Choose a molding resin by the conditions the finished part will face. A material family narrows the search, but grades within that family can differ in impact strength, stiffness, shrinkage and appearance. Use the exact grade’s data sheet and test conditions when making a decision.
Define the operating conditions
Write down continuous and peak temperatures, loads, chemical contact, moisture and the expected service life. Specify where appearance, transparency or color matters. A housing wiped with a cleaner needs a different review from a sliding latch or a component exposed to sustained heat.
Read a grade datasheet against the actual use case
Start with the requirements that could rule out a grade. A cold drop test, prolonged heat under load, repeated cleaning or a tight snap fit can each change the shortlist. Record the test method and conditions alongside the data so that values measured on different specimens or at different temperatures are interpreted correctly.
| Part requirement | What to compare | What to verify on a molded sample |
|---|---|---|
| Stiffness under load | Flexural modulus and the supplier’s data for the expected service temperature. | Deflection at the actual load and support points. For a sustained load, check deformation over time. |
| Impact resistance | Impact values taken using comparable test methods, notch conditions and temperatures. | Drop or impact on the assembled geometry, especially near bosses, clips and sharp transitions. |
| Heat exposure | Temperature limits relevant to the load and duration. Note the test load on any heat-deflection value. | Fit and performance after the expected temperature cycle. |
| Cleaner, moisture or chemical exposure | Compatibility with the actual agent, concentration, contact time and temperature. | Inspect stressed clips and fastener bosses for cracks, swelling, discoloration or loss of fit. |
| Dimensional stability | Grade-specific mold shrinkage, moisture effects and the direction of flow when data are given separately. | Check mating dimensions and flatness after the part has reached its specified conditioning state. |
Compare candidate resin families
ABS is a common starting point for opaque housings. PP may suit parts that need flex or chemical resistance. PC can be considered where impact resistance or a clear grade matters. Nylon is often considered for loaded components, with moisture effects checked where fit is critical. Acetal (POM) can suit sliding features. PEEK is a candidate for demanding thermal or chemical conditions when the grade and cost are justified.
Worked example: a sensor housing exposed to cleaner and drops
A handheld sensor has an opaque housing with screw bosses and a snap-fit cover. It will be wiped with a specified cleaning solution and may be dropped in use. Before comparing resins, record the cleaner and concentration, wipe frequency, service temperature, drop height and acceptable condition after the test.
| Candidate | Check before shortlisting | Confirm on the molded housing |
|---|---|---|
| ABS | Look up impact data at the relevant temperature and ask for compatibility data for the specified cleaner. | Check cracking at the screw bosses after cleaning and drop the assembled housing. |
| PC | Check the grade’s impact data and chemical resistance under stress. | Inspect stressed areas around installed screws and snap fits for fine cracks after repeated wipes. |
| PP | Check the exact grade’s stiffness, shrinkage and low-temperature impact performance. | Verify lid fit, snap retention and screw-boss performance after molding and conditioning. |
Use datasheet figures to screen grades tested under comparable conditions. Run the same cleaning and drop sequence on assembled molded samples. Check that the cover stays shut, the bosses and clips are intact and the housing still fits. Confirm the grade and shrinkage assumptions with the toolmaker before releasing the mold. For an example of how a resin supplier documents this, see Covestro’s polycarbonate chemical resistance guide.
Review how the resin changes the part and tool
Discuss wall transitions, draft, ribs, bosses and gate location with the toolmaker before the cavity dimensions are fixed. The same material family can include filled, impact-modified or flame-retardant grades with different shrinkage and processing requirements. For filled grades, ask how flow direction may affect warpage or stiffness in the finished part. Identify visible faces and features that need a controlled fit before choosing a gate.
Define the exact grade, supplier, color and reinforcement in the material specification. When a project requires documentation for a particular application, obtain it for that grade and intended use. Agree on whether recycled content, a different pigment or an alternate grade can be accepted and what would need to be retested.
Confirm the grade before releasing tooling
Check how the shortlisted resin fills the part and how shrinkage could affect mating features. Specify reinforcement, color and any documentation on the drawing or material specification. If a different grade is proposed after tooling, review fit, appearance and performance again before accepting the substitution.
Check the first molded samples before approving production
- Record the resin grade, lot and molding conditions for the trial parts. Use the specified conditioning and time before measuring fit-sensitive features.
- Check the dimensions and surface features that control assembly or appearance. Examine gate marks, sink, weld lines and warpage where they affect the part’s function.
- Run the application tests on assembled samples, then record the result and any material or tool changes. If the supplier proposes another grade, repeat the checks affected by the substitution.
Review available molding routes in the injection molding process directory before deciding when the grade and tool need to be finalized.
Explore the materials by manufacturing process and injection molding service overview.
