So how do you go about making the absolutely lightest body work possible for a vehicle? Here’s one approach, used to build the aerodynamic fairing for a Human Powered Vehicle.
Introduction
For nine years I have been using foam materials, and in particular Zotefoam, to make lightweight shells for Human Powered Vehicles (HPVs).
Zotefoam is a closed cell cross-linked polyethylene thermoplastic material which, with the application of heat and pressure, enables three-dimensional shapes to be formed from flat sheets. Even at high temperatures, the cross-links maintain the foam structure, and after cooling they allow the foam to retain the shape of the forming mould.
But correct application of heat and force are the keys.
Most composite shells are built by constructing first a male mould and then a female mould, which is labour-intensive and quite expensive. For thermo-forming a Zotefoam shell, I have developed a system which requires only a male mould, and only a minor amount of equipment.
This series describes a system by which you can produce a very light shell to be used as a prototype to check out a new design, or for making a few copies off a low cost mould, or for making a finished product out of a home workshop. For higher volume production, the system is still practical, and the mould can be improved by applying a layer of muslin cloth and plaster to protect the mould from accidental melting during the process of heat-forming the foam.
The Mould
Before describing the mould, the title picture (above) shows the Velotrike built with a foam-shell made using the technique to be described. The shell is 94 inches long and 26.8 inches wide; it weighs about 7 lbs. Total vehicle weight is 41 lbs.
This drawing shows the basic top and side view of the layout. The 13 inch horizontal line from the tip of the nose to the tail is the main reference line at the maximum width of the shell. Other reference lines will be described later.
For the vertical sectional views, I used the bottom bracket, front wheel, shoulder, and rear wheel as reference points. An additional 10 inch section is useful because of the fast-changing nose curves.
The vertical dashed line represents the tail junction. The tail is removable for fixing a flat tyre and for removing the trike from the shell for maintenance or to be used as an unfaired vehicle. The removable tail also reduces the size of a container needed for shipping or transporting in a car.
The basic mould construction material is common 2 inch thick home insulation, which usually comes in 24 x 96 inch sheets. The maximum width of the Velotrike is 26.8 inches. If a 2 inch vertical centre section is used and the sides are made in separate halves, the maximum width for one side is 12.4 inches.
This shows the layout in a sectional view at the front wheel, which is at the maximum width of the shell. When laying out the various stacks of mould foam for cutting, both a wide stack section and one of the less wide upper stacks can be cut from one sheet of rigid foam. The centre section is a side view of the shell; it acts as a convenient reference point for these dimensions when carving the mould foam.
Note that the stack starts on the 13 inch line. The entire stack above and below this is at a known reference dimension. These 2 inch section lines make wonderful references on the actual mould. They also help when generating the initial drawings of the side, top, and sectional views.
I used the front wheel section as the major reference. The position of the trike has to be known accurately to fit the wheel cut-out on the shell, plus accounting for toe and heel clearances, so all the stacks have this important reference drawn in on the back edge, even on the front edge. This helps with alignment when gluing the stacks together.
This shows the vertical reference lines. Another important reference is the 22 inch height line which is the bottom of the entry door. This is about the highest you can comfortably get your foot up and over the shell edge when climbing in and out. The 27.5 inch line is where the canopy hinges on the right side and opens on the left side. It is also the separation line for the Turtle Deck and the colour separation line from the front wheels through to the tail. The line deviates from this dimension as it goes down toward the nose.
The main shell is divided into a separate top, two sides, and a bottom. One reason for this is the sheet size of the Zotefoam, which is around 40 inches wide (and 80 inches long, another reason for a removable tail). The location of the join of the top and sides is carefully chosen to minimize the amount of compound curving needed using a flat sheet of Zotefoam. This is also a convenient place for the colour change, which helps hide the separation line. I made cardboard sectional templates (arrowed) as guides when carving the mould.
This shows the front partially shaped.
Here is the almost finished mould. Note that it is bolted together on the front and rear wheel section lines. Later I added another bolt at the 10 inch section. It is difficult to clamp the rigid foam securely enough to hold it in position because the foam collapses. I even tried 1 inch dowels through the centre section, but they, too, allowed movement. Possibly a 3-4 inch diameter plastic pipe would act as a better locater. I could have glued the sides to the centre, but I wanted to be able to separate them to reduce bulk and weight when transporting or storing the mould.
Note also that the mould can be rotated. This is very important when carving, but more importantly the mould has to be properly orientated when heating and forming the Zotefoam. This shows the centre section with a wooden block through which a pipe can be inserted so the mould can be rotated. A similar one is on the tail. I made sure there was enough height above the table so that the mould can be rotated to the positions required for heating the Zotefoam. The tail block is located much higher than the nose.
Finally, here is the fibreglass nose piece. The nose piece is a high compound curved area. To aid in forming it, the Zotefoam is glued to the fibreglass, making a strong and stable nose area, strong enough for a nose support off the vehicle. First lay some shrink wrap Mylar over the mould foam to act as a mould release, then apply one layer of Crowfoot weave fibreglass, which is enough. This shows the fibreglass on the nose just before epoxy resin is applied. Below it you can see the Mylar, which will be thrown away. Remove the fibreglass to trim the edges, then replace it on the mould and hold it on with a couple of small pieces of masking tape. To remove the main shell from the mould after it is fully formed, that masking tape has to release on its own because you can't get to it.
This is also the time to lay out the colour separation lines.
Colour Layout
The colour separation lines should be laid out as soon as the mould is finished. These lines are needed so the raw Zotefoam can be cut oversized in preparation for heating. With the 2 inch reference stacks on the mould, the 27.5 inch line can be easily located from the front wheel to the tail. Masking tape makes a very good visual guide to mark the curving front section line. Look at the low angles to see if the curve is smooth. Stand back to see proper proportions. If the lines are off, they can be moved. Once properly located, fresh masking tape should be used (the mould can also be marked). Also mark which edge of the tape is the separation line.
Don’t forget to do the green/grey line along the bottom.
Laying out these lines takes a bit of care, patience, and judgment as to where the lines should be in terms of looks and function.
In the smaller radius area of the nose, the masking tape has to be cut every half inch or so to go around the tight corners. The width of this tape must be accurately maintained as a guide for trimming.
Heating and Forming
You will need to order one sheet of Zotefoam for the top, two sheets for the sides, and one sheet for the bottom. As a backup, order a second top sheet, one extra side sheet, and an extra bottom sheet. These should be 0.5 inch thick LD 45. They normally come in 40 x 80 inch, but the green did not, so I had to weld two pieces together to make one side (the joint is the front wheel section line).
This shows a yellow top wrapped over the mould and pulled down against the mould with many bungee cords. When heated, the Zotefoam will normally expand away from the mould. These cords provide even tension so the Zotefoam conforms to the mould shape. The bungees I used pulled a bit too hard, causing thinning of the Zotefoam.
Cut the Zotefoam so there is about 4 inches of excess material below the colour separation line. What often happens is there is some amount of puckering between the bungee cords because of the curvature of the mould. Puckering has to be below the colour separation line (ie, it will later be trimmed off).
Very important: after the top is bungeed, make a registration mark at the end (beyond the canopy end) for forward/back location, and a couple of lines for left/right position on the mould. The end of the top has to cover only part of the open cockpit. Marking is very important so the part can be accurately repositioned for trimming and assembly. Use a removable ink marking pen to make surface marks that can later be removed with water. Other registration marks will be made after trimming.
Here is where the hard part comes in: heating must be done with large area heaters. A hot air gun can heat only a 12 x 12 inch area, meaning the Zotefoam will be unevenly heated, so I used two Marvin Workshop Quartz 7060 overhead shop heaters from WB Marvin Manufacturing Co., 211 Glen Ave., Urbana, OH. The heaters can be switched from 700 to 1200 watts. Each comes with a bracket so it can be mounted on a wall or ceiling and the angle adjusted. That bracket can be modified so two heaters can be coupled together, while each heater can be adjusted independently in angle.
I hang these heaters about 18 inches above the mould on ropes so the height can be adjusted. I also use bungee cords from the ropes to the heaters to allow quick manual adjustment of the height while heating the Zotefoam. This is necessary to heat properly around the edges which are lower down on the sides of the mould.
The arrow shows the lower heater rotated to heat the edges. But note how the upper heater is aimed towards the middle area. This will cause overheating in the middle. It would be better to rotate the upper heater temporarily away from the top and then rotate the entire assembly. Then the upper heater will be aimed away from the centre. When changing the heater angle, pull it away from the mould.
The main rule is to keep the heaters moving at all times. Sweep an area about 36 inches long by the width of the piece to be heated. Let the heat soak in slowly. Keep the heaters about 14–24 inches above the Zotefoam. As the temperature of the Zotefoam rises, it changes colour. Observe where this colour change is occurring, and reduce the heating in this area while increasing the sweeps above the slower areas.
This shows the colour change. One of the challenges is to learn what depth of heating is occurring. Once the colour change occurs, no further visible change can be observed. (Well, not until overheating - then the surface is melted and destroyed!) Feeling the surface will give a clue. Smell is another. But always keep the heaters moving. Practice helps. Overheating also melts the mould; the muslin cloth and plaster help prevent this.
Because the heaters are essentially fixed in location, move the mould to cover another area. After the most of the top is heat-formed, go back over it to reduce stress from uneven heating. For this operation, it is not necessary to bring the temperature as high as the initial heating. Raise the height of the heaters with the ropes when done, because even when off the heaters radiate a lot of heat. When you disconnect the bungees, the Zotefoam should be laying uniformly up against the mould. If it isn’t, then reconnect the bungees and do another reheat.
Note as you come closer to the front, you will need to cut darts in the sides, otherwise the Zotefoam can’t bend without severe puckering.
Here there are not enough bungees to do a proper re-heat. To finish the nose area, use a hand-held hot air gun. This area is too small to use the large heaters.
Heat an area and hold the edges down against the mould until cooled. Don’t touch the heated area that will be the top, because the Zotefoam is soft and you will leave handprints.
I have thought about trying to use a remote-reading temperature gauge to get an idea of the Zotefoam temperature. This could possibly be mounted between the heaters and aimed at the Zotefoam. It would require some experimenting to correlate the surface temperature with the depth of heating, but it may give more reliable results. Zotefoam needs to be around 200 degrees F. If the temperature is too high, the crosslinks stretch too much and may break, weakening the Zotefoam. Also if overheated, the surface can be destroyed. If too low, the crosslinks won’t be able to adjust and the Zotefoam won’t maintain the shape of the mould.
Next: Gluing and installation
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...
|
More of our most popular articles.
|
|
|
|