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Old Feb 11, 2012, 07:23 AM
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Another bladder molded pod/boom build thread

Herein lies the documentation of my pod building efforts. Inspired by Adam, Tom, and the several others, I'm learning along the way. These early posts will be copy & paste from my other posts to limit new typing.

Dan
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Old Feb 11, 2012, 07:24 AM
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Mold making

I gave my first mold half a day's worth of layup work yesterday. Mold keys were made from Kleen Klay snakes cut in half (didn't have enough depth in my parting plane for a routered slot); there may be some incorrect draft angle areas, but I should be able to sand those areas out. Getting the first layer of 1.5oz cloth to fit down in the many nooks & crannies with all those "features" coming out of the parting plane was rough. I resorted to using extra epoxy and stippling smaller pieces of fabric down into the depression areas. We'll see if I got all the bubbles out in a couple weeks after I finish the 2nd mold half next weekend. I used approximately 5 layers of 6oz cloth on top of the surface coat (West, cab, and teal pigment), a 5:1 mix of sand to epoxy approximately 3/4" deep, and another 4 layers of 6oz cloth. I haven't trimmed the flanges down all the way so I can get clamps on the sand yet.


Hey Jesper, the blue stuff, if you're talking about the mold color, is a teal pigment from US Composites. One of my surface coats started to exotherm and discolored a bit greener than the other, but even still I quite like the lighter background color than the previous black pigment I was using for molds. The glass work this weekend was much easier to see when wet and I'm betting carbon will be nicer to work with for the same reason
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Old Feb 11, 2012, 07:24 AM
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Part #1

This was just a glass part to test out the mold and start learning the bladder techniques. Now I can go re-read for the tips I didn't understand at the time.

It took me six bags to finally get one that held any pressure, and I have no idea how to get the fabric seam to overlap as the molds are closed. Whew!

Lessons / questions:
* Sand the mold key down lower ... 1/16" seems scary not quiet enough, but 1/8" seems large. Mine are ~1/4" now
* Have the seam split between the two part or halves or keep them only on the flush mold? How on earth to close the molds?
* Pre-cut the cloth to exact size? Difficult to trim wet in the mold without disturbing the weave and nicking the PVA (ack, hope it releases with just wax!)
* Weld the bolts head to the spanner (Adam, your clamp style is terrific)
* Work out an easier way to prevent the bladder insert from popping out.
* Pre-warm the molds (I don't have a hot-box yet)
* Maybe try inserting the bladder on the stem after bolting closed the halves.

A few pictures of my attempt attached.

Dan

Update: Went downstairs to check on the pressure and noticed I should have put down some protection on my bench because resin is literally dripping out of the seams. I sure hope I waxed the sides of the mold! So I guess the bladder is working its magic. Hard to imagine this is 2x the pressure compared to the vacuum parts I've made. And hard to imagine too that the resin moved some 1.5 inches out of the fiber and up over a mold key. I used 20g of resin which I think I found listed in this thread somewhere.
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Old Feb 11, 2012, 07:25 AM
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Part #1 unveiling

Hot off the SD card, some how-it's-not-done pictures :-)

No seriously, I'm quite happy with the new mold and satisfied with a successful first bladder molding attempt, given the known shortcuts and things I ignored. The pros might cringe at such a terrible part, but certainly the mold's potential is evident.

Mold came apart with no major efforts. Once there was enough gap from a screwdriver prying in wedges I left in the flange, a Popsicle stick around the perimeter popped the two halves apart. There is flashing everywhere ... so much resin moved. Several rounds with a sharpened Popsicle stick to get the flashing to release from the mold corner and from those dinged PVA areas, a bit of pushing inward to separate the part from the local mold surface, and a gentle pull up from the tip of the tailboom. There were some areas of the overlap fabric that clearly were in the flashing rather than overlapping the other fabric half, and those areas understandably burst at the seam. I had three other areas of the part that buckled from the side-loads, 4" aft of the TE, at the TE, and 1" aft of the canopy area (where I dropped a layer of glass in the layup). Good places to start looking at for layering and reinforcements. Some final pushing/twisting/tugging to release the part and viola!

I tried to pull the bladder out from the tailboom tip by twisting a bunch. At some point it stopped releasing from the inside surface (could watch the bag release) and eventually I had to give a tug, which tore the bladder. No biggie, just opened up the canopy area and it simply pulled out from the front. I think a longer inflation stem would help this twisting release process. Inside the fuselage has a characteristic occasional wrinkle texture from a slightly oversized bladder.

Trim off some of the flashing, sand down roughly with 60 grit, and realize it's going to work ... then write this.

In my defense, I made a total hack of wetting out the material. I simply mixed 20g of resin, made no effort to weigh the fabric to know how much I really needed, and made a wet layup. It's a trash part anyway, so no sense in sweating the details. That at least helps explain the obscene amount of resin that moved. A proper resin trap will also help immensely. Wish now I had figured out a more machined mold key so it would look prettier and the overflow would be more evident; lesson learned.

I'm happy enough. Time to figure out how to get the seams to overlap when closing the mold. Figure another glass part is in my future.

Thanks for the interest,
Dan
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Old Feb 11, 2012, 07:25 AM
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Part #2

Well I tried again, this time with real material ... it was going great. Fabric all went in easily, 26g of cloth and 24g of resin, felt like the layers were built up reasonable for a good test. Put together the molds and gradually inflate .... and get that tiny whistle that just doesn't go away and begins to get louder. No pressure on the gauge. Fine, I unbolt, pull it apart and reset the cloth, pull out the old bag, and prep another (I had three new ones, down to two after the first pop). Doing a test mouth-pressure just to listen quietly and pffftt. Pull out the last bag and prep it carefully. Passes the mouth pressure check, no leaks, no obvious stress marks. Lay that into the bag, work the overlap with thread again (wrapping all the way around one mold half with thread worked pretty nicely, removal after closure before clamping takes a simple tug) and do another mouth-pressure test before gradually inflating with tank pressure. .......... I got to 10psi and somewhere near the nose tip burst. Out of new bladders, I cracked the molds and tried inserting electrical tape in the mold flange to act as a gasket, tightening the crud out of my clamps.h it installed just to pre-inflate and hopefully seat the bladder. Then ever so g No joy, it still leaks. It's not zero psi, but it isn't more than five either.

So, back to the cutting board for how to make better bladders.
  • I did not wipe with alcohol before placing the bladder into the mold cavity. I wonder if that would help it slip on sticky resin enough to help the initial seating.
  • These bags were over-sized for sure by about 1/4" compared to the template I was using. I figure wrinkles inside are just fine for these first ones and don't hurt other than what weight is in the epoxy ridge.
  • What should I be looking for in the seaming process? A bead of plastic or a nice wide color contrast as the bags stick to the backer board (mylar, your roofing piece)?
  • What more QC tests can help check out my bladders after I make them before using them in a layup?
  • Is there anything you can think to salvage a burst bladder without cracking it apart to replace it? Run-flat goo for tires is all I can think of...

I also now have a pretty good idea why my bladder burst at the tip of the nose. Turns out instead of pushing the nose back into itself, I folded it over. I have to presume by the time I finally got the bladder in the last time, the resin was pretty sticky and didn't let the fold actually un-fold.

There was essentially no pressure on this part. I think it mostly received the right shape from the five seconds the bladder actually held 10psi, but certainly it isn't very well pressurized. As a result, it's super resin rich.

But you know what, one more step in the right direction.

Most of the seam worked. There are two areas the seam didn't overlap, most likely from an overhang that wasn't as pronounced as it should have been. I ended up making my own version of the thread wrap method. That worked pretty well. It may take extra time compared to the split seam (one on each half) method, but I had good luck with it.

A friend is coming over tomorrow and we're going to lay up another one. After making and testing several more bladders of course :-)

Thanks for your continued help everyone!
Dan
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Old Feb 11, 2012, 07:26 AM
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Part #3

Whew, another one in the bag! Trent came over and we blasted a part out. The layup was tweaked a tad, removing a layer of 0-90 6oz FG from the nose, lengthening the boom doubler, and splitting the overlap to either side (rather than using the thread method).

We also changed up the layup method a tad too. The boom was done as a 3M'd sandwich of one layer 4.7oz uni CF and two layers of 1.4oz FG 0-90. Wet-out was done on a separate wax paper, slightly rolled, and transferred to the mold. The nose layers were then done similarly. Much faster and the edges didn't fray as much either. Mold was ready for the bladder at 30 minutes flat.

This boom tapers to not any larger than 3/8" at the tip, so even with a good vacuum applied and a wipe of alcohol, sliding in from the rear wasn't the best. And that bladder burst. So we broke open the mold and straightened up the fabric, set up another bladder (which was leaking, so grab the last backup) and laid it nicely in one half. Closed the molds and slowly cranked in 30psi. It's still holding :-)

It's funny that you can listen to the resin moving.
Dan
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Old Feb 11, 2012, 07:27 AM
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Part #4

You guys are right, I just need to make a call and try another one. Added another full layer of 4.7oz uni CF and two additional 1.4oz FG 0-90 layers in the nose. Seams down the boom are still hard. Keys are still too tall to use a persuader through the small slit... tough. My patterns are still a bit wide and I trimmed 1/8" off the overlap while wet. I'm saving these to weigh later this time :-) May be heavy, but I'm hoping for a flyable part.

Yesterday's part came out at 45.7g before real cleanup, mission accomplished :-) This one is flight worthy enough to give a try. My seam overlap still needs some technique work because there was about 3" of missed seam at the tailboom tip and at the tip of the nose. But, they're repairable to put some throws on this layup. Wahoo!

Dan
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Old Feb 11, 2012, 07:27 AM
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Part #5 & #6

I think the weights I quoted for fabric in the 4th part were misleading ... I tried an identical layup for #5 as the double-carbon boom #4 with no trimming and got 53g after cleaning off the flashing. That's more in line with Adam's guess and suggests I trimmed off more fabric weight when working the seams than I realized.

The 5th part (another double-carbon boom mentioned above) had some serious closure issues in the back of the boom area. I had seam splits on both sides some 8-12" back from the boom tip. The forward seam area turned out great. But I also realized how ridiculously heavy the double carbon boom is. It's stiff, but makes for a heavy tailboom. You'll also never believe it, but the bladder came out of the 4th part (the one Trent has) unscathed and I used it in the 5th part up to 40psi that I dared try. Didn't survive being twisted out of the 5th part, but served me well. Part of the reason for the extra weight on the 5th part may also have come from extra epoxy usage. I didn't 3M the two layers of 4.7oz CF together because they were hard to wet out together. Installing them individually really was a pain and I know I added a bunch of excess resin...

The 6th part just got pressurized about an hour ago. I dropped back to: 1x 4.7oz CF uni + 3x 1.4oz FG 0-90 for the tailboom with a local 1x 1.4oz FG 0-90 doubler from the rear wing bolt to 3" aft of the wing TE. The nose is 2x 1.4oz FG 0-90 + 1x 6.0oz FG bias + 2x longer 1.4oz FG 0-90 pieces to transition to the wing LE area better. Most things were 3M'd together to keep from fraying the edges so bad. I also didn't press hard when wetting out the boom to keep from expanding the boom fabric (helped tremendously!). Seam width was approximately 1/8". Pressure on the epoxy at 45 minutes after first mixing.

After letting 3rd part cure for more than a week, the single 4.7oz CF uni tailboom is starting to really seem okay. I only put 2x 1.4oz FG 0-90 in the boom interior and the local area with 3x seems much better. This is why 6th part dropped back to 1x layer of 4.7oz in the boom.

A single piece of 6k CF tow 10" long in the corner of the wing fillet with a generous swipe of raw epoxy before any cloth goes down has worked consistently to keep bubbles away.

A piece of 1/8" ply approximately 3/8" x 1/2" square with the corners beveled to 45" have been good hard-points instead of using the bolt alignment holes. These bolt plates are placed in last before the bladder and held to the fabric with a tack of thin CA.

I aggressively sanded down my mold keys to about 3/32" and that made the seaming much easier (further would probably still help). I haven't figured out the best arrangement to see into the slot to ensure the fabric is pushed into the mold cavity, but it was slightly possible now that the keys are shorter. The tailboom tip area takes extra persuasion because the bladder and pressure inflation nipple basically close off that area; I can't insert the bladder into this tailboom tip, so the bladder on the stem has to go in before mold closure.

My mold has held 40psi twice now and I haven't seem any evidence or been scared of cracking/ chipping/ failure. It probably would hold more if I asked it to

The mold flanges need a lot of cleaning after popping a part to ensure there is no spare PVA or drops of resin that would prevent the molds from closing fully. Clear resin drops hide well on the teal.

I cut material for an identical layup for #7. May try to lay that up on Sunday.

Dan

Update: #6 is 41.5g before trimming, 39.7g after trimming the flashing, tail flair, and opening the canopy. Despite the single layer of carbon, the additional hoop strength of the additional layer of fiberglass makes this one feel on par with the #4 double carbon fuselage. Quite happy with the seams as well, with only the last 1" of carbon slightly clamped in the flange yet no seam splitting. I'm going to build this one out and throw till it blows :-)
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Old Feb 11, 2012, 07:28 AM
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Just closed the molds on part #7. I screwed up the nose order of outside to inside, putting the long 1.4oz 0-90 pair down first, a new 1.7oz bias Kevlar layer, and then the 6oz bias + 1.4oz 0-90. Last time I put the 1.4oz 0-90 + 6oz layer down first so I could slather on the epoxy as a surface coat and work it up through the heavy 6oz and soak the easier thinner layers after that, rather than this time when I added a bunch of epoxy before the last fabric went in. It's late, so that sentence probably doesn't make sense.

As an added bonus, ran this one up to almost 50psi ... and she is talking! No bag pops, but definitely lots of resin coming out of both bolt alignment holes. Super glad I PVA'd the outside of the molds in that area. Got pressure on the epoxy at 40-45 minutes after mixing.

Dan

Update:
Nope. Back to seaming problems. What sands sand so I can make the key shorter??
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Old Feb 11, 2012, 07:28 AM
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part #8

Another weekend layup. Needed to add some fabric weight to get up to 45g from 40g. Adding 3g of raw cloth was harder than I thought with the light-weight 1.4oz FG. Found a half gram found by extending the 2x 1.4oz layer from the nose to connect into this boom doubler. Also, I added a piece of boom material cutoff to the transition from the square to circle cross section and back 6" or so into the boom (boom material is built from 4.7oz CF uni + 3x 1.4oz FG 0-90). The nose is built from 2x 1.4oz FG 0-90 + 1x 6oz FG bias + 2x 1.4oz FG 0-90.

Demolding results were disappointing. Even with extra effort to ensure the seam is aligned properly, there were several areas with fabric in the mold flange. Terrible. I have been sticking with the offset method, keeping a lip of fabric on each mold and closing them by aligning off-center and sliding toward the center. This time I then reopened as little as possible and poked at the seam with a credit card through the slit. I'm guessing my mold keys are still too tall and the seaming material is getting caught back between the mold flanges during the credit-card check.

Dan

PS: 44g after clean-up. Right where I wanted to be.
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Old Feb 11, 2012, 07:29 AM
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reserved for part 9 ... see post #21
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Old Feb 11, 2012, 01:03 PM
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Good work.

Good to hear that you are getting consistent results now with the bladder. You can definitely reduce your key height some more. If the key is taller or close to the same height as the seaming flap you are going to have problems closing the mold. If the flap is too tall in the narrow portions of the boom then it will bottom out when the mold is closed. This will cause the fabric to slide around and stick up on the flush side. This will make a fabric pinch on the flush side. Once you have everything dialed the mold closing will take you 20 seconds max.
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Old Feb 12, 2012, 12:15 AM
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I will be watching this one as well Dan. I am learning as much from your experiences as I am from Adams info. If you help me through one problem in the future then well worth the read. Thanks for sharing your efforts.

Rob...
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Old Feb 12, 2012, 10:53 AM
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Great work

I've been playing around with my pod mold, SG style.
What is the height of the seaming flap you are using?
Also, any tips for the nose seam? It is my weak point in finished parts, I don't get good closure there, usually gaps.

Thanks
Dani
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Old Feb 13, 2012, 05:13 PM
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Quote:
Originally Posted by Dani123 View Post
What is the height of the seaming flap you are using?
I'm aiming for 1/8" range. When wetting the carbon (using the wax paper transfer method), my initial attempts really expanded the carbon with too much pressure and made the seam ridiculous (~3/8" in places). On #6 I was really gentle and had much better results. It still frays some during the transfer, but I think that comes with practice.

Quote:
Originally Posted by Dani123 View Post
Also, any tips for the nose seam? It is my weak point in finished parts, I don't get good closure there, usually gaps.
I've gotten lucky so far, with nose splits in a few but not when it mattered. From Adam's video, it seems he adds a small bit of chopped Kevlar mixed with epoxy in each nose tip to be pushed into position by the bladder. That's the best I have for you until I come up with something that works consistently.

Dan
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