Hi all- first of all I just want to briefly introduce myself- I am very much a beginner in the foundry hobby having bought my first propane furnace just a month ago. Always wanted to try metal casting out but it was one of those 'where to start' things that got put on the back burner- I also never realised that decent home furnaces could be got so cheaply and set up so easily. Anyway- here I am- having been experimenting for a few weeks I have a couple of issues that I'm sure are easily solved by someone with experience in the hobby. I messed around a small bit with aluminium but really wanted to do some bronze casting. I am using home made green sand (Bentonite cat litter added to silicia based sand and sieved down- the standard cheap and cheerful stuff) and home made flasks. The sprue drilled into the top and the bronze poured downwards into the mould- as opposed to the usual way of sand casting. But having seen this done elsewhere I don't see the difference this would make. Here are photos of an example of my moulds: As regards vents- I am a complete beginner and kind of unsure about this, but I always carve one or two using a pencil or skewer. As can be seen below on the left: Moulds ready for pouring (vertical as opposed to the usual horizontal method of sand casting) My problems with my first castings are pretty self explanatory with the photos below: Firstly as you can see there is horrible pitting/spitting in areas, and secondly sometimes the whole object doesn't cast: The bottom will get cast, as will the sprue, but an area in the middle just disappears for some reason. As can be seen with the object on the left of the second pic (the mould for this is the one shown above). I'm sure these are beginner issues and there is just one or two simple things I'm doing wrong. My theories are using water based as opposed to oil based sand, vent issues, pouring issues, or a mix of all. I try to get decent pours and for the most part all these casts were the result of a continuous pour. Although sometimes it seems as if the melted bronze cools immediately when pouring which makes it difficult to get that liquid-like pour. Maybe let it sit in the furnace for a while longer before pouring? I always skim off the slag before pouring. The bronze I'm using is good quality so I'm hesitant to suggest that this is an issue. Many thanks in advance everyone and looking forward to my time on this forum. C
I'm no sand casing expert. In fact Ive never done it. But just from a basic understanding of physics, with horizontal casting gravity is being employed to keep the metal in place and also the sand too. With sand and the frame resisting the any horizontal movement of the metal. With your vertical method the weight of the metal will push the sand outwards and there's very little to resist that sideways movement. This may not be a problem with your small castings but with anything large the weight of the metal will just burst through the side of the mould. I think you can start to see this happening with the large piece of flashing on the fourth picture. Also in the first picture it doesnt look like the sand has been compacted hard enough. The ancients cast axes vertically but they used stone or clay moulds which were rigid and could handle the sideways pressure of the metal.
The sand was too wet like Davis suggests: the excess water will generate steam bubbles through the metal. Casting the metal horizontally instead of vertically reduces the effective height so the pressure from the weight of the metal is reduced and less likely to force the metal into the sand which gives an improved texture.
Update guys- switched to some of my Petrobond for an experiment, having never used it before. Did the exact same castings using exact same method and I'm amazed at the improvement. Obviously the water was my main problem which is eliminated here. I still have the odd small hole in these castings though- maybe add more vents?
Or switch to a conventional feed system with tapered sprue, runner, gates, and Cope and Drag flasks. Best, Kelly
Doubtful venting would improve things. What you are doing is a direct cavity pour where there is alot of splashing as the mold fills and that will contribute to all types of inclusions. To reduce the splashing you could try angling the mold a bit so the metal runs down the side of the mold cavity. More head pressure might also help as well as having a clean melt by proper skimming fluxing..
Facts. My suggestion as well. I've made a few bronze axes, though they weren't bronze age replicas. The top gated vertical mold creates a large amount of splashing at the bottom. Splashing is never good. People cast bronze swords vertically because it's hard to get the metal to run so far any other way, but you don't need that to fill castings like those. Petrobond makes a little smoke that needs a lot less venting than greensand, which makes a lot of steam. The sand should be permeable enough to let the smoke escape. So likely you don't need any more vents and could probably do away with the ones you've been using. Switching to a horizontal cope and drag with a gating system with a feeder on top of an in-gate connected to the thickest part of the mold cavity should solve a lot of your problems IMO. Make sure to also be really careful to blow all the loose sand and excess parting dust out of your mold, if not they will also cause ugly pits on the cast surface. Good luck! Jeff
I'm not sure he needs a riser, I'm not seeing a thick enough section( 3/4 inch ) to need it, now that assumes a Silicone or Tin bronze. Brass , Manganese bronze and Aluminum Bronze can give shrink defects on 1/2 inch if you overheat the metal, I'd have to see an inscribed circle to be sure. V/r HT1
