Last edited: 26 May 2016
Cardboard is a popular material used for the construction of solar cookers. Can easily be found for free in developed countries and relatively cheaply in poorer countries. It is ideal for solar cooking because it's lightweight, easy to work with and easy to find. Double-walled corrugated cardboard is stronger and more insulative than single-wall, desirable characteristics for some types of cookers. Its manor drawback is that it is not waterproof.
Boxes from electronic equipment such as computer cases, printers, laptop computers, etc, are often of superior quality and in good proportions for solar box ovens. Stores that sell window glass and doors are good sources of long pieces that are useful for cutting 'planks' for liners or for other applications where long or large pieces are needed. Take a box or razor knife, straight-edge, and cutting board/mat along with you on quests for long/large pieces, so you can cut them down to a size that will fit in your vehicle if necessary. Cardboard cookers can be made more waterproof by painting them with several coats of exterior latex or other paint. This doesn't mean you can store your cooker outdoors full time, but it will make it more resistant to dew or unexpected light rain that might blow in while your cooker is out. For greater strength and water resistance, the fabric/glue/beeswax treatment described on the Tips and Tricks page is very good. You still can't store your cardboard cooker outdoors all the time, but it will take more in the way of dew or an unexpected shower that blows up while you're away from home, and it makes the box very tight and strong.
Working with Cardboard
- Measure twice, cut once! Be sure your line is drawn right before cutting.
- Use a box- or razor knife with a sharp blade.
- Use a straight metal edge to cut against. A rafter square, which is an L shaped ruler, is very useful for measuring, cutting, and making sure angles are square.
- Go over your line repeatedly, rather than trying to cut all the way through on the first pass. The cut will be straighter and easier to control if your first pass is very light and shallow.
- Main article: Glue
- A half and half mix of water and white glue works well for gluing cardboard and foil together.
- Cheap sponge brushes work very well for spreading glue smoothly on cardboard or foil and are suprisingly easy to clean afterwards. A very smoothly folded piece of worn, smooth cotton is the best thing for smoothing out the foil when gluing.
- When gluing cardboard to cardboard, wood glue may give a stronger result than white glue.
- Main article: Paint
- Sponge brushes work very well for a smooth result on cardboard. Get them just a little damp (wet, then press excess water out with hands) before painting.
- Exterior latex is good for the outside of cookers. Use thin coats and let dry before the next coat so your cardboard won't get wet enough to try to warp.
- For painting cardboard black for oven interiors, use black tempera paint mixed with some white glue.
- Main article: Waterproofing
Stephen Pearson reports: "We build CooKits in pieces use eleven small panels of cardboard. Shops even in rural areas have thin cardstock available as scrap. These can be laminated together using local cassava glue (cassava is grown all over Africa) to form stiff panels, thus avoiding the need to import thicker cardboard. Join the eleven panels together with 2" strips of worn out dress or shirt (using the cassava glue). Protect the perimeter of the 11 panels with more 2"strips (1" front and 1" back to stop delamination of the cardboard). Let each panel dry properly. Then turn it over and glue on the aluminum foil to the side with no cloth hinge strips."
Alternatives to cardboard
Stephen and Sheila Harrigan of Solar Clutch, report from Addis Ababa, Ethiopia, where they promote solar cooking, that common solar panel cooker construction materials have always been difficult to obtain. The widely used CooKit has traditionally been made from cardboard, though recent versions are being made from fluteboard, such as the Poly Furnace CooKit variation. These updated cookers last far longer than the cardboard versions, which are considered to be successful if they last six months. For them however, the major problem with providing inexpensive yet durable cookers is the availability of construction materials. Cardboard is scarce, and when it is commercially available, the quality is often poor, or the product arrives torn. Aluminum foil and glue can also be difficult to find. Stephen began looking more closely at what was available locally, and hit upon a product referred to as 'cool roofing'.
Cool roofing is a zinc galvanized metal material, which is fairly reflective. Besides shedding water, it is also intended to reflect the hot sun from the roofs of local homes. And because of its availability, the material cost for cookers is competitive with cardboard. A CooKit can be constructed for about $6 USD worth of the metal roofing. It can be cut with tin snips, and bends are made by folding the metal around a straight board. The metal CooKit has performed well, even if not quite as well as the mylar covered fluteboard version, and its weight would help in windy conditions. Because the material is easily available, and can be made with existing worker skills, local manufacturing of solar cookers offers the possibility for a community business enterprise. Disadvantages include the inability to fold the cooker for storage, and transporting completed cookers will take considerable space and will be relatively heavy. The cooker edges need to be dulled or protected, as the cut metal edge is quite sharp. As solar cookers are more widely promoted and accepted, the durability of inexpensive solar cookers becomes a significant issue to be addressed. The question raised will be, is it better to offer a cooker that can be made locally, or find a way to finance the availability of cookers that are made at a more sophisticated facility, losing the empowering aspect of neighborhood manufacturing?
- November 2007: University students put corrugated plastic to the test - Solar Cooker Review