LSR vs HTV material selection for custom molded parts

Choosing between liquid silicone rubber (LSR), high‑temperature vulcanizing (HTV) silicone, and thermoplastic elastomers (TPE) is a critical decision for engineers who need parts that meet functional, regulatory, and cost targets. Each material family offers distinct advantages in terms of temperature resistance, hardness range, processing complexity, and long‑term durability, so a systematic comparison at the part level helps avoid costly redesigns.

This guide breaks down the key selection criteria—material properties, molding process constraints, and secondary considerations like sterilization and overmolding—using real‑world data from WSR’s production floor. By focusing on tolerances, cure chemistry, flash control, and bonding performance, you can map your application requirements directly to the most suitable technology.

LSR: Precision molding for medical, automotive, and consumer applications

LSR is a two‑part platinum‑catalyzed silicone that cures rapidly in a closed mold at temperatures typically between 150 °C and 200 °C. Because the material is pumped as a low‑viscosity liquid, it fills thin sections (<0.5 mm) and intricate geometries with minimal pressure, yielding tolerances as tight as ±0.02 mm on critical dimensions. Shore A hardness ranges from 0 to 80, and parts can withstand continuous use from –55 °C to 200 °C, with short‑term excursions to 230 °C.

The injection process eliminates most flash; typical flash thickness is <0.05 mm, and parting lines are barely visible because the material does not require high clamping force. Overmolding LSR onto substrates such as polycarbonate or stainless steel achieves bond strengths exceeding 2.5 MPa without primers, thanks to the platinum cure system’s compatibility with many engineering plastics. Sterilization compatibility is excellent: LSR survives >200 autoclave cycles (121 °C, 15 psi) and is resistant to gamma radiation up to 50 kGy.

Property	Typical Value
Mold temperature (°C)	150‑200
Cycle time (s) for 5 g part	15‑30
Tolerance (mm)	±0.02‑0.05
Flash thickness (mm)	<0.05
Sterilization (autoclave cycles)	>200

HTV: High‑strength sheets and extrusions for compression or transfer molding

HTV silicone is a gum‑based material that requires milling or extrusion before molding, then cures via a peroxide system at 160 °C‑200 °C under compression. Because the starting material has higher viscosity, mold filling pressures are higher, which can lead to flash thicknesses of 0.1 mm‑0.3 mm unless venting and mold design are optimized. Tolerances are generally looser than LSR, averaging ±0.05 mm‑0.10 mm for dimensions under 20 mm, though post‑machining can improve critical features.

Hardness spans 30‑90 Shore A, and HTV retains mechanical strength at temperatures up to 250 °C continuously, making it suitable for gaskets in engine compartments or industrial seals. The peroxide cure leaves negligible by‑products, so HTV is often chosen for food‑contact applications that require FDA 21 CFR 177.2600 compliance. Overmolding HTV onto metals is possible but usually requires a primer or surface treatment to achieve bond strengths above 1.5 MPa.

Parameter	HTV Typical
Cure temperature (°C)	160‑200
Mold pressure (bar)	100‑200
Flash (mm)	0.1‑0.3
Tolerance (mm)	±0.05‑0.10
Continuous use temp (°C)	‑60 to 250
Food‑grade (FDA)	Yes (with proper formulation)

TPE: Thermoplastic flexibility for overmolding and rapid cycling

Thermoplastic elastomers blend rubber‑like elasticity with thermoplastic processability, allowing injection molding on standard thermoplastic machines at temperatures between 180 °C and 230 °C. Because TPEs are melt‑processed, cycle times are very short—often 10‑20 seconds for a 5 g part—making them attractive for high‑volume consumer goods. Tolerances are comparable to standard thermoplastics, typically ±0.03 mm‑0.07 mm, and flash is usually <0.08 mm when mold venting is adequate.

Hardness covers a broad range from 20 Shore AA to 85 Shore D, enabling soft‑touch grips as well as structural components. Continuous use temperature is limited to roughly 80 °C‑120 °C depending on the specific TPE formulation (e.g., TPU, TPO, SBS‑based), which restricts under‑hood automotive applications. Overmolding TPE onto rigid plastics such as ABS or PC achieves bond strengths of 1.8‑2.5 MPa without primers due to molecular compatibility. Sterilization options are limited: most TPEs tolerate only a few autoclave cycles (<10) before degradation, so they are rarely used for reusable medical devices.

Attribute	Typical TPE Range
Melt temperature (°C)	180‑230
Cycle time (s) for 5 g part	10‑20
Hardness (Shore)	20 AA‑85 D
Tolerance (mm)	±0.03‑0.07
Flash (mm)	<0.08
Continuous use temp (°C)	80‑120
Autoclave cycles tolerated	<10

Decision checklist: matching material to part requirements

Use this concise table to weigh the most common selection drivers. Assign weights based on your project priorities (e.g., medical sterilization = high weight, cost = medium). The scores are relative; higher totals indicate a better fit.

Criterion	LSR	HTV	TPE
Temperature resistance (°C)	‑55 to 200 (short 230)	‑60 to 250	80‑120
Tolerance capability (mm)	±0.02‑0.05	±0.05‑0.10	±0.03‑0.07
Flash thickness (mm)	<0.05	0.1‑0.3	<0.08
Overmold bond strength (MPa)	>2.5 (no primer)	1.5‑2.0 (often needs primer)	1.8‑2.5 (no primer)
Sterilization (autoclave cycles)	>200	>100 (peroxide cure stable)	<10
Relative mold cost (USD)	High (precision, heated)	Medium	Low‑Medium (standard thermoplastic)
Cycle time (s) for 5 g part	15‑30	20‑40	10‑20

Frequently Asked Questions

What is the typical cost difference between LSR and HTV molds for a medium‑complexity part?

LSR molds generally cost 20‑35% more than comparable HTV molds because they require higher precision machining, heated runner systems, and tighter venting to control flash. For a 150 mm × 100 mm × 50 mm mold with four cavities, expect LSR tooling in the range of $12,000‑$18,000 versus $9,000‑$13,000 for HTV.

Can TPE be used for a part that needs to survive 150 °C continuous heat?

Standard TPE formulations (TPU, TPE‑S, TPO) lose elasticity and may deform above 120 °C continuous exposure, so they are not suitable for 150 °C service. Specialized high‑temperature TPEs (e.g., certain copolyester‑based grades) can reach 150 °C but are significantly more expensive and often still inferior to LSR or HTV in long‑term aging.

How does flash thickness affect sealing performance in a static O‑ring groove?

Flash thicker than 0.05 mm can prevent the O‑ring from seating fully in the groove, increasing leakage risk by up to 40% at 5 bar pressure. LSR’s typical flash (<0.05 mm) maintains seal integrity, whereas HTV flash (0.1‑0.3 mm) often requires post‑mold trimming or secondary machining to achieve reliable sealing.

Conclusion

Selecting LSR, HTV, or TPE hinges on matching your part’s temperature window, tolerance needs, sterilization demands, and budget to each material’s process characteristics. LSR excels where precision, biocompatibility, and repeated autoclave cycles are non‑negotiable; HTV offers a cost‑effective route for high‑temperature seals when tighter tolerances can be relaxed; TPE shines in high‑volume, low‑temperature overmolding scenarios with rapid cycle times. To determine the optimal solution for your specific geometry and performance targets, request a quote from WSR’s engineering team.