Design Considerations for Liquid Silicone Rubber
Content in this Article
LSR is easiest to control before tooling release. Align material, wall thickness, ejection, gating and acceptance criteria so appearance, dimensions and cycle time are considered together.
- Define function, environment, quantity and risk before selecting a material or process.
- Treat typical values as comparison points; confirm the exact grade and supplier capability before release.
- Use a first article or pilot lot to verify dimensions, function and records before scaling.
Define the decision
Document function, environment, quantity and risk so every option is compared against the same requirement.
What LSR Actually Is, in Processing Terms
LSR is a two-part, platinum-catalysed silicone supplied as pumpable A and B components that meet at a static mixer, get metered into a cold barrel at around 15–25 °C, and are injected into a mould heated to 160–200 °C where they cross-link in 10–30 seconds. The part ejects as a finished thermoset elastomer — no cooling phase, no regrind, no second chance if a runner solidifies in the wrong place.
That inversion — cold barrel, hot mould — is the single most important mental model. It dictates gate placement, vent design, cold-slug wells, and how the mould is thermally isolated from the injection unit.
Choosing a Durometer Before You Draw the Part
Durometer drives wall thickness, draft, undercut tolerance, and flash behaviour. A 30 Shore A gasket and a 70 Shore A grommet recommended practice not share design rules.
LSR evaluation flow
Define requirements, narrow the material and process, then verify with a consistent acceptance method.
- 1Requirements
Function, environment, quantity and risk
- 2Options
Material, process, design controls and finishing
- 3Acceptance
Dimensions, functional tests and production records
Compare the practical options
Compare material, process, design controls and finishing together—not unit price alone.
Wall Thickness, Uniformity, and the Cure-Time Trap
LSR cures from the outside in. A 1.
Minimum wall is genuinely thin: 0. 25 mm is mouldable on a 40 Shore A grade with proper venting and gate placement.
Parting Lines, Shutoffs, and the Flash Problem
LSR viscosity sits around 100–1000 Pa·s uncured — low enough that it finds any gap above roughly 5 microns. Shutoff surfaces need to meet at a positive angle (3–5° minimum), not a knife edge, and every parting line must be placed where a visible flash witness will not break the part function.
| Parting-line location | Flash risk | When to use | When to avoid |
|---|---|---|---|
| Flat equator of a symmetric part | Low | Simple gaskets, O-rings, bellows | Parts with sealing surface on equator |
| Offset to a non-functional rib | Low | Valves, diaphragms, medical seals | When rib adds unacceptable thickness |
| Stepped across a wall | Medium | Complex 2K overmoulds | Optical or cosmetic faces |
| Across a sealing face | High | Never by design | Always — relocate or redesign |
Validate before scaling
Before scaling, verify dimensions, function and records with one controlled method across representative lots.
Undercuts, Overmoulding, and Release
Soft LSR tolerates undercuts that would demand a side action on a rigid polymer. A 40 Shore A part with a 0.
Release strategy is rarely a single trick: the typical tool uses a combination of generous draft (2–5° minimum), air blast, stripper plates, and in tight cases manual part removal. Add mould-release coating only as a last resort — it migrates into the part and can compromise biocompatibility certifications.
Pre-RFQ review: recommended practice and common mistakes
| Recommended practice | Common mistake |
|---|---|
| Document function, environment, quantity and risk | Select from a material name or machine specification alone |
| Review material, process, design controls and finishing together | Address manufacturing limits only after design freeze |
| Inspect critical dimensions and function on the first article | Scale production from visual approval alone |
| Keep material, revision and inspection records | Reuse old results after a material or process change |
- Function, environment, quantity and risk are documented
- Material, process, design controls and finishing are reviewed with the supplier
- Critical dimensions, appearance and functional acceptance are on the drawing or RFQ
- First-article or pilot-lot verification is planned
- Material, process and revision changes trigger a new review
FAQ, further reading and sources
What should be defined first for LSR?
Start with function, environment, quantity and acceptance criteria. These inputs narrow the practical options faster than naming a machine or material first.
Can typical values in this article be released directly on a drawing?
No. Use them for early comparison, then confirm the exact grade, supplier capability and first-article result.
When is the process ready to scale?
Scale only after material, process, finishing and inspection are controlled and repeatable across representative lots.
Next, explore 3D printing services, CNC machining, materials, and the related reports linked on this page.
