Liquid silicone rubber is a versatile elastomer widely used across industries due to its durability and outstanding performance under demanding working conditions. This article provides a detailed overview of liquid silicone rubber, its properties, and its various types. You’ll also learn about the advantages and limitations of liquid silicone rubber. Let’s get to it!
What is Liquid Silicone Rubber?
Liquid silicon rubber, or LSR, is a unique two-part platinum-cured elastomer with excellent fluidity. The first part of the LSR contains a platinum catalyst, while the second part contains methyl hydrogen siloxane as a cross-linker and alcohol inhibitor. It is usually injected into a silicone mold cavity to create different parts with precise details.
The liquid nature of LSR distinguishes it from high-consistency rubber materials. This material uses additive curing with platinum and requires special treatment in injection molding. Additionally, silicone rubber parts do not form byproducts or odors which could contaminate the air while curing, making LSR ideal for food and medical-grade applications.
Properties of Liquid Silicone Rubber
Manufacturers use liquid silicon rubber in demanding applications due to its excellent properties.
Tasteless and Odorless
Liquid silicone rubber is ideal for different applications, especially for food and beverage and baby products, because it has no taste or odor. Hence, you can easily formulate it to meet the strict FDA standards for these applications.
Unlike other elastomers, liquid silicone rubber has higher bacterial-resistant properties and doesn’t corrode or stain other materials. After extensive testing, LSR exhibits excellent compatibility with body fluids and human tissue. It is an ideal choice for skin-contact products since it is hypoallergenic.
Liquid silicon rubber materials can withstand oxidation, water, and chemical solutions like alkalis and acids. Besides, it is compatible with sterilization processes such as e-beam, ethylene oxide (ETO), steam autoclaving, gamma, and several other methods compatible with medical and healthcare products.
Transparency and Pigmentation
Naturally, liquid silicone rubber is translucent. Its inherent color is white and suitable for pigmentation for creating custom, molded products in different colors.
Liquid silicon rubber parts possess excellent insulating properties suitable for various electrical applications. Silicon materials can function well in higher and lower temperatures than conventional insulating materials.
Types of Liquid Silicone Rubber
Liquid silicone rubber has different types and compositions. Each type has varying characteristics.
General Purpose LSR
This is a widely used material with extensive applications due to its flexibility, biocompatibility, and heat resistance. Typical uses of general purpose LSR type include seals, gaskets, and industrial components.
High-temperature liquid silicone rubber materials can withstand extreme heat exposure without degrading or compromising its qualities. Common applications of this LSR include industrial equipment, automotive parts, and electronic components.
Medical Grade LSR
This type of liquid silicone rubber is formulated to meet the strict biocompatibility and safety requirements for medical and healthcare products. Medical grade LSR materials are sterilizable and biocompatible, making them suitable for applications that require contact with the human body.
Self-bonding LSR material exhibits excellent properties of adhering to itself without using other adhesives. Therefore, it is perfect for making complex assemblies without depending on traditional bonding techniques.
The food grade is similar to the medical grade LSR. It meets strict requirements for contact with food and beverages. Baby products, baking molds, and kitchen utensils are typical food-grade liquid silicone material applications.
Electrically Conductive LSR
It contains electrostatic discharge (ESD) additives that offer electrical conductivity properties. This grade is commonly used in applications with electrical conductivity and flexibility of silicone rubber requirements, including EMI/RFI shielding and flexible circuits.
Optically clear liquid silicone rubber is suitable for making products like lenses, light guides, and other optical parts and equipment with transparency and optical clarity requirements.
Advantages and Limitations of Liquid Silicone Rubber
Liquid silicone rubber material offers remarkable benefits due to its unique properties.
Advantages of LSR
Below are some of these benefits:
Overmolding Capabilities and Complex Geometries
Liquid silicone rubber is perfect for molding parts with complex shapes and intricate features since it offers low viscosity. At the same time, LSR is helpful in developing challenging elements and overmolding them onto other substrates. Hence, using a single manufacturing method, these silicone rubber parts often possess complex and functional features with different materials.
Silicone rubber parts are more efficient and cost-effective than other parts produced through traditional molding methods. Liquid silicone rubber parts allow shorter molding cycle times and higher production outputs, ensuring an efficient molding process.
Due to their high compatibility and excellent material properties, manufacturers often use liquid silicone rubber parts for different applications. These parts offer superior precision and quality that meet the stringent requirements of various industries. More so, silicone rubber parts provide reliable solutions for industrial applications.
Limitations of LSR
Although LSR offers a vast range of benefits, there are some limitations to using the material.
Liquid silicone rubber parts processing can generate more waste than other rubber materials due to the need for accurate mixing and curing requirements.
Silicone rubber parts take longer time to cure than some other rubber materials. Consequently, this delays the production processes, particularly for complex and large components.
Certain LSR formulations might consist of additives or elements that may be open to regulatory examination, especially applications that involve contact with food, sensitive environments, or medical devices.
Despite its good elasticity, LSR parts have lesser tear strength than other rubber materials. As such, it may limit its use in applications with high tear resistance.
Liquid Silicone Rubber Molding Process: Step by Step
Injection molding is ideal for making precise silicone rubber parts from LSR. It offers parts with accurate dimensions, intricate design, and superior surface finishes. Below is a step-wise breakdown of the process.
Step 1: Material Preparation
LSR material is a two-part platinum-cured elastomer containing a base silicone polymer and a curing medium. These components are combined in a calculated ratio to begin the molding process. You can mix these components manually or with automatic metering and mixing tools.
Step 2: Injection Molding
It would be best to inject the LSR mixture into the mold with specialized injection molding machines ideal for LSR processing. Generally, the mold consists of two cavities made from high-quality steel or aluminum, which determines the shape of the silicon rubber mold. You must inject the liquid silicone rubber into the mold cavity under high pressure, ensuring it fills the required details and features.
Step 3: Curing
Cure and solidify the silicon rubber mold with a chemical reaction prepared by heat. Depending on the liquid silicone rubber’s formulation, you must heat the mold to a particular temperature. The curing process lasts a few minutes as it changes the LSR material from liquid to solid elastomer.
Step 4: Cooling and Demolding
Cooling the mold to a suitable temperature after the curing process is important. It helps to ensure the silicone rubber parts maintain their shape and dimensions. It is safe to open the mold once the parts have cooled properly. Then, you can eject the cured parts from the mold cavities.
Step 5: Post-Processing
Depending on the silicone rubber parts’ requirements, post-processing operations may be necessary for your silicon rubber parts. These procedures may include excess material or flash trimming, checking for defects, and assembling components if the product comprises multiple parts.
Applications of Liquid Silicone Rubber in Various Industries
Product designers and manufacturers often use liquid silicone rubber in different industry applications. The LSR injection molding process is ideal for making prototypes in the final stages to achieve near-production quality, especially in clinical trials and testing, as well as pilot production requirements. Typical applications of silicon rubber include:
The LSR’s excellent performance when exposed to ultraviolet light and specific temperatures makes it well-suited for making different parts in the automotive industry. Typical examples of parts produced with LSR include housings, wire harness seals and connectors, sensors, vibration dampeners, gaskets, and bellows.
LSR is a versatile material that establishes watertight barriers and showerheads resistant to bacteria. Likewise, it is useful for vibration-resistant sealed housings for personal hygiene products, including soft, pliable surfaces for headphones and hearing aid tips, electric toothbrushes, and shavers.
Liquid silicone rubber can withstand harsh environments. Therefore, LSR is an excellent fit for producing gaskets, valves, sealed housing, and other components used in safety devices such as carbon monoxide and smoke detectors.
The medical industry often uses liquid silicone rubber for different medical applications due to its chemical inertness, stability in demanding environments, and biocompatibility. Most products in the medical sector comprise components with intricate features, tight tolerances, heightened cleanliness, and tiny part geometries.
There are many critical applications in the medical industry where liquid silicone rubber is the ideal option, including surgical instruments, respirator masks, diaphragms, and drug delivery devices.
Design Considerations for Liquid Silicone Rubber Parts
There are several essential tips and considerations to ensure the successful production and optimal performance of silicone rubber parts. Below are some of these critical design considerations:
It is advisable to ensure the compatibility of the LSR material with the intended materials when overmolding LSR onto other substrates. Therefore, ensure the process and mold design support proper bonding between the substrate and the LSR.
Tolerances and Dimensional Accuracy
Although liquid silicone rubber can achieve tight tolerances, it would be best to note that potential variations in dimensions may occur due to curing and material shrinkage. Hence, it would help to consult your mold designers to improve the geometry of your silicone rubber parts for accurate dimensions.
Use a balanced wall thickness for your parts to ensure uniform curing and avoid dimensional control errors and deformations such as warping or voids. Therefore, using a minimum wall thickness between 0.635 -0.762 mm on small parts and 1.02 – 1.27 mm wall thickness on larger parts in injection molding is advisable. Also, be careful not to make hasty changes to the wall thickness to prevent stress concentration or inconsistent curing.
Undercuts and Draft Angles
Draft angles are crucial in molding silicone rubber parts because they enable the smooth ejection of parts. Therefore, designing your parts with the proper draft angles and reducing undercuts is advisable to ensure easy ejection.
Surface Finish and Texture
Silicone rubber parts are compatible with various surface finishes such as matte, textured, or smooth. As a result, it is advisable to choose the texture that suits your silicone rubber parts’ functional requirements and aesthetics.
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Liquid silicone rubber is a responsive material that is durable, customizable and safe for specific applications. Silicone rubber parts exhibit remarkable characteristics and perform well in varying working conditions making LSR an ideal material for a broad spectrum of applications. However, it would be best to understand everything about liquid silicone rubber and design considerations for molding quality liquid silicone rubber parts.
What differentiates liquid silicone rubber from silicone rubber?
Liquid silicone rubber parts have no odor, exhibit high transparency, and have a glue injection port. Conversely, solid silicone parts exhibit low transparency with the vulcanization agents or other smells surrounding the curing agents. Likewise, solid silicone products have no injection port.
What are the common uses of liquid silicone rubber?
Liquid silicone is a multipurpose material with an extensive application range. Manufacturers and product designers commonly use it in automotive components, healthcare, electronics, and consumer product manufacturing.
Is liquid silicone rubber resistant to water?
LSR is waterproof and won’t deteriorate upon exposure to chemical solvents. Moreover, it is suitable for applications that require constant water exposure.