LSR
- Keeps its properties from freezer cold to oven heat
- Low compression set, so a seal keeps pushing back after years
- Needs its own dosing unit, heated mold, and cold runner
LSR molding
Liquid Silicone Rubber Molding: A Cold Barrel and a Hot Mold
Two-part silicone is mixed cold and cured in a hot mold. LSR molding and LSR overmolding are in scope when the RFQ names them. Upload CAD for DFM and an instant quote.
The reversal
Hot where plastic is cold, cold where plastic is hot
Most of what makes LSR tooling different follows from one fact: the material cures with heat instead of freezing.
LSR molding runs a cold barrel into a hot mold. Material properties are on the LSR material page . Overmolding is how a seal is scoped onto a rigid part. Design notes for that stack are in the LSR overmold guide . TPE is the thermoplastic soft alternative.
In thermoplastic molding the barrel is hot and the mold is cool. Resin melts, flows, and freezes. LSR runs the other way. The A and B components meet in a static mixer, stay cool in the barrel so they do not start curing, and only react once they touch hot steel. The part comes out solid and cannot be melted again.
That reversal explains the rest. Runners are cooled rather than heated, because silicone that cures in a runner is scrap. The mold is heated, often with electric cartridges, and insulated from the platens. And the mix is so thin when it enters the cavity that it finds gaps a plastic melt would never get into.
Roles
Not sure which silicone or soft route
LSR molding is a cure cycle: cold material, hot mold. Material properties are on the LSR material page. Overmolding puts silicone on a rigid part. TPE melts and freezes instead of curing.
| Page | Use it for |
|---|---|
| LSR molding (this page) | Cold barrel, hot mold, and the cure |
| LSR material | Properties and where silicone is used |
| Overmolding | A seal scoped onto a rigid part |
| LSR overmold guide | What to design before tooling |
| TPE | A thermoplastic soft material |
Station by station
What changes from pellet molding to LSR
Same kind of press frame, different logic at every station.
| Station | Thermoplastic | LSR |
|---|---|---|
| Material feed | Pellets dried and dropped from a hopper | Two liquid components pumped from drums, often with a color paste dosed in |
| Barrel | Heated to melt the resin | Cooled so the mix does not cure early |
| Runner | Hot runner keeps the melt liquid | Cold runner keeps the mix from curing, so little is wasted |
| Mold | Cooled so the part freezes | Heated so the part cures |
| Ejection | Pins push a rigid part off the core | Parts are floppy, so they are peeled, blown, or brushed off |
| After molding | Usually finished at the press | Oven post cure for some grades and markets |
The hard part
Flash, venting, and parts that stretch off the core
Uncured silicone flows like thick oil. The mold has to be built for that.
- Parting lines have to close far tighter than on a plastic mold, or every edge grows a thin fin
- Vents sized for plastic let silicone straight through, so many LSR molds pull a vacuum before injection instead
- Flashless tooling is possible but costs more steel work. The other route is trimming or cryogenic deflashing on every part
- Undercuts that would lock a rigid part often release, because the rubber stretches off the core and snaps back
Silicone onto plastic
When the seal is molded straight onto a rigid part
Two material parts put the plastic substrate inside a mold built for curing rubber.
- 1
Pick a substrate that survives the mold
The plastic part sits in steel hot enough to cure silicone. Low heat resins soften or move, so substrates tend to be PBT, polycarbonate, nylon, or PPS.
- 2
Choose self bonding grade or primer
Self bonding LSR grades stick to specific plastics on their own. Otherwise the substrate is primed first, which adds a step and one more thing to go wrong.
- 3
Plan where the silicone stops
The mold clamps on the plastic along the edge of the seal. A loose shut off leaves a silicone film where nobody wanted it.
- 4
Peel test real parts early
Adhesion problems show up on production substrates, not on test plaques. Mold release left on the plastic part is a common culprit.
Alternative
LSR or a TPE overmold
Plenty of soft grips and light seals never needed silicone in the first place.
TPE overmolded on plastic
- Runs on thermoplastic equipment, often in the same cell as the rigid part
- Runners and rejects can go back into the process
- Softens with heat and takes a set under long compression, unlike silicone
Before you commit
Questions that decide whether it has to be LSR
Answer these before you name LSR on the RFQ.
What is the worst temperature the part sees in use, hot and cold
Does the seal stay squeezed for years, or only now and then
Will it be sterilized, dishwashed, or in contact with food or skin
Is the rigid part molded separately, or does silicone go onto it in the same tool
Which edges can carry a parting line or a trimmed fin
Keep reading
Related guides
The material itself, the soft plastic alternatives, and the parts silicone usually ends up in.
LSR silicone
The material, its grades, and where it fits.
LSR siliconeFluorosilicone
Silicone that tolerates fuel and oil.
FluorosiliconeTPE
The usual soft touch alternative.
TPEOvermolding
Soft material molded onto a rigid part.
OvermoldingSeals
Sealing geometry and material choice.
SealsGlass filled PBT
A common substrate under molded silicone.
Glass filled PBTAll processes
Every molding process and secondary operation.
All processesQuote guides
Quote LSR and the soft-skin alternatives
Material page, overmold RFQ, and when TPE is enough.
LSR material guide
Properties, seals, and how to name LSR on the RFQ.
LSR material guideLSR vs TPE overmolding
Which soft skin to quote first.
LSR vs TPE overmoldingLSR overmold design guide
Substrate, flash, and what to settle before steel.
LSR overmold design guideQuote overmolding or two-shot
Two bodies, resin families, and named bond faces.
Quote overmolding or two-shotOvermolding services
The production page for a soft layer on a rigid part.
Overmolding servicesInstant quote platform
Upload CAD. Name LSR or LSR overmold on the RFQ.
Instant quote platformFAQ
Questions about LSR molding
Do you mold LSR or liquid silicone rubber?
Yes. Liquid silicone rubber (LSR) molding and LSR overmolding are in scope when the program is named on the RFQ, the same pattern as other molding services. Upload CAD and mark silicone-only or overmolded.
What is the difference between LSR and HCR silicone?
HCR, high consistency rubber, is a stiff gum usually shaped by pressing a preform in a heated mold. LSR is pumpable, so it can be injected with short cycles and cold runners. Both cure to similar rubber.
Why do some LSR parts need post cure?
Curing leaves small amounts of volatile byproducts in the rubber. Some food contact and healthcare programs call for an oven post cure to drive them off. Many industrial parts skip it.
Can silicone and plastic be molded in the same tool?
Yes, on a two material press with rotating or transfer tooling. The plastic shot goes first, then silicone cures onto it. It is a specialist setup, which is why many programs mold the plastic part on its own and load it into the silicone mold.
Is LSR the same as TPE?
TPE melts and freezes like other thermoplastics. LSR cures in a hot mold and does not melt again.
Can a silicone seal be molded onto a rigid part?
Yes. That is LSR overmolding. Mark the rigid resin and the silicone on the RFQ and upload CAD for DFM.
What should an LSR RFQ include?
The CAD, whether the part is silicone only or overmolded, and what the seal or flex has to do.
Next step
Quote LSR molding from CAD
LSR molding and LSR overmolding can be scoped on the RFQ. Upload CAD for DFM review and an instant quote.