There are various types of injection molding methods and OEMs may wonder which technique is best for a given molding project. The answer is that it depends on the component being produced, and the desired properties of the final product. It is important for OEMs to have a good understanding of the different molding methods in order to select the best technique for a particular application. Additionally, it is advantageous for OEMs to leverage the expertise of a molding partner, such as ProMed, that has experience in a variety of techniques including overmolding and insert molding – two common molding processes that are often confused.

Each process has its benefits and applications that it is best suited for. While there are some similarities in these two techniques, such as the fact that they are both multi-material molding processes, there are also some key differentiators. Below is a closer look at overmolding and insert molding that will shed light on the commonalities and differences between these two methods.

What is Overmolding?

Simply put, overmolding is when one material is molded over another material. With this technique, a plastic or silicone layer is molded over and around the base layer – resulting in a single, finished product. Oftentimes, the exterior layer is an elastomer that gives the desired surface texture or physical property such as pliability. It is often very desirable to make a composite product which has a plastic or silicone layer molded over some or all of a piece of a different material, thus, overmolding is a very common and versatile injection molding option.

Creating overmolded parts is typically a two-shot (or more) process—essentially a separate molding process for each layer. Since two or more plastic materials are being used to create the final part, material selection is critical to the success of an overmolding project. The materials do not have to be the same but they must be compatible and bond together chemically and/or mechanically in order to avoid mold defects such as distortion or warping. OEMs must carefully check the material compatibility of the materials they wish to combine because not all combinations of elastomers, thermoplastics, and metals are possible.

This technique is employed across a variety of industries including medical, pharmaceutical, dental, military applications, electrical/electronics, and safety. Examples of medical devices manufactured using the overmolding technique are steel surgical instruments with a silicone gripping surface. The soft grip plastic layer is molded over the instrument to achieve the desired grip and aesthetic.

There are many benefits to overmolding and below are the biggest advantages:

  • Increases Cost-Efficiency: overmolding is often a cost-effective option as it reduces the number of production steps. This process allows the second layer to be molded directly onto the base layer, avoiding the separate molding of two parts that need to be assembled post-production – reducing production times and saving OEMs money.
  • Extends Product Life: the addition of a second layer that is typically an elastomer offers many benefits such as sealing, sound absorption, and vibration dampening. These benefits result in a more durable product with an extended life time.
  • Improves Safety and Ergonomics: the use of a soft elastomer on top of a harder base layer provides a non-slip grip on many different products, providing a safer interaction with the product.
  • Improves Aesthetics: overmolding opens up endless possibilities for OEMs to create more attractive products via the use of multiple colors and/or patterns.

What is Insert Molding?

Insert molding is a one-shot process in which a pre-made insert is placed in the tool for molten plastic to flow around. With this process, the plastic encapsulates or surrounds the insert in order to integrate it into a larger injection molded part.

Inserts are often metal and, therefore, must be placed in the mold either robotically or manually prior to the injection of the plastic. The combination of plastic and metal allows designers to capitalize on the weight reduction of plastics and the strength of metal. The insert and the plastic, often a rigid plastic, must mechanically bond together in order for the insert to remain embedded in the plastic. Generally, insert molding results in better and more reproduceable encapsulation than other techniques such as heat staking or ultrasonic welding where the plastic part is melted post-molding in order to add the insert.

Similar to overmolding, insert molding is found in a variety of products across a wide range of industries including medical, pharmaceutical, dental, military applications, electrical/electronics, and safety. Insert molding was developed to place threaded inserts in molded components, and to encapsulate the wire-plug connection on electrical cords; however, this technique has evolved to include inserts as intricate as motors and batteries. Some examples of products manufactured by insert molding include metal knives with plastic handles, and plastic parts with protruding metal screws that allow for repeated fastening and unfastening.

Like overmolding, insert molding offers many advantages such as:

  • Increases Cost-Efficiency: molding an insert directly into the product avoids post-production operations – reducing production time and saving money.
  • Enhances Strength: this method creates a single molded plastic piece that is typically more durable and robust than if the product were created via secondary assembly.
  • Improves Cost-Effectiveness Over Metal: the use of plastic in insert molding decreases the part weight and reduces the amount of metal or other more costly materials needed – decreasing the overall product cost.
  • Increases Design Options: insert molding allows the combination of plastic with metal or other insert materials, increasing the product design options available to OEMs.

Many of the similarities and differences between overmolding and insert molding have already been noted; however, there is one other important distinction: production time. Generally speaking, insert molding is slightly faster than overmolding because the two materials are molded at the same time in a single molding step; whereas, overmolding is a two-step process where the base layer is fabricated first; then, the second layer is poured over the base layer and allowed to cool.

ProMed Molding Capabilities

ProMed has expertise in working with the full spectrum of silicones covering a wide range of properties and characteristics. We will assist in your material selection to help ensure all design requirements are met. Our manufacturing facilities and equipment are designed for a single purpose—to mold medical and implantable silicone, combination components, and bio-material grade plastics with uncompromising quality and service. We currently have four divisions that are located within two manufacturing sites. All are certified class 10,000 / ISO Class 7 cleanrooms. We can identify the right manufacturing solution for any project and have extensive experience in a wide range of molding techniques including:

  • Automated Injection Molding
  • Multi-cavity tooling
  • Micro molds and micro molding
  • Servo-controlled de-molding capabilities
  • Insert molds, overmolds, and automation integration
  • Transfer molding
  • Compression molding

Contact ProMed today at 763-331-3800 to discuss how we can help with your next project.

A Closer Look at Overmolding and Insert Molding
Article Name
A Closer Look at Overmolding and Insert Molding
A closer look at the similarities of overmolding and insert molding, such as being multi-material processes, as well as the key differences.
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ProMed Molding
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