BackyardMetalcasting.com - Lionel's Laboratory

[Abby]

Hi guys thought you might like this picture !

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[HAVEHEATWILLCAST1]

Gun metal, right


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Heat them up, mold them out.

[Bobnova]

Nice!
Those look like they came out quite well.
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[Rowan Patel]

Very nice .

Are these investment cast as normal ?

[HAVEHEATWILLCAST1]

What makes the bolt heads look like they are cast too ?
Are they the same metal for expansion or something ?
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Heat them up, mold them out.

[Abby]

The bolt heads ARE cast HH , the glands are dummies , and the actual seal is provided by an o'ring which is housed in a machined recess between the mating flange faces.
As per usual the wax patterns were assembled by glueing - still on superglue but looking for an alternative- a jig was needed to get the flanges etc concentric to each other , all the holes have been cast in with tapping allowance where needed or bolt clearance. Because the o'ring tolerances are very tight the housing recesses have had to be machined , just could not get them good enough as cast.
These parts are for the valve chests on a 7 1/2" scale german loco.
One problem encountered , which I have come across before when investment casting aluminium , is stress cracking , two of the castings had a crack running from a bolt hole to the edge of the flange - see picture !
These castings are in silicon bronze , cast by my usual method and the finished parts are bead blasted and look a treat.
Although the crack is not detrimental to the overall casting and can easily be repaired by silver soldering the cause is eluding me - any ideas anyone ? I have tried changing the mould temperature , rate of cooling and method of sprue-ing to no avail.
This is the first part of a large project and has involved 3D cad design and rapid prototyping - both new to me - I am still on a steep learning curve but see a great things for this type of production in the backyard foundry and the costs - of course there are costs - are not as high as you might expect and the time saved is huge.
Air bubble attached to patterns result in tiny spheres of metal on the castings , I have been trying to solve this problem for years with no success, I have tried various wax washes , vacuuming and vibration but still the annoying little buggers are there. They come off easily with a gentle tap from a small chisel but it is extra work I could do without.
Non of the investment suppliers or wax producers can offer a solution.

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[Bobnova]

If you can vibrate the hell out of the coating that will get rid of the bubbles.
Most modern game console controllers have decently beefy vibrators in them, or you can go industrial and get one designed for concrete work, they'll dance a table around nicely.

Thats the only thing i can think of bubble wise.

No clue on the crack, sorry.
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[moya034]

If you need a vibrator all you need is an offset weight on a motor. Just keep in mind that the bearings in the motor will have a short life span unless designed for the purpose.
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[Abby]

Thanks for the replies guys , but I have tried both methods without 100% success , I think the vibration frequency is the critical factor.
Understand that a wax " tree" is somewhat delicate so severe vibration is not an option, the vibration also needs to be carried out during vacuuming so the equipment needs to be small and easily attached/detached from each flask.
The process time for mixing and pouring the investment is quite critical so any vibration needs to be done during the final 4 minutes of processing.
These bubbles are attracted to the pattern by some force - static ? and the attraction is greater than their bouyancy.
any dudes out there into physics ? if a flask is vacuumed to a minimum 28" Hg , do these bubbles grow in size and if so does their bouyancy increase ?
Could I use ultra-sonic waves to break them up ?
Here's a "tree" after casting that shows some bubbles , you can see how delicate the sprues are.
Also notice that the bubbles are not neccessarily in areas where there is a trap of any kind , they just appear to be rooted to the spot for no obvious ( to me) reason.

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[Anon]

Stress cracking may be caused by an investment formulation that's too rigid to allow the metal to shrink. That would explain why they appear leading from holes to the edge: the core forming the hole won't crush, and the metal cracks to relieve stress. The only solution I can think of is to get a different investment formulation.
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[Abby]

That was my initial thought Anon , and you may be correct , my first run on these parts had one or more cracks on 25% of the castings which I put down to my impatience to see the results. I quenched the mould in water much sooner than I would normally.
The second run was cooled naturally and had only two cracks, by coincidence I used a stronger investment , having run out of the weaker one.
I am now waiting for a delivery of my usual investment powder so that I can have a third attempt with natural cooling and compare the results.
The site of the crack is in the weakest area , but I have not come across cracks in silicon bronze before.
We live and learn
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[Anon]

Something I thought about with the bubbles:

Vacuum should get rid of most of them, but to get rid of (or at least minimize) the ones that are left, you might relieve the vacuum, or even pressurize the vacuum chamber, after the mold stops bubbling air out. This should compress any remaining air pockets enough that the bubbles they form are very small or non-noticeable. If you have enough investment set time, a combination of vibration, high vacuum, and high pressure at the appropriate times should get rid of bubbles for good.
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[HAVEHEATWILLCAST1]

You could try painting on a thin layer of investment or wall joint compound (I know you'll like that, but it's air dry. ).

It would be best to paint it on before you build the tree up, just leave the "glue" spots. That's a lot of painting and you mite want to do the chisel thing after all the painting. Could be you could really thin some investment and dip them each piece.

The thing I see is to get the " shine " off the wax as it holds a bubble of air the same as it beads water. Haveing a thin waterbased coat on before the investment dip of the tree would let the painted coat melt into the thicker one.

[Daniel Rotblatt]

Abby, bubbles are a bitch. I did a life casting of a hand recently and at many of the hair follicles there were tiny little bubbles (maybe 1 mm) - I sat there for several hours with magnifying glasses, a tiny chisel and chasing hammer tapping them off.

Are you doing ceramic shell, or solid investment? I do shell, but perhaps some of these may help.

For my first dip, I use a combination of a brush and blow compressed air (gentle - at around 5-15 psi) to pop all the bubbles and blow slurry into the grooves and recesses. Even with investment, you might try a predip or two then fully invest them. I also spray the pieces with a coat of shellac which helps the investment flow. There is also a pre-dip - I think mainly alcohol which a friend of mine uses.

For silicon rubbers and plaster I was taught to pour gently and steadily into the lowest corner of the frame and allow the rubber or plaster to flow around the piece - this helped prevent bubbles.

In regards to a vacuum - for silicon rubber molds I vacuum the silicon at a over 29" of mercury. This is my experience with mold rubbers, not investment, but once more, it may help. As the vacuum is applied, the rubber foams up to at least 4 times it's size, then suddenly collapses. It still seems to be filled with bubbles, but once the pressure is released they slowly disappear - by the time I pour the bubbles are gone (and I mean gone - no bubbles). This process takes about 5-8 minutes - so if you have the time to do this before your investment sets it could work. With mold rubbers there is no rush. In your case, I would suggest you mix the investment, vacuum it and then pour it over the waxes. 27" of mercury is probably not enough, you really need a pretty good vacuum. I purchased a AC vacuum pump from EBay for under $100 and made a chamber out of a steel cooking pot and a 1" piece of acrylic plastic for a lid. I routed a circular groove in the acrylic the same diameter as the pot and filled the groove with silicon mold rubber to make a gasket seal. Works great!
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[Abby]

Hi Dan and thanks for the advice,
I did not really expect to find an easy solution to the bubble problem on this or any forum to be honest , but I thought it would be an interesting subject.
Although I am pleased with the response , I have tried EVERY suggestion so far and most of them several years ago.
I vacuum my rubber and my investment at OVER 27" Hg which is the figure quoted by all of the manufacturers , I have a pair of Edwards high vacuum pumps which are serviced regularly and have the oil changed frequently so I know that this is not a factor.
If you look on my web-site you can see how I mix plaster based investment under vacuum.
It is interesting that you also have bubbles with ceramic shell moulding and actually cancels out one or two ideas I have yet to try.
I think the answer is in a combination of both vacuum and vibration applied together but the vibration frequency is very important.
Working on an idea - more after testing.
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