johnsmachines

machines which I have made, am making, or intend to make, and some other stuff. If you find this site interesting, please leave a comment. I read every comment and respond to most. n.b. There is a list of my first 800 posts in my post of 17 June 2021, titled "800 Posts"

Tag: aluminum casting

Casting Aluminium. It is just too light.

When doing my aluminium castings  for the model Armstrong cannon, I noticed that the objects in the wax/PLA tree which were closest to the funnel (i.e. the topmost ones) were the ones which were most likely to have significant deficits, and I wondered whether the pressure of the molten metal at that level was the problem.  The objects at the bottom of the tree were most likely to be successful.

Well, my friend Stuart Tankard is working on a positive pressure system, and I am working out a negative pressure system, to increase the pressure at the higher levels within the tree.

An interesting number is that for every 1″ / 25.4mm increase in the head of molten aluminium, the increase in pressure forcing the molten aluminium into the casting voids, is only 0.1 psi!!!  Bronze, being much more dense (x3 – x4)  would be less problematic, but still less than 1psi.

Some casters use a centrifugal system to increase the pressure on the molten aluminium.  Frankly, that idea frightens the shit out of me.  One episode of molten metal flying around my workshop was enough.

Stuart T is working on a positive pressure system, using approximately 5psi on the melt, to force it into the tree spaces, and he is well along the path of manufacturing the hardware to accomplish that.

I am inclined to use negative pressure to suck the melt down.  I already have a vacuum pump, and I think that it might be easier to seal the hot steel cylinder to the silicon gasket which is required.  There is a YouTube video on the subject.

(oops.  I pasted the wrong VOG video.  The one below is the intended one.)

I have ordered some 3mm thick Silicone sheet.

VOG, in the above video, allows the surface of his casting cylinder to cool to 100ºc before pouring the aluminium melt, so the silicone gasket does not burst into flame, and he has had some excellent results.  It is casting heresy, and he should be burnt at the stake.  But if it works…. hey?!  (maybe the core of the casting cylinder is still closer to the molten aluminium temperature of 710ºc?).

So that is the path which I am following.  Not exactly.  But using the principle.  Watch this space for my results.

Actually, molten bronze is my next pour.   I doubt that it will require vacuum or pressure.

Lost PLA Casting – 3rd pour

Today I attempted another aluminium casting session with trees that I had made 2 days ago.  More wheel forks, and barrel trolley brackets.  16 parts altogether.

And this time I installed air release vents, following my previous poor results, and at the suggestion of reader Rob R.

I also made some 50mm extensions of the pouring funnel, to increase the head of melt pressure.  The extensions were “add ons” rather than designed into the system, and the molten aluminium leaked between the extension and the main flask with the tree, so I doubt that they were very effective.

BUT!  Of the 16 parts on the trees, 14 were good to excellent, and only 2 showed any voids, and I assess one of those as repairable.  So, 15/16 is very pleasing.  I feel that I am closer to getting good results every time, if I can make an effective system of increasing the delivery pressure of the molten aluminium.

PANA3978

These are the extension pieces to the funnels on the investment flasks.  The shape was made with the plastic funnel.  If I had positioned them before the investment plaster had set hard they might have worked better, but as they just sat on top of the already hardened plaster, the join leaked molten aluminium rather badly.  I have a different system in mind for my next pour.

PANA3980

Previous failures were cut up and thrown into the melt.

PANA3994

PANA3997

See the tiny silver dots surrounding the central funnel.  That proves that the air vents functioned as intended.

PANA3999

The aluminium trees.  Not very pretty, but delightful to see.  10 parts on the top one, 6 on the other.  It is odd to see the wax spaghetti turn into aluminium spaghetti.   I will separate the parts tomorrow.

And while the investment burnout and baking was proceeding, I worked on previously cast parts.

PANA3993

The centre columns have beep painted with etch primer.  A little more filling required, then I will use the best 2 on the models.   The 2 bracket and column assemblies on the right were initially considered unusable due to large voids, but I used some aluminium solder to fill the defects, and they might possibly be OK.  The 2 on the left just need some tidying, machining removal of  melt tubes, and minimal filling.

PANA3989

I will probably remake this one, but will continue to salvage it and see how well it comes up.  Note the solder fill on the RHS.   That will not be seen on the model.

One more melt and pour, and that should be the last of the castings made for the model Armstrong cannons.  It has been a challenge, and lots to learn, but very interesting and very satisfying.

IMG_8657

Finally for today’s post…  I noticed some black marks on the normally pristine white wall above the casting bench.  They extend about 4 meters above the floor.  Do you know what they are?  The paint has been melted off the wall by bits of flying molten brass, resulting from the steam explosion 2 days ago!

Many thanks to Rob R for his spot on suggestion about the air vents.