Apparatus For Particulate Coating Of An Elongate Article

Abstract

A substantially closed shell contains an air pervious liner defining an inner chamber for coating an article with thermoadhesive particles. Air flowing continuously through the liner toward the article in the chamber prevents buildup of the thermoadhesive particles on either the top or sides of the chamber. The chamber may be formed with a pair of openings to permit the entry and exit of pipe to be coated.

Description

This invention relates to the coating of elongate articles with resin particles.

It has been known in the past to coat elongate metal articles, e.g., pipe, by heating the article to be coated and passing it into a chamber containing air-suspended resin particles maintained below their melt temperature. When the particles come in contact with the pipe, which is maintained at a temperature higher than the melt temperature of the resin, they melt and fuse on the pipe surface to form a coating. The nonadherent excess particles are then removed by vacuum.

While fusing is necessary to adherently coat the article, the fusing operation can take place in an oven after the resin particles have been deposited on the article. Coating, when external fusing is used, is usually done by placing an electrostatic charge on the particles immediately before they come in contact with the metal article which is grounded.

A further variant used, and indeed preferred, is to coat the articles by using both electrostatic forces and fusing simultaneously in the coating chamber.

Because these finely divided particles provide an explosive danger if allowed to disperse about the building where the coating is performed, coating systems such as that disclosed in U. S. Pat. No. 3,361,111, have been devised which provide a means for preventing escape of the particles from the chamber. The coating process disclosed in the above-mentioned patent is designed to be essentially continuous, and the retrieved particles, after removal from the chamber by vacuum, are recirculated to be used again.

One of the difficulties encountered in all of these particulate coating processes, however, is that the powder used for coating tends to adhere to the chamber surfaces with which it comes in contact. This powder is not successfully removed by vacuum means used to withdraw the uncoated particles. The powder clinging to the sides and top of the chamber tends to fall off and land upon the workpiece producing localized, irregularly surfaced coating areas. While the problem of powder adherence to the sides and top of the coating chamber is a troublesome one for all coating processes of this type, it is particularly annoying for the continuous operations. Periodically, these operations must be shut down and the particles removed from the surfaces of the chamber.

It has now been found that the coating of an article such as a hot metal pipe by a resin powder may be performed without the deposition of substantial amounts of powder on the top or sides of the chamber. This is accomplished by establishing a differential pressure across an air pervious liner which forms the coating chamber. With the pressure on the outside surface of the liner greater than that on the inside surface, particles are maintained about the article to be coated and away from the top and sides of the coating chamber.

The construction of the device may readily be understood by reference to the drawings in which:

FIG. 1 is a cross sectional view of one embodiment of the coating apparatus,

FIG. 2 is a longitudinal cross sectional view of the apparatus depicted in FIG. 1.

The substantially closed shell 10 in combination with the air pervious liner 11 and support struts 12 forms an outer chamber 21 consisting of a series of compartments. (It should be noted that the support struts may be air pervious or otherwise allow the passage of air. When this is the case, a single outer compartment rather than a series of outer compartments is formed with no evident effect on the performance of the apparatus.) A substantially closed inner or coating chamber 22 is defined by the liner 11. The hot pipe 17 enters the chamber 22 through the workpiece entry 16 and is kept in alignment by externally located rollers 20. Thermoadhesive powder is propelled by air through the particle inlets 14 and passes through the nozzles 15 which may be used to electrically charge the particles by equipment not shown. Air enters through a plurality of air inlets 13 and passes through and is diffused by inner liner 11. Particles which have not adhered to the pipe are removed through exhaust port 18.

After the hot pipe is introduced into the chamber it is bombarded with the thermoadhesive particles. When air is introduced through the air inlets, a pressure differential is created between the outer chamber and the coating chamber which causes air to flow toward the pipe continuously from all sides. The air flow prevents buildup on the walls and ceiling of the coating chamber. Particles which do not coat the pipe fall to the chamber floor or are removed through the exhaust port.

As seen in FIG. 1 of the drawing, the floor of the chamber is slanted toward the exhaust port 18. This aids somewhat in particle removal although it is not a necessity. Since the small amount of particles present on the floor of the chamber do not detract from the coating process, they may be allowed to remain till they can be conveniently removed. It may be desirable to include gentle agitation means attached to the floor of the coating chamber, particularly if a slanted floor is used, to impel the particles toward the exhaust port.

While the teachings of this invention are particularly useful in continuous coating processes of the type illustrated in U. S. Pat. Nos. 3,361,111 and 3,161,530, and the improvements described in this application are particularly adaptable for use therewith, the teachings of this invention are not so limited. It is readily apparent that apparatus without particle recycling means can be used.

A further variant contemplated is the use of a semiconductive air pervious layer coated with electrostatically charged semiconductive particles. When the resin particles come under the influence of the electrostatic field created between the charged liner and the grounded pipe, the likelihood of particle attraction to the pipe is increased.

COMPARATIVE TEST

This test provides an illustration of the effect of the air pervious liner on particle adherence to chamber walls. While, in this example, 3/16-inch thick Fluidizing Grade "Vyon" porous polyethylene sheet material was used as the air pervious member, it should be readily apparent that any air pervious substance may be substituted for the polyethylene including such diverse materials as metal screening and kraft paper. It is preferred, however, that the openings in the liner be smaller than the thermoplastic particles.

For this test a small rectangular coating chamber was made with a single air inlet at its top face. A sheet of porous polyethylene was attached to one of the vertical walls of the chamber and to a centrally located support strut depending from the ceiling. A metal plate was attached to the opposite vertical wall and to the supporting strut. The polyethylene sheet and metal together produced a coating chamber with a false ceiling which was parallel to, but lower than the ceiling of the original chamber. Half of this new ceiling was air pervious while the other half was not.

A charging electrode was attached to the particle spray nozzle and a 90,000 volt potential was applied to the nozzle to electrostatically charge the thermoadhesive particles.

Over a period of four hours, one hundred pounds of powder epoxy particles described in U.S. Pat. No. 3,102,043 was electrostatically sprayed into the chamber toward the grounded pipe. An air flow of 4 standard cubic feet per minute per sq. ft. of surface area as measured in the air inlet was used to create positive pressure along the inside of the air pervious part of the new ceiling.

After spraying was complete the ceiling was inspected for signs of powder buildup. It was found that while the metal half of the ceiling was covered with a thick powder coat, the air pervious half was completely devoid of powder.

EXAMPLE 1

The apparatus illustrated in the drawings was used for this example. The shell was a cube of 28 inches per side (excluding the exhaust port extension at the floor). The length of the floor including the exhaust port was 32 inches, and the chamber opening for the exhaust port was 4 inches in width. The porous polyethylene liner was mounted at a constant distance of 2 inches from the outside shell except for the chamber floor. (One end of the chamber floor was attached to the shell at the exhaust port and the other end was attached to the strut 2 inches from the bottom of the shell as illustrated in FIG. 1.)

An air flow of 4 standard cubic feet per minute per square foot of surface area as measured at the air inlet was used to provide the outer chamber with a higher pressure than the coating chamber. The vacuum drawn at the exhaust port was 1,000 cu. ft./min.

After coating was completed, an examination of the interior revealed that the top and sides of the liner were completely clear of powder except for minor amounts which had collected in the area of the struts. Some powder was found in a thin layer on the slanted floor of the chamber but a slight vibration of the floor caused the powder to move toward the collection port and be removed by the vacuum means attached thereto. The coating on the pipe was firmly adherent and uniform along the entire surface.

EXAMPLE 2

The apparatus used in Example 1 was used in this example except that the polyethylene liner was coated to a wet thickness of 1 to 2 mils "Aquadag," a colloidal suspension of graphite particles in water containing 22 percent graphite solids. The graphite was uniformly applied over approximately 80 percent of the surface area of the inner chamber. The areas around the metal support struts were left uncoated to prevent shorting out. Using the air flow indicated in Example 1, three identical segments of pipe were cold-coated as follows. The first pipe was coated with a negative charge at the particle intake nozzle, the second with a negative charge on the graphite and the third with a negative charge on both the graphite and the nozzle. After coating the pipes were heated in an oven to fuse the resin. Coating thickness was measured after the pipes were allowed to cool. It was found that the first two pipes had a well-adhered, uniform coating of 4 mils, while the pipe coated with charges on both the graphite and the nozzle produced a well-adhered, uniform coating of between 5 and 6 mils.

It should be noted that a positive charge can be used on the nozzle and/or the coated liner. The choice of charge is to some extent dependent on the particular resin chosen, e.g., the epoxy resins accept a negative charge more easily than a positive one.

Claims

What is claimed is:

1. Apparatus for coating an elongate article such as pipe with thermoadhesive particles comprising a substantially closed outer shell containing an air pervious liner defining an inner coating chamber and an outer chamber between the liner and the shell, openings in the shell for continuous ingress and egress of said elongate article, means for propelling the particles toward the article, and an exhaust port for removing from the coating chamber particles not deposited on the article, wherein the improvement comprises:

the air pervious liner provides at least the top and sides of the inner coating chamber and is formed with openings affording continuous entry and exit of the pipe,

the exhaust port is directly connected to the inner coating chamber and

means for establishing a pressure differential between the entire outer chamber and the entire coating chamber so that air flows continuously through the air pervious liner toward the article, thereby preventing the deposition of substantial amounts of the particles on the top and sides of the coating chamber.

2. Apparatus for coating pipe including a substantially closed outer shell and an air pervious liner defining an inner coating chamber and an outer chamber between the liner and the shell, openings in the shell for continuous ingress and egress of pipe, nozzle means in the inner coating chamber for propelling thermoadhesive particles toward the pipe and an exhaust port for removing by vacuum from the inner coating chamber particles not deposited on the pipe, wherein the improvement comprises:

the air pervious liner provides substantially the entire inner coating chamber and is formed with openings affording continuous entry and exit of the pipe,

the exhaust port is directly connected to the inner coating chamber, and

means establishing a pressure differential between the entire outer chamber and the entire coating chamber so that air flows continuously through the air pervious liner toward the pipe, thereby preventing the deposition of substantial amounts of the particles on the top and sides of the coating chamber.

3. Apparatus as defined in claim 2, wherein a further improvement comprises:

the exhaust is located at the bottom of the coating chamber and the floor of the chamber is slanted downwardly toward the exhaust to facilitate removal of nondeposited particles.

4. Apparatus as defined in claim 2, wherein a further improvement comprises:

the air pervious liner has broad semiconductive areas and means are provided for maintaining a negative charge on the nozzle means and on the walls at said semiconductive areas.