Pushing Boundaries – Rapid Prototyping Foam

Rapid prototyping is awesome — especially when we get the chance to push the envelope a little. Sometimes while engineering products for our customers, we end up with needs beyond the normal. This project example is one of those, and it comes in the context of needing to rapid prototype a simulation of foam. But it’s more than just foam, it’s simulating various densities of foam with a thick upholstery over it, bonded to it. Can we do that with simple 3D printing?

Rapid Prototyping (aka 3D Printing)

The field of rapid prototyping is expanding rapidly with lots of new technologies. And, the new techniques help us with all sorts of new ways to prototype in product development. We can now 3D print metals and several versions of plastic that approach molded strength. Oh, and the price is down to the point that some parts are even printed for production. Ok, I know this is not new news — hey, some of the parts we designed a few years ago are now in production by rapid prototyping.

Intrusion Deflection With A Bicycle Seat — For perspective, think of a Bicycle Seat. It’s a common example of upholstery bonded over foam for padded body contact. Getting the right “feel” for a body pad is extremely important for comfort.

What is new, at least for us and our rapid prototyping vendors, is simulation of upholstered foam. In this case, for upholstered rest. Can we simulate the right “feel” with 3D printing?

Engineering Feel

We use 3D CAD modeling in all sorts of complex shapes in daily routine. Making it look cool or smooth or stylish is not a problem. However, making the design “Feel” a particular way is another story.

There are a lot of great engineering tools help parts to fit perfect, or have strength in the right areas. It’s easy enough to calculate thermodynamics or stresses and deflections too. However, I don’t know of ways to engineer how something “Feels”, because “Feel” is a combination of touch senses — texture, emissivity, temperature, pressure sensitivity, deflection, etc.. And, it’s an interpretation by the individual. Sure, there are measures like durometer, but numerical guesses don’t always add up to “Feels Good!”

For instance, knowing just how much spring force to get the right “Feel” in a mechanism is tricky. Finding just the right shape and contour for a handle or button (or body pad), for instance, is another tricky task. Thank goodness for rapid prototyping that allows us to make several versions and hone the “Feel” just right.

This project brought on another aspect of engineering “Feel”. Thinking about body padding, it’s something pretty intimate. You lean on it, and if it does not feel right, you know it pretty soon. But it’s more than just “shape” and “fit”. There is a “Feel” aspect that is not easy to define. So, how can we simulate not only the shape for testing, but also the “Feel” of the foam? Can we prototype it quickly?

Materials for Rapid Prototyping

Material is a big part of the “Feel” equation. Yes, they do make various 3D printing elastomer materials, but the elastomer by itself does not simulate the kind of “Feel” we want for a body pad.

Our first attempt was to use various durometers of elastomer for the pad. Even the very elastic materials from a Connex Polyjet don’t really make the simulation. Unfortunately, that prototype attempt failed before it got off the ground. Though it was a good thought, that 3D printing process could not do what we needed. Strike one.

For a second attempt, the prototype uses material of much higher durometer, but hollow, having a supportive fill structure inside. Actually, we didn’t build them, GoProto did the building for us.

3D Printing 3 Block Test — Samples of infill density with rapid prototyping (3D Printing).

The photos show three blocks made with different sizes of “crosshatch” inside. The “Feel” of the block is not like foam, but it is much like upholstery over foam. When you think about the upholstery over foam in a pad, it’s not soft fabric like your couch. Also, the foam under the upholstery on a body pad is much denser than your couch. Put them together, and that’s kind of how this feels.

Rapid Prototyping the Pad

These blocks provided a good direction, but there are limitations. The corners, for instance, create a false stiffness. Cut them away, and the right “feel” starts to come out. That’s good, because body pads don’t have sharp corners.

Through experimentation, we found several important aspects of feel using this technique. Here are the four big ones:

First is thickness of the skin or shell. Too thin and the infill does not support well. That’s probably self explanatory.
The Second aspect is density of the infill. With more inside build, there is a greater resistance to squish. Again, that seems pretty obvious.
Third is the rapid prototyping build direction. One feel is attained pressing perpendicular to the build direction, and quite a different feel is attained pressing along the direction of build. It stands to reason, of course. Pressing into the squares you see in the photos causes buckling (effectively) of these internal “ribs”. Where pressing on the side causes a flex or bending of the ribs. This being said, it’s not as intuitive as you might expect. Yes, there is a different “feel”, but it’s a bit surprising — especially when comparing infills.
Fourth is the material durometer. For what it’s worth, we used much harder durometers than we would have expected. Hard, but rubbery material actually turns out the best. Harder than expected. The same material in solid form feels, well, solid.

There are several other key points that make it work, but we won’t go into them right now. We’re still learning the effects, and the desired settings.

Going beyond the 3D Printing off foam, all these parts require some support — some with mating rapid prototyping “hard” parts, and some with machining.

Stay Tuned

This project is not over, but the learning is just so cool that it’s valuable to share. Please leave a comment below, or on the Contact Us page if you have input to contribute. We certainly do appreciate it. Also, let us know if you like posts about rapid prototyping.

Come back in a while and we’ll update this post (or add a new one) with more information about how the prototypes turn out. In the meantime, The Engineer’s Perspective has a bunch more to read. Until then, be well.