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Author Topic: Help with casting problems  (Read 1045 times)

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Gemscholar

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Help with casting problems
« on: November 11, 2017, 01:27:27 PM »

I am experiencing partial casts.  I do lost wax method using a centrifugal casting machine and I have a poor success rate at full rings.
Often I will get only three quarters of the full ring to cast.   What is this symptomatic of?   The burnout time is standard 5 hours , starting at 300F etc etc. 
Here is an image of a recent failed attempt,  using sterling silver.  IMG_0058.JPG
*IMG_0058.JPG (551 kB . 1632x1224 - viewed 160 times) [ Invalid Attachment ]


any help and or advice will be greatly appreciated.

Krupp
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gemfeller

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Re: Help with casting problems
« Reply #1 on: November 11, 2017, 03:37:02 PM »

Generally, if the burn-out sequence & temperature are correct, and metal is heated to correct casting temperature, incomplete casts are due to sprue problems.  Your image is so large I can only see a small portion of it on my screen, so I can't tell anything about your spruing if it's shown.

Are your sprues the right diameter for the size of the object you're casting?  In general the sprue wax should be slightly thicker than the average section of the model and should be attached to the thickest part of the model; 8 mm. is an average choice.  Are they too long? (Sprues should be kept as short as possible).  Are you using multiple sprues for larger models?  Are you creating fillets or a broadening of the wax where it attaches to the model? 

There are many other considerations, correct metal temperature being a critical one.  If it's not correct it can "freeze" before reaching all areas of the mold.  If it's too hot, pitting can occur.

A correctly sized image of your problem might help in the diagnosis.
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hummingbirdstones

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Re: Help with casting problems
« Reply #2 on: November 11, 2017, 04:06:32 PM »

I've resized the picture for you, Gemfeller.   :icon_sunny:
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Robin

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Re: Help with casting problems
« Reply #3 on: November 11, 2017, 04:42:23 PM »

Thanks Hummingbirdstones.

Yup, looks to me it's likely a sprue problem.  Could also be incorrect metal temp but proper spruing would help a lot.  The main sprue is short enough and you have good fillets but there should have been "feeder" sprues to the heaviest parts of the rings.  I notice on one ring the sprue runs to the narrowest part of the shank.  Another sprue to the heaviest part of the model would help a lot I think. 

The ring in the foreground has a sprue to its heaviest part and still didn't fill.  That makes me wonder if you're trying to do too many models on the same tree; it also makes me think maybe your metal wasn't hot enough. 

Do you keep your torch on the button for a while during casting? It's easiest to do with vacuum casting because there's no pause while a centrifugal machine throws the metal.  Torch melting can be very imprecise.  Do you heat your crucible thoroughly before adding the metal?  Lots of things to consider.  What casting method do you use? 
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Debbie K

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Re: Help with casting problems
« Reply #4 on: November 11, 2017, 08:28:10 PM »

Looks like two or three things went wrong. First, the large casting on the left should have been attached by a large sprue at the base of the shank of the ring centered over the main feeder sprue straight up. The heaviest castings should always have the center, top most position. I like to attach side feeder sprues to things that look like they could use the help.

It looks like your flask may have cooled off too much before you cast. Try utilizing an assistant; you melt the metal in the crucible and then when it is molten, get the assistant to put the flask in position straight from the kiln. Or, do what I do; take it out as soon as the temp goes down to about 1100 degrees and let it cool down in the casting unit a little while you are melting your metal.

If you are melting your metal with a torch, you should make sure it is thoroughly melted before casting. You should be stirring it with a heated graphite rod to make sure that everything is molten. And, this is the most important thing, even when it is molten, often it is not quite hot enough to cast. You need to see a " ring of fire" around the molten mercury-like metal first. Then, the second most important thing, you remove the torch and release the caster simultaneously. I have a problem with this myself, so I lift the torch on the count of "one" and release the caster on "two". I'm always afraid that the caster will hit the torch and this is my solution. Make sure you are using some borax flux, too, as this makes the metal more fluid.

If you are doing a vacuum casting, the sprues and temp of metal can still be the problem. The orientation of the sprues are different for centrifigal casting which should be like a "Y" and vacuum casting, which should be like a "T". I'm too lazy to explain why, but if you think about it, it makes sense. I didn't think about it and had an absolutely heartbreaking failure with five hand-carved complicated wax carvings that looked a lot like your casting when I did my first vacuum casting.

My side sprues are always much thinner than my central feeder sprue, otherwise the clean up is too difficult.

Sorry if you know most of this already, if you do, maybe it will be useful for someone else in the future.

So, position of the sprue on the piece, finest area to heaviest area, in other words, on a ring put the main sprue on the shank. Position of sprue in the flask; heaviest in the middle topmost. Thorough heating of the metal in a hot-enough flask with flux. Finally, releasing the caster at the same time as you raise your torch.

All of that said, it looks to me that your metal or your flask aren't hot enough, maybe both.

P.S. It also appears that you are doing a flask that may be larger than you can get away with for a 5 hour burnout. Remember, when you look at the charts what really matters is the width of the flask, not the height. Again, think about it and it makes sense. The heat is coming from the outside of the flask going in to center, and if your flask is wide it takes longer to come to temp.

Hope this helps.

Debbie K
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Gemscholar

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Re: Help with casting problems
« Reply #5 on: November 11, 2017, 10:07:24 PM »

Thank you all very much, Yes,  I feel that the flask cools off too much , i didnt realize that timing was so crucial. For instance when I get the silver in the crucible to a molten state, and stir it with the graphite rod, I then set the torch down,  retrieve the flask from the oven, put in the centrifugal caster, and everytime,  i have had to re heat the silver.  I guess the silver is not at the right temp.  I will try to enlist the aid of my wife. I like that idea. 

also, i am sure I am using the wrong size sprues now, I felt that was an issue,  but seeing as how I am just starting out,  I didnt know. as for spruing , i really appreciate the advice, the smaller feeders are the way to go, but a PITA to clean up after casting, better than having a failed cast though.

I am very grateful for the advice here,  thank you all so kindly.  I will let you know how it goes next time!! 

Krupp
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Debbie K

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Re: Help with casting problems
« Reply #6 on: November 12, 2017, 11:47:41 AM »

When I look at your photo, you sprues look large enough. Try a few, finer side sprues and make sure your metal is hot enough and that your flask hasn't cooled too much. Also, make sure you are using some borax flux, it really helps.

Look for the "ring of fire" in your crucible. It will be a bright, light yellow/orange color all the way around the silver. If you don't get your silver this hot, it won't cast correctly. And yes, you really do have to move fast. It is critical that the metal not be allowed to cool down even a few seconds.

You'll get the hang of it in no time! Keep up the good work.

Debbie K
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Gemscholar

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Re: Help with casting problems
« Reply #7 on: November 19, 2017, 04:29:03 PM »

Success !!!!   Thanks all for the advice,   i had a successful cast of three rings, I used thinner feeder bands off the sprue and also lessened the time between final heat of the silver and releasing the spin caster.  Worked like a Charm!!   I would post pics but i was so excited I cut the rings off the tree, pickled them and began setting stones. Love it !  thanks again.      :thumbsup:
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bobby1

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Re: Help with casting problems
« Reply #8 on: November 19, 2017, 07:33:49 PM »

It also looks like your rings are almost horizontal to the main sprue. They should be as straight up from the main sprue as can be practically placed. The molten metal wants to flow straight into the mold, not sideways.
Bob
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Debbie K

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Re: Help with casting problems
« Reply #9 on: November 25, 2017, 06:32:18 AM »

I'm so glad you had a successful casting experience! Casting is the only thing I do that I will actually mentally rehearse my actions, and physically check all equipment before I ever take the flask out of the oven. Timing is so critical and molten metal so dangerous that I never have allowed myself to become complacent.

Debbie K
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