Bob Puhakka on Bifilm theory

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

  1. Billy Elmore

    Billy Elmore Silver

  2. I haven't done any simulations but I think I get excellent results with an offset basin. I don't use anything else for aluminum and have determined the casting is much cleaner with a basin, even using ingots from multiple melts. I determine casting inclusions from sectioning and crude microscopic analysis.

     
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  3. I'm going to try both a styrofoam plug, and different thicknesses of steel shimstock sheets for starting the pour on steel castings.
     
  4. Seems like many of you are ahead of commercial casters. Too much of 'but we've always done it this way'
     
  5. Tobho Mott

    Tobho Mott Gold Banner Member

    Replying here in hopes of separating Campbell gating related discussion from the casting defect thread at:
    http://forums.thehomefoundry.org/in...-any-idea-what-happened-here.1776/#post-39501

    It's a Campbell thing. A spinner pattern can have plenty of draft; easy to ram up and draw out. I would say easier to build than a tapered runner pattern. So I don't see it as very complicated. But on the other hand, often I do not bother with it, so I guess maybe it is a tiny bit complicated after all.

    The theory as I understand it is this: the metal does more than just bounce back off the end of a blind runner. The air trapped in the runner extension heats up and expands a lot, and rapidly, causing metal full of air bubbles to squirt violently from gates. There are some pretty convincing xray videos of runners and molds filling that show this happening.

    Spin traps are supposed to centrifuge that air through a generous central vent instead, while also shock absorbing 'brick wall' effects (if any, I guess?) and also slowing down the gate until the spin trap is full. By that time, since of course we always botton gate (yeah right), any jetting from the gate resuming full speed should be submerged deep enough not to roll the surface of the melt over on itself entraining more air bubbles and dirty B-words.

    I tilt my plaque molds slightly with the gate at the downhill end when I pour them too for basically the same reason: avoiding air bubble entrainment.

    Since you asked. I'm not trying to sell anyone else on this stuff, just trying things out to see for myself.

    Jeff
     
  6. I think I get better castings with a spin trap but it is obviously hard to quantify.

    I have also used waste spin trap risers to machine into small parts with good success.:rolleyes:

    The very top end is full of oxides however.

    I think there is a lot to the idea the spin trap inhibits flow into the gate and then slowly accelerates that flow as the riser fills.
     
  7. Tobho Mott

    Tobho Mott Gold Banner Member

    Your glass flask experiments helped convince me to try it.
     
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  8. Billy Elmore

    Billy Elmore Silver

    Yup.. You probably do get better results for many different reasons. Slowing down the initial velocity is probably the biggest benefit. I see turbulence under the plug before the sprue fills. I look at them in slow motion in simulations and I see where it would be happening if I ran a sim on this...which I might do just to show you what I mean if I get a chance. There will always be voids in filling the sprues at initial fill unless you put the down sprue on a steep angle. Any free falling stream will get voids and turbulence. We shape them in an attempt to reduce the separation of the melt and the mold wall which helps. Just the filling of the basin itself creates bi-film as it splashes around filling until it is full enough to start feeding the sprue. I have ran many sims on curved and steep slope sprues and that is the only way to get a fill without creating metal separation at initial filling of the sprue. One note I will add....the porosity created by this is microscopic and it takes quite a few instances of this happening in the same location to actually create a void big enough to be of concern unless you are working for NASA and are doing micro electro radiography....however high velocities at the ingate will create turbulence and wash in sand molding and can also create dross if it is free falling inside the casting which is usually more of an issue than the bi-film created at the top of the mold. I am not downplaying his theory but have found that in general it is kind of a null and void topic in production environments as we generally push all of our boundaries past the limits in order to be the most profitable which often times out weighs creating tooling for optimum gating. We put as many castings as we can reasonably (reasonably is questionable) mold and then figure out how to get metal into the cavities as best we can. Yes...ass backwards...put more than we can properly gate and then repeatedly ask me to properly gate it. I love the test and videos though and I would love to test them against a sim one day.
     
  9. Billy Elmore

    Billy Elmore Silver

    A tapered runner will still get the bounce back wave. I run an extension of the end of the runner which tapers down in all directions...being vertically parted molds I can run the runner extension upwards as it is tapering which effectively stops the metal from flowing and drastically reducing the bounce back wave. Yo
    I have ran sims on the spin traps. They really do more for slowing down initial velocity than anything and after the trap is full the velocity spikes up at the ingate where it could produce jetting, creating wash or turbulence. Just about any directional change in the melt channel will give you oxides...loss of contact with mold wall... unless you have an extremely generous radius at the turns which insures metal to wall contact. In order to absolutely follow his guidelines only the melt front can be exposed to oxygen, there can be no free falling metal and no voids between the melt and the sand molds...hence tapered sprue/ runners...which increase velocity... which increases jetting... which creates bifilm and dross....hence the spin trap to absorb the velocity and to catch any loose sand in the melt front. I do not use a spin trap but do use an extension angled upward at about 20 degrees and tapered down to around 1/8"x1/8" at the end. It effectively stops the melt front in the small area but since I use to chokes to help depressurize the system my runners will not be full and I still get a bounce back wave as the runner fills but the extension reduces it tremendously. I have designed a gating system based on his theories (no loss of contact except for the melt front) but they are not practical unless you have lots of free space in your mold and plenty of extra metal to fill it with. Having said all of this I still use some of his methods and have adapted some of them to be used in more practical production environments. His theories are not wrong but are usually not what is causing scrap or defects in the production world...slight variations in sand and chemistry/temps and poor machine maintenance dominate the real world. I think the biggest reason people see a difference when trying his methods is the reduction of the initial velocity followed by trapping trash at the end of the runner and finally the bigger pouring basin helps them maintain a more smooth consistent pour. I spent way too much time on typing this. I should just post the sims.LOL
     
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  10. HT1

    HT1 Gold Banner Member

    spin traps are not new, and I do not think they are a Campbell/Puhhaka invention and they are only a deal in specific classes of castings (large being the first criteria) for a plaque in petrobond draw a scratch line vent off the end of the runner for the same effect with far less work and less wasted metal ,

    it's been a while , but I only remember spin traps being a thing in cast fe but it's been 30 years

    a few things to remember when talking about Campbell/Puhhaka everything they work on is in some sort of resin sand , alot of which have bad permeability compared to petrobond , so some of their advocated technique are quite important in their applications, but in good green sand , or petrobond, become excessive , not wrong, but excessive .

    Andy follows my instagram, and knows how many plaques I make, and none of my patterns have a spin trap , most of my runners taper, almost imperfectively, flat backed plaques break so many of the casting rules they are in a class all their own, I'm sure if Campbell/Puhhaka deemed to make plaques they would pour them standing up with a bottom fill, too much work

    V/r HT1
     
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  11. Dean

    Dean Copper

    The book is available directly from the publisher Aspect Design for 15 quid plus postage (posts internationally)
    https://www.aspect-design.net/product-page/mini-casting-handbook-2nd-ed
     
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