13
Contemporary Machining Processes
competing companies develop new compositions according to market expectations.
In addition, special modifications of MuCell ® technology are proposed in order to
optimize the process.
However, MuCell ® technology has some limitations, such as reduction of mechanical strength or only matte surfaces obtained (Szostak et al., 2018). Some customers
may not accept the roughness parameters or the microscopic surface discontinuities
caused by bubbles. Nevertheless, the MuCell ® process is primarily used in automotive, consumer electronics, medical devices, packaging, and consumer goods applications (Trexel, 2021), where roughness is desirable to prevent slippage. For this very
reason, in some cases, sand blasting operations may be excluded from mold fabrication, providing further overall savings. When poor appearance is the issue, methods
for surface quality improvement can be applied, such as the co-injection process,
where a skin is injected over a microcellular part, or the heat and cool process, where
control of mold temperature ensures continuity of the surface layer (Lima et al., 2016).
Another way to obtain an integral structure and an improved surface quality
of a microcellular plastic is the gas counter pressure (GCP) process. The effect is
achieved by pressurizing the mold to a pressure higher than that of the foaming gas
prior to injection of the melt (Shotov et al., 1986). Prior to the injection, the mold cavity is filled with an inert gas at a certain pressure, and the polymer melt is injected
against the gas pressure. Controlling the pressure inside the mold cavity, it is possible to prevent foaming during the skin formation process. When integral skin is
formed, counterpressure of the inert gas is reduced starting the foaming process in
the core of the produced part. Some reports say that under GCP control alone, when
counterpressure was greater than 10 MPa, part surface roughness for transparent
polystyrene (PS) was improved by 90% (Chen et al., 2013). The authors proposed a
combination of the GCP and mold temperature control methods, producing molded
MuCell parts with a high-quality surface, thin skin, defect-free surface, and small
and uniform cell size, demonstrating the possibility of enhancing the application
potential for the MuCell process.
It should be emphasized that the MuCell technology is not an alternative to the
traditional injection molding, multicomponent sandwich-like molding for parts with
continuous skin and foamed core, low-pressure foam injection molding, or molding
with gas injection. Application of supercritical fluids can be treated as an additional
technology which may be combined with others and promote further technological
development. Nowadays, new injection molding foaming technologies are emerging,
e.g., technology IQ Foam ® (Gómez-Monterde et al., 2019), Optifoam ® , Ergocell ® , or
ProFoam ® (Xu, 2010).
1.2.3 foam injecTion molding equiPmenT
FIM technologies in general, and MuCell in particular, are based on typical injection
molding machine structures with modified aggregates and some additional equipment. A schematic of the MuCell system is presented in Figure 1.2.
In the microcellular IMM, the injection unit is principally different from conventional solutions. The plasticization unit must have a specific geometry to ensure
Contemporary Machining Processes
competing companies develop new compositions according to market expectations.
In addition, special modifications of MuCell ® technology are proposed in order to
optimize the process.
However, MuCell ® technology has some limitations, such as reduction of mechanical strength or only matte surfaces obtained (Szostak et al., 2018). Some customers
may not accept the roughness parameters or the microscopic surface discontinuities
caused by bubbles. Nevertheless, the MuCell ® process is primarily used in automotive, consumer electronics, medical devices, packaging, and consumer goods applications (Trexel, 2021), where roughness is desirable to prevent slippage. For this very
reason, in some cases, sand blasting operations may be excluded from mold fabrication, providing further overall savings. When poor appearance is the issue, methods
for surface quality improvement can be applied, such as the co-injection process,
where a skin is injected over a microcellular part, or the heat and cool process, where
control of mold temperature ensures continuity of the surface layer (Lima et al., 2016).
Another way to obtain an integral structure and an improved surface quality
of a microcellular plastic is the gas counter pressure (GCP) process. The effect is
achieved by pressurizing the mold to a pressure higher than that of the foaming gas
prior to injection of the melt (Shotov et al., 1986). Prior to the injection, the mold cavity is filled with an inert gas at a certain pressure, and the polymer melt is injected
against the gas pressure. Controlling the pressure inside the mold cavity, it is possible to prevent foaming during the skin formation process. When integral skin is
formed, counterpressure of the inert gas is reduced starting the foaming process in
the core of the produced part. Some reports say that under GCP control alone, when
counterpressure was greater than 10 MPa, part surface roughness for transparent
polystyrene (PS) was improved by 90% (Chen et al., 2013). The authors proposed a
combination of the GCP and mold temperature control methods, producing molded
MuCell parts with a high-quality surface, thin skin, defect-free surface, and small
and uniform cell size, demonstrating the possibility of enhancing the application
potential for the MuCell process.
It should be emphasized that the MuCell technology is not an alternative to the
traditional injection molding, multicomponent sandwich-like molding for parts with
continuous skin and foamed core, low-pressure foam injection molding, or molding
with gas injection. Application of supercritical fluids can be treated as an additional
technology which may be combined with others and promote further technological
development. Nowadays, new injection molding foaming technologies are emerging,
e.g., technology IQ Foam ® (Gómez-Monterde et al., 2019), Optifoam ® , Ergocell ® , or
ProFoam ® (Xu, 2010).
1.2.3 foam injecTion molding equiPmenT
FIM technologies in general, and MuCell in particular, are based on typical injection
molding machine structures with modified aggregates and some additional equipment. A schematic of the MuCell system is presented in Figure 1.2.
In the microcellular IMM, the injection unit is principally different from conventional solutions. The plasticization unit must have a specific geometry to ensure