Liquid Silicone Rubber (LSR) Injection Molding: DFM & Tooling Guide

Liquid Silicone Rubber (LSR) injection molding reverses almost every thermal and mechanical rule of thermoplastic injection molding. While thermoplastics are melted in a heated barrel (200–320 °C) and injected into a cooled mold (20–90 °C) to solidify, LSR is a two-part, platinum-catalyzed thermoset elastomer that is pumped through a water-chilled barrel (15–25 °C) and injected into a red-hot steel mold (170–210 °C) where it chemically crosslinks (vulcanizes) in 10–45 seconds.

Because uncured LSR has a water-like shear-thinning viscosity (10,000–100,000 mPa·s at high shear rates), it will flash into any mold gap wider than 0.004–0.005 mm (4–5 microns)—less than one-fifth the venting depth of a standard nylon or ABS mold. This guide details LSR material physics, cold-deck tooling architecture, flashless DFM rules, and medical/automotive validation criteria.

1. LSR vs Thermoplastic Elastomers (TPE / TPU) vs HCR Compression Molding

Engineering Property / Process Metric	Liquid Silicone Rubber (LSR Injection)	High-Consistency Silicone (HCR Compression/Transfer)	Thermoplastic Elastomer (TPE / TPV / TPU)
Hardness Range	Shore 5A to 80A (plus Shore 00 gel grades)	Shore 20A to 80A	Shore 30A to 75D
Continuous Service Temp	-55 °C to +200 °C (up to +250 °C heat-stabilized)	-55 °C to +200 °C	-40 °C to +85 °C (TPE) / +135 °C (TPV)
Compression Set (22 h @ 175 °C)	10–20% (Post-cured)	18–30%	50–100% (Melts or relaxes above 125 °C)
Typical Cure / Cycle Time	12–45 seconds (Automated flashless)	120–300 seconds (Manual deflashing required)	15–35 seconds
Parting Line Flash Tolerance	0.002–0.004 mm (Flashless capable)	0.05–0.20 mm (Tear-trim flash groove)	0.015–0.030 mm
Linear Mold Shrinkage	2.0–3.5% (0.020–0.035 mm/mm)	2.5–4.0%	0.5–2.0%
Biocompatibility & Sterilization	USP Class VI / ISO 10993; Autoclave, EtO, Gamma	USP Class VI / ISO 10993	Limited autoclave capability
Tooling Investment (4–16 Cav)	$28,000–$85,000 (Cold-runner + vacuum)	$6,000–$18,000	$12,000–$35,000

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2. How the LSR Molding Process Works: 1:1 Dosing to Vulcanization

1. Metering & Static Mixing (1:1 ± 1.0% Ratio): LSR is supplied in 20 L pails or 200 L drums as Component A (vinyl-functional polysiloxane + platinum catalyst) and Component B (si-H crosslinker + cure inhibitor). Hydraulic or servo-electric dosing pumps feed A and B at a 1:1 volumetric ratio (tolerance ≤ ±1.0%), optionally injecting 0.5–2.0% liquid silicone color paste into a 24–32 element static mixer.
2. Chilled Injection Unit (15–25 °C): Once mixed, the pot life at 20 °C is typically 3–7 days, but reactivity accelerates exponentially above 40 °C. The injection barrel and water-cooled shut-off nozzle are jacketed at 15–22 °C, and the screw uses a specialized non-return check valve with tight clearance to handle low-viscosity fluid at 40–120 bar (4–12 MPa) injection pressure.
3. Chilled Cold Runner into Hot Cavities (170–210 °C): Because an LSR runner cures irreversibly if exposed to mold heat (and cured silicone cannot be reground or remelted), production LSR tools use a water-cooled Open-Nozzle or Valve-Gated Cold Runner Deck (maintained at 18–25 °C) separated from the 175–205 °C heated cavity plates by titanium nozzle tips and 8–12 mm glass-fiber thermal insulation boards.
4. Exothermic Platinum Hydrosilylation Cure: Inside the cavity, platinum catalyzes an addition-cure reaction between vinyl and silicon-hydride groups with zero volatile reaction byproducts (unlike peroxide-cured HCR, which releases organic acid odor and blooming). Vulcanization completes at roughly 4–8 seconds per millimeter of wall thickness at 190 °C.

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3. LSR Mold Steel, Venting (0.002 mm), and Vacuum Evacuation

Building a flashless LSR mold requires CNC grinding, wire EDM, and optical polishing tolerances an order of magnitude tighter than thermoplastic tooling:

• Steel Selection & Surface Treatments: Cavities are machined from through-hardened corrosion-resistant stainless tool steels such as Stavax ESR (1.2083), M390, or 1.2343 ESR hardened to 52–56 HRC, often coated with PVD (CrN, TiCN, or DLC) to reduce silicone surface tack and ease automatic demolding without mold-release spray.
• Micron-Level Venting & Vacuum Seals:
  • Air trapped inside an LSR cavity causes incomplete vulcanization (sticky/gummy pockets) and diesel-effect scorch marks.
  • Vent channels are ground to 0.002–0.004 mm (2–4 µm) depth by 2–4 mm width, opening immediately into a 0.5 mm relief channel connected to a high-speed vacuum pump.
  • Before the valve gates open, a perimeter silicone O-ring vacuum seal closes on the parting line and evacuates cavity air down to 20–50 mbar absolute pressure within 0.3–0.6 seconds.

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4. DFM Design Rules for LSR Molded Parts

Because vulcanized LSR has high elongation at break (300–800%) and high thermal expansion inside a 190 °C tool, LSR part geometry follows dramatically different DFM rules than rigid thermoplastics:

Geometric Feature	LSR DFM Specification	Engineering Rationale
Wall Thickness Range	0.20 mm to 8.0 mm (Ideal: 0.8–3.0 mm)	Low viscosity fills 0.20 mm membranes easily; walls transitions can vary up to 4:1 without sink marks
Parting Line Step / Mismatch	≤ 0.005 mm (5 µm)	Prevents flash curtains; locate parting lines off dynamic sealing beads (offset by ≥ 0.5 mm)
Draft Angles	0° to 1.0° (0° standard on straight walls)	High flexibility allows zero-draft cores; 1°–2° aids robotic air-assist brushing
Undercuts (Stripped / Bumped Off)	Up to 15–35% of feature diameter (Shore 30A–50A)	Deep internal ribs, umbrella valves, and sealing grooves strip directly off solid cores without slides
Internal Corner Radii	≥ 0.25 mm (Minimum 0.15 mm at tear points)	Eliminates tear initiation during robotic brush/gripper demolding
Pinpoint / Valve Gate Diameter	Ø0.25–0.60 mm (Sub-gate or cold-runner valve pin)	Leaves a flush vestige ≤ 0.05 mm without tear-out

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5. Two-Shot LSR Overmolding & Post-Curing (200 °C × 4 Hours)

Primerless Self-Adhesive LSR Overmolding onto Thermoplastics

Standard LSR will not bond to thermoplastics without plasma/corona activation and a siloxane primer. However, Self-Adhesive LSR (LSR-SA) grades contain integrated organofunctional adhesion promoters that form covalent bonds directly to PA66-GF30, PBT-GF30, PC, and PPS inside a 2-shot (2K) mold—provided the thermoplastic substrate can withstand a 130–160 °C cavity temperature during the LSR cure cycle without warping beyond its Heat Deflection Temperature (HDT).

Thermal Post-Cure (4 Hours at 200 °C)

While platinum-cured LSR leaves the mold fully vulcanized, medical, food-contact (FDA 21 CFR 177.2600, BfR XV), and automotive connector seals typically undergo a ventilated oven post-cure at 200 °C for 2–4 hours with 100+ L/min fresh air exchange. Post-curing:

1. Drives volatile cyclic siloxanes (D4–D8) below < 0.1–0.5% weight loss, preventing fogging on optical lenses or electrical contact fouling.
2. Reduces Compression Set by 30–45% (e.g., dropping from 28% as-molded down to 14–16% after post-cure) and stabilizes final post-cure shrinkage (adding an extra +0.3% to +0.7% linear shrinkage that must be pre-compensated in cavity steel dimensions).

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Frequently Asked Questions (PAA)

Why does Liquid Silicone Rubber (LSR) have such high mold shrinkage (2.0–3.5%)?

Unlike thermoplastics that shrink due to cooling from melt to room temperature, LSR enters a 180–200 °C mold and crosslinks at that elevated temperature. When the cured silicone part ejects and cools down by 160–175 °C to room temperature (23 °C), the high Coefficient of Thermal Expansion (CTE) of silicone (~250–300 × 10⁻⁶ /K) causes 2.0–3.5% isotropic thermal contraction. Adding thermal post-curing (200 °C × 4 h) increases total shrinkage by another 0.3–0.7%.

Can you mold LSR without flash?

Yes—flashless LSR injection molding is standard for high-volume medical and automotive parts, provided the mold parting lines are lapped to ≤ 0.002–0.004 mm (2–4 µm) gap clearance, cavities are evacuated to 20–50 mbar vacuum prior to injection, and each cavity is fed directly by a piezo or pneumatic shut-off valve-gated cold runner (Ø0.3–0.5 mm orifice) with cavity-pressure switchover control.

Which thermoplastics can be overmolded with LSR in a 2-shot mold?

Because LSR cures at 130–190 °C (even when using low-temperature fast-cure LSR grades), the rigid thermoplastic substrate must have a Heat Deflection Temperature (HDT at 1.8 MPa) above 145 °C. The primary engineering resins for 2K LSR overmolding are PA66-GF30 (HDT 250 °C), PBT-GF30 (HDT 210 °C), PPS-GF40 (HDT 265 °C), PSU/PPSU (HDT 175–205 °C), and high-heat Copolycarbonates (PC-HT, HDT 160–185 °C).

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Scope Note

PlasticsTechnologyAlliance.com is an independent buyer resource and does not manufacture parts, build molds, or certify suppliers. Confirm final tooling architecture, resin grade datasheets, and validation protocols directly with your contract molder and material manufacturer.

Related PTA Guides

• TPU vs TPE vs TPV vs Silicone (LSR) Material Selection
• Two-Shot (2K) Injection Molding vs Insert Overmolding
• Cleanroom & ISO 13485 Medical Device Injection Molding
• Overmolding Material Compatibility Matrix & Bond Physics