Silicone part tolerance: process limits and DFM rules
Silicone is dimensionally stable, but it is not a precision plastic. The achievable production tolerance depends on the molding process (LSR injection, HCR transfer, HTV compression), the cavity layout, the cure profile and the part geometry. This page lists what each process can hold and the ten design rules that keep a tight-tolerance silicone part inside spec.
What the tolerance numbers actually mean
The numbers you see on a silicone part drawing (e.g. "±0.10 mm") are usually the production tolerance — the spread a controlled process can hold across cavity-to-cavity and shot-to-shot variation. It is not the same as the measurement uncertainty of the dimensional inspection, and it is not the same as the mold tolerance (which is usually tighter, e.g. ±0.02 mm, because the steel does not move between shots).
For a tight-tolerance silicone part, the design intent is to drive process variation down to where it is comparable to the inspection uncertainty. The two levers are:
Process control. Cold-runner LSR injection with a balanced cavity layout, a heated tool held within ±5 °C of the setpoint and a flash-free parting line can hold ±0.05 mm on critical features. HCR transfer molding with a manual preform and an open runner system sits at ±0.15 mm. HTV compression molding with hand-loaded blanks and a flash groove sits at ±0.25 mm.
Geometry. Thin walls, long aspect ratios and sharp corners magnify variation. A 0.8 mm wall will move ±0.05 mm under cure shrinkage; a 4 mm wall will move ±0.02 mm. Tighten the part, not the process — the process is already at its ceiling.
Tolerance by process
LSR injection (cold runner, balanced cavity): ±0.05 mm on features up to 50 mm; ±0.10 mm on features up to 200 mm. Multi-cavity tools that share the runner with first-class thermal balance hit ±0.08 mm across all cavities. Tightest on the medical / baby-care lines, where the cost of the tool is amortised over high annual volume.
HCR transfer (preform + closed runner): ±0.10 mm on features up to 50 mm; ±0.20 mm on features up to 200 mm. A second-tier option for parts that need tighter than HTV but cannot justify an LSR cold-runner tool.
HTV compression (hand-loaded blank + flash groove): ±0.20 mm on features up to 50 mm; ±0.30 mm on features up to 200 mm. Used for low-volume, large or thick-walled parts where tool cost dominates the unit price.
Ten DFM rules for tight-tolerance silicone parts
| # | Rule | What it prevents |
|---|---|---|
| 1 | Wall thickness between 1.0 mm and 3.0 mm | Incomplete fill (thin) or voids / sink (thick) |
| 2 | Wall thickness variation ≤ 25 % across the part | Differential cure, warping |
| 3 | Draft angle ≥ 1° on every surface in the mold draw direction | Part sticking in the cavity, mould damage at ejection |
| 4 | No sharp internal corners; radius ≥ 0.5 mm | Stress concentration, tear initiation |
| 5 | Parting line on a non-critical, non-sealing surface | Visible flash, secondary deflashing operation |
| 6 | Undercuts only on flexible materials (Shore A < 50); otherwise split-cavity tool | Side-action cost, mould wear |
| 7 | Self-mating surfaces texturing allowance ≥ 0.05 mm | Surface-to-surface sealing after cure shrinkage |
| 8 | Tight tolerance features on a single cavity of a multi-cavity tool | Cavity-to-cavity variation that the inspector will catch |
| 9 | Gate location on the thickest section of the part | Weld-line voids, air trap on thin walls |
| 10 | Specify critical dimensions per ISO 3302-1 Class E2 (silicone) or tighter; never borrow a metal or plastic tolerance class | Tolerance conflict between drawing and process capability |
What we provide in the DFM memo
Every quote comes with a DFM memo within 48 hours. The memo flags undercut locations, draft angles, shrinkage compensation and which dimensions will drive cost. We also include our recommended tolerance per critical feature based on the selected process — so the drawing and the production reality match on day one, not after the first 1,000 pieces have been scrapped.
If you have a drawing with a tolerance tighter than what the process can hold, we will say so and propose either a process change (LSR instead of HCR), a geometry change (move a critical dimension onto a thicker section) or a different inspection strategy (100 % gauge check vs. AQL sampling).
FAQ — silicone part tolerance
What is the tightest tolerance LSR can hold?
±0.05 mm on a single-cavity tool with critical feature under 50 mm and a balanced cold-runner system. For multi-cavity tools, the cavity-to-cavity variation adds ±0.02–0.05 mm. Tighter than this requires an LSR tool designed for the specific part family.
Why does HTV compression need a looser tolerance?
Hand-loaded blanks vary in weight by 2–5 %, the flash groove lets material escape unevenly, and the cure profile is open-loop in compression presses. The variation is intrinsic to the process, not a tool quality issue.
Can silicone parts hold tighter tolerance with secondary machining?
Not really. Silicone is too soft to machine without distorting the cut surface (it smears, it does not shear). The achievable tolerance is set at molding, not at post-processing.
How do I pick between specifying tolerance per ISO 3302-1 Class E1 vs. E2 vs. E3?
E1 is the tightest, E3 the loosest. Most medical and electronic silicone parts sit at E2. Food-contact and industrial seals sit at E3. Specifying a tighter class than the process can hold guarantees a per-piece rejection at incoming inspection.