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Basic Material Considerations For Overmolding in Medical Devices

Posted by bead electronics on Jul 9, 2021 2:33:46 PM


Imagine having to perform surgery when your tools don't feel right in your hands. With an improper grip or a flawed tool, the outcome of your procedure can be compromised. Any tool for any job needs to have that ergonomic benefit to optimize the outcome and enhance performance.

This is where overmolding comes onto the scene.

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What is Overmolding?

Overmolding leverages a mechanical or chemical bond to join parts together. It takes the substrate (a sub-assembly or previously molded piece) back into the press and has a second plastic or LSR (liquid silicone rubber) or thermoplastic elastomer (TPE) injected over, into, and around the substrate. This provides a second layer over a metal or plastic structure without any added support.

A rubber grip on a surgical instrument is an example of an overmolded tool. In its essence, the overmolding is added to act as a grip or handle without glue or screws, or any other added support, creating a specific, customized shape to components. This second injection can resolve issues like low-impact vibration resistance, slippery surfaces, sub-standard ergonomics, and superficial concerns.

When you look inside of a medical device, like an MRI, surgical cautery equipment, or a defibrillator, overmolding plays a big part in the construction and continued use of these machines. Electrical connectors, from wall outlets to pins in a circuit board, are often overmolded to facilitate a safer and stronger connection that is more flexible and more durable.

The TPE overmold is what gives the tool these added benefits, and the substrate provides structural support. It is used for items on a broad field with the benefits of strong and light parts, replacing machined or cast metal products.

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Before overmolding is done, three items must be considered.

Bonding

A strong bond between the two materials is critical to overmolding, but not always easy to do. In some cases, it may not even be necessary.

Materials

The substrate and overmold materials must be compatible in terms of their physical, chemical, and thermal properties. They must also be suited to the overmold process.

Principles

The principles of moldability apply the same to overmolding, with the extra considerations. These include the wall thickness of the mold, texture, and surface.

Material Considerations

There are often two options for the substrates. First, there are plastic-over-metal substrates that combine materials that are fundamentally compatible. There are few chemical formulations that limit producing a component that is consistent in performing its purpose.

Then, we have plastic-over-plastic substrates that present a greater risk of incompatibility in the material. For that reason, an expert accomplishes this overmold.

Many resins used are both thermoplastics and thermosets. Let's take a look at what these include:

Thermoplastic Polyurethane (TPU)

TPU is noted for its toughness and flexibility, and biocompatibility. This resin is in items for external body contact and implants.

Thermoplastic Vulcanate (TPV)

TPV offers many capabilities of thermosetting resins. It is both lighter and recyclable.

Styrene-Ethylene/Butylene-Styrene Copolymer (SEBS)

SEBS provides a good flow in the mold. It also provides a stiffness as well as a tendency to not warp, and adhesiveness to other thermoplastics.

Thermoplastic Elastomer (TPE)

TPE has the same elasticity as rubber with a high rate of consistency. It's easily colored and more economical to use than other thermosets. A TPE overmold has several added benefits such as safety and performance, comfort, production, and identity.

Liquid Silicone Rubber (LSR)

LSR produces a high lubricity that is highly stable and tolerant of heat for the purpose of sterilization. It is compatible with metal and several different plastic substrates. It is also flexible at lower temperatures and biologically dormant.

Tooling is also a critical step but a little more complex for the overmolded parts than for those made of single resins.

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Applications for Considered Material

There are many applications for overmolding, particularly in the medical field, but in any case, the overmold must improve the overall quality of the product.

In general, LSR seems to be one of the best options with high tension and tear strength. It is also water repellant, flexible, biocompatible, and resistant to both UV light and bacteria. Liquid silicone rubber has one drawback and that is its relatively high molding temperature at 350 degrees Fahrenheit.

At this temperature, LSR is hot enough to soften most substrates. However, it's been found that polybutylene terephthalate (PBT) and glass-filled nylon hold up quite well to it.

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Other Applications

Due to the qualities that overmolded parts offer, the list of medical applications covers a variety of options.

Overmolding can include abrasion resistance to both monitors and handheld devices plus add a degree of vibration control, which can be applied to critical items like handhold instruments, monitors, and instrumental housing.

The nonslip grips and chemical resistance add another benefit to surgical instruments and syringes, as impact resistance helps instrumental housings and monitors.

The ability to act like a liquid or gas seal greatly benefits tubing or Luer fillings.

Overmolding on metal inserts is often the most efficient, cost-effective, and durable option to create custom connectors that address your applications' needs.

Medical Applications

In surgery, an overmold on the medical instruments provides a stronger grip by utilizing the nonslip feature of the overmold over the substrate. This keeps the instrument from falling out of the hand of the physician or causing damage to the patient rather than repairing what was previously damaged.

The medical uses of applying an overmold to tubings or Luer fillings are examples of not necessarily needing a chemical reaction. It provides a seal to liquid or gasses, making it a versatile product.

About Bead Electronics

Bead Electronics, is a global manufacturer of electronic connector pins and has been manufacturing in Connecticut for over 100 years. The award-winning company carries over 500 patents and is best known for inventing its manufacturing process called swaging. This process is a high-speed, virtually scrap-less, cold-forming process capable of producing a wide size range of metal electronic components that are consistent and cost-effective. The family-owned business is led by its fifth generation. Click here to speak to a connector pin specialist today.

Topics: Custom Connectors, Bead Electronics, Quality Standards, Medical, Overmolding