Before requesting a quote for a silicone part, there is a decision that determines cost, lead time and feasibility: moulding or extrusion? They are not interchangeable processes. Each has geometric limitations, tolerance ranges and cost structures that make it optimal for specific applications. Choosing wrong means paying for unnecessary tooling, extending development or receiving parts that do not perform their function.
This article does not explain what moulding is. It explains when to use it, what geometries it allows and what specification errors to avoid. If you need an O-ring, a silicone membrane, a bellows, a washer with specific geometry or any part with a three-dimensional shape, moulding is your process. If you need a continuous profile with constant cross-section, it is not.
1. Moulding vs. extrusion: technical decision criteria
Extrusion produces constant cross-sections in continuous lengths. Moulding produces three-dimensional geometries defined by a closed cavity. This fundamental difference determines when to use each process.
When to choose moulding
- The part has section variations along its geometry
- Closed shapes such as O-rings, membranes or diaphragms are required
- The application demands tight dimensional tolerances in all directions (class M1 or M2 per ISO 3302-1)
- The component needs integrated metal or plastic inserts
- The geometry includes internal cavities, ribs, sealing lips or specific functional surfaces
- A controlled surface finish is required (texture, marking, specific roughness)
When NOT to choose moulding
- The part has constant cross-section along its entire length
- Lengths greater than 500 mm are required for simple geometries
- The design is a profile, tube or cord that can be cut to length
- Flexibility to supply different lengths without tooling change is needed
- Volume does not justify investment in a dedicated mould
2. What problems moulding solves versus extrusion
Moulding is not simply an alternative to extrusion. It solves specific problems that extrusion cannot address:
Three-dimensional geometries
A silicone O-ring has a constant circular cross-section, but it is a closed form: it cannot be manufactured by direct extrusion. A flat gasket with sealing lips, a membrane with reinforcing ribs, a silicone bellows with variable geometry folds, a conical plug, a technical part with multiple integrated functions: all require moulding because their geometry varies in three dimensions.
Silicone O-Rings
Silicone VMQ O-Rings molded by compression or injection. Static or dynamic sealing in critical applications. Hardness 30–90 Shore A. Certified materia...
View product →Tighter tolerances
Moulded products achieve class M1 or M2 tolerances per ISO 3302-1, significantly tighter than the E1 or E2 classes typical in extrusion. For applications where dimensional fit is critical—precision seals, valve components, silicone washers for sealing in process equipment—moulding provides the repeatability that extrusion cannot achieve.
Component integration
Moulding allows overmoulding of metal inserts, bushings, reinforcements or armatures. A shock absorber with threaded fixing, a component with metal core to resist tightening torque, a silicone rubber part with internal structural reinforcement: these are only viable through moulding with insert placed in cavity.
Controlled surface finishes
The surface of a moulded part exactly replicates the mould finish. This allows obtaining anti-slip textures, specific gloss or matte finishes, permanent markings without ink, sealing zones with controlled roughness. Extrusion does not offer this level of surface control.
Series repeatability
Once the process is adjusted, each part comes out identical to the previous one. This consistency is mandatory in regulated sectors: a membrane for a medical device, a food-grade gasket for process equipment, a component with railway certification must maintain constant properties batch after batch.
3. Moulding processes: compression, injection and LSR
There is no single «silicone moulding». There are three main processes, and choosing the right one affects cost, lead time and technical feasibility.
3.1 Compression moulding
The most versatile process. The silicone compound is placed in the lower cavity of the open mould, the press closes applying pressure and temperature, the material flows to fill the geometry and vulcanises in position.
Compression moulding has the lowest tooling cost and accepts virtually any silicone formulation, including high hardness compounds, with special fillers or difficult to process by other methods. It is the standard process for large cross-section O-rings, flat gaskets, robust geometry technical parts, short and medium runs.
The main limitation is cycle time: longer than injection, with inherent flash requiring deflashing. It is not efficient for very complex geometries with fine details or multiple cavities.
Silicone Compression Molded Parts
VMQ silicone compression molding. Manufacturing of large or complex parts, functional prototypes, and short runs. Hardness 20–90 Shore A. Industrial o...
View product →3.2 Injection moulding
The compound is plasticised in a heated barrel and injected under pressure into the closed mould. Filling is more controlled, flash is reduced, cycles are shorter.
Injection moulding is preferable for medium-long runs, complex geometries with precise details, parts where productivity justifies more expensive tooling. Silicone valves, technical plugs, precision components with tight tolerances are typically manufactured by injection.
Not all formulations are injectable—very high hardness silicones or those with abrasive fillers can present problems—and tooling is significantly more expensive than compression.
HCR Injection Molded Parts
High Consistency Rubber (HCR) silicone injection molding. Production of precise, repeatable, and high-performance parts. Hardness 30–90 Shore A. Autom...
View product →3.3 LSR moulding (Liquid Silicone Rubber)
LSR uses two-component liquid silicone that is metered, mixed and injected directly into the mould. Addition catalysis (platinum) allows very fast vulcanisation.
LSR is the process for high-demand medical components, precision parts with very thin walls, high-volume long runs. LSR formulations achieve the most demanding certifications: USP Class VI biocompatibility, ISO 10993, short-term implantable grades for tissue contact up to 29 days.
Entry cost is highest: specific metering machinery, higher precision moulds, more expensive formulations. It is justified in applications where productivity, material purity or regulatory requirements demand it.
LSR Injection Molded Parts
Liquid Silicone Rubber (LSR) injection molding. Fully automated process for high-precision parts, microcomponents, and medical products. Hardness 20-9...
View product →4. Technical fundamentals of the process
Regardless of method (compression, injection, LSR), silicone moulding shares technical principles that affect design and specification.
In-mould vulcanisation
The silicone compound enters the mould in plastic state (uncured) and exits as crosslinked elastomer. Vulcanisation occurs through temperature (typically 150-200 °C) and time. The cycle must be sufficient for the centre of the part to reach the required degree of crosslinking.
Thick parts require longer cycles: heat must penetrate to the core. This has design implications: a 20 mm thick section vulcanises much slower than a 3 mm one, affecting productivity and cost.
Shrinkage
Silicone shrinks during vulcanisation and cooling. Typical shrinkage is between 2% and 4% depending on formulation and geometry. The mould is designed oversized to compensate, but the specifier must indicate final part dimensions, not mould dimensions.
Flash
In compression and conventional injection, some material escapes through the mould parting line forming flash. This flash must be removed by manual, cryogenic or mechanical deflashing. Part and mould design can minimise flash, but not completely eliminate it in these processes.
Demoulding
The vulcanised part must be extracted from the mould. Geometries with undercuts, closed cavities or negative angles complicate or prevent demoulding. Design must consider how the part will exit: draft angles (minimum 1-3°), push surfaces, possibility of elastic deformation to overcome minor retentions.
Parting line
The mould has a closure plane that leaves a mark on the part. This parting line must be located where it does not affect functional surfaces (sealing zones) or aesthetics. The part drawing must indicate where this line can and cannot be.
5. Common errors in moulded part specification
Accumulated experience allows identifying recurring errors that generate rejections, cost overruns or in-service failures.
5.1 Impossible geometry to demould
Closed undercuts, internal cavities without access, geometries that «trap» the mould: these are design errors detected when the mould is already in manufacture or, worse, when the first parts do not come out.
Moulds can incorporate solutions (slides, collapsible cores, split moulds), but they multiply cost and complexity. It is preferable to design for simple demoulding from the start.
5.2 Parting line in functional zone
If the mould closure line crosses a sealing surface of a silicone flat gasket or the lip of a seal, there will be a discontinuity in the critical zone. This can mean leaks or premature failure.
The drawing must explicitly define where the parting line can be located. If not indicated, the moulder will place it where most convenient for manufacturing, not necessarily for function.
Silicone Flat Gaskets
Flat gaskets made from solid or sponge VMQ silicone. Cut by die, laser, or waterjet. Certified materials FDA, EC 1935/2004, UL94, EN 45545-2, USP VI, ...
View product →5.3 Problematic thicknesses
Very thin walls (< 0.5 mm) are difficult to fill and prone to defects. Very thick sections (> 15 mm) vulcanise unevenly: the exterior cures before the interior, generating stresses and non-homogeneous properties.
Abrupt thickness changes create similar problems. If design requires thick zones next to thin zones, transitions must be gradual.
5.4 Metal drawing tolerances
Copying tolerances from a machined part to a silicone rubber part is a common error. Silicone is an elastomer: it has shrinkage, temperature variation, elastic behaviour. It is not a micrometric precision material.
Tolerances must be specified per ISO 3302-1: class M2 for standard applications, M1 for precision. Demanding tighter tolerances increases tooling cost, slows production and generates unnecessary rejections.
5.5 Material without functional specification
Indicating «60 Shore A silicone» leaves compound selection to the manufacturer. They will optimise for availability or cost, not for application performance.
The specification must include:
- Hardness range with tolerance (e.g.: 60 ±5 Shore A)
- Required certifications (FDA, EN 45545-2, USP Class VI...)
- Special resistances if applicable (high tear, low compression set, chemical resistance)
- Actual service temperature range
- Colour restrictions if any (some colourants void food certifications)
5.6 Not evaluating tooling cost
The mould is an investment. A simple compression tool can cost from €1,500-3,000; a multi-cavity injection mould can exceed €15,000-25,000.
For very short runs, mould cost can exceed part cost. Before launching development, evaluate: does a standard product exist that works? Can it be die-cut from calendered sheet? Does forecast volume reasonably amortise the tooling?
6. Formulation selection: technical criteria
Correctly specifying the compound is as important as geometry. «Silicone» is not a single material: there are dozens of formulations with very different properties.
By catalysis type
Peroxide catalysis: the most widespread system. Good cost-performance ratio, wide hardness range (10-90 Shore A), compatible with most processes. Generates volatile by-products that may require post-cure in sensitive applications.
Platinum catalysis: addition vulcanisation, no by-products. Mandatory for demanding medical applications, high-purity food-grade. Superior mechanical properties, especially tear resistance. More expensive and sensitive to contaminants (sulphur, amines, certain metals).
By mechanical properties
High tear: tear resistance > 25 kN/m (peroxide) or > 30 kN/m (platinum). For parts subject to assembly stresses, cyclic flexing, crack propagation risk. O-rings with frequent assembly, pump membranes, bellows.
Low compression set: compression set < 15% (70h/150°C). Critical for long-duration static seals where loss of elastic recovery means loss of sealing.
By special resistance
Fluorosilicone (FVMQ): resistance to hydrocarbons, fuels, oils that standard VMQ silicone cannot withstand. Reduced thermal range (-60 to +170 °C).
High temperature: stabilised formulations for continuous service up to +270 °C or +300 °C. For parts in ovens, engines, thermal process equipment.
Low temperature (PVMQ): phenyl silicones maintaining flexibility down to -110 °C. Cryogenic, aerospace, LNG applications.
By certification
Food contact: FDA 21 CFR 177.2600 (USA), EC 1935/2004 (Europe), BfR (Germany). Mandatory for any part in direct or indirect contact with food.
Medical grade: USP Class VI, ISO 10993. Short-term implantable formulations for tissue contact up to 29 days.
Railway EN 45545-2: certified compounds for fire, smoke and toxicity behaviour. Moulding compounds achieve HL1, HL2, HL3 levels in requirement sets R22 and R23. Relevant technical data: moulding formulations can offer better smoke opacity values (Ds max 45) than equivalent extrusion formulations (Ds max 84.7).
7. Sectors where silicone moulding is critical
Medical and pharmaceutical
Medical device components, infusion pump diaphragms, dosing valves, diagnostic equipment elements. Biocompatibility, traceability, cleanroom manufacturing and ISO 13485 quality system requirements are mandatory.
Medical grade formulations with platinum catalysis and USP Class VI / ISO 10993 certification are standard. For short-term implantable components (< 29 days), specific grades validated for tissue contact are required.
Medical
Piezas, tubos y componentes LSR certificados ISO 13485 y USP Class VI para dispositivos médicos.
Explore sector →
Medical LSR Liquid Silicone
Medical-grade LSR liquid silicone compounds for injection molding in an ISO 8 cleanroom. Compliance with USP VI, ISO 10993, FDA 21 CFR 177.2600, and E...
View product →Food and beverage
Gaskets for processing machinery, food pump membranes, valves, equipment seals. FDA and EC 1935/2004 certifications are mandatory. Autoclave gaskets also require formulations with improved saturated steam resistance.
Railway
EN 45545-2 compliance is mandatory for rolling stock in Europe. Certified compounds are available for both moulding and extrusion, with hardnesses from 30 to 85 Shore A. Typical applications include door seals, cable grommets, HVAC seals.
Railway
Fabricación de componentes y juntas de silicona con certificaciones EN 45545 y resistencia al fuego para el sector ferroviario.
Explore sector →General industry
Automotive (engine gaskets, cooling components), household appliances (oven seals, dishwasher seals), electronics (IP67/IP68 seals, keypads), lighting, HVAC, industrial machinery. Moulded silicone solves sealing and damping problems where other elastomers fail due to temperature, ageing or chemical compatibility.
8. Decision process: is moulding right for your part?
- Does the part have three-dimensional geometry that varies in volume? → Yes = moulding
- Is it a closed form (O-ring, membrane, diaphragm)? → Yes = moulding
- Does it require ISO 3302-1 class M1 or M2 tolerances? → Yes = moulding
- Does it need integrated inserts? → Yes = moulding
- Is it a profile or tube with constant cross-section? → Yes = probably extrusion
- Does volume justify tooling investment? → If < 100-500 parts, evaluate alternatives (die-cutting, machining, standard product)
Need to validate if moulding is the right process?
If you have a part in design phase and need to confirm process, moulding type or formulation, our technical team can help before committing to tooling. We work with engineering departments to optimise designs, select materials and anticipate problems.
Contact engineering →The information contained in this article is for guidance purposes. Final specifications must be validated with our technical department based on the specific requirements of each application.
