10
Remanufacturing and Advanced Machining
Physical blowing agents in a liquid or gaseous state under pressure are injected
into a molten polymer, mainly inside a plasticating cylinder. The counterpressure in
the cylinder prevents the mixture from bubbling, so that gas is solved in the polymer
molt reducing its viscosity. It should be noted, however, that the effect on viscosity
is dependent on many factors. It was demonstrated that mass fractions of blowing
agents propane and carbon dioxide below 2 wt% had little to no effect in regard to
viscosity reduction of a polypropylene melt, but a mass fraction of 3.5 wt% resulted
in significantly decreased viscosity values (Vincent et al., 2020). Injected into a
mold cavity, a polymer melt is foaming due to the lack of counterpressure, forming
pores in the polymer. Usually, supercritical carbon dioxide (scCO 2 ), nitrogen, and
air, but also various ecological gas mixtures are applied as processing solvents. The
Montreal Protocol and its subsequent amendments to the Vienna Convention for the
Protection of the Ozone Layer have led to the introduction of “second-generation”
blowing agents, such as hydrofluorocarbons (HFCs) or hydrocarbons. Zero ozonedepleting hydrocarbon blowing agents comprise n-pentane, isopentane, or cyclopentane (Höfer, 2012). Application of some other blowing agents, such as methylene
chloride, becomes more and more limited under the air toxics legislation within
various states considering it a suspected carcinogen (Kaufman and Overcash, 1993;
Jimoda, 2011).
Among the main shortcomings of the physical foaming agents, the following
should be named:
• The blowing agents, both liquids and gases, require specialized equipment
to be delivered and stored. Moreover, gaseous agents must be turned to their
liquid state before application in a plasticizer. As a result, manufacturing
costs increase.
• Injection unit of the IMM should be redesigned so that injectors may work
in it. That means additional investment into existing equipment in order to
adapt it to foam injection molding.
Microcellular injection molding (MIM) is widely applied in many industries, due to
its excellent flexibility and capability, good scalability and environmental benefits,
low cost, and high efficiency (Zhao et al., 2020). It emerged recently because the
traditional FIM techniques were able neither to assure steady distribution of cells in
the volume nor to provide strict control of the resulting density of a foamed material. Moreover, some foaming agents appeared to be environmentally harmful, and
specialized FIM equipment was not suitable for the processing of bulky polymers.
To analyze the superiority of microcellular porous structures, Zhu et al. (2020)
performed numerical simulations of static mechanical properties of polymethyl
methacrylate (PMMA) microcellular foams. The authors demonstrate that for foams
with the same average cell size (7 ± 1 μm), the compressive strength increases by
144% (the void porosity is from 65% to 37%); for foams with the same void porosity
(64 ± 1%), the compressive strength rises by 42% (the average cell size from 21 to
8 μm). They also compare the structures with spherical, ellipsoidal, and polyhedral
cells and find that the ellipsoids have a superior compression performance, with the
Remanufacturing and Advanced Machining
Physical blowing agents in a liquid or gaseous state under pressure are injected
into a molten polymer, mainly inside a plasticating cylinder. The counterpressure in
the cylinder prevents the mixture from bubbling, so that gas is solved in the polymer
molt reducing its viscosity. It should be noted, however, that the effect on viscosity
is dependent on many factors. It was demonstrated that mass fractions of blowing
agents propane and carbon dioxide below 2 wt% had little to no effect in regard to
viscosity reduction of a polypropylene melt, but a mass fraction of 3.5 wt% resulted
in significantly decreased viscosity values (Vincent et al., 2020). Injected into a
mold cavity, a polymer melt is foaming due to the lack of counterpressure, forming
pores in the polymer. Usually, supercritical carbon dioxide (scCO 2 ), nitrogen, and
air, but also various ecological gas mixtures are applied as processing solvents. The
Montreal Protocol and its subsequent amendments to the Vienna Convention for the
Protection of the Ozone Layer have led to the introduction of “second-generation”
blowing agents, such as hydrofluorocarbons (HFCs) or hydrocarbons. Zero ozonedepleting hydrocarbon blowing agents comprise n-pentane, isopentane, or cyclopentane (Höfer, 2012). Application of some other blowing agents, such as methylene
chloride, becomes more and more limited under the air toxics legislation within
various states considering it a suspected carcinogen (Kaufman and Overcash, 1993;
Jimoda, 2011).
Among the main shortcomings of the physical foaming agents, the following
should be named:
• The blowing agents, both liquids and gases, require specialized equipment
to be delivered and stored. Moreover, gaseous agents must be turned to their
liquid state before application in a plasticizer. As a result, manufacturing
costs increase.
• Injection unit of the IMM should be redesigned so that injectors may work
in it. That means additional investment into existing equipment in order to
adapt it to foam injection molding.
Microcellular injection molding (MIM) is widely applied in many industries, due to
its excellent flexibility and capability, good scalability and environmental benefits,
low cost, and high efficiency (Zhao et al., 2020). It emerged recently because the
traditional FIM techniques were able neither to assure steady distribution of cells in
the volume nor to provide strict control of the resulting density of a foamed material. Moreover, some foaming agents appeared to be environmentally harmful, and
specialized FIM equipment was not suitable for the processing of bulky polymers.
To analyze the superiority of microcellular porous structures, Zhu et al. (2020)
performed numerical simulations of static mechanical properties of polymethyl
methacrylate (PMMA) microcellular foams. The authors demonstrate that for foams
with the same average cell size (7 ± 1 μm), the compressive strength increases by
144% (the void porosity is from 65% to 37%); for foams with the same void porosity
(64 ± 1%), the compressive strength rises by 42% (the average cell size from 21 to
8 μm). They also compare the structures with spherical, ellipsoidal, and polyhedral
cells and find that the ellipsoids have a superior compression performance, with the