Silicone Overmolding- The Ultimate FAQs Guide

Silicone Overmolding- The Ultimate FAQs Guide

Silicone molding is a process used by many industries, from healthcare, automotive, beauty industry, electronics industry to more. Below are all your questions answered so you can better choose what to go for in your needs:

  1. What Is Silicone Overmolding?
  2. Why Should You Overmold Silicone?
  3. What Are The Benefits Of Silicone Overmolding?
  4. What Are The Disadvantages Of Silicone Overmolding?
  5. What Is The Difference Between Overmolding And True 2-shot Molding?
  6. Why Is LSR The Preferred Choice For Overmolding?
  7. What About Substrates?
  8. What Temperature Is Required For Overmolding?
  9. What Are The Applications Of LSR Overmolding?
  10. Overmolding Vs Insert Molding
  11. What Products Can Be Made Using Silicone Overmolding?
  12. What Are The Different Methods Of Silicone Overmolding?
  13. What Are The Steps Involved In Silicone Overmolding Onto Metal/Glass?
  14. How To Ensure Good Overmolding Bonds?
  15. How To Test The Bond Strength Of A Silicone Overmolded Part?

What Is Silicone Overmolding?

There is another injection molding process called silicone overmolding where silicone is molded over, under, or through another material such as plastic, or metal to make the part complete. Since it allows designers to join two materials, choices in durometer and rigidity are supported in this manufacturing process.

For instance, you can mold a soft silicone rubber over a handle that is made of plastic. The plastic will strengthen and give structure to the part while silicone will provide a soft surface.

It is also called an in-mold assembly and supports ergonomics, choices or color, and brand impression when it comes to industries such as baby care, and medical.

Why Should You Overmold Silicone?

The main reason silicone is overmolded is so the quality and performance between components can be improved like assembly and sealing quality. However, the reasons are different depending on each application but include a low assembly price and have a device function like encasing electronics. Silicone also acts as a protective layer against dust, water, heat, impact, and electric shocks. When it comes to implantable devices, it creates a kind of bio-friendly gap between the patient and the medical device.



Insert molding involves placing the substrate into the mold and overmolding the whole or silicone parts of the substrate. To have maximum strength, the injection mold layer may have to preheat the substrate so its surface temperature is closer to the melting temperature of the overmold. Insert molding is cost effective if you have lower units of parts compared to double-shot molding. But, it requires a tool for the substrate and a different core and cavity to create the volume for the overmolded part.


What Are The Benefits Of Silicone Overmolding?

LSR injection molding is worth your money and a flexible process that manufacturers use for various industries and applications. From LSR over-molded handles to complicated electronics assembly, there are various reasons why overmolding is a good choice for your company:

  1. Cost-Effective And Fast: When you mold silicone directly on a metal or plastic substrate, you save money and the time needed to assemble a complicated component by hand. This method will sharply decrease the total manufacturing costs and turnaround time. This results in your product reaching the market in an economic way.
  2. Flexible Design: LSR injection molding is a suitable choice if you have complicated design needs as seen in the automotive and medical industries. Silicone overmolding creates parts that are water-resistant, airproof as well as shock and vibration resistant.
  3. Quality Control: LSR injection overmolding process provides a high tolerance level and rigid quality control resulting in the process being suitable for applications where precision is a must.

What Are The Disadvantages Of Silicone Overmolding?

Below are some of the disadvantages of silicone overmolding

  1. The overmolding process moves the first molded substrate part on the second over-mold which gives it a low tolerance.
  2. The machine setting cost of over molding is very high. The overall prototyping, designing, testing, and tooling need plenty of capital as well as expertise.
  3. Overmolding is the same as injection molding so whatever way you design should work with injection molding. But, silicone allows for adaptability in that area.
  4. You can not conduct secondary practices on the finished parts of overmolding. Any activity or adjustment will stop when the molding material becomes cold.
  5. In cases where production is low, overmolding costs become expensive. It is because a worker has to manually put the substrate into the overmold, increasing the cycle time as well as product cost.

silicone overmolding medicine cutter

What Is The Difference Between Overmolding And True 2-shot Molding?

Both, overmolding and the 2-shot molding process join two substrates during the molding process to create a singular multi-substrate component. In liquid silicone rubber overmolding, the LSR substrate is molded on a metal or plastic part to produce the multi-substrate component. However, in true 2-shot molding, both of the materials are molded in the same process, and machine through cavity-to-cavity alignment for strengthening bonding, a safe precision place.

silicone overmolding SIM card holder

Why Is LSR The Preferred Choice For Overmolding?

LSR is the chosen raw material for a number of overmolding injection molding applications. That is because LSR is durable, tear resistant, and offers high fatigue and insulation. It also has environmental sealing properties if you are to use it for easily breakable parts.

Being hypoallergenic and biocompatible, it is the favored material of choice for the production of medical devices, electronics, and automotive parts, as well as a number of consumer goods.

What About The Risks Involving Substrates?

Substrate carries a huge risk. It is that all substrates mold release oils, or chemicals that discourage bonding like sulfur. Even if there are only trace amounts of these substances, it could create more problems or failures in an overmolding project.

These substrate tolerances have a huge impact on overmolding and should be looked at. When it comes to mold pressure, silicone flashes at 0.0001in (0.0025mm) or less. Since silicone is deformable, tolerances are overstretched which in turn increases the price and challenge of creating it. If the substrate is loose, it will flash while if it is tight, it will crash. This causes damage to the part, tool, and in some cases, both. During designing, take into account how the substrate interacts with the mold.

What Temperature Is Required For Overmolding?

Substrate parts need to be hot because the LSR bonding to the other part occurs due to chemical reaction, it requires that pressure, time, and temperature work together. Typical mold temperatures for LSR are between 300°F to 400°F. The less the temperature, the less time of the cure cycle.

However, when it comes to substrates that are molded in a two-shot molds, temperature plays less of a part. This is because the heat that is left in the substrate from the first shot helps in curing the LSR.

What Are The Applications Of LSR Overmolding?

Below are some applications of where the LSR overmolding is used:

  • Consumer Goods: Toothbrushes, cell phone cases, and thumb drives are created through overmolding. Overmolding is also useful for making two-tone plastic furniture like step stools or patio chairs. Plastic housing for GPS units and navigational devices are also created through overmolding.
  • Automotive Industry: Electrical sensors, gears, fasteners, and other automotive parts that are made up of metal and plastic or rubber material are also created through overmolding.
  • Medical Industry: This industry is very dependent on overmolding because silicone is easier to sterilize then any other material. Defibrillators, medical cables, and wiring are usually encased with plastic using overmolding. This allows it to be cleaned easily while making it easier to use.
  • Electrical Industry: With the help of overmolding, electric wires are made safer as well as waterproof. Companies also coat wires in silicone to make products such as cell phone and computer chargers safer for customers.
  • Beauty Industry: Overmolding is used on a frequent basis to make cosmetics more attractive. compacts, makeup brushes, and perfume bottles are all manufactured using overmolding.

silicone overmolding electronics part

Overmolding Vs Insert Molding

You should go for overmolding when:

  • Your complete part is made of thermoplastics or rubber.
  • Your completed part has multiple layers, materials listed as above and multiple colors.
  • You will create both the second layer as well as the substrate.
  • There will be no need to take apart or disassemble your finished product.

However, you should go for insert molding if:

  • You are going for a prefabricated substrate.
  • It is made of metal, computerized parts, or wires.
  • If the completed part has to be one solid piece.

Overmolding is preferred if you have a large production run but it has a high upfront cost. It is also very time consuming and the overall price to manufacture is high. So, you would require a high production unit to divert these costs.

What Is Silicone Over Molding Used For?

Silicone 0ver-molding is used for molding plastics, silicone, rubber metals, electronics, and medical silicones, etc.

Furthermore, silicone overmolding creates a bio-friendly barrier between the medical device and the patient for implantables.

What Products Can Be Made Using Silicone Overmolding?

The use of silicone overmolding can be seen in the production of many different products, including baby brushes, spatulas, shockproof auto mounts, screwdrivers, etc. Let’s discuss each product in detail:

  • Baby Brushes:

There are certain materials like hair, synthetic fibres, wire etc which are used to make a brush and they are attached to wooden or plastic body.

Some brushes have been designed to function with power equipment for heavy-duty tasks like removing paint or polishing metals.

The baby brushes which are made using silicone molding have their head made up of silicone molding to give them a soft touch.

  • Spatula

Because of its inherent adaptability, the silicone spatula has evolved from a baking tool to a multipurpose kitchen tool.

Whether you’re baking or trying to cook anything, a silicone spatula scrapes the edges of a bowl, pan, or other vessel. And this process is done more cleanly and quickly.

Furthermore, silicone can safely resist high heat without melting, allowing it to be substituted for other traditional kitchen equipment in the kitchen.

That is why, despite its origins as a baking utensil, it has evolved into a versatile spatula for both cooking and baking.

silicone over molded spatula

  • Silicone Floor Drain Check Valve:

Environmentally friendly, made of high-quality silicone that is non-toxic and tasteless.

Your best option for resolving the drainage issue is to use a backwater valve floor drain.

The floor drain backflow preventer can drain and filter while also preventing material from returning, preventing odor and backflow.

Silicone Floor Drain Check Valve


  • Custom Protective Silicone Rubber Sleeve:

A simple type of coupling is the rubber bushing sleeve. This consists of a tube with a hole diameter machined to the shaft’s needed size.

Rubber sleeves and bumpers are available in various forms, dimensions, and materials to suit a variety of uses.

On the basis of its performance, rubber bushing sleeves are classed as impermeable, shockproof, insulated, flame – resistant, generally pro cushion, lubricant barrier, dampening, safety, and exit preservation.

Custom Protective Silicone Rubber Sleeve

  • Hearing Aids Silicone Accessory:

Hearing Aid Silicone Accessories, Hearing Aid Replacement Accessories, Silicone Sleeves Cover, Silicone Loops, Custom BTE Jacket, BTE Clip.

They are used to keep perspiration off of the hearing aid behind the ear.

Moreover, they keep your BTE hearing aid from falling out.

Silicone is used as a structural provides basic support to the body of the product.

There are four parts: one clip, one rope, and two silicone BTE jackets.

Hearing Aids Silicone Accessory


  • Baby Silicone Pacifier:

BPA, PVC, and phthalate-free pacifiers are available. The orthodontic silicone nipple promotes normal mouth muscle development.

Safe and durable pacifier with no odor that maintains its shape during the time.


  • Silicone molding Massage Seat Cushion:

They are made up of high-quality silicone which is also recyclable. The silicone provides a major structure to the cushion.

Moreover, it has a double-sided design which has more than 1000 3D high elastic pillars.

What Are The Different Methods Of Silicone Overmolding?

There are 4 methods of Silicone Overmolding, which are stated as follows:

  • Compression Molding :

When the substrate is a metal or a plastic that can withstand high temperatures, this method is commonly used.

Subsequently, It is economical but after repeated shots, the separating lines will also increase simultaneously.


  • Transfer Molding:    

This method is mainly used when the metal substrate is so small that if the material is vulcanized unevenly, it will easily flow.

Alongside this, there is one drawback of this method that a lot of waste material is produced in this method.

  • Plastic Injection Over Molding:

This method is majorly used for the over-molding of big thick products.

Furthermore, the injection mold can precisely manage the parison’s wall thickness distribution, resulting in a rather uniform wall thickness.

  • Laser Over Molding:

This method is widely used for the molding of silicone rubber parts or any plastic substrate part etc.

Besides its wide application, this method has one drawback,which is it costs too much.

What Are The Steps Involved In Silicone Overmolding Onto Metal/ Glass?

The silicone overmolding process involves two steps which are as follows. Here they are.

Step 1:

In step 1, A LSR mold is used to transfer a finished plastic or metal entity into the mold location.

Step 2:

The LSR is directly over-molded on the component. This will create a layer on the surface.

The ability to employ a non-plastic substrate, such as steel, is one of the advantages of liquid silicone over-molded parts and components.

silicone overmolding over a metal


How To Ensure Good Overmolded Bonds?

Overmolded bonding can be ensured in several ways. As follows

  • Clean and dry surface:

Keep the substrate’s surface clean and dry. Some impurities can even prevent the silicone from curing.

Pollutants like lubricants or molds discharge factors, on the other hand, can prevent the silicone from chemically and mechanically sticking to the metal or plastic basis.

  • Designing Of Features Into Substrate:

This design of features is of mechanical bonding. This is especially true if the overmolding material is Liquid Silicone Rubber (LSR)

In addition, Silicone can flow into the substrate and mechanically lock into it, all this is done due to the loops.

  • Using primers or plasma to treat the surface:

Silicone and base are occasionally incompatible when it comes to chemical reactions.

Primers for medical devices should be used with caution, as they can affect the end device’s biocompatibility.

As a result, plasma surface treatment is preferable because it does not introduce any new substances.

How To Test The Bond Strength Of A Silicone Overmolded Part?

There are several methods to confirm the bond strength of a silicone over-molded part.  They are stated as follows:

  • Make Sample Plaques With Your Substrate:

Molds are produced before the final material is chosen. And most importantly this should be done in the early time.

An OEM, on the other hand, might have to experiment with a few alternative silicone compositions or base surface coating before finding one that meets not just the ductility standards, but also all of the other design specifications

  • Putting The Sample Plaques To The Test:

The best method to assess device prototypes is to subject them to the same destructive testing that the actual product undergoes.

Furthermore, the length of an union between two materials is used to decide the quantity and type of stress required to cause that link to break.

  • Device Prototypes:

Make device prototypes and put them through the same destructive testing as the real thing.

Furthermore, this can be done on early prototypes as well as production-ready items.

  • Incorporation Of Bond Strength Testing:

This can be accomplished in a variety of ways, ranging from quick and easy hand stripping or peeling tests to considerably more precise machine testing.

Furthermore, because strength properties evaluation is harmful, it should only be performed on a sample of pieces in either scenario.

What Are The Tooling Considerations That Designers Need To Make For Silicone Overmolding?

When producing silicone overmolding, there are a few things to keep in mind.

It is not always the price that is to be taken into account. They are stated as follows:

  • Transition:

There is always a need for a sharp transition for silicon molding between the substrate and the overmold.

This is because it can lose its shape or other issues can occur later on.

  • Thinning:

The over-mold is required to avoid thinning because this results in delamination which means several layers can be formed.

  • Minimizing Of Flow Length:

When it comes to gate sizing, the flow length to wall thickness ratio is crucial for adhesion.

Moreover, gates are placed in the thickest wall regions to reduce flow length.

Because air entrapment can induce flash, the mold requires vents that are deep enough to prevent this.

  • Texture:

Another significant design consideration is texture, which improves the feel of the item while also reducing surface defects.

Apart from the textured surface frequently feels softer to the touch than its durometer indicates.

Textured surfaces also release parts more easily than smooth surfaces.

Injection-molded parts can have a wide range of textures, and designers can even include graphics or logos for branding or product identification.

Silicone overmolding process


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