Roto Casting

Sparky · Post by **Sparky** » Mon Mar 13, 2006 12:31 pm

Does anyone have links to their roto-casting setup? I'm looking to see what is needed to do resin roto-casting.

If I use a slow set resin can I make something with a low rpm that will do the trick

novahobbies · Post by **novahobbies** » Tue Mar 14, 2006 12:26 am

I built one for the Negh'var main hull when I was casting that -- it was way too large to solid cast it, and I didn't have the funds to go out and buy one. Mine was entirely people-powered, though. I stood there and turned the frames slowly and evenly to get full coverage. The ones that other companies have used are better, though....motorized, even with a timer to (don't shoot me) "set it and forget it." I have a little video of the process somewhere, but I don't have the capabilities of hosting vids. Basically it was a smaller wooden frame nestled within a larger frame, which in turn was set between a stand. one frame moved X, the other Y, and I got pretty good coverage. The mold itself was clamped to a small sheet of plywood the constituted the back of the inner frame.

Schadenfreudian · Post by **Schadenfreudian** » Wed Mar 15, 2006 7:11 pm

I put the moulds in a plastic bag and swing it round my head - works every time.

Bar · Post by **Bar** » Fri Mar 24, 2006 2:56 pm

Schadenfreudian wrote:I put the moulds in a plastic bag and swing it round my head - works every time.

The old "enough room to swing a cat" technique...

Schadenfreudian · Post by **Schadenfreudian** » Fri Mar 24, 2006 5:00 pm

Bar wrote:
Schadenfreudian wrote:I put the moulds in a plastic bag and swing it round my head - works every time.
The old "enough room to swing a cat" technique...

But ironically in a flat that strictly speaking isn't really big enough to swing a cat...

Seriously, though, I do actually employ two different 'swinging' techniques, depending on the size and type of mould.

So far, all I've done resin-wise is make a kit of a battleship, around 23cm in length, to give an idea of the size of the parts I deal with. The hull is the largest mould, and when the resin's been poured and the mould clamped together, it goes in a bag standing on its head (or bow) and gets swung back and forth like a pendulum - the stern has a channel going into a cavity, also filled with resin, such that the swinging forces any air bubbles up and into the cavity where it accumulates away from the hull.

Small moulds, for turrets, funnel, etc., go in a plastic bag which is then hung from a metal bar held at arm's length in front of my body - I then spin it round and round for a minute or so in a vertical plane, just missing my face (clearly, be careful with the size of bag!). Works a treat.

Bar · Post by **Bar** » Fri Mar 24, 2006 5:07 pm

This is highly interesting. I always wondered how roto-casting was done.
I have two of the Alliance Delta flyers(And i was amazed at how light the castings were).
I am genuinely curious: How do you calculate the amount of Resin required?

Sparky · Post by **Sparky** » Fri Mar 24, 2006 5:32 pm

I sounds like there are two things here, the swing in the bag I think he is using instead of pressure casting. The roto-cast technique (I get the name from websites that sell roto-casting machines) is for making simi-hollow castings.

I think you just make a test cast and add resin as need be. Maybe make a solid cast then reduce resin based on a surface area estimate.

Schadenfreudian · Post by **Schadenfreudian** » Sat Mar 25, 2006 5:36 am

Sparky wrote:The roto-cast technique (I get the name from websites that sell roto-casting machines) is for making simi-hollow castings.

Ah, I see, I hadn't realised that was what he was asking about, I'd assumed just a way of rotating moulds as an alternative to pressure casting.

I'll get me coat...

Bar · Post by **Bar** » Sat Mar 25, 2006 6:54 am

It's all helpful info guys.
Rob(Scifimodels) assures me that the 1/24 F-302 i am mastering(Soon to be completed), will be roto-cast for weight saving(It's over 60cm wide...).
I was just interested to hear about the process, and what it involved, just for my own information(I will never even try casting, as there are many people out there who are good at that.
I'll stick to what i know, but i still like to know how it's done.
So. There are specific machines available for roto-casting?

Bar.

Sparky · Post by **Sparky** » Sat Mar 25, 2006 7:29 pm

BTW the spin casting is good if you want to know how the metal mini's are made. But I thought it needed to be high speed, good to know that spining it by hand is enough to get the bubbles out.

Yikes 'commerial' casters cost!

http://mannetron.com/roto.htm

http://www.plastermaster.com/rotation/protomo/index.htm

They seem to be made for folks casting ceramics/plaster items. A definite for things like lawn art.

Sparky · Post by **Sparky** » Sun Mar 26, 2006 6:17 pm

I setup a hooky rigged caster, one axis and made a test cast with the <A HREF="http://www.kc6sye.com/techmages_5_15_05.html" target="_blank">tank hull mold</A>

Looks good accept I pulled it early and worked to peel the styrene backing off, putting an indent in the turret mounting point. I'll have to cut the bottom open to see the coverage better.

Weights: Solid hull 712 grams, hollow 224.5

I'll snap some pics and try to post them latter.

Go Flight · Post by **Go Flight** » Sun Mar 26, 2006 10:05 pm

There was just an article about out to do it cheaply in either the last issue of AFM or Kitbuilders (I don't have either in front of me right now).

Sparky · Post by **Sparky** » Mon Mar 27, 2006 12:12 am

I would like to see the article, if anyone can get the issue and or scan of the artical for reference I'd like to put it on file.

On a side note I have the second to last copy of the IPMs journal where a fellow setup a vacuum pump using th ~$10.00 venturi effect device harbor freight has, seems to have worked well for him. I have one but found it difficult to get a good setup, jsut went ahead and bought a pump.

Go Flight · Post by **Go Flight** » Mon Mar 27, 2006 1:21 pm

Sparky wrote:I would like to see the article, if anyone can get the issue and or scan of the artical for reference I'd like to put it on file.

On a side note I have the second to last copy of the IPMs journal where a fellow setup a vacuum pump using th ~$10.00 venturi effect device harbor freight has, seems to have worked well for him. I have one but found it difficult to get a good setup, jsut went ahead and bought a pump.

I just took a look and it IS the current Kituilder magazine (#53) - the one with Kong fight T-Rex on it - http://www.kitbuildersmagazine.com/NewIssue.htm

I don't have a scanner but if you can't find that issue, PM me your address and I can make a hard copy of the article.

In short, he took one of those large wooden reels thats used to carry wire, and cut a hole in the center spindle, put the mold filled with resin in the center spindle and he and his buddy rolled the think back and forth in his driveway. He knew that the resin cured in so many minutes, so he just rolled it for that many minutes plus an extra minute or two to be safe. These were for LARGE parts for a project he was doing. I'm sure something smaller could be constructed for smaller parts.

edit - - I forgot to mention the article is entitled Poor Man's Roto Tool by Jason "Spyda" Adams

Sparky · Post by **Sparky** » Mon Mar 27, 2006 8:08 pm

That's good idea, those wire spools can be real tuff built. I took some pics of the rig I have. I'm a bit schizo when it comes to the hobby room, I like to pour the resin and then move off to fiddle with another project.

Right now the system runs off a 12-volt battery, easily replaced with a power supply. But what I really need to add is a timer module. I had to get a lid too so things don't flop out, the tank hull stayed in for almost the entire run before flopping out.

Oh and another set of motors arrived today from allelectronics, so now I need to try to rig those to at least agitate the other axis.

Sparky · Post by **Sparky** » Mon Mar 27, 2006 8:22 pm

Ok with the extra lip and bottom dremelled off she weighs in at 158grams. I wish I'd logged the amount of resin I used, but I did put some in the smaller tank hull and turret mold neil had (another porject, ya neil liky the tankies).

<A HREF="http://www.kc6sye.com/images/images_03_ ... acklit.jpg" target="_balnk">Top of the hull with backlight</A>

<A HREF="http://www.kc6sye.com/images/images_03_ ... _blobs.jpg" target="_balnk">Two big blobs formed right at the bottom plate and model's edge</A>

<A HREF="http://www.kc6sye.com/images/images_03_ ... _vents.jpg" target="_balnk">The side vents are a little thin, so opend them up! they need vent covers anyways.</A>

Ok the 'setup:'
<A HREF="http://www.kc6sye.com/images/images_03_ ... r_btry.jpg" target="_balnk">Surplus dish rotor, from allelectronics, and 12 volt battery</A>

<A HREF="http://www.kc6sye.com/images/images_03_ ... g_bckt.jpg" target="_balnk">5 Gallon generic bucket becomes the casting bucket</A>

Just need a lid and some packaging foam to wedge all the molds in-dar.

BTW the dish rotor's are no longer in stock at all' so I orderd some heavy duty windshield whiper motors, hope one can be added to help with moving the other axis.

Sparky · Post by **Sparky** » Wed Apr 12, 2006 3:36 pm

Bump . . .

Here's the start of how I did it page:
http://www.kc6sye.com/techmages_3_27_06.html

I added a motor with a small throw on it for the x-axis. It's a bit violent/loud. It dint' workout. There's something to be said for a good visual example of what is normally an abstract idea.

My first experience with this was capacitor leakage in Tesla coil circuit. You hear it and see it in calculations but what does it mean. Seeing electric spark/fingers flowing out from the edges of aluminum foil plates to wrap around the edge of plate glass and meet up with the next plate's fingers is well self evident.

In this case the abstract idea of bias is demonstrated. When making calculations with computer software you usually have a round up or round down issue, you pick one and always round that way. The problem is this gives you a bias somewhere above 0, depending on your resolution this can be large.

In the casting experiment the center point of the new motor's arm is behind the center point of the casting bucket by 1/4 to 1/2 an inch, but the arm's total throw is no more than 5 inches (I'll measure it tonight). The net effect is a very thin casting for 2/3rds of the casting, and a weird effect that resulted in a partition like wall of resin forming in the forward half of the hull.

I'll try to take a picture of this casting just for the record.
Next steps:
Modify the contact point of the arm, so that the neutral point of both is equal, a 2x3 looks to do the trick.
External AC power circuit (the battery's cute an all. . .)
Document the new motors mount.
Look into adding a drive system so this motor isn't loaded as much and has more throw (it moves fast enough we can sacrifice rotations for torque).
Look into a timer setup for the system, for the cure times of my favorite resins.

Sparky · Post by **Sparky** » Tue May 23, 2006 2:58 pm

Another bump update:

The extra axis seems mostly useless. It may be that I need slower and greater range of throw to make it usefull. Mostly I just changed the orientation of the mold so that the one axis rotation provides the maxium coverage.

Also I cutup a new base and 'wheel' mount. These are thinner 1/4 inch plywood. Installing them will require a complete dismantle so I've put it off. Maybe when I get a timmer module and power supply setup to test I'll do a full upgrade.

I got another spent mold to play with and see how the roto-caster works with it. Its a completely different [shaped] part than the tank hull. I probably won't get to test casting it till after wonderfest.

Maybe I can pick up another gallon of resin while i'm there. saving on shiping and handling costs makes it worth it.

Bar · Post by **Bar** » Tue May 23, 2006 3:52 pm

I can't wait to see pics.
This is a subject i have mucho interest in.
Bar.

Sparky · Post by **Sparky** » Tue May 23, 2006 3:59 pm

The two pics on that one page is mostly it!

Its a piece of wood mounted by hinges to the end of wood table. This lets the 'x-axis' rotor-motor and bucket assembly hang and swing freely. I added the wipper motor to the leg of the table, but it doesn't have enough throw to be effective and it rotates to fast.

So the bucket sticks out from the end of the table, I throw in the molds and push the lid on enough so it cant pop off then clip the postive lead back onto the battery. There's a charger added when the battery gets low, the wiper motor draws a lot of current even in standby but it's not hooked up anymore.

I'll take some more shots the room is better lit during the day. Those shots were at night when I had finished mounting the thing. I'll also try to take a mpeg clip of it running.

Bar · Post by **Bar** » Tue May 23, 2006 4:16 pm

Sorry. I meant pics of the "fruits" of your roto-casting machine...
My mistake: I should have made that clear.
Bar.

Chacal · Post by **Chacal** » Tue May 23, 2006 5:18 pm

Is the second (inner) motor really necessary? Can't the inner frame be spun by a gearing of the relative motion of the outer frame against the stationary base?

Darth Humorous · Post by **Darth Humorous** » Tue May 23, 2006 5:31 pm

Sparky wrote:There's a charger added when the battery gets low, the wiper motor draws a lot of current even in standby but it's not hooked up anymore.

Say Sparky,

being that the wiper motor is D.C., have ya thought about in-series resistive loads or a rheostat to control its speed? Not elegant, but Q&D to see if it works. For a cheap way to make a step-like rheostat, I've wired old utility porcelain flange sockets in series with one another and used light bulbs for the resistors. You can wire as many as you like, and use different wattages for a variety of resistances. I also placed parallel wired bypass switches for each stage of fixtures for a quick way to reduce resistance. I had this setup once upon a time for testing and temporary control of electrical devices. Works with A.C. and D.C., although for A.C. a dimmer is quicker and more elegant.

I suppose ya could use line power by putting a full wave bridge rectifier on the output of a dimmer, and have that contraption wired in series with the light bulbs for a little better control. In that situaton, one or two sockets with appropriately sized bulbs may be enough.

Hey, if nothing else, this kind of stuff can look cool, like antique electrical experimentation apparatus!

Just a thought.

Mark

Sparky · Post by **Sparky** » Tue May 23, 2006 6:27 pm

The claim on the wiper motor is it won't run with out very near the full voltage. There is circuitry in side that keeps the motor locked in placed. A small ground wire enables it to rotate; if you make the connection briefly it will make one rotation and stop at the neutral point.

Now if you leave this connection attached and just pull the positive lead you can rotate the arm partially. If use of this motor shows that more thorough coverage can be had I'll setup a pulse width modulator circuit to slow it down. I have one configured for dimming a light. I tried this on a motor once and I think the problem is the working range wasn't useful for a motor (duty cycle and frequency).

As far as other means of building the test fixture so one motor can rotate both axis. Well I'm not so good with mechanical trickery. The motor in the pictures is only one motor; itâ€™s a gear-reduced motor driving a rotating platter, which further reduces its speed. This motor will run at 6 volts, since its the same kind we used for the one that drove the space station project.

tonyG2 · Post by **tonyG2** » Thu May 25, 2006 2:38 pm

Adding my own enquiry to the mix here. If you go for the MacGyver appraoch and make you own roto-casting set up - and I just love Schadenfreudian's technique of swinging the mold in a bag about your head

, if you are going for what is essentialy a single axis rotation, how can you acheive even coverage.

I would think that the "swing the cat" method will conentrate the resin at one end of the mold and a single axis "spindle" approach would leave the ends of the mold (at the axis of the spindle) with insufficient coverage.

Chacal · Post by **Chacal** » Thu May 25, 2006 3:51 pm

The roto-casting setup with the frames and stuff is to cast a "shell" inside a closed mold, to lighten up a big part (by making a cavity inside it without connection to the outside â€“ a bubble of resin, so to speak). It is achieved by rotating the mold slowly enough to get the resin flowing inside it without whipping air bubbles into it, but fast enough to get the whole inside covered before the resin hardens. The two-axis setup assures that all of the inside is covered (as long as the rotation speeds of the axes are not the same â€“ 1:1 or in some "easy" ratio, like 2:3 or 2:1, which would not guarantee coverage without too much resin/too thick a shell)

Spin casting is another beast altogether. It is like the swing-the-cat approach: "centrifugal force" used to aid the casting (Train wheels are done like this â€“ the round mold is spun very fast, the molten steel is poured in, the pressure from the spinning makes the dense pure metal go to the edge of the wheel, the light slag and air bubbles float "up" to the hub and are machined away when the hole for the axle is drilled/turned)

tonyG2 · Post by **tonyG2** » Thu May 25, 2006 4:16 pm

Sort of figured I was mixing apples and oranges there.

I've done the spin casting for white metal models on a home made rig using a power saw to spin a turntable with the mold strapped on top.

Although it cast some really nice figures - much better than the pour and pray method - I wouldn't recommend it for home use unless you are comfortable with the notion of white hot metal spraying round the room

I used to have the burn marks on my chest as proof of how stupid an idea it actually was. Fotunately in this case I can honestly say that we live and learn

Schadenfreudian · Post by **Schadenfreudian** » Sat May 27, 2006 12:41 pm

tonyG2 wrote:...I just love Schadenfreudian's technique of swinging the mold in a bag about your head , if you are going for what is essentialy a single axis rotation, how can you acheive even coverage.

I would think that the "swing the cat" method will conentrate the resin at one end of the mold and a single axis "spindle" approach would leave the ends of the mold (at the axis of the spindle) with insufficient coverage.

None of this motorised malarkey for me, I'm a cheap git...

You're absolutely right that my method would give very uneven coverage - I'd actually misunderstood the first post on this thread, and confused 'rotocasting' with 'spincasting' (like, duh...)! What my cheap-as-chips technique does is to spincast (using the term as excellently defined above by Chacal) solid castings. I think if I had a go using my method for rotocasting it'd be a disaster!

BTW, although nothing I'm currently working on lends itself to rotocasting, this has been a very interesting thread, with great tips, and some marvellous ingenuity on display!

TonyG · Post by **TonyG** » Sat May 27, 2006 2:15 pm

some marvellous ingenuity on display!

Although my patent hot white metal concept on a Black and Decker should carry a government health warning

Schadenfreudian · Post by **Schadenfreudian** » Sat May 27, 2006 4:58 pm

TonyG wrote:Although my patent hot white metal concept on a Black and Decker should carry a government health warning

Ow!