Liquid Silicone Rubber (LSR) injection molding has become one of the fastest-growing manufacturing processes in the medical, automotive, baby care, and wearable device industries. Unlike conventional thermoplastic injection molding — where plastic pellets are melted and injected into a cold mold — LSR molding injects a two-component liquid elastomer into a heated mold where it cures (vulcanizes) into a flexible, durable, and biocompatible rubber part.
The unique properties of LSR — extreme temperature resistance (-50°C to +250°C), chemical inertness, electrical insulation, and ISO 10993 biocompatibility — make it the material of choice for seals, valves, wearable sensors, baby products, and medical components that thermoplastics simply cannot match. However, LSR molding requires specialized equipment, specific tooling design, and different processing parameters compared to standard injection molding.
This guide provides a comprehensive resource on LSR injection molding: material fundamentals, tooling requirements, processing parameters, design guidelines, overmolding techniques, quality control, and cost considerations. Whether you are a product designer evaluating materials for a new medical device or an engineer sourcing LSR molded parts from China, this guide will help you make informed decisions.
1. What Is Liquid Silicone Rubber (LSR)?
Liquid Silicone Rubber is a two-part, platinum-cured silicone elastomer supplied in liquid form. The two components (Part A and Part B) are pumped in a 1:1 ratio through a static mixer and then injected into a heated mold, where an addition-cure (platinum-catalyzed) reaction crosslinks the material into a solid elastomer.
Key material properties of LSR:
- Temperature range: -50°C to +250°C continuous, up to +300°C intermittent
- Hardness range: 5 Shore 00 to 80 Shore A (very soft to firm rubber)
- Tensile strength: 5–12 MPa depending on grade
- Elongation at break: 300%–800%
- Compression set: Excellent — under 15% at 175°C for 22 hours
- Chemical resistance: Resistant to water, ozone, UV, many acids and bases
- Electrical properties: Dielectric strength 20–25 kV/mm, volume resistivity >10¹⁵ Ω·cm
- Biocompatibility: ISO 10993, USP Class VI compliant for medical grades
1.1 LSR vs HTV (High Consistency Rubber)
Silicone rubber is available in two primary forms for injection molding:
- LSR (Liquid Silicone Rubber): Low viscosity liquid, pumped through metering systems, ideal for complex geometries, thin walls, and high-volume production. Cure time is typically 15–60 seconds at 160–200°C.
- HTV (High Consistency Rubber): Solid gum-like material, requires preforming and manual or robotic loading. Used for very large parts or low-volume production. Cure times are longer (minutes).
For precision and high-volume applications, LSR is almost always the better choice. Its low viscosity allows filling of extremely thin walls (0.15 mm) and complex micro-features that would be impossible with HTV.
1.2 Common LSR Grades
- Standard LSR: General-purpose grades, 20–80 Shore A, for consumer and industrial parts
- Medical LSR: ISO 10993 tested, low volatile content, for medical devices and pharmaceutical closures
- Self-lubricating LSR: Contains embedded silicone oil for reduced friction (e.g., syringe stoppers)
- Conductive LSR: Filled with carbon or silver for EMI shielding or wearable electrodes
- Flame-retardant LSR: UL94 V-0 rated for automotive and aerospace connectors
- Optical LSR: High transparency grades for LED lenses and light guides
- Fast-cure LSR: Formulated for cycle times under 10 seconds at high temperatures
2. LSR Injection Molding Equipment
LSR molding requires specialized injection molding machines and auxiliary equipment that differ significantly from thermoplastic setups.
2.1 LSR Injection Molding Machine
The molding machine is the heart of the LSR process. Key differences from thermoplastic machines include:
- Barrel and screw: LSR machines use a cold-feed screw (kept at room temperature) to prevent premature curing. The barrel is typically water-cooled to 20–30°C.
- Injection unit: Uses a check-ring or non-return valve compatible with liquid silicone to prevent back-flow during injection.
- Clamp force: Similar to thermoplastic machines. Most LSR parts require relatively low clamp force due to low material viscosity and short flow paths.
- Mold temperature: The mold is heated to 160–220°C using electrical cartridge heaters or oil-heated platens. This is the opposite of thermoplastic molding, where the mold is cooled.
2.2 Metering and Mixing System
LSR is supplied as two separate liquid components (Part A and Part B). Before injection, they must be precisely metered and mixed:
- Metering pump: A 1:1 ratio gear pump system with accuracy of ±1% or better. Incorrect ratio leads to incomplete cure or degraded properties.
- Static mixer: A disposable or cleanable static mixer that combines Part A and Part B through a series of internal baffles.
- Material drums: 20-liter pails or 200-liter drums with follower plates to prevent air entrapment.
- Feed system: The mixed LSR is fed directly into the injection barrel through a feed tube. Pot life after mixing is typically 2–5 days at room temperature.
2.3 Mold Design for LSR
LSR molds share the same basic construction as thermoplastic molds (A-side/B-side, cores, cavities, ejector systems) but require specific design considerations:
- Heating system: Cartridge heaters embedded in the mold plates, with thermocouple feedback for zones of ±2°C accuracy. Insulator plates between mold and machine platens prevent heat loss.
- Shrinkage: LSR shrinkage is typically 2%–4%, much higher than most thermoplastics. Tooling must account for this — a part designed at 100 mm will shrink to 96–98 mm after molding.
- Vent depth: LSR vents are extremely shallow — 0.005–0.015 mm — because LSR viscosity is very low. Deeper vents cause flash; shallower vents trap air causing voids.
- Parting line flash: LSR flash is extremely thin (often <0.05 mm) and can extend significantly due to low viscosity. Mold parting lines must be precision-machined with flatness under 0.005 mm.
- Ejection: LSR parts are flexible and can be ejected from shallow undercuts. However, deep cores may require compressed air ejection or robotic extraction to prevent distortion.
- Runner system: Cold runner systems are standard for LSR. For high-volume production, valve-gated cold runner systems minimize waste. Insulated runners can also reduce material waste.
- Mold material: Standard mold steels (P20, 718H, S136) are suitable. Stainless steel (S136, 420SS) is recommended for medical LSR molds to prevent contamination and resist corrosion from any peroxide byproducts.
2.4 Typical Mold Temperatures and Cure Times
| Part Thickness (mm) | Mold Temp (°C) | Cure Time (sec) |
|---|---|---|
| 0.5 | 180–200 | 5–10 |
| 1.0 | 170–190 | 10–20 |
| 2.0 | 165–185 | 20–35 |
| 5.0 | 160–180 | 45–90 |
| 10.0 | 160–175 | 90–180 |
3. LSR Processing Parameters
Understanding and controlling LSR processing parameters is critical for producing defect-free parts with consistent mechanical properties.
3.1 Injection Pressure
LSR injection pressures are much lower than thermoplastic pressures, typically ranging from 20 to 100 bar (2–10 MPa). The low viscosity of LSR means it flows easily into fine details. Excessive injection pressure causes flash — because LSR can flow through gaps as small as 0.005 mm. The general approach is to use the lowest injection pressure that completely fills the cavity without short shots.
3.2 Injection Speed
Injection speed should be slow and controlled. High injection speeds cause air entrapment and burn marks (diesel effect) when trapped air compresses at the end of fill. A typical injection time is 2–8 seconds depending on part volume. Profiled injection — starting fast and slowing down at the end — is recommended to prevent air traps.
3.3 Cure Time and Temperature
Cure time is the most critical parameter in LSR molding. It is directly related to the thickest section of the part. The relationship follows an Arrhenius equation — for every 10°C increase in mold temperature, cure time is roughly halved. However, excessively high temperatures can cause:
- Surface discoloration (yellowing)
- Reduced elongation due to over-cure
- Reduced mold life due to thermal stress
3.4 Typical Processing Window
| Parameter | Typical Range | Notes |
|---|---|---|
| Barrel temperature | 20–30°C (cooled) | Must stay below 40°C to prevent premature curing |
| Mold temperature | 160–220°C | Higher temps for faster cycle, watch for discoloration |
| Injection pressure | 20–100 bar | Use lowest pressure that fills completely |
| Injection speed | 2–15 mm/s | Profiled — slow end of fill to prevent air traps |
| Cure time | 5–180 sec | Depends on thickest section and mold temperature |
| Cooling/demolding | 5–15 sec | LSR parts can be handled hot; minimal shrinkage after demolding |
| Mixing ratio (A:B) | 1:1 | Metering accuracy ±1% or better |
4. LSR Part Design Guidelines
Designing for LSR requires different thinking than thermoplastic design. LSR's flexibility, low viscosity, and thermal cure process create both opportunities and constraints.
4.1 Wall Thickness
- Minimum wall thickness: 0.15 mm (for short flow lengths under 10 mm)
- Recommended minimum: 0.30 mm for general features
- Typical range: 0.5–3.0 mm for most parts
- Maximum: 25 mm possible but cure times become very long
- Wall transition: Gradual transitions are preferred. Step changes should be limited to 2:1 ratio maximum.
Unlike thermoplastics, LSR does not exhibit significant sink marks on thick sections. However, very thick sections dramatically increase cure time and can develop internal porosity.
4.2 Draft Angles
Because LSR is flexible, draft angles can be much smaller than thermoplastic parts:
- Smooth surfaces: 0.5°–1° draft is sufficient
- Textured surfaces: 1°–2° draft
- Deep cores (>50 mm): 1°–2° plus compressed air ejection
- Shallow undercuts: LSR can be demolded from undercuts up to 0.5 mm per side by stretching the part
4.3 Radii and Fillets
Sharp corners cause stress concentrations and premature mold wear (LSR is abrasive due to silica filler). Always use radii:
- Internal corners: Minimum 0.3 mm radius
- External corners: Match internal radius + wall thickness
- Mold life benefit: Proper radii extend mold life by 30%–50% by reducing stress concentration in steel
4.4 Flash and Parting Line
Flash control is one of the biggest challenges in LSR molding. LSR's extremely low viscosity means it can flow through any gap in the mold:
- Design parting lines on non-critical surfaces
- Specify flash-free zones on part drawings
- Use stepped or tongue-and-groove parting lines for better alignment
- Avoid parting lines across sealing surfaces — if unavoidable, design a 0.2–0.5 mm flash groove to contain flash
- Tolerances on parting lines should be ±0.05 mm
4.5 Tolerances
LSR can achieve tight tolerances despite being a rubber material:
| Dimension Type | Commercial (mm) | Precision (mm) |
|---|---|---|
| Critical dimensions (≤10 mm) | ±0.05 | ±0.025 |
| Normal dimensions (10–50 mm) | ±0.10 | ±0.05 |
| Large dimensions (50–150 mm) | ±0.20 | ±0.10 |
| Wall thickness | ±0.10 | ±0.05 |
| Flatness (per 25 mm) | 0.15 | 0.08 |
4.6 Undercuts and Through-Holes
LSR's flexibility allows demolding from undercuts that would be impossible in thermoplastics:
- External undercuts up to 0.5 mm: Can be stripped from the mold without mechanical release
- Internal undercuts: Require side cores or collapsible cores
- Through-holes: Core pins must be supported on both ends for holes deeper than 3× diameter. LSR's low injection pressure means core pins deflect less than in thermoplastic molding.
- Blind holes: Depth-to-diameter ratio up to 5:1 without support
5. LSR Overmolding and Two-Shot Molding
One of the most valuable applications of LSR is overmolding onto thermoplastic or metal substrates. This combines the structural rigidity of plastic with the soft-touch, sealing, or biocompatible properties of silicone.
5.1 LSR Over Thermoplastics
Common substrate materials include PC, PA (nylon), PBT, PEEK, and metal inserts. The key challenge is adhesion:
- Chemical bonding: Some thermoplastics (PA, PBT) bond chemically with LSR during curing. No primer is needed.
- Mechanical interlocking: For materials that do not bond chemically (PC, metal), design undercuts, holes, or textured surfaces in the substrate for mechanical retention.
- Primer coating: Silicone primers can be applied to the substrate before overmolding to achieve chemical adhesion on difficult materials.
- Self-bonding LSR grades: Specially formulated LSR that bonds to specific plastics without primer — a growing trend.
5.2 Two-Shot LSR Molding
Two-shot (2K) molding combines thermoplastic and LSR in a single molding cycle:
- Machine requirements: A two-material injection molding machine with both a thermoplastic injection unit and an LSR injection unit, plus a rotary table or index plate.
- Process: The thermoplastic substrate is molded first in one cavity, then the mold rotates and the LSR is overmolded in a second, heated cavity.
- Advantages: Eliminates the need for a separate overmolding operation, reduces handling, improves bond strength, and reduces cycle time.
- Applications: Wearable devices, medical sensors, automotive connectors with integrated seals, consumer electronics with soft-touch buttons.
6. Quality Control for LSR Parts
Quality control for LSR parts requires specific tests beyond standard dimensional inspection:
6.1 Visual Inspection
- Flash: Inspect for flash at parting lines, vents, and ejector pins. Flash is the most common defect and must be controlled through mold maintenance and process optimization.
- Bubbles/voids: Internal or surface voids caused by trapped air. Corrected through venting improvements and slower injection speeds.
- Burn marks: Brown or black marks caused by diesel effect (compressed air ignition). Corrected by improving venting.
- Flow lines: Visible lines where flow fronts meet. Corrected by adjusting gate location and injection speed.
- Surface finish: LSR reproduces mold surface exactly. A polished mold produces a glossy part; a textured mold produces a matte part.
6.2 Mechanical Testing
- Hardness (Shore A): Test per ASTM D2240. Typical variation: ±2 Shore A from nominal.
- Tensile strength and elongation: Test per ASTM D412 (die-cut dumbbell specimens).
- Compression set: Test per ASTM D395. Measures permanent deformation after compression at elevated temperature. Critical for sealing applications.
- Tear resistance: Test per ASTM D624 (Die B or Die C). Important for parts with sharp corners or thin sections.
6.3 Post-Curing
Most LSR parts benefit from post-curing in a circulating air oven at 150–200°C for 1–4 hours. Post-curing:
- Removes volatile byproducts (siloxane oligomers)
- Completes the crosslinking reaction (improves mechanical properties by 5%–15%)
- Reduces compression set values
- Is mandatory for medical and food-contact parts per FDA and ISO 10993 requirements
7. Applications of LSR Injection Molding
7.1 Medical and Healthcare
The medical industry is the largest consumer of LSR molded parts. Key applications include:
- Syringe stoppers and seals: Self-lubricating LSR provides smooth plunger action and sterile barrier
- Respiratory masks and components: LSR's softness and biocompatibility make it ideal for CPAP masks, ventilator components
- Implantable devices: Long-term implantable LSR grades for cardiac leads, cochlear implants
- Pharmaceutical closures: Vial stoppers and septa requiring minimal extractables
- Wearable health monitors: LSR wrist straps, sensor housings, electrode substrates
- Surgical instruments: Soft-touch grips overmolded on stainless steel handles
7.2 Automotive
- Connector seals and grommets: LSR withstands engine compartment temperatures (-40°C to +200°C)
- LED headlight lenses: Optical-grade LSR for high-temperature transparency
- Ignition cables and boots: Electrical insulation combined with thermal resistance
- Airbag covers: LSR seams designed to tear in a controlled pattern during deployment
- EV battery seals: LSR gaskets for battery packs requiring IP67+ sealing and flame retardancy
7.3 Consumer Products
- Baby products: Pacifiers, bottle nipples, teething toys (food-grade LSR)
- Kitchenware: Baking molds, spatulas, steam release valves
- Wearables: Smartwatch bands, fitness tracker straps, earbud tips
- Personal care: Toothbrush grips, cosmetic applicators
7.4 Electronics
- Sealing gaskets: IP68-rated enclosure seals for smartphones, outdoor cameras
- Conductive keypads: Carbon-filled LSR contacts for membrane switches
- EMI shielding: Silver-filled LSR gaskets for electronic enclosures
- Cable accessories: Strain relief boots, end caps, overmolded connectors
8. LSR vs Thermoplastic Injection Molding: Key Differences
| Factor | LSR Molding | Thermoplastic Molding |
|---|---|---|
| Material state at injection | Liquid (low viscosity) | Melted solid (high viscosity) |
| Mold temperature | Heated (160–220°C) | Cooled (15–80°C) |
| Barrel temperature | Cooled (20–30°C) | Heated (180–350°C) |
| Injection pressure | Low (20–100 bar) | High (600–2000 bar) |
| Curing mechanism | Chemical crosslinking (irreversible) | Thermal solidification (reversible) |
| Shrinkage | 2%–4% | 0.2%–2.5% |
| Flash tendency | Very high (low viscosity) | Moderate |
| Cycle time | Longer (cure time dominates) | Shorter (cooling time) |
| Tooling cost | 10%–30% higher (heating, vents) | Standard |
| Part cost | Higher (material + cycle) | Lower |
| Recyclability | Not recyclable (thermoset) | Recyclable (re-meltable) |
9. Cost Considerations
9.1 Tooling Cost
LSR molds typically cost 10%–30% more than equivalent thermoplastic molds due to:
- Heating system (cartridge heaters, thermocouples, insulator plates)
- Precision parting line requirements (flatness ±0.005 mm)
- Shallow vent machining (0.005–0.015 mm)
- Cold runner manifold system
Typical LSR mold costs at Huanze Technology range from $8,000 for single-cavity prototyping molds to $80,000+ for 4–8 cavity production molds with valve-gated cold runners.
9.2 Material Cost
LSR raw material costs are higher than most thermoplastics:
- Standard LSR: $8–$15/kg
- Medical-grade LSR: $15–$35/kg
- Specialty grades (conductive, optical): $50–$200/kg
- Comparison: ABS ~$2/kg, PC ~$3/kg, PEEK ~$80/kg
9.3 Piece Price
Despite higher tooling and material costs, LSR part prices can be competitive at production volumes because:
- Low injection pressure allows multi-cavity molds (16, 32, 64 cavities)
- Scrap rates are low with cold runner systems
- Cycle times for thin parts can be as fast as 5–10 seconds
- No drying required (unlike nylon or PC)
10. Choosing an LSR Molding Partner in China
When sourcing LSR molded parts from Chinese manufacturers, evaluate these critical capabilities:
- Dedicated LSR machines: The supplier must have injection molding machines specifically designed for LSR with proper metering and mixing systems. Adapting thermoplastic machines for LSR often results in quality issues.
- Cleanroom capability: For medical and optical LSR parts, ISO Class 7 or Class 8 cleanroom molding is essential.
- Tooling expertise: LSR mold design requires specific experience — particularly in venting, heating uniformity, and parting line precision. Ask to see previous LSR molds and samples.
- Material documentation: For medical parts, the supplier must provide LSR material certificates, ISO 10993 test reports, and full traceability.
- Post-curing capability: Proper post-cure ovens with temperature mapping and circulation control.
- Quality testing: Hardness testing, tensile testing, compression set testing — in-house or through accredited labs.
At Huanze Technology in Shenzhen, we operate dedicated LSR injection molding machines ranging from 50 to 200 tons, ISO Class 8 cleanroom molding for medical components, and a complete tooling shop with LSR-specific mold design expertise. Our LSR molding services cover everything from prototype tooling to high-volume production with 32-cavity molds.
Conclusion
Liquid Silicone Rubber injection molding is a specialized process that enables products impossible to achieve with conventional thermoplastics — from soft, biocompatible medical seals to high-temperature automotive connectors and comfortable wearable devices. While the equipment, tooling, and processing differ fundamentally from thermoplastic molding, the results justify the investment for applications demanding flexibility, temperature resistance, chemical inertness, or skin-contact safety.
Success in LSR molding comes down to three pillars: proper material selection (matching LSR grade to application requirements), precision tooling design (heating uniformity, venting accuracy, parting line flatness), and controlled processing (injection speed, cure time, mold temperature). When all three are executed correctly, LSR molding delivers consistently high-quality parts with unique properties that no thermoplastic can match.
If you are developing a product that requires LSR molding — whether a medical device, automotive seal, wearable, or consumer product — Huanze Technology can help with material selection, tooling design, prototype molding, and volume production. Contact us to discuss your project requirements.