Casting a flywheel in ZL12 alloy.

[A7er]

I have been using aluminium from a Renault car wheel for my recent casting attempts. Today I used an aluminium/zinc alloy called ZL12, which is 88% zinc and 12% aluminium. The flywheel cast from the car wheel weighs 169g, from ZL12 it weighs 385g. The first picture shows a slight depression in the rim of the flywheel. The shrinkage is right above the ingate. I am assuming that the metal wasn't hot enough and it "froze". The dip is less than 1mm (it looks deeper because of the rim sticking out), so it would machine out. I had allowed for the removal of 2mm from the outer faces. The second picture shows the small amount of flashing on the casting. It seems I can either get a casting with virtually no flashing, or with a good surface finish, but not both! The flywheel is approx 90mm in dia by 18mm wide.

[Jasonb]

Certainly a nice surface finish.

The small amount of flash looks to be where sand has come away when removing the pattern rather than poor contact between the two halves

That's a good weight increase using the heavier alloy and only about 50g less than if it were iron

[A7er]

I think you are right Jason. I am still removing my patterns by inverting the flask and tapping it until the pattern falls out. I need to find a better way of extracting the pattern.
Lee

Edit. I am 100% convinced that the flashing is down to the way I have been removing the pattern. I now have something positive to work on!

[Jasonb]

A common way to do it is to lightly tap the alignment studs which just loosen the pattern  and for the other half that has the mating holes use a couple of tapped ones so you can put a screw in and give then a gentle sideways tap just to loosen the pattern and then lift out by the same screws.  You might just be able to leave small holes in the prints and use self tapping screws

[A7er]

Hi Jason,
I drilled two 6mm holes in the back of the hub of the pattern that has the locating stud holes, the other pattern has the locating pins. I filled the holes with car body filler and will drill two holes for small screws. I can then attach a small buldog clip to the screws, and then with the aid of a vertical puller, which I have yet to make a-la Olfoundryman, extract the pattern. I will use the same procedure on the pattern with a central locating pin. The bulldog clip is 20mm wide and will sit on the pattern without rocking. Well that's the plan anyway!
Lee

Edit. The other alternative is a match plate. I am watching videos to get some ideas.

[Alyn Foundry]

You will definitely get better results from any pattern if they are accurately secured to a plate. My masters were done using 10 mm thickness Aluminium plates with all the runners and feeders built in. They also had “ slip blanks “ the term used for a cone and cup system that ensured mould halves were accurately aligned once the plate was removed.

The shrinkage probably occurred due to insufficient “ head “ of metal. Try increasing the height of the sprue. The sprue will always have the hottest metal after the pour so the casting will try to feed the cooling sprue.

Picture of my Hornsby Akryod flywheel pattern.

   Graham.

[A7er]

Graham.
Did you forget to post the picture or can I find it elsewhere?
Lee

[Alyn Foundry]

Graham.
Did you forget to post the picture or can I find it elsewhere?
Lee

Hi Lee.
My apologies, the photo was stored upon another device.

   Graham.

[A7er]

Thanks Graham.
I looked up the price of aluminium plate at 8mm to 10mm thick. The price was far too high for me to contemplate, so I made mine out of what I had available, 8mm plywood. The first casting I did would have been a lot better if I had reversed the order of taking the flasks off the matchplate. I took the one off that was uppermost at the time, it was the flask with the locating dowels in. This left the other flask with nothing to guide it as the matchplate was removed. The casting was still good and at least proved that I had the alignment right. The second casting I turned the flasks over and removed the one with the alignment holes in first. This gave me a much better casting. Now I know it works I will seal the ply with a coat of paint. I think the bare plywood might be sapping a bit of moisture out of the sand in contact with it causing a small area of flashing.
I am looking forward to the next few tries.

[bent]

A7er,

I've looked at zinc alloy casting, but thought it would be too difficult due to the high vapor pressure of the molten metal?  Are you just using green sand molds for the flywheels?

 

[A7er]

Bent.
I am unaware of high vapour pressure with the zl12. I have used it for several different castings in the past, including a small oval nameplate where I wanted good definition of the lettering. I use a home made electric foundry of about 1800 watts on 240-ish volts. I melt the metal at 500c, a bit higher than I would like but if I only use a temp of 450c it takes forever to melt the metal. I use very fine green sand for all my casting, so far. I would like to try fine sand mixed with epoxy resin, I just haven't got round to it yet. The picture is of the nameplate. The lettering is about 6 to 8mm in height (I forget which) and about 1 to 2mm extruded from the surface.

[bent]

Thanks A7er!  I know nothing about zinc casting other than what I've read, so it's good to hear from somebody doing it in the real world.  Will be paying more attention to these posts now that I'm in the 3d printing business.

[A7er]

Bent.
The youtuber "Myfordboy" casts many parts in zl12, especially flywheels. I can't point you to the relevant vids, sorry.
I think the lower casting temp, 450 to 500c is kinder to my little foundry. My next casting will be a conrod in zl12 using a matchplate mould. I am getting to like matchplates! The conrod patterns have a 5 degree draft whereas the previous patterns only had 3 degrees. I hope this makes it easier to seperate pattern and sand mould.
Lee

[Alyn Foundry]

I have been using Zinc alloy for many years now. Mainly for small parts. It is quite dense and really works well as a substitute for Bronze or Brass bearings. I use Steel dies which are heated up during the melting process. The metal shrinkage aids the removal from the die, once cooled. Here’s a short video of casting a Gardner main bearing.

<a href="https://www.youtube.com/watch?v=tXp8hEamWtY" target="_blank">http://www.youtube.com/watch?v=tXp8hEamWtY</a>

Hopefully the attached pictures are self explanatory?

   Graham.

[A7er]

Nicely done Graham.
I had hoped to be casting today or tomorrow, but other things have got in the way. I am now gaining a bit more experience with casting using a match plate and am slowly working towards casting the cylinder head in zl12. The head is 56mm dia with inlet and exhaust tracts, plus sparkplug and a water jacket. I am now looking forward to it.
Did you have to upload your video to youtube before inserting it here? I had thought about posting a video of the conrod casting as well as a few images.
Lee

[Alyn Foundry]

Hi Lee.

Yes I did have to upload to YouTube. I’ll be interested to see how you intend to core out the passageways for your cylinder head. This is something that hasn’t really worked out that well on small scale engines for me. We did have some success when my old foundry was working. They had a special Zircon sand blend that was resin bonded, very low fume. I used it for the Hornsby Akryod Oil engine hot bulb.

   Graham.

[A7er]

Graham.
I have made a mould to produce the core. It is small! It's all a bit of an experiment to see what I can do. The pictures show a water manifold I made some time ago. I cast it in zl12. the water passage is about 10mm dia but it gets very thin towards the left end, no more than 3mm thick, I had to use a piece of twisted steel wire (still less than 1mm in dia) to reinforce the sodium silicate/sand core. The core stood up to the casting even when used with ordinary aluminium poured at 730c. I could also try to produce the core with an epoxy resin/sand mix. That might be a bit stronger than SS/sand. The part was for my half size 1930 Austin Seven engine, which I have now decided should be one third size instead to make it easier to produce the patterns. The work I am doing on the castings should help with the A7 engine. Once I am happy with my casting abilities, I can think about buying a lathe and mill and start learning how to use them. It could take a while!
Lee
Thanks for the info about uploading a video.
Edit. I am also including two more pictures of a ss/sand core for a 30mm dia piston. You can just make out a piece of wire reinforcement in one of the arms which are 8mm dia. The arms make the gudgeon pin holes in the piston to be machined to size later. I have cast a few pistons this way just to test it works. One of the arms looks wonky, it isn't!
A 20p coin is included for scale.

[bent]

Thanks Graham and A7r - I have watched several of Myfordboy's videos, and had not seen him do zinc castings, will go look for them when I get the chance.  I know he had some good videos on preparing your own green sand mix that I'd rewatch when I'm ready, and his demonstrations of the entire process (from patternmaking to molding to casting to fettling and machining) are very good.

[Alyn Foundry]

Good morning Lee.
Nearly 60 years ago a highly progressive school teacher brought in a 1932 Austin 7 open top tourer for us juniors to fix. This was my induction to the world of mechanical restoration. Needless to say over the spring and summer terms we got to drive it on the school playing field. It was such an interesting project that I can still remember us removing the cylinder head, rather brutally, with two pieces of rebar poked through the plug holes!

In later years my good friend Vincent restored a Riley RME for which we made new pistons for using cast Iron dies and loose pieces. I think the die now resides at the Anson engine museum. We made the sprue as an extension of the piston crown to ensure good strength around it and the gudgeon pin bosses.

I wish I could step back in time a little, I would have bought a bag of the sand I used for the Ackroyd project. The resin was activated by heat, the core boxes were made from Aluminium and you just poured it in with a little jug.

   Graham.

[Jasonb]

Bent, this one is with the zinc alloy, I put someone in contact with him that was after a 5 spoke flywheel and he did the pattern & cast it for them

<a href="https://www.youtube.com/watch?v=uohxoOeJpw8" target="_blank">http://www.youtube.com/watch?v=uohxoOeJpw8</a>

[A7er]

Hi Graham.
I have three Austin Sevens. My car that I bought in 1975 and two others left to me by a late friend. I also have a spare engine that I measure to produce the 3D drawings and prints for the model engine I will build.
When I started to get into metal casting I went down the same route as many others did. Sand from B&Q mixed with ground up cat litter for the bentonite. I also used aluminium cans and extrusions for melting and a stainless steel pot as a crucible. I discovered the error of my ways thanks to a local engineer who invited me to watch a pour of 25kg of molten iron. I had proper sand, bentonite and aluminium from him. What a difference! As for core sand, I have a pottery supplier not too far away that I bought some very fine sand from, I think it was over £7.00 per kilo. It didn't last long, he also sold bentonite, again, very expensive. I still use paving sand for core moulds but I filter it through a fine mesh tea strainer, my cores are very small. If I want it a bit finer I will grind the sand with a pestle and mortar. I did wonder about using investment casting medium to make cores.

[bent]

Thanks Jasonb, I just watched that one yesterday during a spare moment.

I'd been taught, or had read somewhere, that you have to pressure-cast zinc, because it has a low boiling point and will tend to develop gas porosity if gravity cast.  But, watching Myfordboy's videos again, I see that he uses tall feeds and risers to help keep the head up on his molds, and from his videos it's pretty clear that any porosity he suffers is minimal.  I do see that he also starts using clamped molds in later videos, to keep the flash and occasional blowouts to a minimum.  Good stuff.

[A7er]

The height above the casting and my pouring basin is barely 25mm, one inch, at the most. I do sometimes wonder if perhaps a bit more height would help. My sprue follows the Ol'foundryman design-ish. 7mm dia at the bottom with a one and a half degree draft to make it a bit wider at the top. Ol'foundryman videos are well worth watching. His pouring basin design is supposed to eliminate air getting into the pour, and his sand and ramming are both top-notch.

[bent]

A7er,

Hmm, interesting.  Watching Myfordboy again, and he seems to use the same high risers on both Al and Zn castings - maybe 6 to 8" or more of height?

- Ben

[A7er]

Ben.
I did use a similar system as Myfordboy when I cast a small aluminium picture frame. It had such a narrow section that the mould wouldn't fill before the metal "froze". The aluminium I was using came from broken engine parts that were probably die cast 90 years ago, not the best metal to use. Myfordboys method got more metal into the mould quicker and it filled. I prefer to use a pouring basin like Ol'foundryman and Swdweeb and others use. Try both methods and see which you prefer.
The picture is of a small, 90mm dia, flywheel I cast recently. It shows the pouring basin and sprue still attached, and the feeder in the middle of the flywheel.
Lee

[bent]

Interesting, will look up those videos too, once I'm ready to become a founder(?).

[A7er]

Ben.
As I understand it, all flasks should be clamped in some way because the molten metal will try to "float" the top flask away from the bottom one. I always clamp my flasks. In the beginning of my casting journey I also weighed the top flask down as well as use clamps. I don't use weights now. I watched a Myfordboy video when he forgot to clamp and had spillage of molten metal. I had that happen once, so I sit my flask in a large steel tray just in case.
Lee

[bent]

Yup, ok.  After watching quite a few of olfoundryman's videos, I am a convert.  The surface finishes on his parts are...well, amazing (compared to the run of the mill types on YouTube).  He also has a video where he critiques a lot of other Youtubers, from alloy choices to ramming techniques to...well, everything.  And gives good, cogent, scientific explanations on his whys and wherefores, with references to literature as well.  Low pouring basins with smooth transitions to narrow fill channels, got it.  And pick good casting alloys and don't overheat them.

[A7er]

His name is Martin and he has only recently started to post videos after being ill for a long time. He isn't in the best of health now, so we might not see a casting video from him for a while. Even though Martin pushes the correct way to do things, in his book anyway, he does sometimes display a few lapses. I have watched a few of his videos where he foregoes a pouring basin and just pours the metal down a large sprue. He also took his aluminium over temp to keep it liquid as it poured. What I take away from this is that there is a "right" way to do things. But sometimes the "wrong" way is OK.

I keep aiming for a finish as good as his, but my aim isn't very good!
Lee