RTV Silicone Mold Low Volume Casting
A room-temperature vulcanized silicone tool copies a machined or printed master, and polyurethane resin is poured under vacuum to make 1 to 50 parts per mold. This page explains the mechanism, the boundaries, and how to judge whether your part belongs in silicone or in steel.
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How an RTV silicone mold copies a master pattern
RTV stands for room-temperature vulcanized. Two silicone components are mixed, degassed, and poured over a master pattern. The silicone cures at room temperature, so the master does not need to survive heat or clamping force. After cure the block is cut along a parting line, the master is pulled out, and an empty cavity remains. That cavity is a negative of the part.
The cavity is only as good as the master inside it. If the master is machined to ±0.005 mm on a 5-axis center, that geometry is what the silicone records. If the master is a rough print with visible layer lines, the silicone records those too, and every cast part inherits them. This is why we machine masters in-house rather than accepting a pattern from elsewhere in the chain.
Pouring happens under vacuum. Mixed resin and the closed mold are evacuated, then the mold fills with resin while air is still being pulled out. Vacuum removes entrained bubbles from the resin and prevents air from being trapped in undercuts or thin ribs. A mold that is not properly evacuated produces porosity, and porosity in a structural part is a fatigue crack waiting to happen.
Cure is a chemical reaction, not a cooling process. The resin cross-links in the cavity at room temperature or in a low-temperature oven. Demolding too early distorts walls. Demolding too late can make the part stick. Both windows depend on the specific urethane system, so the resin data sheet sets the schedule, not the operator's feel.
Where RTV silicone mold low volume casting stops making sense
The economic window is roughly 1 to 50 parts per mold. Below that, direct CNC machining or 3D printing is usually faster and cheaper because there is no tool to build. Above that, the silicone tool starts to wear: the parting line degrades, the cavity stretches, and dimensional drift creeps into later pulls. At some point the amortized cost of a steel tool becomes lower per part.
Silicone is soft and flexible. That helps with deep undercuts, snap fits, and features that would need side actions in steel. It also means the cavity can deflect under resin pressure. Thin, tall walls and long slender cores are the first features to shift. If your part is a 0.8 mm wall inside a 100 mm tall tube, silicone will struggle to hold concentricity.
Temperature is a second boundary. RTV silicone tools run at low temperature, so the cast urethane must cure at low temperature too. If your production material is glass-filled PA66 running at 280 °C, silicone casting cannot reproduce its mechanical behavior. You can get the shape, but not the polymer properties. That is a prototype, not a production part.
Transparency is a third. Clear urethanes exist and cast well, but a silicone cavity with any surface imperfection scatters light. For a lens or a light pipe, the mold face must be polished to the same standard as the part, which raises cost and rejects. For a translucent housing or a colored indicator, the same process is straightforward.
Matching urethane resin and part geometry
Cast urethanes are sold by Shore hardness, and the range is wide. A Shore 40A resin behaves like a soft rubber gasket. A Shore 80D resin is stiff and can substitute for some engineering thermoplastics in a fit check. Between those, you can find impact-modified grades, flame-retardant grades, and heat-resistant grades that hold shape up to moderate service temperatures.
The trap is treating hardness as the only variable. Two resins with the same Shore number can differ in flexural modulus, elongation at break, and heat deflection temperature. For a snap-fit housing, elongation matters more than hardness. For a bracket under load, flexural modulus and creep matter more. We ask which one governs before recommending a grade.
Draft and wall thickness decide how the part releases. A 1° to 2° draft on vertical faces helps. Sharp internal corners concentrate stress during demolding and often tear. A uniform wall of 1.5 mm to 3.0 mm casts predictably. Walls below 1.0 mm can fill, but they are fragile during the pull.
Inserts are common in this process. Threaded inserts, metal bushings, and magnets can be placed in the cavity and encapsulated during pouring. That is a real advantage over machining, where an insert means a secondary press or bond operation. It also means the insert position must be fixtured in the tool, so insert tolerance is set by the fixture, not by the resin.
Why the master pattern decides everything downstream
Every defect in the master is copied exactly, and every cast part carries it. There is no averaging, no self-correction. If the master has a 0.05 mm step at a parting surface, the part has that step. If the master has a scratch on a visible face, so does every part from that mold.
That is the argument for machining the master from aluminum or an engineering plastic on a controlled machine. With 16 simultaneous 5-axis centers and a 4,000 mm maximum processing size, we can cut masters that include the draft, the fillets, and the polish-ready surfaces in one setup. Fewer setups means fewer datum shifts, and datum shifts show up as form error in the cast part.
Printed masters still have a role. For a shape that is hard to machine, like an organic enclosure with blended surfaces, an SLA or SLS master is practical. The trade-off is surface finish and the need for sealing and sanding before molding. We treat printed masters as a separate route with its own prep steps, not as a shortcut around machining.
Whichever route is taken, the master must be dimensionally verified before silicone is poured. Once the tool is cast, the master geometry is locked in. Rework means cutting a new mold, which costs the same as the first one. Inspection at the master stage is the cheapest inspection in the whole process.
Vacuum integrity, mixing, and the defects they prevent
Vacuum does two jobs: it pulls dissolved gas and entrained air out of the mixed resin, and it pulls air out of the cavity as resin enters. If the chamber does not reach the specified level, or if it reaches it too slowly, bubbles survive. Some appear as surface pinholes. Others sit just under the skin and only show up after finishing or after a drop test.
Mixing ratio is a chemical stoichiometry problem. Off-ratio resin may still gel, but its mechanical properties drop and its cure can be incomplete in thick sections. Hand mixing by volume is a common source of variation. Metered mixing, or at minimum weight-based mixing with a calibrated scale, keeps the ratio inside the resin supplier's window.
Moisture is the quiet failure. Urethane components absorb water from humid air. Water reacts with isocyanate and releases carbon dioxide, which forms internal voids. In a humid climate this can happen within days of opening a container. Storage under dry conditions and checking for cloudiness or skinning before use are simple controls that prevent a whole class of porosity.
Cycle discipline matters as much as equipment. Degas time, pour rate, vacuum hold, and demold time should be written down per resin and per part. When they live in the operator's memory, quality drifts with shift changes. When they are on a traveler, the same part comes out the same way next month.
Silicone casting compared with the alternatives
Use this to pick a route, not to rank suppliers.
| Route | Typical batch | Best for | Main limit |
|---|---|---|---|
| RTV silicone casting | 1–50 parts per mold | Functional samples, pilot builds | Tool wears above ~50 pulls |
| Direct CNC machining | 1–200 parts | Tight tolerance, real alloys | Unit cost stays flat, no tooling |
| 3D printing | 1–20 parts | Fast shape checks | Anisotropic strength, layer lines |
| Steel injection mold | 1,000+ parts | Production volumes | High tooling cost, long lead |
| Die casting | 1,000+ parts | Metal housings | Tooling cost, draft and wall rules |
Pick the route by what the part has to prove
If the part must prove fit, feel, and appearance in a real material before steel is cut, use RTV silicone mold low volume casting. If it must prove structural strength in the final alloy or the final polymer, machine or mold it in that material instead, and accept the higher unit cost.
Questions engineers ask before committing
How many parts can one silicone mold produce?
Most RTV silicone tools are planned for 1 to 50 pulls. The first pulls are the most accurate. After that the parting line softens and the cavity can stretch slightly, so late parts drift.
If you need more than 50 identical parts with tight tolerance, plan a second mold or move to a harder tool. Running one mold past its window is how a good pilot build turns into a batch of rejects.
Can cast urethane replace the final production plastic?
For fit, assembly, and appearance checks, yes. For mechanical validation, only if the urethane grade matches the production resin in modulus, elongation, and heat deflection.
A glass-filled nylon housing and a Shore 80D urethane housing can look the same and behave differently under load. Tell us what the part has to survive, and we will say whether the resin can represent it.
What tolerance can I expect on a cast part?
Expect tighter control on the master than on the cast part. We machine masters to ±0.005 mm where the geometry allows. The cast part then adds shrink and demold variation on top.
Critical dimensions should be called out on the drawing. We check them on the master and on the first cast parts, and we report what we measure.
Do I need to supply a 3D model and a master pattern?
A 3D model is enough to start. If you already have a master, we will inspect it before molding and tell you if it needs rework. A master with surface damage or missing draft will show up in every cast part.
Uploads are handled as confidential, and an NDA is available on request.
Can inserts and threads be cast into the part?
Yes. Threaded inserts, bushings, and magnets can be fixtured in the cavity and encapsulated during pouring. This removes a secondary press or bonding step.
Insert position tolerance comes from the fixture, so send the mating part and we will set the fixture accordingly.
What finishes can be applied after casting?
Cast parts accept painting, laser marking, and light sanding or polishing. Laser marking needs a minimum character height of 1.5 mm to stay legible on a cast surface.
Heavy anodizing and plating are metal processes and do not apply to urethane parts. If the part needs a metal finish, it should be machined from metal in the first place.
Send the model and get a route recommendation
We review your geometry, batch size, and tolerance callouts, then tell you whether silicone casting, machining, or a printed master is the sensible route. Quotation and free DFM analysis come back within 12 hours.
12-hour quoteNo MOQNDA on request100% inspection
More process notes from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
