Overmolding in the Plastics Industry

Have you ever wondered the type of techniques in overmolding or the type of materials that you can use for the overmolding process? This article is your definitive guide to acquiring in-depth insights upon overmolding techniques and processes.

Table of Contents

What Is Overmolding?

Overmolding is also referred to as ‘two-stage’ injection molding. Overmolding is a type of injection molding process in which two or more components are molded over one another. The overmolding technique primarily involves covering the substrate which is the first material either partially or completely by the subsequent materials.

Process Explained:

The uniqueness of the overmolding process is such that various materials can be seamlessly combined into a single product or part. 

Some of the most common examples of overmolding are power tools that often use textured materials to add an ergonomic grip. Another example includes manufacturing car tail-lights that encompass multiple color parts.  

The overmolding process generally includes a rigid, plastic-base overlaid with a thin exterior layer. This can be done using either a single-shot (insert molding) or two-shot (multiple-shot molding) technique.

overmolding before
overmolding finished part

The most important aspect of the overmolding process is to ensure that the melting temperature of the substrate is considerably higher than the second molding material. 

There are typically 4 types of combinations that we can implement to develop a product. These are listed below: 

  • Plastic Overmold – Plastic Substrate – In this case, a rigid plastic substrate is first molded after which another rigid plastic is molded around the substrate. Both the plastics that are used could differ in color and the type of material. This is particularly known as ‘overmolding’. 
  • Rubber Overmold – Plastic Substrate – In this case, the substrate is a rigid plastic, upon which, a soft rubber such as thermoplastic elastomer is molded. This gives an ergonomic grip to the rigid part.
  • Plastic Overmold – Metal Substrate – This type of molding is used when we want to include metal components in plastic parts. In this type of overmolding, first a substrate of metal material is inserted into an injection molding tool. The plastic is now molded around the metal substrate. This type of molding is commonly known as insert molding.
  • Rubber Overmold – Metal Substrate – Just like the rubber over plastic overmolding, this type of overmolding is used to get an ergonomic grip to the rigid metallic part. In this case, a substrate of metal material is inserted into an injection molding tool. The rubber plastic is now molded around the metal substrate. 

You must however, note that product developers and engineers are not always restricted to using only 2 materials in the overmolding process. If you are unsure which materials can be combined, don’t hesitate to reach out to us. Our engineers and design team can guide you to combine various materials that are compatible to develop a durable product.

Overmolding Techniques

  • Insert molding also known as single-shot molding primarily involves positioning an insert in the injection mold after which the second material (either the thermoplastic or elastomer) is then injection molded over the insert. 

The total cost of insert molding is low as compared to the other 2 techniques which is why it is widely known as a low cost overmolding technique. The Insert Molding process is a 3 step process: 

    1. The first step is to position the substrate inside the injection mold. 
    2. Once the substrate is positioned in the injection mold, the second material which is either the thermoplastic or elastomer is injected inside the injection mold. 
    3. The third step is to allow it to cool down, after which the part or the product is ejected from the injection mold.
overmolding guide
  • Multiple-shot also known as two-shot molding requires an injection molding machine consisting of tool rotating arrangements and multiple barrels. 
    1. The first step is to inject molten plastic from the first barrel into the injection mold which is allowed to cool. Here, the core and cavities separate upon cooling. 
    2. In the next step, the injection mold is rotated to be in position with the 2nd barrel. 
    3. The mold is now closed and the machine now injects the molten plastic from the second barrel. 
    4. The final step includes the cooling of the material in which the part ejects out of the injection molding machine.

      At VEM, overmolding techniques can also be automated with robotics to varying degrees for precise placement of parts and fast turnaround times. With robotics, molded parts can be automatically placed from one injection molding machine into another which reduces labor and assembly costs.
overmolding part
  • Plastic Co-Injection Molding is a relatively more complex and expensive process than the former 2. It involves injecting 2 or more plastics that have different material properties in 1 mold.

    A high bonding strength between the 2 materials can be achieved as during the co-injection process, both the materials are in a completely molten state. 

Manual overmolding process :
The overmolding process can also be performed manually by taking out part of mold 1 from the injection molding machine and placing it in mold 2. This is typically done for small batches of up to 10,000 pieces.
Manual overmolding is not suited for large scale productions as it is constantly manual. Thus, it is not cost optimized for mass production however, it is the best solution for smaller batches.

Overmolding versus Insert Molding

While the 2 terminologies are very interchangeably used, there are some differences. Overmolding combines multiple materials seamlessly into a single part or product whereas insert molding requires pre-placement of an insert which is typically metal, in the tool. After which, the injected plastic flows around encapsulating the insert. This creates a single molded plastic piece. 

Generally the final part that is made using insert molding is more durable than the one made by secondary assembly.

Overmolding versus Two-Shot Molding

Two-shot molding or 2K Injection Molding is a type of overmolding technique. They are essentially similar. We must note, however, that two-shot injection molding is often used for huge production runs as opposed to smaller production runs.

At VEM, we help you choose between the 2 types. Finding the most appropriate solution for your needs is always our main priority. We ensure to critically evaluate each step of the manufacturing process and your product requirements.

Material Combinations for Overmolding

One of the most important steps in overmolding and the product development process is the ‘material selection’ stage. It is imperative to choose the best-suited plastic resins for the application as this will ensure the combination of required material properties to guarantee a durable and optimally performing product.

overmolding plastic material
  • ABS abbreviated for acrylonitrile butadiene styrene is a commonly used plastic for components in electronic and automotive industries. ABS has high impact strength and a great resistance to chemicals. In addition, ABS has a low melting temperature and excellent tensile strength.

    It also has the ability to alloy with polymers such as high-density polyethylene (HDPE) and low-density polyethylene (LDPE). 

  • PEEK Resin, abbreviated for Polyether ether ketone, is known for its tremendous resistance to high temperatures, hydrolysis and chemicals.
    PEEK also has excellent resistance to fatigue which is why it is popularly used for the production of medical device appliances. Apart from medical appliances, it is also widely used in aerospace, automotive, military and electrical industry.
  • PMMA, abbreviated for polymethyl methacrylate acrylic, is naturally transparent. It is also known as ‘acrylic glass’ or ‘acrylic thermoplastics’. It has a high resistance to wear out and UV rays.
  • HDPE is a strong, lightweight plastic that is also impact and weather resistant. One of the most important properties is that It is highly malleable, thus, it is a popular choice for complex parts.

    HDPE is best suited for parts and products where durability is of the utmost importance such as corrosion-resistant pipes.

We encourage our customers to involve us early on in the process so that they can discuss material combinations. By getting involved in the early phases of the manufacturing cycle, we can help you with the best material selection for your products. 

Scope of Overmolding Applications

Overmolding has an expansive scope in improving the aesthetics and functionality of a product. It can be used to either add colors, textures, shapes, or improve upon the functionality of the product. 

Overmolding process is a technique that when applied, alters the physical properties of the part or product to suit the requirements that may vary from upgrading the functionality of the product to enhancing the aesthetics in order to differentiate it from the competition. The overmolding process is typically used to manufacture innovative products to promote user experience and boost sales. Manufacturing modern and new designs also helps to differentiate a product visually from competitors.

The overmolding process can be applied to a myriad of industries and has diversified applications. Some of these applications are listed below:

Medical Industry

Medical overmolding is one of the most specialized applications of overmolding. Many medical devices are required to have a rigid substrate with a soft overmold. By utilizing medial overmolding, VEM can produce a range of medical devices, and high performance equipment that has a non-slip design.     

When it comes to medical device manufacturing, we ensure to conform to the stringent requirements of the medical manufacturing industry to manufacture the highest quality product. Our mold design, engineering and manufacturing team follows rigorous standards. Our Thailand plant is ISO 13485 certified and has an ISO 7/Class 10,000 cleanroom after ISO 14644-1:2015 certification.

Automotive Industry

The overmolding process is also a very popular injection molding technique in the automotive industry. It is often a preferred technique to produce handles, components of car seats and headrest, car handles, energy absorber mounts, car hood, and dashboard accessories. 

Prior to overmolding, these components were manufactured either solely or largely with metal to maintain durability. Post over-molding, these components can be manufactured to be not only durable but also light-weight and desirable for the consumer.   

When molding automotive products and parts, it is imperative to take into account the numerous factors that are related to vehicle movement. Today, molding automotive products and parts requires material behavior and manufacturing processes to be  considered at an early design stage. This helps us to streamline any obstacles that may arise in manufacturing.

Electronics Industry

Another industry where overmolding techniques are very extensively used is the electronics industry. Many parts are over-molded in the electronics industry to seal the sensitive electronics components and connect it with the cables.

The most significant overmolding applications in the electronics industry are PCB, thermostat and circuit board overmolding. In this type of overmolding, electronic assemblies are positioned within a mold. Once they are positioned, they are then injected upon with molten plastic which then hardens into a protective layer. This ultimately reduces the damage risk by sealing the junction between the wires and connectors. Almost every electronical product that you can see has some degree of overmolding involved in their production. Just take a look at your USB charger or HDMI cable, just to name a few.

overmolding electronics part

When Should you choose Overmolding?

The primary reason to use overmolding techniques is to develop more innovative products and complex designs but in addition to that, it offers various benefits that can significantly impact your manufacturing cycle. 

  • Overmolding process can save both time and money, because it eliminates assembly steps. Instead of manufacturing 2 parts separately and then joining them together, you can overmold the parts and eliminate the need for assembly altogether. 

  • It can significantly improve the material flexibility.

  • It can be used to either add a soft grip surface above the substrate or add flexible areas to a rigid part. 

  • It also helps to develop durable products and parts that match the design language of the product developer. 

  • It also eliminates the need for fasteners or adhesives. Overmolding helps to capture one part inside, thus eliminating the requirement of fasteners or adhesives.

Disadvantages

Though there are various benefits of developing products with overmolding techniques, there are a few disadvantages that we must note as well. Some of these are listed below: 

  • Overmolded parts and products are made in a two-step process. This 2 step process can increase part cycle time significantly making the production cycle more expensive. At VEM, we can suggest an alternative in which our engineers help to create two separate injection-molded parts. If we consider this alternative, it generally takes the same amount of time for overmolding parts and products. These 2 parts are then further assembled. 
  • Debonding – Sometimes, bonding two different materials in an injection mold may result in delamination if the temperatures are not in the optimal range. This may happen if one does not have an adequate knowledge of specific material combinations. At VEM, we eliminate these errors and ensure that such errors are avoided. If there is a chance that we cannot rely upon heat for material bonding, then mechanical interlocks can also be used.

Contact VEM

VEM provides end-to-end solutions in plastic overmolding. We have 7 locations across the globe with state-of-the-art tooling facilities. Our engineers and designers can help you create high quality molded parts and products. To better understand how VEM tooling can serve you, in terms of overmolding, contact us or request a quote today.