Vinyl "microbeads" provide manufacturers of automotive instrument panels, airbag covers, and interior trim with brilliant molded-in color and appealing tactile properties while being simpler to process and less energy-intensive than powder blends and plastisols, according to Teknor Apex.
Only 0.004 in. (0.1 mm) in diameter, microbeads flow freely over the inside of hollow molds in processes like slush molding and rotomolding. After years of producing beads with this micro particle-size distribution for non-automotive markets, Teknor Apex has acquired the rights to market the beads internationally to the automotive industry through an agreement with Bayer MaterialScience AG. Teknor Apex will supply the microbeads under the trade name Tek-Spheres™ and can produce them in Shore A durometers from 55 to 90. Teknor Apex manufactures microbeads by melt compounding PVC resin with color, plasticizer, and other additives, producing a completely homogeneous material that melts rapidly in contact with a heated mold.
The melt compounding process for microbeads also ensures a more uniform dispersion of color and additives, makes possible a broader range of durometers, and provides greater scope for incorporating new ingredients, including solid plasticizers.
Of the various hollow-mold processes available for thin-skin applications on automotive components such as instrument panels and headrests, one of the most commonly used is slush molding, in which a partially open mold rotates on a single axis. Another method, rotational molding, uses a closed mold in a two-axis system. To meet the requirements of these processes, Teknor Apex supplies Tek-Spheres products at different size levels. The direct product of the melt compounding method used for making Tek-Spheres is actually minibeads, with diameters in the 0.020 to 0.060 in. (0.5 to 1.5 mm) range. This is already considerably smaller than the standard 0.125-in. (3.2 mm) pellet compound used in injection molding and extrusion, and it is suitable for various open-mold processes, including slush molding. In a subsequent step, Teknor Apex produces microbeads with a particle size of 0.004 in. This smaller particle-size distribution is more suitable for rotomolding and for slush molding applications involving deep draw areas, severe undercuts, or fine detailing. In contrast to these solid materials, plastisols are liquid formulations of PVC resin in a carrier consisting primarily of plasticizer. Plastisols vary in viscosity in accordance with ambient temperature and other mixing conditions, may contain entrapped air that causes pinholes or voids in finished products, may undergo separation of components during storage, can require up to three times longer for color changeovers, and entail use of volatile organic solvents for cleanup after spills. Another type of conventional vinyl material used in slush molding is that of powder blends, highly plasticized mixtures of PVC and other additives. The high plasticizer content of powder blends places a limit on low-durometer formulation, since this would require adding even more plasticizer, preventing the material from acting as a free-flowing powder.