Information
-
Patent Grant
-
6220486
-
Patent Number
6,220,486
-
Date Filed
Wednesday, July 7, 199925 years ago
-
Date Issued
Tuesday, April 24, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 222 195
- 222 226
- 222 227
- 222 236
- 222 412
- 222 413
-
International Classifications
-
Abstract
The powder paint feeder for powder coating directly is installed to a powder paint tank of a fluidized bed type. In the powder paint feeder, a cylindrical screw casing is inserted into the powder paint tank and fixed thereto and has an introducing port into which the powder paint fluidized in the powder paint tank is introduced and a discharging port through which the powder paint dispersed externally of the powder paint tank is discharged. A screw is accommodated in the screw casing and feeds the powder paint introduced from the introducing port to the discharging port while compacting it. A deaerating device is disposed to the screw casing and deaerates the fluidized powder paint to accelerate the compaction of the powder paint. A dispersing blade is mounted on the rotational shaft of the screw in the vicinity of the discharging port downstream of the screw for dispersing the compacted powder paint. A driving device rotates the rotational shaft of the screw. The powder paint feeder for powder coating is less expensive and arranged such that it can feed the powder paint with an excellent constant quantity feeding property without being affected by the powder level in a tank and can be washed easily and efficiently.
Description
BACKGROUND OF THE INVENTION
The present invention pertains to a powder paint feeder for powder coating. More particularly, the present invention relates to a powder paint feeder for powder coating which is directly installed to a fluidized bed type powder paint tank and discharges a powder paint in a constant quantity at a pinpoint accuracy.
Powder coating for coating a powder paint without using organic solvent has been widely employed from the view point of the protection of the terrestrial environment. In case of the powder coating and, in particular, in case of electrostatic powder coating, a powder paint stored in a paint tank is sprayed onto an object by a coating gun through a powder paint feeder. Fluidized bed type powder paint tanks (hereinafter, referred to as fluidizing tanks) for applying fluidity to a powder paint by supplying air from under the powder paint have been widely used as the paint tank.
Japanese Unexamined Patent Publication No. 8-57365 discloses, as a powder paint feeder used for a fluidizing tank, an apparatus for directly taking out a powder paint, which has been fluidized in a paint tank by agitating blades, by an ejector and feeding the powder paint into a coating gun. However, the apparatus has a problem that the quantity of feed of the powder paint is changed depending upon the height of the powder level (hereinafter, referred to as the powder level) of the powder paint in the fluidizing tank because the powder paint is taken out as it is from the tank as a fluidized bed through the ejector. That is, the fall of the powder level in the paint tank reduces a powder pressure, whereby a powder volume density is also reduced. As a result, the quantity of the powder paint discharged through the ejector is also reduced. On the other hand, when a powder paint is fed into the paint tank and the powder level of the powder paint rises, the powder volume density is increased and accordingly the quantity of the powder paint fed is also increased. Thus, the feed rate of the powder paint is varied depending upon the powder level. Because of the reason mentioned above, the variation of the quantity of a discharged powder paint, which is caused by the variation of the powder level, reaches, for example, ±10-odd %.
Further, the apparatus has a drawback that the quantity of feed of a powder paint and the quantity of air conveyed by the ejector cannot be optionally, separately and independently set because the ejector takes out the powder paint mixed with the air fluidized in the fluidizing tank.
Whereas, Japanese Unexamined Patent Publication No. 8-47656 discloses an apparatus for feeding a powder paint into a coating gun in a constant quantity to prevent the variation of the quantity of feed of the powder paint caused by the variation of the powder level of it. The apparatus is arranged such that the powder paint intermittently fed from a fluidizing tank is stored once in a sub-tank, fluidized air is released from the sub-tank using a vibrator or the like, and the powder paint is fed into a coating gun by a screw feeder disposed at a lower portion of the sub-tank. In the constant quantity feeding apparatus disclosed in the publication, a constant quantity feeding property is improved by adjusting the quantity of a discharged powder paint by the rotation of a screw.
However, the constant quantity feeding apparatus disclosed in the publication has a drawback that since the apparatus is made large in size and complicated in construction by the provision of the sub-tank and the vibrator, the cost of the apparatus is increased as compared with the cost of the above ejector type powder paint feeder as well as a long time is necessary to disassemble and wash the apparatus, although the apparatus can reduce the variation of the quantity of a discharged power paint which is caused by a powder level to about ±2% by the installation of the sub-tank.
Further, there is also proposed a constant quantity powder paint feeder the arrangement of which is simplified by omitting a sub-tank or a hopper unit and directly disposing a screw feeder to a fluidizing tank to improve a disassembling property and a washing property and to reduce a cost.
However, the constant quantity powder paint feeder has a problem that since the feeder is not provided with a sub-tank, fluidized air cannot be sufficiently released, a powder volume density is varied by being affected by a powder level, and a sufficiently high constant quantity feeding property cannot be obtained; that is, a quantity of feed is varied about ±7% based on the quantity of a discharged powder paint by the variation of the powder level. Further, the feeder also has a drawback that when a highly fluid powder paint is used, countermeasures against a flushing phenomenon must be taken by employing a mechanical method such as the installation of a straightening vane, or the like.
SUMMARY OF THE INVENTION
An object of the present invention made in view of the above circumstances is to provide a powder paint feeder for powder coating which is installed to a fluidized bed type powder paint tank, which is less expensive and is arranged such that it can feed a powder paint with an excellent constant quantity feeding property without being affected by the powder level in a tank and can be washed easily and efficiently.
To achieve the above-described object, the present invention provides a powder paint feeder for powder coating directly installed to a powder paint tank of a fluidized bed type, comprising: a cylindrical screw casing inserted into the powder paint tank and fixed thereto and having an introducing port into which the powder paint fluidized in the powder paint tank is introduced and a discharging port through which the powder paint dispersed externally of the powder paint tank is discharged; a screw accommodated in the screw casing for feeding the powder paint introduced from the introducing port to the discharging port while compacting it; a deaerating means disposed to the screw casing for deaerating the fluidized powder paint to accelerate the compaction of the powder paint; a dispersing blade mounted on the rotational shaft of the screw in the vicinity of the discharging port downstream of the screw for dispersing the compacted powder paint; and a driving means for rotating the rotational shaft of the screw.
It is preferable that the screw has screw pitches which are narrowed downstream in the feed direction of the powder paint.
Preferably, the screw pitches in at least part of the screw are continuously narrowed downstream in the feed direction of the powder paint or the screw pitches are narrowed stepwise (in a single step or multiple steps) downstream in the feed direction of the powder paint.
Preferably, a ratio of the minimum pitch to the maximum pitch of the screw is ⅔.
Preferably, the deaerating means is disposed at a portion where the screw pitches are going to be narrowed or have been narrowed downstream in the feed direction of the powder paint.
It is also preferable that the driving means is disposed oppositely of the screw with respect to the discharging port, and the powder paint feeder further comprises a purging air discharging means which is disposed in the vicinity of the driving means for preventing the penetration of the powder paint to the driving means.
It is further preferable that the deaerating means is disposed downstream of the screw and upstream of the dispersing blade in the vicinity of the dispersing blade.
It is still further preferable that the deaerating means is a deaerating device which includes a vacuum pump for sucking and removing fluidized air contained in the powder paint by the sucking pressure, and a duct for transmitting sucking pressure and passing the sucked air, and a filter for sucking only the fluidized air in the powder paint without sucking the powder paint.
Preferably, the sucking pressure of the vacuum pump ranges from −100 mmHg to −10 mmHg on the basis of atmospheric pressure.
It is also further preferable that the dispersing blade has a blade piece group disposed in a single stage or blade piece groups disposed in multiple stages spaced apart from each other in the direction of the rotational shaft.
Preferably, the blade piece groups have the same number of blade pieces and each blade piece group is dislocated one another in the rotational direction of the screw.
The another aspect of the present invention provides a powder paint feeder for powder coating directly installed to a powder paint tank of a fluidized bed type, comprising: a cylindrical screw casing inserted into the powder paint tank and fixed thereto and having an introducing port into which the powder paint fluidized in the powder paint tank is introduced and a discharging port through which the powder paint dispersed externally of the powder paint tank is discharged; a screw accommodated in the screw casing for feeding the powder paint introduced from the introducing port to the discharging port while compacting it, the screw having screw pitches which are (continuously or stepwise) narrowed downstream in the feed direction of the powder paint; a dispersing blade mounted on the rotational shaft of the screw in the vicinity of the discharging port downstream of the screw for dispersing the compacted powder paint; and a driving means for rotating the rotational shaft of the screw.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a schematic view showing an embodiment of a powder paint feeder for powder coating of the present invention;
FIG. 2
is a partly enlarged view showing a dispersing blade in the powder paint feeder shown in
FIG. 1
;
FIG. 3
is a graph showing the relationship between the number of revolution (revolution per minute) of a screw and the feed rate of a powder in the powder paint feeder shown in
FIG. 1
;
FIG. 4
is a graph showing the relationship between the powder level in a tank and the feed rate in the powder paint feeder shown in
FIG. 1
; and
FIG. 5
is a graph showing the relationship between the powder level in a tank and the feed rate in a powder paint feeder provided with a uniform pitch screw.
DETAILED DESCRIPTION OF THE INVENTION
A powder paint feeder for powder coating (hereinafter, referred to as a powder feeder) of the present invention will be described with reference to the preferred embodiment shown in FIG.
1
.
FIG. 1
is a schematic view showing an embodiment in which the powder feeder of the present invention is applied to a powder paint tank of a fluidized bed type.
The powder feeder
10
shown in
FIG. 1
is directly installed to the fluidized bed
8
of a fluidized bed type powder paint tank (hereinafter, referred to as tank)
2
and feeds the fluidized powder paint (hereinafter, referred to as powder) in the tank
2
to a gun
42
through an ejector
40
in a constant quantity. As shown in
FIG. 1
, the tank
2
of the illustrated example is composed of an air chamber
4
, a porous plate
6
and a fluidized bed
8
, and fluidizes a charged powder by supplying pressurized air thereto through the porous plate
6
. However, the powder feeder of the present invention is not limited to the illustrated example and it is applicable to various kinds of known fluidized bed type powder paint tanks.
The powder feeder
10
shown in
FIG. 1
includes a screw casing
24
and a screw
26
which constitute a screw feeder, a deaerating means
30
, a dispersing blade
18
and a driving means
22
. The powder feeder
10
is broadly classified into at least an introducing section
12
, a fixing section
14
, a deaerating/compacting section
16
, a dispersing section
17
, a discharging section
20
and a driving section
23
on the basis of the functions thereof.
The screw casing
24
forms a flow passage from the introducing section
12
where the powder is introduced into the powder feeder
10
to the ejector
40
through the deaerating/compacting section
16
and the discharging section
20
. In addition, the screw casing
24
is a cylindrical body for accommodating the screw
26
extending from the driving means
22
. Only the discharging section
20
is formed by being branched downward from the flow passage.
The introducing section
12
is inserted into the fluidized bed
8
of the tank
2
and introduces the powder fluidized in the fluidized bed
8
of the tank
2
into the powder feeder
10
. At the introducing section
12
, the screw casing
24
is formed to a semi-cylindrical shape having an open upper half portion (180°). The screw casing
24
has a powder introducing port
24
a
formed thereto through which the powder is introduced into the screw casing
24
. The end of the screw casing
24
on the introducing section
12
side thereof is closed by a side end member
27
and a bearing
28
is disposed at the center of the side end member
27
.
The fixing section
14
is formed to fix the powder feeder
10
to a side of the fluidized bed
8
of the tank
2
. At the fixing section
14
, the screw casing
24
has a circular-plate-shaped flange. The flange vertically projects to the outside from the boundary between the introducing section
12
and the deaerating/compacting section
16
and has a plurality of holes formed thereto to permit bolts to be inserted therethrough. Thus, the powder feeder
10
can be simply fixed to the tank
2
through the flange using the bolts and nuts. A method of fixing the fixing section
14
to the fluidized bed
8
is not limited to the method of using the bolts and the nuts. That is, the method is not particularly limited and various kinds of conventionally known methods such as welding and so on may be employed.
At the deaerating/compacting section
16
, the screw casing
24
is formed to a cylindrical shape from the introducing section
12
in communication therewith. The screw
26
to be described later is disposed in the screw casing
24
and the deaerating means
30
is disposed around the outer periphery of it.
At the discharging section
20
, the screw casing
24
is branched downward at the portion thereof between the dispersing blade
18
to be described later and the driving means
22
and forms a flow passage through which the powder dispersed by the dispersing blade
18
to be described later is discharged and reaches the ejector
40
. That is, a space, through which the powder is discharged, is provided in the vicinity of the dispersing blade
18
in the screw casing
24
on the downstream side in a feed direction and the screw casing
24
located at the space is branched downward and communicated with the space so that a discharging port
24
b
is formed to the lower end of the branched portion.
The screw
26
, which is composed of screw blades
26
a
and a screw shaft
26
b
, is accommodated in the screw casing
24
and feeds the powder introduced in the introducing section
12
to the discharging section
20
through the deaerating/compacting section
16
. At the same time, the screw
26
increases the density of the introduced powder (compacts the powder) by applying a pressure thereto in the deaerating/compacting section
16
. The screw blades
26
a
are spirally formed around the screw shaft
26
b
from a position near to the extreme end of the screw shaft
26
b
on the fixing section
14
side thereof to a position upstream of the dispersing blade
18
in the feed direction thereof. The other end of the screw shaft
26
b
of the screw
26
is coupled with the driving means
22
and the extreme end thereof is rotatably supported through a bearing
28
.
The illustrated screw
26
has a characteristic that the pitches of the screw blades
26
a
are formed widely in the introducing section
12
and formed narrowly in the deaerating/compacting section
16
, in addition to the above arrangement. Since the pitches formed narrowly downstream in the feed direction increase the number of windings of the screw blades
26
a
and decrease the volume of the powder which can pass through the respective spaces between the screw blades
26
a
, a pressure is applied to the powder being fed by the screw. The density of the powder is increased (the powder is compacted) by the pressure so that the volume density of the powder can be made constant. With this arrangement, the quantity of feed of the powder can be prevented from being affected by the powder level of the powder in the fluidized bed
8
of the tank
2
. Specifically, it is sufficient that the pitches of the screw
26
are made constant at least in the introducing section
12
of the screw
26
and the pitches of the screw
26
in the deaerating/compacting section
16
are stepwise or continuously narrowed downstream in the feed direction. When the pitches are narrowed stepwise, they may be narrowed in one step as shown in the illustrated example or may be narrowed in many steps of two or more steps. Although it is preferable that the ratio of the minimum pitch to the maximum pitch in a single screw is about ⅔, this is not particularly limited.
It should be noted that the screw applied to the present invention is not limited to the screw described above the pitches of which are continuously and stepwise narrowed downstream in the feed direction as long as the powder paint feeder comprises the deaerating means. For example, the screw having uniform screw pitches may be used. However, the screw having the narrowed pitches is more preferable than the screw having the uniform pitches because of the effective compaction of the powder.
The deaerating means
30
more sufficiently compacts the powder in the spaces between the screw blades
26
a
by increasing the density of the powder by sucking and removing the fluidized air contained in the powder in the deaerating/compacting section
16
to thereby more improve the constant quantity feeding property when the powder is fed. At the same time, the deaerating means
30
improves the responsiveness when the feed of a powder is started or stopped, even if the powder has a high flushing property. The deaerating means
30
is composed of a filter
32
, a duct
34
, a suction pipe
36
and a vacuum pump
31
. It is preferable to perform deaeration in the deaerating/compacting section
16
.
The filter
32
is continuously disposed around the screw casing
24
in a cylindrical shape by partly or entirely replacing the screw casing
24
located at least downstream of the deaerating/compacting section
16
with the filter
32
. The filter
32
is not particularly limited so long as it has fine holes which can suck only the air in the powder without sucking the powder. Specifically, a stainless filter having fine holes of 5 μm is preferably used.
The duct
34
is a member for covering the outer periphery of the cylindrical filter
32
at prescribed intervals defined therebetween and communicating with the suction pipe
36
. When a sucking pressure is transmitted to the filter
32
and a space through which sucked air passes is provided, the vacuum pump
31
can suck air through the filter
32
and the suction pipe
36
The suction pipe
36
couples the duct
34
with the vacuum pump
31
, whereby the suction pressure of the vacuum pump
31
is transmitted into the screw casing
24
through the duct
34
and the filter
32
.
The vacuum pump
31
deaerates the powder compacted through the suction pipe
36
and the filter
32
. The vacuum pump
31
is not particularly limited so long as it can perform pressure reduction of about −100 to −10 mm Hg on the basis of atmospheric pressure, and various types of conventionally known suction means such an ejector and the like can be used. Excessive pressure reduction is not preferable because the powder is excessively compacted and the screw is stopped thereby and the discharge of the powder is made difficult.
The powder being fed by the screw in the screw casing
24
in the fluidized state is compacted while being deaerated by the provision of the narrow pitch screw
26
and the deaerating means
30
with the deaerating/compacting section
16
as described above. Therefore, the powder is charged into the portions between the screw blades
26
a
of the screw
26
in high density and the volume density of the powder is made constant regardless of the powder level in the tank
2
. As a result, the powder can be fed in a constant quantity with the quantity of feed of the powder per one revolution of the screw
26
set constant.
It should be noted that the deaerating means
30
may not be necessarily disposed in accordance with the powder, in case that the powder is fully compacted by only the screw
26
having the screw pitches narrowed downstream in the feed direction and thereby the powder can be fed in a constant quantity.
The dispersing blade
18
uniformly disperses the powder compacted through the deaerating/compacting section
16
and made to the constant quantity and discharges the powder into the discharge section
20
. This operation is performed to improve the constant quantity feeding property of the powder and to sufficiently electrify the particles of the powder one by one.
FIG. 2
shows a schematic view of the dispersing blade
18
of the powder feeder
10
. The dispersing blade
18
of the illustrated example is composed of a cylindrical dispersing blade main body
18
b,
which permits the screw shaft
26
b
to be inserted therethrough at the center of it and to be fixed thereto, and blade piece groups
18
c
and
18
d
each composed of
16
round-rod-shaped blade pieces
18
a
. The blade pieces
18
a
uniformly and radially project from prescribed positions of the dispersing blade main body
18
b
in the axial direction thereof. The dispersing blade
18
is disposed in the vicinity of the end of the deaerating/compacting section
16
on the downstream side thereof in the feed direction, that is, in the vicinity of the end of the screw blades
26
a
of the screw
26
. The blade piece groups
18
c
and
18
d
are dislocated 11.25° in the rotational direction of the screw
26
, are disposed in the direction of the center axis of the dispersing blade main body
18
b
in two steps, and are spaced apart from one another at prescribed intervals. The shape of the blade pieces
18
a
is not limited to the round rod shape and may be formed to a plate shape. In addition, the blade pieces
18
a
may be inclined with respect to the screw shaft
26
b
. The blade piece groups may be disposed in one stage or in two or more stages. When they are disposed in the two or more stages, it is preferable that the respective dispersing blades are dislocated by a prescribed angle. However, the angle is not particularly limited. Further, the blade pieces
18
a
may be directly formed to the screw shaft
26
b
without interposing the dispersing blade main body
18
b
therebetween.
The driving means
22
rotates the screw shaft
26
b
and accordingly the screw
26
. The driving means
22
is not particularly limited and any conventionally known driving means such as a motor and the like may be used. When the motor or the like is used as the driving means
22
, the motor or the like is coupled with the screw shaft
26
b
of the screw
26
by a not shown coupling.
It is preferable that a bearing
21
is interposed between the driving means
22
and the discharging section
20
to rotatably support the screw shaft
26
b
. Further, it is preferable that the bearing
21
closes the driving means
22
from the discharging section
20
as shown in the illustrated example, because the penetration of the powder to the driving means
22
can be prevented thereby.
In addition, it is preferable to interpose a purging air discharging means
44
between the bearing
21
and the driving means
22
to prevent the penetration of the powder to the driving means
22
through the gap of the bearing
21
when the powder is discharged into the discharging section
20
. The purging air discharging means
44
is coupled through a coupling pipe
44
a
which is disposed to the screw casing
24
at the position thereof adjacent to the driving means
22
and discharges purging air from the position into the screw casing
24
. With this operation, the pressure in the portion between the bearing
21
and the driving means
22
is made higher than the pressure in the discharging section
20
so that the penetration of the powder to the driving means
22
can be prevented. Any conventionally known purging air discharging means such as a compressor for compressing air and the like may be sufficiently used as the purging air discharging means
44
so long as it can discharge a very slight quantity of purging air.
The ejector
40
is connected to the discharging port
24
b
of the screw casing
24
and feeds the powder to the gun
42
so that it is discharged therefrom. The ejector
40
is not particularly limited and any known ejector may be used. For the above purpose, the gun
42
is connected to the ejector
40
.
When the powder paint feeder of the present invention arranged as described above is installed to the tank
2
, first, a hole, through which the introducing section
12
of the powder paint feeder is inserted, and fixing holes, through which the bolts
14
a
for fixing the fixing section
14
are inserted, are previously formed to the side of the fluidized bed
8
of the tank
2
. Next, the introducing section
12
of the powder paint feeder is inserted into the hole to cause the fixing section
14
to come into contact with the side of the tank
2
. In this state, the powder feeder
10
is fixed to the side of the tank
2
by inserting the bolts
14
a
through the fixing holes of the fixing section
14
and tightening the bolts
14
a
with the nuts
14
b
through packings.
The powder paint feeder for powder coating of the present invention is essentially arranged as described above. Next, the operation of the powder paint feeder will be described below.
First, a powder is charged into the tank
2
and is fluidized by the air supplied from the air chamber
4
through the porous plate
6
. With this operation, the powder flows into the introducing port
24
a
of the introducing section
12
of the screw casing
24
.
When the screw
26
is rotated by driving the driving means
22
, the powder starts to be fed by the screw
26
rightward in the figure from the introducing section
12
, whereby the powder is taken out from the tank
2
and fed up to the deaerating/compacting section
16
.
Since the pitches of the screw
26
are made narrower in the deaerating/compacting section
16
than in the introducing section
12
, the powder is compacted by the pressure which is applied thereto by the screw blades
26
a
of the screw
26
disposed at the narrower intervals. Further, since the powder is deaerated by the suction means at the same time, it can be still more compacted by the removal of the fluidized air. The volume density of the powder is made constant and a constant quantity of the powder is charged with respect to the pitches of the screw
26
at all times, because the powder is sufficiently compacted while being deaerated. Accordingly, the powder is discharged from the discharge port
24
b
in the quantity which is proportional to the number of revolution (rpm: revolution per minute) of the screw without being almost affected by the powder level in the fluidized bed
8
of the tank
2
, whereby the constant quantity feeding property of the powder paint feeder can be improved. Specifically, even if the powder level is varied, the variation of the feed rate of the powder can be reduced within about ±1%.
The powder having passed through the deaerating/compacting section
16
reaches the dispersing blade
18
. The powder which has been compacted and whose quantity has been made constant is uniformly dispersed by the dispersing blade
18
and is discharged into the discharging section
20
in a constant quantity. The powder having been discharged into the discharging section
20
is fed to the gun
42
in the constant quantity by being sucked by the ejector
40
and fed thereby under pressure. Powder coating is carried out by the powder discharged from the gun
42
.
It should be noted that the distance between the filter
32
and the screw
26
is preferably set at a smaller value, if contact of the screw blades
26
a
of the screw
26
with the filter
32
does not cause damage or breakage of the screw blades
26
a
and the filter
32
. An appropriate value can be selected depending on the powder to be fed and the shape and size of the screw
26
, the type and size of the filter
32
, and the size precision and assembly precision thereof.
Experiment 1
The inventors performed powder paint feeding experiments as to the characteristics shown below using the powder feeder
10
described above and shown in FIG.
1
. Not only an experiment using the variable pitch screw
26
of which the pitches are narrowed stepwise toward the downstream side, but also an experiment in which the variable pitch screw
26
was replaced with the uniform pitch screw (not shown) were performed.
The type of a powder paint, the arrangements and the sizes of the main components of the powder feeder
10
used in the experiment and the operating conditions of the feeder
10
are as shown below.
|
Powder paint
|
Type
Acryl
|
Average particle size
30 μm
|
powder feeder
|
Introducing port shape
opened 180° (upper half)
|
Introducing port length
80 mm
|
Screw outside diameter
27 mm
|
Screw shaft diameter
8 mm
|
Screw pitches
|
Variable pitch screw
|
Introducing section
15 mm
|
Deaerating/compacting section
10 mm
|
Uniform pitch screw
13 mm
|
Filter material
stainless
|
Filter fine hole diameter
5 μm
|
Degree of pressure reduction of filter section
|
−40 mmHg (on the basis of atmospheric
|
pressure)
|
Pressure reducing method
Vacuum pump
|
|
(Controllability of quantity of feed)
To examine whether the feed rate of the powder was proportional to each of the numbers of revolution (rpm) of the variable pitch screw
26
and the uniform pitch screw, the relationship between the numbers of revolution (rpm) and the feed rate was examined. First, the feed rates at respective numbers of revolution (rpm) of the screws were measured by changing the number of revolution (rpm) while performing deaeration (−40 mmHg in the variable pitch screw and −10 mmHg in the uniform pitch screw on the basis of atmospheric pressure). Further, the relationship between the numbers of revolution (rpm) of the screws and the feed rate was examined in the same manner except that no aeration was performed.
FIG. 3
shows the result of the experiments.
As apparent from
FIG. 3
, when deaeration was performed in the powder feeder of the present invention using the variable pitch screw, the proportional relationship of the feed rate of the powder to the number of revolution (rpm) of the screw was better than in the case where no deaeration was performed. It has been found that the feed rate of the power was reduced in the case of the uniform pitch screw, as compared with the case of the variable pitch screw, although a preferred proportional relationship was obtained whether the deaeration was performed or not.
(Constant quantity feeding property)
To examine how the quantity of the powder fed from the powder feeder of the present invention was stable with time, the changes with time of the feed rate of the powder were measured under the respective conditions shown in the examples 1-9 of Table 1 (the pitch type of the screw, the number or revolution (rpm) of the screw, the degree of pressure reduction, the powder level) and a coefficient of variation was calculated from the standard deviation of the changes using the following formula. Further, the change of a feed rate was also measured in the same manner in the case where no pressure reduction was performed in the powder feeder using the uniform pitch screw, thereby calculating a coefficient of variation as a comparative example 1. It can be found that a smaller coefficient of variation exhibits a better constant quantity feeding property. Table 1 shows the result of the measurement.
coefficient of variation=(standard deviation)/(average quantity of feed)
The variable pitch screw was used in the examples 1-8, and the uniform pitch screw was used in the example 9 and the comparative example 1. Further, the measurements in the examples 1-3 were performed under the same conditions except that the number of revolution (rpm) of the screw was changed, the measurements in the examples 4 and 8 were performed under the same conditions except that deaeration was carried out in the example 4 and no deaeration was carried out in the example 8, the measurements in the examples 5-7 were performed under the same conditions except that powder levels were changed, and the measurements in the example 9 and the comparative example 1 were performed under the same conditions except that deaeration was carried out in the example 9 and no deaeration was carried out in the comparative example 1.
As apparent from Table 1, it can be found that the powder feeder of the present invention exhibits a low coefficient of variation under any conditions and is stable with time as compared with the case where no deaeration is performed in the powder feeder using the uniform pitch screw. It is also found from the comparison between the examples 4 and 8, and the comparison between the example 9 and the comparative example 1, that the average quantity of feed in the case where deaeration is performed is more than in the case where no deaeration is performed, whether the screw pitches are uniform or variable, but that the coefficient of variation is reduced to be about one-half. It is further found from the comparison between the example 3 using the variable pitch screw and the example 9 using the uniform pitch screw, both examples showing the numbers of revolution close to each other, that the variable pitch screw is more excellent in the constant quantity feeding property than the uniform pitch screw, since the example 3 shows a more increased average quantity of feed and a much more reduced coefficient of variation at a number of revolution a little decreased, although the powder level is different.
(Effect of powder level)
The changes of the feed rate to the change of the height of a powder surface (powder level) in the tank were measured as to the respective numbers of revolution (rpm) of 37 rpm and 10 rpm in the case of using the variable pitch screw, and of 64 rpm and 11 rpm in the case of using the uniform pitch screw, respectively. The measurement was performed in the same manner except that no deaeration was performed.
FIG. 4
shows the result of the measurements when using the variable pitch screw.
FIG. 5
shows the result of the measurements when using the uniform pitch screw.
As can be seen from
FIG. 4
, when deaeration is performed in the powder feeder with the variable pitch screw according to the present invention, a more constant feed rate can be maintained at any numbers of revolution (rpm) of the screw irrespective of the change of the powder level, as compared with the case where no deaeration was performed.
As apparent from
FIG. 5
, when deaeration is performed in the powder feeder with the uniform pitch screw according to the invention, a more constant feed rate can be maintained at any numbers of revolution (rpm) of the screw irrespective of the change of the powder level than the conventional case where no deaeration is performed.
FIG. 5
shows in particular that a powder paint can be fed constantly in the case of low powder level/high feed rate where the feed amount is apt to vary.
The powder feeder of the present invention is provided with at least one of the variable pitch screw and the deaerating means. As is clear from the above, this powder feeder is more excellent in the feed rate (feed amount), the constant quantity feeding property and the feed constancy with respect to the powder level than conventional powder feeders with uniform pitch screw that have no deaerating means. It is found in particular that constant feed is achieved even in the case of low powder level/high feed rate where the feed amount. is apt to vary. It is also found that the powder feeder of the invention which has both of the variable pitch screw and the deaerating means is significantly excellent in the feed rate, the controllability of quantity of feed, the constant quantity feeding property, and the feed constancy with respect to the powder level.
TABLE 1
|
|
Comparative
|
Examples
example
|
1
2
3
4
5
6
7
8
9
1
|
|
Pitch type of
Variable
Variable
Variable
Variable
Variable
Variable
Variable
Variable
Uniform
Uniform
|
screw
|
Number of
9.9˜
25.1˜
53.1
9.9˜
33.0˜
33.0˜
33.0˜
9.9˜
64.0
64.0
|
revolution of
10.1
25.2
10.1
33.2
33.2
33.2
10.1
|
screw (rpm)
|
Degree of
|
pressure
−40
−40
−40
−40
−20
−40
−60
0
−60
0
|
reduction (mmHg)
|
Powder level (mm)
160
260
260
260
160
160
160
245
140
140
|
Average quantity
3.4
8.5
17.0
3.3
11.0
11.2
11.3
3.1
15.13
14.33
|
of feed (kg/h)
|
Standard
14.5
21.7
12.5
12.0
10.1
20.0
26.9
22.8
65.0
126.1
|
deviation (g/h)
|
Coefficient of
0.42
0.26
0.07
0.36
0.09
0.18
0.24
0.71
0.43
0.88
|
variation (%)
|
|
Experiment 2
Powder paint feeding experiments were performed as in Experiment 1 using the powder feeder
10
as shown in
FIG. 1
to determine the effect of the clearance between the filter
32
in the deaerating means
30
and the screw blades
26
a
the screw
26
on the suction pressure. The type of the powder paint and the structure and operating conditions of the powder feeder
10
were the same as in Experiment 1 except for the structure and size of the screw
26
and the size of the filter
32
. Table 2 shows the structure and size of the screw
26
, the size of the filter
32
, the clearance between the screw
26
and the filter
32
, and other operating conditions and the results obtained.
TABLE 2
|
|
Example 10
Example 11
|
|
|
Pitch type of screw
Variable
Variable
|
Screw outer diameter (mm)
28.0
26.5
|
Screw pitch (introducing
15
15
|
section) (mm)
|
Screw pitch
10
13
|
(deaerating/compacting
|
section) (mm)
|
Filter inner diameter (mm)
35.2
28.9
|
Clearance between filter and
3.6
1.2
|
screw
|
Number of revolution of screw
10
10
|
(rpm)
|
Degree of pressure reduction
−40
−10
|
(mmHg)
|
Powder level (mm)
160
160
|
Amount of fluidized air
35
35
|
(Nl/min)
|
Feed amount (average quantity
3.4
2.5
|
of feed) (kg/h)
|
Coefficient of variation (%)
0.41
0.36
|
|
As is clear from Table 2, the performance could be attained at a similar level, and especially, the degree of pressure reduction to achieve the coefficient of variation could be significantly reduced to be one-fourth by reducing the clearance between the screw
26
and the filter
32
by a factor of 3.
As described above, since the powder paint feeder for powder coating of the present invention is simply arranged so that it can be directly installed to the fluidized bed of the tank, it is excellent in a dismantling property and a washing property. Moreover, since the powder paint feeder of the present invention excludes the effect of the variation of the powder level in the tank by deaerating and compacting a fluidized powder paint and instantly and uniformly disperses and feeds the compacted powder paint, the powder paint feeder can disperse and feed the powder paint with a high constant quantity feeding property imparted to the ejector and the gun without being affected by the powder level in the tank.
Since a discharged quantity of the powder paint can be measured externally of a coating booth, the discharged quantity can be easily adjusted.
Further, the provision of the non-mechanical deaerating mechanism permits even a powder having a high flushing property to be fed by the powder paint feeder of the present invention as well as the powder paint feeder is excellent in responsiveness when the feed of the powder is started or stopped.
Claims
- 1. A powder paint feeder for powder coating directly installed to a powder paint tank of a fluidized bed type, comprising:a cylindrical screw casing inserted into the powder paint tank and fixed thereto and having an introducing port into which powder paint fluidized in the powder paint tank is introduced and a discharging port through which the powder paint dispersed externally of the powder paint tank is discharged; a screw accommodated in said screw casing for feeding the powder paint introduced from the introducing port to the discharging port while compacting the powder paint; a deaerating means operatively associated with said screw casing for deaerating the fluidized powder paint to accelerate compaction of the powder paint; a dispersing blade mounted on a rotational shaft of said screw in a vicinity of the discharging port downstream of said screw for dispersing the compacted powder paint; and a driving means for rotating the rotational shaft of said screw.
- 2. The powder paint feeder according to claim 1, wherein said screw has screw pitches which are narrowed downstream in a feed direction of the powder paint.
- 3. The powder paint feeder according to claim 2, wherein said screw pitches in at least part of the screw are continuously narrowed downstream in the feed direction of the powder paint.
- 4. The powder paint feeder according to claim 2, wherein said screw pitches are narrowed stepwise downstream in the feed direction of the powder paint.
- 5. The powder paint feeder according to claim 2, wherein a ratio of a minimum pitch to a maximum pitch of said screw is ⅔.
- 6. The powder paint feeder according to claim 2, wherein said deaerating means is disposed at a portion where the screw pitches are going to be narrowed or have been narrowed downstream in the feed direction of the powder paint.
- 7. The powder paint feeder according to claim 1, wherein said driving means is disposed oppositely of said screw with respect to the discharging port, and the powder paint feeder further comprises a purging air discharging means which is disposed in a vicinity of said driving means for preventing penetration of the powder paint to said driving means.
- 8. The powder paint feeder according to claim 1, wherein said deaerating means is disposed downstream of said screw and upstream of said dispersing blade in a vicinity of said dispersing blade.
- 9. The powder paint feeder according to claim 1, wherein said deaerating means is a deaerating device which includes a vacuum pump for sucking and removing fluidized air contained in the powder paint by sucking pressure, and a duct for transmitting sucking pressure and passing sucked air, and a filter for sucking only the fluidized air in the powder paint without sucking the powder paint.
- 10. The powder paint feeder according to claim 9, wherein said sucking pressure of said vacuum pump ranges from −100 mmHg to −10 mmHg on a basis of atmospheric pressure.
- 11. The powder paint feeder according to claim 1, wherein said dispersing blade has a blade piece group disposed in a single stage or blade piece groups disposed in multiple stages spaced apart from each other in a direction of the rotational shaft.
- 12. The powder paint feeder according to claim 11, wherein said blade piece groups have same number of blade pieces and each blade piece group is dislocated one another in a rotational direction of the screw.
- 13. A powder paint feeder for powder coating directly installed to a powder paint tank of a fluidized bed type, comprising:a cylindrical screw casing inserted into the powder paint tank and fixed thereto and having an introducing port into which powder paint fluidized in the powder paint tank is introduced and a discharging port through which the powder paint dispersed externally of the powder paint tank is discharged; a screw accommodated in said screw casing for feeding the powder paint introduced from the introducing port to the discharging port while compacting the powder paint, said screw having screw pitches which are narrowed downstream in a feed direction of the powder paint; a dispersing blade mounted on a rotational shaft of said screw in a vicinity of the discharging port downstream of said screw for dispersing the compacted powder paint; and a driving means for rotating the rotational shaft of said screw.
US Referenced Citations (15)