MEDICAL CABLE OVER-MOLD MATERIALS – PART I of IV : A FOCUS ON THERMOPLASTIC POLYURETHANE (TPU)
“Help! Which overmold material is best for my medical device application?” This is a question we hear all too often. As OEM’s come to us in the early stages of their projects, they often feel a push-pull in selecting a material that will meet performance (electrical, mechanical, and environmental), quality and aesthetic requirements. At the end of the day, there are several resins on the market today where you don’t have to make sacrifices in requirements. Here are the most common resins we come across at ClearPath Medical:
- Thermoplastic Polyurethane (TPU) offers excellent mechanical properties including abrasion resistance and tear strength. Disadvantages of polyurethane material include poor resistance to some common cleaning agents and high temperatures making them unsuitable for sterilization by autoclave.
- Thermoplastic Elastomer/Rubber (TPE/TPR) is often referred to by the trade name of Santoprene®. TPE/TPR materials have excellent chemical resistance and are suitable for cleaning and disinfection by most methods. If designed appropriately, TPE/TPE cable assemblies can withstand several autoclave cycles.
- Polyvinyl Carbonate (PVC) is one of the lowest cost cable materials. It is commonly used to insulate conductors as cable jacket material and suitable for molded components. PVC offers good resistance to alcohols, most solvents and alkalis, but is not suitable for steam sterilization by autoclave.
- Liquid Silicone Rubber (LSR) is both very flexible and offers very high flex life. It is the most common choice where a high number of sterilization cycles by autoclave are required. Silicone cables can also be disinfected with most common solutions.
In the first part of the series of over-mold materials, we will take a deeper dive into TPU. Thermoplastic polyurethane resin is a material that softens when heated and hardens again when cooled. This allows the material to be used in over-molding and cable jacket extrusion; two processes used in the manufacture of medical cable assemblies.
Most medical cable assemblies have one or more over-molded components, such as a strain relief or yoke. These components are formed by injecting nearly liquefied mold material under high pressure into a mold cavity where the components to be encapsulated are held in place. The result is an over-molded cable assembly, that provides the finished medical device with a number of strengthening characteristics including: overall mechanical strength of the product, encapsulation of sensitive electronics and components, bond of strain relief to the cable jacket, protection of internal components and an appealing look and finish to the final medical device.
MOLD CHARACTERISTICS OF TPU
The characteristics of thermoplastic polyurethane make it a good choice for insert molding where adhesion to substrates and high tear strength may be required such as in shooting the outer mold of a small part. Polyurethane resins can be nearly liquefied when heated which allows for the mold cavity to be filled correctly forming intricate detail. Insert molding is one of the key offerings at ClearPath Medical.
THE CHEMISTRY OF TPU RESINS
Thermoplastic polyurethanes achieve their unique properties due to the chemical reactions that form the material. TPU resins are referred to as block co-polymers. They consist of softer and rather long linear polymeric chains in block-structures. These alternate with shorter, hard segments and are joined by covalent links to form the block co-polymer.
Polyurethane resins contain physical cross-links, rather than chemical cross-links. The physical cross-links become ineffective when heated and allow the material to soften, liquefy and flow. It is this property of the resin that allow it to be used for injection molding and extrusion. When cooled, the physical cross-links re-engage and the material becomes hard.
PERFORMANCE CHARACTERISTICS OF TPU RESINS
While each thermoplastic polyurethane resin has different characteristics, common to most are the following:
- Good abrasion resistance
- Good elasticity
- Good resistance to common cleaning solutions
- Excellent low-temperature impact strength
- Good flexibility over a wide temperature range
- Good tactile “feel” properties
- Good adhesive properties
- Easily colored
TPU resins are not suitable when sterilization by autoclave is required so we don’t recommend it for steam sterilized applications. Most TPU resins also require a thicker wall section than other over-molding resins to fill cavities and fully encapsulate without risk of voids in molded component.
RECOMMENDED TPU RESINS
We recommend a variety of medical grade polyurethane resins and have included the ones that we commonly use to create medical cable assemblies. All are polyether based and are our preference because they do not breakdown when exposed to water and are more resistant to microbial growth, which is one of many considerations for medical device applications.
Lubrizol Pellethane® is a very durable material with high tear and tensile strength, high abrasion resistance and very good adhesion to many substrates. Pellethane is available in a wide variety of durometers (53D to 90A) and is available in medical grades.
Lubrizol Tecothane® is a TPU resin that is available in durometers as low as 42A. Tecothane has good adhesive properties and is available in medical grades.
Covestro Texin® offers high tear strength, easier to mold, good adhesion to many substrates and is available in medical grades
GLS Versollan® is a TPU alloy used for thin wall applications with excellent adhesion to Polycarbonates, ABS, and PC/ABS substrates. Versollan has low tear strength but provides soft feel and grip and good resistance to chemicals. Versollan is available in low (soft) durometers and medical grades.
Huntsman Irogran® is great to specify for raw cable jacket extrusion. Irogran offers excellent abrasion resistance, tensile strength and cut resistance. These characteristics coupled with the availability of low durometer resins make this material a good choice for suitable cable jacketing medical cable.
SUMMARY
We look forward to delivering more details in future posts on other resins to consider for your medical device application. In the meantime, do not hesitate to contact the team here at ClearPath Medical with any questions. We have the expertise and experience and can recommend the best resin to be used for over-molding and jacketing of raw cable for your finished assembly. If you would like to discuss thermoplastic urethane resins, or any other aspect of using these materials in the manufacture of medical cable assemblies contact us.
