wyowindworks

The options that I listed in post #13 are for molds not for parts.

Many cores used when making molds are really just some type of filler. Poraver, bean bag filler, sand, even rice can be used when mixed with resin. You use just enough resin to get the filler to stick together. I use sand in areas that will be undergoing large compression forces like the clamping flange for example. A practice that will significantly improve a mold is do a reiforced flange mold with a full length indexing key. Essentially half the glass to be used is put down, then a sand/epoxy slurry is pour about about 3/4" thick around the flange (dam required). The sand sinks and the excess resin floats to the surface. You then use this excess resin to wet out the second halve of the layup that goes on top of it. This sandwiched flange prevents the mold from developing deformation near the bolting/clamping locations. The fulll length key prevents the mold from ever stepping. If you use a full length key but don't reinforce the flange the mold can still step because the flanges will deform from the constant localized clamping presure. The reinforced flange mold is my absolute bare minimun when making a mold that includes a seam. These can also handle moderate inflation bladder pressures (30 psi) depending on the internal surface area.

You can mix epoxy and sand together in a really dry packable consistency as well. Lay down some glass, then pack on the epoxy/sand, then more glass. These molds are very hardy, hold heat, are super stable but very heavy.

The lightest filler that can still handle high clamping forces is tooling compounds or tooling dough.

The bean bag filler works great but cannot handle higher clamping forces around the flange.

The downside to steel frames is the expansion differential if you are heat curing in the mold (yeilds the best surface finish on painted parts). You typically need some kind of interface between the mold and the steel. If the mold is large then you need a flexible interface between the steel and the mold. We did a canoe mold that utilized a steel frame. We put the steel frame on with silicone before the mold had fully cured. The epoxy mold shrunk just a little bit and caused the whole thing to bow. We had to cut the steel frame in multiple locations and reweld to remove the stress. Lesson learned.

Another option is the egg crate backing (see photo). The downside to these is handling a clamping technique on molds that get clamped together. Holes in the side rails can work well. I still use 2mm coremat on molds like this. The coremat allows me to use less resin and still get adequate thickness with less labor. 20 ounces of glass, 2 mm Coremat, 20 ounes of glass makes a pretty rigid structure that if faster to construct than laminating just glass to 1/4" thick. On large surfaces the the thickness is really important as it can bow between the ribs on the egg crate.

Since you are building large molds you may find "Advanced Composite Mold Making" by John J. Morena to be helpful. You will pay over $80 for it new. I found a copy used (google) for around $20.

The weights that I quoted where based on techniques that a robust mold will allow you to perform. A simple glass backed mold has it's limitations when it comes to debulking techniques (bladders, presses, vacuum bag) especially if the parts are to be seamed in the mold. The techniques used to process the parts often dictates the requirements of the mold.

I remeber seeing that SR-71 project eons ago. I was and still am quite impressed!