The consulting-producer team had considerable experience with thermoset molding compounds. Thermoset’s natural hardness and chemical resistance appeared desirable in a pump bushing. However, these compounds require fibers, usually glass, to give them strength and toughness. Glass is harder than the metal of the shaft and, as a result, causes shaft wear.
To overcome this problem, they used Kevlar to impart necessary strength to the molded part without the abrasive qualities of glass. Kevlar was not previously used because, in the words of a project engineer, “Nothing sticks to Kevlar.” However, the molding producer had recently developed a resin that stuck to Teflon, so the consultant decided to try it with Kevlar.
Drawbacks of Kevlar
Without adhesion, the fiber reinforcement does not add much to the strength of the part. Without adhesion, when machining parts, the machined surface is fuzzy, therefore eliminating the possibility of holding tolerances.
When using any fiber-filled plastics to mold parts, two separate forces determine the strength of the molded part. The first is the strength of the fiber and the second is the adhesion of the plastic to the fiber.
The ideal condition is for the adhesive force between the plastic and the fiber to be greater than the strength of the fiber, or as great as it can be. When a good adhesive force is achieved, the part can be machined to a smooth surface without protruding (fuzzy) fibers.
The second major drawback of using Kevlar filled BMC as a bushing material is that Kevlar absorbs 7 percent moisture at ambient temperature. This causes problems with Kevlar “bullet proof” vests.
As the Kevlar absorbs moisture, the vest becomes less effective. A molded and machined part used inside a high-pressure pump that absorbs liquid will quickly become ineffective.
New Resin Eliminates the Problems
The new resin, when used to manufacture a Kevlar filled BMC compound, solves both these problems. Molding a near-net shape part and machining to size produces a bushing that meets all the requirements listed in the section above.
In addition, it absorbs water at a rate about the same as a glass-filled epoxy, which is the bench mark for water absorption in a fiber-filled molded part. In other words, water absorption is not a problem.
Parts with the new BMC were molded, machined and assembled into a pump for testing. The molded part had a 5-inch outside diameter (OD) and 0.1-inch wall thickness. When quality control professionals checked the part, they found that the OD was 0.010 inch oversize, or 0.010 inch larger than the hole in the steel part into which it was to be pressed.
Rather than re-machining, the plastic part was positioned above the hole in the stainless steel pump housing, and the two parts were placed into a 20 ton press. Three steel plates were placed on top of the two parts to be assembled. As the press applied pressure, a crack like a gunshot was heard.
“Well, there goes your plastic part.” an engineer commented. After disassembly the stack showed that one of the steel plates had cracked, leaving the molded part perfectly seated. That was a 0.010-inch compressive press fit.
Despite skepticism, pump bushings produced with Kevlar-based thermoset material meet all the requirements for pump wear parts: The near-perfect bushing’s material is tough, soft, hard, flexible, stiff, thermally stable, easily machined, non-absorbent and cost effective.
Pumps & Systems, May 2012