RE: My vacuum bag layup has crinkles, any advice?
I think you'll be disappointed if you don't ensure that each ply is fully wet-out before you put it in the bag, as you're likely to get dry areas and puddled areas. How you get each ply wet out is a matter of preference. An ideal would be to have room temp prepreg (yes, it does exist). What I typically do is brush in a coat of epoxy (often thickened a bit with colloidal silica to make it sticky)... then add plies one at a time. I brush in a thin coat of plain resin between each ply. Small reinforcements are often individually "prepregged" on butcher paper before being added to the layup. Once the layup is finished, I blot up excess resin using paper towels or toilet paper. Then I add peel ply or release film... making sure to remove any wrinkles. Then the breather and bag as usual. As dreadnaut said, start with low vacuum so you can adjust things and remove wrinkles.
Posted on: 12/7/2007 5:18 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=6727052

RE: My vacuum bag layup has crinkles, any advice?
Yeah, I wasn't meaning to contradict you. I use the pink perforated stuff from CST myself... but my point was that if the release film goes in with wrinkles... those wrinkles aren't going to disappear when you apply vacuum.
Posted on: 12/7/2007 5:09 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=6727011

RE: My vacuum bag layup has crinkles, any advice?
Any wrinkles in your drop cloth (or any release film... perforated or not) will result in these resin ridges. The resin fills the wrinkles and stays there. These resin ridges are kind of the nature of the beast. What you can do is... 1) eliminate wrinkles in the release film as best you can... 2) use less epoxy... 3) use more vacuum. You can substitute peel ply for the release film, and applying the peel ply in strips can help avoid wrinkles. Good luck.
Posted on: 12/7/2007 4:00 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=6726747

RE: heat shield ceramic
My point is, if the EGT is 600F at the point of the vectoring vanes, epoxy/fabric composites will not be suitable. Ceramic sheets may work fine, if you can find them. You need air between your "heat shield" and any composite structure. Stainless still looks like the easiest solution.
Posted on: 3/9/2007 5:27 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5536538

RE: heat shield ceramic
You need to define the temperatures before you can start looking for materials.
Posted on: 3/9/2007 5:22 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5536509

RE: heat shield ceramic
What is the temperature of the exhaust gas at the location of the vectoring vanes?
Posted on: 3/9/2007 10:32 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5534866

RE: heat shield ceramic
Use stainless steel for the exhaust ducting.
Posted on: 3/9/2007 1:45 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5533902

RE: How do I make a plug from this heli canopy?
Wax and PVA are so cheap compared to the fiberglass and epoxy you'll be using for the mold and parts... so why cheap out? It's not that much work to do it right... and it will save you a lot of pain and frustration. -David
Posted on: 3/5/2007 2:13 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5512435

RE: epoxy to epoxy?
Yes, that works fine. It's how molded wings are put together, for example. You should scuff the area to be bonded with sandpaper, and clean with acetone if possible. The bond will be fine. It would be better to join the two halves when the epoxy is still uncured, but that's not always possible.
Posted on: 1/28/2007 8:55 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=5337834

RE: Best Prop. Wood? Why?
[quote]ORIGINAL: darock I used to mess around with pitch, but have found that more effort than effect. And a synthetic prop that's out of pitch side to side is just lost money. [/quote] Not entirely true... you can re-pitch most plastic and composite props.
Posted on: 12/14/2006 12:00 AM by Author "davidfee" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=5114012

RE: difficult to use Quicklock
No no no... no mallet. First of all, what thickness is your bagging material? Obviously, that can make a big difference. The easiest way I've found for getting the thing together is to fold the bag back over itself. If you have the bag spread out on each side and you're pressing directly on the tube, then it seems to be much more difficult. I think you're, in effect, preventing the bag from sliding into the slot of the clip. I used to swear by these clips... and I still like them... but tacky tape really does work better in the long run, even though it's a consumable.
Posted on: 11/9/2006 10:30 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4977201

RE: Amount of epoxy to mix for skinning wings
If you want to build light models (they do fly better, after all), you look for ways to maximize the use of your materials, and minimize the amount of excess weight. Sealing balsa with a light coat of sanding sealer for this application is very common in competition circles. The surface is still plenty rough for the epoxy to make an excellent bond... but the porosity is limited so that the resin can't "wick" into the balsa to the same extent. This means more of the applied resin will remain at the bondline... which in turn means less epoxy can be used. And that means less weight for equivalent strength. Certainly epoxy-saturated balsa is stronger than raw balsa... but the increase in strength is more than ofset by the disproportionate increase in weight. You are better off using a layer of fiberglass if you really need the extra "strength." HTH, -David
Posted on: 11/9/2006 9:48 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4976999

RE: Epoxy mixture?
Matt, I'm pretty sure Randy is talking about joining the upper and lower skins of a hollow molded wing. The skin sandwich panels (balsa or foam sheet core) are bagged into their respective molds. The spars, ribs, etc. are then installed and bonded to one of the wing skins (usually with microballoons/epoxy) as a secondary operation. After a final fit-check, the entire assy is bonded together with a bead of adhesive along the perimeter, along the spars and on the ribs... again, typically with micro/epoxy. The molds have guide pins for alignment of the upper and lower skins. It sounds like he's asking you to suggest a potential alternate adhesive. -David
Posted on: 11/9/2006 9:38 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4976947

RE: Alternate tail airfoil
You give the impression that this stab is very highly loaded and, thus, having sufficient spar height is critical. So, take the RAF30 and thicken up the 70-80% region to meet your requirements. Then, if you're really concerned, do it in Profili/xfoil so you can tweak the section to minimize drag.
Posted on: 10/26/2006 9:13 AM by Author "davidfee" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=4916845

RE: thinning epoxy
(covers ears) lalalala ;) Yes, of course you can thin epoxy with alcohol (or acetone, MEK, etc.) Yes, the results can be quite satisfactory. It's just less than optimum and it's generally not necessary. There are products made especially for each of these applications if warming the epoxy doesn't cut it for you. If all you want to do is fuel/waterproof, then epoxy paint comes to mind. You will generally want to seal the surface, so then sanding sealer and lacquer or dope start to sound good. Whatever floats your boat, man. :)
Posted on: 10/20/2006 4:09 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4893299

RE: thinning epoxy
Just say no to thinning epoxy. Warm the stuff up to make it flow. If you are doing a glass tape wrap of a dihedral break, use a heat gun to heat the epoxy and a credit card to work it into the cloth. If you are glassing a wing, fuselage, etc., then use a finishing resin (or laminating resin) as has been suggested.
Posted on: 10/19/2006 11:08 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4890612

RE: Glass/balsa/glass laminate fuselage
Tom, It is called a sandwich panel. It is many, many times stiffer for a given weight than a solid laminate panel. Thus, for a given design stiffness, the product can be made much lighter. Using balsa is nice because it is moldable (by softening the fibers with water) and because it has a grain structure which can be used to increase panel strength and stiffness in the primary loading direction. Other common core materials are rigid foams like Rohacell and of course various honeycombs. Balsa is about the easiest to deal with for most people. -David
Posted on: 10/12/2006 1:13 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4857346

RE: Unequal winghalfs
I've definitely built some rubber-powered FF models with asymmetric wings, but these were indoor models with props that were about 1/2 - 3/4 of the wingspan. The asymmetry was required to react against motor torque. I don't suspect you're dealing with the same thing, but anything is possible. What model designs are these?
Posted on: 9/17/2006 4:04 AM by Author "davidfee" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=4757833

RE: lap joint in molds
You generally make an additional piece (or set of pieces) for your joggle mold after the primary mold parts are built. The "joggle tool" is then removable and the parts can still be joined using the molds and alignment pins. There are several ways to get the correct offset for the joggle tool. I hope other folks will chime in with suggestions, since I've only done it one way. I used the same plug without modification to lay-up my joggle tools, then shifted them inward (towards the center of the part) by the approximate thickness of the part laminate. They were then fixed to the primary mold piece using screws. HTH, -David
Posted on: 9/1/2006 4:29 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4698094

RE: Composite Fin Layup
Those aren't done... time to start polishing! :)
Posted on: 8/10/2006 1:37 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4604769

RE: Epoxy melting blue foam.
[quote]ORIGINAL: dolanosa It kinda stinks so it may be polyester but I'll check tonight. It uses MEK in very small quantities...does that mean it's polyester resin? [/quote] Yes, this is polyester resin. The catalyst is MEKP (not MEK), and the main ingredient is styrene monomer... which is what your blue polystyrene foam is made from. So... there's your problem. Polyester resin and polystyrene foam do not play well together. -David
Posted on: 7/25/2006 9:08 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4544729

RE: K&B paint on plugs?
On Dolphin wax: Yes, it's a nice wax. Is it the end-all, be-all? I don't know. You'll read lots of praises... but my first part made with the stuff stuck in the mold. It was an old mold... with dozens of parts having previously been pulled from it. I didn't clean the mold first with acetone or anything... just applied the dolphin wax over the existing wax. Anyway, I learned something from that experience. When I first got my dolphin wax, it was very soft... with lots of solvent smell. It has since gotten a lot stiffer, since the lid doesn't seal air-tight and the solvents have slowly evaporated. The instructions I was given stated that it should be wiped on and wiped off... without allowing it to dry on the mold surface. In other words, don't wait for a haze to form (it will be very hard to buff out). Because of the relatively high initial solvent content, it is my feeling that I basically wiped the wax clean off the mold surface in the area that stuck (it was a raised portion of the mold). Now that I've terrified you: Dolphin wax works just fine, and it does work as advertized. I would suggest that each application be allowed to "dry" for 30 minutes or so before going back for the next application. You should apply 8-10 coats on any new plug or mold... I don't care what you hear elsewhere... I don't want to hear that your plug stuck in the mold when you got in a hurry and gave up after only applying "one or two coats" of wax. Let the wax "dry" for at least a couple hours before painting. It's good to let the solvents outgas... and it probably ensures a better barrier. I apply the wax with torn up strips of old t-shirts. I also wipe/buff with clean strips of old t-shirts. It works for me. Good luck, -David
Posted on: 7/11/2006 11:48 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4491194

RE: K&B paint on plugs?
Well, I've used a variety of products... all from the auto parts store. I've used No 7 White Polishing Compound, followed by a clearcoat-safe compound from 3M. HTH, -David
Posted on: 7/10/2006 9:22 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4483538

RE: K&B paint on plugs?
I've had very good luck using automotive polishing products on my plugs. If you have a lot of orange peel, start with wet sanding. WEST graphite powder will work fine in your tooling resin. -David
Posted on: 7/9/2006 2:39 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4479275

RE: Want to do fiberglassing... what do I need?
Yes, MGS and Aeropoxy (in fact, virtually every epoxy type) are fully compatible with polystyrene foam (EPS).
Posted on: 6/20/2006 3:44 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4411098

RE: SKIN HINGE ? NEED INFO
[quote]ORIGINAL: Darth Shreck When we make them, we reinforce them with a 4 to 5 oz Kevlar strip that is 1 1/2" wide and about 3" longer than the actual hinge. [/quote] So are you doing the type of "skin hinge" where the skin flexes across some area, as opposed to having a discrete hinge line? If so, what are the advantages you see in doing it this way? -David
Posted on: 6/13/2006 2:30 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4384480

RE: Want to do fiberglassing... what do I need?
What are you trying to do with fiberglass? Just name an example project. What is "endothermic epoxy, not resin"? -David
Posted on: 6/10/2006 8:55 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4376231

RE: White foam melting temp
You're pretty close... the HDT of EPS is typically 180 - 190 °F. The true "melting" point would be higher (about 400 °F), but by then the wing is goo anyway, so it doesn't matter. -David
Posted on: 5/30/2006 10:35 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4334237

RE: Live hinge material
[quote]ORIGINAL: danj31 Peel ply is not suppose to stick to your fiberglass part, it soaks up all of the extra epoxy and you pull it off and throw it away. That would be a bad choice for hinge material. [/quote] The epoxy makes effectively thousands of little rivets through the hinge material. I'd love to see you pull one apart without destroying the rest of the structure first. I can show you pictures of exploded wings (from flutter and collisions with terra-firma) where the peel-ply hinges are still intact. But, whatever floats your boat.
Posted on: 5/24/2006 3:47 PM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4311699

RE: Live hinge material
I guess it depends on what you're after. Nylon peel-ply and Kevlar also work well. -David
Posted on: 5/24/2006 9:17 AM by Author "davidfee" in the forum "Composites Fabrication And Repair"
http://www.rcuniverse.com/forum/fb.asp?m=4310495