Casting Convergent-Divergent Nozzles in Epoxy
Rendering of the 4:1
Results: As of Feb 25 2004 the 4:1 nozzle is ready. Casting results were good. As of Mar 5 2004 all three nozzles are ready. Housing in the test stand is next.
Method: The castings ended up being done in 5 main parts.
1. The wax cone for the inside of the divergent section
2. The plaster mold for the outside of the divergent section
3. The casting of the divergent section.
4. The plaster mold for the inside of the convergent section
5. The casting of the convergent section.
The picture above is an overview of the divergence section. It shows most of the basic tools and materials used.
Part 1. The wax cone for the inside of the divergent section
The first consideration is size. How big will the nozzle be? I elected to make the nozzles as large as possible and still fit inside an FTC. So the exit radius is the radius of the inside of the FTC, which is about 21mm. But my first wax cone ended up a tad small so I bumped the next 3 tries up to 22mm for a perfect fit.
The second consideration is angles. I choose 15 degrees for the divergent section and at first was going to use 60 degrees for the convergent section but decided to use a natural parabolic formed by wax cones (discussed later).
So now we know the size and angle, we need to calculate the hypotenuse of the cone. You can draw this out on graph paper and get good results or you can use a spreadsheet I built just for this purpose. If you have MS Excel, download cone.xls.
The hypotenuse of the cone is the radius of the circle we will draw on paper to form the paper cone, in this case 85mm.
Part1a. The paper cone
Square a piece of standard paper by measuring down the sheet the same length as the width (8.5 inches or 21.5 cm) and draw a line. Draw an X from each corner of the square. Repeat the X on the backside of the paper. Refer to the picture above, far left. For 15 degrees, the lines of the X will be line up lines when folding the cone. Using a protractor, draw a circle with the center at the center of the X and a radius the same as the hypotenuse (85mm in this case). Cut the circle out and in half (one sheet makes two paper cones). Twist the paper into a cone where the edge lines up with the lines inside and out. Tape the inside (neatly) and the outside with transparent tape. Re-tape the tip again as this tends to leak. A little leak is ok but it should not let the wax pour out.
Part1b. The paper cone holder
We need some way of holding the paper cone upright and level when we pour in the wax. I used a piece of FTC and PVC couplings (shown above upper far right) but whatever you build will be fine as long as it's level and won't fall over. Wax won't blister you can it can still hurt and will make a mess.
Part1c. The wax cone
Using the bottom half of a aluminum can, melt some hard wax (quality candle should be fine). Place the paper cone in the holder and carefully fill to the top to where is just trying to overflow but does not. It will bulge up a little due to surface tension and this is a good thing. Allow to cool off and remove the paper. With a bit practice it should look something like the following.
Part 2.The plaster mold for the outside of the divergent section
For this we'll need legos, a 1" PVC coupling, a bit of clay, 2 some thin strips of plastic, plaster of paris, paper, ruler and a protractor.
Part2a. The Lego Casing
Build 2 C-shaped sections slightly higher than the 1" PVC coupling. Using four's and eight's worked out just right.
Put the 2 lego sections together with the thin plastic strips between them. Draw the outline of the block on paper, inside and out. Take the 2 halves apart. Draw an X on the paper corner to corner of the square made by the Legos. Use the protractor and from the center of the X, draw a circle a little bigger than the coupling. This pattern will be used to lay out the mold before pouring the plaster. Put clay in the bottom of the coupling to help it stay put. Center it in the circle. Place the plastic strips up to the coupling as shown above. A bit of Vaseline will help them stick to the Legos.
Apply a thin layer of Vaseline to everything. Put the two Lego halves together lined up on the square on the paper. Put several rubber bands or tape around it to hold it together. Make sure the coupling is lined up on the circle and that the plastic strips are in place.
Part2b. The Plaster Mold
Mix and pour in the plaster to just over the top of the coupling. When the plaster is set, gently disassemble. Sand the top flat and remove any burrs.
Part 3. Casting the divergent section
Lightly Coat the inside of the plaster mold with Vaseline. Using the same plastic strips in between the halves put them together and hold with several rubber band. Place a wax cones inside.
Fill any gaps around the base of the cone with clay to stop any leaks. Place the whole thing on a plastic bag and put clay around the base to hold in place. Fill to just over the desired height with epoxy. Avoid air bubbles when stirring and pouring. My first tries had way to many.
Once the epoxy is very hard (24 hours at least) gently remove the plaster mold. Wash the Vaseline off with soap and water. Place the nozzle in boiling water to remove the wax. Re-cycle the wax. Sand the top and bottom flat. Polish the inside.
4. The plaster mold for the inside of the convergent section
For the convergent section I decided to use a natural parabolic formed by the wax cones when they cool. Each nozzle will get the same parabolic but each will be cut to match the throat. We'll need a length of FTC, a 1" PVC coupling, a 1 1/4" PVC coupling, a wax cone with a nice curve, and some plaster.
Assemble as shown with the wax cone base up. Pour in some plaster.
Once set, remove the cone from the FTC.
Separate the mold from the cone and sand smooth.
To cast the convergent section, place the plaster mold in a short length of FTC and pour in enough epoxy to almost cover the tip of the mold. Also shown is the completed divergent section in FTC.
Repeat for the different size nozzles desired. Here are some results of the 4:1 and the 2:1. Will post more as they are built.
