Silicone Injection Molding
Silicone is the rubber for the extremes. It works from well below freezing to well above boiling, it ignores sunlight, ozone and weather, it holds its compression set better than any other general-purpose rubber at temperature, and it is biocompatible enough for implants. Moulding it usually means LSR — liquid silicone rubber — where two components are pumped, mixed at the machine and injected into a hot mould that cures the part in seconds. That makes cycles very fast and tooling comparatively expensive. What silicone cannot do is resist oil: it swells in hydrocarbons, which is the exact opposite of NBR.
Specification at a glance
- Material family
- Silicone rubber — LSR (liquid silicone rubber) for injection molding, HCR (high consistency rubber) for compression molding
- Cure system
- Platinum-catalysed addition cure for LSR — no peroxide by-products
- Cure temperature
- Roughly 150–200 °C in the mould; LSR cures in seconds at temperature
- Mould temperature
- Hot — the tool cures the material rather than cooling it
- Shrinkage
- Roughly 2–4%, dependent on the compound, cure and section thickness
- Temperature range
- Roughly -50 °C to +200 °C continuous; short-term excursions higher
- Compression set
- Excellent, and retained at temperature — the best of the common rubbers
- Weather, ozone and UV resistance
- Excellent
- Oil and fuel resistance
- Poor — silicone swells in hydrocarbons
- Mechanical strength and tear
- Low — silicone is a sealing material rather than a wear or load material
- Electrical insulation
- Excellent, and stable across the temperature range
- Tolerances
- Wider than plastics, but tighter than other rubbers — LSR holds the best tolerances in the rubber family
- Molding route
- Through our vetted partner network, managed under one quality process; metal inserts machined on our own floor
- Mould tooling
- Quoted per drawing — cavity count, flash control and cure layout reviewed before the tool is cut
- MOQ
- From 50 pieces, samples from a single piece
The extremes rubber
Silicone’s position is defined by two properties that no other general-purpose rubber matches together: the widest temperature range and the best compression set at temperature.
| Property | Silicone | NBR | Neoprene |
|---|---|---|---|
| Temperature range | -50 to +200 °C | -40 to +100 °C | -40 to +120 °C |
| Compression set at temperature | Excellent | Moderate | Moderate |
| Weather, ozone, UV | Excellent | Poor | Excellent |
| Oil and fuel resistance | Poor — swells | Excellent | Moderate |
| Flame resistance | Moderate | Poor | Good |
| Tear strength | Low | Good | Good |
| Abrasion resistance | Poor | Good | Good |
| Biocompatibility | Excellent | Limited | Limited |
Two rows in that table decide most silicone specifications.
The temperature and compression set rows are why silicone exists: an O-ring that has to keep sealing in a hot environment needs to still be pushing back after thousands of hours, and silicone is the rubber that does that. NBR and neoprene both take a set and lose sealing force at temperature.
The oil row is why silicone gets specified wrongly. Silicone swells in hydrocarbons — oils, fuels, greases and hydrocarbon solvents cause it to absorb fluid and expand, which destroys both the part’s dimensions and its sealing force. That is the exact opposite of NBR, which is chosen precisely for oil resistance. A hydraulic seal in silicone is a mistake that announces itself as a swollen, leaking part.
Where a part sees both oil and high temperature, neither rubber is ideal, and that is a genuine specification problem to solve with the compound, with shielding, or with a different material family — not something to solve by picking the rubber that sounds closer.
LSR: what silicone injection molding actually means
Silicone is moulded in two quite different forms, and they are not interchangeable.
LSR — liquid silicone rubber. A two-component material supplied as an A and B part, pumped, metered and mixed at the machine, then injected into a hot mould. The cure is a platinum-catalysed addition reaction that starts when the components meet and runs fast at mould temperature, so LSR parts cure in seconds. This is the injection-molding form.
HCR — high consistency rubber. A firmer, gum-like silicone processed by compression and transfer molding rather than injection. It is behind moulded sheets, profiles and larger parts.
The consequence for a moulded part is in the equipment. An LSR job needs a metering and mixing system, a static mixer at the injection point, and tight control of the A:B ratio — which is why LSR carries a higher setup and tooling entry cost than a compression-moulded part. In return, cycles are very fast, so the piece cost at volume is low. That trade is what makes LSR the right process for a high-volume precision silicone part and the wrong one for a handful of large simple parts.
Two by-products of the cure chemistry are worth knowing:
Platinum cure produces no peroxide by-products. That is why platinum-cured LSR dominates medical and food-contact work — there is nothing to extract or leach.
The cure is fast and uniform, which is why LSR holds the best tolerances in the rubber family. The low-viscosity material fills the cavity accurately and cures evenly, producing a more consistent part than compression moulding.
Where silicone’s weaknesses matter
Silicone is not a general-purpose rubber, and three limitations decide applications.
Mechanical strength and tear resistance are low. Silicone is a sealing material, not a load-bearing or wear material. A part that has to transmit force, take abrasion or resist tearing is the wrong application — NBR or neoprene is tougher. This also limits how much demoulding stretch a silicone part can survive, which in turn limits how severe an undercut can be.
Abrasion resistance is poor. A dynamic seal in silicone will wear faster than the same seal in NBR. Where a seal moves against a surface, that is worth weighing.
Oil and fuel resistance is poor. Covered above, and it is the single most common specification error on silicone.
Design rules
Radius everything. Rubber tears at sharp corners, and silicone’s low tear strength makes that more critical than on any other rubber in this range.
Avoid abrupt section changes. They cure unevenly, and on silicone an uneven cure shows as a property variation across the part rather than only as a dimensional one.
Design for the flash line. Rubber flashes at the parting line, and flash on a sealing surface is a functional defect.
Keep the demoulding stretch modest. Silicone tears more readily than NBR or neoprene, so an aggressive undercut that releases cleanly in a tougher rubber may fail here.
Specify compression set and temperature together. They are the reasons to choose silicone, so they belong on the specification rather than being assumed from the material name.
Do not design silicone as a structural or wear part. Its mechanical strength and abrasion resistance are low by the standards of the rubber family.
Agree the tolerances. LSR holds the best tolerances in the rubber family, but they are still rubber tolerances — wider than plastics, with bands that widen as dimensions grow, and the material deforms under measurement.
Molding route
Plastic and rubber molding run through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Tooling is quoted per drawing, and on a silicone part the review covers cavity count, flash control, cure layout and the parting line position — and, for LSR, whether the part suits the LSR process at the volume in question.
Bonded rubber-to-metal and silicone-to-metal parts are handled the same way: the insert is machined on our floor and the silicone is moulded and bonded to it by our partner, from the same drawing, so the fit between the two is planned before the tool is cut.
Where molding silicone is the right choice
- Seals, gaskets and O-rings where the temperature range or the compression set requirement rules out other rubbers
- High-temperature seals that have to keep sealing after long service
- Medical devices, implants and catheters in the appropriate grades — see medical parts
- Keypads and control surfaces
- Infant and food-contact products in the appropriate grades
- Electrical insulation and connectors — see electronics hardware
- Automotive boots and seals exposed to heat and weather
- Cookware and bakeware
- Optically clear silicone parts in the right grade
Where silicone is the wrong choice
The part meets oil, fuel or grease. Silicone swells. NBR is the oil-resistant rubber.
The part has to take abrasion or transmit force. Silicone’s mechanical strength and abrasion resistance are low.
The part is a dynamic seal running against a surface. Silicone wears quickly; NBR or neoprene is tougher.
The part needs to resist tearing during assembly or demoulding. Silicone tears more readily than the other rubbers here.
The volume does not justify an LSR setup. LSR tooling and equipment carry a higher entry cost than compression molding; for very low volumes or large simple parts, a compression-moulded silicone or a standard part may be the better route.
A standard catalogue part would work. Buying a standard O-ring or gasket is cheaper than tooling a custom one.
What to put on the drawing
| Instead of | Write |
|---|---|
| Silicone | Silicone — and note whether the part is for LSR injection or compression molding |
| Silicone, oil or fuel contact | Wrong material — silicone swells in hydrocarbons. NBR is the oil-resistant rubber |
| Silicone, service temperature | State both ends — this is usually the reason silicone was chosen |
| Silicone, long-service seal | Specify compression set — it decides seal life |
| Silicone, medical or food contact | State the application — the grade and its documentation follow from it |
| Silicone, dynamic or abrasive duty | Wrong material — silicone’s tear and abrasion resistance are low |
| Silicone, sealing surface | Say which surface seals — the flash line must avoid it |
| Silicone, tolerance | Agree it — LSR holds the best rubber tolerances, but they are still rubber tolerances |
The three lines that decide most silicone mouldings are the temperature range, whether the part meets oil, and which surface has to seal. The first is why silicone is on the drawing. The second is where it most often fails. The third decides the parting line, and on a seal that is a functional requirement.
Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the cure system, the compression set requirement, the parting line and the LSR process fit where any of them need settling.
Frequently asked
- What is LSR, and how is it different from ordinary silicone rubber?
- LSR is liquid silicone rubber — a two-component, low-viscosity silicone supplied as an A and B part that are pumped, metered and mixed at the moulding machine, then injected into a hot mould where they cure. It is the form used for injection molding. HCR — high consistency rubber — is the firmer, gum-like silicone used for compression and transfer molding; it is the material behind moulded sheets, profiles and many larger parts. Both are silicone, and they are processed completely differently, so the two should not be treated as interchangeable when a part is quoted.
- Why does LSR cure so quickly?
- Because the cure is a platinum-catalysed addition reaction that starts the moment the two components are mixed, and it runs fast at mould temperature. LSR injected into a mould at roughly 150–200 °C cures in seconds rather than minutes, which is why LSR cycles are much shorter than those of other rubbers. The trade is that the equipment is specialised: an LSR machine needs a metering and mixing system, a static mixer at the injection point, and careful control of the A:B ratio. That equipment is why LSR tooling and setup carry a higher entry cost than a compression-moulded part.
- What is silicone's temperature range?
- Roughly -50 °C to +200 °C for continuous service, with short-term excursions above that. It is the widest range of the common rubbers — NBR covers about -40 to +100 °C and neoprene about -40 to +120 °C. Silicone also retains its flexibility at the cold end rather than stiffening, and it holds its compression set at the hot end, which is why it is the material for seals that have to work across a wide thermal range or at sustained high temperature.
- Why is compression set so important for silicone?
- Because it is the property that decides how long a seal keeps sealing. A seal works by staying compressed against its mating surfaces, and compression set is the permanent deformation that remains after a period under compression — the extent to which it fails to spring back. Silicone's compression set is excellent, and — more importantly — it stays good at temperature, where most rubbers degrade. That combination is why silicone holds the high-temperature sealing market: an O-ring in a hot environment needs to still be pushing back after thousands of hours.
- Does silicone resist oil?
- No — and this is the limitation people most often get wrong. Silicone swells in hydrocarbons: oils, fuels, greases and hydrocarbon solvents all cause it to absorb fluid and expand, which destroys a seal's dimensions and its sealing force. This is the exact opposite of NBR, which is chosen precisely for oil resistance. So a hydraulic or fuel seal is an NBR part, not a silicone one, and where a part sees both oil and high temperature, neither rubber is ideal and the material family has to be reconsidered.
- Is moulded silicone safe for medical and food contact?
- Platinum-cured LSR is widely used for both, because the addition cure produces no peroxide by-products to extract or leach, and the material is chemically inert, odourless and tasteless. That is why LSR appears in catheters, implants, seals for medical devices, infant feeding products and cookware. The specific grade and its supporting documentation still have to match the application and the market — the cure system is the reason silicone is suitable, not a substitute for confirming the grade.
- Why can rubber parts have undercuts?
- Because rubber stretches. A rigid plastic part with an undercut needs a slider, a lifter or a collapsing core, because the part cannot deform enough to release from the tool. A rubber part can be stretched off a simple core, so a form that would be impossible in plastic is straightforward in rubber. The limit is the stretch the part can survive without tearing — and silicone's tear strength is low, so the demoulding stretch has to be more modest than it would be for a tougher rubber like NBR.
- What tolerances can a moulded silicone part hold?
- Wider than a plastic part, but tighter than other rubbers. LSR holds the best tolerances in the rubber family, because the low-viscosity material fills the cavity accurately and the fast, uniform cure produces a more consistent part than a compression-moulded one. Even so, rubber dimensioning follows its own conventions, with bands that widen as dimensions grow, and the material deforms under measurement. Tolerances are agreed per drawing in DFM rather than carried over from a plastic part.
- What is silicone injection molding used for?
- Seals, gaskets and O-rings where the temperature range or the compression set requirement rules out other rubbers; medical devices, implants and catheters in the appropriate grades; keypads and control surfaces; infant and food-contact products; electrical insulation and connectors; automotive boots and seals; and cookware and bakeware. It is also used for optically clear silicone parts, which the material can produce in the right grade.
- What design rules matter for a moulded silicone part?
- Radius everything, because rubber tears at sharp corners and silicone's tear strength is low. Avoid abrupt changes in section thickness, because they cure unevenly — and on silicone an uneven cure is more likely to show as a property variation. Design for the flash line, since flash on a sealing surface is a functional defect. Keep the demoulding stretch modest, because silicone tears more readily than NBR or neoprene. And do not design silicone as a load-bearing or wear part — it is a sealing material, and its mechanical strength and abrasion resistance are low.
Related pages
Other Rubber & Silicone materials
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