Incorporating Metal Hardware in Laminations
July 2016 Issue
I imagine that many of us do not enjoy the process of riveting metal uprights onto KAFOs or the occasional metal hardware that we have to add to our devices. I find it frustrating to have copper rivet heads against the inside or, worse yet, tearing up the pretty foam pad when I install them. To remedy this, I've created my own method of laminating them directly into my device and eliminating any need for rivets. The biggest advantage to this method is a much stronger connection. Instead of drilling holes into my uprights, I simply let the E-R Resin flow and surround the metal. This prevents gaps and movement, which means the joints will not break.
If you have doubts, look at Fawn, a Jersey cow, wearing her KAFO with Becker Polycentric stainless steel knee joints incorporated in the lamination. She weighs around 1,000 pounds. I'm well aware that there are weight limits posted on these devices. My friend, Rudy Becker III, president and CEO of Becker, Troy, Michigan, has informed me I'm exceeding the weight limits. (Thanks Rudy.) Then why don't they break under Fawn's weight?
Because they are aligned perfectly and they are laminated with E-R Resin. Since it's a true epoxy resin, E-R Resin will bond to the metal if you roughen the surface. Vinyl esters or so-called epoxy acrylic resins won't do that; they don't bond as well. By not allowing any movement of the uprights and not weakening them with rivets, we are getting the full benefit of the strength of the steel.
I also incorporate T-nuts in the lamination everywhere I want to install a strap or anything else I want to connect. In Fawn's case, I wanted straps and a channel to form a slot for a bungee cord that I use as an extension assist. Using T-nuts allows me to quickly change a strap without grinding a rivet head off. Simply loosen a screw, replace the strap, and put the screw back on.
Start by pulling padding material on the mold while it is under vacuum, and sealing all seams. I usually use AliPlast or Pelite for this. Make sure there are no seams in what will be the final socket area, and then lightly scratch the outer surface with a sand screen. After that, bond cutoff strips in place near the intended trim lines to allow the project to be cut off without damaging the mold.
Next, laminate a thin, two-to-four-layer socket directly on top of the foam padding. Now the padding is part of the project and will not move around. After the lamination is set, remove the PVA bag and place the socket in a pipe vice. I use a Townsend alignment jig to line up all of my joints; I find it easy to use and it guarantees that the joints are aligned correctly. After the contours are bent to closely match the shape (it doesn't have to be perfect like when using rivets), tack bond the joints to the socket in several places with an epoxy adhesive. Remember, bonding epoxy to epoxy means no roughing up is necessary.
After the joints are lightly tacked with adhesive, pack plumber's putty around the head of the joints wherever the resin shouldn't be. Take special care to get putty under the joint head as well. Brushing a thin layer of petroleum jelly over the joint heads also works. After packing the putty around the joint heads, wrap electrical tape tightly around the device to secure the putty.
In the picture above at right of a pony's KAFO that I fabricated, notice the stainless bolt and extra bracket. This setup was used to apply a downward and posterior force on the pony's hoof section to gradually stretch the tendons. It was all laminated in. The client or the veterinarian could now make minor adjustments by tightening the bolt head and creating tension.
Pack the threads of the T-nuts with plumber's putty and use Super Glue to lightly bond them wherever straps should be mounted. Use just enough glue to secure the T-nuts until you get the layup in place. Now wipe the entire laminated surface with either alcohol or acetone; this cleans the skin oils off the surface. Pull the second layup material over the device. Laminate with caution; don't hurry. Allow the resin to flow under and around all of the metal parts. Once the resin starts to set, turn a household fan on to keep the thicker areas of resin cool. This keeps it from gassing up.
Allow it to cure for several hours or overnight, if possible. Then strip off the PVA bag, wipe the device with alcohol or acetone, and pull a very thin nylon material over it. I place my logo (or the client's or animal's name) and any other identifying codes I want in it, and I laminate it with clear epoxy E-R Resin. Turn the bag inside out to get a shiny surface. Poke little holes in the paper with the logo or other identifying information to allow air to dispel while resin flows through it.
When it's cured, cut along the cutoff strips and gently pull the device off of the model. Use a cast cutter, Dremel tool, coping saw, or hacksaw to gently cut around the joint to free up the various sections. Once they are apart, use air to blow all of the putty out.
Trim the device to allow it to fit back onto the cast. Do not round the edges of the interior foam padding-just lightly smooth the edges. Round the laminated edges and polish them if you like.
Place the device back on the model and pull nylon material over it. Then pull another AliPlast or Pelite over that, making sure to seal all edges. Laminate the complete second half and take care trimming it out so as not to hit the first section. Cutoff strips help with this too, if you are not comfortable using a cast cutter.
I originally developed E-R Resin to help Aaron, a 1,600-pound horse in Austria that required working knee joints and the replacement of about eight inches of his lower leg. Aaron lived another eight-and-a-half years and wore out three devices before passing away. None of the joints ever broke or loosened. His device weighed a whopping 7 pounds when completed. Fawn's hinged right leg brace weighs 5 pounds and she weighs 1,000 pounds. We incorporate all joints in the lamination on all the braces we fabricate, no matter how big or small the user. It's simply a stronger method than rivets.
Ronnie N. Graves, BOCPO/L, CO, CTP, is the owner of Prosthetics Research Specialists, Bushnell, Florida, and has worked in the O&P industry for more than 35 years. He specializes in hard-to-fabricate devices and teaches fabrication techniques. Graves is also president of Florida Disaster Animal Response & Transport, Bushnell, a nonprofit organization that specializes in assisting in animal rescue, particularly in disaster situations.