- Posted in: Blog
- By GreenBanana
Speed, cost, and how closely a prototype approximates the final part design, including material properties and dimensions, are key criteria when selecting a silicone rapid prototyping method. Silicone parts can be prototyped through several different processes, including RTV molding, high consistency rubber (HCR) molding, liquid silicone rubber (LSR) injection molding, and 3D printing.
But it’s most important to understand your objectives for the prototyping stage. Are you looking for approximately shaped components to build rough prototypes for customer feedback? Are you trying to check fit and function? Or are you hoping to check tolerances and reduce uncertainty before production by understanding process control ranges, assessing what tolerances are feasible with the design, and evaluating likely process capability?
At ProMed, we evaluate the available options based on part geometry, material requirements, tooling, and the stage of development. Because many new silicone products are manufactured from LSR, often in the 30–70 Shore A durometer range, the right method should reflect how closely the prototype needs to represent the intended production material, dimensions, and molding process.
A review of 4 methods
RTV molding
Industrial room-temperature-vulcanizing (RTV) silicone formulations are widely available. Making parts from these materials requires a mold, which can be made from a range of metal or plastic materials depending on the geometry, dimensions, surface finish, and number of parts required. Although RTV silicone cures at ambient temperatures, ovens may also be used to accelerate curing depending on the formulation.
Pros: RTV molding has relatively low capital equipment requirements, and raw materials are readily available. Certain RTV materials can provide a reasonable approximation of related LSR materials for some prototype evaluations. Well-equipped R&D labs may also be able to manufacture prototype tooling from plastic or soft metal materials in-house. Once the mold is made, producing parts can be a relatively simple fill-and-wait process.
Cons: RTV molding is relatively slow, with cure times that can extend from about 20 minutes to several hours depending on the material and process. Production rate and cost per part are therefore heavily influenced by processing time. RTV also uses a different molding process than production LSR injection molding, so these prototypes generally won’t provide the same insight into production tolerances, cure behavior, process parameters, or other LSR-specific manufacturing conditions.
HCR molding
Transfer molding or gum stock injection molding can be used to make parts from high consistency rubber (HCR). In either process, the high-viscosity, gum-like material is forced into a heated cavity, where it fills the mold and cures. HCR is a natural fit for transfer molding because gum stock is much more viscous than LSR, although specialized gum stock injection molding can also be used and may allow for more automated processing.
Pros: Transfer molding can use relatively simple tooling, and process development tends to be fairly straightforward. HCR prototypes can also provide useful information about geometry, hardness, flexibility, and other material-dependent behavior when HCR is relevant to the intended production design.
Cons: Complex geometries may not be possible depending on the part design and molding route, and cycle times are generally longer than with LSR. If the intended production process uses LSR, an HCR prototype also won’t reproduce LSR material delivery, flow, cure, or injection behavior.
LSR molding
Production-grade liquid silicone rubber (LSR) is injection molded into heated tooling, often using soft metal prototype molds to reduce tooling costs and shorten lead times. When the intended production part will also use LSR injection molding, this approach can provide a close approximation of both the final material and the planned molding process.
Pros: Tooling can be made relatively quickly, and substantial quantities of prototypes can be produced once the process is established. The resulting parts can closely approximate production parts and can provide useful information about dimensions, cavity filling, cure behavior, shrinkage, flash, demolding, tolerances, and process capability before production tooling and process parameters are finalized.
Cons: LSR can result in a higher cost per part for small prototype runs because the initial tooling and process-development costs are spread across relatively few parts. An LSR prototype also doesn’t establish production readiness on its own; production tooling, final process parameters, inspection, validation, and documentation may still need to be defined or refined as the program moves forward.
3D printing
A variety of materials can be 3D printed, but silicone is inherently more difficult to print because of its heat-cured nature. As a result, silicone 3D printing remains less developed than thermoplastic 3D printing, and printed silicone parts are generally a poor approximation of molded production parts.
Pros: 3D printing eliminates the need for prototype mold tooling and can be useful for early design checks when the main goal is to evaluate shape, basic dimensions, or fit. Non-silicone SLA or FDM materials may also be useful as surrogates when silicone material behavior isn’t part of the evaluation.
Cons: 3D-printed parts generally won’t reproduce the material properties, surface finish, dimensional behavior, or molding conditions of a production silicone component. They also can’t show how the intended silicone will fill, cure, release from the mold, or behave during a production molding process so they are not a true approximation of a molded part.
Silicone rapid prototyping comparison
| Method | Description | Timing considerations | Cost considerations | Approximation to production |
|---|---|---|---|---|
| RTV | Silicone is placed into prototype tooling and cured at ambient or elevated temperatures, depending on the formulation | Timing depends on mold fabrication and cure time, which can range from about 20 minutes to several hours | Tooling and equipment requirements can be relatively simple, but longer processing times can increase the cost per part | Can approximate some silicone material properties and basic part behavior, but the material and molding process may differ from production LSR |
| 3D printing | Parts are printed directly without prototype mold tooling, using silicone or non-silicone materials, depending on the approach | Avoids mold fabrication, although timing depends on the printing method, material, and quantity | Avoids the cost of prototype mold tooling | Generally a poor approximation of a molded production silicone part; non-silicone surrogates are mainly useful for evaluating shape, dimensions, or fit |
| HCR | High consistency rubber is molded using transfer or specialized gum stock injection molding | Timing depends on tooling, molding route, part geometry, and quantity; cycle times are generally longer than with LSR | Requires mold tooling as well as material preparation and processing | Can provide useful information about geometry and material behavior and can approximate an eventual HCR production process |
| LSR | Production-grade liquid silicone rubber is injection molded using prototype tooling | Requires tooling and process setup, but substantial quantities of prototypes can be produced once the process is established | Initial tooling and process-development costs can weigh more heavily on small prototype runs | Can closely approximate the intended production material and LSR injection molding process |
Frequently asked questions
What types of silicone rapid prototyping services does ProMed offer?
ProMed supports several silicone rapid prototyping methods, including RTV molding, HCR transfer molding, and LSR injection molding. RTV molding can be useful for low-volume prototypes and early evaluations of form or fit, while HCR transfer molding can provide useful functional information when HCR is relevant to the intended design.
For prototypes that need to more closely approximate an intended LSR production part, ProMed can use production-grade LSR with prototype tooling such as soft aluminum molds. Depending on the program, prototype work may also include approaches such as 3D-printed molds for RTV or overmolding onto rigid substrates.
How does ProMed determine which prototyping method is best for my project?
ProMed evaluates factors such as timeline, budget, part geometry, material performance requirements, and what the prototype needs to demonstrate. If the goal is to evaluate basic form or fit, a simpler approach—such as RTV molding or 3D printing—may be sufficient.
For later-stage development, molded prototypes using HCR or LSR can provide a closer approximation of the intended production material and process. Using tooling and molding methods that are closer to the planned production route can also support early process development and help assess tolerances, process capability, and cost to de-risk the process before production tooling is finalized.
What are the typical lead times for ProMed’s silicone prototyping services?
Lead times depend on the prototyping method and part complexity. HCR transfer molding generally requires about two weeks for tooling, followed by roughly one additional week to produce initial parts.
LSR injection molding typically takes about three weeks for prototype tooling, with initial parts available within about a week after tooling is complete. Expedited timing or parallel tooling development may be possible depending on the project.
Moving from prototype to production
Prototype results can help identify what needs to change before investment in full-scale production tooling and process development. ProMed reviews the intended material, critical geometry, tooling and molding approach, expected quantity, inspection requirements, and what was learned during prototyping so the production plan reflects those findings.
If material selection is still being evaluated, our guide to common materials for silicone injection molding provides additional context. For programs likely to move into LSR production, learn more about liquid silicone rubber molding.
If you’re preparing to move a silicone component beyond prototyping, contact our team to discuss your part requirements, prototype goals, and intended production process.
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