Another small bronze pour yesterday, and it was my best one yet. No bubbles. No voids. And excellent surface definition. What did I do that was different?
First, the 3D parts were printed already attached to the tree. So the trunk and branches were 3D printed with the parts attached. That meant that I could determine more accurately the bronze flow, the gaps, the spaces. The only “failure” was that I added some wax air vent sprues as an afterthought. And those wax parts were the only part of the pour which failed. Fortunately, the absence of the gas vents did not seem to matter.
Next, I painted the tree with a slurry of investment. The slurry was much more watery than the normal investment, but it was thick enough to leave a thin layer of investment on the surfaces, paying particular attention to the corners and internal edges.
Then I used my new, 1 hp vacuum pump to degas the investment mixture. It took about 15 seconds to reach maximum negative pressure, compared with about 1-2 minutes which the 1/4 hp unit was taking.
Then, after pouring the investment, I placed the full flask containing the tree and investment, and degassed the entire unit. I was shocked at how much extra air bubbled out.
The rest of the process was as usual, drying for 4 hours (except that this time it started at 6am, having put the process on an automatic start timer), burnout 2 hours, and baking 3-4 hours.
While the investment flask was cooking, I experimented with the bevel gears which move the cannon barrel elevation. I had cast some bronze gears, teeth and all, some weeks (or was it months?) ago, but was not happy with the result. So, I had bought some bevel gears on Ebay. They are spare parts for an RC model car. Not quite the correct size, but close. The metal is HARD. Sintered? But, machinable with carbide cutters.
Now before you all start shouting at me to make the bevel gears from scratch, let me just say that I might do just that. Not yet decided.
When bubbles occur inside a casting, the cause is probably in the design of the pouring system, or the way the melt was poured. If you watched the video a few posts ago by Prof. John Campbell you would think, as I did, that our pouring funnels and sprues should be more complex and more carefully designed. Difficult at an amateur level. I have made some changes in this direction, using a side reservoir to tip the melt into, and trying to avoid the gurgling and glugging.
With the sort of castings which I have been making for the Armstrong cannon, I am not so concerned about internal voids. The scaled down model always has an advantage in strength of the part, compared with the full size part, within limits.
But, bubbles which stick to the exterior of lost PLA/wax models are replaced by solid bronze, or whatever metal is used, and these metal bubbles can be tricky and difficult to remove.
I decided to remake the big gears, which were the subject of the last few posts, and the cause of a lot of bad language. I decided that the gear teeth were too skinny and pointy, and redesigned the part using larger module teeth. To compensate for PLA shrinkage and metal shrinkage I printed the PLA parts with a 2mm machining allowance.
I printed the PLA blanks, leaving the gears and brake drums as a blank lump which I will turn to shape after casting. But the internal decorative holes and ribs will be cast. And they are the site of many bubbles in previous castings.
So, to avoid the bubbles, I am trying something different in the current casting session. I am trying a method which was suggested by a GSMEE member, and that is to PAINT the first layer of investment material thinly on the tree components, making sure that no bubbles stick to the parts, then to pour the rest of the investment filling the cylinder. There might be some bubbles in the main volume of the pour, but they should not be sticking to the parts. That is the theory anyway. I am waiting for a bigger vacuum pump to arrive by post, which should be more effective at sucking out the bubbles, but meanwhile, I will try this…
Today, I heated 2 cylinders/moulds in the investment oven, and melted some bronze.
For the pour I tried the negative pressure apparatus.
I did not notice any change in the level of the molten bronze in the reservoirs when I applied the negative pressure, so I doubt that it added much to the process.
The results were like the curate’s egg… some good, some bad.
I am waiting for a more powerful vacuum pump to arrive by post. The 1/4hp one that I am currently using is too slow, when time is critical. I have a 1hp pump on order. No more casting until it arrives.
Just to recap, I made the trees and investment powder moulds about 2 weeks ago.
The first problem was that when I was adding the carefully weighed and expensive investment powder to the carefully weighed water, the f***cking scales timed out and shut down. So I had to guess the amount of powder to add. Then mix furiously. Then a briefer than normal degassing. All to be completed in 10 minutes. 10 minutes sounds like a long time. But it is all too short when there is any hick-up at all. (read.. “stuff -up”). #1.
I knew that the scales had a shut down timeout, and following the Chinglish instructions, I thought that I had turned the timeout off. But apparently I had not. After that I ordered some new scales.
Then I had to wait almost 2 weeks for the 15% phosphor copper to arrive.
Yesterday, after installing a new heater element in the melting furnace the day before, I fired up the investment oven and went through the drying, burnout and baking cycles, about 8 hours. And prepared another tree and mould for the next pour, in a few days time.
The melting furnace with its new element took a lot longer than previously to melt the bronze ingots, about double the time. But that was OK. Just had to ring SWMBO to warn her that I would be very late. (Got home about 9pm. She had organised take away.)
I added the 15% phosphor copper in approximately the recommended amount. And poured the first cylinder. But I forgot to wait the recommended 2″. #2.
Then I poured the second mould. And there was not enough melt!!!! #3. I had underestimated the amount of bronze! Those 4 big gears were guzzling the molten bronze! So what to do? Abandon the second cylinder? Or melt another ingot and just pour it on top of the first one after another 30-45″?. Nothing to lose, so I plopped in another bronze ingot, and waited for it to melt. Meanwhile I put the mould half filled with bronze back in the investment oven to keep it warm at 710ºc. Then, for some reason, the electronic controller of the investment oven shut down, and the element stopped heating. #4.
When the extra bronze melted I added bit more phosphor copper and completed the pour into the now substantially cooler mould which was already half filled with bronze which presumably had solidified. I assumed that the join between the 2 pours would be a problem. And it was.
Forgot my camera. Some photos to be inserted here later.
Somewhat surprisingly considering the guessed amount of investment powder at the original mix, and short duration degassing, the first mould was, apart from a few bubbles, completely successful. The second mould, with its 2 pours, was not a total disaster.
Today I sawed apart the trees, removed the bits of sprue and vents, and partly machined the parts. These are the bits.
The toothed gears and non toothed gears are the biggest gears on the cannon. And they include the drum brake, I was unsure whether the cast teeth would be adequate, or whether I would need to cut the teeth. It was not a fair test, in view of the interrupted pour, and the brief degassing. But I will cut the teeth from the blank wheels to the right. The toothed gears were probably the join between the 2 pours and total failure and will be remelted.
Even the blank wheels are not without problems. I did not allow for shrinkage, #5. and the diameter of the wheels is 57.5mm instead of the intended 58mm. So I cannot cut 56 teeth per wheel. I can cut 55 teeth on the smaller diameter. Still pondering that one. Do I repeat the entire exercise with a machining allowance? Or just be satisfied with one tooth less? I have not decided definitely, but am tending towards accepting a slightly smaller tooth count.
But, at least the brackets to the right came out well. This photo was taken after some belt sanding….not perfect, but not bad at all.
Incidentally, the T Rex is quite good. substantially less surface oxidation, and the oxidation layer that was present came off much more easily. Next pour I will add a bit more of the phosphor copper. And I will wait the recommended 2″.
My bronze castings had been coming out of the investment mould looking like this. I had been expecting to see a wonderful shiny golden bronze colour, and was a bit disappointed in the irregular black coating. But the casting itself was complete, with no voids, and having fine details like fingers, and 3D printing marks showing up clearly. And after laboriously attacking the black coating with Dremel wire brushes, files, sulphuric acid (ineffective), a lot of the black was removed.
This baby T Rex has found a home on my model dredger engine. The variegated skin colouring is reasonably convincing, but was not the shiny bronze colour which I wanted.
Maybe a problem with the LG2 bronze ingot? Something else?
So, I telephoned the supplier, Clingcast Metals, Sydney. Paul answered the phone, and he knew exactly what I was describing. “copper oxidation. Did you add 15 phosphor-copper shot to the charge? (The “charge”, I gather, is the crucible loaded with bronze pieces, for melting?)
“No. What is 15 phosphor-copper shot?”
“Small metal balls. If you add a tiny amount to the charge you will avoid the surface oxidation which you are experiencing, AND it will make the bronze melt thinner and run more freely.”
“Great. Where do I get it.”
“Oh. We have heaps here.”
“Would you post some to me?”
“Sure. How much do you want?”
Quickly thinking….”Maybe a kilogram”.
“That will last you a lifetime. $15, plus postage”.
“Fantastic.” – but I can’t find my credit card. Darn.
Paul. “I will post it. Just pay us when you get around to it.”
So, no more casting sessions until the 15 phosphor-copper shot arrives.
And big kudos to Clingcast Metals.
Meanwhile, I am making up trees and moulds.
Also meanwhile, I noted that Banggood have a special on melting furnaces which seem identical to the one for which I paid $425, and I have seen advertised at up to over $500. Banggood are asking $AUD290 + $20 p&p. So I have ordered one, as a spare. If you are interested I suggest that you check out this special, ASAP.
p.s. 2 Sept 2020. The 15% phosphor copper arrived today. I found this info on how much to add. I think that my 1kg will last a lifetime.
Description: 15% Phosphor Copper Shot is use as a deoxidizing agent for copper, brass & bronze alloys. It will also increase fluidity and inhibit gas porosity. The shot we carry is approximately 1/8″ x 1/32″ in size.
Usage: Melting of brass, bronze and copper should be done quickly in a slightly oxidizing atmosphere. The crucible is removed and surface is skimmed, then phosphor copper shot is added by simply dropping it onto the surface. The reaction is very visible. The metal will become fluid and bright. Allow a minute or two for the reaction to complete, and pour.
The phosphorus is a reducing agent (deoxidizer). This product must be carefully measured so that enough oxygen is removed, yet a small amount remains to improve fluidity. Too much phosphor can cause the melt to be so fluid that it leaks from the mold and penetrates the sand. A little goes a very long way!
The primary reason to use the shot is because the molds aren’t filling well or have gas porosity problems. The literature suggests that 1 ounce of 15% copper phosphor shot be added to 100 lbs of metal. A teaspoon of shot weighs about 1 ounce.
Start with these amounts: 100 Lbs add 1 Tsp. 50 Lbs add 1/2 Tsp. 25 Lbs add 1/4 Tsp.
For lesser amounts start with about 1 to 4 granules per pound of metal
One to 4 granules per pound of melt! It really will last a lifetime. My crucible will melt a maximum of 3kg/6.6lb.
Bevel gears seem to me to be rather difficult, even with CNC control of X,Y,Z and A axes. The bevel gears on the model Armstrong cannon are rather small, being 32mm and 14mm outside diameter.
I read Ivan Law’s book on the subject, and I think that I understand the requirements, and I was prepared to try and cut the gears. But, first, I decided to try to cast them.
Using “Gearotic” to design the gears, and save them as an STL file which was able to be imported into the 3D printer.
Made PLA gears with the 3D printer.
Attached the gears to a wax “tree”.
3 pinions and 3 gears. I need 2 of each. 1 spare of each. Plenty of venting sprues. And a head of about 70mm.
4. Then mixed the investment, poured it into the flask. At least that was the intent. The investment makers specify exactly 40:100 by weight of water:powder. But the bloody scales switched themselves off while I was adding the powder to the water, so I had to guess the quantity of powder. This was not looking promising. First bronze casting pour not off to a good start.
5. Dry the mold flask in the potter’s oven for 2 hours, then 2 hours of burning out the PLA and wax, then 2-3 hours of baking at 750ºc. A few minutes into the burnout phase, the oven died. ?heating coil failure, ? control box failure?, ?thermocouple failure, something else? So I replaced the control unit and thermocouple (I had a spare of each), but problem persisted. I rang my expert friend for advice. “sounds like a broken wire” he says. Suggested 3 or 4 things to try. And the 4th suggestion worked! The oven was working again! Brilliant! Thanks Stuart Tankard. So I restarted the oven at the burnout temperature (400ºc) and continued. Nothing to lose, after all.
6. Melted a couple of bars of LG2 bronze at 1100ºc in the melting furnace. Added a pinch of Borax. Let the investment oven cool to 710ºc for 1 hour to let the core of the mold cool to 710ºc.
7. Without any great expectations of success, considering the various problems, I poured the molten bronze into the mold flask. It seemed a bit more viscous and thick than I was expecting. Oh well. It is experimental.
8. When the mold flask had cooled to 150ºc, I plunged into cold water, and flushed out the investment.
Unbelievable. No voids. Hardly any surface bubbles. ALL teeth intact and complete. 6 good gears! You can see the head of molten bronze between the funnel and the top gear. It did not need vacuum or positive pressure.
I will turn the faces, bore the shaft holes, and if necessary file the teeth.
Totally delighted with this result. Beginner’s Luck.
Setting up for casting molten metals into shapes for my model Armstrong cannon. Still getting ready.
Today I made some moulds for dealing with any left over metal melt. Not a big deal, but it does have to be done before the first melt. No point realising that there is nowhere to put the left over aluminium or bronze during the pour. It has to go somewhere.
So today I made some ingot moulds, in readiness.
The ends of the moulds are sloped to allow easy ejection of solidified aluminium or bronze.
4 ingot moulds. Made from 40mm ID thick wall pipe, with long handles. The diameter of my crucible is 48mm ID, so any ingots made should fit into my crucible later for remelting.
It seems a long time since I have done any welding, and the welding of these items was pretty ordinary. But the joins seem water tight, so hopefully they will be OK.
Today I fired up the casting oven, to 850ºC, and the load was some ordinary food tins. They are the correct diameter for investment moulds. I wanted to see if the tins would cope with these temperatures. (after removing labels of course).
3 ordinary food tins, at 850ºC.
It became apparent, that the tin joins were welded not soldered. And the inside and outsides of the tins were covered with some sort of paint or plastic, because it flaked off. But the metal cans remained intact. Admittedly, when hot they were VERY soft, but when cooled they retained their shape, and were quite stiff. I would be prepared to try these for single use moulding projects.
I have realised that my investment plaster mixing bowl is too big for the vacuum chamber which I had bought. So I have ordered another vacuum chamber, and waiting for it to arrive before starting a mix. I am using the delay to gather items like the ingot moulds above.
It will probably be another couple of weeks before I am ready to cast. Meanwhile my 2mm rivets have arrived at last, so I will get back to the riveting.