Rotational Molding Casting (Centrifugal Casting)

How similar to concrete? How long does it take to set up?

If like concrete it's a mixture. How do you keep the mixture from separating due to centrifugal force as you increase rpm.

Concrete takes a while.

Glass Fiber Reinforced Concrete

It takes about 20 minutes to gain some strength and about 45-60 min to develop good stiffness

So far we have been spraying the material and we have not seen any separation yet

Concrete is cement,sand and some type aggregate. You just added fiber glass?

I remember to have seen a machine for centrifugal casting of concrete pipes (steel reinforced). The form was horizontal and when I estimated the (r*ω^2) it was over "g" I do not remember numbers any more. The concrete flew over a kind of guide which run axially low speed and let it in the form. After pouring the machine was accelerated for a time and after it stopped, the form opened to take the part out to the steam room where it stayed for a number of hours.

You typically rotate the mold in two axis for good coverage.

Below is a basic explanation of the process:

Rotational Molding - MFG 355

This is a picture of a mold installed into the frame that would be attached to the spinning ↓machine that rotates the pot mold in two axis.

name：long flower pot mold

Speed would not have to be too great, since you'll have 20-30 minutes before it hardens, but better to experiment.

Great information it has opened my eyes with all your posts.

Still have a couple of questions like:

RPM's will have to be evaluated through several attempts?

Since I'm going to be casting pots then I would need to do the same thing on the inclination, through several attempts so I can validate which degree will give me the best result.

Because the objective is to have the matrix stick to the sides and the bottom only so I can have the top end open?

Once again thanks.

Any other comments are gratefully received.

You MIGHT be able to do this in a single axis, but.................. you're looking at a much higher spin rate to make the matrix "climb" the walls of the mold.

Typically roto molding coats all inner surfaces of the mold with a consistent thickness of resin.

You're almost back to spin casting a pipe and adding the bottom later. Unless your walls are straight, you'll have thickness issues too.

This is basically the shape of my tool and it does have straight sidewall.

Have you considered just making a matching plug to form the inside?

Then no spinning would be needed.

Yes we have a male insert that forms the part by displacing the matrix.

My boss is challenging us to present this rotational casting too.

It will not be cost effective, unless you already have all the equipment in hand and just want to use the different method.

You can only make one pot at a time, or two with a roto machine. I'd spend the money on more molds and make multiple pours at one time.

The biggest challenge you have is to reduce the "in-mold" time.

Why this material?

Give us more to work with.

GFRC is what we are using because it has showed better performance under extreme conditions like freezing and heating without cracking.

Also it's lighter than regular concrete and resist breakage when dropped at certain height.

If we can find a way to learn the rotational molding it can easily be transferred to other lines of our products like columns.

So far, roto molding doesn't seem to be the process you need.

How many products do you have?

Are they structural?

Practically speaking, roto molding is well suited to building tanks and other closed objects.

Ornamental objects like pots and columns can be made out of styrofoam.

Need more info.

I think columns would be better made in a horizontal position, similar to centrifugal casting of metal tubes. I'm pretty familiar with that process. The main challenge I see for your application would be that you would need to place the cement along the length of the mold; whereas molten steel can just be poured in one end and readily flows all the way to the other end, your material (if it's as stiff as I envision it) wouldn't. And even if it would, you probably shouldn't because of likely component separation.

For your tapered pots, vertical axis would be better. The speed would need to be such that the centrifugal/centripetal force tending to carry the material up the wall to the larger diameter would be balanced by the gravitational force. The 2 challenges I see are 1) placement of your material, and 2) as someone, Lyn I think, pointed out, making the bottom.

It is problem of viscosity. To pour the concrete it has to be very liquid and flow to fill all volume full without entrapped air.

Centrifugal casting does not require such a fluid mixture an due to the radial force there is a pressure which takes air and water out thus shortening the process.

Concrete is not a resin its behavior is more as Bingham fluid, resin is viscous but behaves more as a Newton fluid.

The machines I saw were for ducts about 20 to 40 " diameter and length up to 30'. The final accelerated rotation was to increase density and push water out.

I would suggest you contact the manufacturers of this type of equipment such as Hawkeye-Pedershaub, Besser, Hydrotile- McCracken etc and ask if they can help. Why reinvent the wheel if you don't have to.

Good luck