So how does it all come together?
The first thing to do, after a little question and answer session with the client to determine needs, is to make an impression of the feet. It helps sometimes to know what expectations you are trying to meet. This is not to say that you want to do any sort of foot examination before molding. I used to do this, but I found I was being a little too subjective, perhaps looking for things that were not there. It is better, I think, to just take the impression, as the casting media is entirely objective, and couldn’t care less what I think. It just makes an analog of the plantar surface of the foot. Sometimes you find neat surprises when you don’t start with preconceived notions about how the bones in a foot will behave.
I use a casting unit made by the French company, Sidas. This is similar to a unit sold by Instaprint. There is a pair of silicone bags filled with glass beads or something similar. Once the impression has been made, but before the foot has been removed from atop the bag, the air can be drawn out of the bag with an integral vacuum pump, fixing the casting media in place until the vacuum is released. The really nice thing about this unit is that if you think you have made a poor impression (of the foot, not of yourself on your client), you can simply erase the impression and do another. This can happen if the client moves around on the chair without telling you, or if the heel pocket is not deep enough, or if the impression went too deep and the foot bottomed out in the mold. Either way, you can get a good look at the impression and verify that it ‘looks right’ before committing more consumable materials to the process.
So, position the client in the chair, socks off, and verify that their lower leg length is sufficient to allow the foot to rest in the casting media. Those with shorter legs need to slide forward to the edge of the chair, otherwise the contour will be adversely affected. Position the knee such that it is directly over the foot, and ask the client to hold the knee in that position with a free hand. Make sure that the foot is not excessively plantar or dorsiflexed, as this will also affect the mold. Position the foot in what is called ‘sub-talar neutral’, and carefully push the foot into the mold, holding the foot from below the medial and lateral malleoli (that’s ankle bones to those of you who have not cruised through A and P recently.) The idea is to allow the resistance of the casting media to displace the mobile bones until they have stacked up against the adjoining bones. This will represent the bones in relation to each other when they have been sufficiently supported.
Turn on the vacuum pump, ‘freeze’ the mold, and repeat on the opposite foot. I am in the habit of doing the right foot first. No particular reason, but in my shop the right foot is closer to the convection oven. Not that it means anything…. Sometimes, body morphology/skeletal conformation will require the client to rotate slightly in the chair to avoid tension on the lower leg and foot. Pay attention to detail.
Once you have verified that the casts seem legitimate, turn on the oven and heat up a set of blanks. Blanks come in several sizes. It is to your advantage to use the smallest size possible for a given foot, to avoid excessive grinding to remove the excess. You want to choose a blank that is just slightly wider than the foot, and just a bit longer, taking into account that the contours of the mold will use up length and width.
Heat the blanks in a convection oven at just under 300 degrees (Fahrenheit, in the event you are reading this from Canada or the EU), or whatever temperature your supplier recommends, until they become flexible. If you overheat the blanks, they will shrink a bit and become thicker and harder to work with. At least mine do. Once flexible, quickly and carefully lay the blank into the mold, making sure that it is centered. Lightly smooth the blank into the contours of the mold, and then place the clients’ foot on top of the blank, in the appropriate position. Hopefully, the blank will just barely peek out from the sides of the foot, with a bit more ahead of the toes. Repeat for the other foot. If the client is talkative, I usually put forth a “Hold that thought” until both blanks are securely in the molds. The client should hold their knees over their feet, but should not push down on the molds.
Once the blanks have begun to take a set, it is ok to remove the feet from the molds. This will allow the feet to relax, and also hasten the cooling of the blanks. Time is money, after all.
While the blanks are cooling, cut the posting foam from the bulk sheet. Better to cut a bit big than too small. It is possible to purchase precut posting foam, but feet come in so many shapes and sizes, and I figure you can get more use out of a bulk sheet if you plan and cut carefully.
When the blanks are cool enough to pull out of the casting pillows, find a dusty surface and rock the cooled blank from side to side. The dust will trace a line along the ball of the foot, illustrating the line where the posting foam should terminate at the metatarsal heads. If you post beyond this point, the forefoot may be uncomfortable, and worse, you can cause nerve impingement. Not funny. Use a ballpoint pen and trace over the dust line.
If I am using the harder posting foam, or if the foot is fairly large, I will put the blanks outside for a few minutes of further cooling. Meanwhile, the posting foam goes into the convection oven to soften.
You want the posting foam to be soft enough to take on the contour of the molded blank when you put the two back into the casting pillows. Some formulations of foam will grow when they get hot, others will shrink slightly. Either way, more heat will make for a better contour match, but don’t overdo it.
When the posting foam has softened, quickly lay it into the mold, place the contoured blank back in the mold on top of it, and then place the foot back on the top of the blank. It is okay, in fact beneficial, for the client to lean on the foot slightly, like Rodin’s Thinker. This crude press will ensure that the contours of the blank and posting material match, which makes for a more positive glue-up. Make sure that the foot is lined up properly on the blank, and that the blank is lined up properly in the mold. Mold with the other foot at the same time. Posting foam cools much faster than the molding blanks that I use.
When the foam begins to firm up, trace the outline of the contoured blank onto the foam, again with a ballpoint pen, without moving the foot. Make an index mark at the side of the heel pocket as well. These contour lines and the index mark will make it much easier to line up the two materials when you glue them together. With my glue at least, you really only get one chance. I am right handed, so the index mark goes to the right side of the heel, as viewed with the toes pointing my way.
Once the posting foam has cooled, take a deposit for your work, and send the client on their way. No need for them to listen to the grinders. I tend to do better work when I’m not being watched anyway. Make note of any weird bone deformities, as they may affect the way you do the ‘neutral grind’. For instance, the plantar surface of most heel bones is symmetrically curved. However, I have known a few people with broken heels, and the contour is not even. Same thing for hindfoot deformities. A lot of things show up in the mold that wouldn’t show up otherwise, so look them over before you lose reference points. Don’t make assumptions about metatarsal mobility at this juncture, as excess material can obscure reality.
If you are lucky, the posting foam will pick up the pen line across the metatarsal heads. If not, manually transfer this line, and cut away the foam ahead of this line. Be conservative, as you want this line to match as closely as possible. I like a utility knife with a fresh blade.
When you get a good match, bevel the edge on the drum grinder. The aim is to feather the edge where it meets the contoured plastic, to make later grinding easier, and so plantar grinding won’t pull the posting foam away from the contoured blank. To clarify, you are beveling the outside of the posting foam, the side that does not touch the blank.
Knock away any foam dust, and apply a thin coat of adhesive to the mating surfaces of the foam and blank. Apply the glue evenly, and brush past the pen lines and all the way to the edge of the blanks. Do not apply glue past the line on the blank that marks the metatarsal heads, or if so, not by much.
When the glue is dry, which will vary with the amount of solvent in the glue and the temperature in the room, carefully line up the index and contour marks, and lay the blank into the posting foam. This takes a good eye, steady hands, and some practice. The closer you get, the better the bond. If the materials mesh well, then firmly squeeze the materials together. The edges will often need a second press, as that is where the contour of the posting foam is ‘rolling away’ from the contour of the blank.
Cut away the excess foam from around the periphery with the utility knife. As the glue will not fully set for a while, make your cuts in such a way that the foam is not pulled away from the blank. Do not tolerate a dull blade, as, if you slip, you may cut yourself, or take a slice out of the blank. If you do that, you might have to start over on that foot.
Now it is time to begin removing everything from your block of marble that does not look like a horse.
A footbed is supposed to support the foot, not wrap the foot. Imagine the periphery of the foot, and the contours of the bones under the skin. As you remove the excess material from the sides of the footbed, grind as though the footbed was resting on a flat level surface, and grind perpendicular to that surface. It is again better to cut slowly and conservatively, as you can’t really make a footbed out of foam dust. I tend to remove a bunch from the arch side, then the lateral side, and then around the back. Then I take another pass, and look for the emerging shape of the foot.
As a guideline, you can envision a line that starts at the heelbone, just below the point on the lateral side of the calcaneous where the curve starts to go vertical. Extend that line forward to the same area on the outside of the distal end of the fifth metatarsal. When you have removed almost enough waste material, the contour of the fifth metatarsal will begin to show. It is hard to describe when enough is enough, but again, if in doubt, it is better to leave material.
Employ the same methodology on the medial side of the footbed. Bearing in mind that, depending on the mobility of the first metatarsal, the outer curve of the metatarsal head may not be very distinct. Finish off the contour around the back of the heel, and repeat with the other foot.
(That orange boot isn’t mine. I’m just holding it for a friend…)
Grinding away the peripheral waste is one of those things that become easier with practice. At this point, I let my hands do the work, and, for the most part, just sit (stand) back and observe. Most of the peripheral grinding is done on the 4” flat belt, and 3” drum grinder. I like to use the flat belt to rough out the straight lines along the side of the foot, and use the drum to finish those off and contour the heel pocket. Roughing abrasive is 50 grit.
Set the footbed on a flat surface (not the chair pictured) at eye level and, viewing from the arch side, rock the footbed laterally until the curve at the bottom of the 5th metatarsal head and the calcaneous are the only parts in contact with the flat surface. Note the contact point of the calcaneous, and mark this point with a short line that runs across to the top of the footbed. Repeat with the other footbed.
Point of contact at the heel.
Bear in mind that if you know the foot is more mobile on the lateral side, you would reverse your point of view.
I built a grinding machine that removes most of the remaining plantar surface waste material in one shot. Once the footbed has been positioned in the way I think it needs to be (takes some experience) the footbed is secured to a fixture, which is held steady by linear guide bearings, and lowered onto another flat grinding belt. The fixture has two contact points that bear on the top of the footbed, one at the line that represents the ‘low point’ on the calcaneous, and another at a similar point on the 5th metatarsal head. The heel contact is set first. As the medial/lateral plantar curve of most heel bones is symmetric, I set the footbed in the grinder to represent a vertical heel, using the ‘roll’ of that curve as a reference. This is a little harder to do if the contoured blank has a multi-colored top-sheet. For that reason, and for aesthetics, I spec a single colored top sheet from my supplier. It only comes in red, but who cares. I just think of Henry Ford and the Model T “…Any color you want as long as it is black…”
Position the footbed and bring the carriage down until the heel contact is set. Lock the carriage in the down position and lock in the forward contact. Grip the arch and forefoot with the specially made clamps. I made these out of small vise-grips and dial indicator holders. Bring the backstop forward to stabilize the heel cup, unlock the carriage (spring return), turn on the dust collector and start the grinder. The clamps and backstop prevent the footbed from being dragged backward into the suction port, while the contact points provide enough down pressure to ensure an even grind.
As the belt is bearing on a large area of foam, is a good idea to grind slowly and raise the carriage periodically to avoid overheating the foam. Grind just until the blank begins to show through the foam at the contact points. Turn off the grinder, release the footbed from the fixture, and repeat with the other footbed.
When both plantar surfaces have been rough ground, finish this grind by eye on the 4″ drum. The previous grinder does a very good job in very little time, but the grind does not always come out perfectly flat.
Grind away any waste posting in the triangular area formed by the distal and proximal ends of the 5th metatarsal and the outer edge of the footbed in that area. Excess foam in this area will cause the foot to cramp. Retouch the bevel at the leading edge of the posting foam at the metatarsal heads.
If you do not flatten the contour ahead of the metatarsal heads, the foot will probably go numb, as the plantar tissue will expand forwards when weighted, and, after all, this is a semi weighted cast. Use a heat gun and gradually soften the blank. Once softened, I set the blank right side up on a Teflon cooking sheet to cool. Sometimes I will set a zip-lock bag of steel shot on top of the forefoot area, as the blanks have a bit of a memory and will curve back up a bit as they cool. The mass of the posting foam absorbs enough heat that the contour at the met heads is not distorted.
When the forefoot area has cooled, and if you are satisfied that there is no upward curve ahead of the metatarsal heads, turn the footbed over and draw a straight diagonal line from the distal end of the fifth metatarsal, across and ahead of the first metatarsal. Envision the fill of this area, which will support the first metatarsal, and blend to the point where the blank will lay flat. Again, this is for a foot that collapses medially. If the foot collapses laterally, you can skip this and the step involving supplementary posting material. Repeat for the other footbed, and cut two triangular sections from the sheet of bulk foam. Each section should be large enough to cover the width of the footbed from the contact point of the 5th met head, across to the arch side, and back towards the forward curve of the calcaneous.
Bevel the leading edge of these foam pieces, and apply adhesive to the appropriate areas. When the glue appears dry, heat the secondary posting pieces in the convection oven until they begin to curve. Quickly match up the beveled edge and the corresponding pen line, and squeeze the foam and the footbed together, paying particular attention to the area just ahead of the first metatarsal head where the two sections of posting material overlap. If you do enough of these in one day, the heat will dry out the skin of your fingertips and crack. This can be painful, especially when it is your turn to do the dishes, so take care.
When the foam has cooled, trim off the waste with a utility knife. Using the 4” drum grinder, smooth the periphery of the arch, and grind away the rest of the extra posting material, using the existing plantar plane as your guide. Grind until you can just barely see the blank coming through at the 5th metatarsal.
Finish by grinding away the excess posting under the arch until the contours look even.
This represents what I refer to as the ‘basic grind’. Set the footbeds aside, enjoy a cup of your favorite beverage, and consider calling your client for their next visit.