Realistic tolerances of silicone rubber components typically fall between ±0.1 mm and ±0.5 mm as variable with part size, geometry, material hardness and molding process – much looser than the ±0.020.05 mm commonly allowed by rigid injection-molded plastics or machined metals. The elastic properties of silicone, greater variation in the range of shrinkage (2-3 percent, typically, but sometimes as much as 4 percent), and the nature of the curing process all tend to make it difficult to maintain control over dimensions. Silicone is compressed by the measurement pressure, elasticly recovers and more batch-to-batch-varying unlike rigid materials which maintain shape predictably at given temperatures, pressures and even with material lots.
A lot of designers bring plastic or metal tolerances over because they believe that there is achievable ±0.05 mm across the board. Practically, this ought hardly to succeed except with considerable cost penalty, in the form of increased scrap, rework, or special fixturing. Tolerance of realistic silicone should be adapted to the elasticity of the material, variation of shrinkage and ability of the molding process – not perfect CAD dimensions.
Close tolerances (less than 0.05 mm or higher) are only possible on small, stable-geometry components that have been fabricated through liquid silicone rubber (LSR) injection molding under very stringent conditions. In the application of most parts, and where larger or softer parts are to be used, aiming at ±0.150.3 mm (or wider) is closer to the reality of the process and makes the costs manageable.
Why Silicone Dimensional Control Is More Challenging
Compared to rigid plastic or metals the dimensional stability of silicone rubber suffers due to the basic material properties.
Elastic recovery (the part recovers when it is no longer pressurized after demolding, or it is pressed by measuring equipment), compression at the time of measurement (even light caliper pressure makes softer grades compress unevenly), shrinkage variance (dependent on cure temperature, cure pressure, lot-to-lot variation), and cure consistency (non-even crosslinking causes dissimilar contraction) are the largest.
All these factors together contribute to a greater variation in final dimensions than those that occur to the rigid materials. To give some background, the influence of critical factors on tolerance is as follows:
| Factor | Effect on Tolerance |
| Elasticity | Measurement variation from deformation |
| Shrinkage rate | Size fluctuation (typically 2–3%) |
| Shore hardness | Dimensional stability (softer = worse) |
| Wall thickness | Distortion risk (thinner = higher) |
In cases where engineers do not consider silicone molding tolerances or silicone shrinkage rate, components tend to fall out of spec in spite of the mold being constructed properly. Silicone is not as stable dimensionally over time as it is with rigid thermoplastics and increases in stability as one gains more experience with the material and with fine tuning the process.
Typical Tolerances by Molding Process
Tolerances are considerably better in compression molding (with high-consistency rubber or HCR) than in LSR injection-molding injection methods – there is generally greater repeatability with therofil and more accurate pressure regulation, as well as operator variation.
Predominant ranges in the case of the silicone compression molding process should be larger because of manual preform placing and the increased reliance on operator skill. LSR is good in medium- high volume consistency.
| Part Size | Compression Molding | LSR Injection |
| <50mm | ±0.2–0.3mm | ±0.05–0.15mm |
| 50–150mm | ±0.3–0.5mm | ±0.15–0.3mm |
| >150mm | Case-dependent | Case-dependent |
They are useful ranges of parts designed well under normal conditions based on experience. LSR possesses higher repeatability when automating and controlling injection, though in this case too tolerances increase with size, complexity, or soft compounds. Specs that are too aggressive require a change in the mold or process adjustment leading to an increase in tooling and validation.
How Shore Hardness Affects Tolerance Stability
Compounds with harder silicones are more stable in holding measurements and dimensions due to their inability to be squashed when demolded, being cushioned, or being handled.
Lower grades (2030 Shore A) are relatively easy to compress with little force and cause measurement irregularities and increase out of tolerance measurements. Greater durometers (60 Shore A+) are stiffer and act more like semi-rigid materials as they retain their shape better but lose flexibility to use in sealing or cushioning purposes.
The compression effect of measurement tools is particularly rationalized at values less than 40 Shore A – a typical caliper is capable of compressing the component to an extent of 0.122 mm on softer components.
| Shore Hardness | Tolerance Stability | Risk Level |
| 20A–30A | Lower | High deformation |
| 40A–50A | Moderate | Balanced |
| 60A+ | Higher | Better dimensional retention |
In practice we have seen projects in which the tolerance fallout dropped by half upon changing to 50A instead of 30A with no change of geometry.
Wall Thickness and Geometry Impact
Due to thin walls, deep features and large flat surfaces, silicone molding is prone to distortion.
The thin sections do not cool evenly causing sink marks or warping. Deep sinks entrap air or result in disproportionate sinkage. Great flat surfaces are likely to bow due to residual stresses. Ribs and undercuts are also complex making draft or split tooling necessary, which generates parting-line variation.
| Geometry Type | Tolerance Risk |
| Thin wall | Warping |
| Deep cavity | Shrinkage variation |
| Large flat area | Distortion |
| Rib structure | Uneven cooling |
To reduce the effect of such problems, designers should strive to achieve even wall thickness (preferably 1.53 mm) and high radii. Prima facie angles or high ratios nearly always increase tolerances required.
Measurement Challenges in Silicone Parts
Silicone is more difficult to measure as opposed to rigid parts because it is a yielding substance to probe pressure.
Manual gauges can also introduce compression error – particularly on softer grades – whereas short arm digital tools may be used where adjustable force is required. The most repeatable fixtures are custom fixtures that hold the part in place without giving it any distortion, at the expense of time and cost.
| Measurement Method | Accuracy Risk |
| Manual caliper | Compression error |
| Digital gauge | Improved control |
| Custom fixture | Best repeatability |
Never fail to mention low-force measurement (1020kPa typical of soft grades, of course) in drawings and confirm using several different techniques during PPAP.
When Tight Tolerances Are Feasible
Strict tolerances can only be realistic in special circumstances: small components (less than 50 mm), constant (or nearly constant) geometry, hard materials (50A + ), and automated LSR operations with compensated tooling.
In such instances, ±0.05 -0.10 mm is realizable on key linear dimensions and thus ±0.03 mm on sealing feature in the prototyping and process optimization. More than that, look forward to diminishing returns – it costs a lot more to get the marginal returns.
Common Tolerance Mistakes in Silicone Design
- Use of metal standards of tolerance (±0.01 0.05 mm) without taking into consideration the effect of elasticity and shrinkage.
- Not taking into consideration silicone shrinkage rate in mold design, which causes undersized parts.
- Leaving critical and non-critical dimensions undefined, over-constraining.
- Adding stiffness to flexible objects such as lips or bellows which require complies.
- Omitting initial process ability conversation with the molder.
The consequences of these mistakes are 20-50 percent scrap or costly revision of molds.
How to Define Tolerances Strategically
Establish tolerances by the significance of the functionality, not habituality – consider critical sealing surfaces to or mating surfaces, and permit wider bands at non-functional surfaces.
Teamwork in the DFM phase to match to the capability of the process. Make capability studies (Cp/Cpk) not arbitrary, and provide shrinkage compensation at the outset. The method keeps costs down but ensures quality – unrealistic specifications drive up tooling, inspection, and rework.
Conclusion — Silicone Tolerance Is a Process Capability Decision
Silicone precision will in the end rely upon material behavior, hardness, geometry, and desired molding process – not drawings stolen off other materials. Tolerances that are unrealistic cause upsurge in scrap, rework, and lead times and strategic capacity-based planning enhances yield and general program risk.The optimum approach to performance cost and manufacturability balance is going to be early DFM consultation with an experienced silicone manufacturer. Eliminate the tolerances initially and the production floor operates more easily.