If you‘re sourcing injection molds for a new product, you might assume that a mold is a mold—whether it’s for liquid silicone rubber (LSR) or thermoplastic. After all, both processes involve injecting material into a cavity under pressure, right?
Wrong.
While LSR injection molding shares some superficial similarities with thermoplastic injection molding, the differences run deep—and they start at the molecular level. LSR is a thermoset, not a thermoplastic. It cures through an irreversible chemical reaction, it expands in the mold instead of shrinking, and it can flash through gaps as small as 0.005 mm. Thermoplastic molds are designed to cool the material. LSR molds are designed to heat it.
These fundamental differences mean that an LSR mold is not simply a thermoplastic mold with different settings—it requires entirely different design principles, manufacturing tolerances, and maintenance practices.
This guide breaks down the seven most critical differences between LSR molds and thermoplastic molds, helping you understand what makes LSR tooling unique—and why choosing the right mold partner matters.
This is the foundational difference from which all others flow.
Thermoplastics are supplied as solid pellets or powders. They are melted in the machine barrel (typically 180–350°C), injected into a cooled mold (typically below 80°C), and allowed to solidify through cooling. The process is physically reversible—thermoplastics can be re-melted and re-used.
LSR is supplied as two liquid components (Part A and Part B). The material is chilled and mixed in a cooled barrel, injected into a heated mold (typically 150–200°C), and cured through a chemical cross-linking reaction. The process is chemically irreversible—once cured, LSR cannot be re-melted.
| Aspect | Thermoplastic | LSR |
|---|---|---|
| Material form | Solid pellets/powder | Two-part liquid |
| Barrel temperature | 180–350°C (heated) | 40–80°C (cooled) |
| Mold temperature | Below 80°C (cooled) | 150–200°C (heated) |
| Solidification mechanism | Physical cooling | Chemical cross-linking |
| Recyclability | Yes (re-meltable) | No (permanently cured) |
Why this matters for mold design: An LSR mold must withstand continuous high heat (150–200°C) while maintaining precise temperature uniformity across all cavities. Thermoplastic molds, by contrast, are designed for cooling efficiency.
This is perhaps the most visible difference between the two mold types.
Thermoplastic molding requires cooling the material inside the mold. Temperature control is typically achieved with circulating oil or water through cooling channels. Mold temperatures for thermoplastics commonly do not exceed 170°F (77°C).
LSR molding requires heating the material to initiate the curing reaction. LSR molds must be heated to 320–420°F (160–215°C), most commonly with electrical cartridge heaters. The challenge is not just reaching these temperatures but maintaining them uniformly across the entire mold surface.
Design implications:
Thermoplastic molds: Optimized for rapid, uniform cooling
LSR molds: Optimized for rapid, uniform heating with precise zone control
LSR molds require careful thermal expansion management—materials and components expand differently at high temperatures
Thermoplastic molds require a hot runner to maintain material temperature between shots; LSR molds require a cold runner (cold deck) to prevent premature curing
As one industry expert puts it: “Temperature control systems in LSR machines are fundamentally different. While thermoplastic machines focus on heating the barrel and cooling the mold, LSR machines must maintain precise temperature control in the opposite direction”.
Flash—the thin, unwanted excess material that escapes from the mold cavity—is a challenge in both processes. But for LSR, it‘s a fundamentally different challenge.
Thermoplastics are more viscous and less prone to flashing through tight gaps. Thermoplastic molds can achieve acceptable results with fits between inserts of up to 0.002 inches (0.05 mm).
LSR’s extremely low viscosity means it can flash through gaps as small as 0.005 mm (0.0002 inches). In some cases, LSR will flash even with fits as tight as 0.0001 inches or less. This is roughly 20 times tighter than what‘s acceptable for many thermoplastics.
Design implications:
LSR molds require vastly tighter machining tolerances—measured in millionths of an inch
Parting lines in LSR molds must be maintained with extreme precision
Any wear, debris, or temperature variation that creates a gap will result in flash
Venting in LSR molds must be precisely controlled to allow air escape without allowing material escape
The takeaway: LSR molds demand a level of precision that exceeds most thermoplastic mold requirements.
Thermoplastics shrink as they cool in the mold. Shrinkage rates typically range from 1.5% to 5%, varying significantly by material and process conditions. The part shrinks inside the mold, which must be compensated for in cavity sizing.
LSR behaves inversely. Because it‘s injected into a heated mold, the material actually expands due to heat. The real shrinkage—typically 2.5% to 3%—occurs after demolding, as the part cools to room temperature.
Key differences in LSR shrinkage behavior:
LSR parts expand in the mold; volumetric cavity filling should be approximately 98–99% to compensate
Shrinkage can be influenced by material grade, mold temperature, and curing conditions
Post-curing can add an additional 0.5% to 0.7% shrinkage
Design implications:
Thermoplastic molds: Cavity size = final part size × (1 + shrinkage rate)
LSR molds: Same formula applies, but shrinkage occurs after demolding, not in the mold
Thermoplastic molds commonly use straight-wall ejector pins. These pins can cross over the parting line on some parts, with half the ejector pin on the part and half on the parting line shutoff. In thermoplastic molding, this design typically produces no flash.
Ejector pins can be used in LSR molds, but with significant restrictions. They must have a tapered shutoff and cannot invade the parting-line shutoff area. Any minute amount of debris or rubber that builds up on the ejector valve seat will cause process failure.
More commonly, LSR parts are designed to be retained on one half of the mold when it opens, then manually or pneumatically demolded—often with air assistance. Ejector pins are “normally not used during LSR molding due to the flashy nature of the material”.
Design implications:
Thermoplastic molds: Ejector pins are standard, can cross parting lines
LSR molds: Ejector pins require tapered shutoffs; air ejection is often preferred
LSR part design must accommodate manual or air-assisted demolding
Automation of LSR demolding is more challenging and requires careful design
Traditional thermoplastic injection molding operates at high pressures—typically 100 to 1500 bar. These high pressures can cause mold deformation, particularly for large or complex parts, and require stronger mold materials.
LSR injection molding uses exceptionally low pressure—typically 30 to 100 bar. This is due to LSR‘s lower flow resistance compared to thermoplastics.
Benefits of lower LSR pressure:
Reduced mold wear; mold lifespan can be extended by over 30%
Ideal for molding ultra-thin walls (as thin as 0.010 inches / 0.25 mm)
Lower clamping force requirements
Design implications:
Thermoplastic molds: Must withstand high pressures; require stronger materials and more robust construction
LSR molds: Lower pressure means less stress on mold components, but precision requirements are still higher due to flash concerns
Thermoplastic molds typically use hot runner systems that maintain the material at melt temperature between shots. This prevents the material from solidifying in the runner and allows for continuous production.
LSR molds require cold runner systems (sometimes called cold decks) that keep the LSR material cool in the runner channels, preventing premature curing before it enters the heated cavity. Only the material that enters the actual cavity is exposed to heat and cured.
Why this matters:
Thermoplastic runners: Heat the material to keep it fluid
LSR runners: Cool the material to prevent premature curing
LSR cold runner systems require precise temperature control and thermal isolation
The material remaining in the cold runner can be reused, reducing waste
Your part requires elasticity, flexibility, or rubber-like properties
The application demands high-temperature resistance (-50°C to 200°C+, up to 300°C with special formulations)
Biocompatibility is required (medical, food contact, infant care)
Compression set resistance is critical for sealing applications
Low molded-in stress and dimensional stability are required
Your part requires rigidity and impact resistance
Recyclability is important
Material cost is a primary concern (LSR is typically 5–10× the price of standard PP)
Production volumes require shorter cycle times (though LSR can achieve faster cure times in some cases)
Frequent design changes are anticipated
LSR molds and thermoplastic molds may look similar to the untrained eye, but they are fundamentally different tools designed for fundamentally different processes. From temperature control (heating vs. cooling) to flash prevention (microns vs. thousandths of an inch) to ejector systems (pins vs. air), every aspect of LSR mold design requires specialized knowledge and precision.
