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From: D Yuniskis on 12 Apr 2010 13:13 Hi Joe, Joe Chisolm wrote: > On Tue, 23 Mar 2010 01:46:52 -0700, D Yuniskis wrote: > >>> I've done fiberglass molds before. If you want 20 or so units I would >>> do a female mold. Modeling clay, foam and Bondo work great for shaping >>> the male plug and fixing any problems. >> So you are making the mold out of 'glass, too? > > I make the molds out of glass also. One example I have is an airplane > instrument panel. I cut out the basic shape from plywood. I did a couple > of 3/4" pieces to give me a 1.5" flange around the perimeter. I then > used foam and bondo to form other features. I sanded and filled any > imperfections in the plywood with bondo or resin / microballon filler. > Basically trying to get a very smooth surface. Then I coated this with > several coats of resin, letting it cure and lightly sanding between each > coat. The final product was a male plug, very smooth. OK. I.e., you could use that (suitably painted) for a dog-and-pony (to show client what the "real" part will basically look like) > Wax the hell out of the male plug and lay on a very thick layer of resin > and let it set for just a couple of minutes. Then start laying down the > cloth. This gives you a smooth surface on the female mold. This was not > for some production setup, I did 1/2 dozen panels this way. Any small > imperfections in the mold you can fix with Bondo and a little sanding. > Unless you have a part with very long sides you dont have to worry much Presumably you can flex the part enough to overcome "small problems" with the mold (?) > about draft. The final part is going to shrink some anyway. Wax and > mold release are the key. The instrument panel was about 40+ inches > by about 18" width and popped out of the female mode without any real > problems. > >>> I use tooling polyester resin. Any cheap resin will work. Most local >>> fiberglass suppliers will have some brand. Do not use styrofoam with >>> polyester resin as it will turn into a lump of goo. >> Yes :> >> >>> Use a pvc foam - there are several makes. Use Bondo or some resin with >>> a filler to coat all the foam and make sure and sand smooth. Coat with >>> car wax or some other release agent. Then start to make your female >>> mold from the male plug. Use lots of resin and a fine weave cloth for >>> the first layer. For the other layers I usually use some fiberglass >>> mat and any scrap cloth I have in the bin. Glass in some small wood >>> blocks for stability in making the real parts. >>> >>> Let this all cure and then carefully pop the mold off the male plug. >>> You might have to persuade it a little with a rubber mallet. Clean >> I had thought of trying something like plaster for the mold. I.e., >> something that could either be *dissolved* or *fractured*. > > I did this for the counter weights on the elevators. I wanted to "Elevators" as in "control surfaces" (not "people conveyances")? > increase the hinge to weight distance so I needed to rework the skin. > It works for small one off pieces but generally a PIA. > > There is some polyurethane mold "rubber" you can get. It's a 2 part > catalyzed process. You mix it up and pour it into a container and then > you can put a part into it. It makes a copy of the part as a female Presumably retaining some of its flexibility so you can "peel" it off/out? > mold. I dont remember the name or where I got it now. What I do > remember is it makes one hell of a exothermic reaction! *Always* Cool! :> >>> up any imperfections in the mold with Bondo and some sanding. Dremel >>> with a cutoff wheel works great for cutting the edges of the mold. You >>> can scribe drill marks and cut marks in the mold that will show up on >>> the finished part. I have used a BB bonded to the female mold as a >>> drill "center punch". >> Ah! >> >>> You can get gelcoat from local suppliers or on the net or you can make >>> a poor man's version using some clear casting resin and some pigment. I >>> have gotten pigment and supplies from TAP Plastic in the past. >>> >>> http://www.tapplastics.com >>> >>> For the final part make sure and wax the mold several times. I usually >>> add some form of spray on mold release as the final step. Lay down a >>> nice layer of gelcoat or resin and then use fine weave cloth for the >>> first layer. If needed stiffen the part by using foam and glass on the >>> inside in various locations. For standoffs and screw inserts you can >>> use a small block of foam. I use screw inserts and just fill the >>> threads with a blob of hot glue. Any resin just pops off and the hot >>> glue plug will pull out of the >> Clever! What about just leaving a screw *in* the insert? (and backing >> it out later) > > That works also. I've done that for nut plates and such where some resin > could get into the screws. Vaseline or some paraffin on the threads and > you are good to go. OK. >>> threads. If there is going to be any real stress on the screws you can >>> carve out a pocket in the foam and fill with a milled glass - resin >>> mix. Let it cure for a minute or 2 to thicken and then push the screw >>> insert into the pocket. Biggest problem I had was making sure they stay >>> aligned. What I did for one part was made a wooden jig where I could >>> screw 4 insert onto locater screws and then pressed them into the >>> milled fiber mix. The jig held everything square. >> How did you later get the inserts "unscrewed" from the "locater screws" >> (since the locaters were part of the wooden block)? > > I drilled holes for the locater and had a insert. Basically the locater > was screwed into a insert in the wood. Then I just unscrewed them from > the wooden guide (and the insert now bonded to the part). OK. I imagine strength is related to how much material is *around* it (as well, perhaps, as the exterior shape of the insert? "rotation-deterring") >>> Take a look at http://www.fibreglast.com >>> >>> If you want to go the acrylic sheet method browse US Plastics >>> http://www.usplastic.com >> Thanks! Lots to think about, here... > > There are lots of ways to do tops and bottoms, inserts, etc. For small > qty with minimal stress you can glue nuts on the back of a small flange > to hold a panel in place. Fill the threads with hot glue and bond with > some milled fiber. This is basically finely chopped fiberglass. You > can get it from suppliers in bags. If you cannot find it locally try > wicks aircraft or aircraft spruce. Look in their composite sections. There's a place nearby (non profit) that restores vintage aircraft. I suspect a contact I have there can help me out with more of these details... > You are better off with a small nut plate but finding the correct size > can be difficult. Let's say you have a bottom plate of the box and you > have made a small flange on the inside of the top. You want to screw > the bottom on. You can use the nut plate on the back side of the flange. > If the thickness is not enough, add a few small layers of glass, drill > and be done with it. Again, small qty units, rivet the nut plate in with > counter sunk rivets and a hand squeezer. > > Another method is using simple "brackets". Think of a box, sides and > top and you want to mount a bottom plate > 1) take a 2x4, maybe 8" long and warp tightly with a oven baking bag. ("warp" presumably is "wrap"?) What's an oven cooking bag? (any time I cook in a bag, I do so with just a paper "garbage/shopping" bag -- since usually you're only at ~350F). Is this similar to parchment paper? (I will google) > The resin wont stick to this. > 2) Cut 4 strips of cloth, 4 inches by 7in long fold them in half > length wise to give you a "guide" > 3) Put a coat of resin on the 2x4 and then lay down the cloth. Wet > the cloth out layer by layer. Let it cure and pop it off the 2x4 form. > If viewed from the end of the 2x4 the glass will be about 2" on the > top of the 2x4 and 2" down one the side of the 2x4 So, like you are making "angle iron" out of glass? > What you have is a L shaped piece about 7 inches long. From this cut > brackets maybe 1" wide, so you would get about 6 or 7 brackets. Trim > as needed. You can use a pair of tin snips or a cutoff wheel for this. OK. > Now, mount the nut plate or bond a nut to the inside of the L. > -------- > | Nut plate > | > | > > Now measure carefully and drill holes in the bottom plate located such > that this bracket will be as close to the side of the box as To minimize stress on that connection? > possible. What you are going to do is bond the vertical side of the > bracket to the box. Mount the brackets and box bottom together. > Mix up a thin slurry of resin and milled fiber. Coat the vertical > side of the bracket with this mix. It will be thick enough to hold > in place for what you want to do. Push the brackets and box bottom > in place in the top, tape and let cure. Presto, unscrew the box > bottom and you have aligned brackets bonded to the box sides. > What you have is: > > bottom-------------------- > outer|x============ > outer|x| nutplate > outer|x| > outer|x|<-fiberglass bracket > x=resin and milled glass OK. Though, instead of cutting the "raw" 7" "angle iron" into 1" strips, I could just as easily (?) have used a length of it that was almost as wide as the box's dimensions, right? I.e., so you end up with a "lip" all along the bottom... > This method also works for putting the nut plate on after bonding. Tape > the bracket (or a dot of hot glue) to the bottom in the correct position. > put the resin on the bracket side and place flush and tape. Let cure. > You can then pull the bottom panel off, check everything and drill your > screw holes and mount the nut plates. > > And one more thing. You can use plastic screws to temporary hold nuts, > etc to keep resin out of the screws. It won't stick to the plastic (I assume you mean nylon?) hardware? > Also, polyester resin will not stick to regular clear shipping / packing > tape. Ah, that's worth noting! Thanks for all your time! This has been really informative! --don |