Fix the Problem VI: The Role of Material Microstructure in an Overmolding Application

While working at Ford Motor Company in Sandusky, Ohio – aside: home of the incredible amusement park Cedar Point – I worked with two other engineers on “Integrated Molding and Assembly” (IMA) as a new technology to replace using glue in automotive lighting.  Our first application was on the Ford Econoline headlight, as highlighted here on a portfolio page (alongside a small test piece, which I reference below).  This process was patented; my first patent!

Essentially, this process molded a seal bead of plastic, in the case of the Econoline made from polycarbonate, to join two other pieces – the lens and the body, also polycarbonate – and form a bond that was strong and leak-tight.  The process also worked with using acrylic and ABS bodies and either acrylic or ABS as the seal bead.  In almost all cases, burst testing cracked the substrate parts before the seal joint broke.  Descriptively, this was an overmolding process – a seal was overmolded onto the two substrate pieces.

Giddy with success including a successful launch on a real product (the Econoline), and as the plant was casting around for other potential product lines to bring in to diversify the plant’s product mix, we took a long look at nylon intake manifolds.  The idea was that two halves of the manifold would be molded, and in either a rotary or shuttle mold the two parts would then be joined, and a seal bead overmolded; resin pellets in, complete manifold out.  The patent space was already crowded for this product and similar process technologies, but we and the Ford attorneys believed we had something unique that could be done in a patentable way.

So we molded a bunch of lenses and bodies in our little test mold (again, see my portfolio page, link is above) made out of nylon 6/6 to test the concept; this being the standard material for the application.  We put them in the mold, overmolded a seal bead out of the same material… and we could pop the pieces apart with our hand.  There literally was no adhesion between the seal bead and either of the base parts.

Panic set in.  Everyone was wondering what was going on; it had worked so well before.  And then… I had an insight.  All of the materials we had molded and sealed with enormous success were amorphous plastics.  Nylon, in contrast, is a crystalline plastic.  There is a huge difference between these in several dimensions, and the one that mattered was heat capacity; crystalline materials typically have a higher heat capacity because of the material’s microstructure.

For a strong bond in an overmolding application, the overmolding material needs to remelt a microlayer of the substrate to which it can then bond.  Because crystalline materials have a higher heat capacity, more energy was needed to form that remelted microlayer when we had nylon parts; more energy than the molten seal bead of the same material had available.

To test this theory I arranged to use Amodel, a material that’s similar to nylons chemically but with a higher processing temperature, as the seal bead on our nylon 6/6 substrates.  This time it worked; the assembly didn’t fall apart.  Clearly I was on the right track; it was the crystalline nature of the substrates that was the issue.  When we switched to nylon 6 for the substrates, a nylon also used – though not as commonly – for intake manifolds, it worked even better… because nylon 6 has a lower melt temperature and heat capacity. This allowed for much better formation of that necessary remelted microlayer.

The project was now a “GO!”, but we found out that in the course of talking with one supplier, we had not obtained a non-disclosure agreement (NDA) first.  Ford’s lawyers said this ruined any patent possibility we might have had.  Without the ability to legally protect the technology, the project died.

So here are the key points to take away from this essay:

1. When overmolding, amorphous materials are generally easier to overmold onto than crystalline ones.
2. If overmolding onto a crystalline material, make sure the overmolding material is molded hot enough to overcome the higher heat capacity inherent in the substrate’s crystalline structure (even if amorphous, mold the lower melt temperature material first, and use the higher temperature material as the overmolded material).
3. When overmolding, the more similar the overmold and substrate materials are chemically, the better; here’s a good Rule of Thumb: If they can be alloyed (e.g., PC/ABS alloys), they’re similar enough for a good overmold application.
4. I strongly recommend adding features to physically “back up” the ability of the two materials to engage with each other from overmolding and thus resist whatever forces will be applied. I.e., use keyway features, flanges, and grooves to form a physical interlock.
5. When you have a concept that you even think might be patentable, make sure you protect yourself with NDAs before you talk with anyone outside.  A few days of delay while getting things signed can save a new technology from being ruled not patentable.

© 2014, David Hunt, PE