Silicone Extrusion Tolerances: BS 3734 & ISO 3302-1 Explained
A practical engineer's guide to dimensional tolerances for extruded silicone rubber: what the E1, E2 and E3 tolerance classes mean, the physics behind why rubber can't be held to machined-metal limits, and how to specify tolerances on your drawing so you get the part you need at the price you expect.
Why Extruded Rubber Has Wider Tolerances Than Machined Metal
Engineers coming from a machining background often draw a silicone profile with ±0.05mm limits, because that's a routine tolerance for turned or milled metal. For extruded rubber, it isn't, and the reasons are physical, not a matter of manufacturing effort. Understanding them is the fastest way to write a drawing an extruder can actually quote.
Die swell. Uncured silicone is a viscoelastic material. As it leaves the extrusion die, the polymer relaxes and the extrudate expands, so the profile leaving the die is not the same shape or size as the die orifice. The amount of swell varies with compound rheology, hardness, extrusion speed and profile geometry, which is why extrusion dies are cut deliberately undersized and then iterated against measured samples rather than machined to the finished dimension.
Cure shrinkage. Silicone crosslinks in a hot-air vulcanising oven immediately after extrusion, and as it cures and post-cures it shrinks, typically 2–4% depending on the compound. On a 25mm dimension that is 0.5–1.0mm of movement that has to be predicted and compensated in the tooling. Shrinkage also varies slightly batch to batch with compound lot and cure conditions.
Thermal expansion. Rubber's coefficient of thermal expansion is roughly ten times that of steel. A profile measured warm off the line will read differently from the same profile at the standard 23°C laboratory reference temperature, which is another reason dimensions are verified after conditioning, not at the die.
The material is soft. Finally, measuring silicone is not like measuring steel. Callipers compress the part; the act of measurement changes the answer. Standards therefore assume unstressed measurement at 23°C with controlled contact force, and tolerance bands are set wide enough to be meaningful for a material that deflects under a fingertip.
None of this means silicone extrusion is imprecise. It means precision is achieved statistically, through tooling iteration and process control, and expressed through standardised tolerance classes rather than arbitrary tight limits. That's what BS 3734-1 and ISO 3302-1 exist to do.
BS 3734-1 / ISO 3302-1 Tolerance Classes for Extrusions
Both standards define the same class system for the cross-sectional dimensions of rubber extrudates. Rather than inventing limits per drawing, you pick a class, and the tolerance for each dimension follows from its size.
Class E1: precision. The tightest class routinely applied to extrusions. It demands refined tooling, tighter process control and more frequent dimensional inspection, so it costs more and should be reserved for dimensions that genuinely need it: sealing faces, interference fits, mating grooves. The exception is simple round sections, which run so repeatably that tubing is held to E1 as standard at Viking Extrusions.
Class E2: general purpose. The standard commercial class for regular solid sections: strips and cords are held to E2 as standard at Viking Extrusions. For the overwhelming majority of seals, gaskets and trims, E2 tolerances are comfortably tight enough for the part to function.
Class E3: coarse. Wider limits for complex bespoke profiles (the house standard for profiles at Viking Extrusions), non-critical dimensions, decorative trims, very soft compounds and sponge extrusions, where the economics of chasing tight limits make no sense.
The tolerance for any dimension depends on which size band it falls into: a 2mm wall and a 30mm overall width on the same profile carry different absolute tolerances, even within the same class:
| Cross-Sectional Dimension (mm) | Class E1 (Precision) | Class E2 (General) | Class E3 (Coarse) |
|---|---|---|---|
| 0 – 1.5 | ±0.15 | ±0.25 | ±0.4 |
| 1.5 – 2.5 | ±0.20 | ±0.35 | ±0.5 |
| 2.5 – 4.0 | ±0.25 | ±0.4 | ±0.7 |
| 4.0 – 6.3 | ±0.35 | ±0.5 | ±0.8 |
| 6.3 – 10 | ±0.4 | ±0.7 | ±1.0 |
| 10 – 16 | ±0.5 | ±0.8 | ±1.3 |
| 16 – 25 | ±0.7 | ±1.0 | ±1.6 |
| 25 – 40 | ±0.8 | ±1.3 | ±2.0 |
| 40 – 63 | ±1.0 | ±1.6 | ±2.5 |
Values summarised from the ISO 3302-1 Class E tables for extrudates (cross-sectional dimensions); always confirm against the current edition of the standard for contractual tolerances.
Reading the table is straightforward: a hollow D-section 12mm high with a 1.8mm wall, specified to Class E2, carries ±0.8mm on the height and ±0.35mm on the wall. Note how the E1 band at any size is roughly half the E3 band. That factor of two is the real-world difference between precision and coarse extrusion work.
Length Tolerances: The Class L System
Cross-section is only half the drawing. When extrusions are supplied cut to length rather than coiled, ISO 3302-1 / BS 3734-1 applies a separate set of classes, L1, L2 and L3, to the cut length, on the same principle as the E classes: the longer the piece, the wider the absolute band, and the tighter the class, the tighter the limits. Class L3 is Viking Extrusions' house standard for cut lengths, unless otherwise specified:
| Cut Length (mm) | Class L3 Tolerance | Cut Length (mm) | Class L3 Tolerance |
|---|---|---|---|
| 0 – 40 | ±1.6 | 400 – 630 | ±6.3 |
| 40 – 63 | ±2.0 | 630 – 1000 | ±10.0 |
| 63 – 100 | ±2.5 | 1000 – 1600 | ±12.5 |
| 100 – 160 | ±3.2 | 1600 – 2500 | ±16.0 |
| 160 – 250 | ±4.0 | 2500 – 4000 | ±20.0 |
| 250 – 400 | ±5.0 | Above 4000 | ±0.5% |
Length tolerance behaves differently from cross-section tolerance because rubber lengths are governed by tension and relaxation, not tooling. Silicone stretches slightly as it is handled and cut, then recovers, so a length measured immediately after cutting can differ from the same piece measured relaxed on a bench. Good practice (ours included) is to cut with minimal line tension and verify lengths on relaxed, conditioned parts.
Practical guidance: for short cut pieces (up to a few hundred millimetres), tolerances of ±1–2mm are routine; for metre-plus lengths expect the band to grow with length per the L-class tables. If your assembly needs a seal to finish flush at both ends of a fixed aperture, tell us. It's often better solved with a joined ring or an on-fixture trim than an extreme length tolerance.
Large Bore Tubing: Working Tolerances
Thin-walled tube at large diameters behaves differently from any other extrusion: the bore is large, the wall is not, and the two need separate treatment. Our quality manual applies dedicated working tolerances to large bore tubing from 30mm to 100mm ID.
| Tube ID | Tolerance on ID | 1mm Wall | 2mm Wall | 3mm Wall | 4mm Wall | >4mm Wall |
|---|---|---|---|---|---|---|
| 30mm | ±0.80 | ±0.15 | ±0.20 | ±0.25 | ±0.35 | Class E1 |
| 35mm | ±0.80 | ±0.30 | ±0.25 | ±0.25 | ±0.35 | Class E1 |
| 40mm | ±0.80 | ±0.35 | ±0.25 | ±0.25 | ±0.35 | Class E1 |
| 45mm | ±1.00 | ±0.35 | ±0.25 | ±0.25 | ±0.35 | Class E1 |
| 50mm | ±1.00 | ±0.40 | ±0.25 | ±0.30 | ±0.40 | Class E2 |
| 55mm | ±1.00 | ±0.40 | ±0.25 | ±0.30 | ±0.40 | Class E2 |
| 60mm | ±1.00 | ±0.40 | ±0.30 | ±0.35 | ±0.40 | Class E2 |
| 65mm | ±1.30 | N/A | ±0.30 | ±0.35 | ±0.40 | Class E2 |
| 70mm | ±1.60 | N/A | ±0.35 | ±0.40 | ±0.50 | Class E2 |
| 75mm | ±2.00 | N/A | ±0.35 | ±0.40 | ±0.50 | Class E2 |
| 80mm | ±2.00 | N/A | ±0.35 | ±0.40 | ±0.50 | Class E2 |
| 85mm | ±2.50 | N/A | ±0.35 | ±0.40 | ±0.50 | Class E2 |
| 90mm | ±3.00 | N/A | ±0.40 | ±0.45 | ±0.70 | Class E2 |
| 95mm | ±3.00 | N/A | ±0.40 | ±0.45 | ±0.70 | Class E2 |
| 100mm | ±3.00 | N/A | ±0.40 | ±0.45 | ±0.70 | Class E2 |
All values in millimetres, "unless otherwise specified". Walls thicker than 4mm revert to the standard ISO 3302-1 class shown in the final column. Source: Viking Extrusions Large Bore Tubing Tolerances, Rev 02.
What Affects the Tolerance an Extruder Can Actually Hold
Two profiles specified to the same class are not equally easy to make. These are the factors that decide whether E1 is realistic or E3 is honest.
Profile Complexity
Simple, symmetrical sections (cords, square strips, round tubing) extrude with very stable dimensions. Complex asymmetric profiles with multiple hollows, thin flaps or deep re-entrant features flow unevenly through the die, and each feature interacts with its neighbours: the more complex the section, the wider the honest tolerance.
Wall Thickness Balance
Rubber wants to flow towards the path of least resistance. A profile combining a 5mm solid block with a 0.6mm wall is fighting physics: the thick section drags material from the thin one. Keeping walls reasonably balanced across the section is the single best design decision for tight, repeatable dimensions.
Hardness: Softer Is Harder
Very soft compounds (Shore A 20–30) have low green strength: the uncured profile can sag under its own weight before it reaches the cure oven, and the finished part deflects under measurement. Mid-range hardness (50–70) is the sweet spot for holding tight tolerances.
Sponge vs Solid
Sponge silicone expands during cure, so final size depends on the blow as well as the die. Expansion varies subtly along the length and between batches, which is why silicone sponge extrusions are normally quoted to Class E3 or wider agreed limits, and why critical sponge behaviour is better specified by compression performance than linear dimensions.
Material Grade
Compound choice shifts behaviour too: high-temperature, flame-retardant, metal-detectable and platinum-cured grades each have their own rheology and shrinkage character. A tolerance proven on a general-purpose 60 Shore compound doesn't automatically transfer to a specialist grade, so requalify when the material changes.
Tooling Quality & Iteration
Because die swell and shrinkage can only be predicted approximately, tight tolerances come from iterating the die against measured extrudate. An extruder who makes tooling in-house can run this loop in days; one who buys dies out can't. It is the quiet variable behind every tolerance an extruder promises.
How to Specify Tolerances on Your Drawing
Five habits that get you a functional part, a fast quote and no surprises at inspection.
Put the Class on the Drawing
A general note such as "Tolerances to BS ISO 3302-1 Class E3 unless otherwise stated" (E3 for a profile; use E1 for tubes, E2 for strips and cords) covers every undimensioned-tolerance feature in one line, is unambiguous to any extruder in the world, and beats a title-block default written for machined metal.
Tighten Only What Matters
Identify the two or three dimensions that control function (the sealing face, the groove width, the interference) and call those up to E1 (or an agreed limit) individually. Blanket E1 across a drawing multiplies tooling iterations and inspection cost for dimensions nobody will ever gauge. Honest truth: tolerance is a cost lever, and you control it.
Define Datum Faces
Tell us which face locates in the assembly and which surfaces mate. Dimensioning from a functional datum, rather than chaining across the section, stops tolerances stacking up on the one dimension that actually seals.
Measure Rubber Correctly
Dimensions apply to the part unstressed, conditioned at 23°C, measured without compressing it: optical/profile projection or low-force gauging, not tightened callipers. If your incoming inspection squeezes the part, you will reject good product. State the measurement condition on the drawing if your QA team needs it.
Talk Before You Freeze the Drawing
A five-minute conversation at design stage, covering wall balance, hardness and which class each dimension needs, routinely saves a tooling revision later. Send the draft; we'd rather comment on a sketch than concession a finished drawing.
Agree Special Limits Explicitly
If a dimension genuinely needs tighter than E1, don't just draw it: agree it. Tighter-than-standard limits are a joint engineering exercise between your design and our tooling, settled at quotation, so both sides know exactly what will be measured and how.
Tolerances at Viking Extrusions
Viking Extrusions' house tolerances are set down in our quality manual per shape, because different geometries hold dimensions differently: tubes are manufactured to Class E1, strips and cords to Class E2, and bespoke profiles to Class E3, unless otherwise specified. Cut lengths follow Class L3. Anything with any doubt of attaining house tolerance is flagged at quotation or clarified during contract review, and tighter tolerances are available on a case-by-case basis after engineering review.
| Shape | House Tolerance Class |
|---|---|
| Tubes | Class E1 |
| Strips / cords | Class E2 |
| Profiles | Class E3 |
| Cut lengths | Class L3 |
The capability rests on one structural advantage: all extrusion tooling is designed and made in-house in Worthing. Because die swell and cure shrinkage can only ever be predicted approximately, hitting a tight tolerance means cutting a die, extruding, measuring, and refining. We can turn that loop around in days rather than the weeks it takes when tooling is bought out. First-article dimensional reports are available against your drawing on request.
The same tolerance discipline applies across everything we extrude: bespoke silicone profiles to your drawing, silicone rubber tubing with controlled bore and wall, extrusions supplied cut to length to the L-class system, and silicone sponge extrusions with honestly quoted expanded-material limits.
The official documents our quality team works to are available to download: Extrusion Tolerances (PDF), Cut Length Tolerances (PDF) and Large Bore Tubing Tolerances (PDF).
