Avoiding Plating Headaches, Part I 
 
First of all, there are several references that would be best to tell you about. The design section in Electroplating Engineering by A. Kenneth Graham would be the most readily available. The classic paper entitled “The Relationship of Shape to Effectiveness of Plating” by W. H. Safranek explains why some of the complaints that platers get are beyond their control; in other words, they are built into the part by poor design. If parts had easy to plate contours and avoided complex shapes, a large portion of rejects could be minimized. Another very basic article is the ASTM B507-86 Standard Practice. 
  
Machinists and platers are constantly in a battle over narrowing tolerances and while many advances have been made in CNC machines, not much has helped electroplaters hold close dimensions since Faraday's time in the 1830's, except for electroless plating. If you wish to know more about Faraday's Law, give us a call. It explains what we commonly refer to as "dog boning" and "throwing power". The "dog bone" effect is the term for plating build up on edges and protrusions, including the crowns of threads. “Throw” of finish into recesses and complex shaped parts is another concern due to shape of electrical fields. If all plating were electroless, this distribution concern (that is, dog boning and covering/throwing power) would not be on the laundry list of headaches. Unfortunately chrome, zinc, tin, gold and most other plating operations are not "electroless" and end-up as items 1) and 3) on "THE LIST".  
  
       THE LIST 
  
1) Sharp pointed edges or points. 
2) Lack of either drain holes, handling holes or holes to vent gas from plating operation.            
3) Close tolerance. 
4) Blind holes. 
5) Ultra thin material. 
6) Crimped edges. 
  
In addition to "dog-boning" as a result of sharp points and edges, burning can occur. The sharp edges can cause the current density to be very high, above the allowable range of good operations for the solution, and dulling or dark discoloration occurs.

Another issue that platers have to deal with are holes (laundry list 2):  need to have them, love to hate them. As long as the material is "hefty" we may be able to use hooks and rack thru those holes. If the material is ultra thin (0.001" to 0.005") using holes may lead to damage/distortion of the material around the hole. If modest in thickness and the part is large, the plater has to be cautious about handling in all tanks, including rinses, so that the entry and exit motion does not distort the rack holes. Care must be given if these holes are threaded so as to not damage the threads. On occasion, placing screws or bolts into a threaded hole and racking around the fastener will protect the thread from damage. 
  
Now then, we have protected the threaded hole from damage by using fasteners into the hole, but at this point we have no protection from corrosion of the bare metal in the hole. Masked holes must have fasteners removed, flushed out with rinse water, dried and possibly oiled. Another factor to consider is masking of holes because of tight dimensions. We face the same problems, however, at the end of the plating cycle:  that is, rinsing, protecting and drying to prevent salts build up or corrosion.  When considering masking of holes, threaded holes hold masking plugs far better than their unthreaded counterparts.  Lastly, if you are getting blind holes back with salts or rust in them, your plater is not taking the proper care to avoid this situation. Crimped or folded over edges can entrap cleaning and plating solutions, causing staining, salts or rust leakage. If possible, leave a small gap so our extra special rinsing can penetrate the seam. Another approach is to close the gap after plating with an added operation.  
  
You will note that many of these solutions to the headache list are labor intensive. When possible, we should work together to find solutions to problems that do not increase labor costs. Where possible, create through holes, eliminating blind ones. Do not fold over those crimps so tight so that normal rinsing will suffice. Educate yourself on threaded fasteners. Put that handling hole or gas vent hole in the part.  
  
A major headache for both your and our quality control departments is where to measure thickness, both on the main body of the part and also on fasteners. If a bolt, for instance, calls for 0.0005" thickness, where is this to be measured? Generally we measure the thickness of bolts or nuts on the side of the hex or the unthreaded portion of the shank. On smaller fasteners we use the smooth head or at the end opposite the head. From a machinist's standpoint, it might be prudent to provide go- no / go gauges because the threaded end will build up (it acts like a sharp point) and the lead thread may prevent the entrance of the gage. Dr. Faraday, being the forerunner of Mr. Murphy (Murphy's Law), may dictate that you should get the thickness of fasteners reduced down to the 0.0002" range. This is sometimes allowed under the plating specification. Machinists, keep in mind that that for a 60 degree thread pitch angle a 0.0005" of plating will result in a 0.002" change in the pitch diameter. It would be great if blueprints call out just where we both will measure thickness. One or two per thousand do; most do not. On repeat jobs get that spelled out, so as to prevent later confrontation. It sometimes is also advisable to determine just what type of thickness measurement machine will be used by both of you since there is a tendency for slight variations. 
  
If you would like to discuss a particular project or application, please e-mail Anoplate at sales@anoplate.com or give us a call at 315-471-6143. We would love to help you solve your surface engineering challenges!