In Part 1
of this series I introduced the making of a mould and the heating and shaping of Zotefoam. In this story I look at trimming and cutting the foam, gluing it and making reinforcing braces.
Trimming and Cutting
It’s a challenge to trim the edges of each section accurately so they can be glued together to make a beautiful fine line. Following is the system I developed.
A second piece of tape is placed parallel to the colour separation line. The centre (arrowed) is the actual colour separation line.
This diagram shows a small area of Zotefoam heat-formed over the mould, ready to be trimmed. This represents the yellow top section.
This is a detailed section view of the above. The aim is to rough cut enough of the excess Zotefoam so that the lower tape becomes visible. About half of the tape should be visible after cutting. I use 1inch wide tape.
This shows a system of marking the outside surface of the Zotefoam so that a razor can be guided accurately along this line. Make a marking guide about 1.5 inches long from a piece of wood the thickness of the Zotefoam. Screw a thin piece of metal to the wood, with a hole in the metal to allow a pen to mark the colour separation line. Visually align the guide along the bottom edge of the lower tape and slide it along with the pen, marking the Zotefoam. The bottom tape edge is for trimming the top. When making the cuts for a side, the upper tape is the reference line.
This shows the blade at an angle; it is pushed rather than drawn through. I find this is more accurate because it is easier to see that the razor is following the line – the blade is not covered by the hand. The angle of the razor shown gives a long length so that the razor has less tendency to waver. The razor has to be super sharp. I use a whetstone to keep the blades sharp.
Warning: after trimming but before removing the part, make registration marks so the part can be repositioned accurately for assembly. Make an edge mark at the front wheel section line, for example. All four sections of Zotefoam can be cut and then later assembled on the mould. This system turns out to be accurate enough to give fine separation lines.
This shows a close-up of the nose area with the fine colour separation line. For production, a fiberglass template would be made and placed over the formed Zotefoam so a line could be drawn for cutting.
Gluing
I recommend using DAP Weldwood contact cement, one of the few types of cement that sticks to Zotefoam. Buy two or three 3 fl. oz. (88.7 ml) bottles with the application brush inside. Also buy a larger can of it, and when the cement is used up, refill from the can - much cheaper this way. Caution: be aware of fine spider web-like filaments coming off the brush as you take the brush out of the bottle and when applying the glue to the Zotefoam. Generally there is a static charge, which causes the filaments to zap randomly onto the Zotefoam surface. You can wipe the filaments off immediately, but once dried, they will be annoyingly visible.
Start with the yellow top: prepare it by sanding the edges that will be cemented, using 60 grit paper. Try to stay away from the outside surface edge, because the sanding raises tiny tufts that will be noticeable even after gluing. If you do raise tufts, run the hot air gun fairly quickly over the surface to melt the tufts. Experiment on scrap pieces to determine how much sanding is needed and how to remove the tufts by heating.
Lay the top on the mould and position it carefully with the registration marks made after trimming. Use masking tape to attach it to the mould to prevent moving. Then glue the nose area to the fibreglass nose piece in three steps, not all at once. Put down a layer of contact cement across the rear of the fibreglass mould and a corresponding area on the inside of the top. Let it dry at least 7 minutes or more. When dry to the touch, apply another layer of cement, wait about 5 minutes, and carefully bring the Zotefoam into contact with the fibreglass without losing the registration marks. Then repeat the process until the entire top is glued to the fibreglass. This photo shows the yellow top glued to the nose.
The end of the top stops at the horizontal division right at the tip of the nose. This is because the pipe holding the mould does not allow a continuous nose piece. Cut enough of the top away so it can be formed around the pipe. Later this opening can be cleaned up and used for either a headlight or cooling vent.
While you wait for the glue to dry between the top and the fibreglass, prepare a side by sanding the edges. Then rotate the mould so the side can be placed on it without slipping off. Adjust the side to match the registration marks that were used during the heat forming and trimming process, then tape in several places.
Because the most critical area in terms of fit tolerance is along the fibreglass nose, gluing should not start here, but rather after the fibreglass nose. First, make sure the side does indeed fit well to the top all along the fibreglass nose area before gluing further back. Glue by sliding the side away from the top far enough to brush on the contact cement. Glue about 8-12 inches, using two layers of contact cement. When ready, let the two surfaces come in contact together lightly. Do this in very small, carefully controlled steps (at about 0.25 inch increments) for a total of several inches. One way to prevent accidental contact is to use a piece of paper between the two surfaces.
Make sure the levels of the two outside surfaces are visually even before actual contact. Any differences in height will be noticeable. If a section looks good, then push the edges together more firmly. After the first 8-12 inches are in good contact, push hard with the thumbs down and in towards each other across the division line, hopefully making firm contact all the way to the inside surface which is up against the mould. This will leave thumb indentations on the outside surface. After the glue has thoroughly dried for several hours, go over the indentations with the hot air gun to make the indentations disappear.
Continue this process toward the tail for 12-24 inches. This will help stabilize the side to the top. Then glue the nose area. Because the Zotefoam is curving inward on a compound curve, it will have to be pulled up away from the fibreglass nose, which will take some effort. Pull it just high enough to get cement inside. That is why the nose should be glued later. Lay about half of the nose with glue, including the edges. One way to prevent accidental contact is to pull the side away from the top just enough to allow contact with the edge of the top in small increments. Try to make contact first with the edge, not with the fibreglass. After a section of edge is in good contact, allow the Zotefoam to contact the fibreglass. Again, finish by using the thumbs.
Finish gluing the side to the top all the way to the tail cut-off line. This is easier because everything is stable. Again, pull the side away from the top to control accuracy of contact. When finished, use masking tape to attach the bottom of the side to the mould to prevent movement. Rotate the mould to do the second side. Shown here are the top and right side glued on mould.
I continued the yellow top colour below the horizontal division line at the tip of the nose. I formed this section using a hand-held hot air gun and trimming the part to fit into the sides and top. This was a difficult process due to the compound curve and the tight tolerances. This piece has to fit within the already-glued sides. The better way would be to glue this yellow bottom piece in before the sides are glued on using the trimming process.
I did a simple straight cut-off line running across the width of the shell about 12 inches back from the nose. This turned out to look clunky. It would have been nicer to make a dart shape here.
When both sides are glued and taped, turn the mould upside down to glue the bottom. The bottom has to fit within the two sides, but because of the large dimensions and Zotefoam flexibility, it is not a problem. Start with the joint between the yellow and the leading edge of the bottom. Shown here is the use of paper to prevent accidental contact of glue.
The finished bottom view.
After the bottom is glued, lay out the leading edge cut-off line for the canopy. I used the wheel centre line as a reference. To locate the approximate centre of the axle point that was marked on the mould, cut away a section of Zotefoam in the area of where the axle should be. The area the size of the wheel will be cut away, so you don’t have to be careful here. Using the vertical wheel section line drawn on the mould as a guide, draw in a vertical section line (or use masking tape) to be used as reference to locate the leading edge of the canopy. Use a length of masking tape across the width of the shell to guide the razor blade. The masking tape is an excellent visual guide for a straight line, and it can be adjusted easily.
Next, lightly lay out the lines for the entry door with a removable ink marking pen. I didn’t cut the door out because I didn’t know exactly how far forward the door needed to be to get my feet past it. But I did push a razor through to the inside surface in two spots along the 22 inch line. These marks provide a convenient reference on the inside surface when gluing in the internal brace. Once the trailing edge of the door is drawn in, the line extends up for the trailing edge of the canopy. The canopy can then be cut off and put aside.
Use a blade of wood or old hacksaw blade to draw a circle representing the wheel radius, then cut out the wheel opening in the Zotefoam. I recommend cutting it about 1 inch smaller in case you need to move the trike forward or back in the shell for toe/heel clearance, unless your drawings are right on. Final trimming can be done after everything is mounted and checked out. I didn’t cut the wheel turning butterfly area at this time. Mark a wheel section line on the edge of the bottom to be used later as reference point.
The shell can be removed from the mould. The shell will be very floppy and will sag if laid on the floor. Hang it by the nose. Use some heavy coat hanger-like wire inside the nose hole and a rope to a pulley on the ceiling of your shop. This way the shell will take on a very natural aero shape. It will also allow you to work inside when adding the various internal braces.
With the wheel hole cut, an inside line can be drawn on the bottom representing the front wheel section line. This is a reference point for positioning the trike. This is the wheel axle forward/back location.
Internal Braces
The foam-shell by itself is floppy – one can’t get this kind of lightness without some floppiness. By adding internal braces and attaching the foam-shell to a trike (or bike) frame, the foam-shell becomes surprisingly stiff. I used two types of internal braces: U-shaped and tongue & groove. All the braces for this foam-shell were made from various thicknesses of HD 80 Zotefoam.
To form the U-shaped braces, I made wooden male and female moulds. I used 0.2 inch thick HD 80 for the bigger braces and 0.175 inch HD 80 for the smaller braces, so order a sheet each of 0.2 inch and 0.175 inch HD 80 foam. The smaller braces get plugged into the bigger braces to align the removable tail with the main shell. Here is a drawing of the moulds to make these braces.
The tongue & groove braces are used to support the edges at the door/canopy, tail/shell, and turtledeck/shell junctions. This shows a sectional view of a typical edge support.
This shows the layout of the braces used on my VeloTrike; use this drawing as a main reference.
To form the U-shaped braces, heat about 8inches of Zotefoam and lay it on the female mould. Press the male mould down into the Zotefoam and push it into the female mould. This can be done by hand. Hold for 20 seconds, leaving the male form in the mould. Heat the next 8 inches of Zotefoam sticking out beyond the mould. Then lift the male form out and move the heated Zotefoam over the mould. Press the male back into the Zotefoam and hold for about 10 seconds. As this section is cooling, heat the next section. Repeat the process to make long pieces. After the entire length is pre-formed, do a second pass to make a cleaner, sharper U-shaped form.
With a razor blade, trim the sides of the U uniformly. The step in the mould allows the razor blade to lie flat. Then glue in a bottom piece of HD 80 to close the loop, using the usual gluing techniques. The bottom increases the stiffness, plus it gives a bigger gluing area for the inside of the shell. This assembly is extremely stiff yet very light.
The following work is done inside the hanging shell:
Attach the first internal brace along the left side at the 22 inch line at the bottom of the door. This brace replaces some of the stiffness in the shell that is lost when the door is cut out. I had suggested cutting two marks with a razor blade when the shell was on the mould. Stick two razors in these slots and you will see where the internal line for the brace has to go. Run the brace from the nose area to the tail cut-off. This brace is low enough that your shoes will not rub up against it. Pre-curve the brace to fit the shell shape (cold form it). Hold the brace up against the inside of the shell and draw glue lines on either side of it. Sand this area and use the double glue technique to glue it in place. Lay down masking tape on either side of the lines to prevent the glue from going beyond the edge of the stiffeners. This makes a cleaner looking installation.
A pair of braces should straddle the green/grey line along the bottom. This line runs from the glass nose, interrupted by the spray shields around the front wheel openings, and continues to the end of the main shell. This is will be the lower alignment coupling to the tail.
Another brace is needed on the right side at the 27.5 inch line. This cannot go all the way to the nose because it’s high and your toe would hit it. This brace stops at the rear end of the canopy. An alignment slot will go from the end of the canopy to the shell/tail junction (described later).
With one U-shaped brace at the green/grey bottom line on the right side, a second U-shaped brace is needed halfway from this bottom line to the 27.5 inch line. This could be a short one, about 15 inches long from the rear end of the shell toward the front.
Next: attaching the shell to the frame and all the finishing details
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