Bad casting! Beginner problems

[BClem]

Drillpoint,

I'll jump in here too....I've done some thin section castings in both aluminum and bronze. I have found that Petrobond is too gassey to use for this application. The gas from the bonding oils in Petrobond will work nicely for a reducing atmosphere in the mold but it will also impede and cool the flow in very thin section castings.

Make up a batch of fine Albany green molding sand. Abany is a naturally occurring sand that contains its own bonding clays. You can purchase it in 100# sacks from foundry supply companys. A fine grade of Abany will cast your fingerprints! Mull it with water to the correct 'fracture' and do your mold as you have been but remember that any venting will help even down to a piece of 1/16 brazing wire run into the sand from above the pattern (before splitting) to within 1/8 of the pattern surface will vent without a blemish on the casting. Albany will breath and pour very freely, and the more you use it the better it gets. I have what is left of a 100# bag that I have been using for about 30 years. I believe I added some southern bentonite to it to increase its hot strength for bronze...it will lose some bonding strength over many years of use.

How about another 2¢...

But it looks like you're doing pretty good. You found that the feeders at each end will cause the casting to hot tear on cooling - the feeders will 'fight' for metal as they cool and rip the casting. It's hard to balance a thin section with feeders so the ability of the mold to breath is the best way to insure full feed.

[Harold_V]

You found that the feeders at each end will cause the casting to hot tear on cooling - the feeders will 'fight' for metal as they cool and rip the casting. It's hard to balance a thin section with feeders so the ability of the mold to breath is the best way to insure full feed.

Thanks for that! Something I had not considerered.

Harold

[Fender]

Although you are getting much closer to success, I can't help but think that you need more temperature in your melt before you will have the result you want. A lot of my castings have been what I euphemistically call "non-ferrous alloy", which is basically some kind of mix of brass and copper with a little whatever mixed in. When it is ready to pour, the zinc is burning off, and there is a whitish/greenish cloud being emitted from the melt. I stay up wind. Bronze is naturally less fluid than either aluminum or iron. You really need to sock the heat to it.

[jpfalt]

The three vents in the last attempt are actually pop offs. They are open channels directly from the mold cavity to outside atmosphere.

I did some digging on alloys and confirmed something I remembered from some old info. You will probably want to lay hands on some phosphor bronze. The phosphorous addition improves flowability and allows it to flow into thinner sections with more detail.

[drillpoint]

I have done some digging around online and found Ammen's book on sand casting. Not only is Albany sand mentioned but he goes into great detail of the different formula's of sand mixes for used for various metals and castings. He makes it quite clear that "the devil is in the details". And that often compromises have to be made regarding the strength, surface finish, etc. of the molding sands used. Unfortunately at this time I am limited to using only Petrobond. After reading about all the different sand mixes that have been used I can see now that store bought Petrobond, for all of its advantages, cannot be as versatile. But the one thing I don't get is that Petrobond is supposed to be better at gas permeability and have less cooling effect on the metal. I have found that Budget Casting Supply does offer a phosphor copper shot to improve the metal flow. So far, I have made a reasonably decent full copy with a full 1/8" backing plate and will try reducing that to 1/16". Regarding the temps, I think I have already reached the point of getting it too hot on a couple attempts. It looked gassy where the sprue hits the well. I also think that when melting tiny amounts of metal as I am doing here, the use of a pyrometer will cool the melt unless the pyrometer itself is preheated very close to the melt temp. Does that make sense. Doug

[jpfalt]

For small melts, preheating the pyrometer is essential.

Another consideration is that the bronze will absorb hydrogen from the combustion products of fuel gas. The hydrogen causes copper based alloys to lose flowability and thicken up. This is really pronounced with remelted scrap copper, brass and bronze and was a big reason for us not to remelt machined chips contaminated with oil or coolant. If the material you are using has been remelted several times, your success may be drawing away from you due to hydrogen contamination.

I'd suggest you retry with fresh material with the phosphor copper shot addition to the melt and try a fresh pour.

[ghornbostel]

Doug, First of all you need a pyrometer. You cannot guess what the temp is by looking into the furnace. From the pictures it looks as if you have two things going. Metal probably is being poured too cold and you have a gas problem. The rounded edges on the misrun castings is cold shut. The rounded tops of your sprues and riser are usually gas. If you grind the face of the lettering on the misrun castings and find small cavities under the surface its gas. This name plate should run at about 2200F. You have enough sprue however I like to make a 90 degree turn to the metal before it enters the gate. Its called a runner and should be 4 times the area of your sprue or at least the smallest part of the sprue. This will take the turbulance out of the metal as you aren't pouring into a basin and straight down the sprue. That turbulance can be another source of gas. If the gas is being introduced at melt you probably need to cover the melt with glass. A beverage bottle broken up and added as the melt progress will form a cover. When you get ready to pour you simply draw that cover back with a steel bar. It also controls the slag on the top of the melt. Petrobond...I have used it and have never had good results with it. There is several thousand years of casting experence in green sand to recommend it for this casting including the casting you are using for your pattern. Now, you already have the petrobond so you could start by venting it using a sharp wire plushed into the cope and down to the pattern before you remove the pattern (after you strike off the cope). A riser will do the same thing if you have enough melt. You have to keep the sprue full at all times or it will draw atmosphere into the mold and cause gassing. Get the pour going and the sprue full and watch the riser. When it fills cut the metal off. This will be in very short order as there just isn't very much metal in the total casting. Phos-copper shot will help take out gas at melt and will make the metal more fluid. Use sparingly as too much will cause the melt to act like water and soak into the mold. Hope this helps. This isn't easy as the original name plate was done with a match plate and about 20 more like it in the same mold. It can be done just keep after it and only change one thing at a time.
Greg Hornbostel

[Rwilliams]

This may be a bit late based on the dates of the postings but better late than never.

Much good information has been supplied in order to achieve success. During college foundry classes, it was almost comical to watch the guys do the same thing over and over again without improving their results. I guess they figured they could do the same exact thing and get lucky the next time but it did not work that way, same exact failure. Finally one of the guys realized the instructor was waiting for one of them to use their brain and make a change to result in a decent casting. The pattern in question was a mug which had been poured many times in the past with success. Each group that came along had to figure out what the secret was to make it pour correctly. I knew what the problem was by reading all of their mistakes but never said anything because they thought I did not have a clue. At long last one of the upper class men came to me on the side and said, "You have been too quiet on this project and I suspect you know the answer as he will not tell us."

He was correct and I shard with him my observations of all the successful failures and what I might do if it were my project. He listened well and went after it again with my suggestions. He stayed off to the side and did not make a big deal of how he changed the gating of the casting. When poured, the metal went in faster as there was less restriction. That was the first sign of success. When cooled and opened up the visual evidence was proof that he had listened to the right idea and was the first one to meet the goal of a good mug casting.


Everyone was envious and wanted to know what his secret was as he had already cut the gating system off to hide the changes I had advised him to use. The teacher came to me later and shared with me that I had helped to teach a hard lesson of asking the right guy for help instead of continuing down the path of failure.

This plate casting has been done before so it can be done again. How it was gated is the big secret. Thin sections are a challenge just as in the circular mug project of years gone by. That mug needed more hot metal faster to prevent the freezing before it was completed. In the mug I advised at least two gates to double the metal flow which worked like a champ. In this case a thin flat section needs at least two gates, one either side and perhaps some creative runners beyond the gates to fill first and heat the mold on both sides before actually filling the mold cavity from both sides at the same time. Coupled with some vents it should pour quite nicely. Getting the metal to the mold is half the battle in foundry work and once the delivery is refined, the resulting casting can be made to look easy.

Another trick to speeding the flow of hot metal is to coat the entire mold and gate/runner system with a thin layer of powder graphite easily found at the farm supply stores where it is used in the high speed corn planters to insure all parts are working well and the seed does not build up. They sell it by the case to the farmers at planting time. The graphite will speed the flow of the hot metal and act as an insulator to prevent burning of the details in the petrobond sand. Once you use graphite, there is no going back. I spray the graphite on to the mold surfaces with a old air brush. Or one can sprinkle it on and use a soft brush to remove the excess.

Hope this helps resolve the problem.

[Pipescs]

Another trick to speeding the flow of hot metal is to coat the entire mold and gate/runner system with a thin layer of powder graphite easily found at the farm supply stores where it is used in the high speed corn planters to insure all parts are working well and the seed does not build up. They sell it by the case to the farmers at planting time. The graphite will speed the flow of the hot metal and act as an insulator to prevent burning of the details in the petrobond sand. Once you use graphite, there is no going back. I spray the graphite on to the mold surfaces with a old air brush. Or one can sprinkle it on and use a soft brush to remove the excess.

Could you expound on the use of the graphite? I too am looking at casting long thin parts using brass and bronze.

You mention an airbrush. Is the graphite in liquid or powder?

[Pipescs]

Tried the graphite. Had good results. Bought it at the local John Deere Dealer in a one pound bottle (about the size of a quart) with a glue type nozzle. Cost a little under five dollars.

I sprayed it on with a smaller glue bottle that acted like a dry spray.

Then turned the mold over and blew out the excess.

This was the first time I managed to get the metal to flow the entire distance of eleven inches in an "I" Beam with webs less than one eighth inch thickness.