10.2417/spepro.005005
Foam injection molding using
nitrogen and carbon dioxide as
co-blowing agents
Xiaofei Sun and Lih-Sheng Turng
A novel microcellular injection molding approach using nitrogen and
carbon dioxide significantly improves the morphology and mechanical
properties of melts.
Microcellular injection molding (MIM) is a very special injection
molding process. It injects a gas such as nitrogen (N2 ) or carbon dioxide (CO2 ) in the so-called supercritical state—i.e., simultaneously liquid and solid—into the polymer melt as a blowing
agent to produce lightweight, foamed plastic parts. MIM continues
to attract attention because it saves on material costs and energy
while improving dimensional stability and production efficiency compared with conventional solid injection molding.1 With its unique
properties, MIM has encouraged a range of innovative applications,
such as packaging materials, insulation, filtration membranes, sports
equipment, automotive components, and aircraft parts.2–5 Recently,
Lee et al.6 proposed a method of producing microcellular injectionmolded parts known as supercritical fluid-laden pellet injection
molding foaming technology (SIFT). This method generates gas-laden
pellets from an extruder equipped with a gas-injection device. Whereas
conventional MIM requires modification and additional equipment for
every injection-molding machine used to make microcellular parts,
only one extruder needs modification with an add-on gas pump when
using SIFT technology. The gas-laden pellets produced can be used
by several conventional injection-molding machines without having to
modify them.6
These two technologies work seamlessly with each other, enabling
the introduction of two different types of gases independently into the
injection-molding foaming process. One type of gas (gas A) can be
embedded into gas-laden pellets using SIFT extrusion. These pellets
can then be used in MIM, during which gas B will be injected (see
Figure 1).
Different gases have distinct physical properties and different
benefits in polymer foaming. When used together as co-blowing
agents, their strengths can be combined and fully exploited, to superior
Figure 1. Schematic of the combined supercritical fluid (SCF)-laden
pellet injection-molding foaming technology/microcellular injectionmolding (SIFT/MIM) process.
effect. Carbon dioxide and N2 are the two most commonly used
physical blowing agents because of their low cost and environmentally friendly nature. Carbon dioxide typically shows high solubility in
polymers and, therefore, can be readily dissolved into the polymer melt
at a high concentration. Moreover, CO2 shows a strong plasticization
effect that serves to reduce the surface energy of the polymer melt and
enhance bubble nucleation. Nitrogen, on the other hand, shows a much
lower solubility, which causes a high degree of supersaturation once
the polymer-gas mixture is injected into the mold cavity. This supersaturation is a major driving force for bubble nucleation. Consequently,
N2 usually leads to a finer bubble structure and higher bubble density.
When both CO2 and N2 are introduced into the same foaming process
at appropriate concentrations and ratios, N2 triggers a high bubble
nucleation rate, while CO2 —which can be uniformly dispersed and
dissolved at high concentration—ensures uniform bubble growth after nucleation. As a result, the final bubble structure achieved using
N2 C CO2 as co-blowing agents is much finer and more uniform than
using either type of gas alone.
Continued on next page
10.2417/spepro.005005 Page 2/3
Figure 2. Scanning electron microscopy (SEM) images of nitrogen (N2 ) SIFT C 2.5% (CO2 ) MIM. Gas concentrations are reported as percent by
weight.
Figure 3. SEM images of N2 SIFT C 2.5% CO2 MIM on high-impact polystyrene (HIPS) and polypropylene (PP). Gas concentrations are reported
as percent by weight.
We carried out an initial experiment using low-density polyethylene
(LDPE). We dosed first with N2 in the SIFT process to create N2 laden pellets at three different levels of N2 content (0, 0.19, and 0.24%
by weight). We then dosed with 2.5% by weight CO2 during the
MIM process. Figure 2 shows the foam morphology under these three
conditions. The addition of N2 in the gas-laden pellets improved the
morphology dramatically: bubble size was reduced from approximately
120m on average to approximately 30m.
We performed tensile tests on the molded parts using an Instron
5967 test machine following the ASTM D638 standard (see Table 1).
Continued on next page
10.2417/spepro.005005 Page 3/3
Table 1. Tensile test results of parts molded by MIM and combined SIFT/MIM processes. Gas concentrations are reported as percent by weight.
Properties
Young’s modulus (bar)
Strain at break (mm/mm)
Toughness (bar)
1.0% N2 MIM
503:7 ˙ 54:38
1:07 ˙ 0:10
76.80
2.5% CO2 MIM
583:3 ˙ 33:27
1:31 ˙ 0:09
98.04
0.19% N2 SIFTC2.5% CO2 MIM
623:1 ˙ 23:78
1:30 ˙ 0:09
102.45
Besides the three samples shown in Figure 2, parts molded using only
MIM with 1% by weight N2 are also listed in the table for comparison.
The combined SIFT/MIM approach significantly improved the Young’s
modulus and toughness thanks to the finer bubble structure.
To verify whether this method is suited to materials other than LDPE,
we applied the same process conditions to high-impact polystyrene
(HIPS) and polypropylene (PP), with three different levels of N2
content (0, 0.19, and 0.24% by weight) in the gas-laden pellets, and
2.5% by weight CO2 injected during MIM. HIPS is a widely used
amorphous material with good foamability, and PP is a semicrystalline
material that is typically difficult to foam because of its crystallinity and
low melt strength, which results from its linear molecular structure.7
Figure 3 shows scanning electron micrographs of the morphology of
these samples. Both HIPS and PP showed a significant reduction in
bubble size and an increase in bubble density once both N2 and CO2
were introduced, compared with CO2 MIM alone.
In summary, we successfully implemented a combined SIFT/MIM
process using both N2 and CO2 as co-blowing agents. N2 SIFT C
CO2 MIM produced remarkable improvements in foam morphology
and mechanical properties compared with either blowing agent alone.
We applied this approach with equally successful results to three
polymers: LDPE, PP, and HIPS. We are currently working on using
combinations of different types of SIFT gas-laden pellets to
produce microcellular injection-molded parts, such that the benefit of
co-blowing agents can be realized without the need for gas dosing
during the injection-molding process.
0.24% N2 SIFTC2:5% CO2 MIM
685:6 ˙ 29:8
1:42 ˙ 0:08
109.70
Institute for Discovery. His research interests include MIM, nanocomposites, bio-based polymers, and tissue engineering scaffolds.
References
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Author Information
Xiaofei Sun and Lih-Sheng Turng
Mechanical Engineering Department
University of Wisconsin-Madison
Madison, WI
Xiaofei Sun is a PhD candidate in mechanical engineering. His research
focuses on the process and automation of MIM.
Lih-Sheng Turng is a professor and co-director of the Polymer
Engineering Center, and was selected to lead an interdisciplinary team
to develop innovative tissue engineering scaffolds at the Wisconsin
c 2013 Society of Plastics Engineers (SPE)