Hi,
just wondering what the main differences would be in constructing a large mould out of Epoxy compared to Polyester/Vinylester.
Since Epoxy is stronger and you are most likely going to use Fabric rather then CSM would you get away with a thinner mould? Obviously less thickness would mean more flex but could that be compensated by glassing in eg. wood or steel reinforcement?
What would be the other pros and cons of a Epoxy mould (other then the cost of course)?
Thanks!
The biggest difference with polyester/vinylester to epoxy is shrinkage. Meaning epoxy doesn´t shrink noticable. If I remember correctly polyester can shrink up to 6%. Therefore you have to size the plug 6% bigger. There are polyester tooling resins with no shrinkage, look around in the forum, there is a topic about this.
Mold thickness is depending on your size, the resulting weight of the mold (therefore bending of the mold, the part´s thickness. You can use a sandwich design on your molds as well. Foam core, nomex or paper honeycomb work well as well.
I wouldn´t reinforce with wood or steel, directly. Meaning to laminate it into the fibre glass, but you can build a steel frame and attach it to the mold.
Yes, polyester does shrink 6% or so, but that does not mean you need to increase the size of the plug with 6%. Most of the shrinkage is in the wet state.
Tooling polyesters have additives added which expand as the resin cures, counterbalancing the shrinkage of the resin. This works pretty well, especially if the user follows the instructions…
As for stiffness: moulds are stiffness specific, so thickness always wins. If you plan to use stiffeners, first make sure your laminate is cured well, then install stiffeners bedded into lots of flexible MS polymer putty. The only place that can use stiffeners directly on the mould is on the flange, and hard points, like boat keels, chines, etc. Not on large flat surfaces.
ok thanks, does anyone have any experience in building epoxy moulds with epoxy tooling putty such as http://www.easycomposites.co.uk/products/mould-making-materials/epoxy-mould-making-putty.aspx
Mmmh, your comment about the shrinkage made me scratch my head. I asked a colleague who made stuctures up to 12m out of polyester resin, and he seconded your oppinion. He didn´t notice any shrinkage and valued his thesis after measuring.
Thanks for this, and I keep learning. :o
Fun thing is to make a small laminate, and weigh it directly after lamination, and after final cure. Some 7% of the resin has evaporated in thin laminates (styrene). This leads to shrinkage, but most in the wet state.
I know the lineair shrinkage of one of our vinylesters is 0,1%.
What is your purpose, epoxy/polyester products, high temp, etc?
I’ve build both. polyester is cheaper and faster. For high temps I build epoxy moulds. I preferred epoxy gelcoat with epoxy laminate(same fibre as used in the product for heat shrinkage). I used some kind of putty they are great for adding loads of stiffness fast, but I had problems with moulds deforming.
metal in moulds isn’t a good idea. It will print. almost anything will.
When using heat, core materials in products and moulds can insulate(foamcores) wich can give problems with curing. Using metals will keep heat longer inside, wich can be an issue when demoulding.
I’m more a fan of stiff moulds, backed up with a stiffening frame on the flanges. No problems with heating, cooling, print, or stiffnes.