Bob Puhakka on Bifilm theory

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

  1. Melterskelter

    Melterskelter Gold Banner Member

    PatJ, I really appreciate your efforts to pull together some useful information for the small time caster. There has to be a happy medium between simply guessing at sprue size and runner/gate geometry versus full-on computer simulations and hours of number crunching. Just “gittin ‘er done” has , for me, resulted too often in really frustrating and time-wasting casting defects.

    Simply reading through some of the information referenced in this thread let’s me see some serious conceptual design flaws in
    my pouring basin, sprue, and runner. I intend to address them. In addition, I hope that it is also possible to get some improved numerical guidelines to follow in designing the filling system used for our castings.

    Keep up the good work. Right now I am pretty preoccupied with machining and delivering castings already made. But it would be a big mistake to be so busy felling trees to as to not take time to sharpen my ax.

    Denis
     
  2. Gippeto

    Gippeto Silver

    Very much looking forward to the next installment.

    Base volumetric flow rate on possible "smooth/clean" pouring speed, base sprue areas on that and sprue height. Runner area equals sprue area at the bottom. Seems almost too simple ... or I'm missing something...which does happen from time to time.

    Al
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    He (Bob) hasn't told us where the 1" sprue base area he used in his example came from. Guessing that was just to make the math easier, as that sounds pretty big (to me anyhow) for the stuff most of us are doing. Hopefully the next video with the rest of the gating calcs will clear that up... Some of Pat's links above have me guessing that the sprue base area (therefore runner cross section area since the two are equal) should perhaps match the size of a "sessile drop" of the alloy in question. Next video should be a good one and clear up a few more questions!

    Jeff
     
  4. HT1

    HT1 Gold Banner Member

    1 inch of area is .56 inch radius, the smallest sprue cutter I have ever seen was 3/8 so .375, the Smallest I have been able to Procure is 1/2 inch very confusing


    V/r HT1
     
  5. Gippeto

    Gippeto Silver

    At 3:00 to 3:30, he talks about measuring how fast you can pour and basing the sprue/runner system off of that number. After determining the flow rate, the sprue areas (top and bottom) need to be worked out from the known flow rate and the sprue height.

    @ 5:50 ish, he picks what he calls a "nominal" number of 1in2 for the sprue bottom. I would interpret that as being nothing more than a convenient number to work with.

    Al
     
  6. 1-1/8" diameter (0.56" radius) is a pretty healthy sprue. 1/2" sprue would be about 1/5 square inch (0.2 sq in).
     
  7. Petee716

    Petee716 Gold Banner Member

    The mechanisms he talked about (sprue and runner) and the formulas he gave are showing us how to control the flow rate without introducing air or turbulence. Gating strategy will probably be more of the same. We haven't seen him talk about gating yet so we can't know, but a maximum speed that the metal enters the mold seems to be what this is leading to. There has been mention of jetting and splashing into the mold, and metal moving through a sprue that big and poured that fast is going to have to get slowed down somehow. Once we know how fast a mold should fill and how fast the metal should be entering the mold, then we can apply that information to find the desired flow rate, basin size, sprue size, etc. Maybe.

    Pete
     
  8. HT1

    HT1 Gold Banner Member

    2 was a reference to the height of the Metal in the Basin, what he Labeled Bh
     
  9. HT1

    HT1 Gold Banner Member

    I've been running this math over and over trying to wrap m y head around it , I think I sort of have it, and I'm really glad I will probably only have to make 3-4 of these calculations. But FYI; if I'm right his example is completely out of the Hobbiest range. the sprue he used as an example would suck down 29Lbs of Brass/ Second ... That is my entire crucible... I cannot pour that fast (cries)

    V/r HT1

    P.S. I do really appreciate his work and the Videos
     
  10. Tobho Mott

    Tobho Mott Gold Banner Member

    I'm going to have to watch these all multiple times to wrap my head around all of it. The gating video is out now and I still have questions; I'm just not 100% sure what the questions are yet... It's barely 8am here after all.

    Jeff
     
  11. HT1

    HT1 Gold Banner Member

    Next video

     
  12. Gippeto

    Gippeto Silver

    By 1in2, I was trying to say 1 inch squared.

    Al
     
  13. HT1

    HT1 Gold Banner Member

    for those of you wrapping your head around this... I computed out my most common product, a 3 Lb plaque about 3/8 inch thick, using a 1/2 sprue cutter with a 3.5 inch effective sprue height... I found , well proof of what I have always Known, I'm doing it about right. Now keep in mind I was taught the old school AFS 1 4 4 system, but everyone knew it worked like $hit for small stuff, I'm Near to the Math Bob explained, though my design with the Splash well (bifilm production chamber) is lousy, my sprue runner and gates on most of my pieces would make Bob Happy .

    If I was making real parts on Match-plates I would definitely use his math and design philosophy ( alot of it I knew Maintain volume on split runners t 1/2 on gates Max . all things that AFS used) But for Flat backed Plaques the additional effort is not Justified. But if I get a serious project on th Carving bench you might see me doing this math and showing my work. Keep watching it's HT1 approved shit

    V/r HT1
     
    Tobho Mott and oldironfarmer like this.
  14. Melterskelter

    Melterskelter Gold Banner Member

    It is, for me at least, always important if a new idea or system suggestion is validated by personal observation. I have run into shrinkage problems at my gates and guess what?—-they were greater in thickness than half the casting thickness at their point of entry—-a violation of Puhakka’s principles. Of course, the next step is to reduce the gate height and, hopefully, see that issue never again raise its ugly head. Now I feel like I have a reasonable working theory on which to base my my casting/gating changes . It will be very interesting to see how much of his theories seem to hold up in our collective experiences.

    My thanks to Bob and to Gippeto for getting this thread started.

    Denis
     
  15. Jammer

    Jammer Silver Banner Member

    Just for reference here is the runner system used by Lodge Cast Iron. The sprue runs down the center of the male side and then risers are on the other side. These skillet are about 4 pounds each and the mold fills in about 3 seconds, I stood and watched the process. The gates are knife edge and are very thin.They do use Phos in their alloy, about .26 of one percent.


    IMG_0040.JPG IMG_0040.JPG IMG_0041.JPG 004.JPG
     
    Tobho Mott and DavidF like this.
  16. PatJ

    PatJ Silver

    And so that is what makes for a head scratcher, which is how some people seem to violate many/most of what are claimed to be such critical concepts, and they still get what appears to be a defect-free product.
    So it makes me think there is more than one way to skin a cat.

    No doubt the phosphorus helps a lot with fluidity, but it makes for a bad structural part according to the books.

    I believe what Bob Puhadda says, and I believe in John Campbells 10 Rules for Good Castings, but again some seem to discard much of that information and get high quality castings, or perhaps they are actually adhering to the 10 Rules and I don't understand exactly how they are doing that.

    I would consider those knife gates, and they make perfect sense since the section of the skillet is so thin.
    Any idea what their rejection rate is? I would assume that give the amount of castings they produce every day, their success rate is very high.

    The long central tapered runner seems totally insane. What the heck?
    There has to be a method to the madness I guess.
    It looks like the mold is poured vertically, so I guess that explains the long vertical runner which leads to the bottom of he mold.
    And the mold fills from the bottom, which I keep hearing is a good idea.

    I see a vent on the handle.
    I know ironsides has often stated that you don't need any vents, but I think venting is critical in some cases, and the fact that he can sometimes get away with no vents just means that it works with his sand but perhaps not anyone else's. A topic of debate I guess.

    Much to consider.

    Edit:
    And I notice that there is no basin at the bottom of the vertical runner, and so that makes me wonder if any white papers that detail a sprue basin are using a bad method?
    Bob seems to indicate that a sprue basin is a very bad thing, and I can see why he thinks that.

    Edit02:
    I would think you would get a bad bounce off the bottom of the vertical runner in that dead end section, and Bob talks about avoiding this.

    .
     
    Last edited: Feb 21, 2019
  17. PatJ

    PatJ Silver

    I absolutely love this guy.
    He is so spot-on with his assessment of what goes on in the industry that it is not funny.
    This goes on in the corporate structure too; I can attest to that; I have witnessed it first hand for many years.

    His frankness and honesty is so refreshing, and if it were not such a sad situation, it would be funny.
    The good part is that if you decide to open your own business, and you can think independently, you can run circles around your competition who are using a rigid corporate structure.

    Edit:
    A word of warning; his videos are not safe for work, that much is for sure, but this is golden stuff in spite of his over-the-top style, which I think is intentional for humor and effect.
    I can relate to so much of what he says though.



    .
     
    Last edited: Feb 22, 2019
  18. Melterskelter

    Melterskelter Gold Banner Member

    Just for the sake of discussion: The tapering of the runner may be a bad idea or maybe not. The reason it might be a good idea is that as fluid runs down a long pipe it tends to lose pressure the further away form the source you measure. That means that, to exaggerate the situation (which I find useful when trying to understand physics problems), that, for a long runner there would be a significant pressure drop so the sides would no longer be "hugging" the flowing fluid. Tapering the runner slightly could be contributing to maintaining natural positive pressure in the system. I think Bob is giving us the boiled-down version of his understanding of runners and gates etc. and it could be that for long (greater rations than, say, length:width of 20:1) there may be a case for slight tapering.

    For sure, looking at this Lodge mold (very interesting) and looking at Bob's calculations I have been "sinning" by using runners and gates that are way larger than needed and are not properly proportioned. I intend to remedy that in the next couple days.

    Vents make so much good sense to avoid the blow-back that Bob warns about. Perhaps they are useless due to the permeability of the sand. That said, I wonder what harm they could do.

    Denis
     
  19. PatJ

    PatJ Silver

    I have had air bubbles trapped in the high points of molds, and those bubbles (perhaps 3/4" across and 1/4" deep) ruined the castings.
    Some people paint a false portrait of things like trying to prove that vents are not needed.
    It only proves that they came up with some way to avoid the problems of not having vents (by using a permeable sand), but it does not alleviate the need that many of us have for venting the high points of molds.

    Morgan makes a crucible that has a bottom-fed pouring spout, and so it makes me wonder why a mold can't be poured directly into a runner using a bottom-fed spout, and get rid of the pouring basin and all its problems.

    .
     
  20. HT1

    HT1 Gold Banner Member

    the Lodge Pans definitely break some of Bobs rules,
    1. the runner branches splitting the Flow,
    2. the extension of the sprue is not by definition a splash well because it does not enlarge, it perhaps is intended as a shock absorber to ease the metal turning
    3. I doubt the ingate is 1/2 thickness of casting

    the odd shape of the split runner is to help with the temperature gradient , warms the side and front of mold cavity farthest away from the hot metal at the gates,

    different types of sands require different venting strategies, and on that thin vents would be a great idea

    all that noted, I have not heard Bob mention cast Fe once... cast Fe acts odd amongst metals and even in AFS it was a totally separate issue . for one it is more "liquidy" then any metal , and it is non drossing, and dross is just gross Bifilm, so it may be a separate issue entirely, but a mold for cast Fe with all of Bobs ideas incorporated should cast wonderfully,.

    V/r HT1
     

Share This Page