U.S. patent number RE32,244 [Application Number 06/426,959] was granted by the patent office on 1986-09-09 for methods and apparatus for applying wear resistant coatings to rotogravure cylinders.
This patent grant is currently assigned to Armotek Industries, Inc.. Invention is credited to Dennis Andersen.
United States Patent |
RE32,244 |
Andersen |
September 9, 1986 |
Methods and apparatus for applying wear resistant coatings to
rotogravure cylinders
Abstract
A coating of wear resistant tungsten carbide is applied to a
photo-etched roto-gravure cylinder, the coating having a thickness
in the range of from 15 to 35 microns. Tungsten carbide powder is
supplied to a plasma flame spray gun by means of a powder feeder
which supplies powder particles at a uniform density and at given
sizes. The feeder comprises a hopper having air inlet and outlet
passages and vertically spaced porous membranes which confine the
powder and permit the passage of air. Air flow through the hopper
suspends the powder particles of a desired diameter range adjacent
a spray gun feed conduit. When copper cylinders are to be coated,
the surface is allowed to oxidize to promote a chemical bonding of
the coating to the surface.
Inventors: |
Andersen; Dennis (Medford,
NJ) |
Assignee: |
Armotek Industries, Inc.
(Palmyra, NJ)
|
Family
ID: |
26780602 |
Appl.
No.: |
06/426,959 |
Filed: |
September 29, 1982 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
089474 |
Oct 30, 1979 |
04262034 |
Apr 14, 1981 |
|
|
Current U.S.
Class: |
427/450;
219/121.47; 406/138; 427/419.2; 427/419.7; 427/446 |
Current CPC
Class: |
B05B
7/144 (20130101); C23C 4/12 (20130101); B05B
7/1472 (20130101) |
Current International
Class: |
B05B
7/14 (20060101); C23C 4/12 (20060101); B05D
001/08 () |
Field of
Search: |
;427/34,399,318,330,419.2,419.7,423 ;406/123,138,172 ;156/89
;219/121PL ;428/698 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Newsome; John H.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
What is claimed is:
1. Apparatus for feeding powder to a plasma flame spray gun for
applying a coating to a roto-gravure cylinder, said apparatus
comprising:
a powder hopper defining an internal chamber which includes a lower
conical portion and an upper cylindrical portion, said hopper
including a gas inlet at a lower end thereof for introducing
pressurized gas into said chamber, and a gas outlet at an upper end
of said chamber for exhausting gas from said chamber,
a lower porous membrane disposed across said chamber above said gas
inlet and below at least part of said conical portion such that gas
passing through said lower membrane enters said conical portion and
is caused to swirl,
an upper porous membrane disposed across said chamber above said
lower membrane and below said gas outlet,
said upper and lower porous membranes being pervious to said gas
and impervious to said powder,
powder discharge opening means opening into said cylindrical
portion of said chamber between upper and lower ends of said
cylindrical portion and between said porous membranes, said
discharge opening means arranged to conduct powder to the spray
gun, and
means for supplying pressurized gas to said gas inlet for
suspending powder particles within said chamber between said upper
and lower membranes such that suspended particles of a selected
size range are suspended at a substantially uniform density
adjacent said powder discharge opening means and are discharged
therethrough.
2. Apparatus according to claim 1, wherein said discharge opening
means faces substantially radially.
3. Apparatus according to claim 1 wherein said powder discharge
opening means are disposed in said cylindrical portion.
4. Apparatus according to claim 3 wherein said powder discharge
opening means comprise a plurality of tubes extending into said
chamber.
5. Apparatus according to claim 4 wherein the inlet ends of said
tubes extend radially approximately midway between a wall of said
cylindrical portion and a vertical central axis of said hopper.
6. Apparatus according to claim 1 including driven brush means for
brushing powder from the lower surface of said upper membrane and
the upper surface of said lower membrane.
7. Apparatus according to claim 6 wherein said brush means comprise
a pair of brushes mounted on a common vertical drive shaft, such
brush including a horizontal bristle holder and bristles mounted
therein.
8. Apparatus according to claim 1 including gas pressure regulating
means for controlling gas pressure at said inlet and gas pressure
at said outlet.
9. Apparatus according to claim 1 including heating means for
heating said chamber.
10. Apparatus for applying a coating of tungsten carbide at a
thickness of from 15 to 35 microns to a roto-gravure cylinder,
comprising:
a plasma flame spray gun for emitting a spray of molten tungsten
carbide, and
means for feeding tungsten carbide particles to said spray gun at a
substantially uniform density, said feeding means comprising:
a hopper comprising a lower conically shaped portion and an upper
cylindrically shaped portion, said portions defining an inner
chamber of said hopper for containing tungsten carbide powder, said
hopper including a gas inlet at a lower end of said conical portion
and a gas outlet at an upper end of said cylindrical portion,
a lower porous membrane disposed across said conical portion above
said gas inlet and below at least part of said conical portion such
that gas passing through said lower membrane enters said conical
portion and is caused to swirl,
an upper porous membrane disposed across said cylindrical portion
below said air outlet,
said upper and lower membranes being pervious to said gas and
impervious to said powder,
powder discharge opening means in said cylindrical portion opening
into said cylindrical portion of said chamber between upper and
lower ends of said cylindrical portion and between said membranes,
said powder discharge opening means being connected to said spray
gun, and means for supplying pressurized gas to said gas inlet for
suspending said powder within said chamber between said upper and
lower membranes such that suspended particles of 1 to 8 microns in
diameter are suspended at a substantially uniform density adjacent
said powder discharge opening means and are discharged
therethrough.
11. Apparatus according to claim 10 wherein said powder discharge
opening means comprise a plurality of tubes extending into said
chamber.
12. Apparatus according to claim 11 wherein the inlet ends of said
tubes extend radially approximately midway from a wall of said
cylindrical portion to a vertical central axis of said hopper.
13. Apparatus according to claim 10 including driven brush means
for brushing powder from the lower surface of said upper membrane
and the upper surface of said lower membrane.
14. Apparatus according to claim 13 wherein said brush means
comprise a pair of brushes mounted on a common vertical drive
shaft, such brush including a horizontal bristle holder and
bristles mounted therein.
15. Apparatus according to claim 10 including gas pressure
regulating means for controlling gas pressure at said inlet and gas
pressure at said outlet.
16. Apparatus according to claim 10 including heating means for
said chamber.
17. Apparatus for applying a coating of tungsten carbide to a
roto-gravure cylinder in the range of from 15 to 35 microns
thickness, said apparatus comprising:
a plasma flame spray gun for emitting a spray of molten tungsten
carbide, and
means for feeding tungsten carbide powder particles of from 1 to 8
microns diameter to said spray gun at a substantially uniform
density, said feeding means comprising:
a chamber-defining hopper including an upper cylindrical portion, a
lower conical portion, an inlet at a lower end of said conical
portion for delivering pressurized air to said chamber, an air
outlet disposed
at an upper end of said cylindrical portion for exhausting air from
said chamber, and means for admitting tungsten carbide powder into
said chamber,
a lower porous membrane disposed across said conical portion above
said air inlet,
an upper porous membrane disposed across said cylindrical portion
below said air outlet,
said upper and lower porous membranes being pervious to air and
impervious to said powder,
said lower membrane disposed below at least part of said conical
portion such that gas passing through said lower membrane enters
said conical portion and is caused to swirl,
powder discharge opening means opening into said cylindrical
portion between upper and lower ends of said cylindrical portion
and between said porous membranes,
means for supplying pressurized air to said air inlet for
suspending tungsten carbide particles within said chamber between
said upper and lower membranes such that suspended particles from 1
to 8 microns diameter are suspended at a substantially uniform
density adjacent said powder discharge means and are conducted
therethrough to said plasma flame spray gun,
pressure control means for controlling air pressure at said air
inlet and air outlet,
upper and lower rotary brush means engageable with a top surface of
said lower membrane a bottom surface of said upper porous membrane,
respectively, and
means for rotating said upper and lower brush means about a
vertical axis to brush particles from said top and bottom surfaces
during passage of air through said chamber.
18. A method for applying a coating of tungsten carbide to a
roto-gravure cylinder comprising the steps of:
suspending tungsten carbide powder particles in a gas flow within a
hopper such that particles having a diameter of from 1 to 8 microns
are suspended at a substantially uniform density adjacent a powder
discharge of said hopper,
conducting said suspended particles from said discharge to a plasma
flame spray torch, and melting said particles in said torch and
spraying the melt agains the surface of a roto-gravure cylinder to
form a tungsten carbide coating of from 15 to 35 microns thickness
thereon.
19. A method according to claim 18 wherein said entraining step
comprises the steps of providing said particles in said hopper
between upper and lower porous membranes, introducing gas into said
hopper beneath said lower membrane and exhausting said gas from
said hopper above said upper membrane, whereby said gas suspends
said powder particles between said membranes.
20. A method according to claim 19 including the step of brushing
powder particles from a lower surface of said upper membrane and
from an upper surface of said lower membrane as said particles are
suspended.
21. A method according to claim 18 wherein said molten spray is
applied to a roto-gravure printing cylinder.
22. A method according to claim 18, wherein said cylinder comprises
a copper cylindrical surface, and wherein prior to said melting and
spraying step, a film of copper oxide is formed on essentially the
entire copper surface, and said molten spray being applied to said
copper oxide film.
23. A method of coating a copper surface of a roto-gravure cylinder
with a wear resistant substance comprising the steps of:
cleaning the cylindrical copper surface of said cylinder,
forming a film of copper oxide on essentially the entire
cylindrical copper surface of said cylinder, and
spraying said wear-resistant substance in molten form onto said
copper oxide film to form said coating.
24. A method according to claim 23 wherein said wear resistant
substance is tungsten carbide.
25. A method according to claim 24 wherein said coating is applied
to a thickness of from 15 to 35 microns.
26. A method according to claim 23 wherein said copper oxide film
is preheated prior to said spraying step.
27. A method according to claim 26 wherein said copper oxide film
is preheated to a temperature in the range of from 180.degree. F.
to 220.degree. F.
28. Method according to claim 23 wherein said forming step
comprises exposing said cylindrical surface to air.
29. Method according to claim 23 wherein said forming step
comprises subjecting said cylindrical surface to heat.
30. Method according to claim 24 including the step of entraining
tungsten carbide powder particles in a gas stream within a hopper
such that particles having a diameter of from 1 to 8 microns are
suspended at a substantially uniform density adjacent a powder
discharge of said hopper, and conducting said suspended particles
from said discharge to a flame spray gun.
31. Apparatus according to claim 10, wherein said discharge opening
means faces substantially radially.
32. Apparatus according to claim 17, wherein said discharge opening
means faces substantially radially. .Iadd.
33. A method for applying a coating of wear-resistant material to a
roto-gravure cylinder comprising the steps of:
suspending powder particles of said wear-resistant material in a
gas flow within a hopper such that particles having a diameter of
from 1 to 8 microns are suspended at a substantially uniform
density adjacent a powder discharge of said hopper,
conducting said suspended particles from said discharge to a plasma
flame spray torch, and melting said particles in said torch and
spraying the melt against the surface of the workpiece to form a
wear-resistant coating of from 15 to 35 microns thickness
thereon..Iaddend.
Description
BACKGROUND AND OBJECTS OF THE INVENTION
The present invention relates to the application of tungsten
carbide coatings to photo-engraved roto-gravure cylinders,
especially roto-gravure printing cylinders.
Roto-gravure cylinders are commonly employed as metering or anilox
rolls to transfer uniform coatings of ink or the like, and as
printing rolls to transfer a specific ink pattern. Such cylinders
are generally fabricated by a photo-engraving process wherein a
cellular surface is produced on the cylinder periphery which
functions to receive ink from a source, and transfer it to a web or
another roll.
Since roto-gravure rolls are subject to rapid wear, and to the
corrosive effects of printing inks, it has been common to plate
such rolls with substances such as copper or chrome to maximize
durability. Plating with copper and chrome is very expensive and
does not provide as much durability as a tungsten carbide coating.
Tungsten carbide coatings have been applied with some success to
mechanically engraved metering rolls by a flame spray technique,
but such success has not been achieved in connection with
roto-gravure printing rolls. In that regard, the presently utilized
flame spray techniques result in the application of a coating which
is excessively thick for roto-gravure printing rolls and thus
diminishes the definition of the cell pattern.
For example, the disclosure of a tungsten carbide coating of 0.002
to 0.008 inches on a metering roll in U.S. Pat. No. 4,009,658 would
be excessively thick for a gravure printing roll. Although the
cells are not filled to such an extent as to unduly impair the
performance of a gravure metering roll, a gravure printing roll
coated to this thickness cannot perform at acceptably high levels
of quality in most instances. Accordingly, gravure printing rolls
continue to be coated with more costly and less durable copper and
substances, such as chrome, which can be applied in sufficiently
thin coatings.
When applying coatings of substances such as chrome to copper
roto-gravure printing cylinders, grit-blasting step has been
performed to form a pitted surface on the copper surface to
facilitate adherence thereto of the coating, the latter entering
the surface pits to create a mechanical bond between the coating
and the cylinder. Prior to the application of the coating, the
copper surface is thoroughly cleaned and special precautions are
taken to prevent oxidation of the copper surface since the
formation of a copper oxide film has been herefore considered to be
detrimental. It would be desirable to eliminate the need for the
grit-blasting step which adds appreciably to the fabrication costs
of the cylinder.
It is therefore, an object of the present investigation to provide
novel methods and apparatus for coating roto-gravure cylinders.
It is another object of the invention to enable tungsten carbide to
be applied to roto-gravure printing cylinders, especially
roto-gravure printing cylinders, in a relatively thin layer.
It is a further object of the invention to enable a tungsten
carbide coating to be flame sprayed onto a roto-gravure cylinder at
a thickness in the range of from 15 to 35 microns.
It is an additional object of the present invention to eliminate
the need for a grit-blasting step for the application of
wear-resistant coatings to copper roto-gravure cylinders and
rolls.
It is yet another object of the invention to create a chemical
bonding between coatings, such as of tungsten carbide, and copper
roto-gravure cylinders and rolls.
SUMMARY OF THE INVENTION
These objects are achieved by the present invention which involves
feeding powder to a plasma flame spray gun for applying a coating
to a roto-gravure cylinder. The apparatus comprises a powder hopper
defining an internal chamber. The hopper includes a gas inlet at a
lower end thereof for introducing pressurized gas into the chamber,
and a gas outlet at an upper end of the chamber for exhausting gas
from the chamber. A lower porous membrane is disposed across the
chamber above the gas inlet and an upper porous membrane is
disposed across the chamber above the lower membrane and below the
gas outlet. The upper and lower porous membranes are pervious to
the gas and impervious to the powder. A powder discharge opening
communicates with the chamber between the porous membranes for
conducting powder to the spray gun. Pressurized gas is supplied to
the gas inlet for suspending powder particles within the chamber
between the upper and lower membranes such that particles of a
selected size range are suspended at a substantially uniform
density adjacent the powder discharge opening and are discharged
therethrough.
Preferably, tungsten carbide particles are fed from the hopper to
deposit a coating of tungsten carbide on the cylinder in the range
of from 15 to 35 microns thickness.
The invention is particularly advantageous in connection with the
coating of photo-etched roto gravure printing cylinders because the
thin coating does not unduly impair the printing performance.
Another aspect of the invention involves a method for coating a
copper surface of a roto-gravure cylinder with a wear resistant
substance comprising the steps of cleaning the copper surface,
forming a film of copper oxide on essentially the entire copper
surface, and spraying the wear-resistant substance in molten form
from a plasma flame spray gun onto the copper oxide film to form
the coating. By forming a copper oxide film, a chemical bonding of
the coating is achieved, which eliminates a grit blasting step
which is needed in cases where a mechanical bonding is
achieved.
THE DRAWING
These and other objects and features of the invention will become
apparent from the claims and from the following description when
read in conjunction with the accompanying drawings.
FIG. 1 is a schematic view of a system for applying a tungsten
carbide coating to a roto gravure roll according to the present
invention;
FIG. 2 is a vertical cross-sectional view through a powder feeding
mechanism of the system; and
FIG. 3 is a cross-sectional view taken along line 3--3 of FIG.
2.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A system 10 is depicted in FIG. 1 for applying a coating of
tungsten carbide to a photo-etched rotor-gravure roll 12. The
coating system includes a plasma flame spray gun 14 of a
conventional type which is mounted on a movable carrier 16. An
electric supply conduit 15 and an argon gas supply conduit 17
extend within the carrier and feed into the flame gun. Powder
supply hoses 18 communicate a feed mechanism 20 with the flame gun
for conducting a powder, preferably tungsten carbide, to the gun.
The spray gun itself is well known and further details thereof are
not needed. Suffice it to say that an electric arc is established
by the gun which melts tungsten carbide powder particles that are
introduced into the gun. The melt is then sprayed from the gun by
the argon gas.
As is conventional, the gravure roll 12 is mounted for rotation
about its longitudinal axis and the gun carrier 16 is mounted for
translational motion in a direction parallel to the roller axis.
The rotational speed of the roller and the translational speed of
the gun are correlated so that the entire periphery of the roller
is coated.
As noted earlier, in order for the cells of a gravure printing roll
coated with tungsten carbide to print with required definition, the
coating must be applied thin enough to avoid an excessive fill-in
of the gravure cells of the roll. For example, an acceptable
coating is one applied at a thickness in the range of from 15 to 35
microns, depending upon the cell size of the particular gravure
roll being treated. In this regard, it is preferable that the
selected thickness be controlled within a tolerance of 2.5
microns.
In order to achieve this goal with the flame spray process,it is
virtually essential that the density of the sprayed tungsten
carbide powder particles remain uniform and constant, and that the
size of the sprayed powder particles lie within a given range.
Substantial difficulties have been heretofore encountered in
achieving these requirements. In this regard, it is noted that
commercially available batches of tungsten carbide powders contain
particles of an excessively wide range of diameters.
Accordingly, the powder feed mechanism 20 of the present invention
has been provided which enables a gravure printing roll to be
coated with a sufficiently thin layer of tungsten carbide, i.e.,
within a range of 15 to 35 microns.
The powder feed mechanism comprises a hopper 22 having a
cylindrical upper portion 24 of circular cross section and a
conical lower portion 26 of circular cross section defining an
internal chamber 28. An entrance 30 for a gas, such as air, is
provided at a lower end of the conical portion 26, and an air
outlet 32 is provided in a cover 34 which covers the upper portion
24. An air delivery duct 36 (FIG. 1) supplies a stream of
pressurized air which enters the chamber 28 through the inlet 30
and exits the chamber through the outlet 32.
Extending completely across the cross section of the conical
portion 26 above the air inlet 30 is a lower circular porous
membrane 38, and extending completely across the cross section of
the cylindrical portion 24 below the air outlet 32 is an upper
circular porous membrane 40. The membranes 38, 40 are conventional
and comprise microporous polyethylene with a mesh size in the range
of 5 to 10 microns, which is sufficient to conduct gas flow but
prevent the passage of powder particles. A membrane thickness of
3/8" is preferred.
Disposed in the wall of the cylindrical portion 24 is a closable
loading hatch 42 through which tungsten carbide powder can be
introduced into the chamber 28.
Before a feeding operation commences, powder within the hopper 22
rests upon the lower membrane 38.
A series of powder discharge tubes 44, preferably four in number,
are positioned equidistantly around the cylindrical portion 24 and
communicate with the plasma gun 14 by means of the flexible hoses
18. The discharge tubes 44 lie in a horizontal plane located at a
level approximately midway along the height of the cylindrical
portion 24. The inlet ends 46 of the tubes 44 are positioned
approximately midway between the central vertical axis of the
hopper and the wall 48 of the cylindrical portion 24.
By passing air through the chamber 28, the tungsten carbide powder
therewithin becomes suspended and dispersed within the chamber.
At a given constant air velocity, powder particles of similar
volume tend to remain suspended at a particular level and density
within the chamber. The discharge tubes 44 are positioned at a
level corresponding to a desired particle size and density. By
varying the air velocity, those characteristics of the suspended
powder can be controlled.
The air velocity and pressure are controlled by regulators 50, 52
in the air inlet and outlet lines. In this fashion, it is possible
to regulate the discharge velocity of the particles through the
discharge tubes 44. For example, it is preferable to maintain a
pressure of 20-25 psi through the chamber 28 when feeding tungsten
carbide particles.
In order to prevent the upper and lower membranes 40, 38 from
becoming clogged with powder, a pair of upper and lower rotary
cleaner brushes 52, 54 are employed to "scrub" the
powder-contacting surfaces of those membranes. The brushes 52, 54
each comprise a plurality of radial spokes 56, 58 to which carry
bristles 60, 62. The spokes 56, 58 are connected to a common drive
shaft 64 extending vertically axially through the hopper 22. A
drive motor 66 is attached to the shaft to drive the brushes 52, 54
at selected speeds. The bristles 60, 62 are positioned to engage
the membranes and scrub powder particles therefrom. This assures
that the lower membrane will be unobstructed and available to pass
air to the powder, and that a uniform density level of powder will
be present adjacent the lower membrane for contact with the air
stream.
During the travel of air through the chamber 28, a swirling air
stream is established due to the conical configuration of the lower
portion 26 of the hopper. Such swirling action creates a random
flow pattern of the powder particles, assuring that a uniform
particle density (i.e., particle count per volume) is established
within the chamber 28.
Heavier powder particles may tend to gravitate downwardly along the
sides of the hopper. Such particles are caused to approach the
center of the hopper, due to the inwardly sloping nature of the
wall of the conical portion 26. Those particles are maintained in a
fluidized cloud above the lower membrane and, gradually gravitate
downwardly onto the membrane 38, whereupon they are immediately
propelled upwardly by the air flow through the lower membrane. The
lower brush 54 stirs the particles and prevents undue clogging of
the membrane 38.
The height to which the particles may rise in the chamber, for a
given air pressure, is a function of the size of the particles and
the air velocity within the chamber. That is, heavier particles
will not be lifted as high as the lighter particles per a given air
velocity. It has been found that for a given air pressure through
the chamber, the powder particles will be suspended at a
substantially uniform density, with particles of generally common
size being situated at a respective level in the chamber. Thus, by
suitable regulation of air pressure through the chamber, particles
of a desired size can be discharged through the discharge tubes 44.
Accordingly, it will be appreciated that the feeder mechanism
functions to classify particles according to size.
Moreover, the heavier, unwanted particles will remain suspended at
the lower end of the chamber and may eventually be disposed of when
the coating operation is finished.
A plurality of heating elements 70 are positioned around the
exterior of the cylindrical portion 24 to maintain the chamber at a
temperature sufficient to eliminate condensation which could
otherwise be absorbed by the powder. It has been found that such a
goal can be achieved by maintaining the chamber at about
150.degree. F.
It will be appreciated that the particle feeding mechanism is
adapted to applying a tungsten carbide coating or the like to all
types of roto-gravure cylinders.
In the fabrication of a copper roto-gravure printing cylinder 12,
the cylinder is initially photo-etched in a conventional manner.
Thereafter, the cylinder 12 is mounted in a rotational machine and
is thoroughly cleaned with trichloroethylene (C.sub.2 HCL.sub.3) to
remove soil that may be present on the surface. The cylinder is
then rotated and pre-heated, using the moving plasma torch as a
heat source, until a desired temperature of the cylinder,
preferably from 110.degree.-120.degree. F., is obtained. The
preheating cycle removes moisture that might be entrapped in the
microporous surfaces of the engraved areas on the cylinder. The
cylinder 12 is then cleaned again with trichloroethylene to remove
any oils that may have exuded during the pre-heating cycle.
As noted earlier, care has heretofore been taken to prevent the
occurrence of appreciable oxidation of the copper surface because
oxidation was believed to be detrimental to the application of the
coating. However, it has now been discovered that by promoting
oxidation, a chemical bond can be established between the copper
surface and the tungsten carbide coating.
Accordingly, oxidation is promoted by exposing the copper surface
to air for a selected period, e.g., about fifteen minutes, or by
heating the surface by a flame torch (with the powder feeder 20
maintained inactive). Accordingly, a film of copper oxide is
established on essentially the entire cylindrical surface of the
cylinder, resulting in a chemical bond being created with the
subsequently-applied tungsten carbide coating. While the inventor
does not wish to be held to a specific theory, it is surmised that
a crystal lattice structure is formed between the copper surface
and the tungsten carbide coating to create such mechanical
bond.
The copper oxide film which is established on the copper surface
can be cupric oxide, cuprous oxide or a combination of both.
The chemical bond which is created is significant in that it
eliminates the mechanical grit-blasting operation which has
heretofore been performed in the conventional mechanical bonding of
the coating to the cylinder. The chemical bonding according to the
present invention is more adherent and there is no distortion of
the cell structure as can result from grit-blasting.
It is possible that, heretofore, during performance of the prior
art coating techniques, a slight amount of oxidation may have
incidentally occurred on the copper cylinder surface prior to
application of the wear-resistant coating, despite efforts taken to
prevent same. However, if essentially the entire copper surface has
been oxidized, the coating would not be applied. Rather, the
surface would be re-cleaned before commencement of the coating
step.
After being formed, the oxidized film is preheated with the flame
torch preferably to a temperature in the range of from 180.degree.
F. to 220.degree. F. Preheating is accomplished, as before, by
means of the flame torch 14 with the cylinder rotating and the
powder feeder inactive.
After the preheating step, the torch 14 is returned to a starting
position and fixed at the proper distance from the face of the
cylinder for a coating step. The powder feeder is activated by
forcing air through the air inlet 30 of the powder-containing
chamber 28. Air passing through the container entrains the powder
particles and suspends them at a substantially uniform density
within the chamber, with particles of common sizes becoming
situated at respective levels within the chamber. The inlet and
outlet pressure valves are to achieve a desired air velocity and
powder density within the chamber to assure that particles of a
diameter in the range of from 1 to 8 microns are disposed at the
level of the powder discharge tubes 44. The valves are also
adjusted to achieve a desired powder velocity through the powder
discharge tubes 44. The brushes 38, 40 are rotated to eliminate
powder build-up on the upper and lower membranes.
The cylinder 12 is then rotated at the desired speed and the torch
14 is translated at the desired speed as a plasma stream of
tungsten carbide is emitted from the torch.
Powder particles in the range of from 1 to 8 microns are supplied
at a uniform density to the torch, enabling a coating to be applied
which is of uniform thickness in the range of from 15 to 35
microns, a coating which was heretofore not possible to achieve
with consistent uniformity. After the completion of the coating
operation, the cylinder is transferred to a wet polishing
machine.
The end result is a gravure printing cylinder having a much higher
resistance to abrasion and erosion, and a life expectancy nearly
ten times that of a conventional chromium plated cylinder.
The thin, uniform coating made possible by the present invention is
ideally suited to roto-gravure printing cylinders because the
definition of the printing cells is not destroyed. It will be
appreciated, of course, that the present invention is also
applicable to the coating of roto-gravure metering or anilox
cylinders with tungsten carbide.
The creation of a copper oxide film on the copper surface of a
roto-gravure rolls prior to the coating of tungsten carbide,
enables the coating to be bonded chemically, thereby eliminating
the conventional grit blasting step.
For principles, preferred embodiments and modes of operation of the
present invention have been described in the foregoing
specification. The invention which is intended to be protected
herein, however, is not to be construed as limited to the
particular forms disclosed, since these are to be regarded as
illustrative rather than restrictive. Variations and changes may be
made by those skilled in the art without departing from the spirit
of the invention.
* * * * *