Hello, I have cast 2 Alumilite blanks with clear slow and mica powders. My mold is UHMW and is 10 in. Long, 1.5in deep, and 1.5in wide. At the ends of the blank after pressure pot are somewhat rounded and the vertical corners do not have a sharp edge ( almost 45 degree). However the length of the blank has a good sharp edge. Not a big deal because I will trim and turn turn the blank on a lathe. I am just curious as to why this happens? When I cast the blank - the vertical corners were full.
Usually voids are caused when air has gotten trapped inside a mold so I'm thinking it might be that though I'm not sure if air bubbles get in a pressure pot - I don't use one but I've seen others say that it is supposed to remove the air bubbles.
Air still might be getting trapped depending on how you pour - are you pouring a slow, thin stream down the side of the blank? Sometimes the stream is poured too fast or too thick and the air can't escape from the other side and gets trapped and this might explain why they're rounded and not sharp.
Well, I am no expert for sure - only 2 blanks- but I poured slowly and watch the resin fill the corners and the end - especially on the 2nd blank. Then straight in the pressure pot and 60 psi for 4 hours. And if what you said is the culprit - then why wouldn’t the linear sides do the same thing? It only happened on the ends - vertical corners and the top edge!
My blanks do that too. Mine aren't pure resin though, I put pieces of burl wood in the bottom. I've been blaming it on the pressure pot forcing resin underneath the wood (pulling resin from the corners because they are the last to set, being a lttle cooler than the center of the mold). Now I'm not sure of that.
If you're doing it slowly and they're not showing up along the sides, we're going to have to ask for some more help from others who also use pressure pots since LawnBoy (thank you for chiming in) also has the same problem.
Anyone else use pressure pots and may have had the same problem and can chime in?
Speaking from a technical standpoint a pressure pot does not remove air bubbles. It crushes them down to a much smaller size. To truly remove them you'd have to vacuum degas the resin after pouring. I've noticed this same thing in my blanks as well and it's always at the corners. I have no explanation as to the cause but would hazard a guess at something to do with the temperature of the resin as it cures. For what its worth, the molds this has occurred in range from silicone to HDPE box to steel box.
Thank you for clearing that up, I don't use either one so I didn't know and now I do. So now we know, degas first then pressure pot and maybe the corners will be a little sharper. I normally don't get this problem but I am also using one piece molds that aren't put in either so maybe any fumes that might be letting off during the cure already have a way out and aren't causing me any trouble.