Liquid silicone rubber is moulded from a two-component system cured with platinum, and the sampling sequence differs from thermoplastic moulding in ways that change both the timeline and the failure modes. This guide walks the sequence and marks the gates where a design decision becomes expensive.
The sequence
- 3D model and DFM review. Two to four days. The supplier reviews wall thickness, parting line, gate location and venting. Silicone flows easily but traps air easily too, and the vent design is what prevents the bubbles that appear weeks later as a field failure.
- Prototype in soft tooling or 3D printed mould. Five to ten days. Verifies form, fit and the user's handling of the part. Not a production process, so it says nothing about cycle time or material behaviour.
- T1 sample from hard tooling. Fifteen to twenty-five days from tool release. The first parts off the production steel. Expect dimensional deviation, unvented flash and gate witness marks. T1 is a tool verification gate, not a sample approval gate.
- T2 after tool correction. Five to ten days. The corrected tool, usually with vents added and the parting line adjusted. This is where dimensional data should be measured and recorded.
- Pilot run. A short run at production cycle time to verify the process window, the demoulding behaviour and the material lot consistency.
- PPAP-style approval and first production. Retained samples, material certificate and dimensional report signed off together.
What T1 actually tells you
T1 verifies that the tool makes the shape. It does not verify that the material cures consistently across the cavity, that the part releases cleanly a thousand times, or that the colour is stable between lots. Buyers who approve a programme on T1 alone usually discover the curing problem in the pilot run, when the schedule is already committed.
The design changes that are cheap before the tool
- Wall thickness. Silicone cures from the outside in, and a section above 4 mm needs a longer cure or a different grade. Uniform 1.5 to 2.5 mm walls are the cheapest to cure consistently.
- Undercuts. A mechanical undercut needs a side action or a lifter in the tool. Designing the part to demould without undercuts removes the most common tool complexity.
- Parting line location. Moving a parting line by two millimetres can remove a visible witness line on a user-facing surface, and it is free before the tool is cut.
- Venting. Adding a vent is easy on the tool but impossible after it is hardened without a re-cut. Ask where the vents are and what the last 5 mm before the vent does.
Material decisions at the sampling stage
Hardness, colour and any transparent tint are all confirmed on pilot-run parts, because T1 parts are usually moulded in a natural or un-pigmented lot to isolate tool issues from material issues. Confirm the production material lot before the pilot, not after, because a platinum-cured lot with an off-spec residue level will fail the biocompatibility screen that the programme depends on.
Related reading
Our platinum-cured LSR component programme runs this exact sequence, and the certification guides cover the document set to request at each gate.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
