Coating Chamber

Abstract

A coating chamber having a drum shaped porous wall, rotatable to expose the wall alternately to pressure and suction is employed to create a powder cloud and collect powder not deposited on the article being coated. Powder so collected is re-blown into the cloud, without going thru a conventional collector system. The invention may be used without electrical charging of the wall, in the process wherein parts are preheated, or the wall may be electrically charged for the electrostatic coating process.

Description

This invention relates to coating chambers of the type used to apply a layer of powdered material to a surface. More specifically it is utilized to apply a layer of powdered plastic to an article onto which the plastic is subsequently caused to adhere by heating, thereby fusing the plastic thereto. It is preferably utilized with the electrostatic process but may be used, as readily, in a process using air only as a means of dispersing and directing the flow of powder particles.

While air is the means by which the powder cloud is formed, generally, it will be understood that wherever the term air is used herein any other suitable gas or fluid may be used.

The invention, as shown and described, is intended to be used primarily in a production coating process wherein a conveyor moves the articles to be coated thru the chamber.

The normal coating chamber consists of a fluidized bed having a porous bottom thru which air may flow upward. The upward flow causes powder covering the bottom to rise in a cloud. An electrostatic charge may be imparted to the powder particles to cause them to adhere to the article being coated. The article to be coated may be held in the cloud manually, it may be supported in a holding fixture moving it in desired ways, or a conveyor may be used to transport a quantity thru the cloud.

Above the bed is usually a hood, having an opening to a fan or vacuum system. This is to carry away any powder not deposited on the article being coated, and carries away the pressurized air which has passed thru the porous bottom to convey the powder particles. The hood may, or may not, have walls enclosing the region above the fluidized bed; but preferably will have to prevent excursion of powder outside of the coating area.

Openings are then provided to permit insertion of the articles to be coated, or for a conveyor to carry them thru the powder cloud.

It will be understood throughout that whenever the invention described is to be used without the electrostatic process those features required only for that process may be omitted.

In the normal coating chamber the powder is formed into a cloud by forcing a flow of air from the bottom upward. The airborne particles are then caused to deposit on the article being coated either by electrostatic attraction or by heat fusing with pre-heating before entering the chamber. The coated parts are then conveyed thru a heating cycle of sufficient time and temperature to complete the curing and bonding process.

In some applications accurate control of the coating thickness is not essential. Lack of control can be wasteful of material. In many applications very accurate control of coating thickness is required for mechanical, electrical, durability, or other reasons. The present designs of production coating chambers have been found to have certain inherent deficiencies which are overcome by the present invention.

Not all powders are manufactured with the desired uniformity of grain size. During the coating process the finer powder particles have a tendency to become airborne first leaving the coarser particles in the fluidizing bed. A sensor is used to determine when the powder level falls below a specified level. Visual observation and manual gaging may also be used. When the sensor detects the need for more powder it is fed into the chamber; either manually, in response to the signal to the operator; or automatically, in response to controls actuated in a mechanical feeder. If feeding is intermittent whenever the feeding cycle is operating there is a change in the cloud density causing a change in the coating thickness. With the accumulation of coarse particles in the coating chamber, due to initial dispersion of the finer particles, the cloud density again changes, due to lesser effectiveness of the air stream in causing the powder particles to rise and circulate. Thia accumulation of coarse particles also causes a change in the coating thickness.

Most, if not all, powders are sensitive to moisture content. This is especially true in the electrostatic process. If the coater is not in an air conditioned room, the requirement for which may be an unwarranted expense, any change in atmospheric humidity affects the moisture content of the powder; again causing a variation in uniformity of thickness. In the case of high atmospheric humidity, this condition may have a cumulative affect causing a constant variation in performance of the coater and having an extremely adverse affect upon its ability to take an electrostatic charge.

A drier might be provided in such a system. Its cost, like that of the air conditioned room, might also be unwarranted because of the large quantity of air that would be required to dehydrate in order to dry the powder.

In the present invention the above conditions are eliminated, as will be seen in referring to the drawings and description which follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal view of a coating chamber embodying the features of the present invention therein;

FIG. 2 is a side view through the section defined by arrow 2--2 of FIG. 1;

FIG. 3 is a side view through the section defined by arrow 3--3 of FIG. 1;

FIG. 4 is a full length view through the section defined by arrows 4--4 of FIG. 3; and

FIGS. 5 and 6 illustrate another version of the invention wherein the slip ring and brush are eliminated.

Referring to FIG. 1, which is a longitudinal view thru the chamber, numeral 1 designates a base comprising the bottom of the coating chamber. Numerals 2 and 3 are end walls matching, in FIG. 2, the outline defined by the outer line of walls 11, 12, 15 and 16 and the lower outer wall of tube 22. In FIG. 2 walls 2,4,5,6,7,8,9 and 10 are of equal length and abut the inner faces of walls 2 and 3 of FIG. 1. In FIG. 1 tube 13 projects thru wall 2 and ends in wall 3. Tube 22 fits matching cutouts in the top of walls 2 and 3 as shown in FIG. 2. In a preferred construction walls 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15 and 16 and tubes 13 and 22 would be cemented or welded together to insure sealing and insulation of any high voltage. However other fastening and sealing means may be utilized. Numerals 17, 18, 19, 20 and 21 define a double walled cylinder, numerals 20 and 21 being end rings connected by spacers 19. Said spacers are rigidly attached to rings 20 and 21 by cementing welding or mechanical fasteners. Walls 17 and 18 are made of some porous material which may be metal, plastic or other material such as fine mesh screen. Wall 17 may be omitted with spacer 19 altered to suit. If the material of wall 18 is chosen to be some non-conductive material then it must have a conductive layer, such as metal screen on it's inner surface. Wall 18 will, however be made of porous metal. Walls 17 and 18 should also be attached to spacer 19 along their entire length. This will preferably be done with cement or by heat fusing.

Slip ring 30 is inserted in ring 21 and connected to the conductive surface of wall 18 by connector 31.

Said cylinder is free to rotate inside of seals 45, which for the sake of simplicity are shown herein as conventional O rings but may be any other suitable type such as inflatable tubes or lip seals, or close fitting of the related parts may provide sufficient sealing.

Walls 26 and 27 are attached to walls 2 and 3 respectively by screws 29, suitably spaced. A suitable gasket or sealant may be installed between walls 27 and 2 and between walls 3 and 26.

FIG. 3 is a side view thru the section defined by arrow 3--3 of FIG. 1.

FIG. 4 is a full length view thru the section defined by arrows 4--4 of FIG. 3. This arrangement is duplicated in the same relative position on the opposite side of the vertical center section. Numerals 54 and 53 are rollers rigidly affixed to shaft 43. Shaft 43 is mounted in ball bearings 42 and 44, which are of the sealed type and supported in bosses 48 and 49 on walls 26 and 27 respectively. Shaft 43 is driven by some outside power source, such as an electric motor turning thru a gear box to obtain the desired low rotational speed, perhaps 1 or 2 RPM (to be determined by testing for each application) of said cylinder. Rollers 53 and 54 may also be, instead, gears meshing with teeth cut in or attached to ring 21. Cutouts are provided thru walls 7 and 8 for clearance with rollers or gears 53 and 54. Shaft 43 and tube 66 must be made of some suitable insulating material of sufficient length to prevent high voltage electrical leakage from slip ring 30 and wall 18. In general parts are made of some insulating material, preferably plastic, unless obviously unsuitable for the intended purpose.

Numeral 36 is a tube extending thru wall 27 to convey powder into the rotatable cylinder.

Numeral 34, FIG. 1, indicates a brush urged into contact with slip ring 30 by spring numeral 33. Numeral 35 is an insulating sleeve supporting and guiding brush 34 and spring 33. High voltage, in the range of perhaps 20,000 to 100,000 volts is supplied to brush 34 from regulateable power source numeral 40. Brush numeral 34 and spring numeral 33 shall be adequately insulated to their connection with the cable from power supply numeral 40. This may be with any commonly known method.

Air, or other fluid, is supplied by pressure source numeral 39 to tube numeral 13, from which it flows thru holes numeral 25 into chamber numeral 56, which is defined by walls numerals 4, 5, 6 and 17. The pressure shall be as determined necessary to flow thru porous walls numerals 17 and 18 with sufficient velocity to cause a suitable powder cloud to be formed. The powder particles are then electrostatically charged from the conductive surface of wall numeral 18 and attached to the grounded article being coated, to which the powder adheres due to electrostatic attraction. The air, flowing from chamber 56 thru walls 17 and 18 follows the general path indicated by arrows numeral 14, inside of the rotating cylinder, and out thru walls 17 and 18 into chamber numeral 57, thence thru holes numeral 24 to the inside of tube numeral 22 and to a suction or vacuum source numeral 41. Pressure source numeral 39 and suction source numeral 41, may and preferably will be, a single unit with constant recycling and conditioning for temperature and humidity. They may be regulatable, as required.

Some of the powder following the arrows 14 will tend to flow to the inner surface of wall 18 from which a portion of it will be repelled by the highly charged conductive surface. The remainder, which will get to the wall and tend to restrict the air flow will be constantly blown off as the cylinder rotates to the pressurized chamber numeral 56. It will be seen therefor that this invention has no need for an external vacuum system including powder recovery. It needs only relatively low capacity in the air circulating system.

Powder supplied into tube numeral 36 falls to the inside surface of wall numeral 18, where it carried by rotation to the region above chamber numeral 56. Due to the air fluidization powder spreads evenly along the length of said rotating cylinder and any surplus falls off the ends into passages numerals 46 and 47, where it falls thru holes numeral 55 into hopper numeral 28. Vibrator numeral 52, of any suitable commercially available type, may be attached to hopper 28 to facilitate powder flow therethru to opening numeral 50 from where it is conveyed along the path indicated by arrow numeral 51, thru drier numeral 32, which may be optional, to powder feeder numeral 38 and back to tube numeral 36. Vibrator numeral 52 may also be attached to tube 36, or to any other place where the powder may have a tendency to stick.

The powder conveyor may be any suitable commercial type such as screw, bucket chain or air.

Openings 37 are to permit insertion and removal of the article to be coated. Preferably a conveyor will pass therethru. Said openings, as well as the entire mechanism including the conveyor may be designed to the optimum size and related characteristics, as required by the articles to be coated.

Walls numerals 7 and 8 fit the outer surface of wall numeral 17 as closely as possible to seal against leakage of air from chamber numeral 56 to chamber numeral 57. Other sealing means may be provided such as longitudinal slots forming a labyrinth seal, or other sealing means such as an O-ring numeral 67 or a lip seal may be provided. The spacing of spacers numeral 19 shall be such that there are always at least two in the region overlapped by walls numerals 7 and 8.

Passage numeral 58 conveys pressure air, to the space between seals numeral 45, wall numeral 3 and ring numeral 21, to prevent entrance of powder to the slip ring and brush.

FIGS. 5 and 6 depict a simpler version of the invention wherein the slip ring and brush are eliminated. The wall 18, need not be of conductive material or have a conductive surface applied. A fixed screen or grid, which may or may not conform to the curvature of wall 18, as shown, is supported on walls 26 and 27 by pins numerals 60 and 62. Pin 60 projects thru wall 26 to connector 61, which is connected to regulatable high voltage power supply and suitably insulated by any well known means. Grid numeral 64 is supported by frames numeral 59 which are supported on walls 26 and 27. The wall 3, as shown in FIG. 1 is replaced by the simpler design in FIG. 6 as numeral 63. With the slip ring and brush omitted ring 21 is replaced by the simpler design numeral 65. Other parts, in general except for obvious reasons, remain as shown in FIGS. 1 thru 4.

It will be evident that in either of the foregoing designs with powder being fed into the rotating cylinder thru tube 36 at a constant rate, and being evenly distributed over the length of said cylinder the fluidizing action of the pressurized air from chamber 56 a constant level will be maintained resulting in a uniform cloud; thus insuring consistency in the coating process. The coarser, or heavier particles of powder will be constantly spilled over the ends of the rotating cylinder preventing buildup in the coating chamber. Since all of the powder entering the rotating cylinder is either attached to the part being coated, or spills off the ends to be recirculated in the powder system a high velocity air stream, with attendant high volume is not required to convey powder thru a vacuum recovery system. Relatively little air from the room in which the coater is operated will enter into the coating air system thus negating, to a large degree, the affect of atmospheric humidity, which may be considerable, on the consistency of the coating process.

Claims

I claim:

1. In a chamber for coating articles with powder, a rotatable cylinder having a porous circumferential wall, end rings attached to said wall, spacers separating said end rings and supporting said circumferential wall, a chamber for pressurized fluid external to and exposed to a sector of said wall, a chamber for suction of fluid external to and exposed to a sector of said wall, sealing means between said chambers for pressurized and suction fluids, and said end rings means to provide pressurized fluid to said chamber for pressurized fluid and means to provide suction to said chamber for suction of fluid, means for feeding powder into said cylinder, means for removing excess powder from said cylinder, means for conditioning excess powder and recirculating it back into said cylinder, walls enclosing said cylinder and chambers, said walls having openings for means to convey articles being coated into and out of said cylinder, and means for rotating said cylinder.

2. A coating chamber in accordance with claim 1 in which the wall is a conductive surface, and means for providing a high voltage electric current to said surface.

3. A coating chamber in accordance with claim 2 having a slip ring on one of said end rings, said slip ring connected to said conductive wall, a brush contacting said slip ring, said sealing means surrounding said slip ring and brush, a source of high voltage connected to said brush.

4. A coating chamber in accordance with claim 3 having means to provide pressurized fluid in the space confined by said sealing means.

5. A coating chamber in accordance with claim 1 having a grid spaced within the area enclosed by said circumferential wall, said grid being located so powder fluidized by said pressurized chamber will flow therethrough and being connected to a high voltage electric current.