Martin made a few comments about CNC'd molds that I do not really agree with. The biggest advantage of using a Computer controlled Machine to produce Molds is accuracy . The next big advantage is one that a lot of people have failed to recognize, you can easily produce a fuselage plug in two pieces that is "keyed together" . This means that no splitter plate is required( no fussing with clay ), you simply bolt the fuse half to a "parting line plate" ( something flat , with a very smooth surface , and hopefully stable over time ), put several coats of wax on it followed by some PVA , and you are ready to layup a mold . If you are in a real hurry you can go ahead and make two "parting line plates" and do both halves in the same day. To make a second or third mold only requires you to add another coat of PVA to be ready to produce another mold. This same thing holds true for wing molds. Instead of CNC'ing a set of molds , you instead cut a pair of plugs, ( one top of wing plug, one bottom of wing plug ) for each wing panel/s . In this case the Parting Line is part of the Plug , and once again there is no fussing with a splitter plate required. Spend several hours doing the final finish on the plugs , along with the usual multiple coats of wax, and you are ready to make a wing mold. So why is the "plug method" any better than just making a mold directly ? In this day of perfectly finished/prepainted molded airplanes it only takes a second to PERMANENTLY damage a mold. Drop a pair of scissors etc. onto the surface of a mold and more than likely you will put a nick/gouge/etc. into the surface of the mold. Trying to fix this so that it disappears , cosmetically speaking, can be nearly impossible to do ( in the case of aluminum the only way to fix it is to remachine the entire surface of the mold ). In an epoxy mold you will have to play "Dentist" and carefully make a tiny "pocket" in the mold to allow you to get some repair material to "stick" in the damaged area. Then you will have to carefully blend out the repair material and repolish the mold. If you don't do a perfect job of this , your "repair" will show up in a finished part . Someone with a keen eye for detail will spot this flaw in under a minute. Someone with a lot of experience like Daryl Perkins , Joe Wurts , or Sal Defrancesco will spot it in 15 or 20 seconds. One other reason ( although in this time of here today, gone tomorrow world of whats hot , I'm not sure it applies to us ) is that every time you use a mold to "damage" the parting line a little bit , although most people would call it "wear", the parting line loses that sharp edge . It's that sharp edge of the parting line that you see in a finished part where you see a fine line in a , painted in the mold, fuse. If the Parting line starts to "round over" due to wear, you end up with a pretty "fat" line . Once the parting starts to round over the mold is "gone" and needs to be replaced. Replacing a CNC'd mold is not inexpensive , and something a Manufacturer is reluctant to do since they may never recover the money spent on a second mold. "Concrete Mold Reinforcement of Molds" First off I would not use Concrete to make or reinforce a mold, there are better materials to use, and given that we are not going to need large amounts of material , even a material 5 or 10 times the price of portland cement is not going to be a huge expense. The first material that comes to mind is Ultracal. This is a Gypsum based ( similar to plaster of paris ) material that has been around for along time, and used to be the primary material for a lot of cast molds. It has a very small shrinkage factor compared with common plaster or portland cement. It's also a lot stronger than common plaster. It fell out of favor because of its weight , and the fact that it doesn't bounce well off a concrete floor. Add in some nice reinforcing fibers of glass , or if you can find it Spectra Fibers , and you could have a pretty strong mold without the need for a lot of bulk. I don't see any reason why you could not do a variation of "reinforced concrete" on a miniature scale. 2x2 or 4x4 mesh steel mesh screening would add an enormous amount of strength and allow you to cast much thinner ( and a whole lot lighter ) sections. If you are thinking of using a cement product as a reinforcement to an Epoxy or Polyester Mold then you are going to have to do something about "tying" the two different materials together. Simply pouring a cement product on top of a plastic is not the best method. Even bent up jumbo paper clips glassed unto the mold would make a big difference. One final thought on "concrete" molds, don't expect a long life, the parting line is not going to last a very long time, it will break down pretty quickly. Jim Bonk RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
