle time all contribute to the true cost of the finished component.
8. Assuming the Most Expensive Material is the Best Material
Higher-performance engineering resins can provide impressive mechanical, thermal and chemical properties. However, more performance isn’t automatically better.
Specifying capabilities that the application doesn’t actually require can increase material costs and potentially make the component more difficult or expensive to manufacture.
The objective isn’t to select the highest-performing or lowest-cost resin available. It’s to select the right material for the application and the manufacturing process while understanding its true cost per finished part. That distinction can have a significant impact when a product moves from prototype quantities into thousands or millions of components.
Material Selection is a Manufacturing Decision
The earlier material selection is considered from a manufacturing perspective, the more opportunities there are to improve the finished product. Working with an experienced injection molding partner can add significant value in making proper material selection.
PRECISION works with customers from early-stage product development through tooling, molding, assembly and full contract manufacturing. Our team can help evaluate how material choices interact with part geometry, mold design, processing requirements, production volumes and overall part economics before those decisions become expensive to change.
Sometimes the most valuable manufacturing expertise is applied before the first part is ever molded.
Before You Specify the Resin, Talk to PRECISION
Are you developing a new injection-molded product or experiencing problems with an existing design? Bring our team into the conversation early.
We can review your design, discuss material and manufacturability considerations, and help identify potential production challenges before they become tooling changes, rejected parts or costly redesigns.
Have a challenging part? Let’s talk about it.
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