Precision Grades and Standards for Plastic Injection Molding Parts
Dimensional precision plays a critical role in the performance, assembly, and appearance of plastic injection-molded parts. Different applications demand varying levels of precision, and understanding the relevant standards helps ensure effective material selection, part design, and quality control.
This article introduces the commonly used dimensional tolerance standards in plastic injection molding—including national and international standards—along with specific numerical values for each grade.
1. What Is Dimensional Precision in Plastic Parts?
The precision of an injection-molded plastic part refers to the allowable deviation of its actual dimensions from its nominal (designed) dimensions, typically expressed as dimensional tolerance. The smaller the tolerance, the higher the precision—requiring tighter mold manufacturing, better materials, and more stable processing conditions.
2. Common Standards for Injection Molding Precision
2.1 GB/T 14486-2008 (China National Standard)
This standard applies to both thermoplastic and thermoset molded plastic parts. It classifies dimensional tolerances into 11 grades (IT0 to IT10), with lower numbers indicating higher precision.
📌 Tolerance Values by Precision Grade (Unit: mm)
| Grade | Nominal Dimension Range (mm) | Max Permissible Deviation ± (mm) |
|---|---|---|
| IT3 | 1.0 ~ 3.2 | 0.02 |
| 3.2 ~ 6.3 | 0.03 | |
| 6.3 ~ 10 | 0.04 | |
| IT5 | 1.0 ~ 3.2 | 0.06 |
| 3.2 ~ 6.3 | 0.08 | |
| 6.3 ~ 10 | 0.10 | |
| IT7 | 1.0 ~ 3.2 | 0.12 |
| 3.2 ~ 6.3 | 0.16 | |
| 6.3 ~ 10 | 0.20 | |
| IT9 | 1.0 ~ 3.2 | 0.24 |
| 3.2 ~ 6.3 | 0.32 | |
| 6.3 ~ 10 | 0.40 |
Grades IT0–IT2 are typically used only for high-precision or micro-scale components.
2.2 DIN 16742 (German Standard)
DIN 16742 classifies plastics into material groups based on shrinkage rate, and provides corresponding tolerance values. This standard is widely used in Europe, especially in automotive and industrial sectors.
📌 Material Groups and Shrinkage Rates
| Group | Shrinkage Range | Typical Materials |
|---|---|---|
| Group 1 | ≤ 0.2% | ABS, PC, SAN |
| Group 2 | 0.2%–0.5% | POM, PS, PMMA |
| Group 3 | 0.5%–1.0% | PA6, PP, PA66 |
| Group 4 | 1.0%–2.0% | HDPE, LDPE |
| Group 5 | ≥ 2.0% | Flexible PVC, TPU |
📌 Example Tolerances for Group 3 (Medium Precision)
| Nominal Dimension (mm) | Tolerance ± (mm) |
|---|---|
| 0.5 ~ 1.0 | 0.05 |
| 1.0 ~ 3.0 | 0.10 |
| 3.0 ~ 6.0 | 0.14 |
| 6.0 ~ 10.0 | 0.18 |
| 10.0 ~ 18.0 | 0.22 |
| 18.0 ~ 30.0 | 0.28 |
| 30.0 ~ 50.0 | 0.34 |
High precision tolerances may be reduced by 50%, while loose tolerances may be increased by about 30%.
2.3 ISO 20457:2018 (International Standard)
This ISO standard defines globally accepted tolerances for injection-molded plastic parts. It takes into account nominal size ranges and material shrinkage to guide allowable dimensional deviations.
ISO 20457 is widely adopted in international production and trade, and its tolerance ranges closely align with DIN 16742.
3. Summary Table: General Dimensional Tolerances
| Nominal Size (mm) | High Precision (±mm) | Medium Precision (±mm) | Low Precision (±mm) |
|---|---|---|---|
| 0.5 ~ 1.0 | 0.02 ~ 0.05 | 0.05 ~ 0.10 | 0.10 ~ 0.20 |
| 1.0 ~ 3.0 | 0.03 ~ 0.06 | 0.10 ~ 0.15 | 0.20 ~ 0.30 |
| 3.0 ~ 10 | 0.04 ~ 0.08 | 0.15 ~ 0.25 | 0.30 ~ 0.50 |
| 10 ~ 50 | 0.10 ~ 0.20 | 0.30 ~ 0.50 | 0.50 ~ 1.00 |
4. Key Factors Affecting Precision in Injection Molding
- Mold Manufacturing Accuracy
The accuracy of the mold cavity directly determines the dimensional outcome of the molded part, especially for features like holes and fit interfaces. - Material Shrinkage Behavior
Different plastics have different shrinkage rates, and this must be considered in both mold design and tolerance assignment. - Processing Parameters
Injection speed, mold temperature, holding pressure, and cooling time can all significantly affect part dimensions and warping. - Part Design
Uneven wall thickness, lack of fillets or ribs, and asymmetrical geometry can cause deformation and tolerance deviation.
5. How to Choose the Right Tolerance Grade
- Evaluate the part’s function: Does it involve mechanical fit, motion, or tight assembly?
- Analyze material shrinkage and flow behavior
- Refer to applicable standards: GB/T, DIN, or ISO
- Confirm mold capabilities and injection process stability
- Consult with molders during DFM (Design for Manufacturing)
Conclusion: Precision Should Be “Appropriate,” Not “Excessive”
Unnecessarily tight tolerances increase mold manufacturing costs, reduce yield rates, and complicate production. Always aim to select a tolerance grade that satisfies function while minimizing cost and manufacturing risk.
If you need a tolerance standard template, customized part design consultation, or injection molding support, feel free to contact our engineering team—we’re ready to help.
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