Rotomolding experience anyone?

Sure its not metal but it's not politics or global warming!
I was wondering why rotomolded plastic water tanks are so expensive.
The raw materials are generally regarded as cheap, the manufacturing process
seems on the surface to be simple. I guess transport / distribution of a
bulky product could be costly. Anyone got some ideas?
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The process is simple---bolt a steel mold together, dump in a quantity of plastic powder, heat and rotate/tumble/turn through 3 axis while the powder melts and coats the inside of the mold. Cool, open mold and remove part. On smaller parts like spot sprayer tanks, there may be several molds on the framework at the same time. Big parts like water tanks are one or two
The process is time, labor and energy intensive, in addition to being very inefficient on freight.
Where are you? Might be cost effective to do a pickup at the factory.
Reply to
Bill Marrs
How big? They are formed in ovens MUCH larger than the tank, and the mold cycle times are very long when compared to injection molding.
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Also there are economies of scale involved. Not physical size but volume.
There are probably an uncountable number of guys here who could take make you a machine nut by taking a piece of steel, drilling a hole in it, taping it, and then cutting the outside so you could grip it with a wrench. It would probably take me an hour to make one nut that way in my shop. I charge $80 for my time (I do not make that much by a long shot). There are a few more guys in this group who could take iron ore. melt it, add some other stuff, and then produce your one nut in their shop.
Or, they could buy some production equipment for a few hundred thousand dollars and make your one nut.
In any case the material is cheap. In the first case the labor is expensive. IN the second the equipment is expensive, and for one nut the setup, electric, fuel, and labor are expensive along with the several hundred thousand for the production tooling in the factory. However, if they make one nut or a million the tooling cost doesn't change. In fact the tooling cost probably doesn't start to change much from tool replacement and repair until they have made a few hundred thousand of them.
So in the first case it costs $81 for one nut.
In the second it costs $500,000 to make one nut.
Now add in overhead, additional fuel, insurance, repairs, etc.
The first shop could make a 1,000,000 nuts for $81,000,000 dollars, and they are still $81 each.
The factory could make 1,000,000 nuts for $750,000. Their nuts cost 75¢ each to produce.
Fortunately the demand for nuts is in the trillions in the world, and factories all over the world are geared up to produce billions of them. This production volume drives the production cost down. The consumer demand keeps the volumes high enough to keep the factories operating at their most efficient level, and competition between factories keeps the profit margins charged reasonable.
I would bet real money the demand for nuts is in the trillions. What is the demand for roto-molded tanks?
They might be able to make them for a few dollars each if they could gear upto produce them continuously in the hundreds of millions.
Heck, have you ever seen a blow mold operate? Milk jugs go in as a slug of plastic and come out as a plastic jug and go right into the bottling part of a milk plant. They cost pennies a-piece. But they make hundreds of millions of them. That plant to make them cost a million dollars to setup.
Reply to
Bob La Londe
What everyone else said about time and labor costs.
The interesting thing to note is that unlike injection or blow molding, roto-molding does have some home shop DIY potential since the mold doesn't have to be super strong (like injection molding), doesn't requite complex support systems (like blow molding), and doesn't even require a big oven if you install heaters on the mold itself and so some simple slip rings to get the power to them.
Reply to
Pete C.
Rotomolding costs lots of heat energy. Watched a kayak factory make them. 13' Kayak, 14' mold, 18' long oven. And the the mold was not cheap. And then the equipment has to rock and rotate the mold in this high heat oven to get the plastic to flow.
Reply to
Calif Bill
The DIY aspect is what got me interested, I had a bit of a look on youtube. It seems there are a few of us toying with rotomolding in our workshops. Heating the mold is an interesting idea.
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Thats what got me interested. There is a one man kayak factory round the corner from our workshop - I may go and bother him for a "tour".
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Did a tour of the Raven Industries rotomold facility in Sioux Falls SD in 2000 or so.
Probably a nice place in the winter--it gets COLD in SD, but this was July and it was way past pleasant, especially when they opened up the oven for the cooling cycle. Fun and interesting to watch several molds rotating through 3 axis motion on the mechanism in the oven. Biggest setup was fairly large, had molds on it for large "nurse tanks" used for ag spraying.
Fun part of the tour was the balloon facility. IIRC the assemble table was 800 ft long.
Reply to
Bill Marrs
For DIY applications thermoset resins might be a more cost effective choice. No ovens or heated molds are needed and the molds can be very inexpensive. Especially if only a few castings are needed. Polyester, polyurethane, and epoxy resins are available with a wide range of properties. The cycle times are generally longer but if you only need a few castings it might work fine.
Reply to
Billy Hiebert

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