Increased Machinability And Improved Finish Of Powdered Metal Parts

Powdered metal components, often used in automotive and other applications, are inherently porous. Problems encountered when dealing with powdered metals include machining issues such as chatter, which can diminish tool life; and blowouts or blisters, which can affect surface finishes. But old methods of preventing these problems have traditionally led to inconsistent results. Now, vacuum impregnation is emerging as a solution. This process seals the part internally using a vacuum pressure chamber to impregnate a specially formulated liquid resin into minute voids. Magna-Tech/TechnaSeal (Muncie, Indiana) provides vacuum impregnation services on an outsourcing basis.

Powdered metal components are made by compressing millions of tiny grains together to form a specific workpiece. Because the workpiece is made of a combination of grains, the surface and the internal structure can be uneven or contain gas pockets. As a result, when a cutting tool passes across the surface or into the workpiece, these irregularities interrupt the cut. The subsequent vibration (chatter) and uneven contact cause premature tool wear and can decrease part performance. This unevenness of finish can also alter the dimensions of the workpiece.

Some manufacturers try filling porosity inherent in their parts with a process called “copper infiltration.” Copper is placed on the surface of the part as the component is put through the high-heat sintering process. The copper melts down onto the uneven surface, into the voids, to create a smooth finish. Other manufacturers add manganese sulfide into the alloy before the part is formed. However, according to Magna-Tech/TechnaSeal officials, both of these approaches have been shown to result in a lack of uniformity in the surface. Therefore, chatter can still occur.

Vacuum impregnation is not an outside coating, but it instead seals the part internally using a vacuum pressure chamber to impregnate a specially formulated liquid resin into minute voids, the company says. The sealant is formulated to withstand high heat and corrosive solvents, and it forms a perfectly uniform surface that is both more attractive and easier to work with.

At Magna-Tech/TechnaSeal, parts ready for assembly are placed into baskets and then into a vacuum pressure chamber. A vacuum is drawn, extracting the gas from tiny voids in the component and making the empty areas receptive to filling. The chamber is then filled with resin and repressurized, forcing the sealant into these voids and sealing the part from within. The resin is extracted from the chamber, and a centrifuge spins the parts to remove excess surface resin. After passing through a rinse, the parts are moved into a catalyst activator tank that cures the resin at the surface of each porosity, creating a hardened cap in the outer portion and trapping the remaining resin for anaerobic self-curing. The final rinse removes any residual activator. The parts are then placed in an oven at 150° F, which reduces the drying time to about one hour. The sealant thus cures into a hard, permanent internal seal. Through batch processing, as many as 200 to 1,000 parts can be vacuum impregnated at a time, making results consistent, the company says.

In a study by a major chemical company comparing the benefits of machining parts treated with impregnation versus manganese sulfide, it was found that drill bits lasted longer, performed more consistently and made cleaner cuts on a powdered metal component that was sealed with impregnation. According to the study, “Drill wear was progressive and nearly linear with holes drilled with resin impregnated parts, while drill failures were often catastrophic (50 percent damaged) in tests with manganese-sulfide treated parts, and always catastrophic with parts that had neither.”

Vacuum impregnation can also help solve finishing problems. For parts that require a final coating (such as a nickel plating, chrome plating or paint) vacuum impregnation can prevent blistering, bubbling and blowouts. Because unsealed powdered metal components can contain air pockets, gases trapped beneath the surface can rise and push against the finish coating, causing bubbles. This outgassing can be prevented by sealing with vacuum impregnation, which permanently fills these voids from the inside.