Casting a windmill gear in grey iron

The temperature-related metal color seems to be not much of a factor with respect to "seeing Mercury." I think you almost saw it when you were skimming. That is, when the metal is approaching 2550, as you skim you uncover a shiny surface, but is skins over again with a thin dull film fairly rapidly like ice forming on a lake---in a few seconds at most and, at lower temps, almost immediately the skin follows the skimmer. But, once the magic temperature is reached, the surface remains a reflective mirror that will look blue on a clear day or white if cloudy. But it looks like a pool of liquid mercury in its reflectiveness. At that point regardless of roiling or not, temperature guns, etc you know you are good to go. Usually there is fairly vigorous roiling soon thereafter as the temperature rises further. But the key feature is seeing Mercury in his resplendent glory.

Denis

 

Thanks, Pat. I feel really lucky to get every casting usable, so far. Of course the first gear may not be usable because of pattern size, that's ok.

From what Denis says I'm getting the temperature, I need to shoot if with my gun to

Thanks for the recap, Denis. I was definitely there with having the metal not skin over quickly after skimming. I was concerned with how clean it was that it would be picking up lots of oxygen. I don't think it will ever look blue to me as I'm under a roof. The rolling I was seeing definitely looked like it should be plenty hot.

 

What I wish someone would explain is why does Mercury appear at 2550 or at all for that matter. It seems like the hotter the metal the more it should oxidize. And so it should skin over more, seemingly. Clearly I do not understand the cause of the observed behavior. I have tried to find information by searching online but have had no luck so far. One wise old gent told me that at that temperature the oxygen dissociates with iron. I have not been able to confirm that, however.

Denis

 

I use straight Borax to slag my iron melts. It congeals the slag and I drag it off the melt into a drain pan with a 1 1/2 " wooden dowel. I made a fixture to hold the tongs that keeps them closed and I can tilt the crucible with one hand and slag with the other. The slag will be crumbly to glassy depending on temperature. Sometimes it is the carbon increaser that didn't go into solution when I didn't get hot enough.

 

Thanks, I keep borax on hand for forge welding. Are you using 1.5" diameter dowel or 1/2"? What do you use for carbon increaser?

 

I received a box of pulverized coal today. I don't remember ordering any, so I assume it is from Ed. Thanks, Ed!

 

I have use an old oak 1.5" hand rail or closet coat hanger rod. The size isn't important since it develops a spear point after a little use. I use Desulco as the carbon increaser.
I went out a got some shots of my slagging fixture. When the crucible is tilted the tongs are locked closed in the slot. The coffee can with the ladle is my Borax, and some of the glassy slag shows in the drain pan.

 

First time I've ever heard of someone getting excited about receiving coal for a gift.

 

Here is a chance for some evening entertainment:

Well, I learned about heat tolerance of a 3-D printed pattern today. Or should I say lack thereof. I got around to packing the large half of the pattern in sodium silicate around midday using propanyl carbonate as a catalyst for the silicate. (I also weakened the silicate with sugar and mixed 5% coal in with the olivine sand for improved surface finish.) Being anxious to pull the pattern this evening, after gassing it with CO2, I put the pattern packed in the sand in my kiln set at 150F for 3 hours to kick the sand to full hardness as it is not easy to get the gas to completely diffuse into the recesses formed by the teeth which is just where they would want to stick. Seemed like that should be far below any concern for a plastic part, but OOOPS, when I retrieved the mold and pattern a half hour ago I noted that the pattern appeared to have pulled away from the sand a skosh. I tapped the pattern out of the sand and got a good mold. Then I compared the molded pattern half to the unmolded smaller half. Sure enough, the OD of the "heat treated" pattern was about a 32nd" smaller than the unmolested half.

Then I looked up heat tolerance of 3-D printed parts and was amazed to see that typical printed plastic material starts to become plastic about 15oF!! I should have checked first, of course, and would have had I in any way suspected a problem at that temp.Well, I guess I do not have the luxury of screwing up this mold. On eyeball comparison of the pattern halves it does not appear that other dimensions were affected and the heated pattern is still a snug fit in the mold all except for the tips of the teeth.

I guess this is what they call learning by experience.

Denis

 

Excellente` my friend!!

Bell invented his telephone (or made it successful) when he spilled acid on it. A failed adhesive experiment resulted in Post-It notes for 3M, and there are countless other examples.

You have just created the new Semi-Lost PLA casting method. Print one without draft, pack it in propanyl carbonate sand, let it set a bit, then cook it to shrink the pattern away from the mold.

Genius!!

Doesn't work for all patterns, but then neither does any other method. Another tool in our collective tool box.

Thank you!!

 

I know you are kidding about the Semi-Lost PLA Method you described. But that exact idea occurred to me. I suspect (and would try if I had extra PLA pattern material) that heating the mold/pattern to 200 or so degrees would yield a very soft pattern residual that could be lifted out of the mold without damage to the sand. A little testing would be needed to determine the right temp range.

So, makers of PLA patterns better not leave them on the dash of the car for one hour on a sunny 95 degree day. Here is a quote from a study done on various cars in just such circumstances:
“To investigate the matter, researchers studied how long it takes different types of cars to heat up on hot days. The findings were sobering: Within 1 hour, the temperature inside of a car parked in the sun on a day that reached 95 degrees Fahrenheit (35 degrees Celsius) or hotter, hit an average of 116 degrees F (47 degrees C).

The cars' dashboards got even hotter, reaching 157 degrees F (69 degrees C), on average; the steering wheels climbed to a temperature of 127 degrees F (53 degrees C), on average; and the temperature of the seats hit 123 degrees F (51 degrees C), on average.”

Denis

 

I was not kidding one bit. This is a new process you stumbled upon. It should work well with zero draft and maybe even with a little negative draft or undercut. May be possible to cast coarse threads with it.

I am genuinely about the possibilities. 20% infill might better be adjusted to 10% as well. Maybe a thinner shell. The pattern would need to be just strong enough to not warp and withstand sand packing. And maybe the standard is optimal, but it will be worth evaluating other settings on the printer to promote shrinkage upon heating.

 

Well, then, ahem, I actually had planned the whole thing in advance....

But I do agree it is worth a look for potential application.

Denis

 

Well put! I remember the exchange, but didn't put two and two together that the PLA would shrink enough to be withdrawn, rather than just soften, and worse, stick to the sand.

 

I can hear all the light bulbs coming on. This would have probably solved Pat's issue in trying to pull with no draft. Also could have reduced some of Denis' painstaking steps on smoothing things out.

 

In Malaysia the workers lift the wiper blades from the screen when they park to stop the blades melting and sticking. Maybe a release agent (graphite) and a controlled shrink would overcome the sticky happening. Interesting stuff.

 

Sometimes an answer has been there for a long time but has to hit you between the eyes before you can see it. I did not give DavidF's post 77 serious consideration or realize that it might actually be workable. Now, I can see it probably would be. The only down side is getting only one cycle from a pattern which is not much of a problem if you print them yourself. Me, I am sweating screwing up the mold I have as reusing the cooked pattern might be acceptable but is marginal.

Denis

 

I did photograph the large half of the mold today and did pack up the second half of the silicate mold. I used propanyl carbonate as a catalyst, coal in the sand to prevent burn in and sugar to prevent formation of "concrete" when exposed to iron. Here are a few pics with captions which I think should be pretty self-explanatory.



I will point out that the silicate bound sand is pretty hard and happily durable too. But that hardness means that rapping the PLA pattern to free it up for pulling does not do much other than break the glue-like bond that weakly forms despite graphite or wax or silicone. I am not sure if it is truly stuck so much as closely applied. And raping does cause a minuscule separation line. But to get the pattern out, it has to be taped from the back of the mold. A wood pattern is another matter. I have pulled them successfully as draw spikes engage the wood securely and then pulling raps will slowly break the pattern free. Had this pattern had 3 or 4 degrees of draft, it would have probably have been possible to pull it despite the thin skin. (I must confess to suggesting 2 degrees but that was prior to my learning the pattern is hollow with only about a .125 skin into which a draw spike can be screwed. Likely one could print in "hard points" of solid plastic strategically located just for pulling.)

 

Question for those who use powdered graphite: what are your best/favorite methods for applying it to molds, patterns and cores?
Brush?, shake it on like using talc?, aerosol? I did find "dry" aerosol graphite which really is in a volatile liquid and did not spray on as densely as I would like. the "chicken breading" method worked pretty well, but is messy.

Denis

Added: this is the cheap granite I used.