Processing TP polyimide offers opportunities and challenges.

A new high-performance engineering thermoplastic shows promise as an injection moldable and extrudable material with unique properties for highly specialized niche applications. But early reviews of its processing characteristics portray it as a material that demands rigorous monitoring of process conditions.

Aurum thermoplastic polyimide (TPI) was brought to North America from Japan by Mitsui Toatsu Chemicals Inc., N.Y.C. Distributed by Advanced Web Products Inc., Ingomar, Pa., Aurum was test marketed for several years before its formal launch last year at the NPE show (see PT, May '94, p. 117; June '93, p. 13; and April '91, p. 14). In addition, RTP Co., a custom compounder in Winona, Minn., recently introduced its own 4200 series of glass- and carbon-fiber compounds based on Aurum (PT, June '94, p. 77).

TABLE 1 - INJECTION MOLDING CONDITIONS FOR AURUM TPI

Melt Temperature, F          750-800+
Mold Temperature, F          300-425
Injection Pressure, psi      20,000-30,000
Predrying Time, hr           5
          Temp., F           392
Regrind Tolerance, %         10-20

In applications, material properties, and molding performance, Aurum is most often compared with PEEK, another high-end thermoplastic supplied by Victrex USA of West Chester, Pa. However, Aurum is being aimed at applications that require even higher dimensional stability, long-term thermal stability, chemical/corrosion resistance, and mechanical properties. Many of these uses will be in the electrical/electronic sector.

For example, the Memory Products Div. of Empak, Waconia, Minn., is using Aurum to mold high-precision disk carriers for computer hard-disk drives. Aurum replaced an aluminum alloy that failed in initial development tests. The part must maintain dimensional accuracy to within 0.000001 in.

Mechanical parts also show promise, according to David R. Erickson, materials and processing engineer with Ufe Inc., a custom molder and contract manufacturer in Stillwater, Minn. Ufe is injection molding Aurum into gear drives, thrust washers, and bearings.

Standard versions of Aurum include three non-filled grades and ones containing glass, carbon, or aramid fiber, as well as fluoropolymer or graphite internal lubricants.

Aurum provides inherently good chemical and wear resistance, electrical resistance, and flame retardance. It offers long-term continuous-use temperatures of 350-446 F and low thermal expansion for superior dimensional stability. Aurum has a glass-transition temperature of 482 F, about 100 [degrees] higher than PEEK. Aurum prices generally run in the $35 to $45/lb range.

PROCESSING CHALLENGES

Aurum TPI, like many other high-performance crystalline polymers, presents its share of processing difficulties, especially in injection molding. Processors experienced in using Aurum say it's best suited for those already well acquainted with handling high-end engineering thermoplastics.

RTP executives point out that there's a learning period required for processors of Aurum that closely resembles their initial education with PEEK in the mid-1980s. "It took compounders and molders a long time to learn how to work with PEEK," recalls Joseph C. Arras, RTP marketing manager. "Now, we're all comfortable using PEEK. The same will be true for Aurum."

Experienced molders say Aurum TPI is a high-viscosity polymer with a relatively narrow processing window. It requires molding pressures between 20,000 and 30,000 psi. Special attention must be paid to the potential for weak knit lines along the melt front. Melt temperature range is 750 to 800 F or more, and mold temperatures run 300-425 F.

On the positive side, molders say Aurum exhibits no signs of excessive flashing, sets up quickly, ejects cleanly from the mold, and causes no extraordinary tool wear. The material does require predrying, but molders say it isn't prone to quick moisture pick-up like nylon or PET. They also say Aurum can be molded successfully with hot runners.

CONTROLLING CRYSTALLINITY

Experienced processors emphasize that special care must be taken to control the level of crystallinity developed in order to avoid excessive brittleness and build-up of stresses in finished products. This involves closely monitoring the melt and mold temperatures to maintain the resin's amorphous state during processing.

Ufe sources say part crystallinity is developed via post-mold annealing to achieve the desired levels of properties [TABULAR DATA OMITTED] such as chemical and heat resistance and mechanical strength.

Avoiding TPI's tendency toward premature crystallization and brittleness is the key challenge for stock-shapes extruder Global Plastics Inc., Bensalem, Pa. Last year, Global became the first North American extruder to be qualified by Mitsui Toatsu for producing Aurum stock shapes. V.P. Don Drew says Globe's method is to quickly quench the extruded shape to retain its amorphous condition. Then, a 10-hr oven-annealing process is used to induce a degree of crystallization appropriate to the demands of the application.

Globe is sampling a new Aurum bearing grade formulated to compete for applications now served by DuPont's Vespel thermoset polyimide and Amoco's Torlon polyamide-imide.

Larry Dressen, Empak engineering manager, says the most daunting challenges for his firm in utilizing Aurum TPI has been optimizing part and tool design to compensate for the resin's brittleness and for overpacked and sticking sprues caused by the high injection pressures needed.

Mitsui Toatsu has tested new Aurum formulations incorporating different combinations of reinforcements and fillers to reduce the resin's brittleness, Dressen says. A curious phenomenon here is that the addition of carbon fibers appears to actually reduce the brittleness level, whereas the opposite is usually true for most engineering resins. He says the sprue-sticking problem recently was overcome with a sprue-break stage in molding and a specially designed sprue puller on his presses.

Dressen also cautions against prolonged barrel residence time, as the high melt temperature can cause carbon deposits and quickly degrade additives such as PTFE.