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
Process guide
Liquid Silicone Rubber Molding
Liquid silicone rubber arrives as two drums of paste. It is metered, mixed cold, and pushed into a mold hot enough to cure it in place. Nearly every habit from thermoplastic molding runs backwards.
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.
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.
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 looking for a silicone molder.
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 PBTProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about LSR molding
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.
Does NetProto mold liquid silicone rubber?
No. NetProto focuses on thermoplastic injection molding and overmolding. This page explains how LSR is made so you can judge the silicone side of a program. The rigid plastic parts, and any TPE alternative, can be quoted from CAD.
Next step
Check whether a TPE can replace the silicone
Upload CAD for the rigid parts with notes on the seal or grip. The review covers the plastic parts and whether a TPE overmold would meet the requirement. NetProto manufacturing is ISO 9001 certified.