In recent years, Liquid Silicone Rubber (LSR) injection molding has shifted from a niche process to a core manufacturing method for components that demand precision, long-term stability, and strict safety requirements. Entering 2025, many medical, automotive and electronics manufacturers are redesigning parts specifically around LSR's performance advantages.
This article provides a practical, engineering-oriented look at how the process works, what makes LSR different from other elastomers, and why certain applications benefit significantly from switching to LSR.
What Makes LSR Different from Solid Silicone
LSR is a two-component, platinum-cured liquid silicone system. Compared with traditional solid silicone (HCR), LSR behaves more like a fluid than a rubber during processing. This difference changes everything about how it flows, fills a cavity, and cures.
Key distinctions include:
Material Form
LSR is supplied in drums and pumped directly into the injection unit. HCR arrives as solid blocks and requires manual loading.
Flow Characteristics
LSR's viscosity is extremely low. It can fill thin-wall areas, micro-features and long flow paths where solid silicone simply cannot perform.
Cure Behavior
LSR cures through heat, not pressure. Once the A and B components mix, the material begins cross-linking when exposed to elevated temperatures.
Dimensional Stability
Parts molded in LSR are consistent from batch to batch, making the material suitable for regulated industries where tolerances matter.
Because of these characteristics, LSR has become a preferred material for projects that require ISO 10993 compliance, optical clarity, extremely long service life, or stable performance in harsh environments.
Why LSR Molds Require Cold Runners and Thermal Isolation
One of the first things engineers learn about LSR molding is that the tooling structure differs significantly from thermoplastic molds. The reason is simple: LSR cures when heated, so keeping the runner cold is essential to prevent premature curing.
A well-built LSR mold typically includes:
Cold-runner manifold to maintain material temperature between 5–25°C
Hot cavities (usually 160–200°C) where the curing reaction takes place
Thermal isolation plates to prevent heat transfer between hot and cold zones
Valve gates for high precision and reduced flash
If heat leaks into the runner, the material can cure inside the channel, leading to contamination, downtime and mold damage. This is why the design, machining accuracy and material selection for LSR molds are more demanding.
Flow and Curing Behavior During the Injection Process
LSR behaves differently from thermoplastic materials throughout the injection cycle:
High Fluidity
The material requires very low pressure to fill even the most challenging geometry. Engineers often use LSR for:
Thin walls down to 0.3 mm
Micro-valve components
Complex sealing structures
Parts that require consistent compression behavior
Fast, Temperature-Driven Cure
Once the material enters the heated cavity, cross-linking accelerates quickly. Depending on the grade and wall thickness, full curing typically takes 20–40 seconds. This rapid reaction is one of the reasons LSR is widely used for high-volume medical disposable products.
Where LSR Is Most Widely Used
Medical Devices
This is the sector with the highest performance requirements - and the largest global consumption of LSR components. Common applications include:
Ventilation and respiratory seals
Valves and diaphragms for pumps
Infant care products
Surgical instrument grips
2K molded soft-touch handles
Medical OEMs rely on LSR because it is biocompatible, odorless, and free from plasticizers or extractables. The moldability and consistency also reduce the need for secondary processing.
Automotive and EV Components
The shift toward electrification has created strong demand for silicone components that can withstand high temperatures, chemicals, and long service life.
Typical LSR automotive applications include:
Connector and ECU seals
Sensor boots
Grommets and sealing rings
Transparent silicone lenses for LED modules
LSR maintains stable properties from –50°C to +200°C, making it ideal for under-hood or high-heat environments.
Consumer and Industrial Electronics
Manufacturers choose LSR when a device must remain sealed, soft to the touch, and durable for years. Examples:
Buttons and keypad components
Wearable seals
Waterproof gaskets for handheld electronics
Optical silicone parts for lighting
Demand continues to rise as more products aim for IP67/IP68 waterproof ratings.
5. When LSR Is a Better Option Than TPE or TPU
Although TPE and TPU are widely used, they fall short in performance when:
Continuous operating temperatures exceed 120°C
Long-term UV or weather exposure is expected
Biocompatibility is required
A transparent part must not yellow over time
The assembly must remain elastic for 5–10 years
Compression set needs to remain low
In these conditions, LSR is often the material of choice. It retains its mechanical behavior long after many thermoplastic elastomers begin to degrade.
6. Common Design Issues and Practical Engineering Advice
Even experienced designers occasionally overlook LSR-specific guidelines. The most common mistakes include:
Placing the gate in a location where the material cures too early
Overlooking venting on deep ribs or enclosed structures
Designing parting lines that make low-flash molding difficult
Underestimating the importance of vacuum assistance
Failing to account for hot/cold zone separation in the mold
To ensure a stable process and consistent parts, engineers usually follow these recommendations:
Maintain wall thickness between 0.3–5.0 mm
Keep flow paths balanced to avoid premature curing
Use polished cavity surfaces for optical-grade silicone
Include mechanical interlocks for 2K molding with plastics such as PC or PA
Reserve space for automation if the part is small or destined for medical use
Final Thoughts
As manufacturers push toward tighter tolerances, automated production, and more reliable materials, LSR injection molding is becoming a strategic technology rather than an optional capability. Medical and automotive companies now expect suppliers to understand LSR processing, mold structure, and design rules from the earliest development stages.
For mold makers and OEMs planning for the next three to five years, investing in LSR-specific design expertise and process stability will be essential. The companies that master this technology early will be better positioned to serve high-growth markets across Europe, North America, and South America.