Ideas & Builds
Carbon Joiner Mould
5 January 2014
Fitting a straight circular rod into a very thin wing accommodating dihedral can be difficult. In fact we stumbled in a few issues when designing the evolution of the Omerta ALES ship for the Season 2014. The spar not being straight, at the joiner box, the rod must be at an angle vertically and laterally. The joiner box itself becomes a challenge since it must provide precise locating for the joiner by respect to the two ends of the spar.
In our case the angle is set at 5 degrees and the diameter of the rod to 3/8". You can see a picture of the final result of the mould ready to receive carbon rowings for our first joiner rod!
Hereunder the step by step process to build the mould…go get your balsa scrap box: we are going to use some of that!
Buy a rod with the diameter of the joiner you want to build. We used some brass, simply because we had some with the right diameter sleeping in a drawer. Make sure it is not dented and perfectly straight and round in section. Polish it carefully: each defect at this stage will be apparent on each of the pieces coming out from the mould eventually.
A side note: as a matter of fact this sequence is valid for a generic mould. Whether you are working on a fuselage or anything else it does not matter. Of course, size, type of material must be adapted accordingly.
Make a reference drawing onto which you are going to check at each step. Cut to pieces of the brass tubes and sand the appropriate angle on one side of each. Cut also two pieces of balsa 3/16" thick. This is going to be glued on both side of the tube. It is important that the material is half the thickness of the tube. You want the mid plane of the mould exactly on the diameter of the rod, so that you always have a positive draft angle to extract the finished part from the mould.
Dry fit all the parts on your drawing. No gaps between the rods and the balsa pieces are allowed at this stage, otherwise resin will flow through the gaps ruining everything. You want everything to fit tight together.
The next steps really depend on your material choice. I have used balsa (only for cost and ease of manufacturing) so now I have to fill in the vein so that the mid plane is going to be shiny as a mirror. Light spackling and sanding will do the job.
If one has used hard wood, plastic or another material characterized by a surface with a smooth and/or shiny surface just skip this step.
Proceed bonding all the pieces together on a straight piece of wood. This is going to be the base of the master. Make sure it is sturdy and straight beforehand. CA will do the trick here. Also add some walls all around the mould (see next pictures). This will keep everything together while the epoxy sets, and will make for nice edges for the mould.
NOTE: don't put any glue on the brass tube. Remember that the surface of the tube is going to be reproduced on every part produced with the eventual mould. Put glue on the wooden pieces only. Hold the tube by gently presseverything together. Trust me it will stay there!!! If hard wood is used instead of balsa, you are not going to be able to squeeze everything together, then you can use plastilina clay to achieve the same: keep everything together without gaps.
Now, prepare the glass you need. From left to right: the first ply must be light. It is going to better follow the shape of the protruding portion of the rod. Then you put a couple of heavier glass fabrics. Then chop some scrap glass fabrics in little pieces, we are going to use this to make the thickness of the mould. Then another couple of heavier glass to finish off. You need two sets of these, one per mould half.
Prepare some resin. I mean go for it, weight is not an issue. Put a minor portion aside, and mix into it some tixotropic to make it denser. Fillet the intersection of the rod and the middle plane so that the fabric will be able to follow the resulting surface. You can make a little tool suited to the specific shape of the fillet for your mould, in a matter of few seconds, using a balsa stick beveled as required.
Lay down the finer glass fabric, then the two thick glass plies. Use lots of resin. We do not care about the weight, but we want the whole thing to be bubble free everywhere. Put the chopped glass into resin and place it on both sides of the tube. Even out the surface with your fingers.
Lay down the last two heavier glass fabric plies and the mould is ready for curing.
Make sure the temperature stays higher than 20C during the cure process. Building an oven is very easy. You can use a carton box close to a hot spot. Ventilation is always recommended. So my bathroom fan is venting, while my heater is keeping the temperature at around 35C.
NOTE: the chemical reaction occurring while epoxy resin is curing generates heat by itself. A box with a lamp inside or a garage where the temperature is set higher than 20C is going to work as well. Do not exaggerate with heatâ¦this thing can catch fire!!!
Once the first half of the mould is cured, remove the wood construction but keep the brass tubes in it. Clean the first half of the mould carefully.
Then, wax multiple times your mould. Build walls with scrap wood (I used balsa again) and then repeat the same steps we went through to build the first half of the mould.
Once cured, drill the holes for the bolts clamping the mould closed while fabricating the parts. Do this BEFORE separating the two halves so that holes are perfectly aligned.
Now, remove the walls and separate the two halves. Congratulations...here's your mould!!!
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