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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7/29/2009http://dimensimouldmaker.blogspot.com/2009/07/hard-chrome-plating.html

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.

(arg1001nez)

Posted by dimensi precision team at 21:41

Labels: hard chrome plating

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