Hi, this is Sunny from Silfuion! Today, we’re diving into an exciting topic: designing prototypes and components using Liquid Silicone Rubber (LSR). Whether you’re new to LSR or looking to refine your understanding, we’re here to give you a detailed guide on how to design LSR parts for maximum efficiency, durability, and functionality. At Silfuion, we specialize in LSR injection molding and offer a broad range of services to meet the needs of industries ranging from automotive to medical to consumer goods. Let’s explore the design guidelines and key factors to consider when working with LSR.
What is Liquid Silicone Rubber (LSR)?
Liquid Silicone Rubber (LSR) is a versatile, two-part thermoset polymer commonly used for injection molding. Unlike thermoplastic materials that can be melted and reformed, LSR undergoes a curing process after injection, solidifying into a permanent shape. This makes it a durable, high-performance material that can withstand extreme temperatures, harsh chemicals, and sterilization processes. LSR is also biocompatible, making it ideal for medical devices and products that have direct skin contact.
LSR’s superior characteristics make it particularly suitable for applications in demanding environments, such as seals, gaskets, valves, and cables in industries like automotive, medical, and food appliances. However, designing components for LSR molding requires a slightly different approach compared to traditional thermoplastics. Below, we’ll cover the important design considerations that are unique to LSR.
Key Design Considerations for LSR Parts
1. Part Dimensions
When designing LSR parts, there are size limitations that must be taken into account. At Silfuion, we typically work with parts that have a maximum size of 12 inches (304mm) in length, 8 inches (203mm) in width, and 4 inches (100mm) in height. The depth of a part should not exceed 2 inches (50mm) from any parting line. For deeper parts, the overall size will need to be smaller. Additionally, the maximum surface area we work with is 65.5 square inches (422.6 cm²), and the maximum part volume is 26 cubic inches (426 cc).
2. Wall and Rib Thickness
One of the strengths of LSR is its ability to fill thin wall sections with minimal challenges. Wall thickness can be as thin as 0.012 inches over smaller areas and 0.015 inches over larger areas, depending on the specific design. Rib thickness should be between 0.5 to 1.0 times the thickness of the adjoining wall. LSR’s unique properties also allow for variations in wall thickness with minimal risk of sink marks, making it a flexible choice for intricate designs.
3. Shrinkage and Flash Considerations
The shrink rate for LSR is relatively high but very consistent throughout the part, with a tolerance of approximately 0.025 inches/inch. Flash, the excess material that can appear at the edges of a part, is a common occurrence in LSR molding. However, with careful mold design, Silfuion’s team ensures that flash is minimized, achieving clean, precise parts. Flash typically occurs in gaps as small as 0.0002 inches, so it’s important to design your part with the proper features to reduce it.
4. Parting Lines
Minimizing parting lines—where the two halves of the mold meet—is crucial for ensuring the quality and appearance of LSR parts. Clean, well-positioned parting lines lead to smoother surfaces and faster production. Silfuion can help optimize your design by simplifying the placement of parting lines, ensuring quicker and cleaner results.
5. Undercuts
LSR parts can easily accommodate undercuts, which are features that can’t be molded without the use of additional mechanical actions. While LSR is highly versatile, it is important to note that LSR tends to tear at sharp edges, especially in areas with undercuts. When designing for LSR, we recommend adding radii or fillets to sharp edges to prevent tearing and improve part durability. Silfuion offers tooling solutions for mechanically releasing undercuts and minimizing potential damage during the molding process.
Advanced Molding Techniques: Overmolding and Insert Molding
At Silfuion, we offer advanced molding techniques such as overmolding and insert molding, which enhance the functionality and performance of LSR parts.
Overmolding
Overmolding is a process where one material (typically a thermoplastic elastomer or TPE) is molded over a previously molded part. This allows for combining the characteristics of different materials, such as adding a soft, tactile rubber layer over a hard, rigid substrate. Overmolding is often used in consumer goods and medical devices to enhance functionality and ergonomics.
Insert Molding
Insert molding is used to reinforce specific areas of an LSR part by adding a metal insert or high-temperature plastic material during the molding process. For example, brass inserts can be added for strength or stainless steel inserts for thread support. LSR, however, does not bond chemically with these inserts, so mechanical bonding features are required to ensure a secure connection. Silfuion’s expertise in insert molding can help ensure a robust, durable final product.
Part Ejection: Design Considerations
Ejecting LSR parts from the mold requires careful consideration because LSR tends to flash easily, and ejector pins are generally avoided. Instead, parts are designed to remain on one half of the mold after the molding cycle, where they are manually removed with air assistance. This manual process requires thoughtful design to facilitate easy de-molding without damaging the parts.
Prototyping and Production Volumes
At Silfuion, we specialize in LSR parts for a variety of industries, offering rapid prototyping and production runs of up to 5,000+ parts in as little as three weeks. Whether you need a small batch for testing or a larger production volume, we can meet your timeline and deliver high-quality LSR components that meet your exact specifications.
Conclusion
Designing with LSR requires a deep understanding of the material’s properties and the intricacies of the molding process. Whether you are designing a medical device, automotive component, or consumer product, Silfuion offers the expertise and experience to bring your vision to life with precision and efficiency.
Our team is here to guide you through the entire design process, from prototyping to final production. If you have a project in mind or need assistance with LSR design, don’t hesitate to reach out to us. Contact Silfuion today, and let’s discuss how we can help you achieve your design goals with the power of Liquid Silicone Rubber!
