Designing Multi-Material Products

Are you a start-up with a cool new concept attempting to build your first proof-of-concept prototype? Or a VC funded company translating your technology to production? Or perhaps a design group in a blue-chip corporation with a new set of specifications for a device? When designing a new device (or redesigning an old one), the first challenge a designer encounters is that of material selection.

The obvious starting point when choosing materials for a new device or application are the mechanical properties. For many applications this may be enough, but in modern multi-material devices, where a host of different environments and processing may be encountered, the mechanical properties alone are not adequate and the expertise to support this effort can sometimes not be available. This is where a specialist materials science group can help and support your development process.

Although metals and ceramics have their own set of problems, polymers and plastics are uniquely sensitive to a host of effects that can work against (and for!) their use in specific applications. Understanding that polymers are not simple mechanical materials but have complex behaviors on their own (often non-linear), is crucial to using them effectively. A few key points to remember are:

• Worry about the interface: Modern devices, products or housings are often composed of multiple materials. The surface energy of these polymers, and how they wet each other, will influence strongly how well they adhere. This may cause problems in manufacturing, where two materials cannot be processed together, or it may result in poor end-use where different sections come apart prematurely. You can often see interface failures in low-cost “double-shot” molds where a soft elastomer is cast over a more rigid thermoplastic.

• Thinking about processing: What one does to the polymer during manufacture, or afterwards, can strongly influence behavior. Sterilization can damage materials or enhance their wear properties. A classic example of this is the orthopedic hip cup, which is polyethylene, but crosslinked to enhance wear properties. Likewise poor process control during molding can degrade the properties.

• Environment matters: If your end use is down-well oil drilling, the polymer must resist high temperatures and organic solvents, whereas a casing for a water sensor must resist water ingress. How the polymer responds to these challenges can make the difference between success or failure. Ever tried using a polyethylene tool on a grill?

• Even polymers get old and tired: The impact of age alone can be substantial. Almost any polymer evolves over time, whether through oxidation, elution of plasticizers, rearrangement of molecules or continued crosslinking. These changes can embrittle the material or make it stiffer (or softer!). Ever wondered why plastic toys become brittle and fail?

• Heat plays a role: Generally, heat accelerates the aging process, but only within limits. There is no universal acceleration rule, and even for “well behaved” materials, other transitions in the material can change the acceleration drastically. We have all seen how polymers can discolor and change properties over time, particular in warmer environments.

• Speed matters: The rate of deformation that a material has to operate under must be accounted for by the material choice. Even an elastomer will appear rigid if hit fast enough and this applies just as much to rigid polymers. Think of a PVC material that is perfectly flexible in summer, but stiff and brittle in winter.

• What goes in, can come out: Any processing aid that is used can be prone to elution. This can be in the form of blooming or off-gassing and will often lead to loss of properties. The new car smell is the result of off-gassing of processing aids.

In your push to generate the next device, or the final product, your team will become experts on your core technology and the primary requirements for your device. However, it rarely makes sense to invest time and resources in building the pure material science capability that might be required to assess the polymer components of your product in the critical areas of interface, processing, environment, aging and deformation. This is where a materials science partner can be invaluable for the short term.

If you would like to explore this in more detail, we have a wealth of information on our website at www.campoly.com, or contact me directly at gavin.braithwaite@campoly.com.