U.S. patent application number 10/329709 was filed with the patent office on 2003-06-19 for coated article, manufacturing method therefor and coating apparatus.
This patent application is currently assigned to Koyo Seiko Co., Ltd.. Invention is credited to Kitahata, Kouji, Koyagi, Katsura, Kurokawa, Takanori, Nakamura, Shingo, Niide, Yoshifumi, Suehiro, Atsuo, Taniguchi, Kouichi, Tojo, Takeshi.
Application Number | 20030113475 10/329709 |
Document ID | / |
Family ID | 26543982 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030113475 |
Kind Code |
A1 |
Kitahata, Kouji ; et
al. |
June 19, 2003 |
Coated article, manufacturing method therefor and coating
apparatus
Abstract
After forming a primary powder bed at a toothed portion of an
article, such as a gear or a spline shaft, by electrostatic powder
coating process, the powder bed is removed from crests of teeth in
the toothed portion by a specified thickness by scraping it off
while sucking up the scraped powder. Next, the electrostatic powder
coating process is performed again to form a secondary powder bed
over the entire surface of the toothed portion. Then, the primary
and secondary powder beds are melted or fused by induction heating
to thereby form a coating film of a substantially uniform
thickness.
Inventors: |
Kitahata, Kouji; (Osaka,
JP) ; Tojo, Takeshi; (Osaka, JP) ; Koyagi,
Katsura; (Osaka, JP) ; Kurokawa, Takanori;
(Nara, JP) ; Suehiro, Atsuo; (Aichi, JP) ;
Niide, Yoshifumi; (Aichi, JP) ; Taniguchi,
Kouichi; (Gifu, JP) ; Nakamura, Shingo;
(Aichi, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Koyo Seiko Co., Ltd.
|
Family ID: |
26543982 |
Appl. No.: |
10/329709 |
Filed: |
December 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10329709 |
Dec 27, 2002 |
|
|
|
09660499 |
Sep 12, 2000 |
|
|
|
Current U.S.
Class: |
427/458 ;
427/180; 427/591 |
Current CPC
Class: |
B05D 3/0281 20130101;
F16D 2001/103 20130101; F16D 1/10 20130101; B05D 7/542 20130101;
B05D 3/12 20130101; F16D 2250/0046 20130101; F16H 55/06 20130101;
B05D 1/06 20130101 |
Class at
Publication: |
427/458 ;
427/591; 427/180 |
International
Class: |
B05D 001/12; H05B
006/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 1999 |
JP |
11-259128 |
Feb 17, 2000 |
JP |
2000-39503 |
Claims
What is claimed is:
1. A method for manufacturing a coated article, comprising the
steps of: performing electrostatic powder coating process to form a
primary powder bed in at least a region of an article to be coated,
said region having projections and depressions defined by or
defining said projections; removing the primary powder bed from a
tip portion of each projection by a specified thickness; performing
electrostatic powder coating process to form a secondary powder bed
over said region from which the primary powder bed at the tip
portions of the projections has been removed by the specified
thickness; and melting the primary and secondary powder beds by
induction heating to form a coating film on the article.
2. The manufacturing method according to claim 1, wherein the step
of removing the primary powder bed includes scraping the primary
powder bed while removing the scraped powder by suction.
3. The method according to claim 1, wherein in the steps of
performing electrostatic powder coating process, a powder coating
material having a mean particle size of 5 .mu.m to 100 .mu.m is
used.
4. The method according to claim 1, wherein in the steps of
performing electrostatic powder coating, a thermoplastic
resin-based powder coating material is used.
5. The method according to claim 4, wherein the thermoplastic resin
is a nylon resin.
6. The method according to claim 1, wherein in the step of removing
the primary powder bed, the thickness by which the powder bed is
removed is set such that the coating film to be formed has a
substantially uniform film thickness at any portion thereof.
7. The method according to claim 6, wherein the film thickness is
within a range of from 100 .mu.m to 300 .mu.m.
8. The method according to claim 1, wherein a ratio of a coating
weight A (g/m.sup.2) of the primary powder bed to a coating weight
B (g/m.sup.2) of the secondary powder bed falls within a range of
A/B=20/80 to 80/20.
9. A method for manufacturing a coated gear, comprising the steps
of: depositing a powder coating material having a particle size of
10 to 70 .mu.m onto a gear by electrostatic powder coating process
to form a primary powder bed on a surface of the gear; removing the
powder bed from crests of teeth of the gear by a specified
thickness by scraping the powder bed with a scraper while sucking
up the scraped powder; further depositing a powder coating material
having a particle size of 10 .mu.m to 70 .mu.m over the surface of
the gear by electrostatic powder coating process to form a
secondary powder bed; and melting the primary and secondary powder
beds by electromagnetic induction heating to form a coating film
having a thickness of 150 .mu.m to 200 .mu.m.
10. A method for manufacturing a coated spline shaft, comprising
the steps of: forming a thermoplastic resin-based primary powder
bed on a surface of a toothed portion of a spline shaft by
electrostatic powder coating process; removing the powder bed from
crests of teeth in the toothed portion of the spline shaft by a
specified thickness by scraping the powder bed with a scraper
movable in an axial direction of the spline shaft while sucking up
the scraped powder by a suction means; further coating the toothed
portion of the spline shaft with a thermoplastic resin-based
secondary powder bed by electrostatic powder coating process; and
melting the primary and secondary powder beds by electromagnetic
induction heating to form a coating film.
11. A method for forming a coating film on an article to be coated,
said article having a region containing projections and depressions
defined by or defining the projections, comprising the steps of:
performing electrostatic powder coating process to form a primary
powder bed in said region of the article to be coated; removing the
primary powder bed from a tip portion of each projection by a
specified thickness; performing electrostatic powder coating
process to form a secondary powder bed over said region from which
the primary powder bed at the tip portions of the projections has
been removed by the specified thickness; and melting the primary
and secondary powder beds by induction heating to form a coating
film in said region of the article, said coating film having a
substantially uniform film thickness at any portion thereof.
12. A coating apparatus comprising: coating means performing
electrostatic powder coating on an article to be coated having a
region containing projections and depressions defined by or
defining the projections, to form a primary powder bed on a surface
of the article in said region; scraper means removing the powder
bed from a tip portion of each projection of the article; coating
means performing electrostatic powder coating on the article from
which the powder bed at the tip portions of the projections has
been removed by the scraper means, to form a secondary powder bed
over the entire surface of the article in said region; and
induction heating means melting the primary and secondary powder
beds.
13. The coating apparatus according to claim 12, further comprising
means for sucking up powder scraped off the powder bed.
14. A method for manufacturing a coated article, comprising the
steps of: electrostatically depositing powder onto an article to be
coated, said article having projections and depressions defined by
or defining the projections; and melting the powder by induction
heating the article to thereby make a coating film adhere to the
article.
15. The method according to claim 14, wherein the powder deposit on
the article has different thicknesses, and dimensions of the
article are previously set according to the thicknesses of the
powder deposit so that the resulting coated article has a desired
outer dimension.
16. The method according to claim 14, further comprising a step of
cutting tips of the projections of the article to be coated before
the step of electrostatically depositing powder.
17. The method according to claim 14, wherein thickness of the
coating film is set to be 100 .mu.m to 250 .mu.m.
18. A coated article having a specified outer dimension and having
a coating film, wherein substantially no air bubbles are present in
the coating film.
19. The coated article according to claim 18, wherein the thickness
of the coating film is substantially same in all portions
thereof.
20. The coated article according to claim 19, wherein a difference
between a film thickness of the coating film at a thickest portion
and a film thickness thereof at a thinnest portion is within a
range of from 0 .mu.m to 40 .mu.m.
21. The coated article according to claim 19, wherein said film
thickness is 100 .mu.m to 300 .mu.m.
22. The coated article according to claim 18, wherein the coating
film has different film thicknesses, and the article before being
coated has dimensions set according to the film thicknesses of the
coating film.
23. The coated article according to claim 22, wherein the film
thicknesses are 100 .mu.m to 250 .mu.m.
24. The coated article according to claim 18, further comprising an
undercoat under the coating film.
25. The coated article according to claim 18, wherein the article
is a gear.
26. The coated article according to claim 18, wherein the article
is a spline shaft.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for forming a
coating film at uniform thickness on metallic articles having
projections and depressions defined by or defining the projection,
particularly gears and spline shafts. The present invention also
relates to a method of manufacturing coated articles such as coated
gears and spline shaft, and to a coating apparatus.
[0002] Metallic spline shafts and metallic gears for use in motive
power or motion transmission are formed with a resin coating to
impart thereto wear resistance, cushioning property, soundproof
property or muffling property. In the case where such a resin
coating is formed by fluidization dipping process, the resulting
coating become large in thickness because of large thermal capacity
of the articles (such as gears etc.), while an attempt to make the
coating into a thin film would cause the coating levelness to
lower, making it difficult to form a uniform coating with high
precision. Therefore, for example, after coating to a film
thickness of 300 .mu.m or more (in particular, about 500-600 .mu.m)
by fluidization dipping process, the coating is subjected to
machining process to form a coating film of about 200.+-.20 .mu.m.
However, this method involves the machining process at the crests
and troughs, or roots, of teeth of gears, resulting in not only
lowered production efficiency but also increased cost.
[0003] Meanwhile, it is also practiced that a powder bed is formed
by applying an electrostatic powder coating to gears or spline
shafts, and then a heating process is performed to form a coating
film. With this method, unfortunately, larger-sized gears or spline
shafts would take prolonged time for elevation of the object
temperature by an oven. Moreover, since the temperature elevation
begins with the surface of the powder bed in the oven, rapid
heating, if applied, would involve scaling and/or flaking, namely
detachment, of the coating film and/or occurrence of pinholes, thus
making it difficult to form a large-thickness coating film
uniformly.
[0004] In order to solve the problems with the oven, electrostatic
powder coating process and high frequency induction heating process
are practiced in combination to form a coating film. For example, a
Japanese publication JP-A-10-296182 discloses to form a resin
powder bed (thermosetting resin powder bed of epoxy resin or the
like) by electrostatic powder coating process on a continuous
metallic body of a metal wire, metal tube, metal bar and the like,
and then fuse the inside layer of the powder bed by induction
heating process to have the inside layer adhere to the metallic
body while forming a semi-fused resin layer at the top surface of
the powder bed, and perform further heating to finally form a
completely fused resin coating film. Japanese Patent No. 2725168
discloses a method for manufacturing an anticorrosion-coated steel
stranded wire including the steps of temporarily undoing a steel
stranded wire into a core wire and individual side wires,
depositing resin powder onto the core wire and the side wires,
heating the core wire and the side wires during the process of
re-stranding them so that an incompletely set coating layer is
formed around the outer periphery and between the core wire and
each side wires, and further heating the re-stranded steel wire so
that the coating layer is completely set.
[0005] Further, the Japanese publication JP-B2-56-34153 discloses
an insulating-coating formation method including the steps of
depositing a powder coating material onto an article having an
uneven surface, while the article is being rotated, within an
electrostatic fluidized dipping vat in which a 100-250 mesh powder
coating material is fluidized and electrified, removing unnecessary
powder coating material from projections of the article by a
brush-equipped suction means while rotating the article, and
heating the article by induction heating of about 100-200 kHz to
thereby fuse and fix the powder coating material. In this method,
in order to ensure electrical insulation at depressions as in the
motor rotor while avoiding the formation of coating film at
projections, a thicker powder bed is formed by
electrostatic-fluidized dipping process using a 100-250 mesh powder
coating material (epoxy resin coating material), and then the
powder bed of projections where the coating film is unnecessary is
removed, leaving the thick film only at the depressions.
[0006] However, the electrostatic powder coating process and the
electrostatic-fluidized dipping process, which use electrified
powder particles, inevitably involve increased film thicknesses at
crests (tip portions) of the teeth and reduced film thicknesses at
troughs and side walls due to the electrical repulsion of the
charged particles. In particular, removal of the powder bed at the
projections by a brush-equipped suction means during the rotation
of the article may cause, in some cases, the powder at the
projections to fall into the depressed portions. As a result, even
with induction heating, articles having a region having projections
and depressions (e.g., toothed portions of gears and spline shafts,
etc.) cannot be coated with a uniform coating film.
SUMMARY OF THE INVENTION
[0007] Accordingly, an object of the present invention is to
provide a method of manufacturing a coated article (e.g., gears,
etc.) which is coated in at least a region having projections and
depressions defined by or defining the projections, such as a
toothed portion, with a coating film of uniform thickness even
though the film is a thick film, as well as providing a coating
apparatus for use in manufacturing such a coated article.
[0008] Another object of the invention is to provide a method of
manufacturing a coated article (e.g., gears, etc.) being high in
both muffling property and wear resistance and having a uniform
film thickness distribution, as well as providing a coating
apparatus for use in manufacturing such a coated article.
[0009] Another object is to provide a method of forming a coating
film of uniform thickness, even with a large film thickness, on a
surface of an article in a region having projections or
depressions, such as a toothed portion.
[0010] A further object of the invention is to provide a coated
article manufacturing method capable of obtaining specified
dimensions of a coated article, even if the article has a region
containing projections and depressions, e.g., a toothed portion,
and the thickness of the coating film differs between the
projections and the depressions, by previously setting dimensions
of the article to be coated.
[0011] As a result of energetic discussions to achieve the above
objects, the inventors found out that by conversely taking
advantage of the fact that in the case of electrostatic powder
coating, gears become thicker in coating film thickness at the tip
or crest portions of the teeth than at trough or root portions and
side walls of the teeth, a powder bed uniform in thickness as a
whole would be able to be formed by performing an electrostatic
powder coating process all over a surface of an article to be
coated (particularly, gears and spline shafts) and thereafter
removing the powder bed present at tip portions of projections
(tooth tips, or crests) and further performing an electrostatic
powder coating process over the surface, while a coating film of
uniform thickness can be obtained by heating the thus formed powder
bed from its inside by induction heating. This is how the present
invention was made.
[0012] Accordingly, a method for manufacturing a coated article
according to an aspect of the present invention comprises the steps
of:
[0013] performing electrostatic powder coating process to form a
primary powder bed in at least a region of an article to be coated,
said region having projections and depressions defined by or
defining the projections;
[0014] removing the primary powder bed from a tip portion of each
projection by a specified thickness;
[0015] performing electrostatic powder coating process to form a
secondary powder bed over at least said region from which the
primary powder bed at the tip portions of the projections has been
removed by the specified thickness; and
[0016] melting the primary and secondary powder beds by induction
heating to form a coating film on the article.
[0017] With this method, a coated article (e.g., coated gears)
having a coating film of uniform thickness formed at least in a
region having projections and depressions of the article can be
obtained. Therefore, the method of the invention is useful also as
a method for forming a coating film of uniform thickness on an
article to be coated (e.g., gears) in a region having projections
and depressions defined by or defining the projections (in
particular, a toothed portion).
[0018] The article to be coated that has a region having
projections and depressions defined by or defining the projections
can be exemplified by a variety of gears and spline shafts and the
like. It is to be noted that the terms "gears" and "spline shafts"
used herein should be interpreted in a broad sense, and the "gears"
include, for example, spur gears, internal gears, helical gears,
double helical gears, rack and pinion, bevel gears, screw gears,
skew gears, worm and worm wheel, etc., while the "spline shafts"
include ordinary spline shafts (grooved shafts), serrated shafts
with spline teeth having a shape of chevron in cross section, and
the like.
[0019] In one embodiment, the step of removing the primary powder
bed includes scraping the primary powder bed while removing the
scraped powder by suction, so that the scraped powder is prevented
from falling off and depositing onto the bottom portions of the
depressions or side walls of the projections (roots, faces or
flanks of teeth).
[0020] For the electrostatic powder coating, powder coating
materials having a mean particle size of 5-100 .mu.m (e.g.,
thermoplastic resin-based powder coating materials such as nylon
resins) are usable.
[0021] The thickness of the coating film to be formed by
electromagnetic induction heating may be selected according to the
type of the article to be coated. For the coating of gears and
spline shafts, thick coating films having a thickness within a
range of about 100 to 300 .mu.m, especially about 150-200 .mu.m may
be formed. As described above, the deposition amount or coating
weight of powder coating material is larger at the tip portions of
the projections than at bottoms of depressions or side walls of the
projections. Therefore, with a view to forming a coating film of
uniform thickness, preferably, a ratio of a coating weight A
(g/m.sup.2) of the primary powder bed to a coating weight B
(g/m.sup.2) of the secondary powder bed may fall within a range of
A/B=approx. 20/80 to 80/20.
[0022] A coating apparatus according to another aspect of the
invention, comprises:
[0023] coating means performing electrostatic powder coating on an
article to be coated (e.g., gears) having a region containing
projections and depressions defined by or defining the projections
(e.g., a toothed portion), to form a primary powder bed on a
surface of the article in said region;
[0024] scraper means removing the powder bed from a tip of each
projection (e.g., tooth crest or tip portions) of the article;
[0025] coating means performing electrostatic powder coating on the
article from which the powder bed at the tip portions of the
projections has been removed by the scraper means, to form a
secondary powder bed over the entire surface of the article in said
region; and
[0026] induction heating means melting the primary and secondary
powder beds.
[0027] In the process of powder coating by the coating apparatus,
for such articles as gears and spline shafts, primary and secondary
coatings will be applied to the articles while the articles are
being rotated about their center axis.
[0028] In order to prevent the powder scraped by the scraper means
from falling off and depositing onto the bottom portions of the
depressions and/or side walls of the projections (e.g., roots,
faces or flanks of the teeth), the coating apparatus may further
comprise means for sucking up powder scraped off the powder bed.
The scraper means itself may be equipped with such a suction means.
Also, the scraper means may be movable in the axial direction of
the article.
[0029] A method for manufacturing a coated article, according to
still another aspect, comprises the steps of:
[0030] electrostatically depositing powder onto an article to be
coated, said article having projections and depressions defined by
or defining the projections; and
[0031] melting the powder by induction heating the article to
thereby make a coating film adhere to the article.
[0032] In this method, an article to be coated having projections
and depressions defined by or defining the projections is
induction-heated in the state that powder has been
electrostatically deposited on the article, to melt the powder.
That is, the powder bed is heated by the article itself inside the
powder bed. As a result, air bubbles become more likely to escape
out of the internally melted powder to the outside, so that air
bubbles are prevented from being included in the coating film.
[0033] The powder deposit on the article may have different
thicknesses. Then, the dimensions of the article before being
coated may be previously set according to the thicknesses of the
powder deposit so that the resulting coated article has desired
outer dimensions (e.g., an outer diameter). In this case, a
secondary finishing process (e.g., broaching) is no longer
necessary, allowing the working cost to be reduced.
[0034] To previously set the dimensions of the article according to
the thicknesses of the powder deposit, the method may further
comprise a step of cutting tips of the projections of the article
to be coated before the step of electrostatically depositing
powder.
[0035] In an embodiment, the thickness of the coating film is set
to 100 .mu.m to 250 .mu.m. As a result, working noise (tap tone) of
this coated article can be reduced to a specified desired value or
lower, and wear can also be reduced. With the film thickness below
100 .mu.m, working noise (tap tone) would be over the specified
value. With the film thickness over 250 .mu.m, not only working
noise would not be significantly reduced but also wear would
increase.
[0036] The present invention also provides a coated article having
a specified outer dimension (e.g., an outer diameter) and having a
coating film, wherein substantially no air bubbles are present in
the coating film. The coated article may have the coating film
either partially or entirely. In the coated article, no air bubbles
are present in the coating film. Thus, the coating quality is
improved.
[0037] In one embodiment, the thickness of the coating film is
substantially same in all portions thereof. A difference between a
film thickness of the coating film at a thickest portion and a film
thickness thereof at a thinnest portion may be within a range of
from 0 .mu.m to 40 .mu.m.
[0038] In another embodiment, the coating film has different film
thicknesses, and the article before being coated has dimensions set
according to the film thicknesses of the coating film. In this
case, a secondary finishing process (e.g., broaching) is no longer
necessary, allowing the working cost to be reduced.
[0039] The coated article may further include an undercoat under
the coating film.
[0040] Other objects, features and advantages of the present
invention will be obvious from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0042] FIGS. 1A, 1B, 1C and 1D are process charts showing an
example of a coating process on a toothed portion of a spline shaft
according to an embodiment of the invention;
[0043] FIG. 2 is a schematic perspective view showing a scrapped
state by a scraper means;
[0044] FIG. 3 is a schematic side view of a spline shaft;
[0045] FIGS. 4A and 4B are schematic views showing a process in
which powder is fused by electromagnetic induction heating and
FIGS. 4C-4D are schematic views showing a process in which powder
is fused by oven heating;
[0046] FIGS. 5A, 5B and 5C show an example of coating process on a
toothed portion of a spline shaft according to a second embodiment
of the invention;
[0047] FIG. 6 is a characteristic chart showing a relationship
between film thickness and working noise (relative working noise)
of a coating film in the second embodiment;
[0048] FIG. 7 is a characteristic chart showing a relationship
between film thickness and wear (relative wear) of a coating film
in the second embodiment;
[0049] FIG. 8 is a block diagram showing an example of the coating
apparatus to be used for manufacturing a coated article of the
invention; and
[0050] FIG. 9 is a block diagram showing another example of the
coating apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] Hereinbelow, the present invention is described in detail
with reference to the accompanying drawings.
[0052] First Embodiment
[0053] FIGS. 1A-1D are process charts showing an example of coating
process on a toothed portion of a spline shaft. FIG. 2 is a
schematic perspective view showing a scrapped state by a scraper,
and FIG. 3 is a schematic side view of the spline shaft.
[0054] Primary Powder Bed Formation Step
[0055] In a step for forming a primary powder bed shown in FIG. 1A,
electrostatic powder coating process is performed to form a primary
powder bed on a surface of an article to be coated. In this
example, first, a spline shaft 1 is preprocessed and
primer-processed, and while being rotated around its axis, the
spline shaft 1 is coated by electrostatic powder coating process
with an electrostatic spray gun as a coating means. By these
processes, a primary powder bed 3a made of a nylon resin-based
powder coating material is formed over an entire surface of a
toothed portion (namely, a keyway portion) 2 of the spline shaft 1,
including crests, or top lands, 2a, sides 2b, and roots, or
bottoms, 2c of teeth.
[0056] The preprocessing of the metallic article to be coated
includes surface treatments such as degreasing process, pickling
process, electrolytic process, anodic oxidation process, and
chemical conversion process, as well as surface finishing processes
such as shot blasting. The primer process may be achieved by
applying conventional methods, for example, by applying, to the
article to be coated, organic primers containing such resins as
epoxy resin, phenol resin, polyvinyl acetal resin and rubber or
oligomer, inorganic primers containing such metallic components as
magnesium, manganese and silicon, or mixed primers of these.
Further, if necessary, the article may be offered for electrostatic
powder coating with its non-targeted sites masked.
[0057] As the powder coating material, are available powder
particles based on various types of thermoplastic resins or
thermosetting resins usable as electrostatic powder coating
material. The thermoplastic resins can be exemplified by olefinic
resins (including polyethylene, polypropylene or other olefins
alone or their copolymers, ethylenic copolymers such as
ethylene-vinyl acetate copolymer, ethylene-(meth)acrylate or their
salts (e.g., ionomers, etc.), propylene copolymers such as
propylene-ethylene copolymer or propylene-butene copolymer, and the
like), acrylic resins, vinyl chloride resins, nylon resins,
polyester resins, and the like. The thermosetting resins can be
exemplified by epoxy resins, thermosetting polyester resins,
thermosetting acrylic resins, and the like. The thermosetting
polyester resins and the thermosetting acrylic resins may be
combined with crosslinking agents or curing agents (e.g., amino
resins, such as melamine resin, etc.) to form thermosetting
resins.
[0058] Preferable resins are thermosetting resins (e.g.,
polyethylene resins, nylon resins, etc.) which are high in wear
resistance, shock resistance and durability and high in adhesion to
metal, in particular, nylon resins. The nylon resins can be
exemplified by homopolymerized nylons such as 6-nylon, 66-nylon,
46-nylon, 610-nylon, 616-nylon, 11-nylon, 12-nylon and 13-nylon,
and copolymerized nylons such as 6/12-nylon and 66/12 nylon.
Preferable nylon resins include homopolymerized and copolymerized
nylons containing at least one nylon component selected from
11-nylon and 12-nylon.
[0059] Mean particle size of the powder coating material is
normally smaller than those of powder coating materials used for
fluidization dipping process and electrostatic-fluidized dipping
process. For example, the mean particle size may be selected from a
range of 5-100 .mu.m (e.g., 10-70 .mu.m, more particularly, 10-50
.mu.m).
[0060] In powder coating process, since the amount of powder
deposition at the crests 2a of the toothed portion 2 is larger than
that at the sides 2b and bottoms 2c of the toothed portion 2 due to
the electrostatic repulsion of charged powder particles or the
like, coating film of uniform thickness cannot be formed even by
heating the primary powder bed 3a with induction heating. In
particular, as the coating film thickness becomes larger, the
amount of powder deposition at the sides 2b and bottoms 2c of the
toothed portion 2 decreases considerably, compared with the amount
of powder deposition at the crests 2a of the toothed portion 2,
causing the uniformity of the film thickness to lower
considerably.
[0061] Scraping Step
[0062] Therefore, in the present invention, in a scraping step
shown in FIG. 1B, the primary powder bed 3a deposited on the tooth
crests 2a of the toothed portion 2 is scraped by a scraper 4 (see
FIG. 2), where the powder is sucked while being scraped, thereby
removed, in order that the powder is prevented from falling off and
depositing onto the sides or bottoms of the toothed portion. In
this example, the scraper 4 is made up of a thin base piece 4a
which is semicircularly bent in correspondence to the degree of
curve of the crests (namely, outer peripheral surface) of the
spline shaft 1 and which has at its front end a doctor blade for
scraping the primary powder bed 3a, a slit 4b formed in the base
piece 4a at a specified distance from the edge of the doctor blade,
and a suction portion 4c for sucking from the rear the powder
guided to this slit.
[0063] The scraper 4 is movable relative to the spline shaft and
along its axis so as to efficiently remove the primary powder bed
3a from a plurality of tooth crests 2a.
[0064] It is also conceivable to scrape the powder bed with a
brush. However, using a brush would cause not only the powder bed
deposited on the tooth crests to drop into the trough portions, but
also the powder bed at the troughs to be scraped off, so that the
surface of the toothed portion could no longer be coated overall
with a coating film of uniform film thickness.
[0065] Secondary Powder Bed Formation Step
[0066] In a secondary powder bed formation step shown in FIG. 1C in
which the powder bed 3a of the crests 2a has been removed, the
spline shaft 1 as an article to be coated, while being rotated
about its center axis, is electrostatically powder-coated in the
toothed portion 2 with the electrostatic spray gun as a coating
means, by which a secondary powder bed 3b is formed on the entire
surface of the toothed portion 2 of the spline shaft 1. That is,
the crests 2a of the toothed portion 2 are coated mainly with the
secondary powder bed 3b, while the bottoms 2c and sides 2b of the
toothed portion 2 are coated with a laminated powder bed composed
of the primary powder bed 3a and the secondary powder bed 3b. Thus,
the surface of the toothed portion 2 can be coated over all with a
powder bed of uniform thickness.
[0067] More specifically, use of the same kind of coating material
normally allows the thickness of the coating film to correspond to
the amount of powder deposition or the thickness of powder bed.
Therefore, given a powder deposition amount X1 at the tooth crests
of the toothed portion by the primary coating process, a powder
deposition amount X2 at the bottoms and sides of the toothed
portion (where X1>X2, .DELTA.X=X1-X2), a powder deposition
amount Y1 at the tooth crests of the toothed portion by the
secondary coating process, and a powder deposition amount Y2 at the
bottoms and sides of the toothed portion (where Y1>Y2,
.DELTA.Y=Y1-Y2), and if the primary powder bed and the secondary
powder bed are formed so that a relationship of Y1=X2+Y2 is
satisfied, then a coating film of uniform thickness can be formed
on the surface of the toothed portion because substantially all the
primary powder bed has been scraped off the tooth crest
portions.
[0068] In a region or place having projections or depressions of an
article, the amounts of powder deposition on the crest portions,
trough portions and side walls vary depending on the geography or
configuration of the place (angular acuteness of crest portions,
depth of trough portions, interval and pitch of crest portions and
trough portions, etc.). Therefore, measuring relationship between
the deposition amount of the powder material and the thickness of
the coating film beforehand by experiments makes it possible to
form a coating film of uniform thickness even in a region having
projections or depressions, according to the shape and type of the
article to be coated.
[0069] Powder deposition amounts (g/m.sup.2) for the primary powder
bed and the secondary powder bed in a region having projections and
depressions defined by or defining the projections of an article
can normally be selected, according to the shape and size of the
article in that region, such that the ratio of deposition amount A
(g/m.sup.2) of the primary powder bed to deposition amount B
(g/m.sup.2) of the secondary powder bed falls within a range of
A/B=approx. 20/80-80/20 (preferably, approx. 25/75-75/25, more
preferably approx. 30/70-70/30).
[0070] Further, since the amount of powder deposition corresponds
to the thickness of the powder bed and the thickness of the coating
film, the above relationship in powder deposition amount between
the first and second powder beds corresponds not only to a
relationship between the thickness of the primary powder bed and
the thickness of the secondary powder bed, but also to a
relationship between the thickness of the primary coating based on
the primary powder bed and the thickness of the secondary coating
based on the secondary powder bed.
[0071] The deposition amounts for the powder beds can easily be
controlled by spray amount (discharge amount) and spray time of
powder coating material. For example, according to the thickness of
the final coating film, in the primary powder bed formation step,
the primary powder bed can be formed through electrostatic coating
by spraying powder coating material for about 5-60 seconds (e.g.,
10-40 seconds) from a spray gun, and in the secondary powder bed
formation step, the secondary powder bed can be formed through
electrostatic coating by spraying powder coating material for about
5-180 seconds (e.g., 10-120 seconds).
[0072] Induction Heating Step
[0073] Then, as shown in FIG. 1D, a coating film 5, which is a
nylon resin based thick film, is formed at a uniform thickness at
the toothed portion 2 of the spline shaft 1 through the steps of
electromagnetic induction heating with induction heating means
(induction heater or the like), cooling by radiational cooling or
the like, and if necessary, removal of the masking at non-coated
sites. This coating film 5 is composed of a primary coating film 5a
corresponding to the primary powder bed 3a and a secondary coating
film 5b corresponding to the secondary powder bed 3b, although
there is seen no boundary between the films 5a and 5b if same
powder coating material is used for the first and second powder
beds 3a and 3b. With the toothed portion 2 of the thus obtained
coated spline shaft 1 set into a boss so that the toothed portion 2
is movable along the axial direction, occurrence of noise due to
the engagement of the toothed portion 2 with the boss can be
prevented, and wear resistance and corrosion resistance can also be
improved. As a result, the spline shaft is enabled to transfer
motion or motive power with noise muffled and with security.
[0074] By making use of the induction heating, the primary powder
bed 3a and the secondary powder bed 3b are fused by being heated
from the inside of the powder bed, by which the coating film 5 of
uniform thickness can be formed within a short time efficiently on
the surface of the toothed portion. That is, the nylon resin-based
powder coating material is not only high in melting point or
softening point, but also high in dielectric constant and thus low
in electric insulation property and small in amount of
electrostatic charge, as compared with resins such as epoxy resin.
Therefore, the adhesion by static electricity considerably lowers
with increasing thickness of the powder bed. Thus, heating such a
powder bed by an ordinary oven, which causes the surface of the
powder bed to be melted but its interior to remain in a powder
state, would involve an excessive heating process, making it
impossible to form a coating film of uniform thickness.
[0075] In contrast to this, use of induction heating, which allows
the powder bed to be heated and melted from its interior, makes it
possible to form a coating film of uniform thickness efficiently
even if the powder bed has a large thickness. In applications to
spline shafts and gears, for example, a coating film of uniform
thickness can be formed on the surface of the toothed portion by
electrostatic powder coating even if the coating film thickness is
comparatively large. In particular, coated gears and spline shafts
in which the coating film thickness (mean value) at the tooth crest
portion, bottom portion and side portion is about 100-300 .mu.m
(particularly, 150-200 .mu.m) and in which the difference in film
thickness between the coating film at the tooth crest and the
coating film at the bottom and side is 0-40 .mu.m (preferably, 0-30
.mu.m, more preferably 0-20 .mu.m) in average can be formed
easily.
[0076] With the use of a resin-based powder coating material having
a high electric insulation property and a large amount of
electrostatic charge and a low melting point or softening point,
there are some cases where induction heating causes the interior of
the powder bed to melt and flow out because of its lower melting
point, making it difficult to form a coating film having a large
thickness. Even in such a case, by controlling the heating
conditions of the induction heating means (induction heater or the
like), a thick coating film can be formed according to the
characteristics of the powder coating material.
[0077] The induction heating may be done by using conventional high
frequency induction heating equipment, where the frequency of
induction heating may be, for example, about 0.5-200 kHz (e.g.,
0.5-100 kHz, preferably 1-80 kHz, more preferably 5-70 kHz,
particularly 10-50 kHz).
[0078] In the primary powder bed and/or secondary powder bed
formation step(s), for articles to be coated having a center axis
(e.g., gears), it is advantageous that the article is subjected to
electrostatic powder coating while being rotated about its axis,
for higher coating efficiency. However, it is not necessarily
required that the article is rotated during the electrostatic
coating process. For example, platy articles to be coated having
depressed portions or grooves may be subjected to electrostatic
coating while being moved vertically or horizontally along its
longitudinal direction. Also, the primary powder bed and/or
secondary powder bed may be formed over a surface of the article
only in the region having projections and depressions. It is noted
that either the same powder coating material or different powder
coating materials may be used for the primary coating and the
secondary coating.
[0079] In the scraping step (or selective removal step) for
removing the powder at the tip portions of the projections, various
removal means such as a scraper may be used according to the type
and shape of the article to be coated. The powder may be scraped
off the bed in directions crossing or perpendicular to the
direction in which the projections (e.g., teeth) extend, but
scraping along the projections would be more efficient. The doctor
blade of the scraper may be made of, for example, paper, plastic,
rubber, metal or the like, where it is advantageous to form the
doctor blade from an electrically non-chargeable base material (or
electrically conductive base material) in order to prevent
deposition of charging particles.
[0080] For example, in the case where the powder bed formed at the
projections of a cylindrical rod (article to be coated) having
grooves extending in the axial direction is scraped while being
moved relative to a scraper in the axial direction, the scraper may
be a scraper having a hollow, cylindrical doctor blade curved in
accordance with the curvature of the outer periphery of the
article, or a scraper having a semi-circular or 1/4 circular shaped
doctor blade, or the like. In the case where the powder bed formed
at the projections of a prismatic rod (article to be coated) having
grooves extending in the axial direction is scraped off by a
scraper moving relative to the rod in the axial direction of the
rod, the scraper may be a scraper having an L-shaped or platy
doctor blade. Further, in the case where the powder bed formed at
projections of a platy body having grooves is scraped off, the
scraper may be a scraper having a flat plate shaped doctor
blade.
[0081] In the scraping step, the powder bed of the projection tips
does not need to be removed entirely and may be removed by a
specified thickness or, unless film thickness is largely affected,
may be removed roughly to some extent.
[0082] In the removal step, preferably, the scraped powder is
removed by a suction device in order that the powder bed at the
projections is prevented from falling into the depressions. This
suction device is normally equipped with a motor and a fan for
sucking up the scraped powder through the slit, and a filter for
regulating the passage of the sucked powder.
[0083] The primary powder bed formation step, the scraping step,
the secondary powder bed formation step and the induction heating
step may be carried out independently, but for higher productivity,
it is advantageous to perform those steps as an automatized
sequence of steps. For example, the coating film may be formed by
(1), as shown in FIG. 8, a sequence of steps of forming a primary
powder bed on an article at a primary electrostatic powder coating
station 11, then transferring the article to a powder removal
station 12 and there scraping off the powder bed at the projection
tips, next transferring the article to a secondary electrostatic
powder coating station 13 and there forming a secondary powder bed
by electrostatic coating, and transferring the article to an
induction heating station 14 and there subjecting the article to an
induction heating step, or by (2), as shown in FIG. 9, forming a
primary powder bed on the article by electrostatic coating at an
electrostatic powder coating and removal station 15, then scraping
off the powder bed and further forming a secondary powder bed by
electrostatic coating at the same place without transferring the
article, next transferring the article to an induction heating
station 16, and subjecting the article to an induction heating
step. In the latter case (2), the number of transfer steps for the
article is reduced. Besides, the coating film can be formed by
electrostatic coating without involving any substantial changes or
modifications of the coating line.
[0084] In the induction heating step, the article is heated to a
temperature corresponding to the type of the powder coating
material. If the powder coating material is a thermosetting
resin-based material, the coating film may be crosslinked or cured
by induction heating or by a heating process subsequent to the
induction heating process as required. Also, the article may be
heated in advance before an electrostatic powder coating process is
carried out.
[0085] In this embodiment, the primary powder bed 3a and the
secondary powder bed 3b are formed at the toothed portion 2 in two
steps. However, if there is a large difference in level between the
tooth root and the tooth crest, the film thickness can be
uniformized by executing three times of powder formation steps and
by controlling the film thickness at the individual steps. It also
becomes possible to set film thickness to desired values in
correspondence to individual portions of the tooth. For example,
the film thickness can be set thinner at the tip portion of the
tooth and thicker at the trough portion..
[0086] The present invention is effective as a method for forming a
coating film of uniform thickness (particularly a thick coating
film) on a surface of a region having projections or depressions of
a metallic article by electrostatic powder coating. The types of
the articles to be coated having a region containing projections or
depressions are not particularly limited, and the articles may have
various shapes such as platy, rod-like, cylindrical, disc-like,
umbrella-like shapes. The present invention is suitable for coating
film formation on articles to be coated having regular projections
or depressions, especially gears and spline shafts.
[0087] The thickness of the coating film in the region having
projections and depressions defined by or defining the projections
may be selected according to the type of the article to be coated,
and is, for example, about 100-200 .mu.m, preferably 150-200 .mu.m.
Thickness variation of the coating film at the projections and
depressions is, normally, about .+-.20 .mu.m (particularly, .+-.15
.mu.m).
[0088] The gears include gears in a wide sense capable of
transmitting motive power or motion in mesh with one another,
exemplified by gear wheels (spur gears, internal gears, helical
gears, double helical gears, rack and pinion, straight bevel gears,
screw gears, skew gears, worm gears, etc.). The spline shafts
include ordinary spline shafts (grooved shafts), serrated shafts
with spline teeth of chevron-shape in cross section, and the like.
For spline shafts, the coating film has only to be formed at least
at the toothed portion. The teeth of the gears and spline shafts
are not limited to a particular shape and may be involute or
cycloid. The cross-sectional shape of the addendum (top portion) of
the teeth may be of an acute salience shape or acute curved shape,
but normally may be so shaped as to have a curved tooth crest or
top land with a curvature corresponding to the addendum circle, as
in a grooved shaft in which axially extending grooves are formed
circumferentially at intervals. For the spline shafts, the number
of grooves is not particularly limited and, normally, may be about
4-50 grooves (e.g., 10-20 grooves).
EXAMPLE
[0089] The present invention is described below in more detail
based on an example thereof, but the present invention is not
limited by the example.
[0090] While a spline shaft was being rotated, a 12-nylon-based
powder coating material (mean particle size: 32 .mu.m) was applied
to the spline shaft at its toothed portion (namely a keyway
section, where deddendum circle diameter: 46 mm .phi., length: 80
mm, keyway count: 20) for 20 seconds by electrostatic powder
coating by using an electrostatic spray gun, by which a primary
powder bed was formed. While a sheet curved in correspondence to
the curvature of the addendum circle was being moved upward along
the axis of the spline shaft, the primary powder bed at the tooth
crests was scraped off, and moreover removed by suction. Then, by
using the electrostatic spray gun and the powder coating material,
the toothed portions of the spline shaft were subjected to a
20-second electrostatic powder coating process, by which a
secondary powder bed was formed. Thereafter, the spline shaft was
heated by an induction heater (frequency: 27 kHz), and thus a
coating film was formed. The toothed portion of the resulting
spline shaft was cut in a direction perpendicular to the center
axis of the shaft into pieces, and film thicknesses were measured
at the tooth crest, right and left sides, and the bottom or tooth
root, three sites for each. By taking mean values of the
measurements, the film thickness was evaluated. It is noted that
the film thickness of each cut piece was measured at four sites
shifted from each other circumferentially by 90 degrees.
Comparative Example
[0091] While a spline shaft was being rotated, a 12-nylon-based
powder coating material (mean particle size: 32 .mu.m) was applied
for 40 seconds by electrostatic powder coating onto a toothed
portion of the spline shaft (namely, a keyway area, where deddendum
circle diameter: 46 mm .phi., length: 80 mm, keyway count: 20) by
using an electrostatic spray gun. Thereafter, the spline shaft was
heated by an induction heater (frequency: 27 kHz), and thus a
coating film was formed. Then, film thicknesses were measured in
the same manner as in foregoing Example.
[0092] Measurement results of the example and comparative example
are shown in Table 1 below.
1 TABLE 1 Mean film thickness (.mu.m) Left Right Measuring side of
Crest of side of Root of sites tooth tooth tooth tooth Example 1
167 182 182 154 2 171 171 155 153 3 154 186 169 166 4 173 182 176
154 Comparative examples 1 164 220 160 130 2 142 204 138 138 3 133
202 142 128 4 135 202 151 130
[0093] As obvious from Table 1, in Example, a coating film of
substantially uniform thickness is formed at the tooth crests,
tooth roots or bottom portions, and side portions of the teeth, as
compared with Comparative Example.
[0094] Second Embodiment
[0095] Next, with reference to FIGS. 5A, 5B and 5C in this order, a
second embodiment of the manufacturing method for coated articles
according to the present invention will be described.
[0096] First, as shown in FIG. 5A, a tip portion 53, indicated by
broken line, of each of teeth 54 of a spline shaft 51 as an article
to be coated is cut off by a specified depth. This cutting depth is
set to such a value as can compensate a thickness difference by
which in the subsequent electrostatic deposition step, thickness of
a powder deposit at the tip portions 53 will be larger than
thickness of a powder deposit at sides 57 and troughs or roots 59
of the teeth. For example, this cutting depth is such that the
tooth height is lowered by an amount corresponding to 20% of the
coating film thickness, although the cutting depth may be set
within such a range that the tooth height is lowered by an amount
corresponding to 10%-60% of the coating film thickness, depending
on the shape of the teeth.
[0097] Next, an undercoat 52 is formed on the entire surface of the
spline shaft 51. The undercoat material is made of an organic
primer or inorganic primer. As the organic primer, are available
epoxy, phenol or polyvinyl acetal resins, rubbers or the like, and
those containing oligomers may also be used. The inorganic primer
may be those containing magnesium, manganese, silicon or the like.
This undercoat 52 plays the role of facilitating the deposition of
resin powder 55 to the spline shaft 51 which is a metallic article
to be coated.
[0098] Next, as shown in FIG. 5B, the powder 55 made of a resin
material is deposited onto the undercoat 52 by electrostatic
coating. In this process, the powder 55 is deposited a specified
extent thicker at the tip portions 53 of the teeth 54 than at the
other portions.
[0099] The resin material constituting the powder 55 may be
selected from among thermoplastic resin materials and thermosetting
resin materials. More specifically, among the thermoplastic resin
materials are nylon materials, olefinic materials, fluoric
materials, and others (e.g., PBT (polybutylene terephthalate), PET
(polyethylene terephthalate), PPS (polyphenylene sulfide)).
Adoptable as the nylon materials are homopolymerized materials,
such as 6-nylon, 66-nylon, 46-nylon, 11-nylon, 12-nylon, and
aromatic polyamides typified by PPA (polyphthalamide), or materials
formulated by adding solid lubricants (e.g., PE (polyethylene),
PTFE (polytetrafluoroethylene), MoS.sub.2, etc.) and/or filler
materials (e.g., calcium carbonate, talc, etc.) to nylon resins. As
the olefinic materials, are adoptable PE (e.g., HDPE (high density
polyethylene), UHDPE (ultra high density polyethylene), etc.), PP
(polypropylene), and the like. As the fluoric materials, are
adoptable PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene
perfluoroalklyvinylether copolymer), FEP (tetrafluoroethylene
hexafluoropropylene copolymer), ETFE (tetrafluoroethylene ethylene
copolymer) and the like. As the thermosetting resin materials, are
adoptable epoxy, phenol or other setting resins, and thermosetting
polyester base or acrylic resins using crosslinking agents or
curing agents.
[0100] Next, the spline shaft 51 is electromagnetically induction
heated, by which, as shown in FIG. 5C, the powder 55 is melted so
that a coating film 56 adheres to the spline shaft 51. In this
embodiment, film thickness of this coating film 56 was set to
within a range of 100 .mu.m-150 .mu.m. By such a film thickness
setting of the coating film 56, as shown in the characteristic
chart of FIG. 6, working noise (tap tone) was able to be reduced to
20% or more, compared with the case where the coating film
thickness is zero. FIG. 6 shows the working noise in relative
values, taking working noise for no coating as a value of 100.
Also, according to the above film thickness setting of the coating
film 56, as shown in the characteristic chart of FIG. 7 in which
wear is given in relative values, taking wear for no coating as a
value of 100, wear was able to be reduced by a maximum of 46%,
compared with the case where the coating film thickness is 300
.mu.m. As can be understood from the characteristic charts of FIGS.
6 and 7, a working noise reduction effect and wear suppression
effect can be obtained by setting the coating film thickness to
within a range of 100-250 .mu.m.
[0101] Also, according to this embodiment, with the powder 55
electrostatically deposited on the spline shaft 51, the powder 55
is melted, or fused, by induction heating the spline shaft 51. This
means that a bed of the powder 55 is heated from the spline shaft
51 inside the powder 55. As a result, air bubbles become more
likely to escape out of the internally melted powder 55 to the
outside, so that air bubbles are prevented from mixing into the
coating film 56, and that the quality of the coating film 56 is
improved.
[0102] More specifically, as schematically shown in FIG. 4A, in the
case of induction heating, after powder 23 is electrodeposited on
an article 21 to be coated having an undercoat 22 formed thereon,
the article 21 is induction heated, by which the powder 23 present
inside near this undercoat 22 first begins to melt. As a result of
this, air bubbles among the powder particles 23 become more likely
to be let out, so that a coating film 25 substantially free from
air bubbles can be formed as shown in FIG. 4B. This effect can be
obtained also by preliminarily heating the article and thereafter
performing the electrostatic powder coating.
[0103] In contrast to this, in the case of oven heating, as shown
in FIG. 4C, an article 101 to be coated is heated from outside of
powder 103 deposited on the surface of an undercoat 102 of the
article 101. As a result, melting of the powder 103 begins with the
outermost side of the powder. Therefore, as shown in FIG. 4D, air
bubbles 106 among the powder particles 103 are more like to be
confined inside a coating film 105.
[0104] Further, in this embodiment, the tip portion 53 of each
tooth 54 of the spline shaft 51 before being coated is cut by a
specified amount so that the spline shaft 51 after being coated
comes to have a desired outer diameter. Thus, a secondary finishing
process (e.g., broaching) is no longer necessary, allowing the
working cost to be reduced.
[0105] Whereas a spline shaft 51 to be used as a drive shaft is
adopted as an article to be coated in this embodiment, this spline
shaft 51 may be either female or male type. The article to be
coated may be an intermediate shaft to be used for steering joint.
Furthermore, the article to be coated may also be a hypoid gear as
a motion transmission (speed reducer) component or a pinion as a
pressure generating (gear pump) component. As the material of the
article to be coated, steel materials or non-ferrous materials
(e.g., aluminum alloys, magnesium alloys, etc.) are adoptable.
[0106] Also in this embodiment, the coating film of a specified
film thickness may be deposited either partly on the surface of the
article to be coated or entirely on the surface.
[0107] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *