ibuild

I thought it would be good to do some explanation and fill in some blind spots now that I have had a little break and have done some other things, leaning back and getting a little distance to it makes it easier to view it all with fresh eyes.

About the first thing I did when making the sparcaps I said that I was using epoxy with extremely slow curing time and this is because it actually takes some time do this, I spent about two hours from starting to mix epoxy till it was in the bag. This does not mean that you cannot do this with a regular laminating epoxy and yet get a high quality result, it will just end up a little thicker and not as extremely compressed. There is a couple of things you can do to slow down the curing of the epoxy a bit; open the door (or window) to get the temperature in the workshop down a degree or two if possible, have the epoxy bottles in cold water perhaps (it will make the epoxy thicker but it will also slow down the curing process), when mixing the epoxy mix it in reasonable small batches so that the epoxy is as fresh as possible at all times. Of course it pays off to do everything you can do of preparations before you start to mix the epoxy - make ready everything you can think of that is to be used or that will go in to the bag before you start.

About using fabric at a diagonal angle (-45/+45*) it is important to get the direction of the fibers the same at the top and bottom of the workpiece if it is to become and remain straight and flat, this might be a sandwich structure material plate like I have shown or perhaps a foam core of some sort with fiberglass at both sides. However if it's something like a cowling, a fuselage or a boat hull that have a more self stabilizing shape this effect might not be that prominent and there would perhaps seem to be little or no real difference if you consider this or not. Anyway, the reason is that most fiberglass fabric is not perfectly balanced, it means that there is perhaps 25 fibers (just to say a random figure) in each tow at the length of the roll and perhaps 24 fibers in each tow at the width of the roll. The epoxy contracts by a small amount when it cures and this will cause the fabric to contract slightly differently in the two directions, so you want the direction that contracts the most (or least) to be parallel at the top and bottom (or both sides) of your workpiece.

About making the spar joiner this would typically be made in a complete piston type of mold with clamps or bolts to compress it rather than using vacuum like I have done, vacuum is most beneficial with larger surface areas and a spar joiner like this does not represent a large surface area so you would get a greater pressure on it using clamps. If I was to make my joiner mold again I would make it like the sparcap mold with two molds in one and use a board on top of the foam pieces to increase the area and pressure like I did with the sparcaps, the pressure on the sparcaps have been probably about twice the amount of pressure compared to the joiner pieces because of the increased area that the floor board represents. Anyway, I like to do it this way because it is a lot simpler and less messy than using a complete clamped mold, also those joiner pieces that goes together to make one solid joiner is likely to be just a little stronger than it actually needs to be even with the moderate compression so I think this will be good enough.

To make a simple example on the pressure involved when using vacuum; at -0,8 bar or -800 mBar of vacuum (23,6 inHg or "inches of mercury") the total pressure on one square meter would equal 8,0 metric tons (using the weight of the earths atmosphere). So if you look at my shearweb plate that is 1/10 of a square meter, that makes a total pressure of 800kg or 1760 pounds lbs on that balsa sheet. If you have a small car that would do it too.

About the tool that I'm using - the vacuum pump; the easiest kind of vacuum tool to use is something like a laboratory device, this is typically a relatively small size and quiet running unit with a built in cooling fan and that is designed to be running continuously. Unfortunately these are often surprisingly expensive, there are other alternatives like a vacuum system with a tank and a vacuum switch that works similar to a typical garage compressor but you might have to make it yourself based on plans and instructions and it would mean some effort and time needs to be put into it before you can actually use it. I could make a thread about making an affordable and easy to make digitally controlled system using one or two 12volt mini pumps and a programmable Arduino board at a later time if there should be any interest for it. However, if you have the budget for it I would recommend a device that is designed to run continuously and that is capable of pulling something like 23 inHg or more, this means that you can just plug it in to the wall outlet and start processing immediately.

Finally I would like to say to those of you that gets an unpleasant feeling by thinking all of this is necessary to make anything in fiberglass for your RC hobby; it is not necessary with a vacuum setup, it just makes it easier to achieve a really good result in some cases and it opens up some new opportunities as well.