MEDICAL CABLE OVER-MOLD MATERIALS – PART II of IV : A SPOTLIGHT ON THERMOPLASTIC ELASTOMERS / RUBBERS (TPE/TPV/TPR)
Last month, we kicked off a multi-part series on overmold materials for custom medical cables, starting with Thermoplastic Urethane (TPU). It’s the perfect segue into our next topic of conversation – the use of thermoplastic elastomers and rubbers – also known as TPE/TPR- in medical device subassemblies. As we introduced last time, medical cable assemblies typically have one or more molded components, such as a strain relief or yoke and many of the resins we introduced can be used to satisfy a molded component requirement. During this spotlight, we’ll get into the details of using TPE in these types of applications. In addition to being used for molded components, TPE is often used for the exterior jacket of cable and wire material. It’s why we always like to reiterate how important it is to first understand how the cable will be used and under what environmental conditions. This really helps the team at ClearPath Medical provide guidance into the most appropriate material for the over-mold and cable jacket.
THE SCIENCE BEHIND THERMOPLASTIC ELASTOMERS
Let’s first get into the science behind this resin. Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers or thermoplastic vulcanizate (TPV), are a class of copolymers or a physical mix of polymers – usually plastic and rubber -which yield a material with both thermoplastic and elastomeric properties.
Three essential characteristics are commonly considered in order for a material to qualify as a thermoplastic elastomer:
- The ability to be deformed to moderate degree under stress and upon the removal of stress, return to close to the original form.
- The ability to process as a liquid or semi-liquid at elevated temperature.
- Not significantly deform under continuous stress – also known as creep.
SANTOPRENE™ TPV
One of the most commonly specified thermoplastic elastomers used for medical cable assemblies is the medical grade of Exxon Mobil’s Santoprene™. This material is a mixture of synthetic rubber and polypropylene. It offers similar flexibility as found in natural rubber but over a wider temperature range and increased durability. The 8281 Series of Santoprene are medical grade and are the most common resins we recommend during the design and manufacture of medical cable assemblies.
OVER-MOLDING THERMOPLASTIC ELASTOMERS
TPE is a great candidate for injection-mold processes, especially when base or primary material, like a rigid plastic is in place to protect a sensitive connection or circuit board. Due to a number of benefits, it’s why TPE is a great candidate for diagnostic applications, like medical and dental cable assemblies.
Over-molding with a thermoplastic elastomer offers the following benefits including:
- A soft “velvety” feel
- Durability over a wide temperature range
- Excellent grip characteristics
- Cushioning
- Custom color matching
- Insulation from electrical current and heat
- Excellent cleaning & disinfection characteristics, along with great characteristics for EtO (Ethylene Oxide) sterilization, autoclave and Sterrad
THE FEEL OF TPE
We’re often asked, how will my end product feel once it’s finished? How do the properties of TPE balance that ‘velvety’ softness with the structural durability that’s trying to be achieved? The relative hardness or softness of a material is commonly one of the first considerations when choosing a thermoplastic elastomer. Hardness can also related to other properties including tensile strength (resistance to being pulled apart) and flexural modulus (bending stiffness). It’s why confusion can sometimes arise when discussing hardness due to the variety of measurement scales and its relationship to other material properties. So, it’s why it’s important to also take into consideration a ‘measurement of hardness’.
The most common measurement of hardness (or softness) with flexible material is durometer. For thermoplastic elastomers, two scales are used, Shore “A” and Shore “D.” The Shore “A” scale is most commonly referenced for softer TPEs and the Shore “D” for those that are harder.
A durometer is used to measure the hardness of rubber-lime materials. In this case a probe is used to see how far it will indent into the material when force is applied, thus yielding the durometer number. The “A” and “D” scales to note.
ENSURING THE TPE IS MEDICAL GRADE FOR YOUR APPLICATION
We are often involved with providing our customers on the selection of resins used for over-molding, and even for cable jacketing. For us, several factors influence the selection process and one of those is specifically whether or not the materials are classified as “Medical Grade.” Resins, such as TPE, that are considered Medical Grade commonly must comply with at least one of the following:
- Meeting United States Pharmacopeia (USP) Class VI for plastics
- One or more evaluations for compliance to ISO 10993 requirements for Surface Devices (Typically for patient cables, -05: Cytotoxicity, -10: Irritation & Sensitization)
USP specifications cover blood and body fluid compatibility/contact applications. USP tests are designed to provide information on the biological effects of polymer materials used in containers. There are six classes into which a polymer may be placed, depending on its performance in specific USP biological tests. Each increasing class number, from I to IV, requires that a polymer be subjected to additional tests, with each level using more extraction vehicles than the previous class. There is also a range of increasingly higher extraction temperatures which may be selected to further characterize the material.
DEALING WITH TPE SHRINKAGE
It’s important to note the shrinkage that often occurs with using TPE as an overmold. As TPE cools from their semi-liquid state, the molecules align with each other and shrink the overall size of the molded part. While usually only in the thousandths of an inch per inch range, shrinkage can affect the shape of the molding, removal of the part from the tool and overall appearance of the final part.
If the shrinkage is uneven, a component that is meant to lay flat can bend or warp. Additionally, in applications with tight tolerances (i.e. medical device parts that must connect together), unexpected shrinkage may affect the fit of a part in a finished medical cable assembly. For these reasons, it’s important to take shrinkage into consideration during design and most importantly, production.
CONSIDERATION OF TPE DURING YOUR DESIGN PROCESS
Due to factors we just mentioned related to shrinkage, we always recommend that the insert mold tooling be cut larger than the desired size of the part. Typically, we can’t predict the exact amount of shrinkage that will occur until a specific part is molded, so we recommend being conservative by using a prototype mold to run trials or approaching critical-to-quality features as steel-safe in the initial tooling to better define and qualify considering the potential shrinkage amounts.
Like other elastomer properties, shrinkage often varies with the direction of a polymer’s flow. In this case, it’s important to consider the location of the tool’s gate (the opening in a mold through which the molten TPE is injected into the final part – also the boundary between part and scrap), which will determine the direction of flow into the part and the shrinkage direction. Also, some TPEs are more isotropic than others, meaning that they shrink more in one direction than another, and is another consideration to think about when designing the mold.
SUMMARY
Overall, TPE is a great material and one that we highly recommend for most diagnostic and therapeutic medical device applications. However, it is important to consider some very specific design factors, like shrinkage, but at the end of the day TPE offers many positives that outweigh the design challenges and is a great material to consider into your end design. Don’t let your questions get in the way of a great medical device design – connect with the team at ClearPath Medical for guidance on the right material and over-mold selection for your application.
