# How does roto-casting work?



## PhilipMarlowe (Jan 23, 2004)

I'm sanding on my Anubis Blade Runner spinner, and I can't figure out how they did it. It's 17" long and 6" wide, yet only weighs 1.12 pounds. 

My curiousity was really aroused sanding, I can't find _any_ trace of a pour sprue at all! I mean nothing that even suggest where there was one but was sanded smooth at the factory. The only sign of any of the fabrication is a very slight line that runs around the circumference that looks like a mold part line.

The instructions say it is roto-cast, but offer no other explanation. How is it done, and why don't we see it on more garage kits?


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## PhilipMarlowe (Jan 23, 2004)

Google is a wonderful thing, in case anybody is curious about the first part of the question:



> What is Rotational Casting?
> 
> Rotational casting is the process whereby a liquid or slurry is poured into a mold and rotated by a rotational casting machine to coat the inside of the mold. When the slurry hardens the mold is taken out of the rotational casting machine and opened up to reveal a hollow casting. This process produces parts with a very fine finish. Airholes are eliminated and the finest of details are reproduced.
> 
> The benefits of rotational casting are a lighter weight product thereby reducing shipping costs, less material is used increasing profits, and a cleaner more detailed part is produced reducing patching and finishing time.


And the fact the machines on the site start at over $5,400 probably answers the second part.

Sure makes nice models though!


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## scotpens (Sep 6, 2003)

PhilipMarlowe said:


> Google is a wonderful thing, in case anybody is curious about the first part of the question. . . And the fact the machines on the site start at over $5,400 probably answers the second part.


Yeah, the guys who make this stuff aren't exactly getting rich, you know!

I was just about to post some links to rotocasting equipment websites, when I saw that you had answered your own question. Often someone on these boards will ask a question for which they could probably Google the answer. Is that because it's more fun to discuss things with fellow modelers, or are we all just a bunch of LAZY SLOBS?


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## Babaganoosh (Dec 16, 2004)

I took a class in high school called plastics and we learned how to do plastics manufacturing techniques like injection molding, casting resin, vacuuforming and rotocasting.

You took a mold made of metal what was in halves and filled one half with the materiel of your choice, polystyrene, polyethylene etc. and bolt the two halves together. You then put the mold into a cabinet that heated the mold and turned it in all directions. After a few hours to a day, we had our finished product which was probably a small ball or something.


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## Brent Gair (Jun 26, 1999)

scotpens said:


> Often someone on these boards will ask a question for which they could probably Google the answer. Is that because it's more fun to discuss things with fellow modelers, or are we all just a bunch of LAZY SLOBS?


The problem with googling everything is that it's no fun! 85% of all the questions asked could be answered by googling and then why would we need this BB?

We'd all just search through existing text and have no contact with each other.

On the Movies for Modelers board, pretty much every question is answerable with a two second search. Want to know what extras are on the new FROM THE EARTH TO THE MOON discs (  )? An easy google...then we can shut down the bulletin boards.

Asking each other is more fun.


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## justinleighty (Jan 13, 2003)

scotpens said:


> Is that because it's more fun to discuss things with fellow modelers, or are we all just a bunch of LAZY SLOBS?


The answer is: Yes.


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## PhilipMarlowe (Jan 23, 2004)

I had idly(or lazy slobbily) wondered about it since the kit arrived, but not finding any indication of a pour sprue _really_ got me wondering. The best explanation I could come up with before goggling was there was something (maybe inflatable) in the mold when it was poured, but looking over the one piece body I couldn't figure how they got it _out_!


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## Daikaiju1 (Apr 26, 2005)

*Yummy Roto casting*

Think back to your childhoods, guys. Ever see a pour sprue on an Easter Egg?? That is how they are made. Only chocolate. And cheaper. Umm, better go... :tongue:


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## razorwyre1 (Jan 28, 2004)

there are rotational casting machines that are much cheaper. (i could buy an rotational cassting accessory for my lab oven for a couple hundred bucks) i once saw a photo (it might have been here) of someone who made one from a 5 gallon pail, some plywood, casters, and a 3/8 inch black and decker drill. (on the other end of the spectrum i once visited a mannequin company that had rotational casting units that spun on 3 axises at once!). seriosl all you need is something that you can clamp the mold into that turns at a set rate of speed. 
the tricky part with rotational casting is the resin. you really need a resin that sets slowly and gradually (unlike the usual stuff garage kits are cast from). otherwise you can get a huge lump of resin on one side and very thin areas everywhere else.


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## Styrofoam_Guy (May 4, 2004)

I was wondering about the home made one with the drill. It spins on only one axis and would that be enough or does it have to spin on 2 or more axis to get the resin even through out the mould?


Alex
Styrofoam Guy


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## PhilipMarlowe (Jan 23, 2004)

I'm curious, my initial wrong self-explanation (putting an inflated ballon into the mold before pouring the resin) _seems_ like a pretty simple and elegant solution, albeit to a guy that has zero casting experience.

Since I don't think I'm _that_ smart to think of something nobody else has, what's wrong with the idea?


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## justinleighty (Jan 13, 2003)

PhilipMarlowe said:


> I'm curious, my initial wrong self-explanation (putting an inflated ballon into the mold before pouring the resin) _seems_ like a pretty simple and elegant solution, albeit to a guy that has zero casting experience.
> 
> Since I don't think I'm _that_ smart to think of something nobody else has, what's wrong with the idea?


Nothing, really. That's how you make papier maché things in grade school (or at least we did, making our own piñatas). The problem might be with the resin bonding to the rubber. You'd have a starship with a little balloon belly button.


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## TAY666 (Jan 8, 2000)

Part of the problem would be positioning of the balloon.
How would you make sure it was kept away from the mold surface far enough to get a thick enough pour all the way around it?


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## razorwyre1 (Jan 28, 2004)

tay666 is correct. resins are liquid, and balloons float. simple as that. it would be a nightmare tring to keep it from touching the sides. (im casting up some small cups for a client right now and even positioning the solid silicone core properly is a real bear.)

also the heat given off in the exothermic reaction would make the air in any balloon expand while the resin was still liquid, which would also cause some general anarchy inside the mold.

philip, i think what you have there is one of those ideas that seems good, but once you know even a little about the realities of the project at hand, you realize its not even in the ball park.


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## coldiron (May 19, 2006)

Bump. I joined this forum in hopes that some of you may have found some links or plans for a rotocast machine? Seems the cheapest machine I could find was around 2,900 usd. I am using this machine to make small and medium sized figures. 

Any help would be appreciated.


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## pagni (Mar 20, 1999)

PhilipMarlowe said:


> I'm sanding on my Anubis Blade Runner spinner, and I can't figure out how they did it. It's 17" long and 6" wide, yet only weighs 1.12 pounds.
> 
> My curiousity was really aroused sanding, I can't find _any_ trace of a pour sprue at all! I mean nothing that even suggest where there was one but was sanded smooth at the factory. The only sign of any of the fabrication is a very slight line that runs around the circumference that looks like a mold part line.
> 
> The instructions say it is roto-cast, but offer no other explanation. How is it done, and why don't we see it on more garage kits?


 There is one question that has not been answered here.
Is the kit actually hollow ?


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## TAY666 (Jan 8, 2000)

Yes.
Rotocast pieces are hollow.
That is why they are so light.
The mold spins on several axis so that the materali coats all sides of the mold evenly.

It's done all the time with large plastic items. Like outdoor toys for kids to play on. Those plastic storage units you see at the do-it-yourself home places. And other things along those lines.
That is how they make big hollow plastic parts.
Though the systems are much larger, and a heck of a lot more costly.


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## Atemylunch (Jan 16, 2006)

> I'm curious, my initial wrong self-explanation (putting an inflated ballon into the mold before pouring the resin) seems like a pretty simple and elegant solution, albeit to a guy that has zero casting experience.


You thinking on the right path. The process is called blow-molding. 
It is used to make 2 liter bottles(plastic). You could say balloons are part of the process. You wouldn't use resin, you would start with a thremoformer(Plastic you would use in a vacuum form machine.) Heat it up, place it in a mold, add some air pressure. instant coke bottle. 
Keep thinking like that. It's amazing what you can learn. 

Never be afraid to ask questions. You never know who might be on these boards. :thumbsup:


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## Tyboy4umodels (Apr 26, 2005)

Are roto casted parts more breakable than solid resin casting? How hard is roto casting to work with? Any help is appreciated.


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## big-dog (Mar 16, 2003)

The first saucer of my Alliance Ent-E was rotocast (well they said it was) and it was atrocious. The details were blurred, and the thing was essentially solid except for a hollow bubble toward the front on the starboard side where the resin was so thin you could see light throught it. Thankfully the resin wasn't mixed right and the soft, gooey piece was replaced (after many unaswered emails) with a solid cast saucer. The details on it are perfect. I suppose the blurred details could be due to the improperly mixed resin though.


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## Tyboy4umodels (Apr 26, 2005)

big-dog said:


> The first saucer of my Alliance Ent-E was rotocast (well they said it was) and it was atrocious. The details were blurred, and the thing was essentially solid except for a hollow bubble toward the front on the starboard side where the resin was so thin you could see light throught it. Thankfully the resin wasn't mixed right and the soft, gooey piece was replaced (after many unaswered emails) with a solid cast saucer. The details on it are perfect. I suppose the blurred details could be due to the improperly mixed resin though.


So Roto Cast would not be the way to go then for a Resin Model? The reason I am asking about how hard it is to work with and now because of the problem you have had with it, is because of lighting purposes. Or if it is even possible to light a resin model?


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## capt Locknar (Dec 29, 2002)

Roto Casting is good for very Large volume Resin Models (volume as in displacement not sales). It makes the kit hollow but not lightable (unless you take the bandsaw to it and slice it down the middle. 
As someone above stated, think of a Hollow Roto Cast Model as if it were a hollow chocolate easter egg. Its hollow but unless you crack it open your not gonna get lights in it. 
Smaller volume resin models (again Displacement not quantity) are better left solid cast as hollow cast would present several problems.


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## pagni (Mar 20, 1999)

TAY666 said:


> Yes.
> Rotocast pieces are hollow.
> That is why they are so light.
> The mold spins on several axis so that the materali coats all sides of the mold evenly.
> ...


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## PhilipMarlowe (Jan 23, 2004)

My 17" Blade Runner spinner is hollow throughout, you have to cut into it to light it since it's a one piece body. The walls are a uniform appox 1/4" thick thru out.


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## pagni (Mar 20, 1999)

Thank you for the response !


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## capt Locknar (Dec 29, 2002)

If you read post number 17 You will find that your Hollow question was answered there as well.


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## TAY666 (Jan 8, 2000)

pagni said:


> So, once again my question is:
> Is the kit actually hollow ?


Thought I answered that when I said


> Yes.
> Rotocast pieces are hollow.


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## Tyboy4umodels (Apr 26, 2005)

capt Locknar said:


> Roto Casting is good for very Large volume Resin Models (volume as in displacement not sales). It makes the kit hollow but not lightable (unless you take the bandsaw to it and slice it down the middle.
> As someone above stated, think of a Hollow Roto Cast Model as if it were a hollow chocolate easter egg. Its hollow but unless you crack it open your not gonna get lights in it.
> Smaller volume resin models (again Displacement not quantity) are better left solid cast as hollow cast would present several problems.


Thanks Cappy, I appreciate the help. I was thinking about getting a roto cast model kit eventually and I was wondering about lighting it since it is hollow. I don't really want to cut it in half to do it though,it will ruin the detail of the model,and once that is ruined,well anyone who has done resin kits knows that detail once it gets goofed up is very hard to fix.Again my thanks.


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## pagni (Mar 20, 1999)

Tay and Locknar...
With all due respect.
* No it wasn't.*
What WAS answered was the general rule that rotocast vinyls are hollow.
I already know that. Please reread the posts.
What was NOT answered was the question: *Is the SPECIFIC model in question actually hollow ?* There was really only one person who could answer that question, the original poster to this thread. Phillip Marlowe.
He has answered the specific question (thank you Phillip)


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## PhilipMarlowe (Jan 23, 2004)

Tyboy4umodels said:


> Thanks Cappy, I appreciate the help. I was thinking about getting a roto cast model kit eventually and I was wondering about lighting it since it is hollow. I don't really want to cut it in half to do it though,it will ruin the detail of the model,and once that is ruined,well anyone who has done resin kits knows that detail once it gets goofed up is very hard to fix.Again my thanks.


You don't neccessarily have to cut a rotocast model in half, on the Spinner I cut out a approx. 4" raised panel that was on the bottom of the chassis out of sight, it's big enough to allow access to the whole inside easily. I could have gotten away with a much smaller opening if I had needed too. You will have to cut into the model, how big a hole will depend on the size and shape of the model you are attempting, and on the size lighting system you are installing. 

But you wouldn't want to start by sawing it in half, that'd be serious overkill.

And yes Pagni, the Annubis spinner is hollow thru-out, including the front wheel "booms", which is what I cut into it to check.


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## Tyboy4umodels (Apr 26, 2005)

PhilipMarlowe said:


> You don't neccessarily have to cut a rotocast model in half, on the Spinner I cut out a approx. 4" raised panel that was on the bottom of the chassis out of sight, it's big enough to allow access to the whole inside easily. I could have gotten away with a much smaller opening if I had needed too. You will have to cut into the model, how big a hole will depend on the size and shape of the model you are attempting, and on the size lighting system you are installing.
> 
> But you wouldn't want to start by sawing it in half, that'd be serious overkill.


Thanks for the tip on that,I appreciate it.


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