+xYea looking at the specs it seems like a 12K to me too. 19.75 oz or something like that..Does all epoxy resin shrink that much? I thought after 24 hours at room temp and a 10 hour bake it’s 95% cured or something like that. The hand layup I did with the heavier stuff about a month ago and it’s still like glass (Minus my air pockets from poor saturation with the brush). I did put down an initial layer of resin as a “gel coat” though before I laid the first layer of fabric in. The 3K doesn’t look terrible, but the 12K definitely looks “neater” and more “factory” around the bends and edges. With the 3K in the tight round pockets and corners up front in the mold I could see the orange tooling coat through the fabric where gaps naturally formed in the weave until I got at least 3-4 layers in there. It wants to tighten up at the top and open up at the bottom sort of, or something like that. The heavier stuff did the same thing but never showed any gaps through the first layer of fabric so from the outside it looks much more uniform and symmetrical. Isn’t 12K more like what would be common for a larger structural part like a carbon hood for a car?
Yes. It isn't a lot of shrinkage, you're talking 0.5-1% linear, which is very small. Certainly compared to a polyester or vinyl ester which can shrink anywhere between 6-10% linearly it is very low shrink. So low that it is only affecting the surface finish and not so much the actual dimensions of the component. And yes, doing an elevated temperature cure before demoulding the part will help drastically as this fully cures the part. But there is no such thing as a thermoset resin that fully cures at room temperature. Realistically, they cure enough in 24 hours to build sufficient strength. Then they carry on curing for a long time after that, unless exposed to heat which accelerates the cure. If this happens in the mould in a controlled manner, then you can reach a full cure while the surface is supported and not suffer bad print through (you'll still get a little bit). If the part then ever gets hotter than it was cured, or exceeds its maximum temperature rating, then it will print again.
The issue is more to do with resin content. It is the fibres that print, so the higher the fibre:resin fraction the worse the print-through will be. With a resin rich process like hand lamination, you've got enough resin that it may not become apparent.
Not all 3k's are made equal. 200g's tend to be relatively loosely woven, because that makes it easier to drape - a desired characteristic of a twill. Some 200g's are tighter knit, and handle better. While most 240's are very tightly woven, and are very stable to handle. All of that aside, it is part of the nature of carbon to distort and move as it goes around compound curvature, that is how it works. To keep things neat, you may be better placing a relief cut along the radius rather than trying to do it in one piece. A nice neat seam is usually more aesthetically appealing than distorted fibres.