Making Giant Egg Mould
The first step in making a giant egg is to make a giant egg! From this a mould can be made from which to cast as many eggs are required.
Depending on the size of the egg you want to make will dictate how you go about making the egg form. Since the egg we needed to make was about 60cm tall it was decided to carve it from a block of polystyrene. Since the egg shape is symmetrical it is best to turn it on a lathe.
The polystyrene has to have plywood boards glued to both ends so a lathe faceplate and tailstock centre can be attached so it can be turned on the lathe.
The corners of the block are then cut off at 45 degrees to make turning a bit easier on the polystyrene. If this is not done it is likely large chunks will break off when the lathe chisel is applied to the turning block.
It is very, very messy turning polystyrene on a lathe so ensure the work area is covered with something to collect the shavings and clean up regularly as you work. This will minimise the polystyrene getting into every nook and cranny of your workshop.
Photo above shows the polystyrene mounted on the lathe ready for turning. After a very messy process and lots of vacuum cleaner use the bBasic shape of the egg has been completed on the lathe as shown above.
The completed polystyrene egg is removed from the lathe and the two ends trimmed. Sandpaper is used to smooth both ends until their radius makes a smooth transition around the egg shape. It is important to get the shape exactly right with no obvious flat spots or sudden changes in the radius. The final egg will only be as good as the mould which will only be as good as the shape you use to make it.
To support the egg while making the mould a box was built to put the egg form into so the middle flange could be made with plasticine (see below). Once that had been done it was given a coat of PVA mould release in preparation for fibreglassing.
Above shows the ply box for making the top half of the mould and the lower picture shows the finished plasticine flange around the egg with mould release applied.
After the top half has been fibreglassed a timber support is fibreglassed on the mould so it sits horizontal when put on its side. The egg with the first half of the mould attached is removed from the support box and turned upside down to expose the underside of the egg form. This is cleaned up, PVA mould release applied and the second half is coated with fibreglass as described in previous mould making posts.
After polishing the inside of the mould halves with release wax the fibreglass casts are made and trimmed to the flange line around the mould on both halves.(see below)
To ensure that both halves of the egg are joined together securely they need to be fibreglassed along the inside centre seam of the egg. To do this an access panel is cut in one half that is big enough to get your hand inside the egg shell. Blocks of wood are attached to the inside of the shell around where the access panel will be cut. This allows for easy re-installation of the panel section after the work has been done.
You can see the alignment blocks in the photo below with the access panel removed.
The photos below show the two halves taped together with mylar tape to keep them aligned. They are then fibreglassed on the inside along the centre seam through the access hole. Once this has been done the access panel can be glued back into position and all gaps around the panel opening and the centre egg seam can be filled with polyester filler and sanded smooth.
After sanding smooth the fibreglass is given a primer coat of de-waxed shellac.
The completed eggs can now be painted and used as required.
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