Liquid silicone rubber, or LSR, is a two-component silicone that is metered, mixed, injected into a heated mold, and cured in place. Because the material is liquid at injection, it fills fine detail and thin walls easily, and because it cures by heat, the cycle can be fast and repeatable. LSR is the material of choice for seals, gaskets, medical components, and anything that touches food or skin, but it deserves its own design and validation review rather than being treated as another plastic. This guide covers the material system, the cold runner and venting details that decide part quality, and the validation checklist you should run before production.
The Material System: What the Two Components Do
LSR is supplied as two liquid components that are mixed in a fixed ratio, typically 1:1, at the machine. The mix is platinum-catalyzed and cures by heat in the mold, not by moisture or room-temperature chemistry. That distinction matters: the cure starts only when the material reaches temperature, which is what makes cold runner systems possible and why process control is part of the material's behavior.
Hardness is specified in Shore A and spans from very soft grades for seals to firmer grades for buttons and valves. The hardness affects the feel, the sealing force, and the ejection behavior, so it belongs on the drawing. Cure temperature and time depend on the grade and the wall thickness, and the supplier should quote a cure window for your geometry, because thick sections need longer hold time and under-cured parts fail in compression set.
| Grade family | Typical Shore A range | Typical applications |
|---|---|---|
| Very soft | 10–30 | Light seals, membranes, baby products |
| Medium | 30–60 | Gaskets, keypads, general seals |
| Firmer | 60–80 | Valves, buttons, structural sealing lips |
| Specialty | Grade-specific | Food contact, medical, high-temperature service |
The range is indicative; the exact hardness, temperature limits, and compliance documentation belong to the specific grade, so confirm the grade before the tool is committed.
The Process: Metering, Cold Runner, and Cure
An LSR molding machine uses a dosing pump that meters the two components, a static mixer that blends them, and an injection unit that delivers the liquid into a heated mold. The mold is held at cure temperature, commonly in the range of 120–180 °C depending on the grade, and the part cures in the cavity in minutes.
The cold runner is the process detail that most affects waste and consistency. The runner keeps the liquid silicone below cure temperature until it enters the cavity, so the material in the runner does not cure and can be reused, which minimizes waste. Hot runner systems also exist, and the choice affects tooling cost, cycle time, and how much material is discarded per shot. Ask which system the supplier runs and how runner waste is handled, because it shows up in the unit price.
Mold temperature control is critical. Uniform heating cures the part evenly, while cold spots under-cure and hot spots cause flash or early cure. The tooling design includes the heating layout and the venting plan together, because liquid silicone will find every gap in the parting line.
Venting, Flash, and Parting Lines
LSR is liquid at injection, so it behaves differently from molten thermoplastics: it flows into gaps that would trap a viscous plastic, and trapped air has nowhere to go unless the mold is vented. Poor venting produces incomplete fill, burn marks where air is compressed, or weak weld lines where flow fronts meet.
Flash control depends on the mold fit, the clamping force, and the vent depth. The drawing should mark which edges are cosmetic and which can carry parting-line flash, because deflashing adds a secondary operation and every flash line has a cost. For soft grades, flash can be especially thin and hard to remove, so the acceptance criteria for visible edges should be agreed in the tooling review.
Design the parting line with the venting plan. A part with deep pockets needs vents at the end of each flow path, and the vent positions often force the parting line location. The supplier should propose the layout during DFM, because changing the parting line after tooling is a mold modification, not a drawing change.
Design Rules for LSR Parts
Wall thickness. LSR fills thin sections well because it is liquid, and walls from about 0.5 mm are practical in well-designed parts, but the practical range depends on the grade, the flow length, and the cure time. Thick sections cure slowly and can trap voids, so keep walls as uniform as practical and transition thicknesses gradually.
Draft. LSR's flexibility releases from the mold more easily than rigid plastic, but draft still helps ejection and prevents distortion. Use roughly 0.5–2 degrees depending on the surface, with more on textured or deep features; the exact value depends on the grade and the depth.
Undercuts. Flexible LSR can release from features that would lock a rigid plastic part, so undercuts are easier, but deep undercuts still need careful mold design and can slow the cycle or force a collapsible core. Confirm which undercuts are feasible and where the parting line will sit during the tooling review.
LSR Overmolding and Insert Molding
LSR overmolds onto plastic and metal components to create seals, grips, and multi-material parts, and insert molding with LSR embeds metal fittings or electrical components in a permanent seal. The bond to the substrate depends on the material pair and surface preparation: some plastics need primers or plasma treatment to bond reliably, and a seal that peels from its substrate fails in service. The retention geometry and wall design follow the same logic as any overmolded or insert-molded part; the selection between the two routes is covered in our overmolding and insert molding guide. If your part only needs an LSR layer or an embedded fitting, confirm the adhesion strategy and the inspection method in the DFM phase.
Post-Curing and Secondary Operations
Post-curing is a controlled heat soak after molding that stabilizes the material's properties. It drives the cure toward completion, which improves compression set and long-term elasticity, and it is part of the process for many grades, not an optional extra. Confirm the post-cure schedule, temperature, and time with the supplier and record it per batch, because two parts molded identically can differ in service behavior if the post-cure step is not consistent.
Secondary operations include deflashing, cleaning, and any surface treatment for bonding. LSR parts are often molded flash-free with good tooling, but soft grades can leave flash that must be removed manually or cryogenically, so the acceptance criteria and the deflashing method should be part of the quote.
Validation and Compliance Checklist
Run this checklist before production, and attach the answers to the material qualification so every batch is verified the same way.
- Shore A hardness confirmed against the specification
- Cure temperature and time defined and recorded per batch
- Post-cure schedule confirmed and repeated consistently
- Compression set tested at the service temperature
- Continuous and peak temperature limits confirmed for the grade
- Food contact: grade documentation and migration data reviewed for the application
- Medical: biocompatibility assessment relevant to the application, such as ISO 10993 data or an equivalent supplier statement
- Overmolded or insert-molded parts: bond or pull-out test defined
- Flash and parting-line acceptance criteria agreed on the drawing
- Material lot traceability confirmed for regulated applications
Conclusion
LSR delivers flexible, heat-resistant, and chemically stable parts, and the process details, cold runner, venting, cure, and post-cure, decide whether the part is consistent batch after batch. Select the grade against the environment, design thin uniform walls with a vented parting line, and validate hardness, compression set, and bonding before production. The validation checklist above turns a material data sheet into a repeatable acceptance plan.
FAQs
How is LSR different from regular silicone rubber?
LSR is a liquid two-component silicone that cures by heat in a mold, which supports fast, repeatable, high-volume injection molding. Room-temperature silicone cures slowly and suits casting and low-volume work. The difference shows up in consistency, wall detail, and cycle time.
What temperature can LSR parts withstand?
Many grades handle continuous service around 200 °C with higher peaks and stay flexible at low temperatures, but the exact limits belong to the specific grade. Confirm the continuous and peak limits, plus compression set at the service temperature, before production.
Can LSR be overmolded onto plastic or metal?
Yes, but the bond depends on the material pair and surface preparation. Some substrates need primers or plasma treatment, and a peel or pull test should verify the bond before production. If you are choosing between an LSR layer and an embedded fitting, review the process selection first.
What validation documents should I ask for with LSR parts?
Grade data sheets with hardness and temperature limits, cure and post-cure parameters per batch, compression set data, and for food or medical use, the relevant compliance documentation such as migration data or biocompatibility assessment. Lot traceability completes the package for regulated applications.
Sources
- 6CProto Liquid Silicone Rubber Molding
- 6CProto Injection Molding Services
- 6CProto Medical Manufacturing
- Wacker – Liquid Silicone Rubber
- Wacker SILPURAN – Silicone for Healthcare