**4. Foaming devices**

Extensive research on polymer foaming which was performed at MIT, Massachusetts (USA) received commercialisation of obtained results. For that purpose Trexel Inc. company was established, which has developed MuCell® technology of microfoams manufacturing by means of a supercritical gas [9]. Its main advantage is low consumption of polymer, high mechanical properties (including the impact strength), good thermoinsulating properties, as well as high surface quality of manufactured details. Such characteristics have been received due to the low cell size (5 - 50 μm), their regular shape and high cell density (108 cells/cm3).

Applying MuCell® technology for injection molding requires mounting the gas injector in a proper place of the cylinder (Fig. 25), selecting a screw geometry enabling thorough mixing of a gas with molten polymer and adjusting the gas injection with the overall injection cycle time. Decompression of the gas-melt solution should take place just in a mould. MuCell® allows for a shorter cycle time (15-35%), lower density of details (6-12%), reduction of internal stresses and high dimension stability.

While applying MuCell® technology for extrusion foaming one should consider also the gas injector, proper screw design and well selected die geometry. Proper matching of all parameters should result in a high pressure in the melt, which is crucial for efficient foaming. Foaming extrusion by MuCell® allows a.o. manufacturing HDPE pipes with cell size of 15 µm and material density of 0,54 g/cm3, EPDM gaskets for automotive applications etc.

Shear stress level in the melt also influences pressure in the die. High pressure is beneficial to the cell density, however its too high level may cause the melt instabilities and cells

The nuclei number in a foamed polymer melt may be increased by means of fillers addition (Antunes et al., 2009; Khorasani et al., 2010). Foaming of polypropylene with carbon dioxide and addition of talc has shown that the cell density depends on both the filler and foaming agent content, however the nucleating effect of talc was observed only at low talc loading. Similarly, the positive effect of talc on the cell nucleation process and total cell density has been noted at foaming of PP with isopentane. It has been concluded that the nucleation mechanism depends on a size of the gas particles used for foaming. Recently also nanofillers have been

Interesting properties exhibit cellular plastics manufactured from polymer blends. Basing on a knowledge on polymer melt rheology and blends morphology one can generate bi-modal distribution of cells located in different polymer domains. That idea has been presented for

Another modification of foaming technology presents mixing of polymer with CBA, partial crosslinking of the polymer and than decomposition of the blowing agent with evolution of a gas. Cell structure can be limited by means of the crosslinking level, thus microfoams can be manufactured with such technology (Rodriguez-Perez et al., 2008). Microcellular polyethylene is used a.o. for pipe insulations, gaskets and in a healthcare as the wound

Extensive research on polymer foaming which was performed at MIT, Massachusetts (USA) received commercialisation of obtained results. For that purpose Trexel Inc. company was established, which has developed MuCell® technology of microfoams manufacturing by means of a supercritical gas [9]. Its main advantage is low consumption of polymer, high mechanical properties (including the impact strength), good thermoinsulating properties, as well as high surface quality of manufactured details. Such characteristics have been received due to the low cell size (5 - 50 μm), their regular shape

Applying MuCell® technology for injection molding requires mounting the gas injector in a proper place of the cylinder (Fig. 25), selecting a screw geometry enabling thorough mixing of a gas with molten polymer and adjusting the gas injection with the overall injection cycle time. Decompression of the gas-melt solution should take place just in a mould. MuCell® allows for a shorter cycle time (15-35%), lower density of details (6-12%), reduction of

While applying MuCell® technology for extrusion foaming one should consider also the gas injector, proper screw design and well selected die geometry. Proper matching of all parameters should result in a high pressure in the melt, which is crucial for efficient foaming. Foaming extrusion by MuCell® allows a.o. manufacturing HDPE pipes with cell size of 15 µm

and material density of 0,54 g/cm3, EPDM gaskets for automotive applications etc.

reported as efficient cell nucleants for PP and HDPE foams (Khorasani et al., 2010).

PPE/SAN blend foaming with carbon dioxide (Ruckdaeschel et al., 2007).

dressing or sensitive skin protection.

and high cell density (108 cells/cm3).

internal stresses and high dimension stability.

**4. Foaming devices** 

rupture.

Fig. 25. Gas injection system in MuCell® technology (acc. to www.trexel.com)


Another technology of cellular profiles extrusion offers Sulzer Chemtech (Switzerland). OptifoamTM technology anticipates a gas injector between the screw and extruder head (www.sulzerchemtech.com). Gas (CO2 or N2) is injected into the gas melt through a fluid injection nozzle made of sintered metal. The mixture of a gas and a polymer is next thoroughly homogenized in the static mixer section (Fig. 26).

Fig. 26. Extrusion foaming set for OptifoamTM technology (www.sulzerchemtech.com)

Lightweight Plastic Materials 313

ErgoCell® foaming technology developed by Demag Ergotech GmbH (Mapleston, 2002) assumes two stage process, with injection of a gas into mixing device located between the stationary screw and a melt accumulator equiped with an injection plunger (Fig. 27). ErgoCell® allows for a shorter cycle time, lower weight details (6-25%), reduction of internal

Car manufacturer Mazda also applied solution of the supercritical gas (CO2 or N2) in the polymer melt. According to the Core Back Expansion Molding technology the material is injected into a mold and once the foamed polymer has filled up the mould, its volume is increased by moving the back of the mould (Fig. 28). Weight of the door panel for Mazda2 (Fig. 29) made with such technology is lower for 20%, while its stiffness is for 16% higher.

Biocomposites used in the construction and automotive sector are frequently called "artificial wood" because their many properties and appearance are like wood (Matuana et al., 1998; Migneault et al., 2008; Bledzki et al., 2008). Unfortunately, the density of biocomposites, even if markedly lower than that of glass fiber reinforced composites, is still twice as high as the natural wood density. That drawback can be reduced by foaming of biocomposites that are lighter and feel more like real wood (Rodrigue et al., 2006; Guo et al., 2004, Bledzki & Faruk, 2006, Kozlowski et al., 2010). The earliest known foamed and woodfilled thermoplastics were based on polystyrene (PS) - this amorphous polymer is a perfect bubble catcher. Wood flour itself has been proved as an efficient nucleating filler in polyethylene foamed with azodicarbonamide (Rodrigue et al. 2006). As far as length of natural fibers is concerned, short fibers (75-125 μm) are favorable for foaming, since they do

not disturb the cell growth process, like do the long fibers (4-25 mm).

Fig. 30. Cross section of foamed PP filled with 30 wt.% of wood flour - injection molded

Selection of the polymer matrix is very important for properties of biocomposites. Because cellulose fibers are polar, the hydrophobic matrices (like polyolefines) need addition of adhesion promoters in order to facilitate regular fiber distribution and efficient stress transfer across the composite during deformation in a molten state and during/after solidification.

Cellular biocomposites can be manufactured both by the extrusion or injection molding technology, however the extrusion foaming provides better results (Fig. 30), as it allows for

stresses and surface defects.

**5. Foaming of biocomposites** 

(left) and extruded profile (right)

a more precise process control.

Fig. 27. Mixing unit of the ErgoCell® foaming injection molding (Sauthof, 2003)

Fig. 28. Core Back Expansion Molding principle (www.mazda.com)

Fig. 29. Mazda2 door panel by Core Back Expansion Molding ((www.mazda.com)

Fig. 27. Mixing unit of the ErgoCell® foaming injection molding (Sauthof, 2003)

Fig. 28. Core Back Expansion Molding principle (www.mazda.com)

Fig. 29. Mazda2 door panel by Core Back Expansion Molding ((www.mazda.com)

ErgoCell® foaming technology developed by Demag Ergotech GmbH (Mapleston, 2002) assumes two stage process, with injection of a gas into mixing device located between the stationary screw and a melt accumulator equiped with an injection plunger (Fig. 27). ErgoCell® allows for a shorter cycle time, lower weight details (6-25%), reduction of internal stresses and surface defects.

Car manufacturer Mazda also applied solution of the supercritical gas (CO2 or N2) in the polymer melt. According to the Core Back Expansion Molding technology the material is injected into a mold and once the foamed polymer has filled up the mould, its volume is increased by moving the back of the mould (Fig. 28). Weight of the door panel for Mazda2 (Fig. 29) made with such technology is lower for 20%, while its stiffness is for 16% higher.
