Last month we handled a typical case: a client’s automotive interior parts frequently cracked brittlely at a low temperature of -15°C in winter, with a return rate as high as 15%. Upon inspection, we found that general-purpose PP was used instead of toughened modified PP, resulting in insufficient low-temperature impact resistance. In addition, the mold was not compensated for PP’s high shrinkage rate, which caused stress concentration and cracking during assembly.
This is far from an isolated incident. We have encountered numerous defects on injection molded parts, including sink marks, weld lines and dimensional deviations. Almost all root causes boil down to two key issues: improper material selection and mold design mismatched with material properties. For instance, one client mistakenly used ABS in place of PP, which directly reduced product strength by 40%. This is a typical chain of problems caused by disconnecting material selection from mold design.
Step 1: Select the Right Material
No matter how refined the subsequent molding process is, it will be futile if the wrong material is chosen. We have seen clients use ABS for high-temperature resistant components where PA66 was actually required, and ordinary PE for medical-grade products that should adopt medical-grade PE. These mistakes either lead to substandard performance or doubled costs.
Why is material selection so critical?
Core parameters of plastics, such as shrinkage rate, fluidity, heat resistance and mechanical properties, directly determine whether products can meet application requirements. For example, PP features excellent fluidity yet a high shrinkage rate of 1.5-2.5%, while ABS delivers high strength but relatively poor fluidity. Wrong material selection will result in:
- Performance failure (e.g. brittle cracking at low temperatures, deformation under high heat)
- Processing difficulties (incomplete filling, burn marks caused by trapped air)
- Cost waste (premium materials used in inappropriate scenarios)
Five-Step Material Selection Framework
We have long adopted this set of criteria to help clients eliminate risks at the source:
- Clarify requirements: List core product indicators, including temperature resistance range, load capacity, service environment and surface appearance grade.
- Screen candidate materials: Match materials against requirements (e.g. ABS/PC for medical applications, PA66/PPS for high-temperature scenarios).
- Conduct performance tests: Verify key indicators such as impact strength and heat deflection temperature. Conduct high-low temperature cycle tests when necessary.
- Carry out prototype validation: Produce prototypes via 3D printing or small-batch trial production to simulate actual service conditions.
- Optimize costs: On the premise of meeting performance standards, prioritize cost-effective alternatives (e.g. use HIPS instead of ABS for non-load-bearing housings).
Step 2: Develop Molds According to Material Properties
After selecting the proper material, the injection mold must be tailored to its characteristics. Plastics vary greatly in shrinkage rate, fluidity and crystallinity. Using identical mold parameters for both PP and ABS will inevitably cause severe sink marks on one and out-of-tolerance dimensions on the other.
Key Mold Adjustments & Specifications
- Shrinkage compensation: Precise cavity dimension calculation
Plastics shrink after cooling. The mold cavity dimension is calculated by the formula: Cavity size = Product size × (1 + Shrinkage rate).
Crystalline plastics such as PP and PE have high shrinkage rates, which are highly sensitive to mold temperature — higher mold temperature leads to higher crystallinity and greater shrinkage. Amorphous plastics like ABS and PC feature low and stable shrinkage rates.
- PP: Shrinkage rate 1.5-2.5%. Set cavity compensation at 1.8% (median value); maintain mold temperature at 50-70°C to stabilize shrinkage.
- ABS: Shrinkage rate 0.5-0.8%. Set cavity compensation at 0.6%; maintain mold temperature at 60-80°C to enhance surface gloss.
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Gate design: Adapt to material fluidity
Materials with good fluidity (PP, PE) allow smaller gate sizes (e.g. pin-point gate with 3mm diameter). Materials with poor fluidity (ABS, PC) require enlarged gates (e.g. fan gate with 8mm width), otherwise incomplete filling or excessive pressure loss will occur. For PP, small gates are acceptable, but sufficient holding pressure is required to compensate for shrinkage and avoid sink marks. For ABS, adopt larger gates or multiple gates to balance filling and reduce weld lines.
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Cooling system: Control crystallinity and internal stress
- Crystalline plastics (PP): Ensure uniform cooling. Keep the spacing between cooling channels ≤ 50mm to prevent uneven shrinkage caused by local temperature differences.
- Amorphous plastics (ABS): Adopt rapid cooling for shaping to avoid warpage. Place cooling channels 15-20mm away from the cavity surface.
- Mold steel selection: Adjust according to material abrasiveness and corrosiveness
- Glass-filled materials (e.g. PA66 + 30% glass fiber) cause severe cavity abrasion. Use quenched S136 steel with hardness above HRC50.
- General PP and ABS can use pre-hardened 718H steel (HRC30-35) to balance cost and service life.
Practical Example for ABS
- Material side: Select high-flow grade ABS (e.g. PA-757) to improve filling performance.
- Mold side: Add vent grooves (0.03mm depth, 5mm width) to eliminate weld lines; arrange gates away from visible surfaces.
Conclusion
There are no shortcuts to manufacturing qualified injection molded parts. First select materials accurately, then customize molds based on material properties. Material determines manufacturability, while mold determines finished quality. The combination of the two can prevent over 90% of common defects from the very start. Our team has worked on more than 3,000 sets of molds. We deeply understand that injection molding is not simply manufacturing per drawings, but a collaborative engineering of materials and molds.
