From the Shop: Spin Caster
I have decided, that I need a spin caster to cast smaller parts with lots of detail. I need the caster for my next project. Casting will be in silicone and with zinc-alloys. This silicone is a special heat resistant co
mpound that withstands 500°C (932°F). The zinc's pouring temperature is at about 400°C (750°F). A mold will be good for at least 20 castings.
Zinc is a quite stiff material and it's mechanical properties are comparable to brass. Do not confuse it with white metal. White metal is very soft, and doesn't serve for my intended use. Zinc's only disadvantage is, that it's working temperature must be below 100°C (ca. 200°F) and the high weight.
I won't describe how to make the molds. Simply, because I don't have enough experience (read: one experimental cast). But I'll add that later, as experience grows. But you also should know, that I made one casting in plaster that gave so promising results, that I'm convinced that castings in silicone will come out very good.
So for now, I'll just describe the caster.
What is Spin Casting?
Spin casting is just one kind of pressurized casting. Nowadays, high volume castings with zinc are made with steel molds. Making these molds is very expensive. This pressurized casting with steel molds uses pressures well above 80 bar (ca. 1200 psi). The process of spin casting is much older and is mostly used for low volume (in the hundreds) or even one off parts. It is well known in jewelry work. In the case of spin casting, the pressure is obtained by centrifugal forces.
Planning ahead:
I made some research and asked in news:rec.crafts.metalworking where I got good advice and some links. Based on how others made their casters, I made my design.
I settled for:
Diameter of the disk: 300mm
RPM: 1400
Other numbers I found for the speed were 400 and 800 rpm for about the same diameter, or 1000 for even bigger disks. Another speed was "I don't know".
So rpm is something to think about. A little math will help and show what happens.
As said, the pressure results from centrifugal forces.
ar = w2 • r (radial acceleration = angular speed • radius)
w = 2p • f (angular speed = 2 Pi * frequency)
f = rpm / 60
With r = 150mm and 1400 rpm we get:
ar = (6.28 • 1400 / 60)2 • 0.15 = 3220 [m/s2]
3220 m/s2 are equal to 328 times the G-force!
With the acceleration, we now can calculate the pressure in the mold:
p = F / A (pressure = force / area)
F = m • a (force = mass • acceleration)
m = V • r (mass = volume • spec. weight)
V = A • h (volume = area • height)
Assembling …
p = m • a / A
p = V • r • a / A
p = A • h • r • a / A
p = h • r • a
Units…
m • kg • m-3 • m • s-2 = kg • m-1 • s-2 = N / m2 = Pa
with …
r(zinc) = 7 kg/dm3 = 7000kg/m3
a = 3220m/s2 / 2 = 1610m/s2 (average acceleration from center to radius is half the acceleration at the radius)
h = 0.15m
… we get:
p = h • r • a = 0.15 • 7000 • 1610 = 1690500 [Pa] (that are 16.9 bar, or 245 psi)
What we see from the formulas above is much simpler than it might seem for the physics-challenged:
If we double the radius the mold is rotating along, we get double the pressure. If we halve the radius, we get half the pressure.
If we double the motor's rpm we get fourfold the pressure, if we half the rpm, we get a quarter of the pressure.
So rpm is one parameter to tweak, if the mold either shows too much distortion or isn't completely filled with metal
Building the real thing:
I mostly used square tubing with 30 * 30 * 1.5mm. You could use something lighter like 20*20*1.5, but the extra weight and stiffness only makes things better.
Some sizes:
Box is: 400mm * 400 * 410mm
Base disk is: ø300mm, 8mm thickness
cover disk is: ø315mm, 5mm thickness
Motor has: 400W. I had this one at hand. You don't need that much power. 200W would be OK, but then spinning up will take a few seconds more. With the 400W it takes about 3 seconds to come to full revs (without mold).
The studs holding down the cover plate are M6, 100mm long.
The maximum mold size is ø260, 80mm high. Those 80mm might be a bit on the big size, but time will show.
One more thing that isn't obvious but worth noting:
I built no stand for the spin caster. This way, you can tilt it to one side and balance the whole setup.
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The frame of the spin caster with the brake installed. The brake is the lever reaching out of the cube on the left front. |
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The caster with the motor and the base disk installed. You can see how the plastic brake shoe works with the base disk.
Don't think that the electrical installation was even near to final! The frame is connected to PE of the cable! |
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This is just another view of the spin caster and it's brake. The shot was taken in an earlier stage, when the frame was not jet painted and I assembled everything to see how things fit.
Note that the motor is fully capsuled, so no metal can fall inside. IIRC, it was IP 54. |
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The final spin caster. The lever in the lower left is for the brake. The switch in the lower right is… well, the switch to turn the motor on and off. The hole in the cover is where the tube of the cover plate can be reached to fill the metal in. |
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If you are unpolite enough, you might call the spin caster a centrifugal cat toilet. But beware! Either me or Hamdi -the cat- will bite you!
While we are at being unpolite, I have to admit that the caster looks more like a furniture or some Jack-in-the-box-device. Next time, I will use a more "tool-ish" color. |
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Here is a look into the working room of the spin caster. You can see the base plate with 3 of the 6 (possible) studs installed. The cover disk can be seen from below, with the hole in the center where the molten zinc passes through to the mold.
Also, you can see the grooves that help aligning the mold on the base disk. Because my lathe is not big enough, I had to mill the disk in the mill on a rotary table. |
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The cover plate installed. Now you can see how the quick release works.
The key holes' bigger diameter is big enough for the nuts to pass through. The disk is turned and then the nuts screwed down. These nuts have a recess that fits into the smaller hole of the key hole.
Yes, I will knurl the nuts or make them easier to handle by some means.
The center tube is just some aluminium tube that was hammered to a smaller diameter at the top. This way, the molten metal will not spread all over, but will be forced downwards. |
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This is my first (and for now my only) casting. Note that I used a plaster mold. I broke that mold before pouring and had to glue it back together. So this is the reason for the thick flash I got. You also see on the knob in the right, that I didn't melt enough zinc and that I had a lot of bubbles in the mold.
But if you subtract all the mess I made (say: I wanted to see what I can do wrong), the casting is quite good. The part in the left shows scratches from file strokes that the original had. Also the 2 pits were in the original.
I know, that I will get much better results from a silicon mold. |
Things to improve
For now, I'm quite happy with what I made. But there is always something to make better or to watch out when you are building your version of a spin caster:
- Get a motor that can be (later) regulated.
- Use the formulae from above to get the right acceleration/pressure range. Going higher than I did seems unnecessary.
- I will see whether the hole in the cover needs some modification. I had a longer tube in the upper clamping disk, but the melt seems to cool down a lot in there. Probably, you find some isolation that fits the tube, so yours can be longer.
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