A11102 bTbfl73

NBSIR 87-3574

Wear Due to Printing Inks

L. K. Ives, M. Peterson, A. W. Ruff, J. S. Harris, and P. A. Boyer

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Metallurgy Division

Gaithersburg, MD 20899

Issued May 1987

Final Technical Report

Contract No. N-0283-04

Reproduction in whole or part is permitted for any purpose of the

United States Government

Prepared for: Bureau of Engraving and Printing

QC f Research and Technical Services

2 QQ J C Streets, SW

lirc |ton, DC 20228

Ubb

#87-3574

1987

C.2

.Research Information Center

National Bureau of Standards

Gaithersburg, Maryland 20899

NBSIR 87-3574 AJB.SC

QC/oo

. test?

WEAR DUE TO PRINTING INKS

L. K. Ives, M. Peterson, A. W. Ruff, J. S. Harris, and P. A. Boyer

/HO.

in7

C,2,

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Metallurgy Division

Gaithersburg, MD 20899

Issued May 1987

Final Technical Report

Contract No. N-0283-04

Reproduction in whole or part is permitted for any purpose of the

United States Government

Prepared for: Bureau of Engraving and Printing

Office of Research and Technical Services

14th and C Streets, SW

Washington, DC 20228

a

<

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

WEAR DUE TO PRINTING INKS

L. K. Ives, M. Peterson, A. W. Ruff,

J. S. Harris, and P. A. Boyer

Metallurgy Division

National Bureau of Standards

Gaithersburg, MD 20899

Submitted to:

Office of Research and

Technical Services

Bureau of Engraving and Printing

14th and C Street NW

Washington, DC 20228

FINAL REPORT

May 1987

EXECUTIVE SUMMARY

Laboratory studies have been conducted of wear processes

occurring in BEP currency and stamp printing operations.

Conclusions were drawn on the principal modes of wear involved.

Laboratory work was carried out to develop new wear test methods

that could be used to evaluate materials such as improved

chromium plating, and to determine the abrasiveness of printing

ink constituents.

In the water wipe press, wear of chromium plated currency

printing plates occurred primarily at the wiper roll contact.

Particles in the ink were trapped at the contact between the PVC

covered wiping roll and the printing plate surface. These

particles caused abrasive scratching and polishing of the

printing plate surface. According to our laboratory tests,

factors which had a significant influence on wear rate were

particle size, hardness and concentration, and the roughness of

the PVC surface. Other factors which had a strong effect on

wear were fluid film thickness at the wiper roll-printing plate

contact (determined by contact pressure, ink rheology, and

sliding speed) and the ability of the ink to form a protective

solid film on the plate surface. Some ink constituents were

found to form a transfer film on chromium in laboratory

experiments

.

Particles contained in the PVC wiper roll cover material did not

cause significant wear of the printing plates. Large, hard,

foreign particles which were occasionally embedded in the PVC

material did cause scratching damage.

Among the inks received from BEP for study in this program, the

wear rate of the highest wearing rate ink was 25 times greater

than that of the lowest in laboratory wear tests. This indicated

a potentially large difference in wear rates among printing inks.

For stamp inks a correlation was established between service

behavior and laboratory results. An ink that caused severe

scratching in service was also found in laboratory tests to

produce severe scratching of chromium plating. It was noted that

severe scratching did not necessarily correlate with a high wear

rate.

Three laboratory tests were evaluated for use in the study of

wear in the printing process. The rotating pin-on-disk test was

found to be suitable both for the evaluation of the wear behavior

of different printing inks and of different chromium plating.

Appropriate test procedures were developed for both of these

tasks. Preliminary experiments indicated that the flat-on-ring

test could also be used for these tasks but further development

was necessary. The GTA test was found to be unsuitable for

determining ink abrasivity.
It was demonstrated that with various printing inks the wear rate

of soft chromium plating was substantially greater than hard BEP

type chromium plating. No appreciable difference was found

between a transition type chromium plating and BEP type plating.

Recommendations for future laboratory work were made in the

report. Development of a scratching test that would evaluate

inks and chromium coatings for scratching tendencies is

important. Development of an additional wear test that could be

used both in the laboratory and in printing press operation is

recommended. Improved wear measurement methods are needed

particularly in the low wear regime. Fundamental studies are

needed to better understand the role of all materials -- plate

materials, inks, and wiping materials -- in the wear process.
TABLE OF CONTENTS

I

.

Introduction

II. Wear of Service Components in Printing

A. General Considerations

1. Sliding Conditions

2. Measuring Wear

B. Printing Plate-End Examinations

1. Paper Wipe Process

2. Water Wipe Process

C. Wiper Blade Wear

D. PVC Wiper Roll Cover Properties

E. Printing Ink Characteristics

III. Laboratory Wear Test Methods and Results

A. General Considerations

B. Rotating Pin-on-Disk Test

C. Flat-on-Ring Test

D. GTA Test

E. Transfer of Wear Test Method to BEP

IV. Summary and Conclusions

V. Recommended Future Work

A. Scratching Test Development

B. Additional Wear Test Methods

C. Additional Wear Measurement Methods

D. Fundamental Research Studies

References
I . Introduction

Currency and postage stamp printing operations of the Bureau of

Engraving and Printing (BEP) are carried out using many different

printing technologies. Of major importance are those methods

utilizing chromium plated engraved metal plates or cylinders.

There are several different processes which limit the life of

these metal surfaces; among them are wear, scratching, fatigue

cracking, corrosion, and accidental damage. The study being

reported here has examined the intaglio currency printing process

and has determined that wear-out or unacceptable scratching of a

plate takes place during the wiping step where excess ink is

removed just before the impression is made. Wiping may take

place by a sliding blade, by a rotating cylinder, or by a paper

web. In all three cases, the motion is sliding under high

pressure between the wiping body and the engraved plate. Other

steps in the process such as inking and printing involve rolling

with respect to the impression plate. There is no sliding,

hence, there is no appreciable wear associated with those steps.

Electrodeposited coatings are widely used by industry to minimize

or eliminate degradation caused by wear and corrosion. In the

case of wear, coatings are frequently used to reduce abrasion

which is the most prevalent mode of wear found in industrial

service. While severe forms of abrasive wear are usually

associated with industrial operations such as mining, many wear

problems involve milder forms of abrasive wear caused by low

concentrations of abrasives or by mildly abrasive materials. In

some instances, wear may occur even in a lubricated system where

abrasives are present as a contaminant in the lubricant. Since

the wear rate under mildly abrasive conditions can be several

orders of magnitude greater than the typical value for lubricated

conditions, this form of wear can be a very serious problem. In

many operations wear processes are complicated by the presence of

fluids or gases that cause a certain amount of corrosion to take

place. The rate of corrosion alone may be acceptably low for the

materials involved, but the simultaneous occurrence of wear

creates a significant problem. Corrosion and wear are usually

synergistic in their effect, and the rate of material loss can

increase greatly. The occurrence of both processes at the same

time severely limits the designer's choice of suitable materials.

Studies carried out in our laboratory have shown that abrasion by

the ink in the water-wipe intaglio currency printing process is

primarily responsible for the printing plate wear. The wear is

due to both hard and soft particles in the ink that are- either

intentionally added or accidentally incorporated as contaminants.

There is also some corrosion which may enhance the wear process.

Details of these findings are presented later in this report.

The problem of wear can be minimized by attention during the

printing operation to the wiping cylinder pressure, the wash

solution chemistry and cleanliness, the wiper blade condition

6
(these blades wipe the PVC roll surface continually), and the PVC

roll surface condition. The ink formulation, and ink cleanliness

in terms of particulate contaminants is also of major importance,

as will be described later.

II. Wear of Service Components in Printing

A. General Considerations

Numerous discussions were held with BEP personnel to obtain

information on the printing process and the performance of the

press equipment. The NBS laboratory wear studies in this program

were planned and carried out using conditions that reflect some

of those that occur during printing. In particular, wear contact

conditions such as load, speed, temperature, and environment,

which are very important factors, were chosen so that we could

compare the laboratory wear data with BEP service experience.

Samples were requested of typical worn materials, inks, and

wiping roll covers, in order to conduct a study of the wear modes

involved. A previous study of worn plate surfaces carried out at

the BEP laboratory offered evidence that abrasive wear was taking

place during the printing cycle. The wear rates were relatively

low and this suggested that the abrasive particles were small,

perhaps comparable in hardness to the chromium plating, or were

present in low concentrations. We also obtained specimens of

new and worn wiper blades, used to scrape the PVC roll during

operation. Wear studies on these steel wiper blades were a

useful adjunct to studies of end sections of the plates

-- themselves

.

A study to identify important system considerations connected

with wear during the currency printing process was conducted.

The study considered the sliding conditions present in the

currency printing process, the materials involved, possible wear

modes, approaches to measure wear, and possible wear test

approaches for laboratory use.

1. Sliding Conditions

A schematic drawing of the currency printing press system is

shown in Fig. 1. Essentially, it consists of a rotating roll to

which the printing plates are attached. The plates are chromium

plated and contain the impressions which transmit the ink to the

paper. Three auxiliary PVC rolls are used. One supplies ink to

the plate; one removes the excess ink; and one loads the paper

against the plate to receive the impression. The contact sites

of primary concern are those that might cause the plates to wear.

These are identified as 1, 2, 3, and 4 in Fig. 1 and are

discussed in more detail in the following paragraphs.

7
(a) Inking Roll Contact

The plate roll and the inking roll turn in the same direction

with the same surface speed so a rolling contact results. The

ink temperature is 85 °C so this would be the contact temperature.

The load is adjustable but the actual values are unknown. The

only wear on the printing plates would be due to ink flow or

microslip resulting from the elastic deformation of the PVC roll.

This wear is believed to be very small.

(b) Wiper Roll Contact

The wiper roll moves in a direction opposite to the drum so that

pure sliding results at a velocity of 7.5 m/s. The temperature

can be assumed to be 85 °C. The load is unknown and probably

varies since it is regularly adjusted by the operator. Since the

rolls move in opposite directions there can be little or no ink

film and, of course, none is desired since the function of the

roll is to remove the ink.

Superimposed on the rolling motion is a translating motion which

is parallel to the roll axis. This motion would have little

additional effect on the wear process since the sliding distance

is short. Wear tracks on the plate would merely appear at an

angle rather than parallel to the rolling motion. The ink

contains several different kinds of abrasives whose hardnesses

are compared below with the roll and plate materials.

TABLE 1

Material

Hardness

( kg/mm^

)

Iron Oxide 600

Silica 900

Calcium Carbonate 180

PVC (Shore D50)

Chromium plating 800-1000

These first two materials would cause abrasive wear of the

chromium plating. Thus, primary attention should be given to

abrasive wear at this contact.

In abrasive wear several distinctions should be made:

- Three Body Wear vs. Two Body Wear

- Hardness of abrasive/Hardness of substrate ratio

- Particle size of the abrasive

8
Three body wear occurs when a hard particle is caught between two

sliding surfaces. The particle may roll or slide depending on

its shape and how firmly it is held by one or the other of the

surfaces. With sliding, scratches may be produced and the wear

process is one of normal abrasion. Since the particles are not

rigidly held, the scratches may be short and tend to wander away

from the direction of the sliding bodies. With rolling, wear may

take place by a process of repeated indentations. Surface

material is deformed and eventually broken away to form a wear

particle. In three body wear the indentations are often

concentrated at the converging inlet. If relatively soft and

ductile surfaces are involved, particles or fragments of

particles may become embedded in the material, and the wear

behavior is more like two body abrasion. Under that condition,

wear of the harder surface may be greater than that of the soft.

Embedment is less likely to occur with soft but highly elastic

materials such as the PVC cover used on the wiper roll. With

brittle surfaces, cracking and chipping may occur. Three body

wear rates are usually about 1/10 that of two body wear rates.

For the case of PVC sliding against chromium, wear is strongly

dependent on the hardness of the PVC and the size of the abrasive

particles, because these factors determine the particle

protrusion distance, p, as shown in Fig. 2. With soft PVC or

small particles the protrusion distance is small and very shallow

scratches or indentations are produced. With very hard PVC, p is

large but the particles are not trapped and rolling occurs. This

is a much less efficient process with respect to wear than

scratching.

Many factors other than those mentioned above are important.

Examples are load, particle shape, particle concentration, the

presence of a fluid film, and, of course, the relative hardnesses

of the abrasive particles and the two surfaces. Studies have

been reported which indicate that the amount of abrasive wear

increases with the ratio, Ha/Hm, where Hm is the hardness of the

wearing body and Ha is the hardness of the abrasive (Fig. 3).

Two regions can be defined: soft abrasive wear and hard abrasive

wear. Soft abrasive wear extends up to approximately

Ha/Hm = 0.8. With soft abrasive wear a polishing type of wear

occurs. [2] The ratio for iron oxide and chromium plating is

about 600/900 = 0.67. Since the concentration of iron oxide in

currency printing inks is likely to be much higher than that of

harder particles, soft abrasive wear by iron oxide is probably

significant. For soft abrasives, the particle size is of less

importance than for hard particles since they can be broken up in

the process. Polishing is not strictly abrasion but is also due

to the repeated plastic deformation of the surfaces and to

adhesive transfer of the metal to the iron oxide powder.

Polishing also may be the result of corrosive wear. Ink consists

of a variety of chemicals and additives. In addition, the water

9
wash process uses a solution of chemicals to remove the ink. PVC

contains chlorine which is known to be corrosive in combination

with metals. Corrosive wear rates may be of the same order of

magnitude as abrasive wear rates so either is possibly of major

concern.

(c) Printing Contact

In this contact the paper is pressed against the ink filled

plate. A slight amount of slip is caused by deformation of the

paper and the impression roll. Under these circumstances the

important variable is the amount of microslip resulting from the

deformation. Since the amount of slip is likely to be small,

this contact would contribute very little to the wear of the

chromium plating.

2. Measuring Wear

Mechanisms of wear are determined by identifying all of the

constituents (materials, designs, operating conditions,

environment) in the wearing process and by careful inspection of

the wearing surface and the wear debris. Once the wear mode has

been identified, it is necessary to set up test devices which

will allow studies to be made which will establish the details.

In running such test devices, it is desirable to establish a

correlation with service. Usually this is done by showing the

same relative ranking of a series of materials in both the test

and the actual application. With the printing process this is

almost impossible to do, so some other approach must be used.

One such approach is to measure a wear rate in service and then

to devise a test which nearly duplicates this wear rate. One

possibility is to utilize the wiper blade that slides on the

wiping PVC roll for this purpose.

The wiper blade rubs against the wiper roll, flooded with ink.

This contact is located very close to the wiper roll-printing

plate contact, and except for the load and velocity, the

conditions at the two sites are almost identical. The blade is

about three feet long which imposes some problems with sample

preparation but these are not insurmountable. This would be an

ideal test to evaluate materials but could not be used to

evaluate other factors such as the abrasivity of the inks or the

effects of changes in operating variables. The primary problem

is that the load is not controlled and changes as wear takes

place. Some modifications would have to be made in the loading

system or a sufficient number of tests would have to be run to

provide acceptable statistical control. If the wiper blade was

chromium plated it could also be used to identify wear modes and

to determine the relative significance between soft abrasive,

hard abrasive, and corrosion wear. This blade could also be used

to measure service wear coefficients to compare with bench tests.

The blades wear rapidly so the total wear could be determined by

10
weighing. The load, however, would have to be measured and

controlled.

B. Printing Plate-End Examinations

A detailed understanding of the process by which printing plates

wear is important for several reasons. In the development of

improved electroplatings such an understanding can indicate which

properties should be emphasized. It can indicate whether

corrosion resistance, adhesion, or hardness should be improved.

This is especially important since improvement in one property

may result in the degradation of another. Identification of the

controlling wear process will also assist in the development of a

valid laboratory wear test. In addition, an understanding of the

wear process may suggest relatively minor changes in present

printing procedures that will result in significant improvement

in plate wear life. The plate-ends were studied in order to

develop an understanding of the wear process.

1. Paper Wipe Process

Several plate-ends sheared from plates used in paper wipe

processes were received for study. Some of these plates were

designated as having been removed from service due to chromium

wear and others because of mechanical failure. In a few cases

the chromium plating appeared to be worn away sufficiently to

expose the nickel substrate at high points within color bars

(engraved special features on the plates). In general, however,

the chromium layer was still present over the plates surfaces.

The worn surface of the chromium plating was similar in

appearance to that of the wiping blades but the scratches were

shallower.

2. Water Wipe Process

Seven plate-ends from plates used in water wipe presses were

received for analysis. Information provided with each of the

plate-ends included the ink color, total number of impressions,

and for six of the plates, the reason for removal from service;

no reason was given for one plate. Three of the plates had been

used with black ink (bill faces) and the remaining four with

green ink (bill backs). Only the plate-ends used with black ink

contain color bars. The six plates for which information was

provided were removed from service because of damage other than

wear. Four experienced indentation damage presumably due to

improper feeding of the currency paper, and two of the plates

were removed because of fracture. The number of impressions to

which the plates had been exposed ranged from 457,000 to

3,263,000. Examination of the color bars did not reveal areas

where the chromium plating had been worn away to expose the

nickel substrate nor was the chromium plating worn away at any

other location on any of the plate-ends. The chromium plating

11
thickness was measured at several locations on each of the

plate-ends. The measurements were made on metallographically

polished cross sections cut from the plate-ends. The thickness

in an area within the margins that was not exposed to the

printing or wiping action was compared to the thickness in the

printing area. From these measurements the amount of wear was

found to be quite small even for the plate that had been run for

3,263,000 impressions. Other features also indicated that the

wear was quite small. In particular, scratches that were

introduced during abrasive buffing after application of the

chromium plating were still visible in the printing area.

Curiously however, at the edge of the plates outside of the area

of contact with the currency paper but still within the area of

contact with the wipe roll, the chromium was thinner (in one case

by a factor of 3 for the plate that had 3,263,000 impressions)

and buffing scratches were worn away. This indicates that wear

can be a significant factor in plate life depending on

circumstances of use. Further information on the details of the

printing process are needed to develop an explanation for this

effect.

Our studies indicate that consideration should be given to

establishing the validity of analyses based only on studies of

plate-ends. Wear which occurs in the plate end region may not be

representative of that which occurs in the engraved part of the

plate involved in printing bills. Indeed the wear may be

generally uneven across the plate area. The latter possibility

was suggested by the study of color bars on paper wipe plateends. There it was found that substantially less wear occurred

at the leading edge of a color bar than at the trailing edge.

The chromium plating was completely worn away on the front row of

the color bar pattern but less than 50% on the back row. This

kind of variation may occur in other parts of the plate. Thus

chromium plating may be worn away at one location of the plate

while very little wear is experienced at another location. It

appears that it will be important to measure wear over various

regions of the plating, including within the engraved areas. To

that end it would be important to obtain samples from used

printing plates taken from unengraved regions throughout the

plate area.

C. Wiper Blade Wear

As discussed previously, the primary wiper blade is loaded

against the PVC wiper roll; its function is to remove most of the

ink from the PVC roll before subsequent cleaning stages. It is

made of carbon steel. In service these blades wear rapidly.

Discussion with one of the press operators indicated that the

blades are replaced approximately once every 24 hours due to

wear. Although the sliding speed at the wiper blade contact is

less than that between the printing plate and the PVC wiping roll

12
and the load may be different, the fact that the wiper blade and

printing plates are otherwise exposed to more or less similar

wear conditions against the wiper roll suggests that study of the

wiper blade could provide valuable information on plate wear

processes. Scratches of the order of a micrometer in depth were

present on the edge of the blade (Fig. 4). This indicates that

abrasive wear is a major if not the primary mode of wear

sustained by the wiper blade. The hardness of the wiper blade

(approximately 550 KHN ) is significantly less than that of

chromium electroplating (~900 KHN). Consequently, it would be

worn more rapidly by abrasive particles. Measurements were made

on wiper blades to obtain a wear rate value that might be related

to that for printing plates.

Seven worn wiper blades were received from BEP. A record was

also provided indicating the number of impressions that had been

made with each blade in place. These data are shown in Table 2.

TABLE 2

WIPER BLADES RECEIVED FOR WEAR MEASUREMENT

# Press Ink Impressions

1 402 Green 142,300

2 402 Green 313,396

3 402 Green 212,843

4 401 Black 143,836

5 401 Black 98,631

6 401 Black 52,686

7 401 Black 74,067

The wear of each blade was determined by measuring the wear scar

width (Fig. 5) and using this value to determine wear volume from

geometrical considerations. Actually, the wear scar width was

found to vary along the length of the blade as shown in Fig. 6.

Higher wear occurred at the blade ends than in the center. For

each blade the total volume was determined by adding the volumes

calculated at 2 cm intervals along on the blade. The specific

wear (wear volume /impress ion) for each blade is shown in Fig. 7.

An average value for the blades was 13.8 x 10" mm^ /impression.

The determination of the wear coefficient requires a knowledge

of the applied load in addition to the known quantities -wear

^Defined by the relation:

(wear volume) * (hardness

)

wear coefficient =

( load) *( sliding distance)

13
volume, sliding distance, and blade hardness. Unfortunately, the

applied load is not a quantity that can be determined directly

from printing press operating or design information. However, by

determining the blade wear rate as a function of load in a

laboratory test simulating the press conditions, the service load

could be determined indirectly. With that value, a wear

coefficient for the blades in service could then be calculated.

D. PVC Wiper Roll Cover Properties

Studies were conducted on both new and used PVC roll cover

material using scanning electron microscopy (SEM) and optical

microscopy. Since the wiper roll slides on both the printing

plates and the wiper blades, these studies revealed further

information concerning the wear modes.

The composition of the PVC cover material is shown in Table 3

Two kinds of particles are present, calcium carbonate and

graphite. Both are relatively soft. A photograph of the PVC

surface is shown in Fig. 8. A large number of particles are

seen. An analysis of several different surface regions showed

that these particles were present at an average concentration of

340/mn\2 and had an average size in the range of 4 to 22 urn. Photographs of the worn PVC surface are shown in Fig. 9 and 10.

Several features can be seen: cracks and pitting both of which

are probably due to the wear process. Another feature is that

the particles seem to be raised above the PVC matrix. This would

take place if they wore at a slower rate than the PVC. In other

words, the PVC is being abraded from around the harder solid

particles.

An analysis of the composition was made of a large number of

particles in both new and used PVC by means of energy dispersive

x~ray analysis. It was concluded that most of the particles

were either graphite or calcium carbonate from the original

formulation; however, out of about 50 particles examined, two

were of a different composition. One contained silicon and the

other silicon, aluminum, and potassium (Fig. 11 and 12). These

particles may be contaminants from materials used in the

formulation or they may have entered the PVC during fabrication

or handling. A photograph of the Si-Al-K containing particle is

shown in Fig. 13.

Whatever the nature of the particles in the PVC, the primary

question was whether they could scratch the chromium plated

surface of the printing plate. A simple test was devised to

answer this question. A pellet of PVC was mounted in the chuck

of a hand grinder and run against chromium plating while

lubricated with mineral oil. Although the test was run at

various pressures there were no scratches in the chromium plating

or evidence of wear. This suggests that the PVC itself should

14
not damage the chromium plating. When run dry against the

chromium plating the PVC was worn rapidly. Under these

conditions there were some scratches in the chromium plating.

Although this is not the condition that would be found in

printing, this result does indicate that there may be some

particles in the PVC that are capable of scratching chromium

plating.

TABLE 3

PVC COMPOSITION (%)

Polyvinyl Chloride 58

Dioctyl Phthalate 28

Monomer X980 9

t-Butyl Perbenzoate .09

Ferro 837 1.2

Calcium Carbonate 2.9

Graphite .9

E. Printing Ink Characteristics

Wear of chromium plating is primarily due to particles in the

printing inks. The composition of an ink formulated by BEP is

shown in Table 4. Samples of those ingredients which contained

or consisted of solid particles were supplied by BEP and examined

microscopically. It was found that for all ingredients the

particles were small (<1 ]±m) except amorphous silica and

synthaline green which contained larger particles (>1 urn).

Photographs of particles from these materials are shown in

Fig. 14 and 15. The important question is which of the

ingredients might cause damage to the chromium plating. To

answer this question a test similar to that described in the

previous section was employed. Polishing cloth was attached to

the end of a disk which was mounted in the chuck of a small high

speed grinder. The particles listed in Table 4 were mixed with

mineral oil and applied to the polishing cloth. The chromium

plated surface of a plate-end was rubbed for several seconds. A

new cloth was used for each particle material. At the conclusion

of each test the chromium plated surface was examined under an

optical microscope for evidence of damage. The results are given

in Table 5

.

The only ingredient which produced scratches was the flushed

synthaline green. The titanium dioxide polished the surface

while the calcium carbonate yielded no visible effect. The

yellow iron oxide and the amorphous silica gave a transferred

film. The film from yellow iron oxide was more or less

continuous while the amorphous silica transferred in lumps

15
TABLE 4

Stock

No.

Ink G-4716-MRE

Ingredients Weight

Sub

Total

295 Aqua Wipe Varnish 850-065 160.0 160.0

248 Cobalt Drier 5 1/2% 6.0 6.0

263 Lead Manganese Drier 6.0 6.0

281 Flushed Synthaline Green 28.0 28.0

- #1840 Black Tint 12.0 12.0

292 Mineral Spirits 8.0 8.0

45 Titanium Dioxide 8.0 8.0

204 Yellow Iron Oxide 44.0 44.0

38 Calcium Carbonate (ultrafine) 120.0 120.0

154 Amorphous Silica 8.0 8.0

(Fig. 16). The lumps have the appearance of molten drops. Both

of these materials contain water which reduces the hardness and

increases the possibility of chemical effects. When the same

test was conducted on ink which had been formulated according to

Table 4, as might be expected, scratches were produced. The

above analysis indicates that the source was probably the

synthaline green. For comparison, tests were also conducted on

three other materials. Quartz having a particle size on the

order of 1 p gave scratches. Submicron aluminum oxide on the

other hand gave polishing like the hard (6.5 Mohs ) titanium

dioxide. The polishing effect is due to the small particle size.

Black iron oxide, a principal ingredient of black currency ink,

produced only a film.

This test demonstrates that two possible forms of wear can occur

with inks, abrasive scratching and polishing wear. The relative

amounts will depend upon the size of the particles and their

concentrations at the interface.

III. Laboratory Wear Test Methods and Results

A. General Considerations

In order to devise a simulated laboratory bench test it is

necessary to draw conclusions as to the mechanism of wear in

service. From the previous discussions, a logical assumption

would be that wear of the chromium platings is due to abrasive

scratching and polishing by particles in the ink. The principal

geometry involved is a cylindrical chromium plated surface

sliding against a PVC surface in the presence of ink. A

discussion of the important variables in this geometry follows.

16
* Particles not found in G-4716-MRE

The contact geometry is shown in Fig. 17; cylinder (2) is the

printing plate and body 1 is the PVC wiper roll. The ink is

being drawn into the contact by body 2 and is wiped away by

body 1. The controlling variables in the wear process are the

amount of ink within the contact area, the length of the contact,

the entrance angle, and the pressure in the contact.

The geometry and the materials can be duplicated in the bench

test. The size of the cylinders would, however, be reduced.

This reduction in size would change several important factors

such as contact area and pressure. This change could reduce the

wear rate since a particle passing through the contact would have

much less area on which to act. The actual printing load is

unknown and the pressure distribution across the plate surface is

likewise unknown. This can be measured but would not necessarily

17
remain constant with time. Another important variable is the

amount of ink in the contact area. The purpose of the PVC wiper

roll is to remove the excess ink. Because of its elasticity and

long contact width this can be effectively accomplished; however,

since the plates contain recesses a small amount of ink will be

dragged into the contact and squeezed out. The squeezing out

process contributes to the wear.

The change in stress, contact width, and the unknown amount of

ink which is squeezed out make it difficult to simulate the

printing wear process, that is, to obtain identical wear rates in

a test rig and the application. However, this is not crucial if

the primary purpose is to make a comparative evaluation of

materials, and to evaluate the abrasivity of inks. Under these

circumstances it is only necessary to ensure that the wear modes

are the same and that the wear rate is rapid enough to conduct

the evaluation in a reasonable length of time.

Three different laboratory tests were employed to evaluate wear

and/or scratching of chromium plating by printing inks. These

tests are referred to as 1) the rotating pin-on-disk test, 2) the

flat-on-ring test, and 3) the Gravure Technical Association ( GTA)

test. The test machine, operating procedures, and the results

obtained for each of these tests will be described in the

following sections. Most of the development work was carried out

on the rotating pin-on-disk test, primarily because much of the

necessary equipment and considerable experience with the test had

already been gained in connection with an earlier but unrelated

study concerned with abrasive wear by lubricating greases

containing hard particles.

The flat-on-ring test machine was designed and constructed

specifically for this project. However, it was only available

for use during the final phase of the project, and only a limited

evaluation was conducted. Additional work is required before it

will be known to what extent the test is suitable for the

evaluation of printing inks and chromium plating. Important

features and some possible difficulties with the test will be

pointed out in the section discussing this test.

The GTA test is designed specifically to determine the abrasivity

of printing inks. It is not applicable to the study of different

types of chromium plating. Unfortunately, as our results will

indicate, it is also not suitable for the evaluation of ink

abrasivity.

B. Rotating Pin-On-Disk Test

Test Machine : A schematic drawing of the rotating pin-on-disk

specimen configuration is shown in Fig. 18. A photograph of the

test machine is shown in Fig. 19. The original design of this

machine was due to Schiefer [5]. Schiefer's application was

18
concerned with the wear of fabric materials. A unique feature of

his design was that the rubbing motion was omnidirectional, that

is, the fabric material was rubbed, not in a single direction,

but in directions that were constantly changing as both the

fabric, which was attached to the disk, and the abrading body, at

the location of the pin in Fig. 18, were rotated. As illustrated

in Fig. 18, this was achieved by mounting the rubbing bodies on

two parallel but offset shafts. Rotation of both shafts is in

the same sense. The rotational speeds however are slightly

different. This is important because it changes what would

otherwise be a circular path into a precessing orbit.

The Schiefer machine, configured for abrasion testing of fabric

materials, is available commercially. Its application to the

testing of fabric materials is described in ASTM D4158, "Abrasion

Resistance of Textile Fabrics (Uniform Abrasion Method)". The

pin and disk system which is illustrated in Fig. 18 is not the

configuration used for fabric testing. The design used here had

been developed in our laboratory for the investigation of wear by

abrasive contaminants in lubricating greases [ 6 ]

.

In addition to

the single pin holder which is shown in Fig. 18, a three pin

holder has also been constructed. Photographs of both holders

are shown in Fig. 20. With the three pin holder it is possible

not only to obtain separate measurements of wear on three pins in

a single test run, but by using pins with different types of

chromium plating it is possible to compare the wear behavior of

the different platings under nearly identical conditions.

The operating conditions used in most of the tests are listed in

Table 6.

As mentioned above the pin and disk shafts rotate at slightly

different speeds so that the sliding speed of the pin is not

constant but oscillates between the values given in the table.

Although the tests were conducted at room temperature, friction

and stirring of the ink caused a small temperature rise, but by

no more than 10 °C.

The so-called pin specimens are ordinary 3/8 inch diameter steel

bearing balls which are electroplated with the coating to be

TABLE 6

Rotating Pin-On-Disk Operating Conditions

Pin Sliding Speed

Load

Temperature

Atmosphere

63-64 cm/min

15 N (5 N per pin with three pin holder)

22+/- 1 C

Air, Relative humidity, 30-65%

19
tested. The balls are clamped in the holder so that they can not

rotate. In early experiments the chromium was deposited directly

onto the steel ball. In later tests the ball was first plated

with -.005 inches of nickel (using BEP conditions) before

depositing the chromium. This was done to simulate more closely

the printing plate construction which is chromium on nickel. The

nickel is considerably softer than the steel bearing material and

could result in a different abrasion response, especially with

larger scratches. Also, the structure of the chromium deposit is

influenced by the substrate. Tests were not conducted

specifically to determine if there were differences in chromium

plating wear associated with the substrate used, and in the tests

that were conducted, there was no indication that the substrate

had a detectable effect.

The steel bearing balls serve as a very convenient substrate to

use for the study of chromium plating wear. They can be

purchased for a few cents a piece from bearing supply retailers.

The bearing balls are manufactured from AISI 52100 steel.

Composition, hardness, and surface finish are highly consistent

from ball to ball. Moreover, the bearing application requires

that the dimensions be maintained within very close standard

tolerances. In each test only a small circular area about 1 mm

in diameter is worn, so that a large number of tests can be

conducted on a single plated ball. In one case nineteen tests

were run on a ball, and there was area available for several

more

Initially, measurement of the amount of wear on the pin presented

some difficulties. With a flat wear scar the simplest procedure

is to measure the diameter of the scar and calculate the volume

lost on the basis of geometry. With the soft elastic PVC

counterface that was used in this investigation, the ball sinks

into the surface, and the wear scar has a more or less spherical

shape. The diameter of the contact area does not change much

with wear. A measurement based on weight loss was impractical

because often only a few nanograms were lost in a test run,

particularly with slow wearing inks. However, the depth of wear

was amenable to measurement using an electronic depth gage. Such

instruments are routinely accurate to a fraction of a micrometer.

For our measurements a Brown and Sharpe Model 599-1021 Electronic

Gage * was employed; a similar instrument from another

manufacturer could have been used.

It was deemed important to monitor periodically the amount of

wear as the test progressed. Thus, changes in wear rate that

This particular commercial instrument is identified solely

for the purpose of adequately describing the test system. It

does not constitute an endorsement of the particular instrument.

20
might occur during a test run could be detected. Also, the

effect of induced changes, for example by varying the load or

introducing a different ink, could be determined. To accomplish

this a fixture to hold the gage head was designed that could be

placed directly on the pin holder without disturbing the ball.

The outer surface of the holder served as the fixed reference

level for the depth measurement. Repeated measurements on the

same specimen showed a precision of about + 0.05 urn. An indication of the precision of the measurement method is

demonstrated by the results plotted in Fig. 21. Here the ink

that was used, compared to most of the other inks studied,

exhibited a very low wear rate against chromium plating. The

difference between each plotted point is the incremental amount

of wear that occurred after 10,000 revolutions (1667 m sliding

distance) . It can be seen that wear depth increases

monotonically with increasing distance and that the plotted

points fall on a smooth curve through these points.

Test Results : More than 80 separate rotating pin-on-disk tests

were conducted during the course of this program. A test run

usually consisted of a number of increments of several thousand

revolutions each. At the end of each increment, the thickness of

material worn from the pin was measured as described above. The

test increment was measured directly in terms of the number of

revolutions. For most inks that increment is 10,000 revolutions;

however, for faster wearing inks or more aggressive abrasive

materials, a smaller increment has been used. Generally, it was

desireable that at least a few tenths of a micrometer be worn

away during each increment, but that the amount should not exceed

one micrometer in order to have a sufficient number of data

points to show the progress of wear. Because the ink became more

viscous during testing as a result if drying, it was replaced

with fresh ink after 10,000 revolutions.

For most tests, a total of about 5 urn of the 10-15 urn thick

chromium plated layer on the pin was worn away. Because of the

wide variation in the wear rates of different inks, as well as

other factors that affect wear rate, tests ranging in length from

a few thousand to 1.5 million revolutions have been conducted.

In the latter case, several days were required to complete the

test.

For an overall view of the rate at which printing inks wear

chromium plating compared to other abrasive materials, data on

printing inks, quartz particles, and aluminum oxide particles are

summarized in Fig. 22. The printing inks include both stamp inks

and currency inks and involve inks produced by both SI CPA and

BEP. The test conditions used to acquire the data shown in

Fig. 22 are those given in Table 1. It can be seen that the wear

rates, expressed in terms of volume per 1000 m sliding distance,

extends over a range of nearly three orders of magnitude. The

21
highest wear rate for inks was more than twenty-five times

greater than the lowest. The wear rate obtained for quartz was

substantially greater than that for most inks and the wear rate

for aluminum oxide was greater than for quartz. Results obtained

for quartz and aluminum oxide represent relatively simple systems

consisting of a single particle type in mineral oil. On the

other hand, printing inks consist of several different types of

particles in a more complex fluid medium.

The different wear rates among these materials can be attributed

to several factors. Among these are particle hardness

(exemplified by the difference between quartz and aluminum

oxide), particle size, concentration and shape. In addition to

effects associated with particle properties, as will be discussed

below, the nature of the fluid vehicle and the condition of the

PVC counterface can have a substantial influence on wear rate.

Repeatability ; If a test procedure is to be useful in comparing

the wear response of different materials, it must operate at a

satisfactory level of repeatability. In Fig. 23, results are

plotted for three consecutive tests, all using a currency ink

from the same container, the same chromium plated pin, and the

same PVC counterface. In terms of the measured quantity, depth

of wear, the repeatability appears to be good. In terms of

volume lost the repeatability, + 22%, is not as good but is still

reasonable when compared to typical results obtained from

laboratory wear tests in general. Figure 24 shows results from a

second set of consecutive wear tests. The ink in this case was an

intaglio stamp ink. Here, the wear rates, in terms of volume

lost, differ by approximately a factor of two. Similar tests on

different inks and chromium platings showed similar variations.

Thus, it appears that one can not expect in separate tests to

obtain repeatability that is better than a factor of two. If the

PVC counterface or the chromium plated pin is changed between

tests, even larger differences could be expected.

In a later section, a procedure will be described which largely

avoids the poor repeatability demonstrated in separate tests and

allows a relatively precise comparison to be made of the wear

response of different inks. However, before taking up this topic

and the discussion of the wear behaviors of the various inks

studied in this program, results will be presented regarding some

important factors that have a strong influence on the observed

wear rate in this test.

Factors Affecting Wear Rate ; A series of tests was conducted on

an abrasive slurry consisting of quartz particles in mineral oil.

These tests were initiated for the purpose of finding a standard

reference fluid which could be used for the periodic calibration

of the test. That is, by utilizing a standard fluid it was

anticipated that variations in wear rate associated with factors

such as the use of different PVC counterface materials or

22
chromium plated pin specimens could be detected. However, it was

found that the wear response of the quartz was rather unusual.

Quartz particles and mineral oil were mixed in the proportions,

one part quartz particles by weight to three parts mineral oil.

The quartz particles, designated BCR 70, were a standard

reference material prepared and distributed by the Commission of

the European Communities. Approximately 95% by mass of the

particles were specified to be within the size range, 0.5-12 urn,

with a median size of 3.0 urn. The mineral oil (paraffinic) was

obtained from Fisher Scientific Co. and had an indicated

viscosity of 125-135 SUS. The viscosity of the 1:3 slurry of

quartz particles and mineral oil was not measured but appeared

similar to that of the inks tested when blended 4 parts by weight

ink with one part of the same mineral oil.

Figure 25 shows the results of a test conducted on quartz

particles. The wear rate is initially rapid, then becomes

slower, and finally increases substantially. An initial

decrease in wear rate has been observed in tests on printing

inks, but it is not always seen. Among its causes are the

smoothing of the slightly rough, nodular surface that is

characteristic of the electrodeposited chromium and the geometric

effect associated with the parabolic dependence between depth and

volume when a flat is worn on the surface of a sphere. The rapid

acceleration in wear depth that occurs later in the test,

however, was not seen with inks. For the test shown in Fig. 25,

a fresh mixture of quartz particles and mineral oil was

introduced at the beginning and was not changed for the duration

of the test, since there was not a problem of drying out as there

was with printing inks. When a new test was run with the used

slurry from a previous test, the response shown in Fig. 26 was

obtained. Here the wear rate was rapid throughout the test.

Although measurements were not made, it is conjectured that

during the first test the quartz particles were fractured and the

smaller, perhaps sharper, particles constituted a more efficient

abrasive medium.

The basis for this hypothesis is as follows. To analyze the wear

process one must consider the geometry of the contact and the

particle size in relation to the film thickness, as is

illustrated in Fig. 27. Large particles (A) are not likely to

pass through the contact area and are pushed aside, although a

few of these particles may get trapped in the entrance zone and

be forced through. The occasional, large scratches seen on the

pin surface can be attributed to these particles. Particles much

smaller than the film thickness will pass through the contact

region with little or no effect. Particles (B) with diameters

equal to or slightly greater than the film thickness can enter

the contact region and cause wear. The amount of wear caused by

these particles will be a function of their concentration and

their hardness in relation to the surface they slide against.

Thus, in the above experiments, a large fraction of the quartz

23
particles were probably larger than the film thickness and did

not enter the contact area. However, large particles that did

pass through the contact zone were susceptible to fracture.

With time this could result in an increase in concentration of

particles in the smaller, more damaging size range.

Although the quartz slurry revealed important features about the

wear process, as the above discussion indicates, it probably is

not suitable as a reference fluid. Further work will be required

before such a fluid can be identified.

Effect of Counterface Roughness: The nature of the counterface

surface can have a substantial affect on wear rate. If the

counterface surface is rough, particles can be trapped in valleys

or small holes and cause much greater wear. The number of large

particles that enter the contact region is also likely to be

greater. The first instance where the importance of counterface

roughness was clearly indicated occurred when a test with quartz

particles was conducted on a newly machined PVC counter face.

The as-machined surface was significantly rougher than surfaces

after lengthy periods of testing. In the test, a slurry of

unused quartz particles was employed. The resulting wear rate

was high and nearly constant throughout the test, being similar

to that shown in Fig. 26. By the time a third test was

conducted, the counterface had become much smoother, in fact,

polished in appearance. Correspondingly, the wear behavior was

again like that shown in Fig. 25.

It was not possible within the framework of the present program

to conduct a thorough and systematic investigation of the

influence of counterface roughness. However, several tests of an

exploratory nature were conducted, that do indicate the manner in

which counterface roughness can affect wear rate with different

printing inks. Figure 28 shows wear rate results for tests with

smooth and rough PVC surfaces. To obtain the rough surface the

counterface was scratched with the edge of a file before each

test increment. The ink used in Fig. 28 was one that had been

found to produce a relatively high wear rate. (The results for

different printing inks are discussed in the next section. ) It

can be seen that the wear rate was substantially greater for the

rough counterface than the smooth counterface. This behavior was

not always observed, however. When similar tests were run with a

low wear rate BEP ink counterface roughness had little effect.

It can be hypothesized that the difference in behavior for these

two inks is related to the size of the particles which they

contain. Large particles in the SICPA ink pass through the

contact zone with less frequency when the counterface is smooth

than when it is rough, so wear is less for the smooth surface.

With relatively few large particles, as appears to be the case

with the BEP ink, counterface roughness has little effect.

24
In addition to surface roughness, the mechanical properties of

the counterface are also likely to have an effect on wear rate.

The elastic modulus of the counterface will determine the contact

area, which in turn affects the film thickness. Also the elastic

modulus and hardness of the counterface will determine how well

the particles are held and the force with which they are pressed

against the pin. These factors were not investigated during our

program.

Wear Characteristics of Currency Inks : Currency inks produced

both by BEP and SICPA were received for study on several

different occasions over the two year period of this project.

The inks were usually delivered in pint or in one instance,

gallon size metal containers. Usually but not always the inks

were described by identification and lot numbers. A list of the

inks received is given in Table 7 . Early in the program no

attempt was made to retard the drying out of the inks in the

containers and it was found that the inks were no longer suitable

for use after several weeks. Later the inks were covered with

nitrogen and refrigerated. In this way the inks were suitable

for use for several months. In addition, BEP prepared a special

batch of inks without dryers. Drying of these inks did occur,

however, by loss of volatile solvent.

TABLE 7

Date

Received Color Identification

9/26/84 black Cylinder wipe currency ink black

9/26/84 green Cylinder wipe currency ink green

1/25/85 green Project G-4716-MRE (without dryers)

3/01/85 black Project BK-3280-373 (without dryers)

11/27/85 green SICPA S10

11/27/85 black SICPA S4

3/20/86 green 1-286 Cylinder wipe green S10

W-2181-06 Lot #2-003-2929

3/20/86 black 1-287 Cylinder wipe black S4

W-2180-66 Lot #2-003-3613

5/21/86 green SICPA Intaglio CW green S10

Color 354100A Lot #2-003-4532

5/21/86 black SICPA Intaglio CW black

Color 374101G Lot #2-003-4740

As indicated in an earlier discussion, separately run tests were

not capable of distinguishing between inks unless they differed

markedly in wear rate. Even then care had to be taken to assure

that the PVC counterface and chromium plating were as nearly

similar as possible for each test. With these precautions it was

possible to identify some inks that gave relatively low wear

25
rates and some that gave relatively high wear rates. Inks that

gave low wear rates were the BEP inks without dryers, BK3270-373

and G-4716-MRE, and the SICPA ink, S4 black Lot #2-003-3163.

Inks that produced much greater wear rates were SICPA S4 black

Lot #2-003-4740 and SICPA S10 green Lot #2-003-4532. Separate

tests comparing SICPA S4 black Lot #'s 2-003-3163 and 2-003-4740

are shown in Fig. 29. On a wear volume basis these inks differed

by almost a factor of 20. The same chromium plating, applied

with BEP conditions, was used in both these tests. Among the

other inks studied, there was no indication on the basis of

separate tests that they were markedly high or low in their wear

behavior

The most reliable means to determine the relative differences in

wear rates between inks was by alternating the inks in the same

test run. Results using this method are shown in Fig. 30. Here

SICPA S10 green Lot # 2-003-4532, a fast wearing ink, is compared

with BEP G-4716-MRE w/o dryer, a slow wearing ink. In Fig. 30a

the SICPA ink is used first. It is replaced by the BEP ink in

the second segment and then used again in the final segment. In

Fig. 30b, the BEP ink is used in the first and last segments and

the SICPA ink is used in the middle segment. In both of these

tests the wear rate was noticeably less for the BEP ink. This

assessment can be made without reference to the actual wear rates

of the inks. Thus differences in chromium plating and the PVC

counterface material that might otherwise interfere with the test

results are avoided. (It is of course possible that a different

ranking would occur with different chromium plating or

counterface materials.)

A major limitation of the above method is that it does not easily

lend itself to the comparison of a large number of inks,

especially if the evaluation is to be made over a long period of

time. A means of avoiding this difficulty is to employ a

standard ink or a reference fluid. The fluid may be an ink or

preferably a simple slurry consisting of well characterized

abrasive particles and a liquid. As discussed in the previous

section, an attempt was made to use quartz particles and mineral

oil as such a fluid, but the wear results were not consistent.

It is thought that a fluid containing only a few of the major

constituents of currency inks might be more satisfactory.

Wear Characteristics of Intaglio Stamp Inks : The wear behavior

of three intaglio stamp inks was studied. These inks were

identified as follows:

SICPA LOCKWOOD GREEN

SICPA BROWN

BEP BROWN

Service experience had indicated that these three inks differed

significantly in abrasivity, with the SICPA lockwood green

exhibiting the greatest abrasivity and the BEP brown the least.

26
The availability of these three inks was of considerable

importance to our laboratory wear testing activity, since they

provided the opportunity to study inks with known service

performance. Rotating pin on disk test were conducted on these

inks. The BEP brown and the SICPA lockwood green inks were about

equal in wear rate while the SICPA brown ink was noticeably

greater. Thus the ranking of the inks in terms of laboratory

wear rate appears to differ from that determined by service

abrasivity. However, on examining the worn chromium plated

surfaces of the laboratory test specimens, it was found that very

large scratches were present with SICPA lockwood green, smaller

and perhaps fewer scratches were obtained with SICPA brown and

very few scratches were seen with BEP brown, in apparent

agreement with the service assessment of abrasivity. Photographs

of the worn test specimen surfaces are shown in Fig. 31.

These results indicate that in the determination of ink

abrasivity it is necessary to distinguish between wear rate and

scratching as two separate processes that may limit the useful

life of printing plates. The results also demonstrate that the

presence of large scratches and a high wear rate do not

necessarily correlate. This behavior is consistent with the fact

that a given ink can contain a range of particles of different

types, sizes, concentrations, and abrasivities . A high

concentration of very small but highly abrasive particles may

produce a high wear rate and yet yield a polished surface. On

the other hand, a mixture consisting of a few large abrasive

particles combined with a high concentration of less abrasive

particles may give a lower wear rate but cause severe damage by

scratching. In summary, these results indicate that printing

inks should be analyzed both for scratching abrasion and for wear

rate.

Wear of Different Types of Chromium Plating : In initial studies,

several different types of chromium plating were examined.

Separate tests were conducted using the same ink and PVC

counterface for each chromium plating. With one exception, the

scatter in the results far exceeded any difference that might

have been attributed to differences in the chromium platings.

The exception involved a chromium plating referred to as Armaloy.

That plating was said to consist of an outer porous layer of

electrodeposited chromium over an inner layer of dense, bright

plating. Typical results on this chromium plating are shown in

Fig. 32. The wear rate was initially very high and then

decreased to a low rate. This behavior was attributed to the

lower wear resistance of the outer- porous layer compared to the

underlying, dense chromium plating.

In order to obtain a more precise comparison of the wear rates of

different types of chromium plating, a three pin holder was

designed and constructed. With this device three of the same or

different platings could be tested simultaneously under

27
essentially identical conditions. The only exception is that

each pin follows a different track on the counterface. However,

by periodically exchanging positions of the pins in the holder,

even this possible source of bias can be minimized.

To investigate the extent under rather extreme conditions to

which different chromium platings might exhibit different wear

behaviors with currency inks, pins were prepared with very soft

chromium plating, approximately 500 HN versus approximately

900 KHN for standard BEP chromium plating. Results obtained with

the soft chromium plating are compared with standard BEP chromium

plating in Fig. 33. Here, the wear rate of the soft chromium

plating is clearly much higher than the BEP chromium plating.

When the test was repeated with a different currency ink, the

results in relative terms were essentially the same. The soft

plating wore much faster than the hard plating. These results

clearly indicate that wear rate can be strongly dependent on the

type of chromium plating under certain conditions.

Additional tests using the three pin holder were carried out to

compare the behavior of "transition" chromium plating with BEP

chromium plating. Transition chromium plating is deposited under

different conditions than those used for BEP chromium. As a

result, the crack density, microstructure and hardness differ.

Figure 34 shows results from a test employing currency inks where

two pins were plated with transition chromium and the third pin

was plated with BEP chromium (wear results for the latter pin

were shown previously in Fig. 30a) . The results indicate that

the BEP chromium has somewhat better wear resistance than the

transition chromium. The difference is however relatively small

compared to the expected variability in the test. One other test

gave the same ranking but a third test indicated that the BEP

chromium was intermediate between the two transition specimens.

Therefore, it can not be concluded that there is a significant

difference between these two different types of chromium plating

in the rotating pin-on-disk test with printing ink.

Some comment should be made about the fact that these results

differ both from those obtained by Lashmore et al.[l] and those

obtained by Bolster et al. [4] at the Naval Research Laboratory.

Lashmore et al. found that the transition chromium plating

exhibited somewhat better wear resistance than the BEP type

plating (prepared at NBS) while Bolster found that the BEP type

plating prepared at NBS was substantially better than the

transition plating but was not quite as good as the BEP chromium

plating on a specimen cut from a printing plate. In the latter

case, the chromium plating prepared by NBS according to BEP

conditions was found to be substantially harder than the BEP

plating on the printing plate. This probably accounts for the

different wear behavior in this instance. On the other hand,

assuming that the BEP and transition platings prepared at NBS and

tested by the three groups were all the same, the different wear

28
behaviors which were found are probably the result of differences

in the tests employed. Lashmore et al. used a slurry containing

5 um AI 2O3 particles with the pH adjusted to 4.5 by means of

ferric chloride, while Bolster et al. employed a slurry

containing 30 um quartz particles. Additionally, the load used

by Bolster et al. was substantially lower than that used by

Lashmore et al. These conditions, of course, differ from those

used in the three pin-on-disk test where printing ink was used as

the abrasive medium. Although the reasons for the different

results are not known in terms of the wear mechanisms involved,

the differences do indicate the strong influence that test

conditions can have and emphasize the importance of selecting

conditions that are relevant to the service conditions that are

actually involved.

C. Flat-on-Ring Test

Test Machine and Procedure : The flat-on-ring configuration,

shown schematically in Fig. 35, simulates more closely the

contact geometry that exists between the wiping roll and printing

plate than does the rotating pin-on-flat configuration. In the

press, the sliding direction is nearly unidirectional except for

a small periodic oscillation by a few degrees as a consequence of

the axial motion of the wiping roll. With the flat-on-ring

configuration sliding is unidirectional.

The ring for the flat-on-ring test is prepared from a 2.0 inch

disk cut from PVC wiping roll cover material. The inner black

and center white PVC layers are machined away leaving only the

outer black layer with a thickness of about 0.09 inches. After

drilling the necessary mounting holes in the disk, it is clamped

between flanges on a motor driven shaft. The design is such that

a ring of PVC material extends beyond the flange and comes into

contact with the flat specimen.

A significant advantage of the flat-on-ring machine design is

that the chromium plated flat specimens, which are in the form of

0.250 inch diameter disks, can be cut directly from the ends (or

other regions) of printing plates. Thus, the actual materials

used in the printing process can be used in the laboratory test

and there is no need to prepare special specimens -- unless, of

course, it is desired to study experimental coatings.

An additional feature of the flat-on-ring machine design is that

the ring and flat are enclosed. During a test, the entire

enclosure is filled with ink and only minimal exposure to the

atmosphere occurs. Thus, drying of the ink due to solvent

evaporation and exposure to oxygen in the air is minimized. In

addition, the enclosure contains passages for fluid circulation

to control temperature. Although this feature has not yet been

put into use, it would permit tests to be conducted at ink

29
temperatures that are normally encountered during press

operation.

Other features that have been incorporated are a load cell to

measure friction force and a variable speed drive. The

capability to measure friction force is important in that film

thickness, viscosity and/or ink rheological effects on wear rate

may be investigated. The variable speed drive allows sliding

speed to be studied as a variable that may influence wear rate.

A disadvantage of the flat-on-ring test is that only one specimen

can be tested during a run, while with the rotating pin-on-disk

machine three can be tested. Thus, the flat-on-ring test is not

as well suited to the comparison of different types of coatings

as is the rotating pin-on-flat test.

Operating conditions reported in the tests reported here are

listed in Table 8.

Prior to each test, the PVC ring and surrounding enclosure was

cleaned by swabbing with mineral spirits to remove old ink. The

enclosure was entirely filled with 12 g of a mixture of ink and

mineral oil (three parts ink to one part mineral oil by weight).

The chromium plated specimen was briefly lapped on 6 urn diamond

paste to produce a flat surface, ultrasonically cleaned with

hexanes, and mounted for testing. The test was then started and

run for the desired number of revolutions of the ring.

At the conclusion of the test, the volume worn from the chromium

plated flat was measured. Intermediate measurements were not

made as was done in case of the rotating pin-on-disk test. The

measurement was carried out by means of an automated, computer

controlled profilometer system. A series of parallel traces

20 urn apart was made across the worn region. The volume was then

obtained by calculating the difference between the worn surface

defined by these traces and the original flat surface.

Results : The results of a series of tests using SICPA S10 green

ink (#2-003-2929) are shown in Fig. 36. The wear volume on the

flat is plotted against the sliding distance as measured by the

number of revolutions. The flats for these experiments were cut

from a currency plate end. Each point on the graph represents a

TABLE 8

Flat-On-Ring Operating Conditions

Sliding Speed

Load

Temperature

89 cm/s

5 N

22+/-1 C

30
separate test run for the indicated number of revolutions. The

curve is a linear least squares fit to the plotted data.

A dimensionless wear coefficient, K, was calculated from Fig. 36

using the equation, K = VH/LD, where V is the wear volume, H is

the hardness, L is the load and D the sliding distance. The K

found was 6.2X10“^. This represents a relatively high rate of

material removal and is indicative of significant abrasive wear.

An example of a wear scar obtained in the flat-on-ring test is

shown in Fig. 37. This is a topographic plot generated from

profilometry data. Photographs of the scar and the surface of

the PVC ring are shown in Fig. 38 and 39, respectively. The worn

flat is characterized by grooves in the direction of sliding

having a maximum depth of about 3.0 urn. This is approximately

the size of the largest particles in the ink used in this test.

For example, see Fig. 40.

It should be noted that there are indications of damage in the

area just ahead of the slider (see Fig. 38). This area is shown

at higher magnification in Fig. 41. Here the surface appears to

be pitted or indented by sharp particles; however, at higher

magnification the edges of the pits are not sharp but rounded as

if erosion had taken place. This has the appearance of soft

abrasive wear where the particles are of essentially the same

hardness as the chromium plating. Under these circumstances the

pits would be formed by abrading away the softer areas in the

chromium plating.

The high wear rate in this test deserves comment. The test

emphasizes abrasive wear based on the rough PVC surface and the

fact that particles can easily be trapped in front of the slider.

In the rotating pin-on-flat test it is more difficult to trap

particles since the rotating ball continuously brings in new

surface void of particles, and there is a flow of the

ink-particle mixture laterally out of the contact line.

D. GTA Test

Test Method ; The Gravure Technical Association (GTA) markets a

relatively inexpensive test system for the evaluation of the

abrasiveness of printing inks. The test system includes a test

machine, referred as the GTA Ink Abrasion Tester, necessary

supplies, and instructions for conducting tests. A test machine

together with necessary supplies was provided to NBS by BEP for

evaluation. In this test method the ink to be evaluated is

rubbed against a chromium coated glass slide by means of a felt

covered rider. The amount of chromium coating worn away is taken

as a measure of the abrasivity of the ink. This result is then

compared with results obtained for other inks or perhaps a

standard ink. In this way a relative ranking is obtained. That

is, the test is intended to give relative abrasivity compared to

31
other inks and not a wear rate for chromium. Our analysis which

is summarized below indicated that the chromium coating on the

glass slides was significantly different in tribological behavior

than electrodeposited chromium. Therefore, the GTA test can not

be expected to give the desired determination of ink abrasivity.

A schematic drawing of the GTA Tester is shown in Fig. 42. The

chromium coated slide is held in the bottom of a rectangular

reservoir which contains the ink. The rider to which the felt is

attached is rubbed against the glass slide in a reciprocating

motion. The number of cycles of sliding is indicated by a

mechanical counter which can be set to turn off the machine after

a predetermined number. The applied load and stroke rate are all

fixed, i.e. there are no provisions for varying these quantities.

Values measured for machine provided by BEP are listed below.

Load 9.3 N

Frequency 80 strokes/s

Stroke Length 5.65 cm

Rider Width 1.7 cm

The vacuum deposited chromium coating on the glass slides is

extremely thin and transmits light. It is specified to have 40%

transmittance at 550 nm which indicates that the coating is no

more than a few tens of nanometers thick. Wear of the coating

will result in an increase in light transmittance which can be

measured by means of a photometer.

Conceptually this test method is very attractive. Wear of

chromium plating by printing inks, although a serious problem in

the printing industry, normally occurs at very low rates. Thus

to obtain an easily measurable amount of wear by many methods

will require a rather long test. Not only is this undesirable

because of the time factor, but with a long test the problem of

maintaining constant test conditions is also greatly increased.

Without good control over test conditions, indicated differences

among inks is subject to substantial uncertainty. Because

transmittance is very sensitive to coating thickness, wear can be

measured even when wear rates are very low with the advantage

that a relatively short test is required.

Ordinarily, the transmittance measurement would be made by a

simple photometer and light source. This equipment is not

provided with the GTA system. Since it was noted in preliminary

tests that there was a tendency for the chromium to be removed in

streaks, it was decided to employ a scanning microdensitometer so

that information on the variations in wear across the contact

region as well as the total loss could be obtained. A scanning

microdensitometer was assembled by replacing the stylus and

pick-up head of an automated profilometer system with a light

source and photodetector. With this device light intensity as a

function of position on the chromium coated glass slide could be

32
recorded. Variations in transmitted intensity as well as the

integrated intensity was available from these measurements.

Based on experience with other tests being conducted at NBS it

was known that wear of chromium plating by printing inks can be

quite sensitive to the substrate employed. In particular it was

considered important to simulate the contact conditions in

cylinder wipe presses as closely as possible. Thus, the majority

of the evaluation was carried out using PVC wiper roll cover

material obtained from BEP rather than the felt material that was

provided. In operation, a thin, 1.0 cm wide strip, of the PVC

material was cemented to the slider.

Currency printing inks have the consistency of pastes at room

temperature and furthermore dry out with exposure to air. The

rate of drying is increased by the mixing action associated with

sliding. The GTA test can not operate under these conditions and

it is necessary to dilute the ink with a solvent to make it less

viscous. Literature with the GTA tester recommends a specific

viscosity determined by the Zahn Cup method. It is suggested

that two or three parts solvent to ten parts ink will be

required. Measurements of viscosity were not made in our

evaluation, however, tests were conducted with a series of

different ink dilutions to cover a wide range of viscosities. As

will become apparent, ink viscosity is a very important quantity

in this test.

Test Results ; Figure 43 shows chromium coating wear as a

function of the number of cycles sliding for a green currency ink

diluted with mineral spirits (three parts ink to one part mineral

spirits). For comparison the results are shown for similar tests

conducted with paraffinic mineral oil (125-135 SUS) also diluted

with the same concentration of mineral spirits. The amount of

wear observed in both cases is about the same. The mineral

oil-mineral spirits solution does not contain abrasive particles

while the ink does. In seeking an explanation for this behavior

it was considered that the wear might have been produced by hard

particles in the PVC wiper roll material. The particles could be

there either inherently as part of the PVC formulation or due to

embedment from the previously used printing ink, despite

carefully cleaning between tests. Thus tests were conducted

using the felt material supplied for the GTA tester. In this

case wear with the mineral spirits-mineral oil mixture was even

greater. Photographic prints prepared by using the glass slides

as negatives are shown in Fig. 44. With a 3:1 mixture of mineral

spirits and mineral oil most of the chromium in the contact area

was worn away with the GTA felt material while there was

relatively little wear when PVC was used. Each test consisted of

300 cycles. Several other cloth materials commonly used in

metallographic polishing procedures were tried. It was

considered that such materials would not contain abrasives due to

the requirement that they should not scratch surfaces which in

33
many cases would be much softer than chromium plating. The

materials included Buehler Microcloth, silk, velveteen, and

nylon. All produced wear similar to GTA felt except nylon which

gave a result similar to PVC. When mineral oil without dilution

was employed, the GTA felt produced very little wear after

300 cycles.

The above results indicate that the chromium coating is quite

easily removed from the glass slides, even by very soft

materials. This kind of behavior is certainly not expected to be

representative of electrodeposited chromium on printing plates.

One would not expect a detectable amount of electrodeposited

chromium to be removed from a printing plate after rubbing for a

few thousand centimeters with organic fibers. An observation

made during experiments with the chromium coated glass slides

suggested a direct means of comparing the behavior of glass

slides with electroplated printing plates. It was noted that in

tests with mineral oil and mineral spirits that a dark

discoloration developed on the contact area of the white GTA felt

due an accumulation of chromium debris particles rubbed off the

glass slide. When an electroplated chromium specimen was used in

place of the GTA glass slide, there was no discoloration of the

felt indicating that there was no observable wear. The chromium

plated specimen was obtained by cutting a section from a currency

printing plate end that had been provided by BEP for other

studies. We therefore must conclude that the chromium coating on

the GTA glass slides has very different properties than

electroplated chromium on printing plates.

On the basis of our experiments it appears that the main factor

determining the amount of wear with the GTA slides is the

viscosity of the ink or other fluid. As long as a fluid film

prevents solid contact with the chromium coating wear will not

occur. Given two inks with the same viscosity but with one ink

having particles larger than the fluid film thickness, the ink

with large particles will cause wear of the coated slide whether

they are hard or quite soft particles. Are there conditions

where the GTA test could give good correlation with service? It

may happen that the fluid film conditions in some printing

operations are similar to those in the GTA test. For example,

gravure cylinder and doctor blade wear are known to be influenced

by the lubricating properties of gravure inks. An ink with large

particles, which happen to be hard enough to cause wear in the

press, may be shown to be relatively abrasive by the GTA test,

leading to a fortuitous example where good correlation exists

between the GTA test and service experience. Unfortunately an

ink with large soft particles would also appear abrasive in the

GTA test but might not cause wear in service.

Although the GTA test, as it is presently constituted, does not

appear to be suitable for measuring the abrasivity of printing

inks, the concept of a simple and rapid test of this type is

34
still attractive. By developing a chromium coating for glass

slides, perhaps an electrodeposited coating, that better

simulates the chromium plating on printing plates, it may be

possible to remedy the most serious deficiency of the GTA test.

In fact, it is our recommendation that an effort be undertaken to

improve the GTA test in this way.

E. Transfer of Wear Test Method to BEP

The method developed during the past year employing the rotating

pin on disc wear tester could be transferred for use by BEP

staff. This would require purchase and modification of a

commercial Schiefer Tester into the configuration now used at

NBS. The purchase cost would be approximately $6350; the

modification cost would depend on the place of modification (NBS

or outside private shop) . The test method developed at NBS would

be documented and instruction in its use would be provided.

Methods for preparing the PVC counterface surface, and the

chromium-coated steel balls would be explained. With this

recommended approach both NBS and BEP would have an identical

wear test system so that joint comparative measurements could be

carried out. In this way the transfer of wear test methodology

to BEP staff would be most feasible. There are also possible

additional modifications to the tester to be considered; the

first one recommended would be to enclose the specimen area so

that an inert gas environment could be established during the

test.

The rotating pin-on-disk test method could be used to measure

(1) the abrasivity of different inks currently in use, (2) the

consistency in wear properties of the "same” ink formulation over

a period of time, (3) the consistency of chromium plating over a

period of time, (4) the wear characteristics of new ink

formulation.

The second wear test method developed in the program, the

flat-on-ring method, is not proposed for transfer to BEP at this

time; the method still needs further development before it could

be used in a routine manner. The advantages of this method have

been discussed already, and are significant.

IV. Summary and Conclusions

Studies have been conducted of wear processes occurring in BEP

currency and stamp printing. From this work the following

conclusions have been drawn.

1. In the water wipe press, wear of chromium plated currency

printing plates occurs primarily at the wiper roll contact.

Particles in the ink are trapped at the contact between the PVC

covered wiping roll and the printing plate surface. Sliding

under pressure of these particles at the speed existing in the

35
contact causes abrasive scratching and polishing of the printing

plate surface.

2. According to the results of our laboratory tests which

simulate the conditions at the wiper roll and printing plate

contact, factors which have a significant influence on wear rate

are particle size, hardness and concentration, and the roughness

of the PVC surface. An increase in any of these factors can

result in a substantial increase in wear rate.

3. Other factors which were not studied in detail but which were

indicated to have a strong effect on wear were fluid film

thickness at the wiper roll-printing plate contact (this is

determined by contact pressure, ink rheology, and sliding speed)

and the ability of the ink to form a protective solid film on the

plate surface. Some ink constituents were found to form a

transfer film on chromium in laboratory experiments. Films were

observed on printing plate-ends which had sustained extremely

long service lives without appreciable wear.

4. Particles contained in the PVC wiper roll cover material do

not, in general, cause significant wear of the printing plates.

Large, hard, foreign particles which are occasionally embedded in

the PVC material may cause severe scratching damage.

5. Among the inks received from BEP for study in this program,

the wear rate of the highest wearing rate ink was 25 times

greater than that of the lowest in laboratory wear tests. Since

only a few different types of inks and different lots among these

types were studied, this result indicates that there is a

potentially large difference in wear rates among printing inks.

6. Currency inks that produced high wear rates were:

SICPA S4 black lot # 2-003-4740

SICPA S10 green lot # 2-003-4532

7. Currency inks that produced low wear rates were:

BEP Bk 3270-373 w/o dryer

BEP G-4716-MRE w/o dryer

SICPA S4 black lot # 2-003-3163

8. For stamp inks a correlation was established between service

behavior and laboratory results. An ink that caused severe

scratching in service was also found in laboratory tests to

produce severe scratching of chromium plating. Similarly, inks

that did not cause scratching in service did not do so in the

laboratory. It was also interesting to note that severe

scratching did not necessarily correlate with a high wear rate.

The concentration of abrading particles is clearly an important

factor here.

36
9. Three laboratory tests were evaluated for use in the study of

wear in the printing process. They were the rotating pin-on-disk

test, the flat-on-ring test, and the GTA test. The rotating

pin-on-disk test was found to be suitable both for the evaluation

of the wear behavior of different printing inks and of different

chromium platings. Appropriate test procedures were developed

for both of these tasks. Preliminary experiments indicated that

the flat-on-ring test could also be used for these tasks but

further development is necessary. The GTA test was found to be

unsuitable for determining ink abrasivity. The chromium coating

on the glass slides used to measure abrasivity was easily

scratched by soft materials that caused no detectable damage to

the chromium plating used on printing plates.

10. Using the rotating pin-on-disk test, it was demonstrated

that with various printing inks the wear rate of soft chromium

plating was substantially greater than hard BEP type chromium

plating. No appreciable difference was found between a

transition type chromium plating and BEP type plating.

V. Recommended Future Work

A. Scratching Test Development

Scratching of engraved printing plates during use is a serious

problem and arises from particles in the ink, present either

through intentional or accidental inclusion. One test method for

evaluating inks for scratching tendencies, the GTA method, has

already been evaluated in the first part of this program, and was

found to be inadequate. A new test that overcomes those noted

deficiencies is needed. The new test would examine chromium

plating^on metal substrates so that the relationship to actual

printing plates will be close. One possible method would use a

single stroke of a slider in the presence of ink on a very thin

plating. The depth and number of scratches would be determined

using chemical etching to penetrate to the underlying metal and

microscopy methods of measurement. It is thought that the method

could later be automated for rapid, routine use.

B. Additional Wear Test Methods

One possible test, the button test, would involve a chromium

plated cylinder or sphere loaded against a rotating PVC specimen.

To ensure a rapid wear rate a high stress test should be run with

provisions being made to get an adequate amount of abradant into

the contact area. There are several ways to accomplish this:

(1) a cylinder-shaped specimen would collect more abrasives at

the entrance zone than a sphere, (2) roughened surfaces will

contain polishing abrasives (roughening could be accomplished by

a light grit blast of both surfaces), (3) the PVC surface might

be made porous.

37
The button test could be used on an operating press and in a

bench test rig by making the arm detachable. In this way a

correlation could be established between the test rig and the

press. Further, with the arm attached to the press a correlation

could be established between the wear measurements and changes

observed in the currency bills.

C. Additional Wear Measurement Methods

It would be very important to obtain a value for the wear rate of

printing plates in actual service, and for a range of conditions.

This information would provide a baseline for use in deciding

whether subsequent wear experience is reasonable, or whether

perhaps some part of the printing process is not under usual . control. Such information would also permit a comparison between

laboratory wear data and printing operation wear data; that would

indicate the degree to which the laboratory wear test simulates

the actual use. Several possibilities exist that might permit

obtaining service wear data. Direct measurement of wear on the

chromium printing plates would be best; however, security

precautions make that difficult. Another approach recognizes that

the steel wiper blades that bear on the wiping roll experience

very similar sliding conditions to the printing plates. Chromium

plating of the wiper blades would permit an even closer

comparison to be made, once the loads involved were determined.

The blades could also serve to study other new materials for

possible printing plate use.

As another approach, a simple specimen holder could be added to

the press that would hold a small test specimen against the PVC

wiping roll during operation of the press. It would not

interfere with the usual operation. A possible design has been

developed (Fig. 45). Since it is not necessary to measure

friction such a device could be relatively simple consisting of a

magnetic base and a loaded lever arm. Such a device could be

constructed using a conventional dial indicator, suitable

attachments, and a magnetic base. An advantage of this approach

is that replaceable tips are available which could be chromium

plated for use as specimens. This device would provide accurate

wear data in a short period of time and it would not interfere

with the printing operation. Secondly, it could be used as a

separate wear test by loading it against an external PVC rotating

rod. It would improve the simulation to have the same device

used in service and in a bench wear test.

D. Fundamental Research Studies

Based on the laboratory test results and the understanding of

printing plate wear mechanisms obtained in the past two years, a

program of fundamental research is recommended. It would provide

quantitative relationships between a number of important

38
parameters that influence wear in printing processes. The

results obtained from experiments using appropriate laboratory

wear test devices would provide the basis for determining the

cause of wear problems when they do arise and also for reducing

wear under normal conditions. The test matrix would be

constructed from the following elements:

Particles: type

size

concentration

Fluid: viscosity

pH

film thickness

Test variables: load

speed

temperature

Counterface material: roughness

hardness

In these tests BEP type chromium plating would be used

throughout. Because of the size of the test matrix it might not

be possible to study all the different combinations of elements.

Each of the above elements should be studied under at least one

set of constant conditions.

ACKNOWLEDGMENTS

The authors are pleased to recognize the very valuable assistance

provided in this project by NBS staff: David Lashmore and Cris

Johnson in preparing various plated test materials, Eric

Whitenton in conducting optical measurements, and

BEP staff: Robert Hall and William Newitt in providing

information on printing technologies and specimens of inks,

plate-ends, and wiping blades.

39
REFERENCES

[1] D. Lashmore, C. Johnson, D. Kelley, L. Ives, and A. W. Ruff,

Quarterly Progress Reports to the Bureau of Engraving and

Printing (1985-1987).

[2] J. Larsen-Basse, Fundamentals of Tribology, N. P. Suh and

H. Saka, eds., MIT Press, p. 679 (1980).

[3] D. Lashmore et al.. Final Report to Bureau of Engraving and

Printing (in preparation).

[4] R. N. Bolster, Quarterly Progress Report to Bureau of

Engraving and Printing (1986).

[5] R. F. Schiefer, J. Res. Nat. Bur. Standards 39^, 1-10 (1947).

[6] L. K. Ives and M. B. Peterson, Wear of Materials -- 1985 , p. 355 (1985) .

" — '

40
Figure 1. Schematic drawing of the currency printing process

showing the wearing contacts.
Figure 2. Schematic drawing of the two-body abrasive wear

process

Ha /Hm

Figure 3. Effect of hardness on abrasive wear volume [2].

Ha is the hardness of the abrasive; HM

is the

hardness of the wearing body.
Figure 4. Photograph of the worn surface of a wiper blades

showing abrasive wear scratches.

Figure 5. Photographs of the cross sections of worn and unworn

wiper blades.
SCAR

WIDTH

PER

IMPRESSION

(mm

x

1000)

Figure 6. Wear scar widths on the seven wiper blades as a

function of position along the blade.

Figure 7. Wear volume as a function of number of impressions

for seven different wiper blades.
Figure 8. Photograph of unused PVC wiping roll cover surface

material.

Figure 9. Photograph of a worn PVC wiping roll surface after

use in service.
Figure 10. Photograph of a worn PVC surface showing cracks and

particles

LT= 200 SECS PVC W. R. C. PART. 10

Figure 11. Energy dispersive x-ray analysis of particle in PVC

wiping roll cover showing aluminum, potassium, and

silicon.
LT= 120 SECS BEP PLASTIC PARTICLE

S

I

30K -

20K

CO

ZDO

(_)

10K

10,020 15.000 ENERGY CkeV)

20.000

Figure 12. Energy dispersive x-ray analysis of particle in PVC

wiping roll cover showing only silicon.

Figure 13. Photograph of the silicon containing particle from

Fig. 12.
Figure 14.

Figure 15.

Photograph of amorphous silica particles used in BEP

formulated ink.

Photograph of particles from synthaline green used in

BEP formulated ink.
Figure 16. Photograph of chromium plated surface showing

transfer of amorphous silica particles.
Figure 17

Schematic drawing showing variables in the ink wiping

process

Loud

1

Figure 18. Schematic drawing of the rotating pin-on-disk

specimen configuration.
Figure 19. Photograph of the rotating pin-on-disk (Schiefer)

test machine modified to determine wear of chromium

plating by printing inks.
Photographs of the single pin holder (a) and three

pin holder (b) used in the rotating pin-on-disk test

machine

Figure 20.
6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 REVOLUTIONS (XI 0-6)

Figure 21. Rotating pin-on-ring wear test results illustrating

measurement precision. Depth of wear of chromium

plating caused by a currency ink is plotted as a

function of sliding distance measured in revolutions.

ABRASIVE MEDIUM

Figure 22. Wear of chromium plating by printing inks compared to

wear by other abrasives. Minimum and maximum values

are shown for the ink and quartz media studied.
6

Figure 23. Results of three separate tests conducted under the

same conditions with the same materials are shown to

illustrate good repeatability.

Figure 24. Results of two tests , run under the same conditions

,

where repeatability was poor.
8

Figure 25. Wear of chromium plating by quartz particles in

mineral oil showing rapid increase in scar depth that

occurred in the latter half of the test.

Figure 26. Wear by used quartz slurry. In contrast to Fig. 9,

wear rate was rapid throughout test.
SCAR

DEPTH

(um)

Load

Figure 27. Drawing illustrating the effect of wear particles in

a fluid film.

Figure 28. Rotating pin-on-disk test results showing effect of

PVC counterface roughness on wear rate.
SCAR

DEPTH

(um)

Figure 29. Results are shown for two inks exhibiting markedly

different wear rates in separate test runs.
SCAR

DEPTH

(um)

7

0 20 40 60 80 100 120 140 REVOLUTIONS (XI 000)

Figure 30. Wear of chromium plating by two different inks is

compared by alternating the inks in the same test

run. In (a) the test was initiated with a fast

wearing ink. In (b) the slow wearing ink was used

first.
Figure 31. Photographs of

wear scars on chromium plated

pins after test runs with three

intaglio stamp inks. Note

differences in scratching

produced by each ink.
4.5

0 20 40 60 80 100 120 140 160 REVOLUTIONS (XI 000)

Figure 32. Wear behavior of Armaloy chromium plating is shown as

a function of sliding distance. Porous chromium

outer layer results in rapid initial wear rate.

Figure 33. Wear behavior of soft chromium plating is compared to

BEP type chromium plating. The three pin holder was

employed for this test.
SCAR

DEPTH

(um)

9

Figure 34. Wear behavior of transition type chromium plating is

compared to BEP type chromium plating. The three pin

holder was used for this test.

Load

1

Figure 35. Schematic drawing of flat-on-ring specimen

configuration.
REVOLUTIONS

Figure 36. Wear of BEP chromium by SICPA S10 green ink with the

flat-on-ring test. Each point is the result of a

separate test. Curve is least squares fit to data.

Figure 37. Topography of wear scar on specimen from flat-on-ring

test obtained by surface prof ilometry
Figure 38.

Figure 39.

Optical photograph of entrance zone of wear scar on

specimen from flat-on-ring test.

Optical photograph of surface of PVC ring from

flat-on-ring test.
Figure 40.

Figure 41.

Scanning electron micrograph of a large particle from

SICPA S10 green ink used in flat-on-ring test.

Scanning electron micrograph of entrance zone of wear

scar from flat-on-ring test showing indentation

damage produced by hard particles.
RELATIVE

TRANSMITTANCE

Load

Ink Reservoir

Figure 42. Schematic diagram of the GTA test configuration.

60

50

40

30

20

10

0

O 40 80 1 20 1 60 200 240 280 320

NUMBER OF CYCLES

Figure 43. Wear (indicated by relative transmittance) of

chromium coating on glass slide as a function of

number of cycles for two different fluids.

+ Green Currency Ink and Mineral Spirits (3:1)

Mineral Spirits and Mineral Oil (3:1)
(a)

Figure 44. Photographs of worn areas on chromium coated slide

after test using (a) GTA felt and (b) PVC wiper roll

cover material.

Cr Plated

Button

Figure 45. Schematic diagram of a test device which would be

attached to a printing press to measure wear of

chromium plating.
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 87-3574

2. Performing Organ. Report No. 3. Publication Date

4. JUNE 1987

TITLE AND SUBTITLE

WEAR DUE TO PRINTING INKS

5.

AUTHOR(S)

L.K. Ives (450), M. Peterson (450), A.W. Ruff (450), J.S. Harris (450), P. A. Boyer (450)

6.

PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No. N-0283-04

I. Type of Report & Period Covered

Final

Oct. 1984-Sept. 1986

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Bureau of Engraving and Printing

Office of Research and Technical Services

14th and C Streets S.W., Washington, D.C. 20228

10.

SUPPLEMENTARY NOTES

"1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The principal modes of wear during currency and stamp printing by the intaglio

method were identified. Three laboratory test methods were developed and

applied to a study of different ink materials and different chromium platings.

The importance of abrasive particles in the inks was established. Recommendations

for continued research, both fundamental and applied, were made.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

abrasive; electroplating; inks; polishing; polyvinylchloride; printing; wear

13. AVAILABILITY

| y | Unlimited

I | For Official Distribution. Do Not Release to NTIS

HI Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

I i Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

69

15. Price

$13.95

U SCOMM'OC 8043-P80
*

.

'•