Think Tank

Stephen F. Rudy, CEF Enequist Chemical Co. 100 Varick Avenue Brooklyn, NY 11237 718/497-1200 E-mail: [email protected] Mechanical Processing— An Alternative To Traditional Plating

The fi rst half of the twentieth century hot alkaline cleaner. The cleaned parts 8. Retrieve. Parts are separated from was a springboard to many technical inno- would be rinsed and transferred to the media and collected for post-plate fi n- vations. Manned fl ight, communications, mechanical plating line for additional ishing. energy (nuclear and conventional), health, surface preparation. Alternatively, the 9. Post Finish. Chromate (e.g., clear, blue, medicine, manufacturing, and transporta- parts may, by previous test confi rma- yellow). Silicate seal or lacquer. tion are but a few of these. By researching tion, be tumble cleaned in the mechan- 10. Dry. each, I’m certain specifi c disciplines in ical plating cycle process barrel. plating and surface fi nishing contributed 2. Surface Conditioning (I). This critical Shopping List to the critical growth, success, and expan- fi rst step combines the action of a The operating cycle incorporates the use sion of each. Many treatment and fi nishing special chemical additive and media. of a fresh make-up solution for each step, processes were developed to meet new The balanced chemical and mechani- eliminating the carryover or re-use of con- and tougher requirements. Improvements cal action develops a rust- and scale- taminated solution. Segregating parts in and refi nements continue as “the bar of free surface, readily accepting a metal the same barrel eliminates loss of parts, expected accomplishment” rises. deposit. The chemical additive consists because they aren’t transferred out until the There are systems in place that offer the of surfactants, inhibitors, and inorganic cycle is complete. Solution temperature for fl exibility to build and branch out, while acids. Media is a glass bead type, pri- steps 2Ð6 is about the same, with 65Ð85¡F still retaining their individual cycles and marily composed of silica, carbonate, (18Ð29¡C) preferred. Some process devel- methodology. One of these is mechanical and limestone. This material offers a opment is required to determine the opti- plating, also referred to as peen plating. relatively long service life, contribut- mum conditions for surface conditioning, With its advent in the post-World War II ing very little friction to the operating promoter, fl ash coat, and deposit build-up. years, it has retained the basic sequence of process. Metal powder requirements for deposit steps, but has refi ned the state of its art. 3. Surface Conditioning (II). The second thick ness are measured on the basis of metal step completes this critical treatment powder weight/deposit thickness/surface by depositing a thin, tightly adherent What It Is area of parts. Critical areas include: copper fi lm. Deposition of copper Mechanical plating is simply a tumbling promotes a uniform surface metal pro- procedure by which metallic dusts are cold- ¥ Parts loading, fi le and forms a mechanical barrier welded or bonded onto the base substrate. ¥ Ratio of parts to impact media, between base metal and hydrogen (2Ð7 Normally, parts for conventional barrel plat- ¥ Volume of water, min). ing can be mechanically plated. Deposited ¥ Barrel rotation speed and angle, and 4. Promoter (or accelerator). A propri- metals include: zinc, aluminum, copper, ¥ Desired deposit thickness. etary additive. It standardizes condi- alloys (such as zinc-nickel), tin, precious tions for deposition of the metal of (gold and silver), and cadmium (although The vast majority of fi nishing cycles choice. It will control agglomeration of now much limited due to its toxicity). offer approximate cycle parameters that the yet-to-be-added metal powder and Coated substrates may be ferrous, castings, usually require some minimal fi ne tuning. maintain its cleanliness (2Ð3 min). stainless steel, or copper. Each step in the Because the solutions are acidic, stainless- 5. Flash Coat. The fi rst aliquot of metal cycle differs in varying degrees compared steel barrels, rubber or appropriate plas- powder to be deposited (e.g., zinc) rap- to conventional surface preparation, plat- tic-lined barrels are preferred. Barrels vary idly covers the copper fi lm with active ing, and post fi nishing. The biggest and greatly in handling volume, from 1Ð50 ft3. sites of zinc that will readily accept most critical differences are the use of one Glass impact media size and ratio to parts additional layers of the metal deposit process vessel and fresh make-up solution is very critical. The requirements for media (4Ð7 min). for most steps in the mechanical plating include: cushioning the load, preventing 6. Deposition Build-up. Based on the cycle. The vessel, similar to an oblique scratching or abrasion, and the ability to thickness requirement, several aliquots tumbling barrel or cement mixer, becomes reach into tight geometric recesses and of the very fi ne metal powder may be our primary process “tank.” This column threaded areas. Optimum conditions permit added. Approx 2-4 min between addi- will focus on a typical cycle. the media to effi ciently transfer mechani- tions of metal powder. This promotes cal energy from the rotating barrel to peen uniformity of metal thickness on parts metal dust particles on the substrate, and How It’s Done with accompanying smoothness across build thickness over previously bonded 1. Parts Cleaning. Accomplished in bulk, the entire barrel load. metal layers. Size and diameter of media off line, by immersion in a suitable 7. Rinse. must also be considered to prevent it from

48 Plating & Surface Finishing • January 2002 being lodged and trapped in parts. Media conditioners and promoters are primarily yield more than 1,000 hr of salt spray pro- diameter may range from <0.001 in. to liquid concentrates. As such, they are typi- tection. >0.25 in. Experienced operators develop cally low in solids, only minimally contrib- reference points that offer good confi rma- uting to any sludging during bulk neutral- Mechanical Zinc in Application tion to preferred load performance (e.g., ization of the spent solutions. color of foam or solution, color of parts, The major automotive corporations world- appearance). wide have included mechanical zinc for Related Costs many years in their particular fi nishing spec- Every process retains specifi c expenses ifi cations. ASTM and MIL specs describe Bragging Rights relative to the individual capabilities and cycles and fi nishing parameters. State and Listed are some features and benefi ts of the requirements. Based on the described equip- federal highway and transportation agen- mechanical plating process. ment for mechanical plating, the cost sav- cies also specify mechanical zinc fi nishes. ings when compared to traditional elec- On an industrial scale, many fi nishes call ¥ Eliminates hydrogen embrittlement troplating may range from 65Ð75 percent. for mechanical zinc, or specify it to meet ¥ Post-plate baking is not required Processing of parts also reveals some addi- coating requirements or corrosion protec- ¥ Deposit uniformity of thickness is con- tional economic benefi ts: tion. sistent throughout the part Mechanical plating is an important con- ¥ “Dog bone” effect of thickness distribu- ¥ Handling from step-to-step involves less tributor to quality fi nishing. This process is tion is eliminated manual labor in position to help meet and exceed future ¥ Adhesion is excellent ¥ A complete cycle may be completed requirements as the industry continues to ¥ Up to 1 mil deposit thickness without within 60 min improve its efforts to fi ght corrosion. P&SF harmful stress ¥ Each cycle step (after cleaning) does not ¥ Flat parts are compatible (no sticking, require maintained heating nesting, or masking) ¥ Solution chilling is not a factor ¥ Readily plates hardened steels, powdered ¥ Filtration, other purifi cation equipment, metals, fasteners, nuts, bolts, washers, and methods are not factors clips, and screws ¥ Buildup times for metal deposits are not ¥ Excellent chromatability of plated parts proportional to desired thicknesses ¥ No anodes (consumable or inert) More Benefits ¥ The post-plate baking step is eliminated Compared to conventional electroplating, These acknowledged cost savings are the mechanical plating cycle is much sim- built into total process economics. If aes- pler. Most of the steps are completed in the thetic brightness is desired (depth and lev- same barrel, so tanks and space are greatly eling), mechanical plating does not match reduced in the mechanical process. Being proprietary electroplated systems. At best, non-electrolytic, rectifi ers are not required. the mechanical fi nish develops by some con- Water usage is predominantly a factor of tributory burnishing, a low-level brightness. suffi ciently charging the barrel, unlike a As the mechanical thickness requirement much larger in-place tank. Rinsing uses gets lower, the relative cost versus equiva- suffi cient fresh water for each step. Drag-in lent electroplating thickness gets closer. of solution from a previous step should not occur. The plating solution requires zero maintenance after a run, because it’s Corrosion Protection always replaced. There are no polypropyl- Unlike the electroplating application, up ene plating barrels that require more fre- to 1 mil or 0.001 in. thickness (ex. zinc) quent repair and service. As previously is routinely accomplished. Depending on stated, each proc ess solution is replaced the fi nishing requirements or specifi cations, with a respective new one after a cycle any thickness and most post-fi nishes can be run. obtained. An example of the related corro- By determining the proper additions of sion protection is tabulated below: metal powders and related operating param- eters discussed previously, deposition effi - ASTM B-117 Salt Spray Data ciencies greater than 95 percent are achiev- Mechanical Zinc Plating able. With Blue & Yellow Chromates The effl uent does not present any spe- Zinc cial burden for the waste treatment system. Zn thickness, in. Blue, hr Yellow, hr Mechanical plating uses non-complexing 0.0003 90-95 150-175 and non-chelating chemistries. Being acidic, 0.0005 100-125 200-225 cyanides are not present. Under proper 0.001 200-225 285-310 operating conditions, discharged effl uents 0.002 295-310 390-410 are low in metals and do not contain any chemicals that hinder or complicate most The thickness of the zinc deposit and common waste treatment systems. Because its properties (adhesion, ductility, unifor- chromating is accomplished in a separate mity, etc.), provides a base of strong sup- tank, chrome-containing waters can be port for the overall corrosion protection. appropriately segregated and treated. Aside Application of a chromate and post-seal from the powder metal additives, surface (e.g., lacquer, wax, polymer, organic), can

Plating & Surface Finishing • January 2002 49