Engineering Answers / Material & Selection
The two routes produce the same material on the drawing and very different parts on the bench. The choice is decided by geometry and volume long before it is decided by price.
LSR is a two-part liquid pumped into a heated tool and cured in seconds; HCR is a high-viscosity compound weighed into a cavity and pressed while it cures for minutes. LSR wins on tolerance, thin walls and high volume but needs far more tooling money. HCR wins on thick sections, large parts and low launch volumes, at the cost of dimensional consistency and more finishing labour.
Both routes start with the same chemistry: polydimethylsiloxane chains that crosslink when a platinum catalyst meets a hydride. What separates them is how much filler the compound carries, and therefore how the material arrives at the mold.
HCR — high-consistency rubber, also sold as HTV — is a gum-like compound, roughly the stiffness of a stiff dough. It is mixed with filler and catalyst by the compounder, delivered in slabs or strips, and has to be cut, weighed and placed into an open cavity before the press closes.
LSR — liquid silicone rubber — is supplied as two low-viscosity components, A and B, that are pumped, metered and mixed at the machine inlet in a 1:1 ratio. The mixed material is injected directly into a closed, heated cavity.
That single difference in how material enters the tool drives every other difference: cycle time, tooling construction, achievable tolerance, minimum wall, labour content and scrap rate.
| Decision | LSR injection | HCR compression |
|---|---|---|
| Tooling investment | Highest — cold runner, hardened shut-offs, tighter alignment | Lowest — a two-plate tool and a press is enough |
| Labour per part | Low — metering, injection and demolding are automated | Higher — charge weighing and loading, more handling |
| Cycle time | Seconds; cure happens in the heated tool | Minutes; the press is occupied while the part cures |
| Dimensional repeatability | Good; shot weight held within ±0.1% | Harder to hold; pressure varies cavity to cavity |
| Minimum wall | Thinner sections are feasible | Thicker walls are safer |
| Flash | Controlled by shut-off design, but LSR finds gaps under 0.005 mm | More flash by nature; trimmed or cryogenically deflashed |
| Material waste | Minimal with an automated cold runner | More waste — flash, runners and charge trimming |
| Typical fit | High volume, tight tolerance, medical and food contact | Low to mid volume, large or thick parts, cost-sensitive launch |
A part with a 0.4 mm wall, a 0.5 mm sealing bead and a 0.05 mm flatness callout is an injection part. There is no press setting that makes compression molding hold those features, because the material has to flow a long distance through a thin section under pressure that the press cannot meter precisely.
A part with a 6 mm section, a 300 mm span and a generous tolerance is a compression part. Injection would need enormous pressure to fill it, a much larger machine, and a tool that costs several times more for a feature the customer does not need.
Rule of thumb: if the wall is under about 1 mm, or the tolerance band is tighter than ±0.1 mm on a functional surface, start with LSR. If the part is big, thick, or geometrically simple, start with HCR and check whether the tolerance really demands injection.
Tooling is a fixed cost that has to be recovered per part. LSR tooling may cost several times an equivalent compression tool, so the crossover sits at annual volume — not at order size.
Ask two questions: how many parts per year, and how many years will the part stay in production. A 2,000-piece order spread across three years with a 500-piece annual call-off is a compression story even if the total looks respectable. A 50,000-piece-per-year programme with a two-year life is an injection story every time, because the labour and scrap savings compound on every shot.
There is also a middle route: aluminium rapid soft tooling produces 10 to 2,000 real injection-molded parts in 7 to 12 days for a fraction of steel tool cost, which lets a project validate the design in the production process before committing to a hardened tool.
Medical and food-contact programmes usually push toward LSR for a second reason beyond tolerance: the closed system is easier to validate. Material is delivered in sealed drums, metered inside the machine, and the part is demolded in a controlled environment — our medical LSR programme runs in a partner-facility ISO Class 7 cleanroom. Compression molding can be run clean, but it involves more open handling of uncured compound, and that is more surface for a validation auditor to question.
When the two routes look financially similar, the difference is usually in secondary operations rather than in molding.
Compression-molded parts arrive with flash that has to be removed. On cosmetic edges, cryogenic deflashing — freezing the part so the thin film goes brittle, then tumbling it against non-abrasive media — is a clean, repeatable answer. On a sealing bead or a thin wall it is the wrong remedy, because the process removes material by impact and the geometry that seals is exactly what you cannot afford to lose. That pushes the work to manual trimming, which is labour, which is variation.
LSR parts still flash, and a high-cavity tool is where flash control gets difficult. But the fix is in the tool: hardened shut-off faces held to ±0.005 mm, verified on a CMM at ±0.002 mm resolution, and venting designed at the fill endpoints. That is a one-time engineering cost rather than a per-part labour cost.
This is the reason a part can be cheaper in LSR at 20,000 pieces a year and more expensive at 2,000 — not because the material changed, but because the trimming labour that compression carries does not scale down.
A process recommendation without these five inputs is a guess:
Send those five and a STEP file, and the route recommendation usually comes back with the DFM report rather than as a separate exercise.
Customers usually arrive asking "which is cheaper". The question that actually decides the route is "which surfaces carry the function". A part with one sealing face and no other critical dimension can often be compression molded at a third of the tooling cost, because the single critical surface can be put on a shut-off that the tool controls precisely. A part where every surface is critical has no such luxury, and LSR becomes the only route that holds the print.
Choose LSR when the part is thin-walled, tolerance-critical, high volume, or needs a documented closed material path. Choose HCR when the part is thick, large, geometrically simple, or launching at a volume that will never repay an injection tool. When the two are close on cost, decide on the secondary operations — trimming labour is the line item that quietly decides the winner.
Often yes, but not at the same quality or the same price. A simple gasket with a thick section and a generous tolerance is a legitimate candidate for either route. Once the part has a thin wall, a sealing bead with a defined contact width, or a tolerance tighter than about ±0.1 mm, compression molding starts losing control of the dimensions that matter and the comparison stops being fair.
No — it is more expensive to start and usually cheaper to run. LSR tooling carries the cold runner system, tighter shut-offs and often more cavities, so the up-front number is higher. Per part, LSR removes the weighing and loading labour, shortens the cycle and cuts flash waste. At low annual volume you never recover the tooling difference; at high volume the LSR route is normally the lower total cost.
Because the process depends on the operator and the press rather than on a closed metering system. The charge weight is placed by hand or by a loading robot, the material flows under press closing rather than through a controlled gate, and each cavity sees a slightly different pressure and temperature history. Injection meters the shot within ±0.1% and fills every cavity through the same runner, which is why repeatability is better.
It narrows the practical range rather than blocking it. Very soft compounds in the Shore 20A–30A region are harder to demold and more prone to distortion in compression molding, while very hard grades above 70A flow poorly and need more injection pressure in LSR. We stock Shore 20A to 80A across ten standard grades and will tell you at DFM stage if your target hardness pushes against the limit of the chosen route.