acdii

I am going to make a mold off of the cowl that comes in the CG Chipmunk kit. Question I have, should I first cuat apart and assemble the cowl, or should I make the mold off the cowl as it comes in the kit? My thinking is I make the mold off as it sit, then cut it in half and have the two halves for making the actual cowl as one piece.

MTK

The way I do cowl molds depends on whether the cowl is flanged around its perimeter for stiffness or is a simple non-flanged part that mates around the fuselage.

If the cowl is simple with minimum compound curves, I make a one piece cavitymold.

If the cowl has a lot of complexity with undercuts like flanged mating to the fuse, a two part mold is simpler to work with in the long run. I install a parting board in the middle of thepart being molded and proceed to cast the two piece mold. I create a flange at the parting board so that the two halves can be re-alligned precisely.

Once the first half is curedI remove the parting board only, prep cowl half that's exposed and the new flange I just created and cast the second half directly onto the first half. I don't separate until the second half has cured. But first I add a series of hardware (blind nuts and screws, usually 8-32) to the flanges, then separate the two halves.

When making the actual part, Each half can be worked independent of the other, casting in all undercuts and any specific complex details. I leave about a 1/2" strip unglassed in the crown where the mold flange is, until I put the two halves together and bolt them securely. ThenI add the last strip of glass while the epoxy is uncured. No need for a taped seam which isn't as strong and adds weight. The whole cowl is cast as single piece in a two part mold without a seam tape section. Strong and light and simpler to separate once cured

speedracerntrixie

My Feedback: (29)

Take the ABS parts and use them as the molds. A coat of Partall #2 wax and a light coat of PVA will have you ready to lay in a few layers of 3.7 cloth. Yes the cowl will be a little smaller but not enough to make a difference. I have done this on several airplanes including the CG Chippy.

SCALECRAFT

My Feedback: (13)

The plug (cowl master pattern) should be kept as one piece, complete, set IN a parting board down the center with key way dowels to align the two half molds. This insures a perfect seam and alignment.

You can assemble the mold and make a one piece cowl or lay up two halves and join the halves. The back of the cowl should also have a parting board/trim board to use as a guide when trimming the glass once, at what I have seen called "B" stage, or soft enough to trim clean but not fully cured.... You can also put a small hole in the prop shaft area to blast compressed air into to ease removal of the "part'. It air hole is normally covered with clay before lay up.

The Chipmonk cowl has cooling intakes that are not a problem when separating the mold if not to deep.

See what I mean about parting boards,, page #5, post #107 on this form for "All composite F-104" I am fabricationg

The weight of the glass depends on if the model tends to be tail heavy. This is one of the factors we use in the glass choice. Note: Consider thickness at cowl to fuse joint on the cowl thickness, at that section were the cowl fits over the fuse or the a bulkhead.

Steve

speedracerntrixie

My Feedback: (29)

I would agree if this was for production and one needed a mold for many cycles. For these one offs using the ABS as the mold works just fine. The Chippy cowl would be a really easy one as if I remember correctly, it is a 3 peice deal with the cowl front a seperate peice. The size difference is just about 1/16" so not really a factor.

acdii

Here is the cowl,

SCALECRAFT

My Feedback: (13)

My answer was based on the modelers first sentence .

Steve

speedracerntrixie

My Feedback: (29)

Fair enough but my intent was to point out that making a mold was going to be an unnessesary step unless he wanted to go through the process of learning how to make a female mold.

acdii

Wheres the fun in that?  Challenges baby , CHALLENGES!    I dont want a one off cowl, I want to be able to replicate it in case I nose it in and have to replace the cowl.  Been there, done that, and have an ugly looking T-34 now. 

MTK

This isnot difficult but is redious and a littletime consuming even for a simple cowl. As I explained earlier, do the parting board, essentially splitting the cowl into two sections down it's long side. The material that works well is simple pressed paper board from Home Depot, the ttype with one shiny side...about 1/8" thick, cuts easily with a band saw. Mount the cowl on a large piece of board, shiny side up, then install the the cut/formed piece of board around the center of the cowl. Back the board with plumber's putty to secure. Also, for a really crisp edge, fill the nooks around the cowl and board mating surface and trim the putty with a razor.

Now you are ready for waxing. I use and recommend Part All. You need to wax the parting board also. 3-4 coats is enough, with light buffing between coats. Then spray PVA on the side you want to mold first and let the PVA fully dry an hour or more.

The first layer of epoxy is a filling or surface coat. I simply mix enough epoxy to coat that the exposed area fully. For a smaller cowl, 8" wide and deep give or take, it will take about2 ozs epoxy. I tend to use medium hardener for this layer, 30 to 40 min pot life. Then I add cabosil (fumed silica) to thicken and a half teaspoon of carbon dust, making a black thin paste, about the consistency of yogurt or a bit thinner. I lightly brush the mix into the preppred make part I need molded. This is allowed to set. Make sure you go over the corners several times to avoid holes

ThenI apply reinforcing layers of 9 oz glass directly over the set but still tacky surface layer. You want a total weight of glass of at least 36 ozs for a sturdy,fairly stiff and strongmold. That's 4 layers of 9 ounce glass. Cutting the glass on bias allows easier working. Work theglass onto the parting board also, developing a flange on your mold. Let that rest and allow24 hours for a good cure. If you can place the casting over a heat source like an oil type space heater, curing is faster and more complete.

Remove the center parting board but leave the base parting board. I place a small plastic wedge on an edge of the flange to help separate the parts later. Just make this wide enough to allow a screwdriver blade. Clean the xposed surfaces well and wax the same as before; PVA as before. Repeat the process.....Once cured but before everything is taken apart, I install hardware on the flange. Typically 8-32 nuts and bolts work well. 8-32 blind nuts installed a scrap plywood serve well. Once that's cured, I typically take a belt sander and smooth all the edges of the glass, then pry the pieces apart. I often find that light tapping with a rubber mallet helps separate the parts.

I use good quality laminating epoxy for the surface coats. West Systems, Pro Set, MGS, or even US Composites epoxies are all excellent as surface coats. Cure very hard and maintain a good surface longer. I often will use less expensive second quality epoxy for the reinforcing layers. such as Aeromarine John Greer, or even epoxy from Home Depot. Molding isn't hard and neither is casting the part afterwards. I often will make molds for stuff I want to recast. Recasting into your own mold allows you flexibility in weight reduction, proper reinforcement where you want it, etc.

Fiberglass pants for example are one such casting where the main body is lightweight glass, with carbon reinforcemnets exactly where I want them. Since I also align the attachment pointis in the mold, my spats come out extremely light (one third to half the weight of commercial units) and with all hardware ready installed and aligned....Good luck. It's a great skill to learn and develop

acdii

Did you see the picture above of the cowl?  Its already split down the middle.  It's pretty much what I have in mind what you wrote up, but both halves done at the same time, and just cut it down the middle.  Reason I dont want a one piece, or to assemble the cowl first, then make a mold is so I can get the vent openings at the nose done right and not have a seam  that would have to be filled and sanded when the part is molded, would rather have a slight build to sand smooth than to have to go back and fill any holes. 

MTK

Put in whatever holes you want, it's not a problem with my technique.

When you cast the actual part, there is no seam eventhough the mold is in two halves. Each half is layed up in a such a way as to leavea small strip unglassed near the flange or crown. Once both halves are glassed to this point, the mold is then put together. Thefinal connecting strip ia dded to the crown, as I described in the earlier post. When the mold is done right, only a slight line is left on the part which is removed with very slight sanding. There is no filling involved unless you butchered something...

I've given you techniques FREE that have taken me many years to perfect. Use what you want and don't use what you don't. Either way, the least you can say is THANKYOU

SCALECRAFT

My Feedback: (13)

Along with the comments by MTK. Fully assembled cowl (plug) is the best, most precision reproduction of the original, placed in a parting board. You make a perfectly finished cowl, fill and finish seamon plastic original. Then create a 2 piece mold. Key ways a must. A trim board at the cowl back makes a perfectly trimmed cowl out of the mold. Paper thin flashing at the seam. Minimal clean up.

An example for a flying composite scale prop..

Steve

SCALECRAFT

My Feedback: (13)

RCU pics don't load. That's one reason I am mostly at RCG

Steve

acdii

Thanks guys. I wasn't sure if you could see the pics I posted or not.  So assembling the cowl first is better than making a mold off them as it sits.  That's what I needed to know.  Well that gives me something else to work on until the stuff I ordered arrives. I needed some Dope and other items to start the covering, so I can work on perfecting the cowl in the mean time.

SCALECRAFT

My Feedback: (13)

Yes. That's what I have done, am doing, and will do.

You parts will have all the perfections or flaws as your original. The standard finishing may fall short when the "Mother of All" your other reproductions are made.

Steve

daytonarc

My Feedback: (5)

As stated assemble first and make 2 mold halves. See the videos on this link for simple instructions on how to make plug,mold and lay up and join both halves. http://www.fibreglast.com/fibreglast...old_fibreglass

SCALECRAFT

My Feedback: (13)

Note: If you make molds and join the halves together before laying up the cowl, you can avoid the joining part by having a one piece cowl with a very fine flashing line.

Split your mold open and your done.

The lay up is a bit more difficult. But practice will make it easy. The mold teaches one how to lay the glass and where to use epoxy/cabosil mix ( thickened epoxy) as needed.

Making molds and composite parts of your favorite models make them always available. Never obsolete.

Good luck and stick with it.

Steve

acdii

Got my parting wax in, so its cowl mold time. I know what I will be working on this week.

acdii

Well one half of the mold is curing. Using West Systems epoxy for the mold, and followed CST tutorial for making a cowl mold, graphite for the gel/tooling coat, followed my several layers of glass.

Discovered dont do 8 shots of Epoxy unless you can spread it quickly, Didnt have as much time as I thought and damn does that s**t get hot! Turned solid in the cup, so I wasted at least 3 shots worth of epoxy, and not too happy about that, would have been enough for at least the starter layer for the cowl itself. Whats interesting is the stuff already laid was still fluid, so guess when they say it cures fast when confined in the container, they just aint a kidding! In any case, 8 shots was exactly the amount needed, I mixed a small batch after the hot stuff and was able to finish the first glass layer. I let that one setup a bit and finished off with heavy mat followed by fine cloth for a top layer.

Tomorrow I will know if I did the waxing and release correctly when I remove the parting board. Keeping fingers crossed.

sensei

I think for my own use I would have assembled the two halves, attached a base plate, and performed all the cosmetics prior making my mold. I say this because once you have that you can layup your cowl, de-mold and your ready to install without the need for any assembly work of the cowling, besides it generally comes out a lighter if you one of those that counts grams.

Bob

acdii

I assembled the cowl already, thats why it has taken me so log to actually start on making the mold. Trying to get the surface as smooth as possible on that plastic is a PITA. This is before the layup on one side. I have one side completed, and took it apart to clean up the edges, and prep it for the other half. The mold looks pretty good, and everything separated cleanly. I rewaxed the cowl after cleaning up some errant smudges of Epoxy on the back side, cleaning off the clay, and making the seam nice and clean. Its sitting with a layer of hairspray on it now, need to give it one more coat and take care of any runs, then I can finish the mold.

Amazing how well a $3 can of Suave hair spray can work as a mold release. I found you can use it, and tested it out, and it works perfectly.

acdii

Mold halfway completed, sitting ready to do the other half.



I really like working with the West System Epoxy 105, ordered me another quart so I can do another mold for another plane I have.