What Is Overmolding in Injection Molding? A Clear Guide

You have just picked up a tool or a phone case that is smooth, soft, and comfortable to hold in your hand, yet is strong and durable simultaneously. Now, you wonder, how did it achieve that fine combination of soft and hard?

Plastic overmolding is the answer. 

This intelligent production process involves merging of two materials into a single, smooth component, which makes products safer, durable, and easier to use.

This article will explore overmolding, its operational mechanism, and why it is a game changer in contemporary product design.

What Is Overmolding?

Overmolding is a special type of injection molding, in which a mold of one material is overmolded onto another, resulting in one part. Typically, a hard base (ABS or nylon) is produced first. A softer substance (e.g. a rubbery thermoplastic or silicone) is then molded over it. As a result of this method, the product is strong, flexible and comfortable at the same time.

This technology is common in electronics, automotive, and medical devices industries since it enables the designers to make components that are not only practical but also visually appealing and ergonomic.

Why Use Plastic Overmolding?

The following are the benefits of using plastic overmolding in product design:

1. Enhanced Functionality

Overmolding enables you to produce both rigid and soft parts by layering materials. For example, the hard core provides stability, whereas a soft outerwear provides a grip or shock absorption.

2. No Glue or Fasteners Needed

Since the overmold material is attached to the substrate in the direct process of the molding, no adhesives or screws are needed. That minimizes both the cost and the risk of failure.

3. Better Durability & Protection

To add sealing features (e.g. waterproofing), shock absorption, and chemical/UV protection, overmolding is also useful.

4. Design Flexibility & Aesthetics

Need a contrast of colors, soft touch textures or sections? The overmolding is a flexible process that can be artistically interpreted: it is possible to apply various colors, various durometers (hardness), or various finishes to the same object.

5. Cost and Time Savings

Although the tooling can be more complex, overmolding eliminates the post-molding steps (gluing or assembling), which in the long-run saves on the wages and time used in production.

How Plastic Overmolding Works: Key Processes

It is easy to understand how plastic overmolding works, if you break it down to smaller steps. Fundamentally, it is only a matter of bringing two materials together to create a single thoughtful, practical part in the overmolding process. The following is a simplified breakdown:

Step 1: Make the Base Part

This begins with a hard base, which is known as the substrate. It is typically manufactured using hard plastics such as ABS, nylon or polycarbonate. The base provides shape and strength to the part, which forms the base of the softer layer.

Step 2: Add the Soft Layer

After the base is prepared, a soft material (such as rubber, silicone or thermoplastic elastomer (TPE)) is then molded over the part. This second material goes over the whole part or only certain sections, such as a grip or a button. The soft material unites with the bottom to become a single unit.

Step 3: Two Main Methods

There are two common ways to overmold:

Two-Step (Insert) Overmolding

● The base part is molded first, then manually placed into a second mold.

● The softer material is injected around it to create the overmolded part.

● This method works well for smaller production runs and offers flexibility.

Two-Shot (2K) Molding

● Uses a special machine with two injection units.

● The first shot creates the hard substrate, and the second shot adds the soft overmold while the part is still warm.

● Perfect for large-scale production because it’s faster and fully automated.

Step 4: Cooling and Finishing

After overmolding, the part is cooled and ejected from the mold. Minimal finishing is required because the materials are already bonded, reducing labor and assembly costs.

Choosing Materials for Overmolding

Choosing the right materials is the key to an effective project of plastic overmolding. The blend of the hard and soft components defines the product's strength, comfort, and durability. 

1. Hard Base Materials

The substrate or base gives the structure and stability. Common choices include:

● ABS: Tough and shock absorbing, commonly employed in electrical and automobile components.

● Nylon: This is strong and not easily damaged by heat and can be used in tools and mechanical components.

● Polycarbonate: It is transparent or colored and has high toughness that can be used in high-end products.

These compounds are the base of overmolded components as they provide the shape and strength that the components require. 

2. Soft Overmold Materials

The second material includes comfort, grip or sealing features. Popular options include:

● Thermoplastic Elastomers (TPE): Soft and flexible, which is ideal when one needs grips and buttons.

● Thermoplastic Polyurethane (TPU): This is a tough, wearable material and it is perfect when it comes to producing protective covers.

● Silicone or Liquid Silicone Rubber (LSR): Soft and flexible and is used in medical equipment or in producing seals that are waterproof.

It's important to keep in mind that not every material pairs naturally. Designers must consider:

● Bonding: There are some combinations which will not stick without primers or mechanical interlocks.

● Thermal Properties: The material should be able to expand and shrink equally on cooling to prevent the warping of the material or poor bonds.

Designers pay attention to the choice of materials, which means that overmolded parts are not only useful but also comfortable, durable, and appealing to the eye.

Common Applications of Plastic Overmolding

Plastic overmolding is not a manufacturing trick, but something that we use in products that we touch every day. From tools to electronics, overmolding is used to make parts stronger, safer and comfortable.

1. Consumer Electronics

Consider your computer mouse, phone case or remote control. The device is structurally engineered with the hard plastic but is enhanced with soft overmolded regions to enhance grip and comfort. Eye-catching designs can also be done using overmolding with various colors and textures.

2. Tools and Hardware

Overmolding is commonly used in power drills, screwdrivers and hand tools. Soft grips allow less fatigue on the hand, non-slip surface, and prevent wear and tear in the tool.

3. Automotive Components

Overmolding enhances comfort and safety in cars. Hard plastics are frequently used together with soft, tactile surfaces on the steering wheels, gear shifts, pedal covers, and interior trims to create the high-end feel. Particularly, overmolding is used to protect parts against dust, vibration, and moisture.

4. Medical Devices

Medical equipment, including surgical instruments, grips, or ergonomic handles, are common examples of products which have been overmolded. The soft over-moulded surfaces are comfortable and precise, and the hard core is durable and controllable.

5. Outdoor and Household Products

Overmolding simplifies our daily routine by making everyday objects easier and safer to handle, including kitchen equipment and gardening. Spatulas, scissors or gardening pruners are examples of a hard structure component with a soft grip to provide easier control.

Conclusion

Plastic overmolding gives manufacturers the ability to merge strength, comfort and style in a single part. That is why we can find it in lots of industries to make products safer, functional, and to make them enjoyable to use. 

If you need to design electronics, tools, medical components, auto parts, or any other product: overmolding can improve the functionality and appearance of your product.

For those looking to experience the advantages of overmolding in your upcoming project, please contact Mulan Group. We would like to assist you in creating smarter, stronger and more comfortable products.