There are several techniques that might be used within the broad realm of plastic injection molding, and one of the most common and well-known is over-molding. Referring to the layering of two or more specific plastic pieces, where at least one additional part is molded directly onto the first piece rather than molding them separately and then assembling them, over-molding is frequently carried out to create high-quality parts – and to reduce production time in many settings.

At EnviroTech Custom Injection Molders, we’re happy to help with a wide variety of over-molding services for any component you require. We over-mold a skeleton component with a given polymer resin, then a secondary resin molded around the skeleton to add strength and durability. What exactly is over-molding, why is it done, and what are some of its common uses and examples? Here’s a primer on everything you need to know about this important process.

Over-Molding Basics

As we noted above, the basic process of plastic over-molding involves creating a single molded or cast part – but one that involves two or more plastic and elastomer materials that are combined. The first step will involve molding the initial, or base, part; from here, additional layers can be molded over and around the original part.

For this process, one or more plastic resins may be used – this will depend in large part on material compatibility and the desired texture of the part. This area – material compatibility – is vital for over-molding, and requires a strong base of knowledge from your injection molding professional to avoid errors. If the wrong materials are used, bringing improper chemical and thermal compatibility, the entire product could have deformities or issues with performance. More on materials used in the process later in this blog.

Benefits of Over-Molding

Over-molding is used for a variety of products and industries, some of which we’ll discuss in a moment. It has a variety of benefits that come with it, including each of the following:

  • Reduced secondary operations, plus decreased costs for assembly and labor
  • Creation of waterproof seal for combined parts (in situations where the parts are molded separately and then assembled, this seal is harder to achieve)
  • Creation of electrical insulation within the part
  • Improved grip and overall ergonomics for products
  • Vibration dampening, which absorbs both vibrations and sounds
  • Ability to customize significantly within color and aesthetic themes
  • Flexible mechanical features for fitment and function

Common Uses and Examples

Over-molding is used across numerous industries and applications. It’s often seen in wearables, medical devices and various automotive parts, plus in many items that involve grips – cordless drills, screwdrivers and even toothbrushes. They’re also often found in personal care products like shampoo bottles or shaving razors, which utilize multiple plastic pieces on the same overall part. And as the years have gone on, over-molding capabilities among plastic injection molders have only improved and expanded, making this a robust area that’s widely-utilized.

Process and Materials Used

The over-molding process involves two major steps. The first of these is the production of an original plastic component, known as the base component. This will generally be done using a typical plastic injection molding process, though there may be some tweaks here depending on the specific part being produced.

Next, the product will be placed into an over-molding tool. Within this tool, it will be coated in molten thermoplastic or rubber, depending on the part requirement. It’s important to note that this process is different from insert molding, which involves encapsulating a product in molded plastic.

There are several different thermoplastic materials that might be used for over-molding, from soft plastics like urethanes to hard plastics like nylon and polycarbonate. These can sometimes be combined or utilized together, such as plastic being used in coordination with epoxy, metal or other materials.

When it comes to plastics used, here are the most common for the over-molding process, plus their benefits:

  • HDPE: Short for high-density polyethylene, HDPE is an extremely popular plastic used in several areas of injection molding, including over-molding. It’s highly versatile and known to do well with parts that require great physical performance, such as corrosion-resistant piping, lumber and others. It’s known for its fantastic strength and durability – qualities that are present despite it being lightweight and easy to manage. It’s both impact- and weather-resistant to allow for part quality in harsh conditions, plus is malleable and can be molded into a virtually limitless selection of shapes. It’s often used to replace older, heavier materials that are now outdated within a given application, and is a sustainable, eco-friendly material.
  • PMMA: Another eco-friendly material often used in over-molding is PMMA, short for polymethyl methacrylate acrylic. Often found in lighting or various optical applications, this product is also sometimes called “acrylic glass.” It’s known for its light weight and resistance to shattering, making it an ideal alternative to glass for many applications. Beyond this, it’s naturally resistant to UV radiation and weathering. It also has fantastic refractive index and clarity, light transmissibility and surface hardness – it’s even scratch-resistant. Dyes can be easily added to PMMA to create any color you want.
  • ABS: Short for acrylonitrile butadiene styrene, ABS is also popular in some over-molding applications, especially electronics and automotive areas. It’s cost-effective and has a low melting point, making it easy for injection molding and 3D printing alike. It finishes a glossy, smooth surface that’s easily applicable with glues and paints, plus resists chemical corrosion and physical impacts.

For more on over-molding and the various plastics often used for it, or to learn about any of our plastic injection molding services, speak to the staff at EnviroTech Custom Injection Molders today.