Starting liquid silicone rubber molding is a cross-functional release decision, not simply a request for a soft component. LSR is commonly supplied as two components that are metered, mixed and heat-cured in a mold, so material, geometry, tooling, molding conditions and inspection interact. A CAD review or material data sheet can begin the discussion, but neither proves the finished part will seal, flex, assemble, look acceptable or meet market obligations. This guide gives European, North American and Middle East product teams a practical validation sequence before tooling or production trials. It assumes the buyer owns product requirements and the final market decision, while the material producer and prospective molder provide grade- and process-specific evidence. For medical, food-contact, electrical, drinking-water, automotive or other regulated use, identify the applicable market, intended use and evidence owner in writing; do not infer approval from the word silicone or a general capability statement.
Release only what you can describe and judge: the specified grade, the part’s job and interfaces, the proposed molding route, the measurement method, and the evidence that will decide acceptance.
01
Translate the product job into a validation brief
Begin with the component’s job in the assembled product, not a hardness value alone. A seal, membrane, protective cap, keypad, valve feature and overmolded grip can all be made from silicone, yet they pose different questions about compression, recovery, repeated movement, contact media, thermal exposure, appearance and assembly force. Mark mating surfaces and load direction in CAD; state whether the feature is static or moving; and describe the use environment, including cleaning, storage and transport when relevant. For products sold in more than one region, keep destination markets, end use and customer-specific restrictions in the same controlled brief rather than relying on verbal assumptions.
LSR material selection should be a documented comparison of candidate grades against those conditions. Request the exact proposed grade, colour or pigment approach, current technical data, storage and handling instructions, and the stated scope of any declaration. Ask what part condition the quoted properties represent and whether post-cure, ageing or conditioning is assumed. A generic family description cannot establish the performance of the finished article. For a safety- or function-critical attribute, agree the specimen, conditioning, test method, pass/fail criterion and report format before using it as a release condition.
Use the table as a minimum brief. It makes gaps visible before a tool concept hardens around them and gives engineering, quality and purchasing a common review record.
| Validation area | Buyer should define | Evidence or decision to request |
|---|---|---|
| Function and environment | Load type, movement, mating parts, contact media, temperature history, cleaning and storage assumptions. | A named functional test or a documented reason why testing is not required at this stage. |
| Material identity | Approved grade, colour, allowable alternatives and any prohibited substances or residues relevant to the product. | Grade-specific data and a traceable material-identification approach; verify declarations against the intended market and use. |
| Interfaces and appearance | Sealing surfaces, visible areas, allowable witness locations, assembly datums and any rigid substrate if applicable. | Marked CAD or drawing and agreed visual/functional acceptance criteria. |
| Markets and records | Destination countries, product category, customer requirements and document retention expectations. | A responsibility map for who checks applicable requirements and who supplies each document or test result. |
02
Validate geometry, sealing intent and part release together
LSR injection molding should be reviewed as a flow, cure and demolding system, not as a direct copy of a thermoplastic tool concept. The review should address parting-line placement, gate and vent positions, flash-sensitive areas, runner strategy, and removal of the cured part. Make thin lips, delicate edges, holes, undercuts, captive features and cosmetic faces visible. Ask the molder to show the opening direction, cavity/core split, gate concept and removal method on the current CAD revision rather than describing them only in an email.
For a sealing feature, define its function in the assembly. Is it compressed against a flat or textured counterface? Does a clamp, housing tolerance, lubricant, fluid or thermal cycle change the contact condition? Which dimensions establish compression, location and leak path? The answer may lead to a functional gauge, fixture-based test or measured interface study; it should not be replaced with an unsupported claim that silicone will seal by itself. Identify each dimension critical to a mate by datum and purpose, but do not automatically apply the same inspection burden to non-critical geometry.
Ask for a risk-ranked DFM record that separates recommended design changes from open assumptions and states likely impact on function, appearance, tooling complexity or inspection. The objective is to decide which uncertainties can be evaluated with samples and which must be resolved before cutting steel. Retain the approved geometry and review record as the baseline for later changes.
03
Approve silicone molding tooling as a controlled process proposal
Silicone molding tooling is more than a cavity image of the part. Before release, ask the prospective molder to explain material feed and mixing, mold heating, gate and runner concept, venting, cure assumptions, part removal, and controls that distinguish normal variation from a process change. The purpose is not to prescribe equipment from a purchasing spreadsheet. It is to establish whether the process can make the agreed part and whether key assumptions are visible enough to test during trials.
A sample can look convincing while leaving the measurement plan undefined. Review inspection with the tool concept: the dated CAD/drawing revision, functional datums, features to be measured, fixture condition, visual standard, sample conditioning, reporting units and reaction to an out-of-agreement result. Select a method because it answers a named question. An optical method, fixture, section or functional leak test may each suit a different need. Do not assume a general CMM report proves in-assembly function.
Set trial gates in advance. The first trial can examine fill, cure, release, flash control, cosmetic condition, dimensions and assembly, but should also confirm that the test setup is repeatable. Later samples may be needed after a design change, revised tool detail or conditioning.
| Release stage | What the team validates | Practical output |
|---|---|---|
| Pre-tool review | Part revision, mold split, gates, vents, removal concept and unresolved geometry risks. | A marked DFM record with decisions, owners and open items. |
| Tool-design review | Tool concept, material feed, heat/cure assumptions, cavity strategy and inspection access. | A reviewable tooling proposal tied to the controlled part revision. |
| Trial plan | What will be observed, measured and function-tested; how samples are conditioned and identified. | A sample plan, report template and agreed acceptance decision path. |
| Production release | Approved configuration, allowable changes, material traceability and ongoing checks. | A release record and change-control list signed by the responsible product team. |
04
Separate grade evidence from product and market approval
Material literature is useful for narrowing choices, but it is not blanket approval for a finished product. Dow describes LSR as a two-part, dispensable silicone material that can be mixed and heat-cured, and material suppliers may offer grades for particular applications. That does not establish that a specific grade, pigment, processing route and finished geometry meet every requirement. Request the source, revision and scope of each declaration, identify its technical owner, and check whether stated use conditions match the intended product. A declaration relevant to a particular food-contact use, for example, is not a statement about all consumer or medical uses.
For liquid silicone rubber parts sold across Europe, North America or Middle East markets, build a market-evidence register rather than assuming one region’s document satisfies another region’s rule or a customer specification. Include the final product category, contacting media and duration/temperature of contact where relevant, and the owner of each test or submission. FDA food-contact guidance illustrates why identity, composition and intended conditions of use matter; it is not a certification route for an unrelated product. Where requirements are uncertain, obtain advice from the appropriate regulatory, customer or material technical authority before release.
This discipline also protects future sourcing. Record whether substitution of a material grade, pigment, processing aid, tool insert, gate, inspection method or assembly sequence requires review. A commercially minor change can affect appearance, dimensions, interface behavior or the evidence package.
05
Qualify the handoff and change path before placing the order
The right LSR injection molding supplier is the one that responds to the defined programme with clear assumptions, not the broadest unqualified promise. Ask each candidate to identify the reviewed part revision; proposed material and colour; key tool and process assumptions; open DFM risks; quoted trial scope; material and sample identification method; inspection proposal; and changes that trigger review. Compare these statements before comparing totals, because similar quotations can represent different cavities, runner concepts, test scope, secondary work, packaging or delivery assumptions.
Assign one buyer-side owner for the controlled product definition and one route for supplier questions and deviations. The release pack should contain CAD and drawing, DFM decisions, material designation, cosmetic limits, CTQs, test and inspection plan, sample approval record, packaging needs, markets to be considered, and change-notification expectations. This establishes how the part will be assessed and who decides when an assumption changes.
Practical takeaway
Before you move
to the next review.
- Document the component’s function, mating interfaces, use environment, destination markets and unresolved assumptions in a controlled validation brief.
- Approve the exact proposed LSR grade, colour approach and allowable alternatives, then define what grade-specific evidence and traceability are required.
- Review the latest CAD with the proposed mold split, gates, vents, removal direction, flash-sensitive zones and functional sealing or assembly interfaces marked up.
- Agree the trial sample plan, conditioning, CTQs, functional tests, visual standard, measurement method, report format and acceptance decision owner before tool release.
- Obtain a written supplier assumption list and establish change control for material, pigment, tooling, process, inspection, packaging and any secondary operation.
FAQ
Common questions
Is a material data sheet enough to approve an LSR design?
No. It can support a candidate-grade discussion, but it does not prove performance in the finished geometry, assembly or use environment. Confirm the exact grade, test conditions and limits of the data, then define any part-specific functional, dimensional, visual or market evidence needed for release.
When should a product team involve an LSR molder?
Involve a prospective molder before tooling release, when the team can still act on feedback about part split, gating, venting, release, flash-sensitive features, inspection and trial planning. The molder still needs the product owner’s defined function and acceptance criteria; early involvement does not transfer those decisions automatically.
Can one sample approval cover future material or tool changes?
Only if the controlled agreement explicitly says which changes are allowed and why they do not affect the approved requirements. As a cautious default, treat a material grade or pigment change, a gate or tool change, a process change, a different inspection method or a new secondary operation as a review trigger. Decide whether the change needs documents, new samples, measurement, functional testing or market evidence before it is implemented.
References
Source material
- [1] Dow, Liquid Silicone Rubber (LSR).
- [2] WACKER, Liquid Silicone Rubber (LSR) for 2K Injection Molding.
- [3] U.S. Food and Drug Administration, Guidance for Industry: Preparation of Premarket Submissions for Food Contact Substances: Chemistry Recommendations.
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.