How to Select LSR Injection Molding Machine Tonnage

What clamping force does an LSR machine actually need?

In short: enough to keep the mold closed against cavity pressure during fill and cure, plus a safety margin. Clamping force is calculated as the total projected area of all cavities and runners multiplied by the peak cavity pressure, then increased by 10-20%. LSR runs at far lower cavity pressures than thermoplastics, so silicone parts usually need a fraction of the tonnage a comparable plastic part would.

Cavity pressure in LSR molding commonly reaches up to roughly 300 bar (30 MPa), and unlike thermoplastics the cavity only needs to be filled to about 90-95% because the material expands as it cures to finish the surface. That low, controlled pressure is why a minimum practical clamp of around 290 kN (about 30 tons) is typical even for small LSR tools.

Why does projected area matter more than part weight?

Because clamping force resists the pressure trying to push the mold halves apart, and that force acts over the projected (shadow) area of the part, not its volume or weight. A thin, wide membrane can demand far more tonnage than a small, thick plug of the same mass. Always sum the projected area of every cavity plus the cold-runner footprint when sizing the machine.

What tonnage range fits common LSR applications?

The table below gives realistic starting points. Final tonnage still depends on your exact projected area, cavity count, and target cavity pressure.

 What else should you check besides tonnage?

Tonnage gets you in the door, but four other specs decide part quality and cost per part:

· Shot size and metering accuracy: the dosing/metering pump must hold the A:B ratio (typically 1:1) within tight tolerance for consistent cure.

· Platen and tie-bar spacing: confirm your mold's footprint and daylight fit, especially for cold-runner stacks and multi-cavity tools.

· Temperature control: heated platens and even mold heating to 170-200 C are essential, since LSR cures in as little as ~30 seconds at 170 C.

· Servo/electric drive and energy use: servo-hydraulic or all-electric clamps cut energy consumption and improve repeatability over standard hydraulics.

Is it better to oversize or undersize tonnage?

Neither extreme is good. Undersizing causes flash, dimensional drift, and scrap because the mold breathes open under pressure. Oversizing inflates purchase price, energy draw, and footprint, and can over-compress delicate cold-runner tools. The right answer is the calculated requirement plus a 10-20% margin, with the next standard machine size up only if your roadmap includes larger or higher-cavity tooling.

How TYM helps you size the right machine

TYM engineers LSR injection molding machines, silicone molds, and turnkey automated production systems for medical, automotive, infant-care, and industrial customers. Before quoting, TYM reviews your part drawing, projected area, cavity layout, and target output to recommend a tonnage and metering configuration matched to your project, rather than a generic catalog spec. The goal is precise, efficient, intelligent production with the lowest sustainable cost per part.

FAQs

Q: How many tons do I need for a small silicone O-ring mold?

A: Most small multi-cavity O-ring tools run on 100-350 ton (1,000-3,500 kN) machines. The exact figure depends on total projected area and cavity pressure. A 16-cavity ring tool around 90 cm2 projected area at 250 bar needs roughly 2,250 kN before adding a safety margin, so a 250-ton machine is a common fit.

Q: Why do LSR machines need less tonnage than plastic machines?

A: Because LSR is injected at much lower cavity pressure (around 150-300 bar) and only fills 90-95% before the material expands during cure. Thermoplastics need high holding pressure and pack-out, which demands far more clamping force for the same projected area.

Q: What safety margin should I add to the calculated clamping force?

A: Add 10-20% above the calculated requirement. This absorbs pressure spikes, mold wear, and minor process drift without forcing you into the next machine class. Larger margins are only justified if you plan to run higher-cavity or larger tools on the same machine later.

Q: Does cavity count change the tonnage I need?

A: Yes. Each added cavity increases total projected area and therefore required clamping force. Doubling cavities roughly doubles projected area, so always size tonnage on the full multi-cavity layout plus cold-runner footprint, not on a single cavity.