Inconsistent material ratios, cheap tooling, and improper venting lead to catastrophic seal failures, expensive reworks, and LSR production delays. When an LSR component degrades prematurely, the cost is rarely limited to the part itself. It extends to warranty claims, regulatory exposure, and brand damage.
This guide walks engineers and procurement teams through the complete LSR molding process: how the material behaves, how tooling decisions shape part quality, what happens at each production step, and which certifications matter for your industry.
Key Takeaways
- LSR’s material performance depends on exact 1:1 mix of the A and B components and strict cure temperature control that prevents premature structural instability
- Tooling is the largest upfront investment and the single biggest determinant of part quality. Certified production molds pay for themselves through dimensional accuracy and automated, high-volume cycles.
- Investing in certified LSR production molds ensures long-term dimensional accuracy and cost-efficiency for high-volume, automated manufacturing cycles
- Selecting industry-specific grades like ISO 13485 or FDA-compliant silicone is vital for meeting international regulatory and safety standards.
What Is Liquid Silicone Rubber Molding?
Liquid Silicone Rubber (LSR) is an inert synthetic elastomer built on a silicon-oxygen (siloxane) backbone with carbon and hydrogen side groups. LSR molding is the process used to manufacture medium to high volumes of precision silicone parts from this material.
It is a two-component, platinum-cured system. Component A contains the vinyl-functional silicone polymer and the platinum catalyst. Component B contains the same base polymer along with the hydride crosslinker and a cure inhibitor that prevents the reaction from starting too early. The two low-viscosity liquids are pumped at a precise 1:1 ratio into the injection unit of a LIM molding machine using high-precision silicone pumps.
Although it is processed on injection molding equipment, LSR behaves as a thermoset. Once heat triggers the platinum-catalyzed crosslinking reaction, the part will not melt or lose shape, even at sustained high temperatures. The table below summarizes the material characteristics that make LSR suitable for demanding applications.
| Property | Typical LSR Value (Specs can differ based on provider and project) | Why It Matters |
| Temperature Range | -65°F to +392°F (−54°C to +200°C) | Performs in automotive engine bays and medical sterilization cycles |
| Shore A Hardness | 5 to 80 (customizable) | Tunable softness — from baby-bottle nipple to rigid gasket |
| Elongation at Break | >300% | Parts stretch to demold and flex in service without tearing |
| Biocompatibility | ISO 10993 / USP Class VI-compliant grades available | Qualifies for implantable and skin-contact medical devices |
| Dimensional Tolerances | ±0.002 in. (±0.05 mm) achievable | High-precision fit for connectors, seals, micro-parts |
| Cure Temperature | 180–220°C (356–428°F) | Fast curing in seconds to minutes, depending on part geometry |
| Shrinkage | 1–5% (material/mold dependent) | Must be factored into mold design for tight tolerances |
| Compression Set | <20% (low) | Seal and gasket surfaces maintain seating force over time |
Table 1: LSR Material Properties at a Glance
With this framework, a brief introduction to LSR molding is established. Now, let’s look into the crucial steps that determine the overall molding process:
Step 1: Choosing the Right LSR Grade
LSR is not a single product; it’s a family of formulations and the grade must be specified against the end application before any tooling work begins. The common grades include generic industrial, medical, food contact, optical, self-adhesive, and flame-retardant. The following table will help you understand the divisions. However, consult with our staff at Konark to make sure that your required grade is available or not.
| Grade | Key Standard | Best For |
| General Industrial | ASTM D2000 | Seals, gaskets, grommets, keypads |
| Medical / Implant | ISO 10993, USP Class VI | Implantables, catheters, surgical instruments |
| Food Contact | FDA 21 CFR 177.2600 | Baby products, kitchen tools, food processing seals |
| Optical / Clear | Custom spec | Lenses, light guides, wearable devices |
| Self-Adhesive | Manufacturer proprietary | Two-shot / overmolding without primer |
| Flame Retardant | UL 94 V-0 | Electronics enclosures, aerospace interior |
| Electrically Conductive | IEC / MIL-SPEC | EMI shielding, EV battery seals, sensors |
Table 2: LSR Grade Selector
Step 2: LSR Tooling – The Foundation of Part Quality
Tooling represents the highest upfront capital expenditure. Additionally, this phase of the LSR injection molding process takes the longest lead time, and it is essential because the mold quality determines the quality of the whole part.
Tooling Phases: Prototype → Pilot → Production
Prototype: Soft tools, typically aluminum or 3D-printed molds, are used in this phase to produce small batches for test design iterations before investing in hard steel tooling.
Pilot: Bridge tools or early-stage production tools are used to produce intermediate volumes, usually for validating the final production process and for market testing.
Production: High-cavity, durable tools, generally hardened steel, built for maximum precision, longevity, and automation, are used to run for thousands or millions of cycles.
Check the table below for comparison.
| Tooling Phase | Material | Cavities | Lead Time | Best For |
| Prototype / Sample Tool | Aluminum or P20 steel | 1–2 | 1–4 weeks | Design validation, market testing, and clinical trials |
| Pilot / Bridge Tool | Semi-hardened steel | 2–8 | 4–8 weeks | Pre-production runs, process refinement, first articles |
| Production Mold | Hardened stainless steel | 8–64+ | 8–16 weeks | High-volume, lights-out, automated manufacturing |
Table 3: LSR Tooling Phase
Critical Tooling Design Elements
The following are some of the tooling design elements that are considered during the tooling process.
- Gate Placement: Fed into the thickest section and kept away from critical-dimension surfaces.
- Draft Angles: Angles as low as 0.5° are accepted because of LSR’s high elasticity, unlike thermoplastics. However, LSR can also be easily molded with Zero Draft (0°) due to its extreme flexibility.
- Venting: Micro-vents (around 0.0003–0.0008 in. deep) are made to prevent trapped gas and flash formation.
- Surface Finish: Matte or textured finishes are recommended to ensure that mold-release agent dependence is low. But for optical parts, mirror polish is recommended.
- Parting Line Placement: LSR’s flexibility allows creative parting line placement that is not possible with rigid plastics.
- Cold Runner vs. Hot Runner: Cold runner systems minimize waste and get rid of gate vestiges, which is usually preferred for high-precision work.
Now that we have covered LSR tooling, let’s look at the production process details.
Step 3: The LSR Molding Process, Step by Step
It is actually a thermoset process where two liquid components, the base and catalyst, are mixed (1:1) and injected into a heated mold that is cured to form a heat-resistant solid. There are 7 steps overall that make up the liquid silicone rubber manufacturing process, and the following tabular format will help you understand it better.
| Step | What Happens | Key Parameter to Control |
| 1. Material Prep | Part A (base + crosslinker) and Part B (platinum catalyst) are loaded into supply drums that are kept at room temperature | 1:1 metering ratio; the mixed (A+B) material is kept below 40°F until injection to prevent premature cure |
| 2. Metering & Mixing | Automated metering unit combines A+B at a fixed ratio; static mixer homogenizes the blend | Mix ratio accuracy ±0.5%; no air entrainment |
| 3. Injection | Blended LSR is injected into a preheated, closed mold under pressure | Injection pressure, shot size, fill speed |
| 4. Curing/Vulcanization | Heat triggers platinum-catalyzed crosslinking; liquid becomes a durable elastomer | Mold temperature 180–220°C; cure time 15–60 sec (part-geometry dependent) |
| 5. Demolding | Flexible LSR parts are removed manually or via automated takeout; no ejector pins needed | Part temperature, release agent (if used), robot path |
| 6. Post-Cure (if required) | Some medical/aerospace grades post-cure at 200°C for 4 hrs to drive off trace volatiles | Oven temp, dwell time per material spec |
| 7. Inspection & QC | Dimensional check, visual inspection, functional test per customer spec or ISO 13485/IATF 16949 protocol | AQL level, CMM, or optical inspection |
Table 4: LSR Molding Process at a Glance
Verification Step: Quality Control & Industry Certifications
For buyers and engineers, the certifications a molder holds are not paperwork. They are the audit trail that proves process control. The standards that matter most by application:
- ISO 9001 – Applies to all industries, quality management system
- ISO 13485 – Applies to medical devices, Medical QMS required for FDA-regulated components
- IATF 16949 – Applies to the automotive industry, like the PPAP or FMEA control plans
- FDA 21 CFR 177.2600 – Applies to food contact and medical products in terms of food-contact and skin-contact silicone
- USP Class VI – Applies to medical/pharmaceutical contexts for biocompatibility testing
- ISO 10993 – Applies to medical implants and device materials
- MIL-SPEC/ITAR-aware – Applies to the defense and aerospace industry for US-sourced materials and domestic manufacturing
Get Your Own LSR Project Done with Konark!
Do you have an LSR project? Konark Silicone Technologies can offer you manufactured rapid silicone prototypes, short-run silicone molding, and high-volume liquid silicone rubber processing. Get in touch with us now for industry-specific details and consultations!
Frequently Asked Questions.
1. Can I Overmold Lsr Onto Plastic Substrates?
Yes. LSR overmolding, also called P+R (plastic plus rubber) molding, bonds silicone directly to a rigid substrate, combining the structure of plastic with the flexibility and heat resistance of silicone. The substrate must withstand LSR cure temperatures, so high-temperature thermoplastics are the usual partners. See the overmolding section above for design constraints.
2.How Does Colorant Impact the Molding Process?
The colorants help transform the inherently translucent or white silicone into a customized, aesthetic product. In the LSR process, this happens when the liquid pigment paste is added as a third stream before it enters the mixer.
3.What is the Typical Shelf Life of Uncured LSR?
The usual shelf life of uncured LSR is typically between 6 and 12 months from the date of manufacture when stored in the original, unopened packaging.
4. Are There Size Limitations for Lsr Molded Parts?
Yes. They primarily depend on the material flow characteristics, molding equipment’s envelope, and tooling capabilities.
5. How is “Flash” Managed in High-precision Molding?
Flash is prevented rather than trimmed: optimal clamp force, mold tolerances held within microns at the parting line, calibrated venting depths, and controlled injection parameters stop the low-viscosity material from escaping the cavity.
