Liquid silicone rubber (LSR) is a thermoset elastomer that exhibits relatively high elongation, consistent high‐temperature performance,can be injection molded or cast into silicone rubber sheets. It is available in a wide range of hardnesses (durometer). Liquid silicone rubber is divided into injection molding silicone and liquid silicone rubber for molds according to its use and curing method. Injection molding silicone belongs to HTV silicone rubber, and liquid silicone rubber for molds belongs to RTV silicone rubber.
Types of Liquid Silicone Rubber
In LSR materials, Liquid silicone rubber consists of two parts, an A and a B. Both parts are a liquid, a catalyst is required as part of the curing process. Common cure catalysts are tin (condensation) cure and platinum (addition) cure.
Liquid Silicone Injection Molding
Liquid injection molding silicone are usually platinum (addition) cured silicone. Component A contains a platinum catalyst and Component B contains methylhydrogensiloxane as a cross-linker (also referred to as a curing agent) and an alcohol inhibitor. LSRs are viscous, but pumpable materials, mostly processed through liquid injection molding (LIM). The mixing ratio for parts A and B is often allowed to vary up to 5% off ratio for a 1:1 ratio LSR without significantly impacting the process or final product properties.
Injection molding process
LSR injection molding uses a base material and a catalyst, a vulcanizing agent that turns liquid silicone rubber into a solid material. Most LSRs use a platinum-based curing system where an addition reaction occurs. Heat or pressure can be applied to accelerate curing.
Platinum or addition curing doesn’t produce significant levels of byproducts, such as volatile organic compounds (VOCs), that require removal. This eliminates the need for post-baking in industrial ovens, which saves time and money during silicone parts production. If post-curing is used, it’s generally to stabilize or enhance an LSR’s properties for an especially demanding application.
LSR injection molding starts with meter-mixing. A pump dispenses the ingredients and a static mixer combines the components. This mixture is then transferred to a cooled section of the injection molding machine. Because the material is homogeneous, LSRs support consistency throughout individual parts and from part to part. LSR injection molding also supports automated operations for greater efficiency.
When LSR curing occurs, there’s no loss of material in the feed lines for reduced material waste. Stainless steel molds are more expensive, but molds that are properly tooled produce silicone parts with minimal flash that eliminate the need for trimming. Because LSRs have a low viscosity or resistance to flow, they also support the use of complex, intricate molds in applications such as medical devices.
Note that injection molding of liquid silicone rubber is the inverse of molding thermoplastics; room temperature liquid silicone compound is injected into and cured in a hot mold vs. plastic pellets melted into hot liquid and forced into a cooled mold.
Advantages
- Batches stability (ready-to-use material)
- High level of repeatability, good for tight tolerance / precision components.y
- Direct injection (no waste)
- Short cycle time
- ‘Flashless’ technology (no burrs)
- Automated process
- Automated demolding systems
- Superior clarity, material can be pigmented in-line with material flow to produce colors.
- Closed mold injection supports molding of complex geometries and over-molding.
Disadvantages
Higher start-up / shutdown costs, better suited for high volume applications.
Runner systems can lead to increased gross material weights when cold runner systems or other low waste options are not utilized.
Liquid Silicone Overmolding Molding
Overmolding, two‐shot molding, and insert molding are all processes when LSR is molded directly onto or around components made of other materials such as metal, thermoplastic, another silicone, etc., thereby allowing a combination of material properties and the direct assembly of LSR onto another other material. The first shot or inserted component is typically the more rigid material with the more flexible LSR material being formed around or onto it. LSR overmolding or two‐shot molding involves molding a polymeric component and then molding an LSR material directly onto it while it remains in the same mold. Components within the mold are indexed or rotated to allow the first material (commonly thermoplastic) to be formed and then the second material (typically LSR) to the formed directly on top.
Liquid Silicone Insert Molding
Insert molding involves loading a previously formed part (metal, plastic, etc.,) into the mold and injecting the LSR around it. Operators or pick‐and‐place robots may be used to physically install the insert into the mold prior to injection. In these processes, the other material must withstand the temperatures associated with the LSR molding and curing process while maintaining sufficient rigidity. This typically excludes olefins and other commodity resins, while favoring engineering thermoplastics with superior high temperature performance, other thermosetting resins, metals, and ceramics.
Liquid Silicone for Molds
Liquid silicone for molds is an ideal material for making silicone molds. There are tin (condensation) cured silicone and platinum (addition) cured silicone.
Platinum-cured LSR is typically “cleaner” than tin-cured LSR . Platinum-cured LSR is often capable of meeting FDA and USP Class VI certifications. Platinum-cured LSR can also survive higher temperatures when compared to tin-cured alternatives.
Tin-cured LSR is usually found in consumer and inventor-grade material. This material can cure using condensation from the air and does not require heat. Tin-cured LSR is typically not FDA grade and is not used in more demanding applications.
While tin-cured LSR is more accessible for hobby-grade and various consumer applications, it does not offer some of the distinct advantages and high-performance characteristics of platinum-cured LSR.
This type of liquid silicone will not be described in detail here. For detailed mold making silicone, please refer to the dictionary: Mold making silicone.
LSR Features
Biocompatibility: Under extensive testing, liquid silicone rubber has demonstrated superior compatibility with human tissue and body fluids. In comparison to other elastomers, LSR is resistant to bacteria growth and will not stain or corrode other materials. LSR is also tasteless and odorless and can be formulated to comply with stringent FDA requirements. The material can be sterilized via a variety of methods, including steam autoclaving, ethylene oxide (ETO), gamma, e-beam and numerous other techniques, meeting all required approvals.
Durable: Durable: LSR parts can withstand extreme temperatures, which makes them an ideal choice for components under the hood of cars and in close proximity to engines. Parts fabricated via liquid silicone rubber injection molding are fire retardant and will not melt.
Chemical resistance: Thermoplastic elastomers can resist chemicals in a broad range of applications. But they only offer chemical resistance in moderate temperature ranges. LSR offers low water absorption and excellent resistance to common chemicals even when exposed to extreme temperatures.
Temperature resistance: Compared to other elastomers, silicone can withstand a wide range of high/low temperature extremes (-50°C to 250°°C).
Aging resistance：Aging resistance is an important factor in determining the right material for your injection molding process. Cured LSR materials provide excellent resistance to UV rays, weathering, and aging. They are ideal for use in applications where long-term exposure to the elements is a concern.
Mechanical properties: LSR has good elongation, high tear and tensile strength, excellent flexibility and a hardness range of 0 to 80 Shore A.
Low compression set: A low compression set means that materials can resist permanent deformation under a constant strain. LSRs have a very low compression set, typically in the range of 15%-20%. They are able to retain their elasticity even when compressed for extended periods of time and subjected to different temperature ranges.
Electrical properties: LSR has excellent insulating properties, which offer an appealing option for a host of electrical applications. Compared to conventional insulating material, silicone can perform in far higher and lower temperatures.
Easy to color: Many available standard colors and custom colors for molded parts and molded sheets. Unique colors and visual effects can be achieved through the use of pigments and master-batches.
Liquid Silicone Durometer Range
Liquid silicone rubber is available in a hardness range of 0 to 80 durometer Shore A. 10 durometer LSR is very soft like chewing gum and can feel softer than some extra firm silicone foam and silicone sponge products. 80 durometer LSR is harder than a heel of a shoe and is as firm as many thermoplastic elastomer products. Most applications for Liquid silicone rubber are specified in the 20 to 70 durometer Shore A range. Most commercial LSR materials are categorized by durometer.
LSR Application
Liquid silicone rubber is a highly versatile material. Their steam resistance and low compression set make them an excellent choice for gaskets and hardware in consumer appliances such as microwaves. Their oil and heat resistance offers long-term durability in automotive components. Their excellent release properties and excellent tear strength make them ideal for making silicone molds.
Liquid silicone rubber is a mainstay in electronics and consumer products, gaskets and other hardware, and many other markets, LSR are considered as the material of choice for industries such as Automotive or Medical devices.
Typical liquid silicone rubber products:
- Industrial and food grade silicone mold making
- Valves and gauges
- Membranes and seal rings
- Irrigation and filtration parts
- Pressure exit, check and one-way valves
- Lighted panel buttons
