As important as these previously discussed properties are, one of the hollow glass microsphere’s greatest assets is the contribution it makes to part processability, which, in a filler, is a direct function of particle shape.
Arguably, the hollow glass microsphere’s small, spherical structure is the perfect shape for a filler. Without exception, the mineral fillers available to composites manufacturers are irregularly shaped. That irregularity results in a relatively large surface area, which increases the viscosity of the resin into which the filler is added. By contrast, the hollow glass microsphere’s regularity minimizes its surface area.
Additionally, the hollow glass microsphere has a nominal 1:1 aspect ratio, giving it inherently isotropic properties that composites manufacturers can use to great advantage. For example, in parts fabricated by a resin injection process, chopped glass fiber, with a high aspect ratio, results in ~60 percent less stiffness in the cross-flow direction than in the flow direction because the fibers become oriented in the direction of flow. This alignment of the fibers can contribute to warpage, especially when introduced to crystalline matrices, such as nylon or polypropylene, which have molecular chains that also tend to align along flow lines. Hollow glass microspheres, on the other hand, do not orient and, in fact, tend to obstruct directional orientation of reinforcing fibers and matrix. The result is that stresses are more evenly distributed, enhancing both reinforcement and dimensional stability.
Hollow glass microspheres can act as “mini ball bearings”. The “ball bearing effect” enables the resin to more easily infiltrate complex mold geometries, resulting in faster cycle times. Further, successful infiltration can occur at lower mold temperatures and injection pressures than are possible when mineral fillers are used.
The hollow glass microsphere’s regular shape can contribute to product surface quality as well. Unlike chopped fiber, which tends to migrate to the part surface during processing, hollow glass microspheres tend to remain more evenly dispersed throughout the part. By adding 10 to 15 percent, by weight, of high-strength hollow glass microspheres and reducing the glass fiber to 5 percent, they were able to achieve the desired mirror finish without sacrificing the stiffness required by the part.
No less important is the fact that the spheres help shorten mold heating and cooling cycles. Because the spheres are hollow, there is less mass to heat or cool, which leads to faster overall throughput.