7 Tips for Choosing Injection Molding Materials

Choosing a plastic by name alone is rarely enough. Two parts made from the same resin can behave differently when wall thickness, processing conditions, or operating temperatures change. A brittle corner, a warped panel, or a worn gear can turn a promising design into an expensive revision. Material selection deserves careful attention.

These seven tips for choosing injection molding materials examine the factors that shape real part performance. Consider mechanical strength, heat resistance, chemical exposure, moisture, appearance, production needs, and total cost. Each factor connects to the others. A glass-filled material may improve stiffness, for example, but it can also affect surface finish and tool wear. There is no universal best choice.

Start with the part’s job. Will it sit near a warm motor, flex repeatedly, or face detergent and water? Note the actual conditions, not just the ideal ones. Then compare material data sheets and discuss processing limits with a qualified supplier or molding engineer. Published values are useful, but they do not guarantee identical results in every design. Small tests can reveal surprises.

Good decisions balance performance with manufacturability and budget. That balance is not always neat. A lower-cost resin may work well, while a premium grade may add little value; sometimes the reverse is true. These tips provide a practical starting point for evaluating injection molding materials, asking better questions, and reducing avoidable design changes before production begins.

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1 Set Strength Targets: Typical Unfilled ABS Tensile Strength Is 40–50 MPa

2 Compare Heat Limits Using ASTM D648 at 0.455 MPa

3 Estimate Shrinkage: ABS Is ~0.4–0.7%; PP Is ~1.0–2.5%

4 Check Moisture Sensitivity: Nylon 6 Can Reach ~2.5% at 50% RH

5 Verify Requirements: UL 94 V-0 and FDA 21 CFR 177.1520 as Applicable

Set Strength Targets: Typical Unfilled ABS Tensile Strength Is 40–50 MPa

For unfilled ABS, a typical tensile strength is about 40–50 MPa. Treat this as a starting point, not a guaranteed result. The exact value depends on the grade, test method, specimen geometry, and conditioning. Not a guarantee. When reviewing a data sheet, check the test standard and temperature alongside the number.

Set a strength target based on what the part will actually experience. A housing clipped into place may face repeated bending, while a bracket may carry a steady pull. Tensile strength alone does not describe impact resistance, fatigue, or long-term creep. A part can meet a strength target and still crack at a sharp corner. That detail is easy to miss.

Leave some margin between the expected load and the material’s tested strength. Molded parts may also vary with wall thickness, weld lines, and flow direction. If failure would cause a costly redesign, test molded samples under realistic conditions rather than relying only on a resin datasheet. This takes extra time. It may feel cautious, but a small prototype test can reveal weak points that a clean laboratory value will not..temperature and moisture exposure matter too, so record them when comparing results.

Set Strength Targets: Typical Unfilled ABS Tensile Strength

Typical tensile strength for unfilled ABS is approximately 40–50 MPa. Use this range as a preliminary reference when setting material strength targets.

Compare Heat Limits Using ASTM D648 at 0.455 MPa

ASTM D648 compares how plastics resist bending as temperature rises under a specified flexural stress. At 0.455 MPa, the reported heat deflection temperature can help screen materials for lightly loaded, warm parts. It is not a safe continuous-use temperature. Keep that distinction visible.

Compare data tested at the same stress. Check specimen thickness, conditioning, and test method details, too; small differences can muddy a comparison. A molded bracket near a warm motor may see heat and sustained force, while a loose cover may see heat alone. The test does not reproduce every real part, including its geometry or cooling conditions. Useful, but limited.

Look at the actual temperature margin. If a part operates near a candidate’s reported value, consider creep, repeated heating, and load duration before choosing. A higher D648 result may narrow the shortlist, not settle it. That is easy to overread. Ask for traceable test data, then validate the material in a representative part or fixture. A quick bench test can reveal surprises, though it cannot replace controlled testing.

Estimate Shrinkage: ABS Is ~0.4–0.7%; PP Is ~1.0–2.5%

Shrinkage is the dimensional change between a mold cavity and a cooled part. Typical injection-molding design references place ABS near 0.4–0.7% and polypropylene near 1.0–2.5%. That difference matters. For a 100 mm cavity dimension, those ranges imply roughly 99.3–99.6 mm for ABS, versus 97.5–99 mm for polypropylene. These are starting estimates, not guaranteed part dimensions.

The ranges reflect common material-selection data; ASTM D955-21 and ISO 294-4:2018 describe methods for measuring molding shrinkage. Actual results shift with resin grade, flow direction, wall thickness, packing pressure, and cooling conditions. A long, thin rib may shrink differently from a thick boss on the same part. So a single percentage can mislead. Keep the mold dimensions adjustable where tight fits matter, then verify samples from the intended process. I would not compensate for the maximum PP figure automatically; that can oversize the tool if the chosen grade and process shrink less. Measure in the critical direction.

Check Moisture Sensitivity: Nylon 6 Can Reach ~2.5% at 50% RH

Nylon 6 can absorb moisture from room air; at 50% relative humidity, some grades may reach about 2.5% moisture by weight at equilibrium. Exact uptake depends on grade, temperature, and conditioning time. The pellets may look dry, but appearance tells you little. Moisture matters because wet resin can degrade during melting and cause bubbles, splay, or inconsistent parts. Not every defect is moisture, though.

Tip: Check the resin supplier’s drying specification and verify moisture with a suitable analyzer when quality is critical. Do not rely on a fixed drying time alone. Hopper temperature, airflow, pellet size, and dryer loading all matter. Keep dried pellets covered and feed them promptly. They can absorb water again. It’s easy to trust the timer too much.

Tip: Record moisture readings alongside melt settings and part results during trials. Compare freshly dried pellets with pellets exposed on the shop floor for a known period. Watch for changes in dimensions and surface finish, not just obvious bubbles. A simple test may still mislead if samples sit open. That’s worth checking. Adjust drying based on measured data and grade guidance, rather than applying one number to every nylon 6 job.

Verify Requirements: UL 94 V-0 and FDA 21 CFR 177.1520 as Applicable

Start with the part’s real operating conditions, not a datasheet headline. UL 94 V-0 is a flammability classification under specified test conditions; it is not a guarantee that every molded part will meet the same result. Ask for the tested material grade and specimen thickness. Then compare them with your part’s wall thickness, color, and molding process. Small details matter. A thinner rib or a different pigment can affect performance, so verify the requirement with representative molded samples where practical.

For food-contact use, check whether the polymer and intended conditions fit 21 CFR 177.1520. That regulation covers specified olefin polymers, but a material’s listing alone does not establish suitability for every food type, temperature, or contact duration. Request documentation that identifies the exact grade and applicable conditions of use. Keep it with the drawing and purchasing records. Paperwork can lag behind a formulation change, and that is easy to overlook. Confirm the document is current, and ask for clarification when its scope does not match your application.

7 Tips for Choosing Injection Molding Materials - Verify Requirements: UL 94 V-0 and FDA 21 CFR 177.1520 as Applicable

Material	Typical Strengths	Key Trade-Offs	UL 94 V-0 Considerations	FDA 21 CFR 177.1520 Considerations	Useful Verification
Polypropylene (PP)	Low density, good chemical resistance, and useful fatigue resistance in living hinges.	Unmodified grades can have limited stiffness and low-temperature impact performance compared with some engineering plastics.	V-0 may be available in specific flame-retardant grades. Confirm the exact grade, color, and tested thickness.	PP is an olefin polymer that may fall under this regulation when the specific formulation and conditions of use meet its requirements.	Review the resin supplier’s food-contact documentation and UL classification for the intended grade and part thickness.
High-density polyethylene (HDPE)	Good moisture and chemical resistance, low density, and toughness in many applications.	Lower stiffness and heat resistance than many engineering plastics; dimensional stability and creep may require attention.	Do not assume a standard grade is V-0. Verify a flame-retardant grade and its exact UL test conditions.	HDPE is an olefin polymer that may be covered when the resin composition and intended food-contact conditions satisfy the regulation.	Check the applicable regulatory conditions of use, including food type, temperature, and contact duration.
Polycarbonate (PC)	High impact resistance and good dimensional stability; transparent grades are available.	Can be sensitive to some chemicals and may require drying before molding.	V-0 classifications exist for particular grades and thicknesses; verify the exact material record.	PC is not an olefin polymer under 21 CFR 177.1520. Check for another applicable authorization for the specific formulation and use.	Confirm the required food-contact regulatory basis separately; do not rely on 177.1520 for PC.
Polyamide (PA, nylon)	Good strength and wear resistance; useful where mechanical performance is important.	Many grades absorb moisture, which can affect dimensions and mechanical properties.	V-0 may be available in specific grades. Confirm conditioning, thickness, and the applicable UL classification.	PA is not an olefin polymer under 21 CFR 177.1520. A different applicable authorization may be needed.	Assess moisture conditioning and verify the applicable food-contact regulation for the exact grade.
Polybutylene terephthalate (PBT)	Good dimensional stability and electrical properties; often used for precision components.	Properties and processing behavior vary by grade; drying and mold design can be important.	V-0 options are available for some grades. Verify the exact grade and minimum tested thickness.	PBT is not an olefin polymer under 21 CFR 177.1520. Confirm a separate applicable food-contact authorization if required.	Check supplier processing guidance, UL records, and the regulatory basis for the intended contact conditions.
Acrylonitrile butadiene styrene (ABS)	Good impact performance and surface finish; generally straightforward to mold.	Solvent and weathering resistance may be limited for some grades and applications.	V-0 grades exist, but the classification is grade- and thickness-specific.	ABS is not an olefin polymer under 21 CFR 177.1520. Verify another applicable authorization for food-contact use.	Confirm chemical exposure, outdoor requirements, and the precise UL and regulatory documentation.
Flame-retardant engineering resin grade	Can be selected to meet a defined flammability target while retaining application-specific mechanical properties.	Flame-retardant additives can affect impact strength, flow, color, or other properties; performance is formulation-specific.	Request evidence for UL 94 V-0 at the required thickness and for the exact grade and color where applicable.	Do not infer food-contact suitability from flame performance. Check the exact formulation and applicable FDA authorization separately.	Validate the molded part’s wall thickness, processing conditions, end-use environment, and documentation requirements.

Important: UL 94 V-0 is a material flammability classification under specified test conditions; it is not a blanket certification of every molded part. FDA 21 CFR 177.1520 applies to qualifying olefin polymers subject to the regulation’s composition and conditions-of-use requirements. Verify the exact resin grade, additives, colorants, thickness, and intended use with current supplier documentation.