Hard Chrome Plating
Transcript of Hard Chrome Plating
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Sabtu, 2009 Juli 11
HARD CHROME PLATING
ADVANTAGE OF HARD CHROME PLATING :
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
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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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Posted by dimensi precision team at 21:41
Labels: hard chrome plating
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