Electroplating Without Part Submersion
Reminiscent of the old hair dressing commercial, which sang that "a little dab'll do ya", brushplating has a unique and special niche in the electroplating field. In addition to serving as a repair process, it permits selective plating on parts which are too large for standard tank immersion.
Most of us are somewhat familiar with tank plating where parts are immersed in a bath of solution. A weak electric current is applied causing metal ions to coat or "plate" the part.
A small but important part of electroplating is done without part submersion. This process is known as selective or "brush" plating. It has also been known as swab or contact plating and various other descriptive names.
Very few people are aware of this type of metal plating process, or of the significant advantages it offers the metalworking industry.
At first glance you might think the operator is doing a painting operation as he dips the tool into a solution and wipes it over the part. Actually a series of water-based electrolytes are being used to prepare and plate the part. The plating tool consists of an insulated handle on which an anode, usually made from graphite, is molded. The anode is covered with an absorbent material which holds the solution. Two electrical leads connect from the terminals of a D.C. rectifier to the plating tool and part. When tool and part make contact, the circuit is closed. As the current flows, metal ions from the solution are deposited onto the part forming a molecular bond. The plate applied has all the characteristics provided by immersion plating, but can be selectively applied.
Fully developed brush plating systems began in France in 1938. Commercial use began in Europe in 1947 and Sifco purchased the North American rights covering the United States and Canada in 1959. It became known here as the "Dalic Process" of electroplating and a division was formed, now known as Sifco Selective Plating, to market the process and the equipment needed. Well over one thousand commercial installations are currently being used in the United States. The first commercial specification for brush plating standards appeared in 1956. Hundreds of others have followed. A few in the aircraft industry are: Boeing Aircraft BAC 5849, Grumman Aircraft GSS 8060C, McDonnell Douglas PS 13113, General Dynamics FPS1046, Martin Marietta STP30056, Hughes Aircraft CPS-4-320, Sikorsky SS 8413, Lockheed Aircraft MPS 1118A, Kahr Bearing Div./Sergeant PBS-020D.
A typical job might require the reduction of a 6 inch diameter by 2 inch long bore in a housing which is oversize 0.002" on the diameter. The area to be plated is pre-cleaned and adjacent areas are solvent cleaned to permit masking. If the base material is cast iron, three cleaning steps are used: electroclean, etch and desmut. Several nickel and copper solutions could be used for plating depending on where the part is to be used and its function. The part is then plated to tolerance by controlling the amount of ampere hours passed while plating. The amount of current required is obtained by the formula: AMP Hrs. = F x A x T. Where F is the calibration factor for the particular solution used (0.015 for example); A is the area to be plated (6 x 3.14 x 2 = 37.7 sq. in.); and T is the thickness of the plate required in .0001" increments. For example: T = 12 on this part. It is calculated as follows: 0.0010 per side + 0.0002 inch to bring the area into the middle of the tolerance range. The amp hours required for the part are 0.015 x 37.7 x 12, or 6.79. The 6.79 amp hours can be completed in just a few minutes, depending on the amount of current used. Sifco Selective Plating has developed a microprocessor-based rectifier which prompts the operator with questions and does all the calculations, internally, relieving the operator of having to go through the mathematics of the formulas.
The payback on equipment is usually under six months. A first job payback is not uncommon where expensive parts are being repaired. The cost savings by OEM operations, where a small section of a part can be plated instead of the entire part, can be extraordinary.
Some of the primary advantages of brush plating are:
- the systems is portable
- capable of plating to tolerance minimizes hydrogen embrittlement plates 30 to 60 times faster than tank plating
- permits plating on parts too large for plating tanks
- does not necessarily require disassembly of a unit
- reduces machine downtime and production delays
- easy to operate
A few of its uses on parts are to:
- provide corrosion protection
- improve wear resistance
- improve solderability or brazing characteristics
- decrease electrical contact resistance...prevent galling
- serve as bearing surfaces
- salvage worn or mismachined parts
Seventy-two metallic solutions are available. There is also a solution and method for brush plating with three types of nickel sulfamates.
Brush-plating job shops offer on-site services for OEMs on parts too large to make transporting them economical.
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