Liquid Silicone Rubber (LSR) is widely favored by both manufacturers and consumers. LSR is a silicone-based material featuring excellent elasticity, water and moisture resistance, and strong chemical stability against acids, alkalis, and various corrosive substances. Because of these advantages, LSR is often used as a replacement for plastic products in daily applications.
LSR consists of two components: Part A and Part B. A metering system delivers the two components in a precise 1:1 ratio, and a static mixer ensures complete mixing. The blended material is then injected through the injection barrel to begin the molding process.
By injecting LSR into a hot-runner mold, silicone rubber products can be manufactured with the advantages of one-step molding, no material waste, and full automation.
(1) Characteristics of LSR
Liquid Silicone Rubber (LSR) is a non-toxic, heat-resistant, highly elastic thermosetting material. Its rheological behavior is characterized by low viscosity, fast curing, shear thinning, and a relatively high coefficient of thermal expansion.
LSR is a two-component, platinum-catalyzed, fast-curing system suitable for high-volume, highly repeatable mechanical production through injection molding. LSR products offer excellent thermal stability, superb cold resistance, outstanding electrical insulation, and do not produce toxic by-products when burned.
Therefore, LSR has become an irreplaceable material in the design and production of health products, automotive parts, baby products, medical devices, diving equipment, kitchenware, and sealing components.
(2) Molding Process
LSR is a two-component liquid material (Component A and Component B). The mixing unit blends A and B in an accurate 1:1 ratio. Since some products require coloration, a color pump and metering unit may also be included. After A + B components, additives, and pigments are fully mixed, the mixture enters the plasticizing system.
The plasticizing screw provides both homogenizing and mixing functions. The screw injects the mixed material into a heated mold, where the silicone cures at 170–200°C.
When a cold runner system is used, it is crucial to maintain a sufficiently low runner temperature. To prevent leakage, needle valves are installed on the mold surface. After injection, the needle valve immediately seals the nozzle.
1. Feeding systems
Several types are available:
- Bidirectional pump
Moves up and down during feeding to maintain stable pressure. Because A and B pumps are connected and synchronized pneumatically/hydraulically, this system is reliable and precise. - Unidirectional pump
A general-purpose system that feeds in only one direction. - Synchronized unidirectional pump with check valve
- Metering cylinder system
Typically used in combination with a unidirectional pump.
2. Key components of an LSR injection molding machine
- Due to LSR’s low viscosity, screw sealing is essential to prevent backflow and leakage.
- Needle-valve nozzles must be used to prevent premature curing.
- Mixing and metering units for components A and B are required.
3. Mold design
Common runner systems include:
- Hot runner
Simple and low-cost but wastes more material; used for larger parts. - Cold runner with needle valve
Enables automation and short cycles. - Cold runner without needle valve
Because LSR has a high thermal expansion coefficient, it expands during heating and slightly shrinks when cooling. This makes it difficult to maintain accurate side clearances in the mold. A closed cold-runner system helps keep LSR cool and fluid; each runner uses a shut-off pin or needle valve to meter material precisely.
Silicone rubber typically has a 2–4% shrinkage rate at a curing temperature of 150°C, and it also exhibits compression set behavior.
(3) LSR Flow / Curing Analysis
- LSR requires a specific reaction time for chemical curing.
- Ideal flow: >100 cm through a 2 mm diameter, 170 cm path.
- Minimum achievable part thickness: 1/1000 mm.
- Excessive mold temperature causes premature curing and restricts flow.
- Laminar flow prevents air entrapment.
- High injection speed causes turbulence.
- Viscosity variations alter flow patterns.
- Turbulence leads to white spot defects.
Thus, mold design must consider the following:
- Use small gates to avoid jetting and turbulence.
- Prefer needle-type or fan-type gates.
- Use T-shaped guide pins instead of cylindrical ones; employ air-eject systems and insulating plates.
- Do not use rust inhibitors or oils containing polymerization inhibitors—use toluene, xylene, etc., as acceptable alternatives.
(4) Common Defects and Solutions
- Incomplete curing
- Surface tackiness (surface not fully dried)
- Incorrect hardness
- Contamination or foreign particles
- Color streaks
- Deformation or scorching
- Short shots or sink marks
Additionally, cured LSR tends to stick to metal surfaces. Common demolding techniques include ejector pins and air ejection.
From these considerations, the key issues that LSR injection molding must address are mixing accuracy, metering precision, screw sealing, and proper mold design.
