To follow Emery's comments. The most common method of fabricating HDPE products is rotational molding. A simple female mold is fabricated, usually from welded aluminum. That is then filled with the proper amount of HDPE pellets, mounted on a gimbal arm and the whole thing is placed in an oven. The gimbal arm turns the mold in a complex figure 8 pattern while the HDPE pellets melt and coat the entire inside of the female mold. Takes anywhere from five to thirty minutes. The arm then swings out of the oven, is cooled by a water spray and the mold opened. The part comes out as a hollow, three dimensional object whose outside surface mirrors the simple female mold. It is an inexpensive way to make large parts like gas tanks, water tanks and the like largely because the mold costs are low. Processing times are long so processing costs are high, but it works well for small volume parts where you simply could not stand the tooling costs of more elaborate and expensive two part molds. Most ag areas have rotational molders who will take on custom work like our gas tanks. Anyone willing to make 50 to 100 or more could be very competitive in the market place. Those ag area rotational molders regularly make tanks and similar stuff for farming applications where volume is low but chemical resistance must be very high. A long time ago I made a line of horse/mule back packing boxes rotationally molded that were very popular. Likely 99% of all of those put into operation are still being used. Very easy to add things like threaded inserts, fittings of all kinds, etc. The problem for thinks like the horse/mule back packs is they seldom ever break or fail so the more you make the smaller the market becomes. That would be the same problem for a person offering rotationally molded HDPE GMC fuel or water tanks.

BTW, for our non-US friends, you might be more familiar with the spelling mould and gimble than the more common American english mold and gimbal.

Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR

glwork@mac.com
http://jerrywork.com
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Message: 11
Date: Mon, 03 Sep 2018 17:08:29 -0600
From: Emery Stora
To: gmclist@list.gmcnet.org
Subject: Re: [GMCnet] New plastic gas tanks needed
Message-ID:
Content-Type: text/plain; charset=utf-8

There are thousands of different plastics. Some will stand up to gasoline and ethanol. There are many very expensive ones that totally hold up but you?d have to refinance your motorhome (or house) to buy them. Fortunately a good plastic is cross linked high density polyethylene (HDPE). Both the high density and the cross linking of the plastic polymer is important. Regular polyethylene will not hold up.

I replaced a rusted steel Jeep gas tanks several years back and it seems to have held up well. It is made of high density cross linked polyethylene.

I don?t know who might have ones to fit a GMC motorhome but Jim Kanomata might.

The reason that GMC motorhome tanks rust out is due to water standing in them. That can easily happen from storing a partly filled tank. Water vapor in the air will enter through the vents. It is drawn in when the tanks cool down. However when the water vapor in the air cools it will condense and settle to the bottom of the tank as liquid water. Day after day this occurs as the air temperature warms and cools. A significant amount of water can build up and rust the tanks from the inside.
One way to prevent this is to fill your tanks before storing the motorhome.
However be aware that some gasoline cannot be stored for months. You can add a stabilizer to the gas but the best solution is to use the motorhome frequently and put in more new gasoline.

Perhaps Rob Mueller can tell us what he does when he leaves his GMC in Houston (where it is often very humid) when he goes off to Australia.

Emery Stora
77 Kingsley
Frederick,
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