Silicone over molding has a broad scope in boosting the functionality and aesthetic of a product. You can use it to either add shapes, textures, colors, or enhance the performance of the product.
Keep reading this blog to understand what is LSR over molding and its applications in everyday life.
Difference Between Liquid Silicone Rubber Over molding Vs Two-shot Molding
Both LSR over molding and two-shot injection molding processes fuse two substrates in the procedure to create a single multi-substrate part.
What Is Silicone Overmolding?
Silicone over molding is a process that entails molding liquid silicone rubber over a placed plastic or metal substrate to produce a multi-substrate part.
The process employs chemical or mechanical bond to permanently fuse two parts together, and offers a broad variety of substrate combinations.
You can combine either injection or compression molding with LSR over molding to create intricate silicone parts.
What is 2 Shot Silicone Injection Molding?
The process is also known as double-shot molding. It describes a silicone injection molding technique that facilitates manufacturing of multi-colored/material parts without incorporating additional assembly steps.
Two-shot silicone molding process involves injecting the first material into a mold cavity to form a substrate. Upon solidifying and cooling, the substrate gets transferred to a second mold chamber.
Subsequently, you unlock the mold and spin the section holding the substrate over 180 degrees. This attaches it with the other mold compartment and injection nozzle.
After position the substrate in place, you inject the second material, which fuses with the substrate to produce a strong bond. Once the second layer has cooled, the injection molding machine ejects the final silicone part.
Advantages Of Silicone Over molding
Overmolding silicone onto plastic or metal substrates is an economical and versatile operation that finds applications in many industries. There are many advantages of over molding that makes it a perfect option for your silicone molding projects:
Wide Range Of Applications
Silicone over molding process facilitates combining of various materials with each other. This allows production of many silicone products that is challenging with standard silicone rubber.
Cost And Time Effective
Overmolding silicone onto substrates will help in saving the cost and time needed to manufacture a complex part manually. Being a more complex process, the initial tooling costs may be moderately greater compared to sum of 2 molded and assembled parts.
Nonetheless, over molding absorbs any extra investment through elimination of auxiliary costs. This is in addition to producing more durable and higher quality product.
High Flexibility In Silicone Overmolding Design
This type of silicone molding process allows the manufacturing of LSR products with sealing, anti-compression, waterproofing and other unique features.
The liquid silicone over molding process provides high tolerance and stringent quality control. This makes it ideal for any application in which precision is necessary.
We can perform custom silicone over molding to your requirements, including parts that require production in class VII/VIII clean room.
Substrates for Silicone Rubber Overmolding
The process of over molding silicone enables you to implement more ideas and options. It makes it possible to match several similar or differing substrates with one another.
Integrating a substrate component helps in changing the hardness and appearance of your silicone part as needed. Thereby, silicone over molding gives rise to higher product design value. It provides effects like insulation, shock resistance, and waterproofing for substrate components.
You can categorize liquid silicone over molding based on the different placement positions and materials of substrate parts. The commonly used prefabricated substrates in LSR over molding process include:
- Non-toxic thermoplastic materials like Polycarbonate, PET, PBT, PPS, and
- Metallic materials
However, from a practical perspective, the hallmark silicone over molding substrate is metal. This is because metals have the necessary features of high temperature resistance and usually possesses higher hardness gains.
For unique material fusion, it might be necessary to discharge, plasma, and fire treat the substrate. Moreover, you need to test the adhesion between the silicone rubber and base material. This enables optimal modifications to manufacturing parameters in time.
Four Common Silicone Overmolding Techniques
There are 4 main types of silicone over molding methods that you can employ for your liquid silicone rubber molding applications:
Here, you need an injection molding machine comprising of multiple barrels and tool rotating arrangements.
In double-shot injection molding process, the first shot injects the substrate material and the next is the silicone overmold. The whole operation utilizes one LSR injection molding equipment in a continual process.
- The first step involves injecting the molten substrate material into the mold cavity and allowing it to cool. After cooling, it is possible to separate the cavities and core.
- Next, rotate the first injection mold to align with the second barrel.
- Inject the liquid silicone rubber into the mold holding the substrate.
- Finally, close the mold and allow the materials to cure.
You can automate multi-shot silicone molding using robotics for accurate parts placement and quick turnaround times.
Robotics enables automatic placing of parts from one silicone injection molding machine to another. In turn, this will minimize assembly and labor costs.
Generally, multi-shot silicone molding is ideal for larger volumes because the molds and injection molding equipment are more expensive.
This process is also referred to as single-shot molding. Insert molding operation entails positioning either a plastic/metal insert in a mold cavity. It then undergoes partial or whole over molding with silicone material.
The 3 main steps in insert molding of liquid silicone rubber include:
- Position the substrate in the mold cavity.
- Inject the molten silicone rubber material into the mold containing the substrate.
- Shut the mold and allow the material to cool down, after which you eject the formed silicone over molded part from the mold.
To attain optimal bond strength, you might need to preheat the insert. This helps in bringing its surface temperature closer to silicone melting temperature.
For lower volume silicone over molding applications, insert molding is more economical compared to two-shot molding. Nevertheless, insert molding needs a separate mold tooling for the substrate material and for preparing the LSR overmold material.
Silicone Co-Injection Molding
This is a comparatively more sophisticated and costlier silicone over molding process than the first two. Silicone co-injection molding entails injecting two or more liquid silicone rubber materials with different properties in a single mold.
Because the materials are in molten state, you can achieve a higher bonding strength between them during the operation.
- Manual Silicone Overmolding
It is also practical to do the silicone over molding process manually. Typically, manual over molding is preferable for small batches not exceeding 10,000 units. This type of over molding silicone is not cost-optimized for large scale manufacturing.
Three Important Parameters To Consider During Designing For Silicone Overmolding
There are 3 essential silicone over molding design considerations that you need to pay attention to before starting the process:
- Bonding: A firm bond between the 2 materials is crucial to the silicone over molding operation.
- Materials: The silicone over mold material and substrate should be thermally, chemically, and physically compatible.
- Molding Principles: The moldability principles apply equivalently to silicone over molding, with a little extra considerations to bear in mind.
How To Achieve Desired Adhesion In Silicone Overmolding Process
Attaining the required adhesion between the LSR material and substrate is among the key concerns during silicone over molding. You can perform this in various ways, including through self-adhesive silicones, primers, or mechanical interlocking.
Always keep the substrates/inserts clean and avoid using any chemicals that could interfere silicone cure chemistry. Also, preheat metal and plastic inserts because it aids in attaining a stronger bond. Do not preheat any materials that possess low melt temperatures.
Let’s look at the 3 main ways of achieving required adhesion in over molding silicone process:
Add some undercutting features that will interlock and hold the liquid silicone rubber in position. This is possible when there is the freedom to modify the substrate/insert design.
The other best alternative is to utilize a primer for treating the substrate surface. Allow the primer to dry for about 30 minutes to 2 hours after application. After that, you can then do the silicone over molding and expect a better bond.
The common surface treatments techniques for silicone over molding are corona, plasma and particulate blasting. Corona discharge is the best method for plastic substrates.
Surface treatment modifies the surface chemistry and increases surface area to boost fusion between the two silicone over molding materials.
There are a number of self-adhesive LSR materials that bond to thermoplastic without employing any surface treatments or primers.
These silicone elastomers fuse well with:
- Nylons (PA)
- PolyEther Ether Ketone (PEEK)
- PolyPhenylene Sulfide (PPS)
- PolyEthylene Terepthalate (PET)
- PolyButylene Terepthalate (PBT)
- Polycarbonate (PC).
Applications Of Liquid Silicone Over molding
Silicone over molding is essential in the manufacturing of products for an expansive range of industries. Here are some of the uses of silicone over molding:
Silicone over molding is key in the manufacturing of most household plastic products. This popular silicone parts manufacturing process helps in producing cell phone cases, thumb drives, and toothbrushes among other consumer goods.
Moreover, you can apply silicone over molding to manufacture several multi-color plastic furniture like patio chairs or step stools. The process is also integral in manufacturing plastic housings for navigational and GPS devices.
Fasteners, electrical sensors, gears, and other automotive components comprising of plastic/metal and rubber material often go through over molding process. The technique is also key in creating multi-colored interior components like knobs, handles, dash panels, doors, and various controls.
The medical sector depends highly on silicone over molded parts since they are easier to sterilize than computer and electrical components. Defibrillators, medical wiring and cables are commonly encased through silicone over molding. This protects the sensitive internal components and facilitate easier cleaning of the finished gadget.
Silicone over molding process is also essential in the manufacturing of implantable pacemakers and camera equipment for internal imaging.
Manufacturers make electrical wires safer and, in some instances, waterproof by over molding silicone exterior. Often, wiring components of computer and cellphone chargers have LSR coating to make them safer.
Cosmetic industry employs silicone over molding technique to make product packages more attractive. It allows designers to produce custom packaging having multiple colors and unique surface textures.
Moreover, manufacturing of products like perfume bottles, makeup brushes, and compacts can be done through over molding silicone.
We offer end-to-end solutions in silicone over molding. Our team of designers and engineers can assist you create top-quality LSR products. Contact us today to better find out how we can serve you in your next silicone over molding project.
Injection Molding – Source: Science Direct
Liquid Silicone Rubber Injection – Source: LegenDay