sign in sign up
<<
Home , Mechanical plating, Parts cleaning, Washer (hardware)

Finishers’ Think Tank

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

Mechanical : A Beneficial Process to Consider

Mechanical plating is essentially a marily composed of silica, carbonate 7. Rinse. tumbling process, by which - and limestone. It offers a relatively lic dusts are deposited on to a metal long service life, contributing very 8. Retrieve. Parts are separated from substrate. The terms cold-welded and little friction to the operating process. the media and collected for post-plate bonded are used to describe the mecha- finishing. nism. Normally, parts for conventional 3. Surface conditioning II. This barrel plating can also be mechanically step completes the critical treatment, 9. Post-finishing. Chromate, sealer, plated. Deposited may include by depositing a thin, tightly adherent lacquer, waxes. , , , precious metals copper film. Deposition of copper (e.g., gold and ), aluminum and promotes a uniform surface metal pro- 10. Dry. . Coated substrates include file. It also forms a mechanical barrier ferrous, castings, copper and stainless between the basis metal and hydrogen. The operating cycle uses a fresh make- . Each step in the cycle differs in (Time, 2 - 7 min) up solution for each step, eliminating varying degrees compared to conven- carry over or re-use of contaminated tional surface preparation, plating and 4. Promoter or accelerator. This solu- solution. Segregating parts in the same post-finishing. The biggest and most tion consists of a proprietary additive. barrel eliminates their loss, because critical difference is using one single It standardizes conditions for deposi- nothing is transferred until the cycle process vessel for most mechanical tion of the metal of choice. The pro- is complete. The solution for steps 2 plating cycles. The vessel is similar to moter controls agglomeration of the thru 6 is preferred at 65 - 85°F (18 an oblique tumbling barrel, and is the yet-to-be-deposited metal powder and - 29°C). Some process development primary process “tank”. maintains surface cleanliness. (Time, 2 is required to determine the optimum - 3 min) conditions for steps 2 thru 6. Metal A typical cycle powder requirements for deposit Flash coat. 1. Parts . This step is usu- 5. This is the first addi- thickness are measured on the basis of ally accomplished in bulk, off-line. tion of metal powder to be deposited. metal powder weight / deposit thick- Traditional hot alkaline cleaning, Typically, the powder is zinc, as ness / surface area of parts. Important mechanical or solvent clean- zinc is the most common mechani- considerations are parts loading, ratio ing are employed as to effectiveness, cally-deposited metal. The flash coat of parts to impact media, volume availability, economic feasibility, etc. rapidly covers the copper layer with of , barrel rotation (speed and Alternatively, the parts can be tumble- active sites of zinc, readily accepting angle) and desired deposit thickness. cleaned in the additional layers of this metal deposit. The majority of finishing cycles offer barrel. Satisfactory cleaning proceeds (Time, 4 - 7 min) approximate cycle parameters, requir- to the next step. ing minimal fine-tuning. Because 6. Deposition build-up. Based on the solutions are acidic, rubber or 2. Surface conditioning I. This criti- the thickness requirement, sufficient -lined barrels are additions of metal powder are added. preferred. Barrels vary greatly in han- cal step utilizes the action of a special 3 chemical additive and media. The bal- Usually, 2 - 4 min are allowed between dling volume, from one to 50 ft . Glass anced chemical and mechanical action additions. This promotes uniformity of impact media size and the ratio of parts develops a part surface free of and metal thickness on parts, with accom- to media is very critical. Requirements scales. The chemical additive consists panying deposit smoothness across the for media include cushioning the load, of surfactants, inhibitors and inorganic entire barrel load. preventing scratching and abrasion, acids. Media is a glass bead type, pri- and reaching into tight recesses and

32 Plating & • August 2010 August 2010 • Plating & Surface Finishing 33 geometric areas. Optimum conditions discharged effluents are low in metals Mechanically Zinc Plated with permit the media to transfer mechani- and do not contain any chemicals that Chromates cal energy efficiently from the rotating would affect standard waste treatment ASTM B-117 barrel to peen metal dust particles on systems. Post finishing, such as chro- the substrate. Thickness continually mating, requires the same treatment Zinc Blue Yellow builds over previously bonded metal as for an line effluent Thickness (hr) (hr) layers. Size and diameter of the media for treatment. Aside from the powder (in.) is important to prevent it from being metal additives, surface conditioners 0.0003 90-95 150-175 lodged and trapped in the parts. Media and promoters are primarily liquid 0.0005 100-125 200-225 diameters may range from below concentrates. Therefore sludging is not 0.001 in. to above 0.25 in. Experienced a post-treatment problem. 0.001 200-225 285-310 operators develop reference points that 0.002 295-310 390-410 offer good confirmation of preferred Related costs load performance. These include color Based on the described equipment for Application of a chromate and post of the foam or solution, and color and mechanical plating, the cost savings seal can provide over 1,000 hours of appearance of the parts. may range from 65 to 75% compared salt spray protection. to traditional electroplating. Other eco- The major international automotive Features and benefits nomic benefits include: companies specify mechanical plating By using mechanical plating: • Less manual labor step-to-step. for certain finishes. ASTM and MIL • is elimi- • Able to complete an entire cycle specifications describe cycles and fin- nated,. within 60 minutes. ishing parameters. State and Federal • Post plate baking is not required; • No heating required after cleaning. highway and transportation agencies • Uniformity of deposit thickness is • Filtration and purification not specify mechanical zinc finishes. On consistent. required. an industrial scale, many finishes • “Dog bone” thickness distribution • No anodes, consumable or inert. require mechanical zinc for thickness • is eliminated,. Post-plate baking eliminated. and corrosion protection. P&SF • Excellent deposit adhesion is obtained. Corrosion protection • Up to 1 mil deposit thickness is Unlike the electroplating application, deposited without harmful stress. up to 1 mil or 0.001-inch deposit • Flat parts are compatible (no stick- thickness (zinc) is routinely obtained. ing, nesting or masking), thanks to Depending on finishing requirements the media. or specifications, any thickness and • The process readily plates hardened post-finishing can be obtained. , powdered metals, fasten- ers, nuts, bolts, washers, clips and . • It allows excellent chromating of plated parts.

Compared to conventional electroplat- ing, the mechanical plating cycle is much simpler. Most steps are com- Answers to I.Q. Quiz #463 pleted in the same barrel. Conventional From page 29. polypropylene plating barrels are not eliminated. This greatly reduces allo- 1. cast , high and carbo-nitrided steels cated space requirements. Rectifiers are not required, as mechanical plating 2. b, d, e does not use current. Rinsing uses suf- ficient water for each step. The plating 3. Higher conductivity solution does require any maintenance, since it is replaced after every process 4. Highly corrosive; lower throwing power; brittle, less pure deposit; run. By determining optimum condi- sensitive to iron contamination; requires close pH control. tions, over 95% efficiency is achiev- able. 5. True. Mechanical plating uses non-com- plexing and non-chelating chemistries. Under proper operating conditions,

32 Plating & Surface Finishing • August 2010 August 2010 • Plating & Surface Finishing 33