Bob Puhakka on Bifilm theory

Its not the gating, it is the entire metal delivery system that causes problems, as follows:

1. If you don't use the correct shaped pour basin, or you pour directly down the spure, and you aspirate and entrain a lot of air into the metal stream.

2. If you have an abrupt transition from the bottom of the sprue to the runner, you get turbulence and more air entrained into the mix.

3. The mold has air in it (generally), and so the molten metal is creating the oxide film as it goes.
With excessive velocity, the metal folds back upon itself as it flows, churning lots of oxide films into itself.

4. If the velocity of the metal is unrestrained, it shoots into the mold cavity in a jet stream, which causes a lot of turbulence, and churns a lot more air and oxide films into the melt.

5. Excessive velocity also can entrain sand and slag into the metal flow, causing inclusions.

6. The gates need to be arranged and sized so they fill the mold evenly, and that may mean a tapered runner.
It may also mean a special type of trap on the end of the runner to catch inclusions, air, air bubbles, and I would assume the trap needs to be vented.

So what does olfoundryman do right in the video, according to John, Bob, and some other sources:

1. He keeps the pour spout of his crucible very close to the top of the sprue.
This is very critical in that it keeps the pour from falling too far and fracturing, which would entrain a lot of air.
As David mentions, he is actually forming an oxide sleeve between the crucible lip and the top of the sprue, and this is a good thing because it does keep the air away from the hot metal, and thus limits oxidation as you pour.

2. He does not fill the mold very quickly, and thus he is limiting his velocity, not breaking up the oxide coating on the metal stream, and not entraining air into a turbulent high-velocity flow of metal.

3. He keeps the riser hot with his exothermic material, however, this material is not necessary if the riser is sized correctly.
I think he uses it as a precaution, but not as an absolute necessity.
I have never used it, and I have had some great aluminum pours without shrinkage.

4. You can see that the metal fills his riser slowly just like what is shown in Bob's videos at his foundry.
This prevents the turbulence that churns the oxide films into the melt.

5. He fills the sprue fast enough to choke it off and minimize air aspiration, but does not pour so fast that he causes turbulence in the flowing metal.

6. He keeps the sprue full at all times so that air is not aspirated.

Whether he knows it or not, he is using many of the 10 rules for good castings.
He may be aware if the 10 rules.


.

 

On February 2nd, 2019, Gippeto started this thread, and added a video by some crazy guy named Bob Puhakka.
If you have not seen the video, go back to the very first post and watch that video in its entirety.
It is well worth the time.

So I had never heard of Bob, and initially I thought this is just some wacky dude with a lot of tats some bizarre casting ideas.

Then I watched some videos posted on swdweeb's video channel, also of Bob explaining his casting methods to the backyard guys.

Again I thought this Bob guy is wacky, but I started to pay attention to what he was talking about and it started making a lot of sense.

Then I found out that John Campbell sort of schooled Bob Puhakka about how to cast metal, and indeed in John's book at the beginning, he credits the genius of Bob with applying his methods and making castings that are basically aerospace grade or better, but using bound sand techniques and pouring the mold with a crucible.

Then I saw the video of Bob in his factory making tons of perfect castings.
I sent Bob an email, and basically said "Holy shit Bob, I had no idea".
And Bob sent back those x-ray photos, and said "Yeah, and most of the casting world has no idea either", and I fully believe him.

But basically John Campbell developed the Cosworth casting method for the Ford racing engines, and Ford went on to win most of the big auto races for the next 20 years.
Ford and GM adopted the Cosworth method for their engine block castings, and still use it to this day.

So I am guessing that Bob uses the roots of the Cosworth method, plus a lot of his own inovative ideas.

Bob does make some videos that many would consider a bit out of the mainstream, but the more I listen to him, the more I find that I have in common with him on so many levels.
I am a professional engineer, and so I can't use the language that he uses.
When I started my first engineering job, I had actually worked in some very rough places prior, such as factories, gas stations, and with some very rough (but often good) people.
And I remember distinctly one of my early conversations with an extremely conservative (highly highly religious, unbeknowst to me) person, and I let loose a stream of profanity that would make a prostitute blush seven shades of red. It was as if I had hit the man hard in the side of the head with a 4x4 timber.
He stumbled backwards into the bookcase, with a look of horror on his face as if he had seen the antichrist, and said "I had no idea.............", (ie: I had no idea you were a totally uncouth idot).
He staggered off without saying anything else, and I don't recall him ever making the mistake of conversing with me again while I worked at that firm.
So I had to clean up my act in order to fit in (when in Rome, speak as the Romans speak).

I think you are missing the point entirely.
It is really a joy to have a casting turn out well.
It is a bigger joy to contemplate how to layout out a sprue/runner/gate/riser system correctly, and then often see that system work well.

If you make bound molds with non-recyclable commercial sand, it is very time consuming and somewhat expensive to have a mold failure from a partial fill, inclusions, bubbles, gas, etc.
With bound sand molds, you really want to do it right the first time, every time.
I use bound sand because the parts are very close to their final size, the casting quality is often excellent, and I don't have problems with parts of the sand breaking off or falling out of the flask.

But I know two backyard guys who do a lot of iron, and I know about the problems they have had with inclusions, gas, shrinkage, etc., and the problems can be extremely difficult to rectify.
One guy runs his own iron foundry business, and so it is a matter of survival of the business that he not have such problems with iron.
You can bet his is actively working on ways to improve his casting yield, because yield is money, and poor yield is little or no money.

For me, it is about predictability.
I want to be able to lay out a feeder system and have a 95% chance of it working right the first time.
I also want consistency, and if I ever decide one day to make 1000 of the same part(s), I want every part to be consistently and reliably high quality.

I mean who does not want consistency and high quality castings?
I know there are those (I won't name any names) who really don't care much about anything as far as foundry work, and they just cast metal for jollies, and sometimes just to make videos that draw a lot of attention, but they never really make anything of significance. They may have mountains of castings, of random stuff and shapes, but it is really just a big ingot farm in many cases.
I guess some people have wine cellars, and some people have shelves and shelves full of ingots and trinket castings.

Its all good of course, but always remember that what is good for thee may not be good for me.

.

 

Gating does not come into play other than it is just one part of many pieces.
What is important is the entire metal delivery system, including the pour basin, the sprue, the runner(s), riser(s), catch basin(s), filters (if used), and other aspects of the delivery system that are critical, and they all have to interact correctly, or should interact correctly for optimum castings and maximum consistency and quality.

We have several examples of people here who ignore much of what John Campbell preaches and have some very good castings.
Some of that has to do with how thin many of our castings are, and how quickly they solidify, and also a bit of blind luck.

I am not convinced that Bob uses inert gas, but it is possible because I suspect that inert gas has been used in NASA-like casting settings since the 60's or before.
If Bob were using an inert gas, it would have to be heavier than air, else it would just rise up out of the mold.
But I don't think inert gas is required to get good castings, and I have never used it and have gotten some excellent castings.

I have seen people use vacuum-assist on a mold, but never on a regular basis.
Commercial foundries have all sorts of special ways to introduce metal into the mold, from the bottom and all sorts of strange ways, but they all follow John's 10 rules, with the big one being don't create a lot of turbulence.

.

 

Thanks for the info.
They may be universal, but not in my head; I am clueless.

.

 

My understanding is the gvcf is multiplied by the area of the runner to give you the area of the gate/mold interface. So, the gvcf can be altered only by varying the sprue height.

 

Thanks, I'll do better next time.

 

This is a pretty cool experiment (post #202).
I think I understand the stepped runner, but why two different gate sizes?

.

 

My understanding is still very general.
I get the basic concepts about velocity, turbulence, etc., but digging into exact formulas is going to take me a while to hash out, and unfortunately I am a very slow learner.
Hopefully between what you post and what I can dig out of John's book I can get some working material.

Edit:
This a a very long-winded thread, but I think it is very valuable material, and indeed I feel like I have learned more about metal casting in the last month from John and Bob than I have learned in the previous seven years studying the subject.
Its like being in a dark room, and somebody comes in and turns on the light (Bob Puhakka) and says "Hey, what are you doing stumbling around in the dark".

.

 

I've been hacking my way through thick cobwebs and rusty fragments in my mind. Every once in a while I find an active brain cell.

 

Maybe you can form a sprue top indentation in the foil thus giving you an accurate target when placing the basin.

 

I have watched this video previously, but did not pick up on all the details of his pouring basin, sprue, filter, runners, risers, etc.

He does pay close attention to metal velocity and keeping the sprue full (although he had trouble keeping the sprue full on I think his first two pours).

His tapered sprue is also interesting and quite advanced I think (not that I would know), and he uses a 1:4 ratio for the sprue bottom/runner area relationship.

I don't think he needed the gigantic gates on the side, but perhaps that was to allow the risers to work and eliminate shrinkage.

Its interesting that he recommends pouring as slowly as possible.

 

And this photo shows his sectioned riser, and it looks like a bunch of unfurled bifilms with expanded air bubbles, but that is just my uneducated guess.

It does seem to look a lot like the photos in John Campbell's book, and also is exactly as Bob Puhakka describes in his video.

If you asked most people, they would probably say "That is just shrinkage", but I suspect it is bifilms opening up as the metal solidifies and the trapped air expands.
According to Bob, you can have shrinkage and the part may get a little smaller, but without bifilms in the melt, you will not see the metal opening up like you see in the photo below.

It seems like I read that a filter can churn bifilms into the melt, and he uses a round ceramic filter as a choke.

 

OK, that makes perfect sense, to expand the runner into the gate to slow the flow. However I have not noticed that, I'll have to look closer. What you're saying is that 52 in/sec may be acceptable in the sprue/runner?

 

What I'm saying is 52 in/s is the approximate velocity you will find in a system with a sprue height of 3.5 inches. The example Bob spoke of in his videos assumed a sprue height of 12 inches which results in a velocity of 96 in/s.

 

This is a pretty cool experiment (post #202).
I think I understand the stepped runner, but why two different gate sizes?

I had my head down, or up it as it were, when I constructed the runner and gates based on the special Pecan square sprue I had just made and did not notice that mistake until I posted the answer to one of the questions. IMHO though, there was still enough choke in the runner to create the oxide sleeve that the rest of the metal ran through without forming additional oxide which could have created a bifilm fold in the casting.
Do brain farts add to global warming, or is it the other way around? Show your work.
I'm having fun here and appreciate all the input.
On a more serious note, Bob notes that a piece of pure zinc will shade blue in when held in the furnace vent if the flame is oxidizing. Does anyone have any experience with this?

Good idea on dimpling the foil. I also thought I could just mark around the basin sans foil to align the sprue, and then place the basin back in that spot.

 

I thought any step like the one in your runner reduction would break up the film through eddys and contribute to the issue. Comment?

I'm thinking a deep dimple would be easy to see when setting the basin, assuming accuracy counts, that should be more accurate.

I'm with you now. And those velocities assume no friction in the sprue/runner system.