The Assmann Difference is the The
Way We Process our Plastic Storage Tanks
1.
Rotational Molding of Polyethelene Tanks and Plastic Products
 Our unique and
efficient large ovens, which rotate end-over-end, receive convection
heat from an adjacent stationary furnace. The oven shape and foil
design create optimum airflow and uniform heat to set the first
stage for the highest quality parts possible.
We never exceed
500° F in the molding process. Using higher temperatures will speed
up the cycle time and cut costs but will compromise the results,
particularly in XLPE and thicker wall parts.
2.
Cooling Without Water Allows Cross-Linking Process to Complete All The Way Through the Polytank Wall
We have separate
chambers using only very high volume air movement to assure a gradual
cooling of the mold and the part inside. This allows the cross-linking
process, still taking place upon exiting the oven chamber, to complete
all the way through the tank wall. This is essential to optimize
the properties of the cross-linked part. (The whole idea is to achieve
as close to 100% cross-linking as possible).
Even in 1.45"
thick walls, we consistently attain gel test results (indication
of % of cross-linking that has taken place) in the high 70's and
low 80's. (ASTM minimum gel requirement is 60%).
It is also important
to mention that tanks molded of linear resins, and processed in
this manner, will also reach their optimum cure rate. We introduce
no water into our cooling cycle. Water spray or mist will speed
up the process, but will abruptly stop the cross-linking and set
up the stresses that will shorten the service life of the tank.
3.
Tooling Of Plastic Storage Tanks -
Assmann Does Not "Shield" Tank Walls.
 Equally important
in our process is that we do not "shield" the outside of
our tank molds. We exceed ASTM design hoop stress minimums for the
lower sidewall and carry this wall thickness as uniformly as possible
throughout the entire tank for maximum structural integrity.
In a competitive
marketplace it has become a common practice to wrap and weld layers
of sheet metal (shielding) around the outside of the mold
to reduce heat transfer and thereby reduce thickness over as much
as 80% of the area of the tank. Upper sidewalls, domes and tank
bottoms are often reduced to less than 25% of the thickness of the
lower sidewall.
Some of the
obvious consequences of this practice:
- There is
greater damage when tank restraint systems are put to the test.
- Tank domes
may collapse, even under snow weight.
- Stress cracking
agents along with UV deterioration will accelerate dome failure.
Also, it is
important to note "shielding" triggers a loss of control in the
process resulting in uneven cooking and significant variations in
gel percentages over the tank structure. This will also shorten
the service life of the part.
Those using
this technique sell it as "state-of-the-art" by marketing with such
terms as "tapered", "engineered", "strati-formed walls", and "strategic
material placement". The intent is only to reduce raw material costs.
The savings, however, are rarely passed on to the end user.
Download the PDF Sheet for this page.
 Our
Position
Since
the rotational molding process uses no pressure it will yield a
stress free tank or part, but only if basic processing rules are
followed.
We at Assmann
Corporation are constantly finding ways to increase efficiency in
our process but will not abandon successful methods for any cost-cutting
measure that would compromise the quality of the product for which
we have become known.
Our plant personnel
are our "experts". They have pride in the plant and their workmanship.
All of us extend a welcome to visit our plant in Garrett, Indiana.
For information,
quotations or plant tours, please call us at 1-888-357-3181. |
