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Hard Chrome Plating on Plastics Tooling: Now More Than Ever
By
Marshall DeLaPuente
 

INTRODUCTION

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 anti-spalling. 

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. 

DISCUSSION

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: 

    1. Hardness and resistance to wear
    2. Resistance to corrosion and heat
    3. Low coefficient of friction (non-adhesive properties)
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

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 
800 Brinell 

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 conditions of: 

    1. dry friction
    2. friction under high pressure
    3. friction under revolving conditions
    4. by abrasive action
    5. by wear at high temperature
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 molding cycle. 

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. 

RECOMMENDED THICKNESS

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 below. 

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. 

    1. Tooling Components

    2. Core pins 
      Sleeves 
      Mold inserts (shim deposit) 
      Heater probes 
      Ejector pins 
      Slides 
      Cavities 
      Sprue bushings 
      Mold valves 
       
    3. Plastics Inspection Tooling

    4. Plug gauges 
      Slip gauges 
      Support fixtures 
       
    5. Plastics Cutting Tools

    6. Drills 
      Taps 
      Reamers 
      End mills 
      Files 
       
    7. Plastics Process Equipment
Sealers 
Granulator blades 
Mixing shafts 
Extruder screws 
Nozzles 
Tie bars
LIMITATIONS
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. 

SUMMARY

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.

END

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