Rack plating on plastics involves the following important steps:

Conditioner-To enhance surface porosity, chromic acid is utilized as one of the chemical ingredients.

Catalyst-Contains a palladium/tin complex. The palladium is the principal metal used to allow the electroless nickel or copper to make its initial deposit.

Accelerator-Removes the tin from the palladium/tin complex. This solution usually utilizes an acid base.

Electroless nickel-Contains a selection of chemicals to include the nickel metal to deposit on the palladium surface. The deposit acts upon the exposed palladium and is activated by one of the chemical ingredients, usually sodium hypophosphite.

Electroless copperóIs more difficult to control as compared to electroless nickel Also, its relative cost factors are higher. The advantage, however, is increased conductivity, and the copper-plated parts can be held over (in cases in which the pre-plate and electroplate are processed separately, with a long delay in between) for plating at a later time without the copper oxidizing. Oxidation is a problem with electroless nickel, which does passivate with time. The ingredients and control for an electroless copper system are usually proprietary in nature.

Acid copper strike-Used for the initial deposit; low current at about 10 A; 1.5 V; plating time 5 to 8 min. or until good coverage is achieved. This initial strike is necessary to begin the plating process, utilizing a lower metal content and a higher concentration of sulfuric acid.

The copper strike is significant because the parts should be fully covered with copper before they enter the actual plating tank where higher currents are used. Burn-off is sure to occur if the copper strike is not used.

If electroless copper is used instead of electroless nickel, the copper plating sequence will perform with greater efficiency because the electroless copper will transmit greater current without contending with nickel-passive films or poorer conductivity factors.

After electroless plating, we now defend on the plating rack contacts to transmit the lower current to begin copper slating without experiencing burn-off. When burn-off occurs, the electroless nickel has "burned away" because of the excessive current transmitted through the prong contact, which causes the electroless nickel film to dissipate, dissolve or disappear. When this situation occurs, it indicates that there is no longer any metal film to transmit the current on the surface of the part. As a result, plating stops. This may not be experienced with electroless copper, as explained previously, but cost and operational factors must be taken into consideration.

If burn-off occurs partially or totally, the plastic part will go through the entire plating process with either partial or no plating taking place. As a result, each contact has its own potential, and each contact prong will or will not transmit current and accept plating. Each contact prong may function independently of each other.

Acid copper plate-Used for a build-up of copper to level and brighten the part prior to any subsequent plated finish. The copper acts as a "go-between" softer metal deposit over the plastic substrate and under the bright nickel plate to improve efficiency performance of coefficient of thermal expansion factors when plated parts are subjected to cold or hot environments. The copper will expand (under heat) and contract (when cold) in a wider range than other metal deposits, and therefore perform a valuable contribution to multiple metal deposits. Plating time is from 20 to 35 min. at an average of 40 A/ft2.

Bright nickel plate-Valuable as a bright and hard base-metal deposit with a depth of white color as an undercoat for subsequent chromium-, brass- or gold-plated finishes, particularly for decorative plating. The amount of bright nickel is held to a maximum of .0005 in. because the deposit is hard and brittle. With thicker deposits, the film may be subject to stresses and will be reduced in value of nickel adhesion to the copper deposit.

After bright nickel plating, proper rinsing and subsequent plated deposits should be performed as soon as possible, to reduce passivation. Because these solutions use proprietary brighteners, passivation factors are increased. Longer dwell times prior to subsequent plated finishes increase the passive film problems, and different types of problems evolve.

When nickel passivation occurs prior to chromium plating, the results are usually burned edges, smears, white streaks, blue haze and other poor quality effects. The same passive nickel film may not have any effects with brass or gold plating. If problems occur, an analysis must take place to see how they can be resolved. Passive nickel films prior to chromium plating can be reduced by using a nickel activator of some type.

There are proprietary solutions available to remedy passivation, but there may be other attributable factors that should be evaluated in this portion of the process before forging ahead to resolve the problem-especially when it is believed that passive nickel could be a problem source.