When you pour there is a tendency for the sprue to stop taking the metal until it kind of burps. I’ve been attaching the sprue to the part at an angle. The sprue is vertical and the part slopes downwards. I am thinking that the sprue angled downwards at about 45 degrees would work better. The angle would allow the hot metal to flow down the bottom side of the sprue and under the foam allowing the hot gases to go up the sprue without causing a burp of metal. The part should slope the same direction so that the metal goes smoothly all the way to the bottom.
Well so much for that theory. Took my time tonight, did a pour had dinner and checked it. Incomplete fill again. Fired up a second batch and poured a second part. Incomplete fill, and this time I poured until it overflowed a lot. You can that this pour with the sprue still attached filled down bottom side of the sloped sprue, then the bubble of hot gas apparently stopped it completely leaving a solid block of metal at the top. This melt was 6061 with 7% silicon added. You can see that the parts look great with fine detail of the foam visible. What would help. Vertical sprue? Longer sprue? Larger sprue?
Are you pouring into compacted sand with sheetrock mud on the foam? It looks to me like the extra gas is not going through the coating on the foam. What kind of foam are you using?
Gary there is a reason for the hesitation in rate at which the part takes metal as you note, but I must confess, I have a very difficult time following your posts and consequently offering any advice. You mention it's an incomplete pour but I have now idea what I'm looking at. Looks like a plaque of aluminum. What was it supposed to be? What temperature did you pour at? Was it coated? If so with what? What was the duration of the pour in seconds? Try video'ing your pour. You have previously posted about a pouring basin. Did you use one? If so, what? Best, Kelly
If I was casting that, without knowing what it's for, I'd just sprue straight down into the top side without angling anything, no coatings (I use super fine sand instead), with the sprue being at least as thick as the part in hopes of the sprue not causing shrink in the part by freezing first. This is based entirely on what has worked for me in the past after a few years of trial and error; I'm well aware it flies in the face of how things are "supposed" to work. If you prefer using coatings or the parts are meant to be structural in any way, then Kelly's your guy to get advice from - my lost foam castings usually work out as planned these days, but it has mostly all been strictly decorative other than a couple of clamps I cast for my muller. ...I guess the lost foam skull belt buckles I've made could be seen as somewhat "structural" too, depending on how tight I want my belt and how much I overdo it at suppertime. Jeff
Sorry for a lack of info guys. I have been so busy lately I am exhausted much of the time so getting pictures video and such just hasn’t happened. However I read and absorb everything you guys write, and I think you have identified the problem. Following Kellys lead I started dipping the parts in drywall mud, not painting it on. The coating is much more uniform, but much thicker. My first couple of pours where I was concerned the coating was too thin, that went well, I noticed turned the sand very black. These last couple not nearly as much. So everyones comments here would explain that I need a much thinner coating.
You can check out my dip coating thread, and I may have mentioned it in one of your other threads, but I did find dip coating drywall mud slurry needed to be much thinner than brushable coating. Adding that much water made it tend to not wet the hydrophobic foam pattern. To combat this I added Dawn dishwashing soap to the slurry and spritzed the foam pattern with a dilute soapy water before dipping to act as surfactant. This made the thin slurry lie down and self level nicely on the pattern. However, the dip slurry wasn't much good for brushing, because it would cover well with one coat. Drying times are a function of coating thickness and water content but overnight was/is always sufficient for me.....much faster with forced air and modest heat. Best, Kelly
Kelly, Yes the part I missed was making it very thin. I did add the Dawn. I have thought of the mud as kind of a refractory like investment. I realize now it’s purpose is mostly to smooth out the sand and still allow the hot gases to pass right thru. I’ll bet my next pour comes out great.
