Liquid Silicone Rubber (LSR) Injection Molding Process
Liquid Silicone Rubber (LSR) Injection Molding Process with Dr. BOY Injection Molding Machines
Liquid Silicone Rubber (LSR) injection molding is an advanced elastomer processing technology that combines the flexibility of rubber with the short cycle times and automation level of thermoplastic injection molding. In applications requiring high thermal resistance, excellent chemical stability, and consistent mechanical properties, LSR has become a highly efficient alternative to conventional thermoset rubber, thermoplastics, and standard elastomers.
Injection molding machines from Dr. BOY provide an ideal platform for LSR processing thanks to their precise control, compact design, and exceptional process stability.
What Is LSR Injection Molding?
LSR is a two-component, platinum-cured silicone elastomer supplied in liquid form. The material consists of component A and component B, which are mixed in a 1:1 ratio immediately before processing. Once mixed, the material remains flowable until it is injected into a heated mold cavity, where curing (crosslinking) is initiated by heat.
Unlike thermoplastics, LSR does not melt when heated. Instead, it cures and solidifies through a chemical reaction. This makes precise temperature separation between the injection unit and the mold absolutely critical.
LSR materials combine rubber-like elasticity with thermoplastic-like cycle times. Today, thousands of LSR components are used in modern products, especially in automotive, medical, food-contact, and electronics applications.
Material Characteristics of LSR
LSR exhibits extremely low viscosity in its uncured state, allowing it to fill very thin wall sections and complex geometries with minimal injection pressure. At the same time, this low viscosity requires highly precise machine and mold design to prevent flash and uncontrolled flow.
Typical LSR material properties include:
• Hardness range from 10 to 80 Shore A
• Elongation at break up to 300-800 percent
• High thermal resistance up to approximately 180 °C
• Excellent UV, ozone, and aging resistance
• Odorless, tasteless, and suitable for medical and food-contact applications
• Very low emission during curing
These properties make LSR especially attractive for sensitive and high-performance applications.
LSR Plasticizing and Dosing on Dr. BOY Machines
In LSR processing, plasticizing differs fundamentally from thermoplastic injection molding. The objective is not to melt the material, but to convey, homogenize, and dose it at low temperature without triggering premature curing.
Dr. BOY injection molding machines use specially designed LSR plasticizing units with optimized screw and barrel geometry. The injection unit is actively cooled, while the mold is heated, ensuring a strict thermal separation between material preparation and curing.
Servo-controlled hydraulic systems enable highly precise control of screw rotation speed, back pressure, and dosing volume. This precision ensures stable flow behavior and consistent shot weights from cycle to cycle.
LSR Mixing and Metering Systems
A critical element of the LSR process is the mixing and metering unit. The A and B components are delivered from 20-liter containers or 200-liter drums and mixed in an exact 1:1 ratio. Optional level monitoring systems ensure continuous and accurate dosing, directly contributing to consistent part quality.
One centralized mixing unit can supply multiple injection molding machines simultaneously, offering both technical and economic advantages in serial production environments.
Injection and Mold Filling
During injection, the mixed LSR compound is transferred into the mold cavity under controlled pressure and very high injection speed. Dr. BOY machines provide closed-loop injection control with extremely high pressure accuracy, which is essential for filling thin sections and multi-cavity molds uniformly.
To prevent defects such as air entrapment, burn marks, or incomplete filling, injection speed is typically set very high, while venting and vacuum systems are used to evacuate air from the mold cavities.
Curing and Mold Temperature Control
Curing takes place entirely inside the heated mold. The stable clamping system of Dr. BOY machines ensures uniform contact pressure between mold halves, enabling consistent heat transfer and homogeneous curing.
Precise mold temperature control is essential, as even small temperature deviations can significantly influence shrinkage behavior, surface quality, and mechanical properties. Dr. BOY machines support accurate mold heating integration and stable thermal conditions throughout the curing phase.
Cycle Time and Process Efficiency
One of the key advantages of LSR injection molding is its short cycle time compared to conventional rubber processing. While curing time dominates the cycle, the highly stable and repeatable machine movements of Dr. BOY injection molding machines allow overall cycle optimization without compromising quality.
This results in high productivity, low scrap rates, and competitive cost per part.
LSR Mold Design Considerations
LSR molds differ significantly from thermoplastic molds. Due to the extremely low viscosity of uncured LSR, the material can penetrate gaps as small as 0.01 mm. Therefore, mold design requires exceptional precision, stress-relieved manufacturing, and specialized ejection concepts.
Cold runner systems are strongly recommended for LSR processing, as they minimize material waste and improve process stability. Proper thermal separation between cold runner, injection unit, and heated mold is essential for controlled curing.
Automation and Part Removal
LSR parts are typically soft and elastic after demolding. Standard ejector pins used in thermoplastic molds are generally unsuitable. Instead, stripping plates, air ejection, or mechanical demolding systems are used.
Dr. BOY machines offer excellent accessibility to the mold area thanks to their tie-bar-less clamping design, making automation and special demolding solutions easy to integrate.
Typical LSR Applications
LSR components produced on Dr. BOY injection molding machines are used across a wide range of industries, including:
• Medical devices such as seals, syringes, breathing masks, and tubing components
• Automotive components such as ignition seals, cable grommets, and connectors
• Electrical and electronic insulation parts
• Food-contact products such as baby bottle nipples and sealing elements
• Consumer and industrial elastomer components
Why Choose Dr. BOY for LSR Injection Molding?
Dr. BOY injection molding machines provide the precision, thermal separation, and process reliability required for demanding LSR applications. Stable clamping behavior, highly accurate injection control, compact machine design, and easy integration of LSR-specific equipment make Dr. BOY an excellent choice for liquid silicone rubber processing.
For manufacturers seeking short cycle times, consistent quality, and a highly automated production environment, Dr. BOY offers a proven and future-ready platform for LSR injection molding.