Silicone Overmolding – The Ultimate Guide

How do manufacturers produce the glossy & beautiful keys and electronic devices? Well, they do it through overmolding. Silicone overmolding is a process that injects a thin silicone layer over a substrate to make it smooth and provide a seamless finish.

It is critical for boosting durability, enhancing aesthetics, and improving the product function. PBT, PA + GF, PPSU, and PC are the typical secondary overmolding materials with high-temperature resistance and thermoplastic features. Silicone overmolds can deliver excellent results than bonding & assembly. Read on to learn more.  

Silicone Overmolding Overview

Liquid silicone rubber overmolding is a distinct process for producing durable and pliable parts in high volume. The method utilizes the LSR (liquid silicone rubber), a platinum-cured silicone with high purity, high & low-temperature resistance, good stability, and low compression set.

It may require special treatment due to its thermosetting features through processes like intensive distributive mixing before placing it into the vulcanized and heated cavity. In general, silicone rubber is a thermoset elastomer that can alter vinyl or methyl side groups, oxygen, and silicon atoms.

One unique feature about these rubbers is that they retain their mechanical components over wide temperature ranges. Meanwhile, silicone rubbers combine with methyl groups, making them hydrophobic and suitable for electrical surface solutions.

Silicone rubbers suit applications that need high precision, like multi-pin connectors, electric connectors, sealing membranes, and seals. It can also apply to smooth surfaces of products like spatulas, baking pans, medical apparatus, and bottle nipples.

Liquid Silicone Overmolding Process

You can buy liquid silicone rubbers in barrels. In the meantime, they feature low viscosity, making it possible to pump them through tubes and pipelines to the vulcanization stage. Hence, you can meter pump the components through a static mixer. 

The typical catalyst for the manufacturing process is platinum-based. At this stage, the manufacturers can add additives like the coloring paste before it flows into the static mixer process. Next, the components hover towards the cool metering part of the injection molding. 

The static mixer produces a homogeneous material that remains consistent from part to part. The process could deliver different results, unlike the solid silicone rubber that you can buy, partially vulcanized and pre-mixed. You can also process hard silicone rubbers through transfer molding, which leads to high surface variability. 

Liquid silicone rubbers require perfect mold cavity seals since they have a low viscosity index that results in a burr-free finish. Considering natural shrinkage and steel dilation at the LSR injection design stage is critical since it involves high temperatures. 

After the metering process, the injection moves through the cooled runner and sprue systems. Next, it moves into the heated cavity for the vulcanization process. The cooling section is vital since it has no waste products, reducing material costs by eliminating the trimming stage. Here is a complete guide to how the process takes place:

  • Material Formula Test Lab

The first step would be best to test the material via a reliable lab testing formula or using professional equipment to measure its precise DFM+CPK. Critical tests at this stage include rubber processing analyzer tests, elemental analysis, phenyl & vinyl content, and polymer material evaluations. This step enables the process to give a pure end product. 

  • Mold Workshop

You have over 1400 mold stocks to compare. Thus, it is possible to craft custom molds based on your preferences. But ensuring that the design suits your personalized silicone application is critical. Manufacturers design the 3D CAD files depending on the applications’ suitability using advanced EDM machines and CNC tooling.  


  • Injection Molding

You can choose from up to ten sold silicone injection lines. The process uses high heat and pressure to mold the silicone rubber by pressing it. The strategy has flexible features to suit design complexity, consistency, and extreme precision demands. Nonetheless, this tech fits high-volume production since the process costs more. 

  • Cleaning & Packaging

The ideal packaging & cleaning option will vary based on factors like the freight type, type of application, and price range. Cleaning is a critical step for rendering the final silicone rubber products in the best conditions. 

Overall, manufacturers supply the liquid silicone rubbers in containers ranging from 55-gallon drums to small tubes. Pumping the end product requires high pressures, varying from 500 to 5000 psi based on the material durometer. 

You can retard the curing procedure using a chiller to supply the jacketed fittings with cold water. Here, you can add a color pigment through a color injector at the material pump before the static mixer process. 

Silicone Overmolding Kit Equipment

You require several mechanical components and equipment to complete the liquid injection procedure successfully. For instance, the molding machine works with an injection unit, a static or dynamic mixer, and a metered pumping device. 

It is possible to integrate different processes for efficiency and proper precision. Here are the critical tools for the injection molding process:

  • Mold Clamp: The clamp is vital for securing the mold through the injection molding procedure and also opens the mold when the process is complete.
  • Nozzle: You need a nozzle to deposit the mold mixture. It has an automatic valve that prevents mold overflowing and leaking.
  • Mixers: A dynamic or static mixer helps to combine the material once they leave the metering unit. Hence, the mixers use high pressure to transport the compound into a designated mold.
  • Supply Drums: The plungers or supply drums are the primary container for material mixing. The pigment container and the supply drums are connected to a pumping system.
  • Metering Units: You will require a metering unit to pump the base-forming silicone and the catalyst that forms the liquid material. But it is crucial to maintain a constant ratio when you pump it concurrently.
  • Injectors: The injector helps to pressurize the liquid material, supporting the injection process by pumping it. The operator can adjust the injection rate and pressure level at any time.

Silicone Overmold Bonding Methods

Attaining a permanent bond in silicone overmolding is crucial. Therefore, you must invest in the best silicone bonding methods. Here are the typical bonding approaches:

  • Adhesive

The silicone surface appears non-compatible with the substrate surface, but it is critical to keep everything uniform. Therefore, you might require conduit application to attain a permanent bond between the surfaces. The most popular adhesive for the process is a compatible primer. 

The silicone primer may feature a reactive silane group in a solvent base carrier. Once you apply it to the substrate and expose it to moisture, it activates the silane groups. Hence, it boosts the substrate and silicone’s adhesion. 

Primers are typical in medical applications since well-established companies offer LSR primers that can work well for medical functions. Although you can access the primers easily on the market, they feature numerous drawbacks. You must devote extensive engineering resources and time to evaluate the primers for reliable products. 

LSR bonding using primers is a reputable process for high-production scale silicone overmolding. The primers serve as a balance between the optimum cosmetic look, moldability, and adequate bonding. 

  • Prepping the Substrate

Careful prepping can help maximize the primers’ effectiveness on ceramic, metal, or plastic surfaces. Besides, you should condition the surface in a way that results in active bonding sites for the primer to adhere to the surface. 


You could use different techniques to ensure a uniform adhesion on the bonding surface. A highly wet substrate can result in more uniformity. Other effective means to treat the surface include cleaning it with corona treatments, plasma, flame, or solvent and rubbing it. 

Roughening it or applying textured material will also promote your results. But ensuring balanced extensive experimentation is critical for better results from processes like plasma treatment. You must define the material type and understand the plasma process for excellent results. 

  • Applying Primers

Applying primers after prepping is also crucial. You can do it in several ways. Spraying, dipping, and brushing can all work, but each method has its drawbacks. For instance, the primers’ uniformity may limit your approach if wicking or pooling occurs. Spraying would deliver excellent results, but you lack control over priming unwanted parts. 

It may also demand consistent maintenance of the atomizing components. In any case, typical spray patterns and cleaning can help you boost the outcome. On the other hand, brushing and dipping also pose several challenges to the application process. Without any automated techniques, these options require intensive labor and time. 

Most silicone overmolding primers require humidity for activation, which makes moisture crucial for the procedure. Thus, you must isolate the primed substrate into a humid environment, requiring a capable production technology. You can use controls to accurately time the overmolding process before you remove it from the moist setting. 

Overmolding Design Considerations

Since the overmolding process is complex, you should plan before you begin designing your products. Do you have the molds ready before you start? You can research upfront how to load the substrate onto the machine and unload the final overmolding product. Here are other critical factors to consider:

  • Material Selection

Although overmolding suits an array of materials, the material you use impacts the results. The substrate may feature a plastic blend or a hard plastic to attain favorable properties. 

Typical materials for the overmolding process include nylons, Polyethylene terephthalate (PETG), polystyrene, Acrylonitrile Butadiene Styrene (ABS), and Polycarbonate. 

Meanwhile, overmolding works well for TPEs (Thermoplastic Elastomers) since they have desirable physical features like easy bonding, resilience, flexibility, impact & wear resistance, and high-temperature ranges. 

  • Bumboffs & Thickness

Maintaining a uniform thickness of about 0.060 to 0.120 inches would be suitable. Smoothing out the surface is critical when you want different thicknesses to avoid abrupt bumboffs. Yet, it is possible to attain a smooth surface due to the high elasticity in overmolding materials. 

  • Edges & Shrinkage

You should account for shrinkage to avoid wrapping and ensure proper substrate fixation to the cavity. Thus, you must keep the surface layer thinner than the substrate. Avoid slowing the subsequent cooling and flow during the injection process since thin edges can tap due to the adverse effects on the overmolding & substrate adhesion. 

  • Material Bonding

Bonding between the two materials is critical for excellent results. Mechanical and chemical properties impact the bonding process. You can use the geometry product’s mechanical properties to prevent overmolding cracks. 

It is possible to bond the overmolding materials with natural shape geometries that enhance deformation & bonding. The typical method for overmolding bonding relies on chemical properties involving the materials’ molecular interaction. 

Therefore, the materials require chemical compatibility. You can also apply an adhesive to activate the bonding process. 

Types of Silicone Overmolding Materials

  • Liquid Silicone Rubber (LSR)

LSR involves brushing the substrate’s primer before the LSR overmolding process. Brushing helps to boost adhesion. The process requires a valve-type cold runner and a needle to deliver excellent results. 


Hence, the mold cavity may attain a vacuum for sealing with glue, which makes the cleanup process easier. But since the product does not discharge the mold cavity gas, it can result in a poor appearance since the process will likely have glue overflow. 

  • High Consistency Rubber (HCR)

HCR is a heat-curable, ready-to-use silicone rubber blend that suits applications like pigments, vulcanizing agents, modifiers, filters, and bases. They offer various fabrication options like calendaring, compression molding, extrusion, and injection molding. 

Meanwhile, they have shortened production processes, excellent electrical insulating & mechanical features, and optimized capabilities. 

  • Fluorosilicate Rubber (FSR)

FSR is a highly fluorinated and silicone-based elastomer that features excellent resistance to wear & tear due to harsh chemicals, solvents, oils, and fuel. 

They have various engineering features, superior extreme-temperature performance, and easy fabrication through calendaring, compression molding, extrusion, and injection molding. 

  • Room Temperature Vulcanizing (RTV)

RTV silicone is a silicone rubber that cures easily at room temperature as a one-substrate product or a mixed combination of a curative and a base. It is available in various hardness levels since you can use a catalyst like tin compounds or platinum. RVT’s typical applications include lenses, reproducing molds, and automotive adhesives.  

  • Electrically Conductive Silicone

Conductive silicone describes a carbon-silicone product with a low amp conductivity suitable for protecting and preventing static charge build-up. Its products and material also suit functions like eliminating radio frequency and electromagnetic interference. 

  • Thermally Conductive Silicone

Thermal conductive silicone materials can help dissipate heat into the heat sink. They protect objects against harsh thermal shocks, vibrations, and the environment. 

Overall, it has great attributes such as suitable hardness & viscosity levels, high-temperature resistance, and up to 3 W/Mk thermal conductivity properties. It is common in LEDs, automotive, and electronics

  • Medical Grade Silicone

Medical grade silicone is critical in manufacturing implantable devices for medical purposes thanks to its purity. The material is typical in various products like skin & food products, water pipes, scuba mouthpieces, baby bottle nipples, and menstrual caps. 

  • Self-Bonding Silicone

Self-bonding silicone produces silicone tapes that are handy for repairing parts with watertight 0r airtight seal requirements. You do not need an adhesive to apply since it quickly binds. It has numerous favorable features like solvents & chemical resistance, heat & cold resistance, versatility, elasticity, flexibility, and water resistance. 

The self-fusing tape works by building a permanent bond to its seal. It has a thin silicone gel layer, and cured rubber compounds that enable it to fuse quickly. Typical applications include aerospace wiring, boat rigging, irrigation systems, swimming pools, air pipe seals, pipe joints, and steam cleaners. 

Applications of Silicone Overmolding

Manufacturers utilize inter molding and overmolding for various products across various industries. Here are the common applications where these processes are vital: 

  • Beauty Industry

Insert molding and overmolding are critical in routine cosmetics packaging since they have an attractive finish. They allow product package designers to use custom packaging designs with multiple colors & unique surface textures. Products like perfume bottles, makeup brushes, and compacts sometimes require overmolding. 

  • Electrical Industry


Inserting a molding LSR exterior contributes to waterproof features in electrical wires. They coat it with rubber to make the wiring components safe for applications like computer & cell phone chargers. They can also help differentiate the current flow using contrasting wire colors. 

Since plastics have better sterilizing features, overmolding can deliver reliable results for the medical industry. Besides, medical cables and defibrillators have plastic cases to ensure easy cleaning and safeguard fragile interior parts. 

Insert molding is critical in medical devices like pacemakers that doctors implant into the body. Medical tools manufacturers use overmolding in numerous cameras that suit internal imaging. A good example is cameras for colonoscopies and endoscopies tasks. 

Automotive parts like fasteners, gears, and electrical sensors combine rubber or plastic material with metals through overmolding. It is also crucial in creating two-tone interior pieces for various controls, knobs, handles, and dash & door panels. Overmolding is also useful in some batteries and motors. 

  • Consumer Goods

Many consumer products in homes have several layers & colors of plastic surfaces. Well, overmolding is a critical step in manufacturing these products. The process is also vital in manufacturing plastic containers, storage bins, gaming pieces, and bowling pins. 

Insert molding and overmolding are important in producing thumb drives, cell phone cases, and toothbrushes. It is typical in two-layer plastic furniture like patio chairs and step stools. Other devices with overmolding include plastic navigational and GPS units. 

Benefits of Silicone Overmolding

A considerable benefit of LSR overmolding is the production process can use non-plastic substrates like steel. Unlike LSRs & thermoplastics, it is the only method to incorporate materials into the injection molding procedure. 

Plastic overmolding is an affordable and flexible procedure for large-scale applications and industries. Also, liquid silicone overmolding produces complex electronic assemblies and simple LSR handles. Here are the benefits of silicone overmolding:

  • Quality Control & Design Flexibility

The LSR overmolding process & accuracy controls give excellent precision to securely-bonded, high-quality, fragile parts. Overmolding with silicone rubber creates airtight and waterproof components that resist vibrations and shocks. 

  • Cost and Time Saving

You can bypass secondary molding in the assembly process by directly overmolding silicone onto a plastic or metal substrate. Thus, you save time and money that you would spend throughout the assembly process. 

  • Non-Plastic Substrates

Apart from exclusive benefits like insulating, sealing, and rigidity & flexibility, the overmolding process allows you to incorporate materials during the injection overmolding procedure. 

  • Increased Durability

Overmolding also boosts the products’ durability since the products have resistance against deformation, warping, and discoloration. It is resistant to fading, chipping, and cracking. 

  • Enhanced Aesthetics

Besides improving the products’ functions, overmolding boosts aesthetics, making it suitable for a seamless and smooth finish. It is ideal for boosting product texture and adding to its color. 


  • Improved Product Function

Several functions can record better results thanks to overmolding. For instance, it can protect products against noise & vibration, provide electrical components insulation, improve the knob & handle grip, and create a gasket or seal on components. 


LSR products are durable and suit various applications, including medical functions. However, you should use high-quality products from trusted vendors for good results. Are you looking to buy products with custom silicone solutions? If yes, look no further!

LEGENDAY is the best platform for purchasing high-quality LSR products, including beauty products, medical devices, and electronics. Moreover, you will get expert-level guidance from the company’s team to ensure you attain your objectives. Are you ready to trade with the best of the best? Contact us Now!

Frequently Asked Questions

  • What is the difference between Overmolding & Shot Molding?

Although 2-shot molding and overmolding combine substrates to create one multi-substrate product, they differ. In a 2-shot molding, you will mold the two materials through the same machine & process with secure, precise placement and superior bonding due to cavity-to-cavity alignment.

On the other hand, LSR overmolding involves molding an LSR substrate over a plastic part or metal mass to form a multi-substrate product. Therefore, the LSR overmolding process allows various substrate combinations.

  • What is the Difference Between Insert Molding and Multi-shot Overmolding?

In multi-shot overmolding, the substrate would be the first material, and the mold would contribute the rest of the content. The process uses pressure from several barrels to squeeze the two resins into one mold in the same injection tool. 

The entire process occurs in one machine, which makes multi-shot molding ideal for large-scale production. Meanwhile, insert molding places the substrates like plastic or metal into a mold and oversells the whole substrate or partial parts. 

Heating the inserted part is critical for better adhesive strength before coating. Although you need a cavity mold or a substrate for insert molding, it is ideal for small-scale overmolding.

Update cookies preferences
Scroll to Top