What materials are optimal for micromedical devices?
That is the topic that Patrick Haney, our R&D Engineer, addresses this commonly asked question in this MPO VideoBites with Sean Fenske. Watch now (or read the transcript below) for key insights!
Here are some highlights from the interview, edited for length and clarity.
Sean: Today we’re focusing in on micro molding, which is obviously very important to device manufacturing, and we’re specifically looking at materials. Patrick, maybe you can tell me: What materials are optimal for micro molding for medtech, and which ones should be avoided?
Patrick: That’s a good question. At MTD, I help with nearly all of the material selection processes with customers, and I actually get this question a lot. Honestly, my answer is that all plastics—all polymers—have their place in medical devices, particularly in micro-medical devices. It’s just a matter of finding its place.
For example, if you have a material that’s going to be implanted in the body, then looking out for things like leachables is of the utmost importance, right? So maybe a material like plasticized PVC goes away because the plasticizers might leach out. Similarly, acetal often can’t be implanted because of concerns of formaldehyde degradation products. There are a lot of material degradation considerations you have to make.
It really comes down to the geometry and the application demands—essentially, your part. To paint in broader strokes: when you’re dealing with ultra-thin mold features in micro molding, materials like polycarbonate or acrylic might have more trouble coercing into those tight windows.
Beyond that, you really just have to worry about what the material is going to do to the manufacturing equipment. If your detail in the mold is only a few thousandths of an inch in diameter and you’re using a material with a high glass filler content—or something that could release corrosive gases when it degrades, like a fluoropolymer—you have to be very concerned about mold maintenance.
But overall, really, every plastic has its place. It’s just about the nuances: whether the material is right for the specific geometry, the application, and the manufacturing equipment.
Sean: So moving toward that application aspect, can we talk about design considerations? Specifically, are the micro molding design considerations driving material selection, or is the application really driving material selection, with the design adjusted as necessary? What’s the best approach?
Patrick: It’s really both at the same time. Ideally, you want your design intent and application to drive the material selection. That’s always where I start: I ask, Where is it going? How long is it going to be there? What’s the temperature while it’s there? — questions like that.
Then we narrow it down further: What does the geometry look like? Is it possible? Is it going to be feasible to not only mold that design out of that material once, but can you build a robust, repeatable process window to validate it?
Oftentimes in micro-medical molding, a single-point process isn’t quite good enough. We need a window of processability for protection. So if you start by saying: Here’s our application. What should we do with the material? That’s the ideal place to start.
For instance, if you need a very clear, transparent part and want to make it out of polycarbonate, oftentimes there needs to be some compromise between what your minimum wall thickness is. In micro molding, if a cross-section of your part is surrounded by so much tool steel, and there’s not so much plastic melt there, it’s not going to take very long for the plastic flow front to freeze off. And then it’s a lot harder to mold.
So that’s the molding standpoint. The other consideration—and perhaps the biggest major limitation—is mold design.
Let’s say you have a couple of standing ribs in your part, and there’s only 12 thousandths of an inch between each rib. That would require a 12-thousandth blade of tool steel to be in the mold cabinet to create that feature. And if it’s cantilevered because they’re standing ribs, now you have injection pressure—maybe tens of thousands of psi—that’s influence a 12-thousandth-thick piece of hardened tool steel.
A lot of times, things can get lost in looking at a SolidWorks model. You may think, Okay, this looks good to me; I meet my nominal wall thickness and all my regular plastic design rules. But when you shrink it down to the micro scale, you have to be careful not to create what we call a “bad steel condition,” where mold maintenance becomes a nightmare—or not even practically possible to keep up with at all.
All plastics—all polymers—have their place in medical devices, particularly in micro-medical devices. It’s just a matter of finding its place.