Liquid Silicone Rubber (LSR) Injection Molding
Liquid Silicone Rubber (LSR) has existed since the late 1970s, and has become the material of choice for manufacturing rubber parts and products. The distinctive qualities of liquid silicone rubber injection molding make it different from molding conventional high-consistency silicone and other thermoplastic injection molding processes.
Liquid Silicone Rubber injection molding has thousands of applications across a variety of industries, including medical, automotive, infant care, general industrial markets, aerospace, electronics and many other specialized industries. While liquid silicone rubber injection molding works best for short- and long-run part production, customers can employ the process in the final stages of prototyping. The process is also suitable for the evaluation of different material configurations before green lighting a project for full production.
As one of the global premier injection molding companies, SIMTEC works exclusively with liquid silicone rubber injection molding. We have produced billions of liquid silicone rubber parts for Fortune 100 and other leading businesses worldwide. We have the manufacturing expertise to provide services that range from prototyping to serial production of high-precision components.
SIMTEC’s ability to provide fast, high-quality service has led to a global reputation for commitment to customer satisfaction and our team of liquid silicone rubber injection molding experts can assist you through the entire process. Contact your SIMTEC representative today and learn how we can help you with your liquid silicone rubber injection molding needs.
Characteristics of LSR
For nearly 40 years, many companies in the major industries have chosen liquid silicone rubber to manufacture their rubber parts because of the manufacturability of the raw material. The injection molding of liquid silicone rubber provides businesses ease of processing, high-volume molding, consistent part quality and improved productivity. Liquid silicone rubber has become the ideal material for very demanding applications because of its unique properties, which include:
- Biocompatibility: Under extensive testing, liquid silicone rubber has demonstrated superior compatibility with human tissue and body fluids. In comparison to other elastomers, liquid silicone rubber 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.
- Durable: Liquid Silicone Rubber 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— a serious problem for components created through plastic injection molding methods.
- Chemical resistance: Liquid Silicone Rubber resists water, oxidation and some chemical solutions such as acids and alkali.
- Temperature resistance: Compared to other elastomers, silicone can withstand a wide range of high/low temperature extremes. For example, Silastic LSR by Dow Corning for continuous use can remain stable through temperatures ranging from –60°C to +180°C.
- Mechanical properties: Liquid Silicone Rubber has good elongation, high tear and tensile strength, excellent flexibility and a hardness range of 5 to 80 Shore A.
- Electrical properties: Liquid Silicone Rubber 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.
- Transparency and pigmentation: Liquid Silicone Rubber possesses a natural transparency, this attribute makes it possible to produce, colorful, custom, molded products.
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Part Design and Materials
By adhering to the fundamental design rules for Liquid Silicone Rubber parts, which are similar to injection molding, manufacturers can produce a component that will be stronger and easier to fabricate and assemble. The injection molding of liquid silicone rubber involves a complex task that consists of numerous functional and structural factors the designer must address, including:
- The intended use of the part.
- How the part fits in an assembly.
- The loads the part will experience in use.
The form that the features of the component must take depends on how the LSR material enters, fills and cools within the mold to create the part. Planning for the appropriate tolerances and incorporating additional features in the design can minimize stress and reduce flash. The ability of LSR to fill thin wall sections with minimal challenges and accommodate variations in wall sections makes the issue of sink virtually non-existent.
Similar to thermoplastics, liquid silicone rubber comes in a variety of types and grades. Depending on the application, customers can choose from a material with a rubbery or slippery feel and from a range of hardness.
General purpose Liquid Silicone Rubber does not have a high amount of silica, which makes it appropriate for the fabrication of parts that require only the minimal physical properties. Mixing additives and other fillers can make LSR suitable for applications that need to endure higher temperatures or fluid atmospheres. The invention of self-adhesive LSR provides the material required for over-molding applications.
The perfect, precision LSR part begins with the mold. Building the mold entails a combination of CNC milling and CNC electrical-discharge machining. It takes time and a great deal of accuracy to build a high-temperature aluminum tool mold capable of holding up to the LSR molding process, and it can be the largest investment in the silicone rubber molding process. However, the ability to flow LSR into very thin cross-sections, tight radii and thick/thin features make tooling design for liquid silicone rubber simpler.
In addition, tooling surfaces do not require a highly polished finish, and draft angles are not necessary to help remove the finished component from the mold. Once the solidification phase comes to completion, the molded part ejects from the mold without concern for distortion or damage.
Liquid Silicone Rubber Part Production
The liquid injection molding (LIM) manufacturing process molds silicone rubber into a broad array of parts and products. While the traditional reaction injection molding (RIM) process focuses on a pressurized impingement mixing process, Liquid Injection Molding is comprised of a mechanical mixing method that blends a two-component compound together in a mold.
The primary difference between LIM and standard plastic injection molding relates to the material delivery. The tool is loaded into an advanced LSR-specific injection-molding press, which is designed for precise control of the shot size and enables the consistent fabrication of liquid silicone rubber components.
Here are the primary steps involved in the liquid silicone rubber injection molding process:
- The uncured liquid silicone starts in two containers, Compound A and Compound B, which connects to the pumping system. Barrel A holds the base-forming material, and Barrel B contains a catalyst. A metering unit automatically releases the two substances at a constant one-to-one ratio, as well as any pigmentation additive.
- The molder can program and customize the automated injection-molding machine and have the injection setting deliver the appropriate shot size for each job. The platens and pistons, which align and close the mold, have settings for the temperature, pressure, injection rate and cycle time adjusted in accordance with the customer’s specifications
- After the completion of the setup, the machine heats the mold to the appropriate temperature and applies the appropriate clamping force. The mold closes in coordination with the machine’s injector, pump and supply. The injection mechanism then pushes the material into the mold and cavities.
- A combination of heat and pressure applied to the liquid silicone rubber cures the material until it solidifies. Upon completion of the cycle, the mold opens, and the part and flashing can be removed automatically or by a worker. Then, the mold closes and repeats the process.
- After the removal of the item from the mold, the post-molding process may include de-flashing, post-curing, inspection and packaging.
Fabrication of LSR components requires creatively solving the problems that will invariably arise. Your manufacturer must have the expertise to meet this challenge, as it is critical for your production process and will save you time and money.
Benefits of LSR Injection Molding
SIMTEC uses the liquid silicone rubber injection molding to create a full range of precision, near flashless silicone parts and over-mold assemblies. Liquid silicone rubber molding has several advantages, including:
- High-volume production: Advanced LSR materials offer manufacturers the opportunity for cost-effective, high-volume production of complex liquid silicone injection molding parts.
- Low chance of contamination: The LSR fabrication process takes place in a closed system, which reduces the possibility of contamination because the operator does not have to touch the material, and it is not exposed to the environment.
- Higher level of automation: While the silicone gum rubber process is very labor intensive, LSR allows for the automation of injection equipment, tooling and ejection equipment. This results in minimal labor cost to monitor the system and change out material drums.
- Quick cycle time: The cycle time for LSR components depends on the following elements:
- Eliminates flash and waste: Liquid silicone rubber does not produce scrap from flash because of the ability to create molds that operate flashless.
- The temperature of the mold and possible inserts.
- The temperature of the material when it reaches the mold.
- The geometry of the component.
- The general vulcanization characteristics.
- The chemistry of the curing material.
Silastic LSR allows for faster curing by preheating the injection barrel and cold runner to 40-80°C.
- Enhanced safety: The automation options make it unnecessary for operators to enter the molding area. Parts are removed from the injection-molding machine with conveyor belts, chutes or robots, which reduces the chances of burns or other safety concerns.
Liquid silicone rubber injection molding provides customers with a consistent, efficient and cost-effective processing method appropriate for a wide variety of applications.
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Industries and Applications
Regardless of the industry, the liquid injection molding process allows for the fabrication of rubber parts or components that must withstand extreme conditions and make your products more effective. Industries and applications include:
- Automotive — LSR has durability, tensile strength, high thermal stability, fatigue and tear-resistance and other properties, which make it an excellent choice for components used under the hood of vehicles. Many OEMs and 1st and 2nd tier automotive suppliers prefer injection molding liquid silicone rubber to create a wide variety of standalone parts, including O-rings, buttons, components, gaskets and wiring harness components. LSR can also be molded directly onto plastic or metal parts.
- Consumer products — The numerous properties found in liquid silicone rubber provide the raw material diversification required to manufacture a broad array of consumer products. These products range from kitchen bake ware and plastic showerheads to electronic gadgets and bottle dispensers.
- Specialty applications — The numerous types and grades of liquid silicone rubber makes the material suitable for a wide range of other uses, including:
- Valves and gauges
- Membranes and seal rings
- Irrigation and filtration parts
- Pressure exit, check and one-way valves
- Lighted panel buttons
- LSR overmolding — This fabrication process provides a cost-efficient solution for complicated assemblies that require the use of a gasket, seal or fragile rubber parts. This liquid silicone rubber injection molding technique can also provide components for demanding applications that require reliable performance with no room for errors. Along with enhanced part performance, LSR overmolding also generates significant savings in assembly costs. The ability to bond LSR, chemically and mechanically, to thermoplastics without the use of primers makes LSR overmolding an increasingly popular solution. The process also works for applications that require bonding to metals, with the use of an integrated adhesive and/or mechanical bonding that can create a durable bond.
LSR Quality Standards
Whether it’s product design, tooling, prototyping or new product build, your silicone rubber molding manufacturer must have the capabilities to satisfy the most demanding specifications and an unwavering commitment to delivering the quality you expect. The most rigorous quality assurance standards for liquid silicone rubber injection molding should include the following elements:
- Stringent in-process inspection procedures: LSR parts receive constant monitoring for defects, damage or any other inconsistencies.
- Database control number: The data control number is used to assign and record the call number for the material batch of every part manufactured.
- Packaged product history: This tracks each product back to manufacturing date, fabrication processes performed and the materials employed.
Quality programs need to get the part production right the first time to save you time, money and your reputation. As an ISO 9001:2008, ISO 13485:2003 and ISO/TS 16949:2009 certified company, the SIMTEC quality assurance system has demonstrated to Fortune 100 companies as well as other businesses from around the globe the proven ability to produce high-precision parts of impeccable quality.
Custom LSR Solutions For Your Business
SIMTEC has dedicated its focus exclusively to silicone rubber manufacturing and has mastered the production of both high-quality LSR parts and LSR 2-Shot components. Our expertise in this area allows us to tailor the entire process to your specifications, yielding true innovation. SIMTEC provides Extraordinary LSR Solutions™, taking “custom” to a new level. Since 2001, SIMTEC has been a leading Liquid Silicone Rubber parts manufacturer, serving some of the world’s largest companies. Our advanced capabilities encompass all aspects of the design and production processes. We can help you choose the right grade of LSR for your part and develop economical tooling and packaging solutions that keep your overhead costs low.
We produce custom designed and manufactured, high-quality LSR parts and LSR 2-Shot components. We produce parts for the automotive and industrial markets, as well as medical and consumer applications. Contact us today for a free quote, or download our informational guide to LSR.
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