Moulding the blades   Back to Wind Power
 
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Using balsa wood to make the blades is ok, but very time consuming. The following describes my first attempt to make and use a mould to create fibreglass, carbon fibre and epoxy blades.

MAKING THE MOULD


The first step was to prepare a positive, used to make the female mold. I had a 100mm blade which I rejected because it started to absorb moisture, so I worked on it with sandpaper for a while to get it smoother.
Next I created a ridge around the flat edge with some square wooden beading and panel pins. This was to create an overhang in the mould so that the complete leading edge would be included in the form. The mould material is flexible enough to allow this to be peeled back easily.
A box was then needed to contain the mould material, and I made one using a meranti pine plank for the base and thin board for the sides. Polyurethane two part foam was used to fill up the spaces to reduce the amount of mould material needed.
The foam was easy to trim to shape with a long knife, and modelling clay was used to pad the positive a bit where needed. Finally a water based mould release agent was painted on the positive. The foam is porous, so Vaseline was smeared thickly on to it to ensure the mould would separate after setting. Vaseline was also used to make a smooth trailing edge.

The mould material selected was silicone rubber, which is a thick gooey liquid mixed with an activator, which then hardens at room temperature overnight. This is quite expensive, and in retrospect, polyurethane rubber might have been a better choice.

Once it had set, some of the foam was cut away, some modelling clay added as padding, and the whole lot covered with more foam. This formed a base to support the rubber once it was flipped over and the positive removed.

Here is the partially completed base on the left, and the finished mould on the right. Some two part polyurethane was used to fill the corner to remove the sharp angle which would have been a weak point (the white triangle on the right).

USING THE MOULD - FIRST ATTEMPT

The first step was to spray the silicone mold since epoxy will stick to it. Next, a layer of epoxy resin was painted on, and two layers of 4oz fibre glass laid down. Then the leading edge was filled with epoxy filled out with cotton flock. Next a series of carbon/glass fibre tape was added: 100cm, 80cm, 60cm, 40cm, and 20cm, each starting at the root.
Now the leading edge needed to be built up to support the flat windward side of the blade. This was done using a thick paste of epoxy and cotton flock. A small jig made of wood helped smooth out the epoxy/cotton paste. Another layer of 4oz glass was then laid over the result.
A 1.5mm balsa sheet was cut to fit, and the inside face coated with glass and epoxy. Before that set, the trailing edge and tip were filled with some more epoxy/cotton paste, and rough ribs were formed. To allow expansion of the air in the cavities, small tubes were embedded in each rib. Finally the balsa sheet was put in place, and weighted down to set. There was a slight difficulty at this stage, as the balsa sheet is very flexible and weighted by the epoxy, so it tends to sag, making a concave shape.

A few steps left... The outside of the balsa sheet was coated with some ultra fine glass, and the root built up in stages. A piece of pine plank is added as shown to reduce the weight and amount of epoxy required. Its important not to make the epoxy too thick at any stage, as it can overheat as it sets.
Two layers of glass were added over the pine, and then after building up the root more, a piece of 5mm plywood was added, and epoxy added to fill to the top of the mould.
Finally, the blade could be removed from the mould. The triangle of white polyurethane stuck to the blade, but was easily removed.
This was a convenient way to support the blades while the final epoxy coat dried. Initially they weighed between 976 and 1004 grams. By a process of sanding, filling, and weighing the tips, roots, and total weights, after the final coat that became 1031 to 1039 grams. That difference was small enough to deal with in the final static balancing of the blades.

Up and flying! One of the concerns with long blades is that they flex in high winds and touch the pole. This would shatter the blades. These performed well however, and show no flexing even in strong, gusty wind. Even if they do crack for some reason, the strong carbon fibre spar should hold the pieces together.

A big advantage of using a mould is the accuracy of the blades. When looking at them spinning side on, it can be seen that each tip travels through exactly the same point.

The next set of blades will be made in a similar fashion to the above. However, I will experiment with using less material to see if they can be lighter while keeping sufficient strength. I also have an idea to prevent the sagging of the top balsa sheet. Using balsa sheet on both faces of the blades may also improve the shape, and give a more visually pleasing result.

Watch this space!

Record of amounts of material used

Since I use this site as a record, here is a table showing the amount of epoxy and cotton used at each step. This is useful so that I can make all the blades as close to the same weight as possible. Admittedly, this is probably quite boring for you!
Step Description Epoxy (g) Hardener (g) Cotton flock (spoons)
1 Glass 2 layers 60 15 -
2 Leading edge 36 9 4
3 More leading edge 60 15 9
4 Carbon fibre tape 60 15 -
5 Leading edge ridge 60 15 16
6 Glass over layer 32 9 -
7 Glass inside of balsa sheet 32 8 -
8 Trailing edge 40 10 8
9 Ribs every 15cm 40 10 12
10 Glass (light) outside of balsa 48 12 -
11 Build up root dam wall 20 5 7
12 Fill root, add wood 20 5 3
13 Build root to top 20 5 7
14 Fill around root 20 5 2
15 2 x glass at root plus 5mm ply 20 5 -
16 Build up ridge 16 4 5
17 Final filling 20 5 2
18 Fill end after weighing 24 6 7
19 Final coat 40 10 -
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