Liquid Silicone Rubber Micro Molding

Stockwell Elastomerics uses liquid silicone rubber micro molding to produce high volumes of very small and extremely precise parts, sometimes called micro molded parts. Liquid silicone rubber (LSR), a group of two component elastomers, supports the use of additives such as colorants and uses platinum curing instead of peroxide curing so that organic acids that can cause contamination aren’t generated.

Liquid silicone rubber micro molding can produce parts that weigh less than a gram and are smaller than a few millimeters in diameter or size. It has some important differences compared to liquid silicone injection molding, but both processes leverage the advantages of LSRs and are used in  applications that include medical and healthcare, aerospace and defense, and electronics.

What is Liquid Silicone Rubber Micro Molding?

Liquid silicone rubber micro molding creates physically small products, but part size is just one of its characteristics. Typically, micro molded parts have dimensional tolerances that exceed those of traditional silicone rubber molding. Micro molded parts also have very thin walls and may incorporate tight tolerance features such as ribs, grooves, or micro holes.

Liquid silicone rubber micro molding begins when two liquid components are mixed, typically with additives. The LSR is heated and injected into a mold or tool that’s held in a press under high pressure. Pressure is maintained until the micro mold is filled. The LSR is then heat cured or vulcanized. After the micro molded parts cool and solidify, they’re ejected from the mold.

Stockwell Elastomerics uses a horizontal press for LSR micro molding because it ensures the more consistent filling of small, intricate cavities. This machine can also accommodate large molds with many small cavities and supports the use of automation for efficient high volume production. Since gravity won’t interfere with part ejection, there’s a reduced risk of damage to micro mold products.

LSR Micro Molding vs. Liquid Silicone Injection Molding

Both LSR micro molding and liquid silicone injection molding can produce high volumes of complex components. With micro molding, however, the level of precision is greater because the part features are smaller. Even when precision tooling is used, other injection molding methods cannot achieve the tiny, intricate features in LSR micro molding.

Liquid silicone rubber micro molding is also different because the press uses a special injection unit and the shot size is significantly smaller. Often, the maximum amount of material that’s injected into a micro mold cavity is less than a gram.

Stockwell Elastomerics provides both LSR micro molding and liquid silicone injection molding services. Typically, LIM is used with small-to-medium sized components and micro molding is used for very small parts. Small parts that don’t have miniature features or tight tolerances may be faster and more cost effective to produce with liquid silicone injection molding.

Advantages of Liquid Silicone Rubber for Micro Injection Molding

LSR micro molding combines the smaller sizes and higher precision of micro molding with the many benefits of liquid silicone rubber (LSR), a category of thermoset resins that combines superior heat stability and low temperature flexibility with resistance to aging and chemicals. LSRs also provide electrical insulation and have a long shelf life and low compression set.
Compared to other moldable materials, LSRs offer key advantages. For example, unlike most thermoplastic elastomers (TPEs), liquid silicone rubber (LSR) retains its properties up to 400°F (~204°C) and remains flexible and elastic at -60°F (~15°C). Compared to silicone high consistency rubber (HCR), liquid silicone rubber (LSR) cures faster for shorter cycle times.

Among its advantages, liquid silicone rubber (LSR) is available in USP Class VI grades for medical, and healthcare applications. Because LSR is free of sulfur and plasticizers, it’s suitable for use with electronic assemblies. Liquid silicone rubber is normally translucent, but it can be made in custom colors for either function or aesthetics.

Additives in LSR Micro Molding

Additives enhance the properties, performance, or processability of the LSR materials that are used in micro injection molding. There are many types of additives, and Stockwell Elastomeric selects products that are compatible with the base material and will not compromise either micro molding precision or part quality. These additives can be divided into the following categories.

• Reinforcements and fillers improve mechanical strength, stiffness, and dimensional stability. Some, such as metal particles, can impart electrical conductivity.
• Colorants or pigments can meet functional requirements such as part identification or aesthetic requirements such as support for a product’s color scheme.
• UV stabilizers extend the lifetime of LSR micro molded parts by preventing degradation from exposure to ultraviolet (UV) light from the sun or from UV lamps.
• Flame retardants improve fire resistance and are available in formulations that meet specific flame ratings such as UL 94V-0.
• Antistatic agents minimize the buildup of static electricity, which can cause sparks and ignite flammable materials.
• Antimicrobial agents enhance silicone’s antibacterial properties.
• Optical modifiers can enhance optical clarity, a measure of transparency, or optical diffusion, a measure of light scattering.

To enhance processability, the liquid silicone rubber (LSR) that’s used in micro molding can contain lubricants and slip agents for reduced resistance, or release agents that facilitate demolding by reducing adhesion between the mold and the part.

Applications for Liquid Silicone Rubber Micro Molding

Micro molding with liquid silicone rubber (LSR) is often associated with the medical and healthcare industries; however, its applications also include electronics, automotive, aerospace, defense, and emerging technologies. Stockwell Elastomerics can micro injection mold high volumes of parts for these and other industries here in Philadelphia, Pennsylvania (USA).

Medical and Healthcare

LSR micro mold products include high precision seals for medical devices, wearable components for medical monitors, and covers for small sensors in diagnostic equipment. USP Class VI silicones are biocompatible, but they may not be required for applications that do not involve direct patient contact. Micro molded LSRs are also in microfluidics applications such as biochemical assays.

Electronics

The electronics industry uses liquid silicone rubber micro molding to produce precise, flexible buttons for keypads and switches. Additional applications include micro connectors for miniaturized electronics and high performance enclosure seals for small components.

Automotive

Like electric vehicles (EVs), cars and trucks that use internal combustion engines have an increasing amount of electronic content. Liquid silicone rubber micro molding can create small gaskets for sensors that need to withstand high temperatures in advanced driver assistance (ADAS) systems.

Aerospace and Defense

Aerospace and defense applications for liquid silicone rubber micro molding include small, high precision seals for components and equipment that are used under a wide range of temperatures. In addition, radar and optical systems need micro molded LSR covers to protect sensitive components from their environment.

Emerging Technologies

Finally, liquid silicone rubber micro molding is used to produce components for fluid transport in lab-on-a-chip devices. Additional emerging technology applications include high precision components that help protect the flexible sensors in industrial robots, cobots, and Internet of Things (IoT) devices. LSR micro molded parts are also used to seal miniature actuators.

Terminology Note: Micro Molding or Micromolding?

Typically the two word version “micro molding” is used, though sometimes it is called “micromolding”.