Double shot, double trouble

In the October issue, I suggested that the use of a common chassis with overmolded detail could allow multiple part numbers without a major redesign or mold investment.

One of the major advantages of molding parts in two stages is the ability to use a cheap, commodity resin as a substrate, and overmold with an engineering grade or high-value resin on top where the end user can see and feel it. Although overmolding isn’t new, getting the second shot to stick in demanding applications like automotive OEM parts with integral seals or gaskets can be a frustrating experience.

A personal “favourite” in overmolding problems is shooting onto heavily filled resins. There are lots of good reasons for designing with filled substrates, but they generally boil down to cost.

With the right fillers, cheap resin can perform like many engineering grades. And with the ability to shoot a thin layer of specialty polymer on top, issues like surface finish, hardness/stiffness and colour are relatively easy to solve.

The real issue is at the interface between the substrate and the overmolded resin. The two materials bond by different methods. The “best” is at the molecular level, with the contact zone forming a sort of blend or alloy of the parent resins, while the “worst” uses mechanical interlocking of the two polymers by holes, channels or undercuts in the substrate.

The ideal method naturally depends on the materials, process (two-shot or insert), part shape, cost, et cetera, but from a production perspective, your ability to alter the process is pretty limited once the engineers have made their choices. Two-shot is the “Cadillac” — or, more accurately, the “Concorde” — technology if speed and volume matters, so making the overmold stick quickly is also a concern.

In our filled resin substrate example, it’s common to run the primary shot a little hot. While this is often thought of as a way to increase “tack” before the overmold, the key is to make sure that the filler is evenly distributed throughout the resin before it hardens. Why? The overmold needs to see polymer, not filler to adhere consistently, and as anyone who runs heavily filled commodity resins in high-speed operations knows, it can be surprisingly difficult to keep the filler evenly dispersed thorough the bulk of the part.

While there is a lot of physics and chemistry going on, I like to distil press-side issues into simple strategies. Did the process work before? The obvious first step is to find out what’s changed since the last high-quality run. New supplier? Different resin lot? Any changes to machine settings?

I’ve worked on problems that took hours to troubleshoot that could have been solved in minutes with the SPC data locked in the QA manager’s computer. If the application of brainpower doesn’t help — not uncommon with a new job — seek help starting with the supplier of the overmold resin, not the substrate.

That search begins with a scan of the product literature to determine baseline machine settings. I like to open a “problem” file, making the troubleshooting process like a miniature engineering project. This is great when you get conflicting recommendations from in-house and supplier engineering staff. I’ve seen two different engineers from the same supplier make different recommendations, and nothing cuts through the confusion faster than a note that says, “Bill recommended 420 degrees at the centre on Wednesday the 21st.”

Provide the overmold resin people with the full specs on your substrate resin. Yes, they had it when they spec’ed the product in the first place, but how do you know if the troubleshooting engineer has that information? Ask the substrate resin compounder about other companies that are using their product in two-shot operations, and give them a call. Unless they’re a direct competitor, this kind of dialogue can pay off for years.

Unless, of course, they hire away your best troubleshooter. But that doesn’t happen in our business, does it?