Re: Rotomolding experience anyone?

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.
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Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com

Calif Bill wrote:
"Bob La Londe" <nospam@xxxxxxxxx> wrote in message news:gsnb8e$7v9$1@xxxxxxxxxxxxxxxxxxxxxx
"Nutz" <nutz@xxxxxxxx> wrote in message news:49ef1305$0$12582$5a62ac22@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?
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.

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.


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