Thermoplastic elastomers (TPE; a.k.a. thermoplastic rubbers), are a physical mixture of two materials, typically a plastic and a rubber. The two are combined to produce a final product with both thermoplastic and elastomeric properties.
Here is a little bit about the desirable material properties in a TPE:
Thermoplastic materials are plastics that can be heated to their melting point, cooled, and re-heated again without significant degradation. Instead of burning, thermoplastics liquefy, allowing them to be easily injection molded and then subsequently recycled. This is in contrast to thermoset plastics that can be heated only once at which point an irreversible chemical change “sets” the material. Attempting to reheat a thermoset plastic after it has set (hardened) will result in burning rather than melting the material.
Elastomeric materials, typical of rubbers, are very elastic or flexible. Accordingly, they have low tensile strength and weak intermolecular forces. Think of a rubber band stretching very easily but yet breaking with relatively low applied force. Elastomeric materials are most often thermoset but can be chemically altered, often via the addition of sulfur, into a thermoplastic material.
Thermoplastic elastomers are often used for overmolded part design because they have advantages typical of both thermoplastic and elastomeric materials.
We are currently working on a few products for a client that will be hitting the market probably later this year or earlier next year that involve the use of overmolded TPEs. Here are a few tips from the shop floor:
TPEs are usually too soft to be machined. As a workaround, they can be approximated by casting with either silicone or polyurethane rubber. There are two different ways in which we overmold at Creative Mechanisms.
When we prototype TPEs we typically design and then CNC machine plastic molds in order to use them for casting a polyurethane rubber part. We can overmold over a rigid substrate, or we can just make a part 100% from TPE. The polyurethane rubbers can be specified to achieve certain hardnesses and colors. The downside to this method is that it can be time consuming and costly because the molds need to be designed and fabricated, and the materials need to be cast (which can take up to 24 hours to fully cure). The process is messy and requires quite a bit of equipment (which is not an issue for us but might be a problem for DIY prototypers).
Another alternative available is to use Polyjet 3D printing. A part created from Polyjet can be printed in many different colors and hardnesses. Complex geometry can be fabricated that is otherwise unachieveable with typical casting processes. Additionally, some Polyjet machines even let you print two materials at once, allowing you to print a rigid substrate and a soft overmold as a single unit. These are great for some applications, but the mechanical properties of Polyjet materials are not really comparable to production grade TPEs. In particular, the Polyjet materials have very low tear resistance and very little elastic memory.
In summary, the right process for prototype overmolding will depend on the needs of the particular job. For those jobs where time and money is less of a constraint, using CNC plastic molds and polyurethane rubber with a rigid substrate is typically the highest quality product. Conversely, more complex geometry and rapid prototype production can be achieved with Polyjet 3D printing. For those instances where material property replication isn’t a huge issue in the prototype stage, this is likely to be the best solution. We can help with both. Please feel free to contact us with questions.
For a full description of overmolding and a few examples of products that are frequently overmolded, read here.