Making the Glass Fibre Dome

To construct the dome panels two glass fibre master moulds were needed, one for the side panels and one for the shutter panels. To make the master moulds, I constructed two formers out of timber and plaster. It was not hard to cut and shape the timbers to form the curve in one plane, but plaster was needed to form the curve in the other plane. Timber framing covered with hardboard was constructed to predetermined dimensions and then plaster was added and shaped by using a curved running-mould. Once set, the plaster was sealed and the whole former painted with a release agent to prevent the glass fibre sticking to the timber and plaster.

The process of glass fibre construction involves laying down alternate layers of resin and glass fibre matting until the required thickness is achieved. A hardener or accelerator is added to the resin before you use it, to make it harden. The more you add the quicker it hardens! Temperature will also affect the time it takes for the resin to harden; the warmer it is the quicker it will harden. It is essential to work in a well ventilated area and to wear protective clothing.

The master moulds needed to be strong as they were going to be used several times, so they were made up using three layers of 600 g/m2 (2 oz/ft ) glass fibre matting. It is easier and quicker if laying the glass fibre is carried out by two people. Each master

Figure 11.3 The first five dome panels assembled to enable overall dimensions to be checked prior to erecting the panels on the observatory.

Figure 11.3 The first five dome panels assembled to enable overall dimensions to be checked prior to erecting the panels on the observatory.

mould took about two hours to make (with the assistance of a colleague), and was left overnight to cure.

It is essential that each master mould has at least one removable side, which can be made of 12 mm (2 in) plywood or chipboard. This allows you to release the panels from the mould. When the master moulds had hardened and were taken off the formers, they were trimmed up and the inner surfaces buffed up to a high gloss using slipwax, which also assists in releasing the panels from the mould.

The moulds were now ready to be used to produce the dome panels.

Each dome panel was cast in the same way. First we gave the mould a coat of release agent, which was allowed to dry. The first resin coat (the gel coat) was mixed with a pigment to give the panel its colour and then applied to the mould. The exposed surface of the gel coat remains tacky, so as to form a good key with following coats. A layer of 600 g/m2 (2oz/ft2) glass fibre matting was then laid on top of the resin and rolled with a laminating roller to make sure that no air was trapped between the resin and the matting. The resin should totally penetrate the matting.

Once you have done this, you add another resin coating, followed by a further layer of matting. I used two layers of glass fibre matting in all the dome panels. After each panel was released from the mould I trimmed its edges with a hacksaw.

As production of the dome panels progresses, it is a good idea to bolt the panels together on the ground as a check that the overall dimensions are correct, prior to final assembly (see Figure 11.3).

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