Bob Puhakka on Bifilm theory

Could the dogleg in the horizontal runners be intended to adjust/reduce the velocity?
That is the only logical explanation I can see.
Otherwise why not just make it straight across?
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Here's a pattern for a horizontal pour of some smaller skillets, a different manufacturer. The little handles are for lids.

 

That is an interesting layout.
Horizontal runners, no sprue basin, multiple ingates per pan, no runner dead ends.
Very typical of many of the older runner/gate layouts I have seen.

 

as I mentioned above, they are keeping the Sprue and runner an equal distance away from the casting so the mold wall will be equally hot at a distance away from the ingates ... the techniques has some term I forget. it's very valuable in thin castings ... it's sort of the reverse of using chill sand .

V/r HT1


P.S. the runner moving up and down like that would cause turbulance, just before the Gate, very bad, but would catch any dross, or loose sand up to that point, probably a questionable trade off

 

ok Folks,, just a reminder CAST IRON gates differently then any metal , because it acts differently. be very careful when examining cast iron gating systems, they will confuse you by breaking rules , while you will miss critical elements important to getting the correct mechanical propertys into the cast Fe

Pattern design in Cast Fe is MUCH more critical then other allows and will cause the average hobbiest a lot of trouble, cast Fe really needs to be of a uniform thickness, or changes must be handled very carefully or you will have cracks tears etc.

Cast iron is a separate subject ... which I would love to See Bob discuss some, even though I avoid it , but I suspect Bob will shy away from it because it is an in-depth subject unto itself... (time Sink)

V/r HT1

 

I was thinking as a practical matter that they put the dogleg in the horizontal runners just as a method to avoid the handle that protrudes out, but then they could have just made the vertical sprue longer to do that, so I have to believe you are correct in that they did it for a very specific reason, as you say.
I am pretty sure nobody would randomly build in a dog leg without a good reason to do so, and the heating effect makes sense.

In many of the photos I have for matchplates for iron castings, the runners and gates seem to be rather haphazard and somewhat non-uniform, which makes me think you can get away with a lot when doing iron work.
The above match plates were used to make thousands of engines.

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I played with a vertical parting several years ago before I added oil fired capability to my furnace . At the time I wanted to cast these miniature lid lifters. There are a number of problems with this layout based on what I have gleaned from Bob's videos and Campbell's book.
To use petrobond, one end of the flask is attached and the mold is rammed conventionally. Then for this test I sandwiched the cope and drag between bottom boards, stood the assembly upright and removed the end plates to expose the pouring basin. Looking at these shots seems to show turbulence in the sprue and probably a lot of bi-film with the poor shape of the basin. With the oil fired furnace I can pour these in CI conventionally so there has been no need for this method. As an aside I really like the small flasks for my miniature parts for speed of molding, and the reduced sand requirement.
If I were to do this again, a square pouring basin that could be formed in one half, a square tapered sprue with or without draft and bottom feed with a vent at the top of the pattern. With draft the sprue could be drawn before the end plates were removed, otherwise it would require replacing the plates and then cutting the gates. Jammers photos show draft on the gates and runners so the match plate can be lifted from the drag/cope.

Pat, the bottom pour would not help as far as bi-film is concerned because the point of the basin is to give the oxides a chance to consolidate before reaching the sprue. For those of us pouring cast iron the bottom pour helps by eliminating the little bits of slag we cannot remove from the surface of our melts by drawing from the clean subsurface. I always liked the idea, but never sprung for one.

 

Here's an experiment. I made a sprue with a 8 deg. inclusive angle; it's conical. Underneath the 1/2" sprue is a well.
The mold is a 6" OD ring with 4" ID. I'm using an 8" square flask.


The drag looks like this.


All clamped together with the obsolete pouring basin.

Results: the bottom well shows a bifold layer or two

Pretty nasty looking where the sprue meets the horizontal disk.


The casting looks okay to me. I was expecting defects in the lower portion and perfection in the top portion.
Lathing and closer inspection to follow.

 

Apologies if it was me that gave you the bum steer...I used the wrong height to calculate that 8degree included angle. HT got IIRC 1.5 degrees

Pretty hefty looking wheel.

Al

 

No apologies required. I was thinking that water flows with no turbulence at a 4-5 deg. angle. So, I thought I would start with that angle. I'm seeing how to go about calculating the proper angle using the sesille drop theory but, well, I just had to pour some metal. My main question is can you tell the difference? How do we measure defects?

btw - the total weight of casting plus sprue, etc. is 4.75 lbs. This casting will eventually be the thing I need to drill holes in long things.

 

Here is Bob Puhakka's foundry.
What a great place for making castings.
This guy is the godfather of good castings.
I guess John Campbell is really the godfather of good castings, but this is his adopted son so to speak.

 

According to Bob's videos, this is what I understand to date (which is very incomplete, but the image is slowly forming):

1. Pour as fast as possible, but don't exceed the critical meniscus velocity.

2. The pouring basin must have a specific shape in order to prevent air aspiration, and the basin must remain full when you are pouring.
The basin works best with a stopper that allows the basin to fill before the metal flows down the sprue, and the metal level in the basin must not fall too much when the stopper is pulled, else air will be aspirated.

3. You should not use a basin at the base of the sprue.
The bottom of the sprue appears to be basically the same area as the runner, and if the runner branches, then the total area of the branches needs to match the area of the bottom of the sprue.

4. John Campbell's 10 rules for good castings must be adhered to.

5. The metal should enter the bottom of the casting through one or more gates.

6. There is some high tech gate designs that are a bit complex to discuss here.

7. You may need a filter somewhere in the system, or else design for a system that traps trash.

8. Some companies use a square tapered sprue just because that is easier to manufacture, and can be made without a lathe with taper attachment.

There is a lot too it, but I absolutely believe John and Bob's methods can be applied to backyard castings, and I think the basic calcs can be done with a spreadsheet.

Bob also emphasizes that you can't just adopt some of the system, it has be adopted from beginning to end, else it will not work.

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I interpreted it much the same Pat, but not entirely so. My take on it, if you see something you disagree with let's discuss it.

1) You need to determine how fast YOU can pour without causing problems...literally pour out a crucible and time it. This flow rate will be used to calculate the rest of the system. In the case of swdweebs and Martins tapered sprues, they're in the 5cuin/second range.(calculated by working their sprue sizes backwards)

2) Where Q is the flow rate, the pour basin should be a minimum of 1Q, John Campbell suggests 2Q for critical castings, the fellow who wrote one of the articles you linked to (Wade Marquardt) likes 3Q...so pick yer poison but it will effect total sprue height so pick a basin first. One thing Martin and Perry have working for them is a "standard" size basin and sprue...makes sense and for most guys a single "standard" will cover most of what they will ever do. With that in mind...not sure a spread sheet is really necessary, just work it out once and be done with it.

3) Bottom of the sprue is calculated from the flow rate and total metal height (fill level to sprue bottom). Top of the sprue from flow rate and distance from the fill level of the pouring basin to the start of the sprue.

4) Curved portion and the runner have the same cross sectional area as the bottom of the sprue. I haven't read it anywhere, but Wade does touch on sessile drop height, and it makes sense to keep the height of the curved portion and runner less than the drop height...also one reason why rectangular runners make sense to me, doubtful most home gamers need to worry as we're talking about .412" for aluminum and the runner will likely not be that high in the first place.

5) Total gate area should be sized according to the velocity of the metal at the bottom of the sprue with the goal of reducing metal velocity into the mold to or below sessile drop height velocity. Gating is ideally from the bottom. Multiple gates would have equal area.

6) From some comments Bob made re step gates (video since taken down, but mentioned/pictured in Wades article)...cross sectional area of the step is equal to the runner cross sectional area. Surge chamber is 1Q with equal diameter and height. Bob said the slot gate was sized to the casting, but I did not understand and did not follow up. Mea Culpa. It makes sense to me that once the surge chamber is full, the metal velocity into the casting is going to be the same as the runner velocity (ie too high) unless we size the slot gate with an eye to reducing that velocity to sessile drop height velocity.

7) I did note in Wades article a picture where a stepped surge chamber was on the end of the runner and the gate in the middle. I rather liked that idea and did some further hunting. Most any air (bifilms, bubbles etc) entrained while the system (sprue) is priming should end up in that surge chamber...and not as part of the casting. Seems such a simple way to improve overall casting quality. I did find an animation where the surge chamber was at the end...although not a stepped one. Fills rather nicely I thought...looks like Bob agreed back then too. (see comments)



John's "Ten rules" certainly give some good guidance as for direction.

I believe that working towards a "better" process is always going to be the right direction. Pretty much all of this should be possible for the home gamer, but it is going to take a little more work than hammering a large sprue and riser into the mold.

I can't say that I agree where it's all or nothing though...applying even the pouring basin and sprue have made improvements for some folks, and that's a REAL easy change to make. The hardest part I've seen is going to be the curved transition from sprue to runner, and I may be over thinking that. Putting the pattern in the cope (or cheek piece and cope) along with a surge chamber should be reasonably straight forward....no I've never used a cheek piece...FNG here and I know I've a lot to learn lol.

Interesting times around the corner here...

Al

 

I am still highly confused about it all, and will need to study it for a while and then test some configurations to at least try to prove that they work.
One of the problems is that sometimes people who ignore all of these items have perfect castings, and so what do you compare it with.

Luckily I know someone who is doing a lot of iron, and in various shapes and sizes, and has had some problems in the past, and so perhaps he can try some of these things since it would really make a big difference to him in getting solid and consistent castings.

From the Sorelmetal paper I was using to set up a spreadsheet, I was under the assumption that the gates (in one scenario) were used to control the metal flow.
In the other scenario, they mention using a choke point in the runner to control flow.

It still seems reasonable to me to use the total gate area to control flow, since if the gates are spilling out metal into the mold cavity faster than the runner can provide metal, then you will perhaps get voids in the metal?

And Bob says don't use a basin at the end of a runner, but others do show one there.

I think what has to happen is that the types of systems used in large castings need to be looked at, and then a system that can be scaled down and used on a small scale has to be determined, and that may or may not match the large scale systems.
If you don't intend to use a filter like Bob does, then that could change everything.

I have much reading to do.
I don't know enough to really get into much of a discussion yet, but some things are starting to emerge as Al points out above.

I do use bound sand like Bob does, and so I can make any sort of shape anywhere in the mold relatively easily, and it will be a self-supporting shape.

Since I have a 3D printer, I will print out the runners, gates, pour basin, sprue, etc., if I can figure out the size and configuration.
Most of my castings seem to fall in the 10-20 lb range, so I am hoping I can make some standard shapes that will work across that range.

I have some 3" round ceramic filters, and have used them a couple of times, but they impeded the iron flow too much and caused partial mold fills. I probably did not have the melt hot enough when I used the filters, and I am guessing they may work with hot iron and a tall enough sprue.

A quick scan of John Campbell's book shows some very interesting stuff for sure, but it is 1028 pages of stuff, so this may take a while.

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Not so sure that those people have "perfect" castings Pat. Just my thoughts on it, but for the most part all a hobbiest is looking for out of a casting is a shape. Wouldn't think most folks do any testing on a part aside from a little machining. If there's no obvious surface defects or holes in a machined surface, a fellow in his garage would likely be quite satisfied by a part that might fail xray miserably.

As for the sprue/pouring basin, Perry did give me a link to some STL files for the sprue he's using that you might find useful for a quick test...it's in the comments. In the link to Martins video on page 1 of this thread, he mentions in the comments that he has STL files available for the basin he uses.



Al

 

In another thread PatJ said: "I am going to have to relearn basins, sprues, runners, gating and risering completely and start from scratch, but at least I feel like I understand the bifilm theory, and also understand the 10 rules by John Campbell."

I am right in there with PatJ and I think a lot of other folks on this. I imagine we are going to be seeing some "interesting" results as our accustomed patterns are disrupted (I think in a good way) and many of us try to institute these ideas to the extent practical. It will take a while for us to sort through what works, has to be modified and is impractical for our use. In the mean time I expect some chaos, at least at my foundry.

One of the ideas I would like to incorporate would be the sprue/basin plug. As many of you know, I work solo. So I need to come up with a triggering technique that will
1) provide a brief---maybe a second or so---delay from the time the metal reaches 3/4 full in the pour basin
2) not introduce undesirable metal debris like lead, aluminum (less bad maybe), copper etc. into the metal stream
3) be simple and reliable
4) not require of me additional physically coordinated steps like stepping on a pedal at the right moment. I have my hands full just accurately and smoothly poring into the basin with my trolley as it is.
5)not be too abrupt in withdrawal so as to avoid splashing
6)not dislodge sand into the metal stream. I will need to be careful to pack the basin good and tight. And I may spray in a little sodium silicate to harden the green sand basin where the plug will sit.

My current best thought is to use a sodium silicate-bound plug triggered by a thin (thinner than typical tie-wire) monofilament steel wire. The lifting force to be supplied by a simple weight on an arm with the lifted arm maybe 1/3rd the length of the counterweight arm. Gravity should provide a (reliable ;-) ) and smooth lifting force and even if a tiny bit of fine steel wire enters the stream it should dissolve quickly.

I will use a "needle" to push the wire through a small hole in the side of the flask, angling upward through the basin and then on toward the back of the flask where it will be connected to a trigger to cause the counter-weighted arm to fall when the wire goes slack. Through experience I will learn what gauge of wire provides about the right delay----or not!

I am throwing this out for comment.

I'd love to kick this around a bit as some really good ideas often surface in such discussions.

Denis

 

What you'd be gaining is faster priming of the system by vastly increasing the complexity. That initial slug of metal/air/oxides might more easily be sent on down the runner to a surge chamber and trapped there.

If you didn't notice...I really like the idea of that surge chamber lol.

Also layed awake for a good part of the night thinking about the transition from sprue to runner. It's the inside radius that develops a low pressure area under flow that aspirates air, so if we form that radius manually and leave the "back" side at the sprue taper, the back side will become a high pressure area under flow, while the inside radius won't aspirate. Remaining issue is maintaining cross sectional area...a thought in progress.

There will be turbulence from the high pressure area which will cause mixing, but once primed it would only be mixing clean metal with clean metal...perhaps a non issue? While not the "ideal" of the completely radiused transition, certainly easier to fashion and employ.

Al

 

Al,

You may have an inflated view of my pouring skills. ;-) What I don't like is the sort of sputtering start of a pour that seems to occur on my castings especially with the first mold of a given heat as the crucible is 75 to 80% full and tipping out the first bit of stream and having it to go just where I want is somewhat problematic. Once the stream is established and (hopefully) slight aiming errors are corrected, a good steady stream is established and the pour usually goes quite smoothly. But those first few seconds of somewhat inconsistent stream I don't like. The plug would allow for some initial errors and would release as the smoother part of the pour is in progress, by my way of thinking.

As a refinement of the thin wire idea, I think passing a loop of very thin wire from the side of the mold into the basin and then out again while snaring a heavier wire loped on its end might make concerns about tramp steel getting into the stream a bit less likely. That way only a short segment of thin wire (maybe 1/4") would be exposed to the basin. That thin wire would fail, releasing the heavier wire which would not yet have had time to melt.

I appreciate your thoughts. It might be that this solo plug attempt won't be worth it. But I am enthused about giving it a try.


Denis

 

As I've been digesting all this bifilm theory stuff, the thing that is bugging me the most is the lack of analysis of pressures. There is a lot of concern about the vortex in the sprue sucking in air. This can happen only if the pressure at the top of the sprue is less than atmospheric which means the pressure at the bottom of the sprue is less than the static pressure of the height of the melt in the sprue.
I encountered this problem on project where our circulating pump kept getting air-bound. We couldn't figure out where the air was coming from. It turned out to be the pump inlet pressure was lower than what the static pressure should have been and subsequently the pressure at the high point was a vacuum.
I think we should also look at the pressures generated from air expansion due to temperature changes. Bob briefly mentioned this variable in his recent gating video but did not elaborate.

The surge chamber idea looks attractive to me, also.

 

Bob Puhakka is actually making perfect castings.
He had a video which he made an aluminum casting and sent it to Loyd's to have it certified.
The thing is stronger than high strength steel, and has twice the ductility of other people's aluminum castings, and xray's show it free from defects.
This is not a one-off casting or a fluke; this is Bob's typical production run.

I started reading John Campbell's casting handbook last night, and right there on the first page is the dedication to Bob Puhakka, as follows:
"To Robert Puhakka, For his dedication to the 10 rules and the living proof that they work".

I knew these guys were close, but I did not realize just how closely they had worked.
Bob Puhakka mentions in his factory tour that he produces "defect-free" castings as a matter of routine, and John Campbell also mentions this in his book.

Some hobbyists are looking for a shape, but some are looking for much more, such as a strong casting with tight tolerances, and without defects, that will hold up in an engine under load without breaking.
And I know a guy who does production runs of iron, and while he moved from hobbyist to actually running a commercial foundry, the money he makes depends totally on his rejection rate, and right now his rejection rate is unacceptably high.
My thoughts are that if there are relatively simple methods that can be adapted in order to make high quality castings, then why not use them?
Yes, it will take doing some homework, and some experimentation, but that is what many/most hobby guys do already and have always had to do.

I have some 2" diameter round graphite, and I am going to sand it into a pyramid shape, drill a hole through the center, and use that for a plug.
The pour basin will be bound sand, and that could either be sodium silicate or resin bound.

An alternative to the pour basin and plug arrangement is what John Campbell mentions in his book, which is a quiescent pour, where the sprue is eliminated, and the lip of the crucible is put in contact with the opening in the cope, and there is no drop down a sprue.
This assumes you can fill a mold without using the height of a sprue, but I think ironsides uses this method with great success.

Bob recommends not using a surge basin due to air expansion and forcing air back into the mold cavity, but with a vented surge chamber that is in a tangential configuration like Wade uses, I think it would work fine, and probably eliminate the need for the filter that Bob uses.

I am not sure that the radius is that critical, rather it is probably more important that there be some sort of radius at the sprue/runner transition, in lieu of just a basin at the bottom of the sprue which splashes metal everywhere.

Some of this is just going to require some testing.
As stated before though, if you have an old-style system, and it works well, then why bother with this new stuff.
But resin bound molds are not necessarily cheap or easy to make, and so I am very interested in the first time every time good casting, and I have been getting that with aluminum and resin bound molds.
I just need to get that set up with iron.
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