My plastic spoons on my Pelton turbine died after only a few days of use. The vendor said the spoons were good to 150 psi, but at only 65 psi this was the result:
My water supply is clean, filtered and put through a settling tank so I ruled out contamination in the water.
The spoons were made of unreinforced HDPE. Reinforced nylon spoons from another vendor are rated at only 130 psi. Because I am going to reposition the turbine lower on the hill which will raise the pressure to about 100 psi, I am casting the replacement spoons in aluminum.
I built a furnace with charcoal as the fuel source (more on this later). Just pouring ingots here:
I am using lost wax in a plaster mold to cast the spoons. The surface finish is good as opposed to that obtained in sand casting. But what made me use this method is that there is an undercut on the spoon that makes it impossible to get it out of the sand without damaging the mold. It is more time consuming to use the lost wax method, but I think the end product will be superior.
More about the furnace: I used charcoal to fire it since I have a large supply already for the gasifier. But charcoal is a pain to use in the furnace. Propane would be easier. Than that got me thinking (dangerous), maybe I should try using the fuel gas from the gasifier to run the furnace. My concern is that it just might not be hot enough. Love to hear from anyone who has tried this.
The only time I have seen it used was a guy at a renfest, and there is a video, I think I posted it. He used wood, in sort of a cross draft fashion but for the âblowerâ, he had his propane on. With a nozzle that pointed towards the fire similar to a number of forge furnace burners like the Frost T where the gas pressure pushes the air. It doesnât burn as hot, but he used a lot less propane because he was able to keep the furnace hot inbetween demonstrations. He cranked up the propane for the demonstration.
If you get it to work, I would love to see it. I had issues controlling the temp with charcoal.
Hi, i just think it would work, iâve wanted to try that too.
Among the first gasifiers in the world (1800s some) was used to melt iron, this was for using wood directly, instead of making charcoal first.
By the way: very nice work casting
I just completed my experiment using fuel gas from the gasifier to run the furnace. Initially I tried to get air into the gas stream by way of a venturi.
That did not work. The flame was cool and searching for oxygen within the furnace. (Sorry for the out of focus videos - old eyes you know. Will get better ones manana).
I then opened the air valve for the generator allowing air to the fan inlet. I wasnât thrilled about a combustible mixture in the lines, but there were no problems and I made sure to close the air valve first when shutting down to purge air out of the lines. The flame became a blowtorch.
It took at least three times as long to melt the aluminum (after all Iâve melted steel nozzles in the gasifier under similar conditions). With charcoal I got the crucible up to orange hot, but with gas it only got to dull red. Good enough though. I didnât have to fuss about adding charcoal and repositioning the crucible each time. Also, I donât risk having the crucible fall over when the charcoal burns out underneath it. Donât ask me how I figured out that potential.
I poured a couple of spoons - you can see the flasks with the plaster molds heating up in the furnace above.
The spoons came out alright, but I am hoping for a better surface finish. I think that the wax burnout in the gas fired furnace may turn out better than with charcoal. The flasks today were previously heated with charcoal. The charcoal gives a very intense heat and that may be cracking the plaster and causing spalling, etc. Tomorrow Iâll try with fresh molds.
I did some more casting today. I think the furnace is a bit hotter than I thought at first. I adjusted the fuel/air mixture and let it run a little longer.
There is a slight improvement, but the finish is not what it could be. I think I am over firing the plaster molds still. It is a fine line between over firing and not getting all the water out. I still have plenty more spoons to cast and perfect my procedure.
Did some more casting today and I think Iâm getting closer to what I want. I was over firing the molds. The finish on the spoons is better although not perfect yet.
The tang is still rough, but I think that may be due to there not being enough pressure. The tang sits at the highest point in the mold beneath the sprue. I think I will try to increase the sprue height and see if that makes a difference.
So the first batch of spoons went well, but the second set had problems. The gas wasnât burning as hot. There was a ton of water coming out at the water trap on the radiator. I removed 2 cups worth during an hour run. When I opened the cleanout door on the gasifier, there was a lot of water just inside the door. I figured the water cooled nozzle sprung a leak. It has about 40 hours on it so I removed it to take a look.
Itâs fine and there is no wear. It has some minor deposits but nothing that interferes with the air inlet.
Apparently, when I refueled before the second run, I must have had some really moist charcoal. I will definitely have to get some drums to store the charcoal in rather than feed bags.
Spin casting them would reduce the shrinkage and allow using slightly lower temperatures. I have a commercial made spin cast machine. I used spin casting for making jewelry from sea horses, star fish, and sea urchins 50 years ago. Mine simply uses a wind up spring loaded mechanism.
Thanks Ron for the tip. I have seen those machines and considered building one, but I didnât relish the thought of something going wrong and spraying molten aluminum everywhere. Maybe if lengthening the sprue doesnât work, Iâll get brave and try to make one.
There should be a containment ring that the spinner fits inside of. The one I used was made of sheet metal. The top of the ring had a inward facing rim that prevented any metal from escaping the containment.
The containment ring eventually had a mechanism that tripped the spring latch. I just reached down inside and push on the lever and quickly withdrew my hand. I donât know if the mechanism and containment ring were supplied by the company that made the machine or was made in the shop. I have a new spinner and supplies at my shop. My dad didnât have an overly protective learning environment. Heck - he had a cyanide bath for heat treating. The students at the vocational center were primarily 11 and 12th grade high school with some adult education students.
A riser or large sprue can be used as a surge tank of sorts to allow having a void of volume that can accommodate some excess metal.
Of course - the approximate volume of metal needed to fill the space of the pattern, sprue, vents is all computed. It is possible to be very accurate at this by using a displacement method in a media such as water. I used this method as a training aid for other students when I cast 2 items using gold. I bought 1/4 ounce with 3 weeks of earnings from my paper route. Then gold prices climbed so quickly I quit making jewelry.
I cast yet again today and I think I have my surface roughness licked. I lay awake last night trying to figure out a centrifuge for casting with materials on hand, but came to the conclusion that I didnât have what was necessary and that it would probably only be used a few times. I decided instead to lengthen the sprue and see if that would help. It did:
The porosity on the tang moved up onto the sprue, leaving a good spoon casting. Only 4 more to cast.
Now I have to decide weather to re-cast the marginal spoons or just call it a day. (Itâs a real pain to get a good wax impression).
Next step is to drill out the mounting holes and mount on a disk. Then on to balancing.
Iâve read about it, but never used it. Sounds like a nice product, however it would be hard for me to find it where I live. I tried regular casting sand, but the splitter on the spoon has an undercut and I couldnât figure a way to get the model out of the sand without disturbing the mold. I determined I would need a core and it just got too complicated for me as a novice.
It sounds like your best bet is a silicon mold to make the wax portion. It is pretty flexible so you can get it out. then do the dip in plaster, and heat to get the wax out, etc. like you already are.
The problem with the silicone mold Iâve been using is the thin section of the spoon cup. I got the silicone from China and it is a bit softer than I would prefer. It is difficult to get it to align well and if it is a hair off, one side of the spoon is thick and the other paper thin. Lately though, Iâve been having more success when I warm up the silicone before closing the mold.
marty, great work you are doingâŠ
have i missed something? i have not always internet connection⊠how and from what material you prepare the form?
great also your water cooled nozzle!
ciao giorgio
Given where the original spoons stressed and broke, I would be making the thin section thicker anyway, and then adding a portion to the back of the spoon that can be removed to fine tune the weight of them after polishing/machining and such.
FOR the spin casting, the old ones use springs. It is really the speed of the first revolution or so to get the metal while it is still liquid pressed as far as it can go as fast as possible and drive all the air out. This is about as low budget as I have seen. The older ones, I am guessing like Ohler has, use a spring instead of the motor. There was one design, that used a portion of a 55 gallon barrel for the splatter guard.
As soon as the plaster firms up, the flask is ready to go into the furnace to drive off water from the plaster and melt out the wax. Then it is ready for the aluminum to be poured into it. Iâve found that when the wax stops smoking out of the sprue, it is ready for pouring. I was leaving it in the furnace way too long before and over firing it. I could look down the sprue and see the plaster glowing red. This led to a lot of hairline cracks in the plaster. It didnât detract from the spoon other than having a network of âhairsâ on the surface of the casting.
Iâve finally perfected my technique after about 10 tries. Some of the earlier casts are passable, but I am recasting about 8.
The OTHER way I have seen it done is to lightly coat the object in plaster, and take the shell, do the burn out, and pack sand around it. Then it isnât as thick and you arenât using as much plaster.
The other thing is you want to slowly heat the up the mold during the burnout similar to a ceramic kiln where you step up the temperature over a longer period of time to a specific temperature. then you slowly bring it back down. It reduces cracking from the temperature differences of the inside and outside. You might be doing that alreadyâŠ
That has been out for a few years. Usually they leave ash, so it is nice to see one that doesnât. It opens up a lot of possibilities, but you still have issues where you can see the layering of the 3d print if it is showing up on your print. And if that doesnât matter, then does a little bit of ash, and lost PLA is the same process but the filament is a lot cheaper.
It all depends on the use case. I do like the note of basically âkeep the end tightâ or else it springs back and causes knots in the filament. lol
I had issues controlling the temp with charcoal.
Maybe if lengthening the sprue doesnât work, Iâll get brave and try to make one. 
