One of the main goals when designing a part, a tool, and a processing method utilizing hollow glass microspheres is to minimize shear stresses to avoid crushing the spheres. Lack of proper attention to this factor may result in sharply reduced properties in the end product and increased part weight.
A single-screw design for incorporating iM30K microspheres into thermoplastic resins should contain a dispersive mixing element, which typically serves to break up agglomerates of fine particles. Examples of such mixing elements are the classic Maddock mixer (a fluted cylinder) or Saxton mixer (a densely flighted screw with a crosscut through the flights), though many others are available. The screw design should also have a distributive mixing element, which usually involves pin mixing sections.
In single-screw extruders, the iM30K microspheres should be added at a downstream feed port after the resin has been melted, just before the beginning of the metering zone, to minimize potential breakage of the spheres. They are added before the distributive mixing elements, in the middle of the compression section of the screw.
To mold polymers filled with hollow glass microspheres, a general-purpose injection screw is best. Other types of screws—like barrier, double-vane, or vented—are not recommended for processing hollow glass microspheres. The minimum diameter of the screw should be 1.5 in.
When molding with hollow glass microspheres, low backpressure of around 10 to 50 psi should be used. The hollow glass microspheres within the molten resin are apt to break when exposed to excessive injection speed and pressure. The injection speed should be kept low to medium. Unlike with previous microspheres, which limited cavity pressures to 10,000 psi, iM30K spheres can withstand 20,000 psi or more.
A variety of gates can be used, but to retain the hollow glass microspheres’ integrity, minimum gate width should be 0.06 in. As stated earlier, S-7 and H-13 type mold steels are recommended for producing parts filled with hollow glass microspheres.
