Liquid silicone rubber (LSR) injection molding is a specialized class of injection molding used to produce flexible, tough parts that are thermally and chemically resistant, biocompatible, and can be made to be optically clear.
This article will describe what LSR is, how the LSR injection molding process works, and detail the various applications of LSR.
Table of Contents
What Is Injection Molding of Liquid Silicone Rubber?
Injection molding is a process whereby molten thermoplastic is rapidly injected into a mold cavity at high pressure to form a component. Liquid silicone rubber injection molding follows a similar process. However, specialized injection molding machines are required and the raw material is a two-component thermosetting polymer that must be mixed just before injection into the mold cavity, where it cures in a heated mold before being ejected.
What Is Injection Molding of Liquid Silicone Rubber Used For?
Injection-molded silicone rubber is used for applications that can benefit from its unique properties. For example, silicone is bio-compatible, making it ideal for medical applications. In addition, it has excellent thermal resistance while also being soft, making it ideal for high-temperature gaskets.
How Is Liquid Silicone Rubber Manufactured?
Liquid silicone rubber is an inorganic polymer consisting of a “backbone” of alternating silicon and oxygen atoms, with a variety of potential molecular groups (e.g., CH3 groups) attached to the remaining available bond locations on the silicon atom. This polysilicon chain is also reinforced via crosslinking to other molecular chains. LSR is produced by mixing two liquid components, namely the polymer and a catalyst. The catalyst is most often platinum-based but other options like 2,4-Dichlorobenzoyl peroxide are available. To solidify the silicone, these two liquids must be thoroughly mixed, which then forms bonds between the molecular chains. This is the process referred to as cross-linking.
How Is LSR Injection Molding Used in Manufacturing?
LSR is used to produce a wide range of specialized components for manufacturing industries, from high-temperature gaskets to vibration isolators.
How Does Liquid Silicone Rubber Molding Work?
The liquid silicone rubber molding process is similar to normal injection molding. Still, it has a few key differences:
1. The injection molding process consists of injecting molten plastic into an enclosed mold cavity. The plastic is melted in a heated barrel with an enclosed screw. As the screw rotates, it feeds plastic pellets into the barrel. The screw pitch becomes smaller as it nears the mold, forcing the plastic to shear against the screw and the barrel wall. This process is what provides most of the heat required to melt the plastic.
Once a sufficient amount of molten plastic has been metered out by the screw, the screw retracts and then pushes the plastic into the mold at a very high pressure.
The molds are designed to withstand the extremely high injection pressure involved and are often temperature-controlled to reduce cycle time and increase part quality.
Once the part has cooled, the mold opens and the completed parts are ejected.
2. The raw material in LSR consists of two parts, namely the polymer and the catalyst. These are typically in liquid form. Once mixed at the correct ratio, they will harden into silicone rubber. In some cases, a third component, usually a pigment, is also added to color the material. Because it consists of two liquid components, to begin with, LSR does not need to be melted.
An LSR molding machine has a mixing head that is designed to thoroughly mix the separate components of the silicone before it enters the barrel with the screw. Poor mixing will result in uncured wet spots on the part. In addition, the screw is kept cool during LSR molding to prevent premature reaction of the polymer with the catalyst. The runners leading from the barrel to the mold are kept cold for the same reason.
LSR molds are kept hot to help increase the curing rate of the LSR. In addition, LSR injection pressures are much lower than those used for injection molding, which means that the molds do not need to be as robust as those used for normal injection molding.
What Is the Process for LSR Injection Molding?
The LSR injection molding process involves several steps, as detailed below:
Material Preparation: Liquid Silicone Rubber (LSR) is a two-component material, typically consisting of a liquid silicone base and a catalyst. The two components are carefully measured using dosing pumps and mixed inside a static or dynamic mixer.
Injection: The mixed LSR material is injected into a heated mold cavity via the screw. This screw is different from a normal injection molding screw, as it is not designed to heat the material through shearing. The screw is also cooled to prevent premature polymerization. Once the required amount of LSR is ready in the screw, it is injected into a heated mold.
Curing: LSR is a thermosetting polymer that solidifies via a chemical reaction. As such, the reaction must be allowed to finish before the part is ejected from the mold. In some cases, heated molds are used to help accelerate the curing process.
How Long Does the LSR Injection Molding Process Take?
The exact amount of time required to complete an LSR injection mold cycle depends on several factors, including such as: the type of LSR used, mold temperature, and the complexity of the part. However, in general, LSR is slower than standard thermoplastic injection molding. This is due to the need for the silicone to cure in the mold. This curing can, however, be accelerated using temperature-controlled, i.e. molds are heated to reduce cure time by helping add energy to the reaction. Some LSR formulations can take as little as 20 seconds to set while others can take as long as 418 seconds, but it is important to note that these cure times are heavily dependent on mold temperature and LSR grade.
How Does Injection Molding of LSR Differ From Other Molding Process Types?
Injection molding of LSR requires a dosing system to measure out the correct quantities of each component of the two-component polymer, which are then mixed in a static or dynamic mixer. The equipment used is similar to normal injection molding but different enough that the machines are not interchangeable. Other rubber molding processes, such as compression molding, work by pressing rubber or silicone into the part shape using a hydraulic press.
Is LSR Injection Molding Better Than Injection Molding?
No, LSR injection molding and normal injection molding are designed for different use cases. If a thermally resistant and flexible part is required then LSR is better suited, but if a rigid and low-cost part is preferred, then normal injection molding is preferred.
What Kinds of Molds Are Used for Liquid Silicone Molding?
LSR injection molds are similar to standard injection molds, and can be manufactured from tool steel or aluminum. LSR molds do not experience excessively high pressures. Therefore, they do not need to be as robust as normal injection molds. LSR molds typically have cold runners instead of hot runners (as is the case with normal injection molding) to keep the material from curing before reaching the mold cavity. The actual LSR molds are heated to help accelerate the curing process.
What Are the Materials Used for Injection Molding of LSR?
Optical-Grade Silicone: Optical-grade silicone maintains its transparency both in the visible and UV (ultraviolet) spectrums with excellent optical clarity. LSR does not discolor as some plastics do when exposed to UV light.
Fluorosilicone: Fluorosilicone is a high-performance grade of silicone that has improved thermal resistance, chemical resistance, and mechanical properties. It must, however, be noted that fluorosilicone is significantly more expensive than standard LSR.
Medical-Grade Silicone: Standard platinum-cured LSR can be considered medical grade, since it does not release any harmful chemicals once fully cured. Medical components are often baked after injection molding to ensure that the polymer is fully cured.
Standard Silicone: Standard LSR can be mixed with multiple additives and pigments to enhance its properties. In most cases, platinum-cured silicone is used for standard injection molding.
What Are the Applications of LSR Injection Molding?
LSR has a wide range of applications across many different industries, as described below:
Medical Devices: LSR is widely used in the medical field due to its excellent biocompatibility. Typical applications include prosthetics, stents, and catheters.
Automotive Industry: Automotive applications of LSR make use of its excellent thermal and chemical resistance. Typical applications can include seals and gaskets inside the engine bay.
Consumer Goods: LSR is a tough and robust material, making it ideal for consumer goods—for example, toys and baby bottles.
Electronics: LSR is an electrical insulator. Typical electrical applications can include cable connector sheaths and keypads.
Aerospace: LSR can withstand both high and low temperatures, making it ideal for aerospace applications such as seals and vibration dampers.
Industrial Equipment: LSR’s flexibility and thermal resistance make it ideal for flexible membranes in industrial sensors such as pressure gauges.
What Is the Quality of Injection-Molded Silicone Products?
As with any injection molding process, final part quality depends on numerous factors, such as mold design, mold finish, and process control. If properly designed and implemented, LSR products can have excellent quality, both in terms of aesthetics as well as part performance. LSR is often used to produce parts with fine features due to its low viscosity during injection.
How Long Do Silicone Injection-Molded Products Last?
Silicone injection-molded parts have excellent longevity with some parts lasting up to 20 years. However, this depends entirely on the application and the environment in which the product is used.
What Are the Advantages of LSR Injection Molding?
Listed below are some common advantages of using LSR for molding:
High-Temperature Resistance: Silicone is often used in applications that require good thermal resistance. Service temperatures over 200 °C are possible.
Chemical Resistance: LSR is resistant to a wide range of chemicals, such as alkalis, inorganic acids, alcohol compounds, and long-term water exposure.
Biocompatibility: LSR parts can be used in medical applications due to their excellent biocompatibility.
Flexibility and Elasticity: One of the primary benefits of LSR is its excellent flexibility and elasticity. For that reason, it is often used for shock absorbers.
Precision and Tight Tolerances: LSR has a low viscosity when injection molded. For that reason, it can be used to produce parts with small features and thin side walls.
Longevity: If properly cured, LSR parts can last as long as 20 years, depending on application and environment.
Hygienic and Easy to Clean: LSR is chemically resistant and inert. For that reason, it does not give off any chemicals. In addition, its high thermal resistance means it can be cleaned at high temperatures.
What Are the Disadvantages of LSR Injection Molding?
Listed below are some typical disadvantages of molding with LSR:
High Initial Costs: LSR is an expensive material and requires purpose-built injection molding machines. Regular injection molding machines cannot be used for LSR molding.
Curing Time: Liquid silicone rubber has a longer cycle time than normal injection molding due to the need for the part to cure in the mold before it can be ejected.
Mold Maintenance: LSR molds require regular maintenance to ensure parts are produced with the required dimensional accuracy. This is especially important for small, complex parts.
Waste Material: Once cured, liquid silicone rubber cannot be recycled like normal thermoplastics. Any wasted material must be discarded, resulting in increased cost per part.
Is Injection Molding of Liquid Silicone Rubber Expensive?
Yes, parts molded from liquid silicone rubber are generally more expensive than typical thermoplastic injection-molded components. This is due to the higher initial raw material cost. However, this may be offset by the improved performance of LSR.
Is LSR Injection Molding Worth the Cost?
Yes. If soft, robust, and thermally resistant parts are required, then LSR is worth the additional cost.
XTJ is a leading OEM Manufacturer that is dedicated to providing one-stop manufacturing solutions  from prototype to production. We are proud to be an ISO 9001 certified system quality management company and we are determined to create value in every customer relationship. We do that through collaboration, innovation, process improvements, and exceptional workmanship.
