How to Make a 1500 Liter Ferro-Cement Tank Mold By Thomas Rach

How to Make a 1500 Liter Ferro-Cement Tank Mold
Thomas Rach
Construction Materials you will need:
12 – 1.00 meter (39 ½”) lengths of 1 inch angle iron.
8 - 1.20 meter (47 ¼“) lengths of 1 inch angle iron.
6 - 2.44 meter (8 foot) sheets of corrugated aluminum.
500 - Pop rivets.
3/8 inch Bolts (1” and 1 ½”) and Nuts
4” (10 cm.) Spacers of Rebarb (1/2”)
Detailed Instructions:
The mold is made out of four (4) identical sections.
Detailed Instructions: The mold is made out of four (4) identical sections.
You will first construct the frame.
Each section of the frame will use:
Angle Iron:
Three (3) 1.00 meter pieces.
Two (2) 1.20 meter pieces.
Corrugate Aluminum Roofing:
One and a quarter 1.20 meter sections.
Pop Rivets (100+)
Cutting Stage:
Cut the angle iron into twelve (12) 1.00 meter lengths.
This will be used to make the three “curves” of each section of the mold.
A in the picture above.
Cut the angle iron into eight (8) 1.20 meter lengths.
Each section of the mold will use two 1.20 meter pieces (straight) of angle iron.
B in the picture above.
Bending Stage:
Draw a circle with a radius of 65 cm (diameter of 130 cm) on a flat cement floor or other
flat surface.
Bend the 12 one meter lengths of angle iron to conform to the curvature of the circle.
Note: The curved metal can not be twisted, it must lay flat!
Note: Each one meter piece of iron will naturally lengthen about a 1cm. each after bending.
Take a close look at the picture below and you will see how the angle iron looks after being
bent. It is a smooth curve, it is not twisted!
The angle iron ring at the top of your mold will need a flat edge facing up to help you make
the lip of the tank.
The angle iron ring at the bottom of the tank mold needs to have the flat edge down, so that
the mold will sit on the ground correctly.
As you can see, if the angle iron is bent correctly, you will have one flat edge facing down
(setting on the ground).
The other edge is facing out so that later you can pop-rivet the aluminum to the angle iron.
Very Important Note:
Because bending the metal is the hardest part of making the mold, I (Tom) sometimes get a
welding shop to bend the metal for me.
They have more experience and patience for bending the angle iron correctly in most
Welding Stage:
The following picture is a completed section of the mold frame (one of the four sections),
inside and outside.
Each panel is made separately by welding two 1.20 meter lengths of angle iron to the 3 bent
(curved) pieces angle iron ring (top, middle, bottom).
The arrows show that there are one and a half sheets of sheet metal roofing used per
Remember that the top has to have the flat side up and the bottom, the flat side down.
The middle curved angle iron rib should be placed 60 cm from the bottom
As you can see you only need to weld six joints on each section of the mold. But on each
joint you need to weld four places, (2 lap joints and 2 butt joints)!
The angle iron is just overlapped to make lap joints, which are stronger then just two butt
This extra strength is important because of the abuse the mold will take when removing it
during the third day of the tank construction process.
Attaching the Metal Roofing
The aluminum will be attached to the frame with pop-rivets. Each section will use one full
sheet 1.20 tall plus another one ripped length wise.
You will need to drill holes through the frame and aluminum to rivet it together. Use
plenty of rivets, one about every 10 cm.
Use the medium size and length pop rivets, be sure the head is not too small and the body
of the rivet too short. Sometimes it must pass though two layers of angle iron and the sheet
Also each section of the mold will use a little more then one width of roofing. As you can
see in the above photo, there is about six inches of another aluminum roofing sheet patched
on. The union is smooth because only the factory cut edges are visible.
See above the inside of the section of the mold. The black line is the rough-cut edge of the
roofing sheet. The sheet metal on the outside of the mold will only have factory-cut edges
In order to keep the outside edge of the mold smooth, you need to attach the factory-cut
edge of the patch piece to the angle iron. Then the rough-cut edge of the patch piece will
be tucked (toward the inside of the mold) behind the factory edge of the un-cut piece of
roofing metal
Note: You use corrugated metal roofing because the corrugations help ensure that the
finished tank is the correct thickness.
Drill Holes:
You will need to drill holes in the upright pieces (See the picture) for bolting the mold
pieces together. The holes should be 10 cm from the top and 10 cm from the bottom.
The hole should be a little off center to leave enough space to tighten and loosen the nut
We use six 3/8 inch bolts that are 1” and two 1 ½” long. The holes, of course, must be big
enough for your bolts.
Number the Sections:
You MUST permanently mark the mold section with some type of permanent mark like
welding points, or cutting notches in the angle iron. Look closely at the angle iron below
the bolt in this picture and you will see the notches drilled into the iron.
Note: I have had the entire mold built for $75 here in Chiapas, Mexico. The cost of the
materials was about $40 dollars. The mold, if cleaned and well oiled after each use should
last for many years of use.
I have built over 50 tanks with one mold and it is patched up where the rivets pull through
occasionally, but still functional. This tank design can be enlarged to fit your needs, I have
both a 3000 and a 6000 liter tank mold which works well for places dependent on rain
Mold Maintenance:
VERY IMPORTANT: Before using your tank mold, it must have the aluminum
(facing out) drenched in used motor oil. If it isn't treated, it will be next to impossible to
remove your mold. Also oil the angle iron to prevent cement from sticking to the metal.
AFTER each tank, the mold needs to be cleaned (NEVER USE WATER); use a wire dish
pad. The wire pad works best, you can feel the specks of cement and conform the pad to
the corrugations of the metal. All the cement should be cleaned off the mold.
ALWAYS re-oil the mold after use. If you keep your mold in good shape, you won't be
tearing the aluminum sheeting, losing pop-rivets or ruining a tank. If your mold sits for a
long time between use, it will need to be re-oiled before using. It is safer to use too much
oil than not enough.
Mold Repair:
Every once in a while, even if the mold was well oiled, you will loose a few rivets while
taking the mold out. This usually leaves a hole bigger than the head of the pop rivets.
Then you will need a patch!
The patches are made out of scrap sheet metal roofing. Cut hexagon shaped pieces about
the size of a nickel. Then punch a hole in the middle of this patch with a large nail. Feed
the new pop rivet through the patch, the sheet metal and the angle iron and then pop rivet it
to the mold.