| |
By Thierry Marchal and Cathy Gomez, Fluent Benelux
View the pdf of this article
Mold and parison for an automotive water tank
Courtesy of MANN+HUMMEL GMBH
When designing the different components of a car, the gas and water (windshield washer fluid) tanks are often the last ones to be considered, because their shapes can be adjusted easily to fit the available space. The constraint is usually rather simple: maximize the inner volume of the tank by using as much of the remaining space as possible. This is why gas and water tanks often have complex, sometimes unusual shapes. For gas tanks, safety regulations require that the thickness of the walls be larger overall than a minimum value in order to avoid any failure in the event of an accident. If the gas tank walls are too thick, however, it needlessly increases the weight of the part and uses an excessive quantity of polymer, which impacts the cost. To balance these requirements, several companies are now using POLYFLOW software to simulate - and optimize - the blow molding process for gas and water tank manufacturing.
The blow molding process consists of two stages:
the extrusion of a cylindrical tube of polymer, also called
the parison, and the blowing process, where the material
is forced to conform to a mold. During the extrusion
of the parison, gravity and die head motion allow
the designer to adjust the parison thickness profile, which
is normally non-uniform. Once extruded, the parison
is positioned in between the two halves of the mold.
These molds have the complex imprint of the desired
tank. The top and bottom of the parison are squeezed
by knives to seal the tube so that air can be blown into
it. The air inflates the parison, just like a balloon, while
the mold gradually closes. When the two halves of the
mold are completely closed, an increase in pressure ensures
that the blowing parison takes the exact shape of the
mold, which is the desired gas and water tank shape.
Thickness distribution of the EVOH layer for an automotive water tank;
this low permeability material is one of several layers used in the parison
Courtesy of MANN+HUMMEL GMBH
Using POLYFLOW, the free surface of the deforming
parison is tracked as it is inflated. The material is allowed
to stretch and thin until contact between the deforming
free surface and moving mold is detected. The result
of the numerical simulation illustrates not only the final
shape of the part, which is similar to the mold, but more
importantly, the thickness map of the blown parison.
Often, the final thickness of a blow-molded product shows
regions where the blown parison is too thin (dark blue)
to be acceptable for safety reasons, or too thick (yellow
and red) to be economically attractive. To compensate
for a non-optimized blown parison, POLYFLOW has tools
to suggest a different profile thickness for the extruded
parison that will lead to a more uniform final thickness
map for the blown product, independent of the
complexity of the shape. Other results of the simulation
include an extensions map, which indicates how
much the material has stretched to reach the final shape,
the permeability of the finished product, the weight of
the flashes (waste material at the edges), as well as the
inner volume.
|
|
|
