lsr-vs-htv-silicone-tooling-decision

Most silicone sourcing decisions are made in the wrong order. The buyer compares price per kilogram, picks the cheaper compound, and discovers six weeks later that labour and scrap have eaten the difference. Material price is the smallest line in the budget. Tooling, cycle time, deflashing and post-cure labour decide the landed cost, and all four are fixed by the process rather than the polymer.

This article is written for buyers who are about to commit to a mould. It covers what actually separates the two routes, where the break-even sits, and which part geometries remove the choice entirely.

The Difference Sits in the Machine, Not the Polymer

A 50 Shore A LSR and a 50 Shore A HTV are both polydimethylsiloxane. Both survive continuous service at 200°C. Both resist the same dilute acids, alcohols and glycols. On a datasheet the two columns look almost identical, which is why buyers assume the decision is about material.

The real difference is how the compound arrives and what it takes to turn it into a part. LSR arrives as two low-viscosity components, usually a few hundred thousand mPa·s each, and cures in seconds once the platinum catalyst meets the crosslinker. HTV arrives as a 20 kg slab at roughly 20 to 40 Mooney units and needs heat and time to vulcanise.

That single fact cascades. Because LSR cures fast, it can be injected into a heated cavity and demoulded in under a minute, which makes automation viable. Because HTV cures slowly, it sits in a press for minutes, which means every cavity is occupied and every cycle needs a person to load and unload it.

What the Mould Actually Costs, and Why

An LSR cold runner mould for a 4 to 8 cavity part typically runs 8,000 to 30,000 USD depending on cavitation, gate type and part complexity. The premium is not the cavity insert. It is the cold runner block and the valve gates.

Because LSR scorches within seconds at cavity temperature, the runner system has to be held at 20 to 25°C while the cavity runs at 170°C. That means an insulated cold plate, a separate tempering circuit, and needle valve gates that open and close on a timer to drop a shot into each cavity. A shop that has not built this hardware cannot quote LSR competitively, and a shop that quotes LSR tooling at HTV prices is usually planning to hand-press it, which defeats the point.

An HTV compression mould for a comparable part typically runs 3,000 to 10,000 USD. It has no cold runner, no valve gates, and looser shut-off tolerances because the compound is viscous enough not to flash through a 0.02 mm gap. What it does have is a longer cycle and a deflashing operation downstream.

A Worked Break-Even on a 12 g Part

Take a 12 gram part with a simple cylindrical geometry and no undercuts. On the HTV route a 4-cavity compression tool at a 5 minute cycle produces 48 parts per hour, and one operator can mind two presses. On the LSR route an 8-cavity tool at a 45 second cycle produces 640 parts per hour, and the machine runs unattended once set.

The last row is the one buyers miss. At 200,000 pieces a year the HTV route does not fail on cost, it fails on capacity. Four thousand machine hours is two presses running a full double shift for a year on a single part number, which no factory will dedicate to one customer.

Geometry Removes the Choice Entirely

Several part features make one route impossible rather than merely expensive.

Wall under 0.8 mm. HTV compound at 20 to 40 Mooney will not reliably fill a thin section without short shots. Below 0.8 mm, plan on LSR.

Overmoulding onto plastic or metal inserts. LSR injects at low pressure and low temperature, so inserts survive. HTV compression at 170°C and high clamp force distorts most engineering plastics.

Undercuts and long unsupported cores. Compression moulds handle these with simple split lines. LSR tools need side actions that add cost and scrap.

Parts over 500 g. LSR metering units top out around 2 to 4 litres per shot on common machines, and the tooling becomes very expensive. Large parts go to compression.

High mix, low volume. Ten variants at 800 pieces each cannot justify ten cold runner tools. One compression tool with interchangeable inserts usually can.

Six Questions Worth Asking Before the Deposit

These are the questions that separate a supplier who runs the process from one who brokers it.

How many cavities, and what is the projected cycle time at that cavitation on your machine?

Is the cold runner block included in the tooling price, or quoted separately?

What is your scrap rate on the first 5,000 pieces of a comparable part, and what causes it?

Do you deflash in house or subcontract it, and is that cost in the unit price?

Which post-cure schedule do you run, and can you supply the oven log for our batch?

What is the mould life in shots, and what is the refurbishment cost at half life?

What Post-Cure Changes, and What It Costs

Post-cure is where medical and food contact buyers get caught. LSR as moulded carries a volatile fraction that shows up as fogging on a heated lens and as extractables in a cytotoxicity test. A 200°C post-cure for 2 to 4 hours drives most of it off.

The cost is not the oven time, it is the handling. Parts go in loose, come out loose, and need a second inspection before packing. On a 200,000 piece order that is a real line item and it belongs in the quotation.

HTV usually needs post-cure regardless of application, because peroxide cure leaves decomposition by-products in the part. If a supplier quotes HTV with no post-cure line item, ask where the volatiles are going, because the answer is usually into the customer's first heat cycle.

Where Anysil Fits

We run both routes on the same site, which means the recommendation is not steered by what equipment happens to be free. Our liquid silicone rubber range covers general purpose through self-lubricating and high transparency grades for injection work, and our solid silicone rubber range covers compression and extrusion compounds from 30 to 80 Shore A.

For parts where wall thickness or transparency is the deciding factor, the high transparency LSR for consumer products is usually the starting point. For profiles and gaskets, the extrusion HTV range is the correct comparison.

Next Step

If you send us the part drawing, annual volume and target unit price, our engineering team will return a process recommendation with indicative tooling cost and cycle time for both routes within three working days. No quotation is issued before the mould concept is agreed.