Hey guys, i recently working on a project to rebuild a historical helmet found in valsgärde from the migration period. these helmets were heavily decorated and had great crests. I printed the model of the crest, used tons of primer and they were ready to cast. ive used petrobond sand and casted it several times in brass. But i never got a perfect result, Some times they have small porosities in the area of the sprue, like in the pictures attached. Some had bigger defects on the top of the mold. Are they from gas or lose sand? Ive tried a tapered sprue with a small diameter and a basin a couple of times but it seems that the molten brass solodifies too quick before filling the complete mold. Ive tried different spots for the vents and sprue but nothing really worked. My questions are: is it possible to fill the porosity somehow? i cant grind it all away. Maybe brazing or welsing or somerhing i havent thought about? If not how would you design the mold for this complex shape? Or what would be your procedure to cast this thing? Im relatively new to casting stuff, i built my furnance in january this year. Thanks a million and best greetings from germany!
Welcome vendel. That looks like a great project. I don’t have definite answers but there may be a couple of things contributing to the porosity. One is sand wash as a result of turbulence. From the pictures it looks like the metal is going to be squirting right against the side of the mold cavity directly from that huge sprue. I would go back to a smaller tapered sprue, no more than 12mm at the bottom (look at me speaking metric, lol) into a runner that runs lower than the pattern. 9mm wide, 12mm deep. Runner in the drag, pattern and ingate(s) in the cope. That way most of the initial inrush of metal will be subdued a bit and will enter the mold more gently. Metal too hot may also be a factor. I don’t have any experience with brass but I have successfully repaired Everdur (silicon bronze) with very similar defects by brazing. HT1 is our resident brass/petrobond guy. Hopefully he’ll chime in. Pete
hey Pete, thanks a lot for your assistance! really appreciate it. will try a new pour with your specs. best would be to just repair the porosity. i will see. and find a picture of my prototype vendel helmet that i have built to learn all the techniques. best, chris
He cast that in BrASS! How are you with a torch? Safety-sil here you come! I think you should reconsider recasting this. OR cast this in bronze with ceramic shell. Bronze porosity problems are not any problem at all. I tig those areas and work them back. The little porosity gun in the video above will not work on brass. Perhaps watching the video I did might explain why welding doesn't work. Your part is pretty cool and maybe the sand guys here can get you closer and more successful on your next attempt with your P-bond. I'm a shell guy so I seldom have those problems. But believe me, you are not fixing those issues without a shitload of gas welding and many hours reshaping it.
pete is right, I dont see any porosity, I see washing, erosion and scabs , start by letting us know what metal you used, if it is brass, remelt it and start again, the scabs, which is metal oxides at the surface caused by turbulence or dirty metal ( one of the Campbell fans will talk about that, basically poor gating system) the oxides are a pain to repair over, unless you have a sand blaster to remove them completely lets see how you gated it, because good metal poured correctly would not have caused all those problems, OK I'll stop myself there, the last picture is extremely telling, looks like you poured directly into a riser , all that black garbage is oxides , this is a NOGO technique in brass, you have to properly gate brass, if the metal roles over itself, you will get oxides (enough we would call it dross), and if they stack up on themselves, they will play havoc with your part, oxides and loose sand, are lighter then the metal, so they float to the surface, and wreck your part, that last picture, shows one of 3 things: you agitated the metal terribly while you poured your gating system, agitated the metal terribly while you poured or you did not skim the dross off the top of your crucible, and directly poured dross into the mold you defiantly had loose sand in the mold, you have to get all the loose sand out before you pour . someone made a youtube video, mentioning and detailing how I blow out molds i'm sure they will chime in, the instructions I gave here are buried in the middle of a thread, Tobho Mott probably remembers, I believe I was answer a question for him shoot us a picture of your pattern, and I will doodle you a gating design ( probably several ) V/r HT1 P.S. I will point out, if you had poured this in Evendure or any Silicon bronze rather then Yellow brass, it would have been better, you would not have gotten as much dross, but the loose sand would have still been an issue P.P.S are you sure this would have been cast on the original pieces, seems like something that should have been worked in sheet brass repousse, because of the concern of weight ??? P.P.P.S here is my post on parting and blowing out molds http://forums.thehomefoundry.org/index.php?threads/larger-moya.780/page-6
Sodium silicate solution was the old timey fix for casting porosity. Put the casting in a bucket of solution, wait a few hours, pull it out and rinse it off. Fine scale defects will be filled with metal silicates. Said to recover castings that won't hold pressure. (It's the same reaction as those 'chemical gardens' from when you were a kid. Plastic tank of solution, drop in some crumbs of stuff, colored nodular towers develop. The solution was sodium silicate, the crumbs were metal salts.) On the pit welder topic, a recent development that might work is the miniature spot welders for battery terminals. $60, fits in your hand, makes a very localised weld in seconds.
the sodium silicate fix does not work for returning surface detail on a part that needs to be buffed out, if he was going to paint it maybe, it's still terribly ghetto, and a short term fix, sodium silicate will occasionally save a part that has been machined exposing a granular defect this si casued by an area, needing just a little more riser, tso the grain structure of the metal starts to not fill in, if exposed while machining, you will get a part that weeps or sprays water, if you soak that part in SS, it will fill in the grain structure, and maynot leak if the pressure is low, peening the part, will give the same results, but the part still needs replaced in the long run V/r HT1
Hey HT1, thanks a lot for your detailed answer. indeed i am a noob and try to gain knowledge. ichose brass because the orher parts of the helmet (rivets, sheet brass, wire..) is easily available. the metal comes from plumbing parts, a kilogram sqare part brass and scraps from rivets, wires and stuff. i skimmed the dross each time before pouring. will read you post about blowing the molds, thanks! originally it was repousse on sheet mostly, but there were casted crests found as well. i dont intend to wear it, just for eye pleasure ive shot a couple of pictures of the mold design, i hope it is clear from the pics. the pour with the basin and tapeted sprue didnt work out because the animal heads at the ends of the crest werent filled with brass, it seemed that it cooled down before reaching them. till now i don’t have a kiln and the other stuff you need for shell casting. will update my shop if i have more cash for it thank you all dor your great help!! chris
OK first, you have chosen a VERY difficult early project, so be patient and keep with it! refer to the doodle below, I would place a runner paralleling the part on the interior , I would gate through 3 risers , one behind or close to behind each of those "heads" they appear to be the heaviest sections, that is what you have to feed the "heavy sections " the second doodle shows you roughly how to size your risers measure or estimate the cross sectional area of the part at the heaviest area draw a circle around it and add about 30% that is the Diameter of your riser if the riser is smaller then one inch you probably dont need one at all the top of an open riser needs to be tapered out, a little to make a funnel, not because you are pouring in it, but because you need the extra metal for heat dont go nuts, just a little, on small risers, rounding the top out with your finger is probably enough, in your case I would go a little farther, as soon as the top of the riser solidifies , the riser stops working , if that happens before the part fills, the riser may suck metal out of the part
Thanks a million mate, this is incredible! how deep and wide would you make the runners? tomorrow ill be heading to africs dor a business trip for two weeks, after that i will definitely try your mold layout as soon as possible. thanks again for your great help!!
Cool casting though! One thing you can do to help with sand inclusions is to open the ingate area up to reduce velocity and pressure. Make them longer to spread the metal out over a larger area of the casting and I am kind of thinking you may not need risers at all but hard to say with just pics. I know that the originals were way over sized and not in the right location. HT1 has you on the right track ....and post some pics of the results!
Hey folks, it's been a while and i tried to cast the crest like 15 times. I never got it REALLY perfect. Either minor inclusions or some shrinkage. But after a LOT of grinding and sanding i got a pretty nice result and i could finish the helmet. Thanks a lot for all of your great help. Here are some pics.
