I'll document my progress on fabricating a two piece mold for a composite nose cone on this page. I have made molds for composite nose cones before using a pour in or brush on silicone mold compound, see the process here http://www.thefintels.com/aer/casting.htm. But this is a much larger nose cone for 5" diameter rocket, and the silicone mold process likely wouldn't be very effective here.
After some consideration and some practical testing of using a foam turned core nose cone plug, I decided I'd like to try the more common two piece mold process. In the foam core plug process, you make a male plug just like a regular nose cone, only it is built slightly undersized, then the composite is laid over the plug to build it up to its final size. The foam plug can then be removed with a solvent, or left in place depending on the application. The problem with process is that it is fairly time consuming and you're left with only one chance at a nose cone. This process also leaves a rough surface finish that would require a lot of finish work, and the dimensional tolerances are not very good.
The two piece fiberglass mold is more time consuming and expensive to build. But you are left with a plug that can be used to make future molds from, and a mold that should last indefinitely for making future nose cones relatively quickly and easily. To get started I ordered a mold parting wax and PVA mold release from Fibre Glast. From the research I've done, you can get by using a pure carnuba wax, and use certain types of hair spray as a mold release, but I really didn't want to take a lot of chances, so I ordered the products from a company that specializes in composites.
The first part of the process involves making a plug to the exact outside dimensions of the desired part. This is pretty easy for a nose cone if you have a lathe. I did, so I glued together 7 pieces of knot free pine 1x6's to form a block of wood. After the glue cured overnight, I trimmed it square, then I screwed it to my 5" face plate for my wood lathe.
This is the start of the nose cone plug. I've turned it down and done the initial rough sanding.
I removed the wood plug from the wood lathe and I'm creating a tip from epoxy car body filler.
From this point on, I'll have to chuck up the shoulder end of the plug in my big metal lathe as I will no longer have a pivot point on the tip of the nose cone plug. While I had the body putty mixed, I filled in any deep scratches or holes with the putty. After the putty had set up, I chucked the nose cone plug into my big South Bend lathe and did a little final rough sanding with some 40 grit paper. Yeah, you read it right, 40 grit paper! That really takes those high spots down fast. Then I used some emery cloth tape, and lastly some 220 grit paper to get a fairly smooth surface while it was in the lathe and easy to sand.
Next was an initial primer coat, after the primer had dried for 30 minutes or so, I used automotive spot putty to fill in any sanding marks or pin holes in the Bondo putty.
The plug after sanding the first coat of primer and spot putty. I start sanding the primer down with 220 grit paper, then move on to 320 grit paper.
After the second coat of primer.
This is pretty much the same process I use to finish any surface. Prime and spot putty, sand, then do it all over as many times as needed. In most cases I'll go with 4 or 5 coats of primer, sanding after each one. Each time I prime and sand, I go with a finer grit sandpaper. The final coat or two of primer will be wet sanded with 400, 600 then 800 grit paper. While I do like nice smooth, shinny nose cones, this plug needs to as close to perfect as possible to keep the laid up fiberglass mold from sticking to the plug. One area of concern is the lip on the shoulder of the nose cone. I used clear pine, but pine isn't a very fine grained wood and the end grain on the lip is going to be a hang up spot for the fiberglass mold. I may be able to clean it up some with a polishing wheel on my rotary tool, but I may have to come up with something else as well. Perhaps the PVA mold release will fill some tiny holes, but I may have to rub wax in the grain to seal it...
I did end up rubbing some paraffin into the end grain at the shoulder, as it turns out, the parting wax I'm using has paraffin listed as one of it's ingredients so I'm think I'm OK. The parting wax and PVA was ordered from Fibre Glast and arrived in short order. So I set about waxing the nose cone plug with Rexco Partall Paste #2, six coats in all. Buffing well by hand after each application.
Here's the plug after a couple coats of wax.
Next I cut out the silhouette of the nose cone in a piece of masonite. This masonite board will form the parting line of the mold.
I reinforced the bottom of the board for stability and to keep the nose cone from rolling about on the work table. The 1" x 3" boards were hot glued to the masonite, the boards themselves were screwed together.
I sprayed the masonite surface with contact glue, then applied a layer of "mylar like" clear plastic. It's really mylar without the metal coating. The plastic was cut to form the nose cone silhouette with a razor blade.
This may have been the hardest part yet, getting the nose cone properly positioned in the parting board frame. The plug needs to be properly positioned exactly on the plugs' center line, if it isn't, the mold won't come off the plug. I used masking tape to loosely hold the plug in place, then held it in the proper position with one hand and applied hot glue with the other hand. The hot glue needs to stay on the back side so it doesn't affect the up side of the mold.
Once there was enough hot glue to hold the plug in place, I started filling the cracks around the plug with modeling clay. In this picture you can see the hot glue tacks, and the clay forced in from the bottom side. I used Permoplast non hardening modeling clay. I have no idea if this is the best clay for this application, but it seemed to work fine.
Once all the gaps were filled from the bottom side, I went from the top and applied more clay. I used a small plastic tool to scrape away the clay and leave a reasonably smooth surface. While the clay is a little messy to use, it's about the only way to get a really good seal between the plug and the parting board. The epoxy that will be applied really won't stick to the clay, making removal easier.
Now it was time to apply the PVA mold release. The best way to apply is using an automotive style spray gun, which I have, but I understand it could be brushed on as well. The spray gun will likely result in a better finish. I applied three medium coats, enough that the surface formed an even, glossy surface after each application. I waited about 5 minutes between coats.
Now that the plug and parting board were ready, it was time to apply a gel coat to the surface. You can buy special gel coat epoxy, but I simply thickened some epoxy with graphite dust (I've got plenty of that!) and some talcum powder. The idea is to get the epoxy thick enough that it can be painted on the plug surface and not run off. No glass is applied with this first coat of epoxy. The black graphite dust not only thickens the epoxy, but make a nice black surface that makes it easier to see how the white fiberglass cloth is wetting out later.
I allowed the gel coat to cure to the full tack stage, in other words, not cured but not wet either. Then I applied the first layer of glass cloth. Really, you're supposed to use a very light weight glass cloth for this first layer, to make sure it lays into all the cracks and corners. But I used my standard 6 ounce fabric, cutting it into strips where required to fit into tight spaces. I allowed this first layer to cure overnight, then rough sanded and cleaned. I applied 8 more layers of glass cloth in two lay ups, allowing the epoxy to tack up again before the last set of 4 layers were applied.
The next day I removed the parting board to reveal the uncovered half of the plug.
This was a critical stage in the mold making process. I wasn't sure how well things would come apart... I learned a good lesson too, don't use any more hot glue than absolutely needed! I had a terrible time getting the hot glue off the plug. Most of the time hot glue just pulls off a part. Not this stuff, it had a firm grip on anything it touched. I do believe a removable hot glue is made, and that would have been the way to go, or find another method of attaching the parting board to the plug.
But I was "stuck" with what I had done. So I ended up using a hair dryer to heat and soften the glue to where I could scrape it off with one of my plastic clay modeling tools. In the process I managed to scratch, dent and in one place cut out a small chuck of the nose cone plug. Which means now I'll have to do more touch up work on the finished nose cone later. Well, live and learn. The good news is, the parting board came right off after the glue and clay were removed. In the picture above you can see the parting plane is perfectly smooth, almost glass like. I was tempted to remove the nose cone plug from the mold and turn it over, to use the good side I hadn't dinged up. But I decided against that.
This is the parting board after removal from the plug. I'm peeling up a layer of the PVA film left behind. This stuff really works!
Now the other side of the plug was prepared in the same way as the first side, a couple more coats of wax on the nose cone plug and the dividing plane. Then three coats of PVA. Then I applied the gel coat and 9 layers of glass cloth.
Once the final layers of cloth were cured, I drilled 8) 1/4" holes around the flange of the mold. These holes will align the mold again when assembling it, and also help hold it together when a nose cone is laid up inside the mold.
I trimmed the edges of the flange so I could get in to separate the two halves from the plug.
I used wood wedges once a corner was lifted to pry the mold apart. This was another critical stage, I've heard horror stories of broken molds and damaged plugs trying to remove the plug.
The first half of the mold came off with only light pressure, so far so good.
The second half came off just as easy!
It's not perfect, the lip at the shoulder isn't very clean, so I'll have to work on that. But all in all it's a pretty good looking mold. I cut off the excess at the shoulder end of the mold and washed it up in warm soapy water. From what I've read, you're supposed to finish sand the inside of the mold, but this is so smooth, I can't imagine getting it any better with sand paper. So I'm going to wax and spray it with PVA prior to laying up the nose cone. I can't believe how much epoxy and cloth went into this mold. I could have made a couple of 3.5" diameter x 6' long fiberglass rockets with what I used! It looks like I have just about enough epoxy left to get one nose cone made, I ordered more epoxy to be on the safe side.
The mold making process is done. Next is lay-up of the nose cone itself, which should go a lot faster. I suppose I should let the mold cure for week or so before I use it, but I'm not sure I can wait that long!
To start the lay up of the nose cone. I polished the inside of the mold with two coats of parting wax, then sprayed with two coats of PVA. Next I mixed up a batch of gel coat, this time thickening it with talc rather than graphite because I wanted the nose cone RF transparent. I did use some carbon fibers in the very tip of the nose cone to reinforce it there. I chopped up some carbon fabric, mixed it with epoxy and applied it to the top 2" of the tip. After allowing the gel coat to firm up to the hard tack stage, I applied the first layer of glass cloth. I was worried about letting the epoxy start to set up, and creating a gap at the parting line. So I went ahead and assembled the two halves right away.
Here is the mold bolted together and the first layer of glass/epoxy curing.
I allowed the first layer of glass to set up to hard tack again, then cut strips of glass cloth, some strips triangular, and some in rectangles to lay up inside the nose cone. I brushed on epoxy inside, then applied the strips of glass. This process didn't go as well as I had hoped for, it was very difficult to get all the way into the 28" long nose cone, and get the strips of cloth to lay flat. I should have about 3 layers of cloth over the entire inside surface at this point. I think once cured, I'll demold it at this point just to play it safe. Should I run into problems, I'd rather ruin the nose cone than the mold. If the nose cone comes out and looks good, I'll lay up more layers of glass after it's out of the mold.
The next morning I pulled the mold apart, again I used a couple of wood wedges and the mold came apart easily.
The final demolding of the nose was a piece of cake! Here's the nose cone out of the mold. You can see the excess PVA at the parting line joint. This nose cone will still need a couple more layers inside for added strength and some touch up work, but it looks pretty good.
As I indicated at the start of this project, the mold making process is pretty involved but straight forward. And this proved to be the case. It's a lot of work, and fairly expensive when you factor in all the glass cloth, epoxy, PVA, parting wax and other odds and ends you go through when working with epoxy. But I'm left with a mold to make future nose cones from, and a very nice, strong, lightweight nose cone.
Here's a look at the nose cone finished and installed on the Defiance Rocket. What looks like dust on the nose cone is reflections from the ceiling and floor.
Here's the nose cone shortly after it's flight to 29,000' and almost 1,400 mph. Notice the blistered paint from air friction heating on the nose cone. The blistering was at it worst where there was spot putty filler under the paint.