Technician's Corner
KAFO in Two Pulls
By Steve Hill, CO
There are times when vacuum forming a KAFO in one
pull just isn’t an option.
Your oven may not be big enough to accommodate a large enough sheet of plastic to cover the cast. Maybe you’d like to pull the proximal section with co-poly and the distal section with polypro. Perhaps the other tech in your lab is out, so you need to pull the plastic on a long leg all by yourself and you can’t afford to mess it up.
There may be some times where you can figure out how to avoid spending the extra time needed for two pulls. For example, you may simply want two different types (or thicknesses) of plastic at the top and bottom. Well, you don’t actually have to pull the plastic twice. You can just heat the two sheets of plastic at the same time with a slight overlap, fusing them together and then pull as one piece.
Other times there may just be no way around it and the job must be done in two pulls. I’ve seen techs accomplish two pulls in many different ways, but all accomplish the same three basic principles of vacuum forming.
Your oven may not be big enough to accommodate a large enough sheet of plastic to cover the cast. Maybe you’d like to pull the proximal section with co-poly and the distal section with polypro. Perhaps the other tech in your lab is out, so you need to pull the plastic on a long leg all by yourself and you can’t afford to mess it up.
There may be some times where you can figure out how to avoid spending the extra time needed for two pulls. For example, you may simply want two different types (or thicknesses) of plastic at the top and bottom. Well, you don’t actually have to pull the plastic twice. You can just heat the two sheets of plastic at the same time with a slight overlap, fusing them together and then pull as one piece.
Other times there may just be no way around it and the job must be done in two pulls. I’ve seen techs accomplish two pulls in many different ways, but all accomplish the same three basic principles of vacuum forming.
Vacuum forming principles
Traditionally, a vacuum source is connected to a vise, which allows air to be withdrawn from around the vise mouth. A mandrel (usually a pipe) coming out of the top of the cast is then inserted into the vise and heated plastic is draped over the cast. When a seal is established, the vacuum evacuates the air from the space between the mold and plastic. Once the air is gone and the plastic has cooled, it hardens again, taking on the shape of the mold.
The same principles apply when vacuum forming a KAFO in two pulls. However you do it, you need to:
Here’s how I do it.
Traditionally, a vacuum source is connected to a vise, which allows air to be withdrawn from around the vise mouth. A mandrel (usually a pipe) coming out of the top of the cast is then inserted into the vise and heated plastic is draped over the cast. When a seal is established, the vacuum evacuates the air from the space between the mold and plastic. Once the air is gone and the plastic has cooled, it hardens again, taking on the shape of the mold.
The same principles apply when vacuum forming a KAFO in two pulls. However you do it, you need to:
• create a seal between two points,
• create vacuum between these two points, and
• seal a heated sheet of plastic over these two points to form it to the shape of the mold.
• create vacuum between these two points, and
• seal a heated sheet of plastic over these two points to form it to the shape of the mold.
Here’s how I do it.
Vacuum forming in two pulls
Vacuum forming the proximal section can be accomplished easily enough. The set up for the vacuum doesn’t change; it remains attached to the end of the vise, with the mouth of the vise wrapped in electrical tape.
What is different is the need for another point at which to seal the plastic—let’s call it seal point two. I usually select somewhere just below the distal trim of the proximal cuff or, more often, at about knee center. Make sure this seal point is relatively smooth. Sureform or sand it until there are no big dips or bumps.
Wrap black electrical tape circumferentially around the mold to create a smooth tape coating approximately 3 inches wide. Apply one end of a stocking to the vise as you normally would. Tape the other end of the stocking to the black electrical tape at seal point two. Apply at least two layers of stocking so that you create an adequate air wick.
Add another layer of electrical tape around seal point two and wrap a layer of foam around this area as well, but only tape the side of the foam facing the foot of the mold. Leave the foam exposed at the top so that the plastic will have something to stick to. Again, make sure the foam is sealed to seal point two. Now just drape the heated plastic over the thigh and seal it to the vise at the top and the foam at the bottom. Cut the plastic off after it cools.
To form the distal cuff, prepare seal point two the same way. This time, however, add a golf-ball-sized hunk of molding clay where you want to attach the vacuum line. Apply a six-inch metal tube about midway along the clay, so that one end faces the area to be pulled and the other end can be attached
to the vacuum line. I prefer copper tubing because it’s more bendable.
Wrap it with another layer of black electrical tape and apply a wad of stocking over the end of the vacuum tube to prevent the plastic from cutting off the air flow. It can be held in place by the stocking, which is drawn up from the foot and taped at seal point two, just as described earlier.
Add the layer of foam around it so the plastic has something to stick to and then apply the vacuum tube. The whole thing should be airtight once the plastic is sealed over it. Vacuum form the mold as usual.
Vacuum forming the proximal section can be accomplished easily enough. The set up for the vacuum doesn’t change; it remains attached to the end of the vise, with the mouth of the vise wrapped in electrical tape.
What is different is the need for another point at which to seal the plastic—let’s call it seal point two. I usually select somewhere just below the distal trim of the proximal cuff or, more often, at about knee center. Make sure this seal point is relatively smooth. Sureform or sand it until there are no big dips or bumps.
Wrap black electrical tape circumferentially around the mold to create a smooth tape coating approximately 3 inches wide. Apply one end of a stocking to the vise as you normally would. Tape the other end of the stocking to the black electrical tape at seal point two. Apply at least two layers of stocking so that you create an adequate air wick.
Add another layer of electrical tape around seal point two and wrap a layer of foam around this area as well, but only tape the side of the foam facing the foot of the mold. Leave the foam exposed at the top so that the plastic will have something to stick to. Again, make sure the foam is sealed to seal point two. Now just drape the heated plastic over the thigh and seal it to the vise at the top and the foam at the bottom. Cut the plastic off after it cools.
To form the distal cuff, prepare seal point two the same way. This time, however, add a golf-ball-sized hunk of molding clay where you want to attach the vacuum line. Apply a six-inch metal tube about midway along the clay, so that one end faces the area to be pulled and the other end can be attached
to the vacuum line. I prefer copper tubing because it’s more bendable.
Wrap it with another layer of black electrical tape and apply a wad of stocking over the end of the vacuum tube to prevent the plastic from cutting off the air flow. It can be held in place by the stocking, which is drawn up from the foot and taped at seal point two, just as described earlier.
Add the layer of foam around it so the plastic has something to stick to and then apply the vacuum tube. The whole thing should be airtight once the plastic is sealed over it. Vacuum form the mold as usual.
Find your own solution
There are a great many other ways to do this. I’ve seen techs cut the KAFO cast in half and fill the two sections separately. This has to be done very carefully in order to retain the relationship between the two parts and maintain proper alignment.
Another method uses a hollow pipe down the center of the cast when it’s filled. Attach the vacuum to the top of the pipe. That way you can just drill a hole through the plaster and into the pipe anywhere you need vacuum. It’s not always easy to hit the pipe, but it can be done.
There are other ways as well, too numerous to mention in just one article. Different circumstances may require unique solutions. They all have the same basic ingredients. You need to be able to make a seal, create a good air wick and apply vacuum. If you accomplish those three things, you can vacuum form plastic anywhere you like.
When more traditional methods don’t accomplish your goals, just keep those objectives in mind and troubleshoot your way around the problem.
There are a great many other ways to do this. I’ve seen techs cut the KAFO cast in half and fill the two sections separately. This has to be done very carefully in order to retain the relationship between the two parts and maintain proper alignment.
Another method uses a hollow pipe down the center of the cast when it’s filled. Attach the vacuum to the top of the pipe. That way you can just drill a hole through the plaster and into the pipe anywhere you need vacuum. It’s not always easy to hit the pipe, but it can be done.
There are other ways as well, too numerous to mention in just one article. Different circumstances may require unique solutions. They all have the same basic ingredients. You need to be able to make a seal, create a good air wick and apply vacuum. If you accomplish those three things, you can vacuum form plastic anywhere you like.
When more traditional methods don’t accomplish your goals, just keep those objectives in mind and troubleshoot your way around the problem.
Steve Hill, CO, is secretary of OPTA and CEO of
Delphi Ortho, located in Asheville, N.C. and on the Web at www.delphiortho.com.
