Introduction to tight tolerances and plastic parts
Every engineer eventually asks the same question: “How tight can we hold this?” Tightest tolerances achievable in injection molding can reach impressive precision, but tolerance is never just about the mold. It’s the combined result of material behavior, tooling accuracy, process control, and design geometry.
In other words — you don’t hold a tolerance; you design for one.
Typical Tolerance Ranges by Feature Type
| Feature Type | Safe Production Tolerance | Tight-Tolerance Capability |
|---|---|---|
| General Dimensions | ±0.05 – ±0.10 mm | ±0.025 mm |
| Critical Diameters / Fits | ±0.025 – ±0.05 mm | ±0.013 mm |
| Micro Features (<0.5 mm) | ±0.02 mm | ±0.01 mm |
| Flatness / Parallelism | 0.05 mm per 25 mm | 0.02 mm per 25 mm |
| Wall Thickness | ±10% of nominal | ±5% |
These represent process-capable numbers under controlled conditions — clean tooling, stable temperature, consistent resin moisture, and calibrated machines.
What Drives Tolerance Capability
Material Shrinkage
Different resins shrink anywhere from 0.1% (LCP) to 2.5% (PP). Controlling shrink variation through tool steel compensation and consistent cooling is critical.
Tooling Precision
Mold manufacturing tolerances can go as tight as ±0.005 mm, but that only matters if the tool steel stays thermally stable. Micro-mold cavities often require temperature-controlled inserts to maintain precision.
Part Geometry
Uniform wall thickness, minimized rib intersections, and short flow paths all reduce internal stress and dimensional drift.
Process Control
Holding a tolerance once is easy; holding it for 100,000 cycles takes:
- Stable melt and mold temperature
- Consistent fill time and pack pressure
- Regular tool maintenance
Resin Performance vs. Tolerance Stability
| Material | Dimensional Stability | Shrink Variation | Notes |
|---|---|---|---|
| LCP | Excellent | ±0.05% | Best for micro molding; near-zero warp. |
| PEEK / PPSU | Very good | ±0.1% | High stiffness; good for load-bearing parts. |
| PC / ABS | Moderate | ±0.3% | Sensitive to cooling and flow orientation. |
| PP / PE | Fair | ±0.5% | High shrink; use only where flexibility offsets tolerance needs. |
| TPE / TPU | Low | ±0.5%+ | Not dimensionally stable; rely on functional fits. |
LCP consistently sets the upper bound for injection-molded precision, potentially capable of ±0.005 mm features in controlled micro-molding environments.
Realistic Precision by Scale
| Part Size | Typical Process Capability (Cpk ≥ 1.33) |
|---|---|
| <25 mm | ±0.01 – ±0.025 mm |
| 25–100 mm | ±0.025 – ±0.05 mm |
| >100 mm | ±0.05 – ±0.1 mm |
Smaller parts don’t inherently mean tighter tolerances — they just magnify variation. The tighter you go, the more you depend on tool precision and process repeatability.
When “Tight” Becomes Too Tight
Designers often specify tolerances just because. Here’s what happens when you over constrain:
- The molder chases variation with pressure and temperature.
- Yields drop.
- Parts pass CMM on Day 1 and fail on Day 30 due to humidity or regrind shift.
Tolerance without DFM insight creates false confidence.
Need help reaching the tightest tolerances possible in injection molding?
Designing to ±0.01 mm is one thing. Holding it for a million cycles is another.
We can help you bridge the gap between ideal CAD and real-world consistency.