P&SF  FINISHER’S THINK TANK Mass Finishing Review: Burnishing

Stephen F. Rudy, CEF Hubbard-Hall Inc. 563 South Leonard Street Waterbury, CT 06708 E-mail: [email protected]

This is perhaps the most popular mass processed without media for purposes of diluted working solution, is metered in at finishing application. Process attributes deburring and enhanced luster. a fi xed rate, continually delivering fresh, include: Burnishing compounds are available as active working solution to condition the • Applicability to all base metals, stamped, powder and liquid concentrates. Effective parts. Equipment requirements are rather drawn, molded parts. burnishing compounds should contribute simple. The drum or tote of concentrated • Produces a bright, smooth, highly refl ec- these operating benefi ts: stock solution is connected to a calibrated tive surface. • Provide a thin lubricating fi lm. Lubricity pump, set to deliver the prepared solution • Removes surface organic soils and is very important. at the preferred fl ow rate, diluted with the scales, facilitating surface preparation • Water hardness tolerance required volume of water. Depending on before a plating cycle. • pH buffered for the application desired quality and effectiveness, the “one • Can be non-aggressive, where there is • Rust and inhibition pass” solution may be recycled, until deter- very little metal removal from the sur- • Flexible foaming characteristics (high mined to be spent. Other than the enclosed face. and low) horizontal barrel, parts can be easily sam- • The focus is honing and removing sur- pled to examine progress of the cycle. In face imperfections. Active components of the burnishing horizontal barrels, the concentrate is added • The base metal surface is leveled and compound work on the part surface, leav- along with water and the optional media. polished prior to additional fi nishing. ing a residual film. This is usually seen Final preparation is made before the barrel • Improves brightness and leveling of the by the water break condition after post- burnishing cycle begins. All the benefi ts plated fi nish. In some instances the plat- burnishing rinsing. Standard soak clean- and simplified operation are contingent ing deposit thickness can be decreased. ing should remove this film as part of a upon proper cycle development. This is surface preparation cycle before plating. true for any mass fi nishing cycle. Burnishing can be quantitatively Application features for burnishing sys- expressed as a measure of the distance tems vary somewhat between liquid and Application: Powder type between high and low points on a metal powder concentrates: • Preferably added to horizontal and surface (peaks to valleys). This can be oblique barrels. The use in vibratory instrumentally measured and interpreted as Application: Liquid type equipment is compatible, especially for an RMS (root mean ). The lower the • Signifi cant handling and control benefi ts shorter cycles. RMS value, the fl atter or more leveled the in fl ow through vibratory systems. • Addition rates of 1 to 4 oz/gal of bur- surface. For example, an RMS value of 1 • Easily used in horizontal and oblique nishing compound per gallon of required describes a more highly burnished surface barrels. water. than one having an RMS of 5. • Can be metered as pre-diluted or in ratio • Working solution containing parts and There are two classifi cations for burnish- with water. optional media tumble for a desired ing: • The working solution concentration may time. 1. Burnishing. Parts are processed with range from 0.5 to 2.0 vol%. • The working solution pH range may be either or porcelain media, • In vibratory equipment, the solution mildly acidic, but usually more toward a or without media (part on part). The works through the load and drains out at pH range of 7 to 11. expected fi nish is for general brightening a continuous fl ow rate of 1 to 5 gal/ft3/ and leveling. hr. The powdered burnishing compound 2. Ball burnishing. Parts, usually softer • Per the surface fi nish requirement, the is added new for each cycle run, thereby metals such as aluminum and brass, solution pH may range from 3 to 11. insuring fresh, optimized chemical mix are processed with case-hardened steel to condition the parts. Powdered systems or media, producing A liquid system offers several ben- offer benefits in that particular active maximum brilliance. This can be accom- efits. Direct handling of the concentrate components may be formulated into the plished before transfer to a plating cycle is minimal, benefiting worker safety. In blend, exceeding the amount that can be or after plating, to improve the final some applications a suffi cient amount of dissolved into a liquid product. Soaps, cer- luster of the fi nish. product may be consumed, permitting the tain conditioners and surfactants are some use of returnable containers, such as totes, components affected in this way. Powdered Shine rolling is another mass fi nishing thereby eliminating packaging disposal burnishing compounds are equivalent in process that has some similarity to bur- requirements. The concentrate as a pre- effectiveness to their liquid counterparts. nishing. In shine rolling, parts are usually

28 Plating & • August 2007 Trouble Shooting Tips

Problem Possible Cause Correction

Dull, hazy fi nish Oil and grease in fi lm Preclean before burnish

Dull (no oily soils) Scale Descale fi rst

Parts stained, rusty, tarnished Insuffi ciently dried parts Dry quickly after rinse

Media and parts fl oat Excess compound, water Adjust concentrations

Parts cushioned Excess compound Adjust concentrations

Rust, corrosion Very low compound conc. Adjust concentrations

Scratchy, dull Very low compound conc. Adjust concentrations

Media, parts corrode Very low compound conc. Adjust concentrations

in relation to chemistry and concentration Handling, pre-dilution, metering and appli- Media of the burnishing process and mechanical cation is a benefi t of the liquid burnishing Six shapes of case-hardened steel media action of the equipment, are factored into formulations. are normally used: ball, ball cone, diago- optimum media-to-parts ratio. nal, oval, pin and wheat. The ball is best Additional facts used, unless specific geometric shapes • Burnishing cycles typically range from result in lodging or insuffi cient media to Equipment 20 to 60 minutes. Longer cycles may be part surface contact. Keeping steel media Horizontal and oblique barrels are charged required based on specific testing and clean and protected with a rust inhibitor with media, burnishing compound, and evaluation. will prevent pitting and rusting. suffi cient water. For many process cycles, • Rust inhibitors protect steel parts. In par- Ceramic and porcelain types are avail- loading the barrel to 50 to 60% of total ticular, they protect steel media for long, able in surface finishes that permit their capacity is sufficient. This loading and dependable service life. use in deburring or descaling and burnish- barrel speed are critical to obtaining the • Inhibitors prevent tarnishing and darken- ing, without the need to change the media. best slide zone. This is where parts, media ing of brass and copper alloys, as well as These types may exhibit a smooth, glazed and active chemicals slide together, facili- corrosion of zinc and aluminum. surface, containing a fixed amount of tating optimum action of media and chemi- • Acidic burnishing effectively removes selected . Different shapes pref- cals on the parts. Since this only occurs on smut and carbon deposits off steel after erentially work on and condition specifi c the downward slide in the barrel, testing descaling and deburring, before the fi nal geometrically formed parts. Keeping the is required to determine proper loading. burnishing treatment. media clean is key to effective burnishing. Barrel speed must also be set to prevent • Descale, de-tarnish, and burnish copper, In general, media-to-parts ratios of 10:1 to centrifugal forces (usually at too high a brass and bronze in one step. 20:1 are used in burnishing cycles. This, rotation speed) that would prevent the parts from sliding. Vibratory equipment makes use of vibrational and rotational energies to work Answers to I.Q. Quiz #433 with media and burnishing chemicals. Mechanical work is constantly being per- formed on parts. The effect is faster action. 1. Silicones - they can devastate the entire plating line. Polyacrylate foam Trial evaluations should be conducted to suppressants are fi ne. optimize energy setting of equipment in relation to chemistry type, concentration 2. Faraday’s Law states that 96,500 coulombs (one Faraday) will deposit the and media-to-parts ratio. P&SF gram equivalent weight of a metal

3. False. Magnetic gages are used for the non-destructive measurements of the thickness of non-magnetic deposits on steel,

4. One of the primary reasons is that, depending on the anodizing conditions, the electrolyte can dissolve some of the coating.

5. Amphoteric. For example, aluminium hydroxide in reaction with acids will + +3 Mark your calendar now and behave as a base: Al(OH)3 + 3H Al + 3H2O and in reaction with bases it - - will act as an acid: Al(OH)3 + 2OH AlO2 + H2O don’t miss the 2008 NASF Washington Forum, April 22-24.

Plating & Surface Finishing • August 2007 29