In Search of ... Symmetrical Fins

When dealing with F3K, they're trying to hold surfaces to a 1-2 thousandths tolerance, so the tooling reflects that. The vacuum hold down allows the second side of the wing to be cut without any (or very little) manual clean up afterward. Since you don't need that kind of tolerance, you can be a little more liberal with your mold making. If you're going to use MDF as a mold material, I recommend using laminating epoxy to coat the surface. That should give you a good enough surface for a clean release.

Well I've done a bit more. I actually put a slab of MDF on and gave it a whirl. So far the results are a bit mixed, with the negatives being operator induced. So it's part of the learning curve.

One of the bigger disappointments is the the recess for the ball doesn't fit. So either the hole or ball or both are out of spec. I got the digital calipers out and mic'd the ball and it came out dead on 0.500". Sigh. It's the G-code writing software. While it may be great at prototyping, maybe it's not so good at hitting the mark. Sigh again. I have found it to not work with circles (I've had to come up with my own tests and calcs to get that to work) perhaps this is part of the same weakness. So the workaround works, it's just a bit of a pain to do when the software should nail it for you. The problem is that spherical voids are much harder to resolve than 2D circles. Don't know what I'm going to do about that. How many workarounds does one do before you shelve it and "bite the bullet" and move on to something else (and spend money)?

Anyway, here is a pic of the left-side, practice mold.



Greg

GregGleason said:

Well I've done a bit more. I actually put a slab of MDF on and gave it a whirl. So far the results are a bit mixed, with the negatives being operator induced. So it's part of the learning curve.

One of the bigger disappointments is the the recess for the ball doesn't fit. So either the hole or ball or both are out of spec. I got the digital calipers out and mic'd the ball and it came out dead on 0.500". Sigh. It's the G-code writing software. While it may be great at prototyping, maybe it's not so good at hitting the mark. Sigh again. I have found it to not work with circles (I've had to come up with my own tests and calcs to get that to work) perhaps this is part of the same weakness. So the workaround works, it's just a bit of a pain to do when the software should nail it for you. The problem is that spherical voids are much harder to resolve than 2D circles. Don't know what I'm going to do about that. How many workarounds does one do before you shelve it and "bite the bullet" and move on to something else (and spend money)?

Anyway, here is a pic of the left-side, practice mold.

View attachment 176181

Greg

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What kind of bit are you using here? It looks like a ball end mill, but tapered from the shank to the ball end.

That's a 1/16" tapered ball end mill with a 1/4" shank. Using that one for the LE and TE ops. I think I'm using about 5 different flavors of end mills for the mold.

Greg

Getting back at this again.

Here is another attempt that I am mostly happy with.



Greg

I had a simple fin design, that I think is a little more field friendly where I fly.

Now that I have one mold half finished (at least the ready for prep) I have a little bit more confidence that I can make a second mold half.

Again, I'm a noob at this so I am learning as I go.



Greg

Here is a blank that has been decked and is ready for milling.



If everything goes ok, this will be the other half of the mold.

Greg

So here are the mold halves apart ...



... and here are the mold halves together.



They seem to fit exactly as I hoped.

After lightly sanding the mold surfaces, I coated the surfaces with Aeropoxy laminating epoxy and let it soak in (the MDF is like a sponge) and placed 2 additional coats on. Then I took a paper towel made damp with acetone and quickly buffed the surfaces to remove any surface excess. I'll let it cure 24 hours or longer before I do further prep on the surfaces.

Greg

Personally I think symmetrical fins are somewhat over-rated.......

BABAR said:

View attachment 269111View attachment 269112View attachment 269113View attachment 269114View attachment 269115View attachment 269116View attachment 269117

Personally I think symmetrical fins are somewhat over-rated.......

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LOL! Well, to each his own. Those are very creative designs BTW.

Greg

After giving the mold halves cure for a couple of days in 110 F heat in the curing oven (aka, the garage), it was time to start sanding the surfaces to get them ready to be waxed.

I started with 600 grit as the roughest because at this stage I am not interested in taking off wood as I am getting it to a polished surface.

At this point it is just classic "old school" sanding. Nothing glamorous about it.

After the 600 grit, the sequence was repeated for 800 grit, then with 1000, 1200, 1500, and finally 2000.

While the surface is not a mirror finish, I can read larger text reflected in the surface.

I then put a coat of "PARTALL" paste #2 mold release wax, let it dry a bit and then buffed it out.

By that time it was dark, so I called it a day.

Greg

I finished the application of the third coat of wax.

Below you can see what it looks like after the 2nd coat of PARTALL:



The first coat I let sit overnight (mostly because I just didn't want to anymore that evening) and let an hour go by between the second and third coat. I used cosmetic cotton pads for the application and buffing. I did use a Dremel with a buffing wheel (low speed, light pressure, kept it moving), but it didn't seem to make any difference that I could see. It's all part of the learning process.

After the second coat, I tried to evaluate the ability to not stick by using a piece of blue tape. It stuck a little, so I am hoping by the fourth coat it won't want to stick much at all.

After the fourth coat of wax, I plan to finish the molds with a coat of PVA.

I do plan to vac bag the fins. Perhaps a 2 or 3 oz fiberglass for the outside, followed by 2 or 3 layers of 6 oz FG for the inner layers. At least that's what I'm considering at the moment.

Greg

GregGleason said:

I finished the application of the third coat of wax.
After the fourth coat of wax, I plan to finish the molds with a coat of PVA.

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I've always used three coats of was and one application of PVA, but I think you're smart being extra cautious with a more porous mold.

JohnCoker said:

I've always used three coats of was and one application of PVA, but I think you're smart being extra cautious with a more porous mold.

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I think your right John. While the surface is very smooth and planar, there are some surface irregularities. I think that a follow on coat of epoxy would have perhaps nailed down the remaining bits, but that's water under the bridge. So I'll go with a fourth coat of wax before the PVA.

In the future, I may go with Corian® as a mold material. But for now I'm content with MDF as part of the "dues paying" as I learn this stuff. Actually I'm somewhat amazed at the progress I've had.

Out of curiosity, what is your method of application of PVA? I've wanted to stay away from spraying, as that seems like it may be more trouble than it's worth.

Greg

GregGleason said:

Out of curiosity, what is your method of application of PVA? I've wanted to stay away from spraying, as that seems like it may be more trouble than it's worth.

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I have always sprayed it, but you can also brush it on.

JohnCoker said:

I have always sprayed it, but you can also brush it on.

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Thanks John.

I put the fourth and final coat of wax on. But before I did that I thought back to my motorhead days (from decades ago) that when you waxed a car there was a trick that if you used corn starch it would help the wax buff out better. So I knew that tricked worked then since it worked then. I just happened to have corn starch on hand so I gave it a go. It worked. It definitely helped the buffing out process so that was a "win".

So the next step will be the application of the PVA.

Greg

In researching the application of PVA, I found this advice:

The trick to PVA is the application. Use an old white T-shirt and cut a little 2" x 2" piece from it. Fold this piece by the corners, like you're wrapping a ball-bearing up. Dip into the PVA and wipe it all over the mold. Let sit for about 1min, depending on the temp and humidity. Then use the same cloth to wipe again, keep rubbing until you have mirror finish. Let dry.

https://www.rcgroups.com/forums/showt...&highlight=PVA

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So I took an old T-shirt and cut it like above and did according to the instructions above. It's virtually invisible so I'm hoping that it is just a thin coat.

Now I need to think about how I'm going to cut the fabric.

Greg

I made a paper guide to help with the fiberglassing the fins.



I cut rectangular pieces of the fiberglass and tacked them to the back of the sheet with 3M 77. I then used a straight edge and an Olfa "pizza style" fabric cutter to cut the longer straight sections from the paper. The shorter lengths were cut with scissors.



The good thing about the paper backing is that it keeps the FG from fraying. The bad thing about the paper backing is that it is very stuck to the FG. A trick I just learned is that if you apply denatured alcohol to the paper and let it sit for 2 to 5 minutes between paper towels the 3M backed paper will come right off. The interesting thing is that acetone will not do the job. I don't know if isopropyl alcohol will work like denatured alcohol.

Greg

GregGleason said:

I made a paper guide to help with the fiberglassing the fins.

View attachment 269586

I cut rectangular pieces of the fiberglass and tacked them to the back of the sheet with 3M 77. I then used a straight edge and an Olfa "pizza style" fabric cutter to cut the longer straight sections from the paper. The shorter lengths were cut with scissors.

View attachment 269587

The good thing about the paper backing is that it keeps the FG from fraying. The bad thing about the paper backing is that it is very stuck to the FG. A trick I just learned is that if you apply denatured alcohol to the paper and let it sit for 2 to 5 minutes between paper towels the 3M backed paper will come right off. The interesting thing is that acetone will not do the job. I don't know if isopropyl alcohol will work like denatured alcohol.

Greg

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The airplane guys use waxed paper and an extremely light coat of 3M77. I mean, I've sprayed it at 12" away over 4 feet for less than a second a pass and it's enough tack to get the job done. Even then, sometime it is difficult to get off. Food for thought.

I agree. You definately want to go light with the stuff. I would be happier if it had less than half of it's stickiness, but it works for what I want it to do.

I hope to do the layup in the next few days, so I'll know soon if it was a good idea or not.

Greg

Vac Bag Day

Let me say that this was a bit of a challenge since the last time I did vac bagging was on plate material and that was perhaps 5 years ago. So to say that I'm a little rusty would be an understatement. I toyed with the idea of doing both fins at the same time but quickly decided that would be a bad idea because of my lack of experience.

I placed the cut FG items on paper towels and applied the denatured alcohol. On the larger items, it took 2 dowsings of the alcohol to free it from the paper. On the "skinny" FG items it was a fail. There is a minimum width and my guess is that width is about 1/2" (12.7 mm). So I had to do manual replacement cuts on the fly.




I mixed the 1-hour Aeropoxy laminating epoxy for this layup. I set aside some of the epoxy and mixed it with microballoons to do a bit of a gel coat on the surface. Then I applied the FG and used the neat epoxy. I then put the peel ply on, followed by the perforated bag material, then used paper towels for the breather medium. One tip on YouTube said they put the vacuum hose connector fitting on some cardboard, so I did that since it sounded like a good idea. Then I put the vacuum film on top and made a port for the fitting. Then I hit a problem.

The film wouldn't hold the vacuum and was leaking all over the place.

I tried and tried for several minutes. Nothing I tried worked. I called my oldest son over to give me a hand, and after trying some things for about 5 minutes he left me to get something and came back a minute later with a gallon ZipLoc bag. I punch a small port hole in the plastic and we slid the assembly into the bag and closed it up. The bag immediately sealed and I was getting a good solid 29" of vacuum on the dial. My son saved the day.




I want to keep it under vacuum for at least several hours. I think it will get to the point that after x number of hours there is essentially nothing more the vacuum pump to do.

Greg

I thought that 6 hours under vacuum in 100 F heat was sufficient, so I turned the pump off and went to bed. If folks think that is too soon then please let me know.



In the morning, I took the assembly out of the bag. After a little coaxing, the breather layer and the peel ply came up and this is what I was left with ...



While the inside looks unattractive, what is pleasing is that the overlapping FG parts are virtually indistinguishable. That means that there was a thorough compression of the parts, and when I tap on it has a subtle ring and is not a dull thud. So it's solid. What is unknown is how the surface looks, but I want to wait on breaking it out on the mold. I want to see if I can do the FG trimming of the part in situ. If I can, it will make bonding the fin halves a bit easier.

Greg

As an aside, I am reviewing Dave Triano's 3rd DVD, which is a "How To" on vac bagging rocketry parts. For those of you who don't know who Dave is, he is the owner of ShadowAero rocketry, and makes the Mercedes Benz of rocket kits and are priced accordingly, at least IMHO. I think I got the DVD set about 10 years ago, and it's still the best thing I've found with regard to understanding composites as they relate to rocketry.

Greg

GregGleason said:

I thought that 6 hours under vacuum in 100 F heat was sufficient, so I turned the pump off and went to bed. If folks think that is too soon then please let me know.



In the morning, I took the assembly out of the bag. After a little coaxing, the breather layer and the peel ply came up and this is what I was left with ...


While the inside looks unattractive, what is pleasing is that the overlapping FG parts are virtually indistinguishable. That means that there was a thorough compression of the parts, and when I tap on it has a subtle ring and is not a dull thud. So it's solid. What is unknown is how the surface looks, but I want to wait on breaking it out on the mold. I want to see if I can do the FG trimming of the part in situ. If I can, it will make bonding the fin halves a bit easier.

Greg

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Since you are heat curing your parts, 6 hours may be sufficient. It is difficult for me to do so, but I always let my parts go at least a day before I mess with them. Since you need to do four parts in your molds, YMMV, of course.

As far as trimming the parts, run a sanding block right on the corner where the parting plane is. It will weaken that part of the FG (where you want to cut it anyways) and give you a nice line to follow.

TheAviator said:

Since you are heat curing your parts, 6 hours may be sufficient. It is difficult for me to do so, but I always let my parts go at least a day before I mess with them. Since you need to do four parts in your molds, YMMV, of course.

As far as trimming the parts, run a sanding block right on the corner where the parting plane is. It will weaken that part of the FG (where you want to cut it anyways) and give you a nice line to follow.

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Thanks for the sanding block tip. I can see how that would work.

What I thinking about doing is placing the mold back on the mill and see if I can get the bit to trim the area. It may work or it may not. I think it will be one of those things where it will work well or be a disaster. I'll keep thinking about it.

Greg

A little late for this advice now, but on the subject of surface finishing MDF female molds:

What has worked well for certain prototype molds done in MDF that I buy from a local supplier is to seal the MDF with a couple of coats of clear lacquer type material and then spray on a nice coat of DuPont K-36 type catalyzed sandable primer. I say type, as the K-36 has gotten horrendously expensive, but English Color and other companies have a similar product that is much more affordable.

Much easier to get a clean release from this material as a surface coat than from epoxy coated MDF, unless you build up the epoxy until no wood grain is in contact with the surface. We found that the K-36 coated molds gave us more parts with little or no mold damage than any other finishing material.

Best of luck with your fin molds.

tab28682 said:

A little late for this advice now, but on the subject of surface finishing MDF female molds:

What has worked well for certain prototype molds done in MDF that I buy from a local supplier is to seal the MDF with a couple of coats of clear lacquer type material and then spray on a nice coat of DuPont K-36 type catalyzed sandable primer. I say type, as the K-36 has gotten horrendously expensive, but English Color and other companies have a similar product that is much more affordable.

Much easier to get a clean release from this material as a surface coat than from epoxy coated MDF, unless you build up the epoxy until no wood grain is in contact with the surface. We found that the K-36 coated molds gave us more parts with little or no mold damage than any other finishing material.

Best of luck with your fin molds.

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Thanks for the tip. I'll look into it.

Greg

I trimmed up the mold last night.



I took a few thousands off at a time. The remaining part is very thin, and I can live with a little flash after the fin halves are joined. It can be trimmed off and cleaned up after de-molding.

Greg

GregGleason said:

That's correct. I will need to make a left and right mold half. After the layup is cured, then join the halves together with an epoxy/microballoon/fumed silica concoction.

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I thought you might just use the hollow mold to make cast epoxy fins.

ThirstyBarbarian said:

It's remarkable that the Easter Islanders mastered this technology 1,000 years ago, but then the technology was lost.

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Hmm, a flying full size Easter Island head. Maybe at LDRS or BALLS next year.