Making a mold of a Bob Martin SR-7 Fuselage - RC Groups
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Nov 17, 2014, 09:26 PM
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Making a mold of a Bob Martin SR-7 Fuselage


So Iíve been wanting to try my hand at molding a fiberglass fuselage. Iíve never done this sort of thing before and have several fiberglass fuses Iíd like to reproduce for spares and who knows what else.

I been studying up on techniques for the last 3 or 4 years. Reading up on anything I could find on the internet , asking questions, reading tutorials, etc. Iím a motivated do-it yourself kind of guy, but Iím not afraid to ask for help if Iím stumped.

I aready completed the mold and made some fuses so I will post this as trial and error tutorial and will document my progress with a "Day 1", "Day 2" basis to reflect the process. It spans several days as I did it between working my regular job.

Here's a list several items that I determined I would need to accomplish the task.

1. The plug - in this case I have a good fuse that I will use as a plug to make the mold

2. Jig saw

3. 3/4in Plywood for a base

4. 1 x 4 x 8ís enough to make a Parting box frame and for several cross supports to elevate and hold the fuselage.

5. 1 5/8" Screws to screw the box together

6. Some masonite hardboard large enough to give me at least 4ins of space around the item to be molded.

7. Stick on plastic bumpers to be used for alignment pins.

8. Carnuba wax, a parting wax

9. Partall Film #10 parting film solution

10. Clear Mylar plastic for a parting surface on top of the masonite Aircraft Spruce has this in 4 ft lengths bought by the foot.

11. A 2lb package of modelers clay, the stuff that doesnít harden.

12. Polishing cloths - Anything cotton like towels, t-shirts, cloth diapers work great.

13. A good epoxy laminating resin system, not the 5min or 30min stuff, but a resin designed for laminating.

14. Cab-O-Sil fumed silica as a thixotropic additive to the epoxy to make the epoxy fuction the way a gel-coat product would if you were using polyester resin. You add it to the Epoxy resin. Aslo available at Aircraft Spruce.

15. WEST SYSTEM 423 Graphite Powder. Wests Graphite Powder is a fine black powder that can be mixed with epoxy to produce a low-friction exterior coating with increased scuff resistance and durability. This is used as an additive with the Cab-O-sil. In this case as an extra measure to help the molded part release from the mold and for making a tougher durable mold surface.

15. Epoxy Clay just for the mold. I found out there are several products which you can buy that you mix together to make a dough or clay that are epoxy based. This will save you from waisting resinon fiberglass mat or fiberglass filler and wasting time and resin doing multiple layups to get a good enough thinkness on yor mold. The resin maufacturers love to sell people stranded mat cause it uses up and really wastes the resin.

16. A couple of yards of 6oz standard woven fiberglass cloth for the mold only. The least expesive 6oz cloth that is epoxy compatible.

17. Mixing cups. I found that Smart & Final has 3 1/4 oz serving cups that work great. I also picked up some 8oz serving bowls I guess youíd call them for spreading the mixed epoxy into so it has a large surface area to keep it from curing to fast after you mix it.

18. Acetone to clean off the epoxy before it hardens.

19 A package of the thick shish kabob sticks for mixing the resin.

20. A mini spray gun with at least a 1.0 tip to spray the PVA (PartAll #10)

21. A can of 3M ô Super 77 ô Multi-Purpose Spray Adhesive to stick the clear mylar to the hardboard.
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Nov 17, 2014, 09:34 PM
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Day 1


Day 1

After practicing on using a few cowlings as plugs and making a mold from them, then reading up over and over again molding threads and tutors, I felt I was ready to tackle a more complex molding project.

So the fuselage Iím molding is an Bob Martin SR-7 Limited Edition Sloper.

The first thing I needed to figure out was a simple and accurate way to trace the fuse onto the hardwood parting board for a cut line.To do this I needed to get the fuse aligned to the hardboard parting surface.

Using modeling clay I cut several chucks of it off and used them to form pillers to support the fuse so I could get the original parting line parallel to the parting board.. If you warm it up abit with a heat gun on one side it will stick quite well. I propped up the fuse on the clay and squished it down using the fuselage seam line as guide for leveling it until the nose and tail were parallel to the parting board.

Next I used my 6in Square to align the fuse at the wing saddle to get the saddle perpendicular to the parting board.

With the fuse aligned to parting board I next needed to make the outline I would use to cut a hole for the fuselage to sit in.

Day 1 to be cont.
Nov 17, 2014, 09:39 PM
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Day 1 cont.


To make the trace I first tried to use a sharpie but after marking the initial outline I relized that the outline I just made was going to be at least a 1/4 inch larger then the fuse I was tracing. This wasnít going to work as I would either have a huge gape to fill if I used the sharpie outline as a cut line or while cutting I would have to follow imaginary line a 1/4 inch inside the outline.

I thought about for while and remebered the certain ball point pens used a insert with a spring to retract and extend the ink cartridge. I searched around and found one I could use to make the tracing. I also decided to use a new parting board so I replaced the one I marked with a Sharpy so I had a clean canvas.

Day 1 cont. next post
Nov 17, 2014, 09:43 PM
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Day 1 cont.


Using some packing tape I taped the ink cartridge to one end of my 6 inch square with about a 1/16 of an inch of the ball pint extending past the bottom corner. The only way get a more accurate outline would be if I used a pencil lead by itself.

I don't know why I didn't think about before but if you extend the ink cartridge down further, thereby tilting the 6 inch square at more of an angle, you can get a perfectly matching outline as doing that would effectively move the ink cartridge in, Doooogh!


End day 1

to be cont.
Last edited by Roguedog; Nov 26, 2014 at 05:05 AM. Reason: added second sentence
Nov 18, 2014, 12:24 PM
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Nov 18, 2014, 05:30 PM
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Day 2


Day 2

Ok, Day 2 consisted of making a box frame with formers to support the fuse.

When I cut out the parting board I cut it 12 inches by 48 inches so I needed a Frame that would fit those dimensions. To make the frame and former I used 1 x 4ís form Home Depot. I cut two 4ft lengths and 5 10.1/2Ē formers. Three of the formers are to support the fuse.

I first made the box using the two 4ft sides and two of the 10 1/2Ē formers and screwing them together with the gold colored drywall screws. Before I add the other 3 formers (fuse supports) I needed to cut out the fuse outline so I screwed done the parting board to the 4inch frame and used a Jig Saw to cut it out.

I stayed about a 1/16 of and inch on the inside of the line I scribbed in an effert to minimize any gaps. This worked fairly well but next time I do it Iíll try tt at an 1/.8 of an inch then sand the edge to fit the fuse better. Iíd rather have an opening that is a little to small and then sand to fit.

You can try to cut out the center in one cut but I like to break long cuts up into smaller chunks,
Nov 18, 2014, 05:40 PM
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Day 2 cont.


Day 2 cont.

After cutting out the fuselage outline the next thing to do was to make 3 templates for knotching the fuselage support formers using a contour gauge. The spacing of the formers worked out such that I could space them 12 inches on center across the fuselage. Next I removed the parting board so I could add in the formers then screwed them on with the same 1 1/4in gold drywall screws I used before then re-attached the parting board so I could make the templates mark the edges of the formers for the next few steps.

To make the templates I sat the fuselage on the parting board surface lined up with the outline and then used the contour gauge at the 12in locations to get the specific contours I needed to make the templates. I transfered the contours onto pieces of paper to be used as the templates transfering onto the formers.

After using the fuselage cut-out to mark where I would knotch the formers I again disassembled the parting board and the 3 formers. I now needed to transfer the contour shapes onto the 3 formers.
Nov 18, 2014, 05:45 PM
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Day 2 cont.


Day 2 cont.

Next I needed to cut out the tracings I made to make the templates. Be sure to make label the top and bottom of the formers and to mark the templates so you donít accidentally make a reverse tracing then cut it out backwards. If you have ever tried making paper templates you know what a PINA it can be tryin to use just a piece of paper as a template. The papers thin and the papers edge isnít thick enough to use as a good template. If you on the other hand take box tape and tape several layers of the tape over your tracing then cut it out it has a decent edge that you can use to trace the contour on to another surface. So I placed 3 layers of box tape over the tracings that I made and cut them out.

Next I used the newly created paper and tape templates to transfer the contours onto the 3 formers and then used my Scroll Saw to cut out the knotches. Itís important to note hear that I did not even think of trying to accurately knotch the fromers for a perfect fit. Instead I cut out the knotches about and 1/8 to a 1/4 inch deeper.

The reason for this is you could really spend alot of time trying fit the the fuse perfectly to the knotches when thereís an easier way. Just cut them out a little deeper then use modeling clay to accurately center and align the fuselage to the fuses original parting line. So much easier as you can add or remove as much modeling clay as needed to get half the fuse lined up on the parting board.

End of Day 2
to be cont.
Nov 18, 2014, 06:43 PM
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Very informative tutorial!!!
Thanks for showing Everything and explaining the details.
I always wanted to try this and you have already answered many questions I had.
Looking forward to the rest...

DM
Nov 18, 2014, 07:00 PM
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Hey David! Thanks a lot.

It was a real cool learning experience.

More to come soon.
Nov 18, 2014, 08:26 PM
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Day 3


After gettin the formers knotched and reassembling the molding box and again re-installing the parting board i needed to add a sheet of mylar (plastic sheet) that I got from Aircraft Spruce as the actual parting surface and then get the fuselage centered on the original seam or as close as possible so that one half of the fuselage would be sticking out.

There are a great many tutorials on how to make parting boards and I just could have used the surface of the hardboard tempered panel, as itís really smooth to begin with, but using the mylar would give me a really smooth surface that the mold wax and PVA would not be able to permeate. I also wanted to use every means possible to protect my original SR-7 fuselage from getting stuck ether to the wood or the epoxy I would be using to cover it. So mylar sheet for the final parting surface instead of just the hardboard tempered panel.

I cut a piece of mylar to fit the top of the parting box then used 3M77 spray adhesive to stick the mylar to the hardboard. I sprayed a thin layer on the hardboard and the one side of the mylar, you donít need much. Waited 10 minutes then carefully aligned the mylar to the parting box and pressed it down. Make sure you line it up accurately before placing as you only get one shot at it.
Nov 18, 2014, 08:31 PM
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Day 3 cont.


Day 3 cont.

With the mylar properly attached in next needed to cut out the fuselage profile again. No power tools for this jsut an X-acto knife. While at the craft store to pick up some molding clay I spied a really cool looking xacto knife made by Fiskars itís a finger tip craft knife and works great. So I used that to trim out the center again.

Next I needed to get the fuse leveled such that only 1/2 half on the fuse would protrude though the parting surface. As I mention previously i knotched the former supports about an 1/8 to 1/4 inch deeper so I could use the modeling clay to accurately align the fuselage. This worked perfectly.

To apply the modelers clay I used a couple of hardboard scraps and a piece of 1 1/2 PCV as a rolling pin to roll out strips of clay then layed the strips across the bottom of the notches. Next using an aluminum 6 inch Square against the wing saddle of the SR-7, I pressed the fuse down to seat it on the clay to align the fuse perpendicular to the parting surface.

End day 3.

To be cont.
Nov 20, 2014, 04:16 PM
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Day 4


Day 4

Before going any further and i need to address how I prepped the fuse. Since i will be building my kit eventually I decided to prepare it for molding as best I could. I had took pics of this but the SD card got swallowed up somewhere and has yet to show itself. So no pics on this part.

Again, I’ve read and watched a lot of tutorials and the concensus is that you need fill any defects or gaps so the epoxy tooling coat doesn’t get or leak through, then wax the mold with a mold release of some type, and then for extra protection use a mold release film.

Depending on what you are going to use for your plug you’re going to want to make sure that the plug you are using has all the surface defects suct as pinholes, layup defects such as holidays (air-bubble sockets), or any other defects that could lock the mold to the plug, filled, so that the fuse will easily release later.

First thing was to determine if there where any holes that went all the way through the fuse from the outside to the inside. Easiest way to do that was fill it up with water. There where a lot of them. If the epoxy was able to run into the holes too the inside of the fuse this would surely lock the fuselage to the mold. I marked the pinhile locations and then looked for surface imperfections that could possibly cause the same thing to happen. There were a couple of spots where the fuse was joined together that didn’t get filled properly when the original fuse halves were combined. They needed to be filled as they were open to the inside of the fuse. I also found a lot of air bubble pockets all over the outside of fuse that needed to be filled as well.

To fix those I simply made up a mixture of 30 minute epoxy and cab-o-sil with the consistance of peanut butter and squeegeed it into the holes. some of the pinholes proved to be a PINA and several wouldn’t fill with the epoxy peanut butter mixture. To fill them i ended up just mixing up a small batch of laminating resin, brushed it over the pinholes and then used the nitrile gloves I was wearing as the squeegee to force it through those holes. Let it dry overnite and then wet sanded those spots with 400 grit until smooth. To completely check the fuse for any other bad spots I compltely wet sanded it lightly with 600 then 1200 grit.

Next I decided to close up the fuse completely as if it where a solid plug. The method I used works ok for any flat area like the where the canopy opening is but not so for the wing saddle area. What I did is took a piece of foam rubber and cut it just a little oversized to fit under the lip of the canopy opening and the wing saddle opening. I did the canopy area first and was able to use a single sheet of 1/8inch balsa inserted through the wing saddle opening. The foam rubber kept the balsa flat against the lip and I was able to use a few drops of super blue to glue it in place. Next i used bondo on top of the balsa so that it was level from side to side.

The wing saddle area gave me considerable grief. Because it is curved following the contour of the wing and since there was no other open in which to slide in a single sheet of balsa cut to fit I had to come up with another way. I want to say before had that I found a better way but this is not how you should do it. What i did was again use foam rubber fit a litttle oversize inside the fuse and then cut sections off a 3 inch wide sheet of balsa to piece in. At first I thoght this was going to work great. It tool about 5 pieces and I cut them oversize so that they could slide in under the lip of the wing saddle between the lip and thefoam rubber. Tacked all the pieces together and then used bonbo again to level it off side to side. I let the bonda dry and sanded the area smooth with 320 grit dry sandpaper. I now had what I thought was a solid closed off plug. More on this later.

to be cont.
Last edited by Roguedog; Nov 20, 2014 at 05:08 PM.
Nov 20, 2014, 04:26 PM
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Day 4 cont.


Day 4 cont.

After prepping the fuse and getting it centered and aligned the next step is to prepare the fuse and parting board for the epoxy tooling coat.

First step was to wax the plug. This is another step that I seriously reccomend. I used a pure Carnauba wax that can be found almost anywhere. No need for paying for a hi dollar mold release wax.

To protect the SR-7 fuse I gave it 3 coats of wax before I placed in the parting board. I next again waxed the fuse and the parting board surface (mylar plasic sheet) together with 3 coats of wax.

I read somewhere that you should wait 30 to 45 minutes between coats. I think that is just ridiculous to have to wait that long. It would take forever to accomplish anything that way. So I did 3 coats one after another.

The only wait time was a few minutes after applying the wax to polish it out. I recommend that while you are waxing the plug and the parting surface to rub the wax in like your sanding it. This forces the wax into nooks or pinholes that you canít see.

The directions on the can say to do not like the wax sit for more that a couple of minutes before polishing. If you have ever left wax on you car to to completely dry youíll understand why. Completely dry it takes forever to polish off not to mention the polishing cloth acts like velcro til you get through the dryed wax.

to be cont.
Nov 20, 2014, 04:49 PM
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Day 4 cont.


Day 4 cont.

Fillling the gaps between the fuse and parting board was the next step. For this I used the modelers clay. At first I used my hands try to make little rolls of clay to squish in between the fuse and parting board. Using my hands was ok but they got covered rather quickly with melted modelers clay after rolling up several pieces.

Instead of using my hands I got a piece of mylar and cut about a 6 inch square of poster board to use to roll out the clay this worded great and I could easily roll out uniform 6 inch lenghts and 1/8th of an inch thick.

To squish the clay into the small gap between the parting board and fuse I used what looks like some kind of dental tool I picked up at a local swap meet. This thing worked great and wedged the clay in nice and tight. To square off the clay I found these really cool rubberized caulking scrapers at Home Depot http://www.homedepot.com/p/DAP-PRO-C...9125/100666143. The kit comes with 4 little tools for scraping up caulking The blue square tool in the pictures has a regular corner and 3 different scraping profiles. This was the perfect tool for removing the excess clay that was left over as it won't scratch the surface of the fuse or the mylar.

This profile scraper one way can be used as a finishing tool to help smooth out the clay and as a tool for scraping the clay off.

I'll be using this tool for epoxy and micro balloon fillets later on wing saddles and for the vertical stabs.

to be cont.
Last edited by Roguedog; Nov 20, 2014 at 05:15 PM.


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