HTV Silicone Rubber Molding Methods Compared: Compression, Transfer, Extrusion and Injection
Quick answer
Choose the molding method from the part, not from the machine you already own. Thin, high-volume, tight-tolerance parts belong in injection molding. Thick or large parts with modest volume belong in compression molding. Long constant-profile parts belong in extrusion. Transfer molding is the fallback when you need better dimensional control than compression gives but cannot justify an injection tool. Getting this wrong usually costs more in scrap than the tooling difference.
What HTV silicone rubber is
HTV - high temperature vulcanizing - is the classic solid silicone rubber supplied as a workable, gum-like compound. It is mixed with fillers and a curing agent, shaped, then vulcanized at elevated temperature. Two families dominate:
Precipitated (fumed) silica filled compounds - the mainstream choice for general molding and extrusion
Platinum-cured systems - for medical, food-contact and optically clear parts
HTV is often confused with LSR. The practical difference: HTV is handled as a solid gum and shaped under pressure, while LSR is pumped as a two-component liquid and injected into a heated mold.
The four methods at a glance
| Compression | Transfer | Extrusion | Injection | |
|---|---|---|---|---|
| Tooling cost | Lowest | Medium | Low (die only) | Highest |
| Typical cycle | Long (minutes) | Medium | Continuous | Shortest |
| Dimensional control | Moderate | Good | Profile-dependent | Best |
| Best for | Thick, large, low-volume parts | Medium precision, medium volume | Tubes, seals, profiles, strips | Small, thin, high-volume parts |
| Scrap rate risk | Higher | Medium | Low | Lowest |
| Flash | Often significant | Less | N/A | Minimal |
Compression molding
Compound is placed directly into a heated cavity, the mold closes and pressure plus heat cure the part. It is the most forgiving method and the cheapest to tool.
Use it for: gaskets, pads, large mats, thick sections, prototypes, and anything where annual volume does not justify a better tool. Because fill is manual or semi-manual, expect flash and some weight variation.
Transfer molding
Compound sits in a pot above the cavities and is pushed through a sprue into the mold. This gives better dimensional repeatability and less flash than compression because the material enters under pressure.
Use it for: parts with inserts, multi-cavity work where consistency matters, and medium volumes that do not justify injection tooling.
Extrusion molding
Compound is forced through a shaped die and cured continuously - typically in a hot air tunnel or a salt bath. There is no cavity tooling, so the economics are excellent for constant cross-section.
Use it for: tubing, cord, sealing strips, sleeving and profiles. The main quality levers are die design, screw temperature profile and cure line speed. Fumed-silica grades with high green strength extrude more cleanly than precipitated grades.
Injection molding
For HTV, injection molding means a screw or ram injects heated compound into a closed, hot mold. Cycles drop to seconds rather than minutes, flash is minimal and tolerances tighten considerably.
Use it for: high annual volumes, small precision parts, and anything where labor content in compression molding would dominate the part cost.
Platinum vs peroxide curing
| Platinum (addition) cured | Peroxide cured | |
|---|---|---|
| By-products | None | Organic residues |
| Post-cure need | Minimal | Required to remove volatiles |
| Transparency | Excellent | Limited |
| Food / medical | Preferred | Case by case |
| Cost | Higher | Lower |
| Process sensitivity | Sensitive to sulphur, tin, amine contamination | More forgiving |
The rule of thumb: if the part touches skin, food or a patient, specify platinum. If it is an industrial gasket, peroxide-cured compound will usually do the job for less.
How to decide in practice
Work through these questions in order:
Is the cross-section constant along the length? If yes, extrusion is almost always the cheapest answer.
How many parts per year? Under roughly 10,000, compression or transfer is usually correct. Above that, run the injection numbers.
How tight is the tolerance? If you cannot measure it with a simple gauge, you probably need injection or transfer.
Does the part touch food, skin or a patient? Specify platinum cure and ask for the migration or biocompatibility file.
What is your tooling budget, and how long is the product life? A cheap tool on a five-year product is a false economy.
FAQ
Is HTV or LSR better for my part? HTV wins on thick sections, low volumes and low tooling cost. LSR wins on thin walls, high volumes, automation and consistent hardness out of the box.
Can one compound be used for both molding and extrusion? Some dual-use grades exist, but they are always a compromise. Dedicated extrusion grades have higher green strength; dedicated molding grades flow better in a cavity.
Why does my compression-molded part have a weak knit line? Usually insufficient flow before cure, or a compound that has partly scorched in storage. Check mold temperature uniformity and compound shelf life.
Do I need a post-cure oven? For peroxide-cured parts, yes - post-cure drives off volatiles and stabilizes properties. For platinum-cured food or medical parts, a defined post-cure is still normal practice.
Can you recommend a grade for my process? Yes. We produce general purpose molding HTV, high heat resistant extrusion compound, the ASH7050 series for compression molding and platinum-cured compound for compression and extrusion, all from a 6,000 m² plant in Guangzhou with 300–400 tonnes monthly capacity.
Next step: send us your part drawing or profile, annual volume and cure-system requirement. We will recommend a compound, quote a trial quantity and ship a sample with a written processing sheet.