To follow Emery=E2=80=99s comments. The most common method of fabricating =
HDPE products is rotational molding. A simple female mold is fabricated, u=
sually 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 th=
e HDPE pellets melt and coat the entire inside of the female mold. Takes a=
nywhere 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 ho=
llow, three dimensional object whose outside surface mirrors the simple fem=
ale mold. It is an inexpensive way to make large parts like gas tanks, wat=
er tanks and the like largely because the mold costs are low. Processing t=
imes are long so processing costs are high, but it works well for small vol=
ume parts where you simply could not stand the tooling costs of more elabor=
ate and expensive two part molds. Most ag areas have rotational molders wh=
o will take on custom work like our gas tanks. Anyone willing to make 50 t=
o 100 or more could be very competitive in the market place. Those ag area=
rotational molders regularly make tanks and similar stuff for farming appl=
ications 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 m=
olded that were very popular. Likely 99% of all of those put into operatio=
n are still being used. Very easy to add things like threaded inserts, fit=
tings of all kinds, etc. The problem for thinks like the horse/mule back p=
acks 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 rota=
tionally molded HDPE GMC fuel or water tanks. BTW, for our non-US frie=
nds, you might be more familiar with the spelling mould and gimble than the=
more common American english mold and gimbal. Jerry Jerry Work Th=
e Dovetail Joint Fine furniture designed and hand crafted in the 1907 for=
mer Masonic Temple building in historic Kerby, OR glwork ht=
tp
://jerrywork.com Message: 11 Date=
: Mon, 03 Sep 2018 17:08:29 -0600 From: Emery Stora =
To: gmclist Subject: Re: [GMCnet] New plastic gas tank=
s needed Message-ID: Con=
tent-Type: text/plain; charset utf-8 There are thousands of differen=
t plastics. Some will stand up to gasoline and ethanol. There are many ver=
y expensive ones that totally hold up but you?d have to refinance your moto=
rhome (or house) to buy them. Fortunately a good plastic is cross linked hi=
gh 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 s=
eems to have held up well. It is made of high density cross linked polyethy=
lene. 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 d=
ue 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 dra=
wn in when the tanks cool down. However when the water vapor in the air coo=
ls it will condense and settle to the bottom of the tank as liquid water. D=
ay after day this occurs as the air temperature warms and cools. A signific=
ant 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 Fre=
derick,
