Polymer Castings take on metals

Terry Capuano, P.E.
President
Accures Casting LLC
Chagrin Falls, Ohio

The structures of equipment such as lathes, milling machines, coordinate-measuring machines, and pump bases are typically cast-iron or steel weldments. The iron-casting process produces a rough casting, which is then stress relieved, precision machined, stress relieved again, and finally painted, a process that may take four months or more.

But polymer-composite (PC) castings are cast to tolerance, demolded, and ready to use, all within days, and cost about 30% less than steel or cast-iron equivalents. They can be cast in color to eliminate painting as well.

A mixture of reactive, high-strength resin and mineral aggregates makes as-cast machine components to 0.0005-in./ft flatness, with hole diameters to ±0.0001 in., and feature dimensions to ±0.0005 in./ft.

The mineral aggregates are precisely graded and range in size from a fine powder to particles 0.375 in diameter. Precise grading minimizes air voids and resin use and promotes stronger castings. In general, castings incorporate coarser aggregates, though fine aggregates better fill thin sections. Mechanical properties of PC are about the same, regardless of aggregate size.

AGGREGATES
High-hardness mineral aggregates including quartz, basalt, and granite are typically used, though recycled glass is another option. Granite, for example, leaves a jagged edge when it is broken up so it better grips the resin. But the jagged edges can also hinder flow into mold features. In contrast, high-strength, high-purity (99.5% SiO2) quartz aggregate has a more round shape that improves flow and compaction. Vibratory compaction during the molding process tightly packs the aggregate together, which boosts part strength. The quartz has a Mohs hardness of 8 (diamond = 10) and makes up about 92% of a part by weight.

RESINS
Resin systems include epoxy, polyester, vinyl ester, methacrylate and furan, the most common of which is epoxy. Epoxy has excellent chemical resistance and long-term stability, as well as good mechanical properties. The addition of wetting agents improves resin/aggregate adhesion, while antifoaming agents reduce trapped air. Volumetric shrinkage of epoxy is minimal and additives can nearly eliminate it.

Epoxy makes sense when precision and high strength are key design considerations. A downside of epoxy is that it takes several hours to cure. Polyester, in contrast, has a high shrink rate and is therefore inappropriate for high-precision parts. It's also mechanically weaker than epoxy. But it can cure in just 10 min, making it ideal for small, high-volume parts.

The PC casting process blends resin, hardener and aggregate in a batch or continuous mixer. A batch mixer is preferred because components can be accurately weighed prior to mixing. The mixture pours into a mold and cures in just a few minutes or perhaps several hours, depending on the resin system and formulation. Curing typically takes place at room temperature, though some resin systems are heat treated for added strength and stability. Features such as tapped holes are cast in place. This a big advantage over machining that depends upon the machine locating the holes properly each time. Casting eliminates the need to inspect hole locations, once established.

Unlike metallic castings, wall thickness can vary without inducing internal stresses. Resins have less mechanical and impact strength than steel or cast iron, and tend to be stronger in compression than in tension. Highly stressed sections are simply made thicker. Thin sections in tension are internally reinforced with steel or fiberglass.

MOLDS
A big advantage of polymer casting is the ability to use molds of various materials. Wood molds work well for extremely large parts or when short lead times are important. Parts cast from these molds can be held to relatively close tolerances. Common methods to form precision surfaces on bulk castings include secondary machining, grouting, or casting in premachined steel components.

A fiberglass mold made from an existing part simplifies engineering and shortens production time. These low-cost tools can cast several hundred parts, provided they are properly prepared and handled. Molds for high-precision parts should be built from steel and incorporate significant reinforcement to withstand vibration compaction. In any case, be sure to hold mold tolerances tighter than that of the part to ensure final-accuracy specs.

Surface finish is another consideration. Obviously, a highly polished mold surface will produce parts with the same finish. Parts can be made with a mat finish or cross-hatching by applying the features to the mold itself.

The polymer-casting process also makes possible cast integral surfaces with properties that are different than those of the basic casting. The epoxy-based special surfaces chemically lock to the primary casting material. For example, it is possible to make sections magnetic, electrically conductive, heavier, or lighter. Parts can be cast with varying density as well. In one case, a lift-truck manufacturer casts the truck main frame from an extremely heavy aggregate in the rear, and a less-expensive, standard aggregate up front.

In another application, low-friction polymer way surfaces are cast simultaneously with the basic casting. Specially formulated epoxy way surfaces provide high lubricity and longer life than those of cast iron or steel. Some users report a cast-iron box way — rub-bing against a polymer way instead of brass — lasts 10 to 15% longer based on sliding wear. The epoxy in this case acts a lubricant.

PC CUTS VIBRATION
Improved wear, lower costs, and ease of manufacture are why many users choose polymer castings over metal counterparts. But the primary reason is improved vibration damping. Tests show polymer composites dampen vibration 10 better then cast iron and 45 better than steel.

Some grinding-machine makers claim polymer-composite bases help grinding wheels last 30% longer and produce smoother ground surfaces. Makers of lathes and milling machines using polymer bases report similar results with cutting tools. In other applications, better vibration dampening lets high-speed printing machines and scanners deliver higher resolutions at faster operating speeds. Makers of computer chips use polymer bases for high-speed inspection machines and for chip bonders that perform 400 welds/min.

MAKE CONTACT Accures Casting LLC, www.accurescasting.com