Designing for dimensional accuracy and consistent performance
Injection moulding is a highly repeatable manufacturing process, but plastic materials behave differently to metals or other rigid materials. Two critical factors that design engineers must consider are shrinkage and tolerances. These directly affect the dimensional accuracy of a moulded part and can have significant implications for assembly, function, and overall product performance.
At CJ Tool & Mouldings, we work with product designers and engineers to anticipate shrinkage and define achievable tolerances early in the design process, ensuring parts are manufactured efficiently and to specification.
Understanding Shrinkage
All plastics shrink to some degree as they cool from molten to solid state inside the mould. Shrinkage rates vary depending on the polymer, the part geometry, and processing conditions. If not properly accounted for in the design and tooling stages, shrinkage can lead to dimensional inaccuracies, warpage, and poor fit in assemblies.
Key factors influencing shrinkage:
- Material type: Semi-crystalline polymers (e.g. Nylon, PP) generally shrink more than amorphous polymers (e.g. ABS, PC).
- Wall thickness: Thicker sections cool more slowly, resulting in greater shrinkage.
- Moulding conditions: Cooling rate, pressure, and temperature all influence final shrinkage.
- Part geometry: Uneven wall sections or poorly designed ribs can cause differential shrinkage, leading to warpage.
At CJ, we use our knowledge of polymer behaviour and mould flow analysis to predict shrinkage and adapt tool design accordingly.
Designing with Tolerances
Tolerances specify the allowable variation in a part’s dimensions. In injection moulding, tolerances need to balance functionality, manufacturability, and cost. Overly tight tolerances can drive up tooling and production costs unnecessarily, while loose tolerances may compromise part performance.
Design considerations for tolerances:
- Fit & Function: Define tolerances based on how the part will interact with mating components.
- Material behaviour: Account for the expected shrinkage of the chosen polymer when setting tolerances.
- Tooling precision: Modern CNC and EDM machining allows for very precise tools, but tolerances must reflect the realities of the moulding process.
Typical achievable tolerances in injection moulding range from ±0.05 mm to ±0.2 mm, depending on part size, geometry, and material. Our team works closely with customers to ensure tolerance specifications are realistic and achievable.
Balancing Accuracy and Efficiency
It’s important to remember that tighter tolerances are not always better. They can:
- Increase cycle times due to slower cooling requirements.
- Drive up costs due to more complex tooling and higher rejection rates.
- Limit material choice if only certain polymers can achieve the required tolerance.
By identifying critical dimensions that require tight control and relaxing non-critical dimensions, engineers can reduce costs without sacrificing quality.
Partnering with CJ on Dimensional Control
Tolerance and shrinkage are part of the fine balance between design intent and manufacturing reality. With years of experience across a wide range of polymers, CJ Tool & Mouldings ensures shrinkage is accounted for and tolerances are specified at the right level for both function and production efficiency.
From tool design through to production, we provide the expertise to ensure your moulded parts fit, function, and perform as intended.
