Throwing power is the ability to plate into low current density areas with the same thickness as in the higher current density areas. Coverage is the ability to obtain a chrome deposit in a difficult area regardless of thickness. In practice, these two terms are used interchangeably and generally refer to missed and bare spots also known as skip plating. Chrome has a relatively poor throwing power so high current density areas will always receive a thicker deposit than low current density areas will. This is the reason that conforming anodes, shields and robbers are frequently used as they assist in controlling deposit thicknesses. Poor throw, poor coverage and skipped spots include:
Bath chemistry imbalance:
Low Dura Additive, low chromic acid, either a high or a low sulfate (improper ratio) or bath contaminants that are too high will cause this problem. Send a bath sample to Plating Resources, Inc. for a detailed analysis. Make bath adjustments based on the analytical results.
Too low a current density.
Increase the current to 2 ASI for best results. The use of a surge current will also help with both throwing power and chrome coverage.
Bath temperature too high.
Bath temperatures much above 145 degrees F. tend to reduce throw and coverage. Lower the temperature to either 130 or 140 F. and maintain it within +/- 2 degrees F. if possible. The use of adequate bath agitation and a closed loop cooling system will be of use here.
Parts need to be properly cleaned BP to remove all traces of oils, organics and surface debris. Refer to both the Process Cycle and the Basic Cycle for suggestions. The best suggestion is to Dura Prep scrub, for smaller lot quantities, if no other remedies works.
Poor electrical contacts.
Check and clean all buss connections as well as anode and cathode hook contacts, check for gassing, clamp if needed.
Anode scale prevents the proper flow of current and causes poor throw and coverage. The anodes need to have the proper brown - black lead peroxide film for maximum electrical transfer. Wire brush to remove any heavy scale and clean in Ano Best 104.
Also, check the gassing to ensure that the hook contact is solid; clamping may be needed.
The part needs to be both chemically clean and active before it is loaded into the chrome bath and certain base metals tend to be passive in nature. If the part is not properly cleaned or if it has a passive surface the chrome deposit will not throw or cover adequately. The best suggestion is to Dura Prep scrub, for smaller lot quantities, if no other remedies works.
Improper anode spacing.
An anode that is too far away from the part being plated will greatly reduce it’s throwing and covering power. The use of conforming anodes will be of great benefit in controlling both chrome coverage and deposit thicknesses.
A bi-polar condition can result from parts dropped in the tank and from heaters, coils, and tank walls that may be in circuit as well as any shorts in the buss bars. These will steal current that should be directed towards the part being plated. Check for stray currents and eliminate them if any are found.
Improper use of robbers and shields:
Using robbers or shields that are either too close or too large.
Open holes in cathode.
A bath with poor throwing power tendencies will have a difficult time plating around holes in a part; this can also cause a non-uniform deposit on adjacent areas. The use of a Dura Bath should overcome this difficulty.
Hydrogen gas entrapment.
Hydrogen can become trapped in inverted cup shaped areas and build up as the plating continues. This prevents solution and electrical contact in these areas. The remedy is to fixture parts so that the gas can escape properly.
Excessive reverse etch.
An excessive reverse etching time can passivate certain base metals. Check the proper reverse etching times for the base metal and hardness being plated. Also, over etching can damage the base metal and bring carbon to the surface which causing pitting.
Too close to the solution level.
Parts placed too close to the solution level will not receive the proper amount of current which can cause a loss of throwing and covering power. It is best that the top of the part be no higher than 4" below the solution level.