|The most basic appeal of thermoplastics has been their ability to make parts lighter and less costly along with providing performance benefits over conventionally used materials like glass, ceramic, aluminum, steel, zinc and lead. The development of plastic material with specific gravity from 1.8 to as high as 15 is in conflict with inherent advantages for which plastic became very popular. Some of the reasons for higher specific gravity are:
• Balancing or ballast roles to be submersible in water (fishing line sinkers)
• Image of enhanced value by higher weights/specific gravity
The relatively new category of high-gravity compounds (HGCs) consists of thermoplastics containing 40-96% by weight of mineral filler or metal powder. HGCs are far denser than existing plastics. High cost fillers make them costly- between US$10-40/kg. New grades can be formulated with lower viscosity which results in improved flow during processing. Lower viscosity can help with filling thinner walls, more intricate designs and can improve surface finish of molded pieces. Lower viscosity can improve thermal stability, as it takes less shear heat to get the material to flow. Performance improvements are the most dramatic in the highest specific gravity materials which are typically in the density ranges of 9-11 gm/cc and are compounded with nylon. This technology has been expanded to improve viscosity in other resin systems, including thermoplastic polyurethane elastomer, polypropylene, and even PEEK.
LNP, GE Plastics, PolyOne and RTP are few of the compound suppliers that offer HGC.
A broad range of base polymers is used to make HGCs. Polypropylene and Polyamide are favored, as they offer superior flow and easier acceptance of high filler loadings than amorphous polymers such as ABS. These compounds have higher abrasion and wear in molds, screws, and barrels. Compound suppliers strongly recommend use of hardened steel tooling. Some note a tendency for higher wear in gate areas and urge use of mechanical shutoffs in hot runners.
HGC processing is comparable in difficulty to running glass-reinforced materials. High-gravity compounds can tip the scales from metal to plastic in niches where injection molding offers greater design freedom, reduced manufacturing steps or an alternative to toxic lead. HGCs have also opened up possibilities in replacement of toxic lead in some potentially large applications such as small-caliber projectiles. Compounds of polyamide and tungsten metal (19 SG) can produce compounds that exactly match the SG of lead and surpass it in some key properties such as rigidity. HGCs can eliminate disposal and remediation costs associated with lead.