11
Contemporary Machining Processes
compressive strength higher by 8.2% (Zhu et al., 2020). New research into partially
melted crystal structures and high melt strength polypropylene demonstrates it is
possible to decrease the average cell size to approximately 2 μm and increase the cell
density to 1.49 × 10 11 cells/cm 3 (Fu et al., 2020). There are also reports of changes
from microcellular to nanocellular bubbles in poly(lactic acid) foam (Ni et al.,
2020), but many results for corresponding foam densities after cell size reduction
remain either too high or close to the density of non-foamed materials (Okolieocha
et al., 2015).
Microcеllular Foam Technology MuCell ® has been introduced by the company
Trexel Inc. (USA) and is considered one of the most promising injection molding
techniques (Szostak et al., 2018). Influence of the processing parameters on cell
structure and mechanical properties has been investigated in a wide range of plastics, such as PP, PC, PS, PA6, PLA, LDPE, PET, and ABS (acrilonitrile-butadienestyrene), as well as reinforced polymers with nanoclay, talc particles, or glass fibers
(Gómez-Monterde et al., 2015). It can be stated that the introduction of microcеllular
foam technology has initiated an important new development stage in the processing
of most polymer materials.
In the microcellular technology, the application of a supercritical fluid (SCF)
is crucial. SCF is defined as a substance for which both pressure and temperature
are above critical and a chosen SCF must be non-toxic, non-flammable, and chemically inert, and its residues should be easily removed (Villamil Jiménez et al., 2020).
Commonly available atmospheric gases, such as carbon dioxide (CO 2 ) or nitrogen
(N 2 ), are used as blowing agents. Foaming with SFCs is conceptually simple, but in
practice it is a complex dynamic process requiring a full appreciation of the thermodynamics, physics, and chemistry of solutions and interacting species, polymer
sciences, and process engineering (Di Maio and Kiran, 2018). A supercritical fluid
is injected with high precision into a molten plastic in the plasticating cylinder of the
injection molding machine. The gas and the plastic are mixed together at continuously controlled SCF injection pressure, polymer molt pressure in the SCF injection
area, and temperature of the molt in the plasticating section of the screw pump.
It should be noted that both the plasticating section and the screw pump must be
designed in a specific way different from that for bulky plastics, which has an effect
on the overall costs of the method. The injected gas plasticizes the polymer upon its
solubilization and reduces its apparent glass transition temperature or melting point
to the processing temperature (Nalawade et al., 2006). It is reported that the addition
of scCO 2 reduces the viscosity of PMMA by 70%, that of polystyrene by up to 60%
at a processing temperature of 170°C, while the viscosity of polydimethylsiloxane
(PDMS) is reduced by 60% at 50°C (Kazarian, 2000). Among others, this phenomenon allows for better filling of thin wall cavities, especially in case of flexible element formation.
This single-phase polymer-gas solution is quickly injected into a mold changing its thermodynamic state, so that gas bubbles separation begins. These form
microcells with an inner pressure, required to fill in the mold cavity completely. As
a result, holding pressure is almost not required, because in all “centers” of cells
growth, pressure increases equally, even in the cavity areas distant from the IMM
Contemporary Machining Processes
compressive strength higher by 8.2% (Zhu et al., 2020). New research into partially
melted crystal structures and high melt strength polypropylene demonstrates it is
possible to decrease the average cell size to approximately 2 μm and increase the cell
density to 1.49 × 10 11 cells/cm 3 (Fu et al., 2020). There are also reports of changes
from microcellular to nanocellular bubbles in poly(lactic acid) foam (Ni et al.,
2020), but many results for corresponding foam densities after cell size reduction
remain either too high or close to the density of non-foamed materials (Okolieocha
et al., 2015).
Microcеllular Foam Technology MuCell ® has been introduced by the company
Trexel Inc. (USA) and is considered one of the most promising injection molding
techniques (Szostak et al., 2018). Influence of the processing parameters on cell
structure and mechanical properties has been investigated in a wide range of plastics, such as PP, PC, PS, PA6, PLA, LDPE, PET, and ABS (acrilonitrile-butadienestyrene), as well as reinforced polymers with nanoclay, talc particles, or glass fibers
(Gómez-Monterde et al., 2015). It can be stated that the introduction of microcеllular
foam technology has initiated an important new development stage in the processing
of most polymer materials.
In the microcellular technology, the application of a supercritical fluid (SCF)
is crucial. SCF is defined as a substance for which both pressure and temperature
are above critical and a chosen SCF must be non-toxic, non-flammable, and chemically inert, and its residues should be easily removed (Villamil Jiménez et al., 2020).
Commonly available atmospheric gases, such as carbon dioxide (CO 2 ) or nitrogen
(N 2 ), are used as blowing agents. Foaming with SFCs is conceptually simple, but in
practice it is a complex dynamic process requiring a full appreciation of the thermodynamics, physics, and chemistry of solutions and interacting species, polymer
sciences, and process engineering (Di Maio and Kiran, 2018). A supercritical fluid
is injected with high precision into a molten plastic in the plasticating cylinder of the
injection molding machine. The gas and the plastic are mixed together at continuously controlled SCF injection pressure, polymer molt pressure in the SCF injection
area, and temperature of the molt in the plasticating section of the screw pump.
It should be noted that both the plasticating section and the screw pump must be
designed in a specific way different from that for bulky plastics, which has an effect
on the overall costs of the method. The injected gas plasticizes the polymer upon its
solubilization and reduces its apparent glass transition temperature or melting point
to the processing temperature (Nalawade et al., 2006). It is reported that the addition
of scCO 2 reduces the viscosity of PMMA by 70%, that of polystyrene by up to 60%
at a processing temperature of 170°C, while the viscosity of polydimethylsiloxane
(PDMS) is reduced by 60% at 50°C (Kazarian, 2000). Among others, this phenomenon allows for better filling of thin wall cavities, especially in case of flexible element formation.
This single-phase polymer-gas solution is quickly injected into a mold changing its thermodynamic state, so that gas bubbles separation begins. These form
microcells with an inner pressure, required to fill in the mold cavity completely. As
a result, holding pressure is almost not required, because in all “centers” of cells
growth, pressure increases equally, even in the cavity areas distant from the IMM