Hollow glass microspheres have been used as a weight-reducing additive in plastics for years. They come as a dry, white-looking powder and are made from a water resistant and chemically stable soda-lime-borosilicate glass. These hollow glass microspheres are water resistant, noncombustible and nonporous so they do not absorb the resin they are mixed with. Older grades range in sizes from 70 to 35 microns with densities from 0.1 to 0.3 gm/cc and strengths of 250 to 2,000 psi.
Hollow glass microspheres can replace resin and fillers without requiring added machinery or new, expensive tooling. Due to their low density, hollow glass microspheres are formulated by volume rather than weight. We suggests a maximum practical loading at 50% volume of resins without fillers. An example is 30 volume % substitution of polypropylene to create 13% weight savings. Other resins that can be substituted include nylon, ABS, acetal, rubber, plastisols, and other engineered thermoplastics. The more hollow glass microspheres that replace resin, the lower the density and stiffer the part.
When fillers are used to reinforce a plastic, hollow glass microspheres should not be substituted for all the filler. The hollow glass microspheres are spherical and have an aspect ratio of 1, which does not create the same strength seen with a high-aspect ratio filler such as glass fiber because the fiber does not align in the flow direction. To reach a 5 to 10% weight savings, we recommend a 10 to 20% substitution of filler if it’s important to maintain physical properties (impact strength, modulus, elongations and heat-deflection temperature) for structural applications.
To use hollow glass microspheres in a process like extrusion or injection molding, we recommend adding the bubbles downstream in a side-stuffing operation when the resin is molten. For injection molding specifically, it is important to lower the back pressures, slow down screw speeds, and use larger gates and runners. The hollow glass microspheres are still susceptible to breaking during high shear processing as in gear pumps or two-roll mills.
Because hollow glass microspheres are lightweight, their use creates a shorter cooling time because they reduce the mass of the part. In addition, glass expands and contracts less than most resins so the plastic’s coefficient of linear thermal expansion (CLTE) improves. Because the hollow glass microspheres have an aspect ratio of 1, they create an isotropic filling with more volume loading capacity. All these variables help improve cycle times by reducing warpage and by helping hit target dimensions for parts that are molded, exposed to vibration or need snap fit.
This article comes from machinedesign edit released