Hard Chrome Plating on Plastics Tooling: Now More Than Ever
Hard chrome plating on plastics tooling was for many years, only a means
for salvaging mold components of intricate shape and usually for increasing
shut-off dimension; more or less a shimming approach by the mold maker.
The thought of creating a more efficient tool by virtue of chrome plating
was unheard of.
Then, as the onslaught of high performance tooling demanded tighter
and tighter tolerances manifestation of words such as scoring and galling
became more reason to take advantage of hard chrome's intrinsic physical
properties: anti galling low coefficient of friction; anti-fretting and
Finally, with the advent of highly corrosive and highly erosive materials
coupled with both optical and aesthetic quality requirements, the need
for properties such as only chrome can offer have become the everyday talk
of not only the mold maker, but the mold designer & the molder as well.
The success of hard chrome in plastics tooling and industrial applications
may probably be attributed to its unique combinations of properties not
possessed by any other material available commercially.
The hardness alone would not be sufficient to secure widespread use
in plastics tooling applications, because a number of other hard materials
and other hardening processes are available. It is the combination of very
great hardness with extremely good corrosion resistance (equal or superior
under most conditions to that of such noble metals as gold or platinum),
and very low coefficient of friction or unique surface qualities which
gives such remarkable results in plastics tooling applications of hard
chrome. To this should be added the ease of stripping and replating for
repeated salvage where the plate wears beyond permissible limits.
The benefits of the hardness of chrome deposits is not effectively obtained
unless the coating is deposited on a sufficiently hard basis metal, and
to a satisfactory thickness. Even a relatively heavy deposit of hard chrome
may be crushed or indented on a soft metal such as copper or aluminum.
The best possible adhesion is also important in many uses where the surface
may be subjected to severe stress and shock Both conditions being prevalent
in the molding cycle.
The low coefficient of friction and desirable surface properties of
chromium are realized for the most part only on relatively smooth surfaces.
The deposits can be ground and lapped to size.
The deposits are easily ground but are sensitive to the heat generated
and should be ground with very light cuts, soft wheel & plenty of coolant.
In most cases bright deposits are obtained on highly polished molding
surfaces, with no further mechanical treatment required. By means of careful
operation it is possible to plate to size within very close limits.
Overall the mold designer, mold maker, and molder have at their disposal
a unique combination of physical properties, which for the most part offer
some level solution for the most common of plastics tooling problems. How
does one, then, determine when, where, and how much.
APPLICATION BEFORE THE PROBLEMS
The mold designer is the one individual who must initially realize his
creation. And though the most common of concerns have a variety of "rules-of-thumb"
solutions that are justified, the word "plating" seems to be some what
black balled in the plastics tooling vocabulary.
However, .0002 in. of hard chrome on a mirror finished core (especially
a large core) can save hundreds if not thousands of dollars in production
just by preventing oxidation from forming on a sweating mold. It won't
appreciably affect any dimensions; if maintenance forgets to oil it down
before storage, it won't rust; and with today's methods of plating adhesion
qualities (from qualified sources) are approaching perfection. Now why
doesn't the mold designer know this? Because, to the average mold designer
hard chrome means someone made a mistake: salvage.
Again: .0002 in. of dense hard chrome on a large L/D ratio core, with
minimum draft can make enough difference in friction reduction that the
core without chrome may never eject acceptability.
A great many of the plastics tooling applications of hard chrome are
derive from the fact that the deposit has a very low surface energy, and
because of the very low coefficient of friction it is ideally suitable
for many engineering applications.
These are just two cases in which a very thin deposit could make a world
of difference. The total list, on the other hand, can be endless. The application
mostly fall in the same general categories, specifically abrasion resistance,
corrosion resistance and friction reduction.
In order for the mold designer or mold maker to effectively specify
and successfully benefit from the various properties of chrome, he must
first of all accept it. In order for this to be a success, somewhat of
a crash course is in order!
PROPERTIES OF HARD CHROME
Chromium (chrome) is a grayish white metal, hard an brittle and capable
of being polished to a mirror finish, in which condition it seems to assume
a bluish tinge.
Some of the characteristics of chrome in the metallic state are as follows:
Atomic Weight 52.01
Melting Point 3362 F°
Crystal Form Body Centered cubic (bcc)
Deposits of hard chrome have a hard lustrous appearance, the deposit
is not affected by heat up to 800° F to
any great extent, and it resists corrosion extremely well, and is attacked
by few chemicals or substances.
The main properties for which it is applied are:
Hard chrome plating is an electrochemical process by which the metal chromium
is deposited on to a conductive substrate by virtue of electrolysis; at
very controlled (low) temperatures and current values.
Hardness and resistance to wear
Resistance to corrosion and heat
Low coefficient of friction (non-adhesive properties)
HARDNESS AND RESISTANCE TO WEAR
The extreme hardness of the electrodeposit has led to the adoption of the
term "hard chrome" in industry. Average figures obtainable gives this hardness:
850 -1000 Vickers P.N.
70 Rockwell C
The resistance to wear of hard chrome is based essentially on
this hardness and also on the fact that the deposited metal has a low surface
energy. Unfortunately it is very difficult to lay down specific means of
testing and measuring wear, although it is a definite fact the hard chrome
performs in a superior manner under most conditions.
In plastic tooling applications the main sources of wear met with, under
RESISTANCE TO CORROSION AND HEAT
Hard chrome deposits are utilized extensively in a vast number of applications
to resist corrosion of the basis metal by extremely superheated
gases. These same conditions exist within the plastics tooling during the
friction under high pressure
friction under revolving conditions
by abrasive action
by wear at high temperature
The protective qualities of the deposit depend on a number of factors,
the most important of which are the fine grain structure of the deposited
metal, its resistance to oxidizing and reducing agents and very high temperatures
which are necessary to oxidize the metal.
Unfortunately one inorganic acid gas, which results commonly from the
degradation of PVC, is beyond the resistance of the plate. Usage in such
an application should be very limited with at least .003 deposit.
In plastics tooling applications, heat is hardly a factor to
consider as possibly affecting the chrome properties. Hard chrome resists
the effects of heat well up to 800° and
then only changes slightly up to 1000° F.
It is generally found that hard chrome is first attacked in the cracks
forming the network. With standard plating solutions and proprietary solutions
as well. These cracks are again very related to temperature control; and
in order to satisfy the requirements of a thin dense deposit the
solution temperature tolerance is in the order of plus or minus 5 Fahrenheit
degrees. This performance can be utilized in two ways: either a fixed thickness
is deposited for improved resistance or the deposit is greatly reduced
to give the same performance. A large number of experiments have been carried
out to determine the optimum deposit thickness for various applications.
These summaries are available as technical publications. From these it
has been found that .000050 in. to .0002 in. chrome can give good protection
to parts subjected to light corrosive conditions. .0002-.0004 in. can withstand
wear and atmospheric corrosion and .0005 in to .002 in. provides superior
protection against chemical attack.
LOW COEFFICIENT OF FRICTION
One of chrome's main attributes is its low coefficient of friction. Being
the lowest over all other metals it has found its place not only in the
tool room of plastics manufactures, but in the compounding and mixing departments
as well. In addition, deposits in the order of .000050 in. are being successfully
applied on taps and reamers used in second operations of molding to increase
wear life by virtue of its low friction properties. This is illustrated
Chrome on white metal .16
Chrome on steel .19 Static coefficient
Steel on white metal .26 of friction
Steel on steel .31
White metal on white metal .55
It is also a fact that chrome can be ground or polished to a high finish.
This is a great advantage in applications where the minimum of draft is
mandatory hard chrome prevents sticking thereby reducing the amount of
deformation upon ejection of the molded part.
The approach in the aforementioned uses should be such that final mirror
finish polishing is accomplished on the plated surface, so as to avoid
repetition of labor.
APPLICATIONS OF HARD CHROME
The applications of hard chrome in the plastics tooling industry grow daily
as new ideas occur for the use of the deposit.
The following list is given as a guide for the successful use of the
deposit in the more established applications.
Mold inserts (shim deposit)
Plastics Inspection Tooling
Plastics Cutting Tools
Plastics Process Equipment
In its over seventy-five years of finding a steady growth of applications
hard chromium plating is not without limitations. Both the customer and
the plating service must be acutely aware of the shortcomings of such a
versitile process. Mainly, its inability to uniformly cover recessed and
intricate surfaces. With the help of specialized soft tools such as conforming
anodes, some of the limitations can and are routinely overcomed. This is
where the knowledge and ability of the plater can make the difference between
success and failure. Many a case exists where the impossible to one plater
is the everyday to another.
The revolution that is taking place with the applications of hard chrome
offers a variety of opportunities to the plastics tooling industry.
Yet like any revolution it is subject to mistakes by, or often disaster
for, those who are overcome by the thrill of new horizons but are blind
or careless to its pursuit.
No longer should hard chrome be an ugly word to the mold designer, mold
maker, or molder.
Faced with the ever rising cost of labor plus a need for increased profits,
the applications of hard chrome for both salvage and performance requirements
must be developed and concerned with design and engineering.
The designer of tomorrow's molds must realize that the benefits of hard
chrome must not be overlooked. They must be incorporated in the initial
plans where problems in processing are anticipated. However, the designer
must be aware of specification, as well. Just to say "flash chrome" will
no longer suffice, or as one might have it, will continue to bring about
the same results.
Hard chrome applications in the engineering discipline are by far no
new toy. It is almost now sixty years since deposition was offered to the
engineering industry deposition was offered to the engineering industry,
and during this time the importance of the process has steadily increased.
The application of hard chrome is now a recognized means of prolonging
the life of all types of metal parts subject to corrosion, friction and
abrasion. And it should be well borne by the mold designer, mold maker
and molder that they will benefit by learning a little more about the practical
side of the job. There is no doubt that very many applications have reached
a point where hard chrome has become an indispensable tool.