Working With Polyurethane Foam

Within the O&P community 3D printing has generated a lot of interest lately. With exciting design possibilities, greater availability, and scanner portability, digital workflow has definitely arrived. While interest in printing is at an all-time high, I believe that the workhorse of computer-aided manufacturing is still carving polyurethane foam positive models for fabrication. Full disclosure: I work for BioSculptor Corporation, and we manufacture polyurethane foam blocks. To get further confirmation about the continued popularity of carving polyurethane, I checked with Rick Noel, director of PVA Med, maker of both carvers and printers. Noel tells me that he sells more printers than carvers, but he doesn’t see interest in carvers going away. “Many people still need to terminate on a positive model they can use as a fabrication mold, and I don’t think carbon fiber is going away anytime soon.” While polyurethane foam may be the less exciting uncle of the O&P CAD/CAM world, its use is still widespread and growing. Keeping in mind those fabricators not yet on the cutting edge, I thought that I could share some tips from my two decades of experience of fabricating sockets with foam. Cost is a critical element for every fabrication operation but be aware of the functional elements of the foam blank properties when selecting your foam. Blanks are available in all shapes, sizes, and densities. Larger blanks and denser foam will cost more. Not only does it make sense to use the smallest block possible, but also to choose the least dense foam that you are comfortable with. While many labs are successful with 2 lb. foam, I find foam that soft will deform easily while handling and can result in some unsatisfactory results. A comfortable density to work with is 4-6 lb. The amount of foam that winds up in your dustbin should also be a concern. For this reason, I find a tapered conical blank better matches a prosthetic limb shape, and elliptical better matches a spinal shape than cylindrical or rectangular shapes. Another functional consideration is to mold a work mandrel shape into your foam blank. This feature will help during the fabrication process. A mandrel with a repeatable zero position will aid you in identifying the line of progression of your socket and allow that model to be remilled if needed. Mold preparation is also critical to your success, and there are some significant differences between fabricating over foam and fabricating over plaster. First, verify that your finished model is exactly what you are expecting when you remove it from your CNC machine. This is less of a concern when using a plaster cast as you can always “trust the cast.” Do not get into the habit of trusting your carver, software, or scanner. This is the time to check your measurements and modifications, not when you are trying to get the socket on the patient. There is no shame in adding plaster to your work at this stage to head off an ill-fitting socket. Should you find any air pockets in the foam, you can quickly fill them with plaster or clay. If you are thermoforming, you will need to drill some additional holes around the trimline. Suction does not pass as evenly over the polyurethane foam as it does over a smooth plaster surface. For lock dummies or Grace plates, an additional hole through the distal center will ensure a nice flat surface. The most common complaint I hear from people is the stocking sticking to the plastic when forming over foam. The two-part solution is a double layer of stretch nylon and making sure that nylon is wet when you drape the plastic. The moisture of the nylon creates steam and gives the inside surface a nice finish. This works great for polypropylene, polyethylene, or PETG, but does not apply to Proflex or ThermoLyn sockets. For these other materials you can pull a PVA bag or latex bladder over a couple of nylons, carefully poke some holes throughout, and powder the model last. Check the inside of the socket for spikes; the plastic can be sucked into pinholes creating an uncomfortable spike. You can knock those off with a little sand screen and minimal effort. Demolding the socket can be surprisingly easy or as tedious as the traditional method of knocking plaster out with a hammer and chisel. Personally, I like the easy way. Wait for the plastic to cool enough that you can comfortably touch it with your hand. Drill a ¼-inch hole in the distal end and release it with a burst of compressed air. Exercise caution when using the compressed air and do so in short bursts. I have seen sockets shoot across the room using this technique from too much pressure. In cases where there is a dramatic undercut like a supracondylar wedge, you can make a vertical cut on the posterior of the socket proximally to the trimline, one medial and one lateral. This will allow the plastic to spread enough for the release. If this is a laminate, cutting and removing the material above the trimline will help, even with carbon sockets. Occasionally, air alone is not enough because the material is too rigid or the undercut too pronounced, and the socket will get stuck halfway off. This is where it’s helpful to either have foam with a T-nut inside of the foam and a threaded work mandrel (Figure 1) or a corkscrew mandrel (Figure 2) to secure the foam model to the bench vise. You can then tap the model off with a block of wood and a hammer. Once a socket with a strong undercut is demolded, the integrity of the mold is compromised and should not be used to fabricate another socket. [caption id="attachment_262400" align="alignleft" width="291"] Figure 1. Threaded work mandrel[/caption] I have also modified my lock dummies to make demolding easier. Drill a small hole all the way to the center of the dummy through the side of the lock where the bolt for the plunger release is. After forming or when the lamination has set, grind off the material from the surface of the bolt and remove it. Insert your air gun into this hole to shoot the socket off the mold. If you followed my tip earlier by drilling a center hole for suction, make sure your mandrel is solid and tight on the model, so you generate enough pressure to release it. [caption id="attachment_262397" align="alignright" width="308"] Figure 2. Corkscrew mandrel[/caption] There is another option available. The folks at Prosthetic Design Inc. have a very clever system that utilizes a push plate to push the mold out of the socket. This system is pricey, but well thought out and worth the cost. Despite the best efforts of the most experienced technicians, the socket does not always release. In this case, CAM can take a back seat, and hopefully you will be lucky enough to have some old pineapple rasps. Set your router at a slow speed, be cautious of your nylon, and hog that socket out like it’s a wood socket and it’s 1960. Chris Cummings has been working in O&P for 32 years. He is currently directing CFab operations and new product development at BioSculptor Corporation. He can be contacted at ccummings@biosculptor.com . Photographs courtesy of Chris Cummings.

Working With Polyurethane Foam

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