in the process I'm familiar with - I've seen a couple of things that can give rise to surface defects.
Firstly, there is the moulded parts themselves - if they aren't packed well you can get small defects in the final finish that look like bits of dirt. Upon investigation, we found that the moulding had pockets of air and or dirt/regrind in them which could be removed by good tool maintenance and strict material controls.
Secondly, we had a problem initially plating the component with copper, prior to putting on a bright chrome finish. In our case, we were over heating the moulding when plating it with copper, and although the resulting surface appeared ok - when it was plated with chrome, we had areas where the chrome finish appeared dull.
Thirdly, we had some problems moving components between copper and chrome plating tanks - there was too much opportunity for dirt to fall on them. This gave a condition similar to the first effect. We got round this through preventive maintenance (Cleaning and lots of it !!) putting a local roof over the plating tanks - and also washing the components thoroughly between each tank. One point though - be careful what you wach it with !!! We initially tried tap water (yeh i know NOW !!!) but we had a lot of trouble with submerged solids and salts which gave us some oclusions when final plating.
Finally, we altered the thickness of the chrome plating to make it less susceptible to oclusions, we increased this from 0.07 to 0.12 nominal and found that it gave what the customer described as a 'deeper colour' and made it more tolerant to the problems outline before. The was probably the hardest change we implemented though, because it meant the customer had to stump up some more money, which he wanted to offset against a reduction in scrap !!
Great advice. I will come at this from the molding end. There are several things in part molding that can cause plating issues. Any mold release agents used or touching the parts without gloves can cause oils to be deposited on the surface which will affect plating. Any excessive molded in stress will cause issues with plating (cracking, delamination, etc). This can be examined by submerging the part in Glacial Acetic acid using the following test method:
ASTM D 1939-84, "Determining Residual Stresses in Extruded or Molded Acrylonitrile-Butadiene-Styrene (ABS) Parts by Immersion in Glacial Acetic Acid"
