My membership has just been approved. I googled and stumbled on your thread about Puhakka and bifilm theory so I thought I could join. I have no backyard foundry experience, only worked in regular industrial foundries => but I plan to start home pouring from time to time just for fun I have theoretical knowledge and metal casting simulation experience. Looking forward to future discussions.
Welcome CLR. I had never heard of Bob Puhakka or the bifilm theory until someone posted a link to one of his videos a month or so ago. At first I thought it was a bunch of snake oil, and then I found out about John Campbell and his 10 rules for good castings, and found out that Bob basically took John's 10 rules and built a very nice commercial foundry that makes great castings. I bought John Campbell's book, and have been reading it (on and off), and I must say I have a much better understanding of the metal casting process than I did since I started home casting 7 years ago. I still have much to learn, but the whole converstation about Bob and bifilms that has gone on here has been very positive because it is making people think critically and objectively about how they do basins, sprues, runners, gates, and risers, and I think some here have already discovered quite a few things they were unaware of (I certainly have). Thes guys here do some impressive work across a wide range of casting/metal types, and they are a good sounding board for ideas too. The gray iron work going on here seems to have taken on a life of its own, and I am super thrilled about that. Grab a crucible and a pouring shank and jump into the middle of all this. .
Thank you PatJ. I have been following prof. Campbell's work for over 15 years, and consequently Bob's work from the "start" and his cast differently blog. I think that prof. Campbell sometimes exaggerates effects of the bifilms to "sell" his theory, but he is definitely right about using naturally pressurized gating systems. They are most important for "reactive" alloys (like Al and especially Al containing Mg), but I personally found bifilm defect in cast steel, and have seen research for cast irons.
I am looking forward to your input. It sounds like you have a lot of knowledge and experience (more than most of us) in the foundry world. We are all trying to figure out how the rubber meets the road in the home foundry with respect to Puhakka and Campbell. Glad to have you joining in. Denis
Very good experience. Welcome to The Home Foundry! What I struggle with is how much bifilm theory is as asset to what I do, but one has to try to understand the effects. And, if you don't investigate the results, you don't know what you did. Obviously keeping air from being ingested into sprues has to be good, so I'm striving for at least that level. I made the centrifugal separator too big, but am eager to look at microstructure to see whether anything appeared to be separated. I'll be interested in hearing your take on results.
That may very well be true, but even if you toss the bifilm theory totally, the 10 rules for good castings seem to be golden stuff, at least from a backyard casting guy's perspective. .
Well, looks like your off to a great start! Lots of good folks here. I'm a complete noob, I came here to learn.. Welcome!!
Sorry, but I think that your centrifugal separator is not doing what you want it to do. Similar centripetal trap designs exist for ferrous metals for dross elimination. For Al alloys such designs do not to work. They seldom work even for ferrous or copper castings. Even if it did work for Al, I would be surprised if you saw bifilms on hobby optical microscope (they require higher magnification and more importantly high-quality optics in most cases => but you can see dross and "regular" oxides/inclusions if they are present). You pour small castings and you do not need to use elaborate mechanisms for your castings (you are not making parts for NASA). Just use basic calculations as shown in prof. Campbell's books or by Bob. Maybe simple vertical runner extension to trap first melt and dampen the first flow of metal. At least that is my take.
What did I want it to do? I was trying out a design and hoping to separate out liquid metal from any heavy contaminants. I can understand a cyclone separator but was having trouble visualizing how to get centrifugal separation and at the same time avoiding mixing air in. Getting a separator started without creating a lot of turbulence seemed difficult so I tried a design with the hope of seeing whether there were bubbles in the metal. I was hoping to create a circular weir with a small enough sprue under it that it would be filled quickly as the cavity filled. The microscope I purchased only goes to 1,600X magnification, I'm thinking that is enough to see reasonably small cavities. I had not thought about trying to discern closed bifilm "seams". Dross and inclusions is what i want to see. I absolutely agree for the stuff I'm casting I don't need NASA quality, but I would like to improve quality if possible. I think I've made a lot of improvement by flooding the sprue.
1. Provide a good quality melt 2. Avoid liquid front damage 3. Avoid arrest of the liquid front 4. Avoid bubble damage 5. Avoid core blows 6. Avoid shrinkage damage 6. Avoid convection damage 7. Plan segregation distribution 8. Control residual stress 9. Provide location points 10. Don't spill your beer
Daves top ten... 1 Know what alloy you are melting 2 Know your furnace (environment) 3 Properly prepare your mold (varies according to process) 4 Understand solidification (cooling) traits 5 Dont give Jason your address !! 6 Repeat steps one through 5 .... Ill come up with more one I get some feedback, and sleep... Been a rough week at work...
