Liquid Silicone Rubber Overmolding

Liquid Silicone Rubber is a two-part synthetic elastomer consisting a rubber base of liquid silicone and a curing agent or catalyst. Combining these two components induces a reaction, crosslinking the material to a solid-rubber form.

Liquid Sillicone Rubber Overmolding
Liquid Sillicone Rubber Overmolding

What is Liquid Silicone Rubber Overmolding?

LSR overmolding is a technique that utilizes LSR in encapsulating or covering a thermoplastic or metal finished part or substrate. Injection molding is used to deposit LSR over the substrate in the overmolding process, resulting in a multi-material product.

The silicone overmolding process can be summarized in two steps:

  • Placement: The molded part made of plastic or metal or other material is positioned within the mold cavity prior to LSR injection.
  • Injection and Formation: The mold is closed and LSR material injected, filling the mold cavity. This effectively overmolds the substrate resulting in an integrated component.

Why Choose LSR Over molding Process

The overmolding silicone process affords you several benefits in your application. The following reasons are why you should choose this process:

  1. Fast Cycle Time

Using LSR overmolding for part production involves less turnover time compared to say two-shot molding. Upon completion of the molds, the overmolding process is straightforward and can facilitate large production volumes.

  1. Design Flexibility

Silicone overmolding can accommodate various design options and complex shapes. It allows for the combination of different materials, colors, finishes and textures making innovative product designs.

  • Excellent Bonding and Adhesion

The bonding quality of LSR is exceptional, allowing strong and durable bond with various substrate materials. Consequently, the resulting adhesion is reliable eliminating the need for extra adhesives or fasteners.

  • Protective Sealing

LSR overmolding can result in effective seal creation that prevents entry of moisture or dust alongside other contaminants. The resulting protective barrier enhances product longevity and durability.

  • Chemical Resistance

LSR has outstanding resistance to oils, chemicals and other solvents ensuring protection of overmolded components. As such, these components can be utilized in harsh environments involving interaction with various chemicals without degradation.

  • Ergonomic

LSR overmolding can enhance user comfort by providing non-slip grips and surfaces with cushioning and soft touch. This is thanks to LSR’s flexibility and elasticity ensuring production of user-friendly products.

  • Cost-Effective

Overmolding silicone can streamline part production process by doing away with supplementary assembly steps. This cuts on the overall manufacturing costs and time required with separate part production and assembly.

  • Electrical Insulation

Overmolding LSR results in products with excellent electrical insulation offering protection against electrical currents. This is essentially useful where electrical conductivity is not desired.

  • Heat and Cold Resistance

LSR maintains its characteristics across a wide temperature range capable of withstanding extreme heat and cold. As such, LSR overmolded parts can tolerate temperature fluctuations without significant changes in appearance.

  • High-Quality Finish

LSR overmolding offers products a smooth and high-quality surface finish enhancing the value and aesthetics.

  • Vibration Protection

Overmolding substrate materials with LSR offers components shock absorption and protection from vibration. It eliminates air gaps reducing the tendency of parts to move and thus resulting in associated damage.

Properties of LSR Material Best for Overmolding

The overmolding process can be undertaken with different materials to form finished components of desired properties. However liquid silicone rubber finds popular use thanks to the following qualities:

i. Chemical Tolerance

The ability of LSR to withstand chemical environments without damage is incredible even at extreme temperatures. This makes its use for various applications subject to different conditions popular.

ii. Electrical Resistance

Corona discharge is an issue many manufacturers face when selecting materials for overmolding. LSR displays high electrical resistance finding use especially in making electrical parts and applications with high-voltage values.

iii. Highly Durable

LSR has a relatively long shelf life compared to other materials like thermoplastic elastomers. LSR can remain useful even after a year of storage without a drop in material performance.

iv. Low Compression Set

Liquid silicone rubber portrays low compression set making it capable of resisting strain-induced permanent deformation.  With a set value between fifteen and twenty percent, LSR can remain elastic under sustained compression even at different temperatures.

v. Incredible Thermal Stability

LSR can function at temperature extremities reaching 180 oC without exhibiting creeping, melting. They also maintain flexibility at low temperatures reaching -50 oC without hardening and cracking.

vi. Resistance to Aging

Aging can be caused by exposure to UV radiation, weathering and sustained use over time. LSR offers outstanding resistance to the effects of the above mentioned factors offering long term use.

Factors to Consider Before Silicone Overmolding

It is essential to consider different factors prior to undertaking LSR overmolding for execution of a successful process. This way, you can make informed decisions and optimize the LSR overmolding process to a higher degree of success.

These factors include:

  • Cost Considerations

Material costs, tooling expenses, and production efficiency all attribute to the general cost of silicone rubber Overmolding process. These costs should be evaluated to ensure they aligns with the projected requirements and budget needs.

  • Design Compatibility

The substrate part design and intended overmold should be compatible for effective bonding and integration. In determining feasibility, evaluate factors like part geometry, draft angles, undercuts, and surface finish.

  • Material Compatibility Testing

Especially when handling large production volumes, testing material compatibility by molding sample parts is necessary. This way, you can correctly ascertain the bonding characteristics and process success of the overmolded parts.

  • Material Selection

It is important to assess the compatibility of the LSR with the substrate material. Bonding and long term performance is influenced by factors such as thermal expansion coefficients, adhesion properties, and chemical resistance.

  • Process Optimization

When conducting LSR overmolding, certain process parameters like injection pressures, curing times, temperatures, and cooling rates are essential. Conducting process optimization is necessary to achieve consistent and high-quality parts.

  • Quality Control and Inspection

Implementing adequate measures for quality control ensures the overmolded parts are highly reliable and consistent. Undertaking visual inspection, checking dimensional measurements, and procuring adhesion tests are useful in assessing final product quality.

  • Surface Preparation

Preparing the substrate surface for sufficient bonding with the LSR is ideal. This can be done by cleaning the surface, priming, or applying treatments to enhance bonding and adhesion.

  • Tooling Design

The mold design and tooling is critical requiring proper considerations for mold cavities, gate placement, venting, part ejection and cooling channels. The design of the mold should fully accommodate the component and allow effective flow and bonding of LSR.

Design Considerations for LSR Overmolding

The design process for silicone overmolding is essential to the success or failure of your project. There are three main design considerations you need to evaluate prior to undertaking LSR overmolding.

  • Material Consideration

LSR combines well with certain substrate materials such as plastics and some metals with its softness influencing resistance to bending. Furthermore, where grip and non-slip resistance is desired, the coefficient of friction is equally important.

  • Overmolding Technique

The primary techniques used in LSR overmolding are: two-shot molding and pick-and-place molding. The former utilizes a single mold while the latter technique utilizes a mold pair. Two-shot molding is also used for high volume productions and pick-and-place effective for low volume production.

  • Bonding

Bonding in LSR overmolding can occur in one of two ways: chemically or mechanically. Chemical bonding, while stronger than mechanical bonding, is harder to attain occurring at molecular level.

It is influenced by factors such as wetting degree, surface treatments and presence of additives all influencing bond strength. Mechanical bonding manifests at the materials interface and attained by grooving or making cuts and holes for LSR flow.

Types of LSR Overmolding Processes

Overmolding when using liquid silicone rubber can be conducted via different processes depending on factors like substrate material and volume of production.

The common types of LSR overmolding processes are:

Two-Shot Overmolding

In two-shot overmolding, a part is created in one molding machine before insertion into another mold where LSR is injected. It requires use of two injection molding machines simultaneously with the hard part first.

The two-shot overmolding method provides a reliable mechanical bond, resulting in increased strength and stability for the products.

Two Shot Overmolding
Two Shot Overmolding

2K Overmolding

Also known as the two-color overmolding process, it involves two steps. First, the material injection and then, a switch to a matching mold cavity, with the component as an insert.

The inserts are thereafter injected with LSR creating finished products incorporating two different materials. Using a twin-cavity injection machine can achieve the same result simultaneously.

An advantage of 2K overmolding is the first material injection remains hot when entering the second cavity, enabling effective bonding. However, care must be taken to prevent surface ablation caused by the high-temperature LSR melt and moisture absorption.

Injection Overmolding

Commonly executed as dual-injection molding and attracts the most in cost and complexity. Unlike other techniques, this method does not feature injection of an overmold over a substrate.

Instead, both the substrate and the LSR overmold materials are simultaneously injected. This results in optimal adherence thanks to both materials being in liquid state.

Injection Overmolding
Injection overmolding

Insert Molding

This method is preferred for its simplicity and cost-effectiveness involving division of the plastic components into tools before insertion of LSR. The flexibility of the materials facilitates adhesion with proper temperature control crucial for effective bonding.

Insert Molding
Insert Molding

Liquid Silicone Rubber Overmolding Process

The process of LSR overmolding can be summarized in the following steps:

  • Material Preparation

This involves preparation of the substrate to facilitate proper adhesion with LSR by undertaking processes like cleaning, priming or surface treatment. LSR is prepared by mixing a liquid silicone rubber base with a curing agent or catalyst.

  • Making the Mold

Typically fashioned from steel or aluminum, the mold is designed to accommodate both the substrate and LSR material. It features channels allowing proper flow of LSR for enhanced bonding with the substrate.

  • Injection Molding

The substrate part is positioned in the mold before injection of the liquid silicone rubber into the cavity. The LSR material fills the mold flowing around the substrate component, initiating a bond between the materials.

  • Curing and Solidification

Heating the mold initiates LSR curing by activating the catalyst inducing crosslinking and solidification of LSR. By carefully varying the heat and time, you undertake the curing process at elevated temperatures.

  • Demolding

The demolding process succeeds the cooling process upon solidification and curing of LSR. In demolding, you remove the overmolded part from the mold.

LSR Overmolding Process
LSR Overmolding Process

Overmolding LSR onto Thermoplastics

Overmolding LSR onto thermoplastics affords you several benefits such as improved thermal and chemical resistance as well as high flexibility. However, the challenge is in achieving good adhesion between the thermoplastic substrate and LSR.

For effective results, the following guidelines are essential when overmolding LSR over thermoplastics:

  • Clean the substrate surface ensuring it is dry before LSR overmolding especially when not using two-shot molding.
  • Carry out a preliminary test to determine the efficiency of the bond between the thermoplastic substrate and LSR.
  • Undertake the LSR overmolding process over hot thermoplastic substrate parts for enhanced adhesion as it is a chemical process. The necessary parameters are pressure, time and temperature typically between 120 and 200 o
  • When carrying out the overmolding process, implement some form of mechanical bonding between LSR and thermoplastic substrate. This ensures the bond achieved between the two materials is much stronger than the self-bonding chemical technique.
  • When using additives, avoid those with amines or sulfur as they constrain the curing process of LSR resulting in weak adhesion.
  • Carefully demold the integrated part of LSR and thermoplastic to prevent deformation just in case curing is incomplete. Lining your mold with PTFE is a useful design improvement to aid in demolding.

Bonding Techniques in LSR Overmolding

When bonding liquid silicone rubber over a substrate material, there are two techniques you can implement. Often, these methods are used together to achieve a stronger bond.

Physical Approach

Involves utilizing techniques such as surface tapping, buckle design and surface rolling with a second material in directly molding a part. This method results in strong adhesion internally with external adhesion limited.

Chemical Method

Involves utilizing chemical means to combine LSR with the substrate material based on molecular affinities resulting in a singular part. This method establishes a more reliable and flexible connection between the materials.

Low-pressure and High-pressure LSR Overmolding Compare

Overmolding silicone finds use in various instances offering protection and comfort depending the application. Both low and high pressure silicone overmolding implement similar strategies but differ in the execution.

High-Pressure LSR Overmolding

This particular overmolding process is executed under high pressure and temperature reaching up to 25,000 psi and 300 oC. Consequently, the LSR injection process is faster appropriate for high volume productions with enhanced efficiency and durability.

Utilizing high-pressure overmolding can however damage delicate substrate materials and tear up components. Additionally, the chances of ending up with flaws is higher stemming from stresses caused by the faster rate of filling.

Low-Pressure LSR Overmolding

The temperatures as well as pressure during LSR injection in this case are lower reaching 180 oC and 100 psi respectively. This technique finds ideal use for delicate parts whose exposure to high pressure can be damaging.

This process allows LSR to completely fill the mold reaching hardly accessible areas enabling greater uniformity and part precision. The process is however slow and only suitable for low volume productions.

Liquid Silicone Rubber Overmolding Applications

The unique properties of liquid silicone rubber overmolding allow their use in various industries. Some common applications include:

High Performance Parts

Silicone overmolding is utilized in high performance industries such as aerospace and defense to make parts like grips, seals and gaskets. The thermal stability of LSR at extreme temperatures and resistance to chemicals and UV radiation also contribute to its use here.

Automotive Components

The excellent durability, sealing properties and chemical resistance of liquid silicone rubber overmolded parts allows for their use in automotive components. Here, they protect these components from automotive fluids, moisture and dust.

Consumer Products

Silicone overmolding is utilized in the manufacturing of consumer products ranging from electronic devices, hygiene and beauty products, to baby products. Keypads, connectors, toothbrushes, pacifiers and toys utilize overmolding silicone in furnishing safe and ergonomic products with enhanced resistance qualities.

Industrial Tools and Equipment

Tooling components, protective covers, handles and grips for tools utilize LSR overmolding. This improves features like grip and ergonomics while providing protection against harsh environments.

Electrical Products

Lighting fixtures and other electrical components such as cables are fashioned from o overmolding silicone rubber. This manufacturing process offers these products thermal resistance, insulation and moisture protection enhancing reliability and longevity.

Medical Use

In the medical sector, LSR overmolding is used in the creation of medical devices like implants and wearable products. Since LSR is biocompatible, its meets the safety threshold for devices in contact with the human anatomy.


As you can see liquid silicone rubber Overmolding plays an integral role in modern rubber molding process. At LegenDay, are your best partner in silicone rubber parts in China.


More resources:

What Is Medical Grade Liquid Silicone Rubber? – Source: LEGENDAY

Step-by-step Liquid Silicone Rubber Injection Molding Process – Source: LEGENDAY

Top 10 Liquid Silicone Rubber Manufacturers in the World – Source: LEGENDAY

Why only Hollow LSR parts Float on Water – Source: LEGENDAY

Step-by-step on How to Dye Liquid Silicone – Source: LEGENDAY

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