Views: 0 Author: Site Editor Publish Time: 2025-09-08 Origin: Site
The most fundamental principle in injection molding tolerances is that the larger the part, the larger the permissible tolerance. This is primarily due to material shrinkage—a phenomenon that becomes more challenging to control uniformly over larger distances.
The following table serves as a general reference guide. It is an excellent starting point for design reviews and initial discussions with your molding partner.
| Nominal Size (mm) | General Tolerance (± mm) | Precision Tolerance (± mm) | Remarks |
|---|---|---|---|
| < 3 | 0.05 - 0.08 | 0.02 - 0.03 | Small features (thin walls, pins, small holes) |
| 3 ~ 10 | 0.08 - 0.10 | 0.03 - 0.04 | Common range for small components |
| 10 ~ 30 | 0.10 - 0.15 | 0.04 - 0.06 | The most common size range |
| 30 ~ 100 | 0.15 - 0.25 | 0.06 - 0.08 | Medium-sized parts |
| 100 ~ 200 | 0.25 - 0.40 | 0.08 - 0.12 | Large parts; shrinkage is a key factor |
| 200 ~ 500 | 0.40 - 0.70 | 0.12 - 0.20 | Shrinkage and mold accuracy are main challenges |
| > 500 | 0.5% - 0.7% of size | 0.2% - 0.3% of size | Very large sizes; %-based calculation is used |
A table is a good start, but intelligent tolerance specification requires understanding the variables at play:
Material Selection (The Biggest Driver):
Crystalline polymers (like PP, PA) shrink significantly more and are less predictable than amorphous polymers (like ABS, PC). Holding a precision tolerance in polypropylene is far more difficult than in ABS.
Always consult the material's datasheet for its specific shrinkage range.
Part and Mold Design:
Wall Thickness Uniformity: Variations cause differential cooling and shrinkage, leading to warpage and missed tolerances.
Gate Location & Type: Determines how material flows and packs into the cavity, directly affecting shrinkage.
Cooling System Design: Uniform cooling is essential for maintaining dimensional stability, especially on large parts.
Injection Molding Process:
Holding Pressure and Time: This is the most critical process parameter for controlling shrinkage. Insufficient hold pressure leads to excessive part shrinkage.
Cooling Time & Temperature: Must be strictly controlled to ensure consistency from shot to shot.
Use General Tolerances for non-critical cosmetic parts, consumer goods, and internal components where fit and function are not highly sensitive to minor variations. This is the most cost-effective approach.
Specify Precision Tolerances only for critical features: interlocking components, parts with mechanical gears, optical elements, and medical devices. Be prepared for a longer process optimization time and a higher mold and part cost.
Define Critical Dimensions: Not every dimension on a drawing is critical. Clearly identify and call out only the critical-to-function (CTF) dimensions with their specific tolerance. Use a general note like "UNSPECIFIED TOLERANCES PER ISO 20457-GENERAL" for all others.
Communicate Early: Engage with your injection molding partner during the design phase (DFM). Their expertise is invaluable in determining what is manufacturable.
Consider Material First: Choose a material with low and predictable shrinkage if dimensional accuracy is a priority.
Prototype and Validate: Use initial sampling (T1 trials) to measure actual part dimensions and refine the process before full-scale production.
Specifying the correct tolerances is a balance between design necessity, manufacturability, and cost. The reference table provided here offers a solid foundation for setting realistic expectations. Remember, achieving tight tolerances is a collaborative effort between a thoughtful design and a highly controlled manufacturing process.
Always partner with an experienced molder who can guide you through this complex but crucial aspect of part design.
