7
Contemporary Machining Processes
higher stiffness-to-weight ratio, or higher energy consumption during biaxial bending tests (Kastner and Steinbichler, 2020).
The main advantage of foam casting of polymers consists in the unproportional
dependence between bulk material density and the strength of final products. In particular, it has been demonstrated that structural foaming saves from 10% to 20% of
an expensive polymer used to produce an element similar to a bulky one and showed
ca. 70% higher strength (Dryakhlov, 2009). Other advantages of foamed plastics
over non-foamed polymers include ease of foam formation, ease of molding, and
lower dielectric constant (Ebnesajjad, 2003).
What is the essence of the foam casting process? The foaming process of polymers
can be divided into three stages: cell formation, cell growth, and cell stabilization
(Jin et al., 2019). First, a blowing agent is added to a molten polymer under certain
conditions and dissolved in the polymer. Counterpressure in the cylinder prevents
the rapid expansion of bubbles and foam formation. Next, the molten polymer is
injected into a mold cavity, where it occupies ca. 80% or 90% of the forming volume.
Now the counterpressure is absent, thermodynamic instability is generated from the
supersaturation of a blowing agent. The foam structure is then produced by expelling
the dissolved blowing agent from the polymer/gas mixture (Nofar and Park, 2018). The
foam increases in volume and occupies all the space inside the mold. When it comes
in contact with the colder walls of the mold, the high-quality surface of the produced
element is formed. During further cooling of the molten polymer with the saturated
blowing agent, residual pressure in foam cells promotes homogeneity of the structure.
It should be noted that these peculiarities of the process require much smaller injection
pressure and holding pressure, since a molten body possesses its own inner pressure.
As a result, in the practice of foamed polymer casting, forces in the mold are reduced
by 25% and even 50% compared to the parallel quantities in bulky polymers.
This technique can turn a majority of plastics into foams (Rosato et al., 2004).
On the one hand, there are obvious savings in raw materials and energy. On the
other hand, there is room for efficiency improvement to injection molding machines
(IMM). For instance, it is possible to apply larger molds than nominally dictated by
the mold clamping force, or to increase the number of cavities in a mold. Structural
foaming often allows for increasing by up to 10–20 times the forming surface per 1
ton of the clamping force. This way, the range of produced element dimensions can
be widened for the same IMM models (Vovk et al., 2018). However, limiting dimensions of molding machines should be considered, too.
1.2.1 Technical advanTages of foam molding
Apart from the above-mentioned functional advantages of foam plastics, the application of blowing agent substantially overcomes the standard processing limitations.
Its benefits can be listed as follows (Dryakhlov, 2009):
• Considerable savings of the polymer material
• Formation of larger and more complicated products and components without a need for further machining or assembling operations
Contemporary Machining Processes
higher stiffness-to-weight ratio, or higher energy consumption during biaxial bending tests (Kastner and Steinbichler, 2020).
The main advantage of foam casting of polymers consists in the unproportional
dependence between bulk material density and the strength of final products. In particular, it has been demonstrated that structural foaming saves from 10% to 20% of
an expensive polymer used to produce an element similar to a bulky one and showed
ca. 70% higher strength (Dryakhlov, 2009). Other advantages of foamed plastics
over non-foamed polymers include ease of foam formation, ease of molding, and
lower dielectric constant (Ebnesajjad, 2003).
What is the essence of the foam casting process? The foaming process of polymers
can be divided into three stages: cell formation, cell growth, and cell stabilization
(Jin et al., 2019). First, a blowing agent is added to a molten polymer under certain
conditions and dissolved in the polymer. Counterpressure in the cylinder prevents
the rapid expansion of bubbles and foam formation. Next, the molten polymer is
injected into a mold cavity, where it occupies ca. 80% or 90% of the forming volume.
Now the counterpressure is absent, thermodynamic instability is generated from the
supersaturation of a blowing agent. The foam structure is then produced by expelling
the dissolved blowing agent from the polymer/gas mixture (Nofar and Park, 2018). The
foam increases in volume and occupies all the space inside the mold. When it comes
in contact with the colder walls of the mold, the high-quality surface of the produced
element is formed. During further cooling of the molten polymer with the saturated
blowing agent, residual pressure in foam cells promotes homogeneity of the structure.
It should be noted that these peculiarities of the process require much smaller injection
pressure and holding pressure, since a molten body possesses its own inner pressure.
As a result, in the practice of foamed polymer casting, forces in the mold are reduced
by 25% and even 50% compared to the parallel quantities in bulky polymers.
This technique can turn a majority of plastics into foams (Rosato et al., 2004).
On the one hand, there are obvious savings in raw materials and energy. On the
other hand, there is room for efficiency improvement to injection molding machines
(IMM). For instance, it is possible to apply larger molds than nominally dictated by
the mold clamping force, or to increase the number of cavities in a mold. Structural
foaming often allows for increasing by up to 10–20 times the forming surface per 1
ton of the clamping force. This way, the range of produced element dimensions can
be widened for the same IMM models (Vovk et al., 2018). However, limiting dimensions of molding machines should be considered, too.
1.2.1 Technical advanTages of foam molding
Apart from the above-mentioned functional advantages of foam plastics, the application of blowing agent substantially overcomes the standard processing limitations.
Its benefits can be listed as follows (Dryakhlov, 2009):
• Considerable savings of the polymer material
• Formation of larger and more complicated products and components without a need for further machining or assembling operations