Overmolding

Overmolding

Overmolding as a production technique is used in manufacturing to combine different materials to produce an excellent finish. The process simply involves adding a soft outer layer to improve the durability of everyday products.

Overmolded parts usually have soft grips, better resistance, and multi-material or dual-color designs. The cost of combining materials rather than using separate materials makes the result more user-friendly and visually appealing.

At Premium Parts, our material experts offer practical guidance to get your manufacturing projects up and running. This in-depth guide covers the basics of overmolding, its applications, best design practices, and factors to consider when selecting materials for overmolding.

What is Overmolding?

Overmolding is an injection molding technique where a softer material (such as TPE) is molded on a rigid plastic component. The external layer is typically the overmold, whereas the tougher portion is the substrate (which can be made from other materials like metal).

The materials undergo a chemical bonding process with no adhesive required, but through reactions over precise temperature and pressure control. Then, the process outputs a composite that guarantees improved product usability, aesthetics, and comfort.

Also, overmolding usually involves soft materials like thermoplastic elastomers(TPE) layered over rigid inner plastic or metal. Combining materials at a molecular level will translate to products with soft grips, better resistance, and improved usability.

This cost-effective manufacturing procedure finds applications in many industries ranging from medical and consumer electronics to advanced automotive and engineering. In today’s manufacturing landscape, high-quality mold design is required for reliable part quality and consistency in appearance.

Some of common materials (plastic) used in overmoulding

Take a look at some of common materials (plastic) used in overmoulding:

Thermoplastic elastomers (TPE)

TPEs are constantly in demand for their maximum flexibility, softness, and ease of processing. When used as overmold finishing for products, TPE provides better grip and impact resistance, made possible by an ability to bond well with several substrates.

Thermoplastic polyurethane (TPU)

TPU has high elasticity, abrasion resistance, and durability properties. It’s a good fit in overmolding applications where toughness, flexibility, and chemical resistance are required.

Polypropylene (PP)

PP is also becoming as popular as TPE. Its low density, excellent chemical resistance, and dimensional stability make this material widely sought after in medical device manufacturing.

Polyethylene (PE)

This is another thermoplastic material notable for its low cost, lightweight nature, and excellent impact resistance. Polyethylene is used as an overmold to make handles, grips, and protective materials.

Acrylonitrile butadiene styrene (ABS)

ABS is commonly used as the substrate in overmolding operations. The strength, impact resistance, and dimensional stability of the material make it a solid base structure for overmolding. In practice, ABS is often overmolded with TPE or TPU for a soft-touch finish.

These are just the most popular materials available for overmoulding. To get the best from a wide range of options, it’s essential to gauge the specific requirements of the output based on desired flexibility, hardness, chemical resistance, and appearance.

You can consult material suppliers or experts to select appropriate materials for overmolding based on the desired properties and performance criteria for the product.

Overmolding process step by step

The overmolding process is a sequential manufacturing technique. During this multi-step injection molding process, a base component known as a substrate is first molded and then allowed to cure.

Usually, the substrate is a rigid component base composed of plastic. Then, a softer second layer, referred to as the overmold, is added over the base. If required, more than two components are molded on top of one another.

There are all kinds of overmolding processes, but the difference lies in how the substrate is loaded into the mold. The method adopted creates two separate overmolding operations:

Manual Molding

Manual molding is a simple operation where the substrates are loaded by hand. The operator will remove the finished part from the mold, inspect it, and package the parts. Here, no secondary equipment is required as it's a common procedure for low-volume overmolding.

The only downside with manual molding is that it creates more labor for operators. As production orders increase, the need to invest in automated production cycles becomes more apparent. That’s where two-shot molding comes in.

Two-Shot Molding

Two-shot molding is an advanced overmolding operation. It uses a complicated mold and robotics tech to mold the substrate and then transfer it to the other component.

Furthermore, two-shot molding uses machines that allow the running of two different polymers at the same time. This technique is best for more sophisticated molding and is used to realize a lower product price.

Insert molding is another type of overmolding operation. A simple application of overmolding in mainstream production can be found in the making of a toothbrush. To make the toothbrush firm on the grip, a top layer of rubber-like material is directly formed over a base layer for the plastic handle.

The overmolding process usually happens via a mechanical or chemical bond. Different materials can be used as an overmold due to their mechanical properties, but the most commonly used include thermoplastic elastomer(TPE), polypropylene (PP), and polycarbonate (PC).

However, the entire overmolding process can be summed into four steps:

  1. Forming the substrate
  2. Placing and heating of the pre-shaped substrate
  3. Overmolding of the soft exterior part
  4. Removing the completed part

Another thing to keep in mind is that overmolding can be soft or hard. Soft overmolding, as the name indicates, means applying a soft or flexible material (like TPE or silicone) onto a rigid base structure. Hard overmolding is the exact opposite—molding a rigid material over a flexible base.

Either technique (soft or hard overmolding) finds use dependent on the level of structural support, reinforcement, or protective cover required for the finishing.

The benefits of Overmolding

Overmolding is useful in several industries because it offers a variety of advantages. Combining materials in an overmolding process will enable the following:

High material flexibility

Overmolding enables production companies to assemble different components in one piece. Producers can utilize all the materials used to increase the flexibility of the part created. Following the overmolding design guide can also increase flexibility.

No adhesives

Overmolding allows diverse parts to fuse naturally without requiring adhesive. The end result is a more robust, more durable material. From a business perspective, overmolding also significantly decreases the overall cost of production.

Custom plastic parts

Overmolding facilitates the design and production of unique plastic parts for any need. Either through a plastic-plastic or metal-plastic mold, overmolding has offered the flexibility and option that every designer requires.

Reduced cost

Every producer genuinely cares about how much it costs to design, manufacture, and assemble a product. Introducing overmoulding can lower manufacturing costs while accelerating the overall process since it literally eliminates the post-assembly stage.

Enhanced functionality

Introducing high-quality plastic over a rigid base structure generally improves a product's performance. Overmolded products have material advantages over regular products in terms of ergonomics and comfort. They also appear more appealing to the eye.

Applications of Overmolding

As stated earlier, overmoulding has applications in a variety of industries. They are generally used in the design of components for grip, vibration dampening, sealing, and sound absorption. Here are some practical applications of the overmolding technique:

Hardware parts

Overmolding is perfectly illustrated in the manufacture of blades, pocket knives, hammers, and other tools. The base structure is mostly metal, such as steel, which must be handled safely. That’s where the softer layer (usually plastic) via overmolding comes in.

In addition to tools, overmolding optimizes common household items like toothbrushes, electric fans, mirrors, and multi-port chargers.

Medical products

Precision and product safety are very critical when designing equipment for the medical sector. Health practitioners should be able to rely on the safety of medical devices used for medical procedures.

For instance, healthcare equipment should be fabricated for safe and ergonomic use. It should also be sterile and simple to clean and store.

Cookware

Overmolding is very useful in fabricating cookware, food containers, and kitchen utensils. It regularly finds use in spatula handles, pot covers, knives, cutting boards, and more.

Furthermore, the use of high-quality polymers also preserves the flavor or freshness of food, guaranteeing customer safety during handling.

Factors to consider when selecting materials for Overmolding

Overmolding allows manufacturers to fashion products with enhanced functionality, improved aesthetics, increased durability, and better user experience.

This molding technique also helps producers meet market expectations by creating functional products within financial constraints.

Producing high-quality products, especially for processes like overmolding, requires meticulous design considerations. For an optimal design, the components must be carefully selected to reap the benefits of this overmolding technique.

Key considerations to bear in mind when selecting materials for overmolding include:

Functionality

Before selecting the kind of overmold material, you need to understand the core function of the part to be molded. Since the overmolding process has a variety of applications, the function will ensure your design is perfectly tailored to its purpose.

For example, when molding a seal on a water-resistant case, the high-priority function is sealing. This ultimately requires selecting a material designed to withstand wet scenarios.

Another case example is choosing a material to add to a handle (like a rubber-like plastic). The reasons include a better grip, ergonomics, or vibration absorption.

Consistent wall thickness

Most overmolding materials are plastic, so it’s essential to ensure a steady thickness for the overmold. The thickness of the material will ultimately determine how well the material properties will perform.

Normally, the overmolding should stay within the range of 1.5 to 3 millimeters(0.06 to 0.12 inches). For instance, a thicker-walled section of TPE means more vibration absorbance and a softer feel. On the contrary, a thin wall of TPE decreases the material’s ability to absorb vibrations.

Material Compatibility

Material experts advise that you check the chemical and physical compatibility of materials (between the substrate and overmold). Compatibility will determine how well the material adheres to the base, ensuring the bond is solid and durable.

TPE plastics are commonly used because of their flexibility—the ability to expand and compress without permanent damage. If your production requires over-molded features that stretch and return properly, TPE is a great selection.

Mechanical properties

Essential properties to watch out for include flexibility, tensile strength, impact resistance, and resistance to fatigue. It’s important to factor in extremes in the end-use environment, such as UV exposure, moisture, and humidity, that can compromise the strength of material.

The ability of a material to resist harsh conditions when exposed will ultimately affect its durability. Since you’re designing for longevity, you should choose an overmold that covers these bases.

Aesthetics

What is material design without its aesthetic appeal? When selecting an overmold, choose material with a nice texture, bright color, and smooth finish.

Even from a design perspective, the goal is to make the product look good. Apart from adhesive tendencies, be sure that you have the right mold of plastic over plastic that improves its overall appearance.

Regulatory requirements

When selecting materials for the overmold, consider whether they comply with food contact safety, electrical insulation standards, and flame retardancy. Plastics should be food-safe, especially when used in the production of kitchenware and cooking parts.

Some materials used as overmold are required to extend the shelf-life of products that are contained. It’s essential that these materials do not have adverse effects on the products. The design should be practical enough and compliant with stipulated industry standards.

Partner with an Overmolding Expert at Premium parts

Overmolding is a powerful procedure in product design, but it requires extreme care and careful material selection to be productive. While overmolding may seem to increase production expenses at first, it will save producers long-term manufacturing costs and raise the value of the final output.

It begins with selecting the right pair of materials (overmold to go with the substrate), and then selecting a suitable overmoulding procedure to effect the design. Design cost requirements and material selection must be considered accordingly.

Our professional team at Premium Parts can help you incorporate cost-effective processes into your design phase. If overmolding doesn’t suit your design requirements, we can provide alternative routes that will still give sustainable results.

Subscribe to Our Newsletter

Please read on, stay posted, subscribe, and we welcome you to tell us what you think.