10.1002/spepro.000059

Morphology for microcellularinjection moldingJingyi Xu

Injection molding makes it possible to better control the structural

alterations in microcellular foam, which can result in minimal mate-

rial property changes for plastic parts.

Microcellular foaming technology was originally conceptualized and

invented at the Massachusetts Institute of Technology in 1984.1, 2 The

idea is simply to add tiny bubbles that are smaller than the preexisting

flaws of the material into a polymer, which reduces the amount of ma-

terial while maintaining toughness.1 Suh defines microcellular plastics

as foamed plastics with a cell size that is less than 30 microns.1 In fact,

this technology offers the potential to manufacture transparent supermi-

crocellular foams with cell sizes<0.05 microns. Microcellular plastics

were not successfully used in industrial applications until 1998, when

we developed the first reciprocating injection-molding machine.3, 4 To-

day, cell sizes typically range from 5 to 100 microns.

Microcellular technology has already had a significant impact on the

worldwide plastics industry. It requires precisely metered quantities of

atmospheric gases (nitrogen or carbon dioxide) in any of the three most

common thermoplastic conversion processes (injection molding, extru-

sion, and blow molding) to create millions of nearly invisible micro-

cells. Fabrication on this scale brings a wide array of benefits, including

reduced weight, smaller amounts of material needed, and lower cost, as

well as compatibility with environmental friendly blowing agents. The

microcellular injection-molding process is primarily used where foam-

ing has not historically been deployed, producing less-expensive preci-

sion parts with consistently high quality and exceptional dimensional

stability.

Our research has revealed that numerous factors influence the quality

of the microcellular foam. For instance, the pressure drop rate must be

high enough for the necessary nucleation. In addition, a minimum gas

percentage is required, and the first stage of gas mixing is critical for

uniform cell distribution in the molded part. (Specifically, CO2 gas can

create close-packed cell structures more easily than N2 gas can.)2–11

Since microcellular structures measure about half the diameter of a

strand of human hair, they cannot be seen by the naked eye. As a result,

the scanning electron microscope is the most popular way to define

the morphological changes of different microcellular parts. Examining

Figure 1. Morphology of polystyrene microcellular foam. Average cell

size: 25 microns. Cell density: 8.1×107cells/cm3.5 White bar is 100

microns. WR: Weight reduction. (Image courtesy of John Wiley and

Sons.)

cross-sections requires the use of a carefully broken section, which is

typically done cryogenically. For instance, liquid nitrogen is used to

deep-freeze the sample, which is then broken along a predetermined

direction to reveal a flat fracture section view. This in turn is magni-

fied about 200 times or higher to see the microcellular structure.12 The

amorphous material polystyrene, shown in Figure 1, has an average

cell size of 25 microns and a cell density of 8.1×107cells/cm3. Other

amorphous materials will have a similar structure.

Injection-molded crystalline and semicrystalline materials have be-

come increasingly popular in a variety of industries. Typical examples

include polypropylene, polyethylene terephthalate, and polyamide. In

these materials, crystallization during cooling may expel gas near the

crystalloid, and the cell structure may not be as uniform as that of an

amorphous material. Our work has shown that amorphous material has

a better cell structure than crystalline material, whose morphology is

also greatly influenced by mold temperature.

Continued on next page

10.1002/spepro.000059 Page 2/2

Figure 2. Morphology of a PC-ABS part injection molded with N2 gas.

(a) Molded part. (b) Air shot sample.5 White bar is 100 microns. (Im-

ages courtesy of John Wiley and Sons.)

Figure 3. Fiber orientation in microcellular polybutylene terephthalate

with 20% glass fiber. (a) Microcellular part. (b) Solid. White bar indi-

cates 100 microns.9

One of the most widely used thermoplastic materials on the market

is polycarbonate-acrylonitrile/butadiene/styrene (PC-ABS). It is well

suited for microcellular processing because it improves heterogeneous

nucleation. Figure 2(a) shows an excellent cell structure with an aver-

age cell size of 10 microns that distributes uniformly in the molded part.

As a comparison between two different morphologies in two stages of

processing, Figure 2(b) shows the structure of an air shot sample that

has uniform cells of 3 microns in diameter, proving that the gas-mixing

quality in the first stage is excellent.

Microcellular foam is typically reinforced using glass-fiber material,

and we have found that, in general, fillers and glass fibers are good for

morphology. The challenge is in achieving the correct fiber orientation,

which microcellular injection molding helps to ensure.10 For example,

Figure 3(a) shows the structure of the fiber distribution in a microcel-

lular part where processing resulted in fiber disorientation in the center

foamed core. Figure 3(b) displays the morphology of a solid part with

strong fiber orientation in the mold flow direction.

When key processing factors are taken into account, microcellular

foam can benefit numerous applications. Injection-molding techniques

offer advantages in controlling and improving the quality of material.

Our future work will focus on ways of enhancing the surface finish

of microcellular foam and making cells smaller. This is an essential

requirement in developing supermicrocellular foam, which would rep-

resent a major advance for this technology.

Author Information

Jingyi Xu

Bekum America Corporation

Williamston, MI

References

1. N. P. Suh, Innovation in Polymer Processing, F. Stevenson James ed., ch. 3,Hanser/Gardner Publications, Inc., 1996.

2. J. E. Martine-Vvedensky, N. P. Suh, and F. A. Waldman, Microcellular closed cellfoams and their method of manufacture, US Patent 4,473,665, 1984.

3. J. Xu and D. Pierick, Microcellular foam processing in reciprocating-screw injectionmolding machines, J. Inject. Mold. Technol. 5, pp. 152–159, 2001.

4. J. Xu, Methods for manufacturing foam material including systems with pressure re-striction element, US Patent 6,579,910 B2, 2003.

5. J. Xu, Microcellular Injection Molding, John Wiley and Sons, to be published.6. S. Doroudiani, C. B. Park, and M. T. Kortschot, Effect of the crystallinity and morphol-

ogy on the microcellular foam structure of semicrystalline polymers, Polym. Eng. Sci.36 (21), pp. 2645–2662, 1996.

7. L. S. Turng, Microcellular injection molding, ANTEC, SPE, pp. 686–690, 2003.8. J. Xu and L. A. Kishbaugh, Simple modeling of the mechanical properties with part

weight reduction for microcellular foam plastic, J. Cell. Plastics 39, pp. 29–47, 2003.9. J. Xu, Methods to the smooth surface of microcellular foam in injection molding,

ANTEC, SPE, pp. 2089–2093, 2007.10. J. Xu, Process of glass fiber reinforced thermoplastic for microcellular injection mold-

ing, ANTEC, SPE, pp. 2158–2162, 2008.11. J. Xu, Effect of injection molding process parameters on the morphology and quality of

microcellular foams, ANTEC, SPE, pp. 2770–2774, 2006.12. J. Xu, Morphology study for microcellular injection molding, ANTEC, SPE, 2009.

c© 2009 Society of Plastics Engineers (SPE)