Achievable Tolerance Bands in Precision Plastic Injection Molding
How to read this sheet: These are typical achievable ranges for production parts under controlled conditions, based on our experience with precision molds and high-precision machining. They are reference values only, not a guarantee. Every project requires a DFM review against your actual drawing, material, tool layout, and measurement method. Final tolerances are agreed in writing before tooling starts.
A. By Material
| Material | Typical Shrinkage | Standard Tolerance | High-Precision (with compensation & process control) | Notes |
|---|---|---|---|---|
| ABS | 0.4–0.7% | ±0.10mm | ±0.02mm | Stable, low shrink; good for tight work |
| PC | 0.5–0.7% | ±0.10mm | ±0.02mm | Low shrink, but sensitive to moisture — must be dried |
| POM (Acetal) | 1.8–2.5% | ±0.13mm | ±0.02mm | Excellent dimensional stability after stabilization; widely used for precision pins/gears |
| PBT / PET (glass-filled) | 0.4–0.8% | ±0.10mm | ±0.015mm | Filled grades reduce shrink variation; watch anisotropy |
| PPS (glass-filled) | 0.2–0.6% | ±0.10mm | ±0.015mm | Very low shrink; preferred for connector housings |
| PEEK | 1.0–1.5% | ±0.15mm | ±0.02mm | High-temp process; requires experienced molding |
| PA6 / PA66 (unfilled) | 0.7–1.5% | ±0.13mm | ±0.03mm | Moisture uptake shifts dimensions after molding — must specify conditioning state |
| PA + GF30 | 0.2–0.8% | ±0.10mm | ±0.02mm | Better than unfilled nylon, but flow-direction shrink differs from cross-flow |
| PP / PE | 1.0–2.5% | ±0.15mm | ±0.03mm | High, variable shrink — not recommended below ±0.02mm |
| TPE / TPU | 1.0–3.0% | ±0.20mm | ±0.05mm | Soft, elastic — measured dimension depends on contact force |
| LCP | 0.1–0.3% | ±0.08mm | ±0.01mm | Lowest shrink of all; best candidate for ultra-tight connector parts |
| PMMA | 0.2–0.8% | ±0.10mm | ±0.02mm | Optical grade; also used for light-guide features |
B. By Feature Size (nominal dimension)
| Nominal Size | Realistic Achievable Band | Comment |
|---|---|---|
| < 1 mm | ±0.005 – ±0.01mm | Small features can hold tight absolute tolerances; limited by mold insert strength & EDM corner radius |
| 1 – 5 mm | ±0.005 – ±0.01mm | The practical zone for “±0.005mm-class” work: pins, bores, sealing lands, insert pockets |
| 5 – 30 mm | ±0.01 – ±0.02mm | The sweet spot for most precision connector & sensor parts |
| 30 – 100 mm | ±0.02 – ±0.05mm | Cumulative shrink variation dominates; tolerance the critical local feature, not the envelope |
| > 100 mm | ±0.05 – ±0.10mm | Overall length/width held to commercial class; locate critical features to datums within the part |
Rule of thumb: the larger the dimension, the wider the band. If you need ±0.005mm on an 80mm overall length, the realistic answer is to redesign so that the 80mm is a non-critical envelope and the mating interface is a small, locally toleranced feature.
C. By Feature Type
| Feature Type | Typical Capability | Why |
|---|---|---|
| Dimension contained within one mold half (no parting line) | Best | Not affected by parting-line mismatch or clamp tonnage variation |
| Bore / hole diameter | Good | Core pin is a single hardened insert; stable and easy to compensate |
| Pin / boss OD | Good | Same as above |
| Distance between two cores (same half) | Good | Both features move together with temperature |
| Dimension spanning the parting line | Reduced | Affected by parting-line wear, flash, clamp force, mold deflection |
| Wall thickness | Moderate | Depends on packing; better controlled by process than by tool |
| Flatness / straightness of a large face | Moderate–poor | Warpage-driven; control by rib design, gate location, and fixturing, not by tightening the number |
| True position of a hole pattern | Good with GD&T | Positional tolerance is often easier to hold than size tolerance when controlled properly |
| Thread (molded) | Moderate | Pitch diameter typically ±0.05mm; use thread ring gauge, not CMM |
| Undercut / lifter-driven feature | Reduced | Lifter/slide repeatability adds variation |
| Thin wall (<0.5mm) | Reduced | Flow and warpage dominate |
| Texture / grain-covered surface | Reduced | Texture depth masks the nominal surface |
D. Mold Steel Capability vs. Injection Molded Plastic Part Capability
| Item | Our Capability | How Verified |
|---|---|---|
| CNC cavity/core machining | Sub-0.005mm on critical features | MIKRON HEM 1000, MIKRON MILL P 900 high-speed machining center; single-setup finishing; thermal-stable spindle design |
| Fine-finish EDM (sinker) | Mirror finish Ra 0.1–0.4μm; corner radii at single-digit micron level | Makino EDGE3i; rough → semi → finish passes with electrode wear compensation |
| Mold assembly & fitting | Parting-line alignment, interlocks lapped, guide pins matched | In-house toolroom |
| Process control | Melt temp ±2°C, injection pressure consistency ±1%, cooling time ±0.5s, cavity temp uniformity ±2°C | Documented per job |
| Dimensional verification (in-house) | CMM MPE in the 0.005mm range | Hexagon DEA CROMA bridge-type CNC CMM, calibrated, temperature-compensated |
| Verification for ultra-tight features | Third-party high-accuracy CMM / functional gauges / calibrated masters | Arranged per customer requirement and stated in the inspection plan |
Important: Because our in-house CMM’s uncertainty is a significant fraction of a ±0.005mm band, we do not claim to prove ±0.005mm on a plastic part with our own equipment alone. For features where customers require verified sub-0.01mm data, we agree the measurement method up front and, where needed, supply third-party metrology reports. We’d rather tell you this now than argue about it at FAI.
E. Conditions That Must Be Met
Tolerances in this sheet assume:
- Measurement temperature stated and controlled — plastics expand 5–10× more than steel per °C. We record ambient temperature on all critical reports.
- Parts stabilized before measurement — typically 24–48 hours after ejection; conditioning time agreed with the customer.
- Material lot consistent — shrinkage compensation is tuned to the actual resin, not a generic table.
- Cpk ≥ 1.33 target on critical dimensions over a sustained run (Cpk ≥ 1.67 for automotive/medical).
- GD&T applied correctly — size, form, orientation, and location all controlled; datum scheme defined.
- No post-molding secondary operations that distort the part unless specified.
If any of these are missing, the numbers in this sheet do not apply.
F. What To Do Next
Send us your drawing. We’ll mark each critical dimension as:
✅ Achievable — standard capability for the material/feature
⚠️ Achievable with compensation — requires mold-flow analysis, adjusted shrinkage compensation, and Cpk validation
❌ Not recommended — we’ll propose an alternative (local feature tolerancing, material change, or GD&T revision) that gives you the same fit at lower cost
Quotation & Report Delivered Within 2 Working Days, No Charge, No Obligation.
FAQ
What tolerance can you hold on plastic parts?
It depends on feature size, material, and tool layout — not a single number. Small features under 5mm (pins, bores, sealing lands) in low-shrink resins like PC, POM, PPS or LCP can be held to ±0.005–0.01mm with compensation and process control.
5–30mm dimensions typically run ±0.01–0.02mm;
30–100mm ±0.02–0.05mm;
above 100mm we hold the envelope to commercial class and datum critical features internally.
High-shrink or elastic materials (PP, PE, TPE/TPU) are not recommended below ±0.02mm. Dimensions crossing the parting line, driven by slides/lifters, or large-face flatness are downgraded. Mold steel can hold sub-0.005mm; the part is what drifts.
Do you provide CMM reports and Cpk data?
Yes. In-house inspection uses a calibrated, temperature-compensated bridge-type CNC CMM (Hexagon DEA CROMA). For sub-0.01mm features on plastic parts our machine’s uncertainty is a significant fraction of the tolerance band, so we agree the measurement method up front and arrange third-party metrology or functional gauges per the inspection plan.
Cpk data available on request for critical dimensions. Reports include ambient temperature and conditioning state.