Metal-filled compounds for hot-runner molding.

A new line of thermoplastic compounds highly filled with copper and stainless-steel powder for metal injection molding (MIM) has been introduced by Mortile Industries Ltd., a Toronto-based compounder. These materials are said to be suitable for hot-runner systems, and scrap can be recycled like any thermoplastic.

As we have reported previously (see PT, March '91, p. 65), MIM is an up-and-coming process for producing metal parts while taking advantage of the efficiency of injection molding. The full MIM process involves mixing metal powder with a small amount of thermoplastic binder, injection molding the mix into a "green" part, burning off the plastic in a debinding oven, and then sintering the porous metal preform at higher temperature to yield a solid metal finished product.

While Mortile's new compounds, dubbed "Metalfil," presumably could be used in the above manner, the company is not actively marketing them that way. Rather, their applications to date have been for use as-molded, without debinding and sintering. This eliminates much of the time and expense of producing powdered-metal parts, and avoids some of the most technically challenging production steps.

Though not as strong as a sintered powder-metal part, the MIM resins will target niche applications with requirements between those for machined-steel bar stock and glass-filled plastics, such as munitions, gears, and underhood automotive components. Metalfil grades offer much of the strength and conductivity of powdered metal, plus the lubricity of the polymer matrix. They would be less expensive in applications where sintered metal parts are overdesigned. And the supplier also points out that Metalfil grades offer greater dimensional stability than sintered powder-metal parts, which must be designed to compensate for high shrinkage.

WHAT'S AVAILABLE

Frank Mortimer, president and CEO, says the two product lines, given the preliminary technical designation MIM CU93 (copper) and MIM SS93 (steel), will eventually span a range of filled thermoplastics, including nylon, ABS, acrylic, LLDPE and EVA. The presently available grades both use a nylon 11 matrix.

Mortimer says the materials can be compounded to meet customer processing and performance specifications. Grades can be specially tailored for filler levels up to 96% by weight of metal powder, attaining a specific gravity of greater than 5.6. Typical particle-size range for the filler is 5 to 150 microns, with a mean of 60 microns.

Mortile uses a proprietary twinkneader batch compounding system, which reportedly provides tightly controlled dispersion of metal powder in the resin melt prior to pelletizing. Stabilizers, impact modifiers, wetting agents and non-wax lubricants are included in the mix.

Initial pricing for the two new powder-metal lines is about $8.50/lb tl. Plastex Inc., Westmont, Ill., serves as the U.S. distributor for the Mortile MIM grades. Plastex also offers product and process support services. Processing these compounds requires some degree of machine adaptation, mainly use of an abrasion-resistant screw and barrel, according to Edmund Q. Sylvester, Plastex president. (CIRCLE 59)