MARS projects - VFS - Welding Zote Foam
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Welding Zote Foam

Sheets of Zote foam are not long enough to cover a typical shell length. In order to withstand the heat and stretching that occurs when vacuum-forming, welding is required. Contact cement and other glues do not work. Welding basically consists of heating the edges of two surfaces of foam to a melting temperature, then physically pushing the two surfaces together and holding them until cool. Done properly, these joints are so strong it is impossible to pull them apart by hand.

I first experimented with a Weller instant soldering iron. I fabricated a thin blade from 1/8” thick copper that replaces the normal soldering tip. The blade part is machined down to 0.045” thick x 0.63” high x 1.1” long, with two posts that are plugged into the soldering iron. I purposely made it short so curved welds can be made. Manufacturers typically use a long straight bar to weld two sheets together – simple, but not aesthetically pleasing on a streamliner

These first experiments went well, so I removed the transformer and mounted it on a parallel hinge arrangement. Here you can see the bare transformer with the welding blade at the top. Heat is controlled with a Variac (a variable transformer). I run the transformer at about 60% power, depending on blade construction/resistance.

This assembly was then mounted under a tabletop. Here you can see a section of a foot-operated string that pulls the welder blade up through a slot in the table. A stop post controls blade height. The hand-held unit I first built could possibly be used as a hand-held welder, but would require two people. I find these welds are not as good as the tabletop version, as control is not as smooth.

Step one of the welding process is checking the blade temperature. Touch a piece of foam to the welder bar. When the surface seems to melt and smokes a little, the temperature is OK. Too low, and the surfaces will not adhere. Too high, and the surfaces will melt away, leaving a gap. Caution: the soldering iron transformer is designed for short burst operation. During the welding procedure, it will be used at a lower power but for a much longer time. It can overheat. Maybe a small fan could help cool the transformer.

Preparing the foam: I cut an S-shape for the separation of the main shell from the tail on the VFS (see Vacuum Process Results). I lay out and cut the first sheet, then traced that line onto the second sheet. I cut the second sheet carefully, and then put the two sheets together to check the quality of the match. There should be no gaps, as gaps are too hard to close while welding. Carefully trim (or if needed, re-cut) the entire second sheet to get a good fit. Use short reference lines across from one sheet to the other so repeat alignment can be assured. I find that trying to hold a razor blade absolutely vertical is difficult. I generally push the razor rather than pull it through because it is somewhat easier to see the blade angle, but I also feel it's better when guiding on a line because your hand doesn't block the view of the cut line.

But how does one hold and control two very long and unwieldy sheets of foam? This shows two prick boards that have small brads nailed through the flat plate such that the nails protrude ~1/16”. These nails do a very good job of grabbing the foam. Yes, they pierce the foam skin, but this is on the inside surface of the shell.

This shows two pieces of foam straddling each side of the welding blade. Test samples are shown here, since it’s too hard to take photos while concentrating on a serious weld. Start by holding the prick boards so there is a slight angle on the foam sheets in relation to the blade. Then bring the foam sheets in light contact with the blade and slide them slowly through the blade. As they exit the blade, make sure to squeeze the foam sheets together as they cool. Cooling for ~10 seconds is sufficient, but the weld is still fragile and could come apart, so treat it carefully.

Once the process is started, continue pushing the foam along the blade in a smooth manner. Do not stop. This operation takes a bit of time-sharing: (1) seeing that the blade is in the exact center of the two edges of foam so that both surfaces are properly heated, while (2) continuing to push the foam through, and (3) holding the foam sheets together after they have exited the blade.

To stop the process, retract the blade. Continue squeezing the foam sheets together until the weld is cooled enough to be stable. Push the welded foam away from the hot blade below to speed cooling. Temporarily stopping the process gives the operator time to move the prick boards to a new location.

To restart the process, pull the foam sheets apart and raise the blade without touching the sheets. Move the sheets so the blade goes partially back into the previous weld to reheat it; then bring the sheets up against the blade and start pushing the sheets across the blade. Stopping and starting can cause uneven areas in the weld, so do it only when absolutely needed. I find that while leaning over a 37” wide weld, I need to stop and reposition myself and the large sheets of foam at least once. I start by curling the foam up so I don’t have to lean so far.
It is possible to move one prick board quickly to a new location (about half the length of the board) by stopping the motion of the foam very briefly. Then continue sliding the foam to prevent overheating, and quickly move the second board. This way there is less need to come to a complete stop. But remember to maintain your concentration on centering the blade and squeezing the weld.

The entire foam sheets have to be rotated to follow the curved separation line. This adds another chore for the operator. Twist the prick boards to force the sheets to rotate, while guiding and squeezing the weld. Luckily, the speed is slow, ~ 5-7 seconds to pass the blade. Tables positioned on either side of the welding table support and allow the foam sheets to lie flat and slide. I find that two helpers are also handy for supporting the sheets of foam so they can be rotated without running into things in the shop. My shop is small. Make practice runs before firing up the welder.

If the weld is done well, it is impossible to pull apart. But the weld has to do more than physically hold the sheets together: the weld has to be airtight when vacuum or pressure-forming. A typical problem occurs when a section of the weld was not held together tight enough, causing a very weak joint which pulls apart during cooling, or possibly opens up during the heating/vacuum process. This also happens if the joint was overheated, causing excess melting. So, after the welding is complete, carefully inspect both sides of the sheet for uneven weld joints. If one is found, then cut the entire two sheets apart and do the weld over again. Do this now instead of mounting and sealing the foam on the flanges of the mold, and taking a chance that the weld will not hold.

If a small hole does appear, which will destroy a vacuum, then contact-cement a thin piece of foam over the hole. HD80 works well here (see Vacuum Process Results). When heated, the contact cement will not hold, but the vacuum will pull the patch into the hole and hopefully give adequate vacuum. A hole in the weld can be detected by feeling for a hot spot on the mold.