A Molding Process That Runs Backwards

Liquid silicone rubber molding looks like injection molding and is not. In thermoplastic molding you heat pellets until they flow, inject into a cool mold, and the part solidifies as it loses heat. LSR does the opposite: two liquid components at room temperature are metered, mixed, and pumped into a hot mold at 300 to 400 degrees F, where they cure chemically in seconds. The barrel is chilled, the tool is heated, and every intuition about gates, vents, and cooling has to be re-learned. That inversion is why LSR quotes surprise people and why the wrong toolmaker delivers a mold that flashes uncontrollably on the first shot.

How the Process Actually Runs

LSR arrives as two drums, A side and B side, typically 1:1. A metering pump feeds both through a static mixer, optionally with pigment, into a chilled screw and then the tool. Cure time depends on wall and durometer but is short: a 0.08 in (2 mm) wall commonly cures in 20 to 45 seconds, competitive with thermoplastics and far faster than compression molding high-consistency rubber.

Because uncured LSR is watery it finds any gap in the tool. Parting surfaces are ground to a far tighter standard than on a thermoplastic mold, vacuum is often pulled on the cavity before the shot, and cold-runner systems are common to avoid curing material in the sprue. Demolding is frequently automated, since silicone parts are floppy and tacky and do not eject like rigid plastic.

When LSR Is the Right Answer

Silicone is not a cheaper rubber. It is chosen when a thermoplastic elastomer cannot survive the environment or cannot pass the regulatory requirement.

  • Temperature extremes. LSR holds its properties from roughly -70 to 400 degrees F. TPE and TPU turn brittle at the cold end and soften at the hot end well before that.
  • Biocompatibility. Medical grades are the default for skin-contact and short-term implantable parts, and are well characterized for the testing in ISO 10993 biocompatibility testing.
  • Food and infant products. Bottle nipples, valves, and seals rely on silicone's inertness and absence of plasticizers, a different qualification path from FDA food-contact plastics but the same conversation.
  • Sealing and compression set. Silicone recovers after years under load far better than most elastomers; gland geometry still follows O-ring selection and gland design.
  • Outdoor exposure and optics. Silicone does not chalk, crack, or yellow under UV and ozone, and optical grades mold high-clarity lenses that tolerate LED heat.

If none of these apply and the part just needs to feel soft, a thermoplastic elastomer overmold is almost always cheaper and simpler. That comparison, and the two-shot tooling it implies, is covered in overmolding and two-shot molding.

What Changes in Part and Tool Design

Several thermoplastic design rules reverse outright.

Draft is often unnecessary. Silicone's elasticity lets it strip off features that would be undercuts in a rigid part. Zero draft on many walls is normal, and modest undercuts can be demolded directly, which removes side actions a rigid part would require.

Wall thickness is far less constrained. Thick sections do not sink the way thermoplastic does, so the uniformity discipline from wall thickness for injection molded parts relaxes considerably. Thick sections extend cure time, and walls down to about 0.01 in (0.25 mm) are achievable.

Shrinkage is higher and durometer-dependent. Expect 2 to 3.5 percent, varying with the grade and the cure. Toolmakers steel-safe the first cut and adjust after the trial.

Flash control is the whole game. Parting line placement, vent depth measured in ten-thousandths of an inch, and vacuum sealing determine whether parts come out clean or need a deflashing operation that can double the piece price.

Durometer must be specified, not assumed. Common LSR grades run 10 to 80 Shore A, and the number changes both feel and sealing force substantially. Choose it deliberately using the Shore hardness guide and confirm it on physical samples, because durometer is nearly impossible to judge from a drawing.

Costs, Volumes, and Lead Times

Tooling for a simple single-cavity LSR mold typically runs $12,000 to $30,000, with multi-cavity production tools and cold-runner systems reaching $40,000 to $80,000 and up. That is roughly 20 to 50 percent more than a comparable thermoplastic tool because of the tighter grinding, the heating system, and the vacuum sealing. Hardened steel is the norm; the aluminum shortcut discussed in aluminum vs steel injection molds is rarely viable for LSR at the required parting-line precision.

Material costs $8 to $25 per pound for standard grades and considerably more for medical and optical grades, so LSR parts do not benefit from the cheap resin economics of commodity plastics. Build time for a production tool is usually 8 to 14 weeks.

The practical crossover is volume. Below a few thousand parts, cast silicone from a soft tool is faster and far cheaper, as described in casting soft silicone parts. Above roughly 10,000 to 20,000 parts per year, LSR's fast cycles and automation amortize the tool quickly. In between, run the numbers rather than assuming.

What to Ask an LSR Molder

Ask how many LSR tools they have built, not how many injection molds. Ask whether they run cold-runner systems in house, whether they pull vacuum on the cavity, and what their standard flash allowance is. Ask their post-cure policy, since some grades need a bake to drive off volatiles for food and medical use. And ask to see untrimmed first-shot parts from a recent program, which tells you more about tooling quality than any capability deck.

Decide Between Silicone and a Thermoplastic Before You Cut Steel

Projects House takes elastomer parts from requirement to production: material and durometer selection, part geometry for moldability, tooling specification, molder qualification, and first-article approval. Describe the environment and the volume your part has to meet through our contact form and we will tell you whether LSR is worth its premium.