Bob Puhakka on Bifilm theory

Discussion in 'Links to useful information' started by Gippeto, Feb 2, 2019.

  1. Melterskelter

    Melterskelter Silver Banner Member

    So, from the above my understanding is that you have incorporated some of Puhakka's concepts successfully without using software simulations and without CAD building of molds.

    It would be very interesting if you could expand on that a bit describing which ideas not requiring elaborate software you have used and found useful. Perhaps others could attempt to also implement them. Casting defects are a real pain whether in a casting for personal use, commercial sale, or aerospace.

    And i also wonder if you could say what you had in ,mind "for the student" with respect to bottom gating. So far, no one has really weighed in on that and I, and I am sure others, are curious as to what you had in mind.

    HT1 and Al Puddle like this.
  2. HT1

    HT1 Silver Banner Member

    Hell Yeah +1
  3. joe yard

    joe yard Silver

    Just my 2 cents. After watching the film with great interest, I noticed one thing that I think might be the most critical to what is happening. Without getting into the math which I admittedly know nothing about. It would seam that all the problems are caused by oxides being trapped in solidifying metal.
    Oxides float! When the metal is liquid the oxides rise to the top so if you make a reservoirs such as he did. It alowes the oxides time to rise to the top. His riser is a reservoirs that is filed to where the metal is never depleted and can not pull material from the top layer containing the oxides. The reservoirs becomes a basin much like a mud pit for a well driller. The small dam would cause a reversal of the metal direction further slowing the material and helping with separation of oxides. The metal flows over the little dam and into another reservoirs with a plunger that is then flooded and the oxides are once again allowed to rise. The plunger is removed and the mold is fed in such a way as to never let the reservoirs go dry. You are puling the metal from the bottom of the reservoirs. It is bottom fed into the mold pushing the oxygen in front of the metal as it fills the cavity and finally venting against the top.
  4. ESC

    ESC Silver Banner Member

    I've got too many projects on my plate right now to delve into this deeply, but I'm inclined to take some of HT's advice and make a smaller sprue cutter for starters. I made two with the old school idea to get the metal into the mold quickly, but always grab the smaller one when it is time to cut the sprue. One of the ideas from Campbell is that the ideal sprue is not straight walled, but rather has a parabola shaped wall to match the theoretical shape of a free falling stream. It would be easier to do it with a sprue pin, but I don't like to ram around them. The pouring basin would best be done with a core since the strength of the dam is critical. It would seem that bottom gating would require a core which would have to match the pattern, or the ability to pull a true horn gate.
  5. HT1

    HT1 Silver Banner Member

    the Dam Basin or offset weir is not new or innovative , referenced in 1957. nor is the use of a stopper, the first can surely be incorporated by the hobbiest. the second also can if you have help.

    very good read below , that someone should snag up before it disappears. please note that concerning sprues it only seems concerned that they taper downward, a very specific angle I did not locate
    Gippeto likes this.
  6. ESC

    ESC Silver Banner Member

    I based mine on Ammen's 1/4" per foot and then machined a tapered shaft. I can wrap the brass sheet around it at the desired outlet size and solder it up. What is the smallest cutter that you use?
  7. PatJ

    PatJ Silver Banner Member

    This is also mentioned in the Sorelmetal paper that I referenced, and they mention that this is why the gate should come off the bottom of the runner, not the top (the slag etc floats on top the melt).
    I figured it it worth a try at least.

    I looked at some of the preview of "The Complete Casting Handbook, 2nd Ed" by John Campbell, and it is heavy on forumulas, graphs and scans, but it very long and highly technical.
    The Sorelmetal paper is a very quick and condensed "how-to" white paper that focuses only on the forumulas that are needed to lay out the dimensions of the pour basin, the bottom of the sprue, the sprue basin, the runners, the gates, and the risers, so it is information that is immediately usable and understandable.

    I put the Sorelmetal formulas into a spreadsheet so that I could use them on multiple layouts/parts.

  8. HT1

    HT1 Silver Banner Member

    Ok I'm going to have to get technical here:::
    but if the Gates come off the Bottom of the runner , the gates have to act as a significant choke to get the runner to fill, which would never happen if the "choke" is at the bottom of the Sprue, additionally, Traditionally we are warned against pressurizing the gates as metal will SPRAY into the mold . I find this silly myself spray??? but it is what we are told .

    now to me the Campbell system seems to advocate the use of horn gates which are tricky to say the least but they get metal directly underneath the casting , but the technique I was taught was to put runner in the Drag and a gate in the cope that fills the Horn gate, this means the runner must completely fill before the Horn gate starts to fill, the horn gate starts to completely fill before the casting starts to fill. which should reduce and trap dross, but only if the Horngate is well enough designed to not be creating dross itself... but that is all way over my paygrade,

    The Only Sprue cutter i use has a 1/2 od at the base . Back when I was being taught AFS gating we did the math on several castings and Up untill about a 100Lb casting or one with some serious thin sections, this was enough metal . I have not done any of Campbells math, but it seems he would advocate smaller, or another manner in which to choke the flow

    V/r HT1
    oldironfarmer likes this.
  9. ESC

    ESC Silver Banner Member

    I rolled up another sprue cutter with a 1/2" small end. I previously used a 5/8" which was the small one I referenced above. No pours scheduled, but I see if I can tell any difference.
  10. PatJ

    PatJ Silver Banner Member

    The Sorelmetals white paper raises some questions in my mind too, and having the gates on the bottom of the runner are an obvious departure from normally accepted foundry practices, but as I read the various papers/books, the casting failure rate in many commercial foundries is extremely high, but more importantly I am reading that by using the correct techniques, the failure rate can be reduced to a very low level.

    So I think it is unwise to dismiss any idea just because it is new/different.
    I don't have hard data on what does and does not work, but apparently some do have that data, and the data would seem to suggest that the methods used 50 years ago (and the methods that many still use today) do not work well in industry and cause a lot of defects.

    My slant on the gate being at the bottom of the runner is that metal will follow the path of least resistance (just like electricity), and so metal would not be prone to run into a small restrictive gate when it has a long runner with perhaps a basin at the end to fill with little or no resistance to flow.

    As far as spraying into the mold cavity, I think it depends on metal velocity, head pressure, and metal viscosity.
    Obviously a high viscosity molten metal is not going to spray anywhere, but I think the real intent is to avoid turbulence, which churns slag, oxides and air (and thus defects) into the metal.

    If the gate on the bottom of the runner were a bad idea, why would it appear in a what appears to be a major publication by the foundry industry?
    Not that anything/everything in print is to be believed, but they use to insist that the earth was flat too.

    So I guess the question is "Can any of these ideas be proven/disproven in a backyard setting?".
    Too many times I have been told I can't do what I do, or it can't turn out like I have found that it does turn out.
    Discussions are good, but talk is cheap, and results that can be readily proven and verified are what matter.

    I am going to try the gate on the bottom of the runner, but with the understanding that what do I compare the results with?
    Unless I try multiple gate/sprue/runner configurations on the same part, and then cut up the parts and analyze every section with a microscope, and strength test the castings, then how do I know exactly how good the parts are?

    I have seen all sorts of riser/runner/gate methods used in a hobby setting, and most produce what appear to be solid castings, but I have also seen "solid" castings that are sectioned, and they turn out to have very significant defects.

    And to further complicate things, if I don't pour at exactly the same rate for every casting, then I can pour either too fast or too slow, and thus negate all other factors, and produce a poor quality casting.

    So what is a person to do in order to make good castings?
    My suggestion is to find a general layout that works well for the metal you are using, and then stick with that layout; ie: don't fix something that is not broken just because someone else has a better idea.
    But I am also open to new idea too, with the understanding that technology has changed over the years, and our understanding of the casting process is much better than it was 50 years ago.

    The computer simulations are obviously a good start as far as determining what is most likely to occur inside the mold with an given sprue/runner/gate combination.
    Lacking a computer program, I am going to try the Sorelmetal layout/calculations, since the calcs are simple enough, and can easily be tried.
    Ideas that are too complex to be implemented consistently by backyard folks are not useful, not matter what the claims of success may be.

    Just a few thoughts.
    I don't pretend to have the answers to it all, but hope to find a few things that work well.

    I have had conversation with several people who do a lot of iron pours, and they do often experience defects, and these defects can be tricky to diagnose/fix.
    I am also aware of some who do a lot of iron work, and who simply hide their defects (or neglect to mention them) as if they did not happen (if a tree falls in the forest, but nobody is around.....thing).
    Iron can be very tricky to cast without defects, that much is for sure, and getting consistently high quality iron castings is my ultimate goal, not view count, likes, karma points, or anything else that may paint a false picture of the realities of working with iron.
    Last edited: Feb 11, 2019
  11. HT1

    HT1 Silver Banner Member

    I hate to Say this Pat: but Cast fe is probably the worst metal to experiment with because it has a slew of additional variable ... Im rushed, do some experiments with something that dosses badly and visibly like a Bronze. then apply your lessons learned to cast FE, in cast Fe you have to Deal with temperature gradients to Make a machinable product , so you might not be able to use all you learn, But I'm very interested in what you discover

    V/r HT1

    P.S. the Gates at the Botton of the Runner is not as big of a deal I know Campbell was not big on the splash well so you bring the sprue directly into a runner on the Underside of the Cope . then the Gates go into the Drag, Now the question is how do you accomplish that , some sort of gentle transition partially in cope and drag , with the Metal eventually popping up out of the in gates into the Pattern entirely in the cope? we want the metal under the casting of course, you will see alot of molds with cope drag and cheeks ( middle sections) as the only thing in the drag will be gates... alot of extra sand involved there, along with specialized flasks, cheeks normally have to be a very specific thickness
    Last edited: Feb 11, 2019
  12. PatJ

    PatJ Silver Banner Member

    For the example I posted previously, I put the entire part in the cope, and also put the runners and gates in the cope.
    The only things in the drag are the sprue basin and the basins at the ends of the runners.

    The reason I put the entire part in the cope was that it seemed like it would be easiest to mold that way, I have done something like that before with a similar part that filled upwards, and the white papers/books indicate that you should not fill a mold cavity from the top, else you get a waterfall effect that churns air and contaminants into the melt.

    From a molding standpoint, I think it would be pretty simple to have most of the stuff in the cope.
    I don't really see putting the gates in the drag arrangement because that changes nothing as far as the mold cavity/runner/gate/sprue relationships, but it does seem to make it more difficult to mold.
    I don't think it makes any difference where you put what as far as cope or mold; it is the overall layout and relationships that matter (my guess).

    If a sprue basin is used (so far I have been using one), it may be a good idea to start filling it somewhat slowly to prevent splashing, and then once it is partially full, pour at a faster rate, but it would also be critical to pour fast enough to fill the sprue quickly and prevent air aspiration.

    I will probably vent the end of each runner out the top of the cope, but will not vent the blind risers. The taper on the blind risers is supposedly critical as far as limiting early solidification of the risers, so I guess I will make a two-piece mold for the risers, with the top part of the riser in one small mold, and the lower part in another small mold, and the two cemented together, and then cement that assembly to the top of the cope.

    The elbow at the bottom of the sprue is certainly worth considering if you could figure out a geometry that worked, and a geometry that could be molded with relative ease.

    I use bound sand, so I basically cement the various parts together.
    I would make a round mold for each blind riser, and cement each of those on top of the flat main mold for the part.
    I also make the sprue section separately and cement it on.
    Making lego-style pieces that can be cemented together saves a considerable amount of bound sand, and is fairly easy to do.
    And with a mold that consists of modular pieces, if one piece has a flaw in it, it is easy enough to mold another piece without disturbing/damaging all the other pieces.
    Generally, if you set things up with the right draft angle, you don't have problems with the sand. I will post an example below, which shows five pieces cemented together, with a snap flask being used to create multiple molds from a single pattern.
    I made a lot of bound molds for one particular engine, and I don't recall a single mold that had to be discarded because it did not mold correct.
    I had a slight breakout on one mold due to not enough draft, but it did not create a problem since it just extended one edge slightly, and you just trim that part off of the casting.
    I do use weights on top of the mold since otherwise occasionally the cement bond at the edges of the cope/drag may fail, especially with metals that are more dense than aluminum.

    All of my flasks are custom made from wood, but making a wood flask is the fastest and easiest thing I do, an it literally just takes a few minutes.


    Last edited: Feb 11, 2019
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  13. Melterskelter

    Melterskelter Silver Banner Member

    Would you mind outlining the actual method you used to produce horn gates in green sand. I have seen several references to them but have not come across an actual step-by-step description of a practical way to make them in green sand. And it is hard for me to imagine a practical way to do this.

    I am not planning to have horn gates as my new go to. Mainly curious

  14. HT1

    HT1 Silver Banner Member

    A horn gate pattern is made from a single piece of wood , it has draft running one end to the other, it is removed from the sand like a fishhook because it is the arc of a circle ( that can be fudged)
    Here is a picture of a reversed horn gate, which is a huge nogo, as it will definitely aspirate air into your casting, a Horn gate has to follow the same rules as a sprue : which I just located, see below

    V/r HT1
  15. Melterskelter

    Melterskelter Silver Banner Member

    HT-1, Thank you.

    Makes sense now. But also means the curve of the horn has to be a true circular arc. And there has to be adequate clearance to remove it. So, not a casually accomplished technique.

    By the way, the linked page had some good information and some rather sketchy embedded/attached links requesting "software updates" --- good to steer clear of that stuff.

  16. HT1

    HT1 Silver Banner Member

    Pat, do you consider yourself a hobbiest??? because resin Bonded sand is not a hobby skill. it allows you to do things entirely out of the scope of anyone save the lost wax guys ... OK alot of what you are advocating and doing I can do in green sand, But i'm not a hobbiest "Duh!!!" I'm an AFS Journeyman Molder. Lets discuss a couple of things that can be done

    the Sprue elbow " can be done quite easily as a woodworking joinery project this hidden tenon is perfect the part running Horizontally in th Picture is your Sprue , sized as you desire, tenoned into the runner you shape the back edge of the runner to get the perfect radius, ever so slightly taper the runner you would create the perfect slightly pressurized sprue and runner, get that under your casting easily enough and you are golden
  17. DavidF

    DavidF Administrator Staff Member Banner Member

    do you beat you chest like king kong when you say that??? ;):D:D:p
  18. PatJ

    PatJ Silver Banner Member

    Well of course I do; there is a wide range that covers the hobby, and no hard and fixed definition of what that term means.
    I get what you are saying, but I don't limit what I say in order to cater to one group or another, I just throw out ideas, and things get discussed, which I think is the best way to do it because it exposed people to a more complete range of options that are available.

    I value your knowledge and experience, no doubt about it.
    You are one of the few people I pay close attention to; always have and always will.

  19. HT1

    HT1 Silver Banner Member

    Stop By My office we can Discuss it in detail
  20. HT1

    HT1 Silver Banner Member

    It's not as hard as it sounds and it doesn't have to be a true circular arc because it gets smaller at one end, Just like a bulls horn, so once you move it a smidgen, it gets loose, but somewher in there there has to be a circular arc (im sure there is some High speed explanation of the concept)
    Melterskelter likes this.

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