Understanding achievable tolerances in injection molding is critical for designing parts that are both functional and cost-effective. Specifying tolerances tighter than necessary increases tooling cost, cycle time, and scrap rate without adding value to the part.
The most widely used tolerance standards for injection molded parts are:
| Dimension | Standard Tolerance (mm) | Precision Tolerance (mm) |
|---|---|---|
| Up to 10mm | ±0.15 | ±0.05 |
| 10-30mm | ±0.20 | ±0.08 |
| 30-60mm | ±0.30 | ±0.10 |
| 60-120mm | ±0.50 | ±0.15 |
| 120-300mm | ±0.80 | ±0.25 |
| 300-500mm | ±1.20 | ±0.40 |
Every plastic material shrinks differently during cooling. Semi-crystalline materials (PA, POM, PP) shrink 1.5-3%, while amorphous materials (ABS, PC, PS) shrink 0.3-0.8%. Glass-filled materials shrink less but more anisotropically. The mold must be designed with the correct shrinkage factor for the specific material.
Thin walls cool faster and shrink less than thick walls. Parts with uniform wall thickness achieve tighter tolerances than parts with varying thicknesses. Features like ribs, bosses, and snap-fits require careful gate placement to ensure consistent packing and shrinkage.
Number of cavities, gate location, runner system, and cooling channel design all affect dimensional consistency. Multi-cavity molds must be carefully balanced to achieve the same tolerances across all cavities.
Consistent process parameters (melt temperature, injection speed, pack pressure, cooling time) are essential for maintaining tolerances across production runs. Scientific molding with documented process windows produces the most repeatable results.
Flatness is one of the most challenging tolerances to control in injection molding. General guidelines:
Tightening tolerances from standard to precision typically increases mold cost by 30-50% and can increase cycle time by 10-20%. For every halving of the tolerance range, manufacturing cost approximately doubles. Always specify the loosest tolerance that satisfies functional requirements.