Silicone Overmolding

Silicone Overmolding

Silicone overmolding is a widely used silicone injection molding process in various fields. It is a specific method of silicone injection molding. Common toothbrushes and device handles are all made of silicone overmolding.The advantages, mechanism, operation process and application of silicone overmolding are described in detail. If you want to customize silicone products, this article can help you better understand this process.

What is silicone overmolding？

Silicone overmolding is a multi-material and multi-step injection molding process. It is also known as in-mold assembly, which mainly involves directly injecting a second layer of material onto a pre-formed substrate. Liquid silicone rubber(LSR) or high temperature vulcanized silicone rubber(HTV) is coated on the substrate through secondary injection molding, ultimately forming a single but composite functional component.

Silicone combines with the substrate in a molten state, achieving a long-lasting and seamless connection between the two materials through chemical bonding, microscopic mechanical interlocking and other methods. This manufacturing process supports different toughness and softness levels and the combination of multiple materials. It can produce many components that combine durability, corrosion resistance and strength. And it is widely used in many fields.

Advantages of Silicone Overmolding

Tactile Feel Improvement

The most obvious advantage of silicone overmolding is the extremely different tactile experience it brings to products, especially for some consumer goods.

Shore hardness is usually used to measure the hardness of non-metallic materials.Silicone has a wide range of Shore hardness, which can offer both a gel-like soft touch and an elastic yet tough feel. Especially on some tool handles or wearable devices, the properties of silicone can relieve the fatigue and stress caused by long-term contact. This in turn improves the comfort of users’ usage.

Additionally, the touch of silicone is closer to the temperature of human skin, thus giving people a warm and smooth texture. The bio-compatibility of silicone makes it more friendly and non-irritating when in contact with the skin. Moreover, silicone can still hold firmly when exposed to moisture or oil. This kind of friction can improve the convenience and safety of product use.

Aesthetic Value

Silicone overmolding can achieve seamless and integrated appearance design. This injection molding process eliminates the gaps that appear in the traditional production of components. It is precisely this process that significantly improves the appearance and texture of the product.

Moreover, silicone can be flexibly color-adjusted, which can add a splash of color to products. What’s more, the surface of silicone can be treated in various ways, such as frosted, textured or customized textures. Different surface treatments can enrich the visual layers and recognition of the product.

Durability

Silicone is not only used for decoration but also as a giver of product performance. The excellent elastic recovery ability of silicone can effectively alleviate collisions and impacts. Therefore, it can be used to protect precision components from collision and extend the service life of the product.

Moreover, silicone can form an excellent sealing layer, achieving a high level of dust and water resistance. The heat resistance, UV resistance and ozone resistance of silicone can protect the substrate from aging, guaranteeing the stability of the product.

For electrical components, silicone, as an excellent insulator, can provide reliable insulation protection for electrical elements and improve electrical safety.

Manufacturing Efficiency

Silicone overmolding reduces the production steps of parts assembly and bonding. It can directly integrate the production process into one or two injection molding steps. This greatly reduces the complexity of parts production assembly, production management and labor cost.

Canceling the adhesion of parts also means saving curing time, inconsistent bonding strength and other problems. This makes the entire production process more controllable. One-piece construction also significantly reduces the defect rate of the entire batch and can achieve the integration of complex functions.

Bonding Mechanisms between Silicone and Substrates

The combination of silicone and the substrate mainly occurs through the following two ways. The specific method needs to be determined based on your material combination and production process.

Chemical Bonding

For applications requiring high sealing performance and dynamic bending, chemical bonding is the key to achieving a high-strength, seamless and durable combination.

It mainly relies on adhesives or direct reactions to establish covalent bonds between silicone and substrates. This is actually a connection at the molecular level.

Thermal Bonding

Usually, the use of specific LSR does not require adhesives. It can achieve direct thermal bonding with specific plastics under precise temperature and pressure. This kind of chemical reaction that occurs at the micro level has strict requirements for the process.

Mechanical Interlocking

In addition to the above two bonding methods, there is also the most fundamental and reliable connection mechanism, mechanical interlocking. This method is suitable for material combinations that are difficult to bond directly chemically.

Mechanical interlocking refers to the design of some specific microstructures in the covering area of the substrate, such as some holes, grooves, etc. Then, the molten silicone needs to be injected into the mold under high pressure and fill these structures. After curing, the silicone can form a physical lock with the substrate.

Material Compatibility Considerations

Chemical Compatibility: The substrate and silicone must be compatible and not repel each other or undergo harmful reactions. You need to verify it based on the recommendations of the material suppliers and experiments.

Surface Energy: Only substrates with high surface energy can better fuse with molten silicone.

Thermal Expansion Coefficient: The coefficient of thermal expansion of the substrate and the silicone should be as close as possible. Excessive differences may cause the final product to crack or delaminate.

Silicone Overmolding Classification

Silicone overmolding typically uses two forms of silicone: LSR and HTV. The choice of which type of silicone to use can directly determine which equipment, mold and process route to select.

LSR Overmolding

As a two components low-viscosity liquid, LSR needs to be cured by heating to undergo a platinum-catalyzed addition reaction.

This overmolding method features higher precision, a shorter molding cycle and a higher degree of automation. It is suitable for making complex and thin-walled fine components. This material has extremely high requirements for purity and is highly suitable for the medical and food industries. Common products mainly include watch straps, precision medical devices and baby products, etc.

HTV Overmolding

HTV is a high-viscosity base polymer. It needs to be vulcanized to form an elastomer. The specific type of vulcanization also needs to be determined based on your budget and application scenarios.

This kind of overmolding can first inject the substrate, and then the compound softens, flows and vulcanizes in the mold to form. It can also be injection-molded with the substrates first and then overmold with silicone in another mold.

The physical performance range of HTV is wide and its cost is relatively low. It is more suitable for making thick-walled and large-volume silicone components. If you choose this material, the production cycle is relatively long and the degree of automation is low. The dimensional accuracy of the products produced is not as good as that of LSR. Common HTV products include industrial equipment buttons and large-sized handles, etc.

Silicone Overmolding Process

Silicone overmolding is a complex production process with multiple intricate steps. The design needs to take into account all the details in the production process. This article takes two-shot

injection molding as an example to elaborate on the entire process of silicone overmolding in detail.

Preliminary Design and Material Selection

Key Considerations for Design

First of all, engineers need to conduct a design for manufacturing(DFM) analysis based on your product’s 3D data. Specifically analyze the design of the bonding area between silicone and the substrate. This requires checking whether the structure of the mechanical interlock of your product is reasonable. And whether the wall thickness of the silicone area is uniform and determines the boundary position between the substrate and the silicone.

Additionally, the different shrinkage rates of the two materials also need to be taken into account. And it is necessary to guarantee dimensional accuracy after the combination of the two materials. Finally, the manufacturer also needs to determine whether to adopt two-shot injection molding or multi-material injection molding based on your production volume, cost and precision requirements.

Mold Structure Design

For two-shot injection molding, manufacturers usually need to design two sets of molds. One set is used to produce the substrates, and the other set is used to overmold the silicone. Molds overmolded with silicone must be designed with hundreds of precise insert positions and fixing systems.Besides, if you choose different silicone materials, the design of the runner and exhaust system of the mold will also be designed accordingly.

Mold Materials and Surface Treatment

Molds are usually made of steel with high hardness and high wear resistance. The interior of the mold may need to be polished or coated with a non-stick coating to better improve the demolding property of the silicone.

Silicone Overmolding Primary Process

Substrate Forming and Pretreatment

The substrate components are first produced on an injection molding machine using conventional materials. Make sure that the size of the substrate is stable, without deformation or warping during production. The formed substrate still needs to be cooled for a period of time to release the internal stress. Before bonding, the substrate also needs to be cleaned and surface activation by plasma treatment.Finally, through accurate spray or brush coating a layer of uniform primer can enter the next production link.

Overmolding

The preprocessed substrate is precisely embedded in the specified cavity of the silicone injection mold.Guarantee that the substrate does not move under high temperature injection to prevent uneven coating or glue overflow.

The mold is locked under high pressure, and the silicone is injected into the cavity at high speed and high pressure. Under the pressure, silicone fills the cavity and adheres closely to the surface and microstructure of the substrate.

Bonding and Vulcanization

At this stage, the heat from the high-temperature mold and the silicone itself is rapidly transferred to the surface of the substrate. Under the influence of heat and pressure, silicone has a chemical reaction with the primer on the surface of the substrate. At the same time, the silicone also begins to vulcanize, gradually changing from a liquid or gel state to an elastomer.

Overall, this process is a stage where physical and chemical reactions occur simultaneously. The temperature, pressure and time of the mold all have a direct impact on the vulcanization rate and bonding reaction.

Demolding

After vulcanization, open the mold and remove the formed mold from it. The speed of taking out should be steady to prevent the product from deforming or tearing.

Post-Processing

Secondary Vulcanization

If your customized products have high application requirements , the product may still need to conduct secondary vulcanization. Secondary vulcanization can eliminate residual volatile components, stabilize material properties, and strengthen the temperature resistance and mechanical strength of the product.

Trim and Clean

Products made with HTV usually have thin flash. It also needs to remove flash through cold trimming, mechanical trimming or manual trimming. The product can be assembled and packed after undergoing quality inspection and cleaning. Assembled and packed to your specifications, the product is finally delivered to you.

Application

Consumer Electronics

The use of silicone overmolding in electronic consumer goods can elevate the user experience and endow the products with a high-end texture.

Take smart phones and wearable devices as examples, such as the side buttons of smart phones, the connection points of smartwatches’ straps, sensor modules, and the head-mounted parts of headphones. These designs not only do not irritate human skin, but also provide users with a better touch. Moreover, these parts can extend the service life of the product.

In personal items, toothbrush handles, hair dryer handles are also common silicone overmolded products. The good touch of silicone provides users with a more comfortable usage experience. And it is not easy to slip when in contact with water.

Moreover, its application in electrical appliances can effectively insulate heat and prevent scalding. The aesthetically pleasing and easy-to-clean silicone perfectly satisfies the usage requirements of the product.

Medical Devices

Silicone overmolding can strictly meet the biocompatibility and precision sealing requirements of medical devices.

For instance, in some diagnostic and testing devices, silicone can protect the internal components from surface erosion and also reduce vibration. Silicone medical materials can also be repeatedly disinfected and cleaned.

Some injection adjustment buttons, keys or pipe interfaces can prevent accidental touch and provide good sealing performance. For the valves and handles of surgical instruments, silicone can insulate electricity and provide a good grip.

Automotive Components

The packaging and sealing of various sensors in automobiles, as well as the sealing parts of gearboxes, have extremely high requirements for high-temperature resistance and oil resistance. Sensitive electronic components also need to be waterproof and shockproof to guarantee the stable transmission of electronic signals. Silicone overmolding can meet these requirements.

The various silicone buttons inside the car can provide a clear button feel and improve the interior quality. Moreover, silicone is UV-resistant and wear-resistant. Even after long-term use, it will not turn yellow, become sticky or crack.

Industry Equipment

In the industrial field, silicone overmolding mainly produces some handheld professional tools, connectors or sealing and encapsulation of some cables. Besides, there are also some buttons or protective cases on the control panel.

Silicone can reduce fatigue and injury caused by prolonged holding. And as a coating for equipment,it is also dust-proof, water-proof, corrosion-resistant and impact-resistant.