Hard Chrome plating is a very important surface treatment method for the plastic processing industry.Hard Chrome layers are precipitated electrochemically from standard or mixed acid baths.The mixed acid baths used of late have the advantage of a higher precipitation rate and the disadvantage of being more aggressive,so that problems may accor in the masking of molds.In contrast to other surface treatment methods,hard chrome plating is performed at relatively low temperatures,and the as a consquence it offers some advantages.
Hardening without distortion
Wear resistance
Surface smoothness
Low coefficient of friction
Little tendency for materials to adhere
Resistance to chemical environments
Corrosion resistance
No oxidation at elevated temperature
FACTORS AFFECTING THE PROPERTIES OF CHROME SURFACE LAYERS(CHEMICAL AND ELECTROCHEMICAL ).
To achieve all the desired properties,hoever,a great deal of experience is necessary.Factors influencing the properties of the chrome layer.
MATERIAL : Analysis ,Method of production,Surface quality.
CONDITIONING : Surface machining,Cleaning,Degreasing ,Activating.
TREATMENT : System used,Treatment bath,Precipitation conditions.
FINISHING : Mechanical treatment,Heat treatment.
Its clear that great importance must be attached to the base material to be hard chrome plated.It should exhibit a relatively high of hardness in order to provide adequate support,as otherwise flaking may occur under high specific loads because of the relatively high hardness of the hard chrome plating.The material used for the mold should also be as clean as possible in order to avoid defects in the chrome layer deposited.Any pores present on the surface are retained as defects in the chrome plating.The heat treatment of mold that are to be hard chrome plated must be performed in accordance with very exact guidelines.The surface must be completely neutral i,e there must be no carburizaton or scaling.Similiar effects can occur if,for instant,partial oxidation takes place as a result of overheating in the course of surface grinding.It can be seen that foreign elements with a detrimental effect on the hard chrome plating process have diffused into the surface.Analysis with a microprobe indentified these elements components of the salt bath.
Plastics processing tooling particular should exhibit as little surface rougness as posible,since the surface of the regions that determine the part shape usually must be polished.It can be expexted,however,that with hard chrome plating a certain increase in surface roughness will result.When surface roughness becomes exessive after hard chrome plating.polishing is necessary.
In order to generate optimum chrome layers. its absoluttely necessary that the baths be controlled precisely with regard to electrolyte composition,temperature,current density. etc. If this is not done,a milky white layer without the required adhesion or the desired Vickers hardness of abouth 1100 may result.The relation between current density,electrolyte composition and hardness.Normally the electrolyte temperature is 50 to 60 C.The layer growth rate is then 0,004 to 0,005 mmm/h.The layer thicknesses used may range up to 0,10 mm.
When a parameter is changed within relativelu narrow limits,very significant effects can be expected .The mold design is also important.Its generally known that corner will grow at sharp edge transitions if external countour are involved.A high current density occurs at sharp edges,so that chrome is deposited rapidly on them.Gives a few guidlines for molds that are to be plated.It should be noted that sometimes a redesign is necessary in order to achieve uniform chrome coatings.
Since hard chrome plating is an electrochemical process,hydrogen is liberated and diffuses into both the steel and the hard chrome layer.This causes hydrogen embrittlement,which may be intensified if,for instance,hydrogen has been incorporated into the surface during the pickling that precedes the chrome plating.Its thus especially necessary that this embrittlement be reduced for parts in which tougness is required.Hydrogen annealing has proved especially useful in this regard.The workpiece is annealed at a temperature of about 200 C for a long time.Annealing can take place in an oil bath,a batch type furnace or a circulating air furnace.The reduction in gas concentration as a function of time and temperature.
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