Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-09-03
2001-07-31
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S475000, C427S485000, C427S317000, C427S393000
Reexamination Certificate
active
06268022
ABSTRACT:
This invention is directed to a method for electrostatically applying a coating powder to a heat-sensitive substrate such as wood and particularly fiberboard, and fusing or fusing/curing the applied coating powder to form a continuous coating. In particular, the invention is directed to an electrostatic method for completely coating edges and corners of workpieces to a uniform thickness.
BACKGROUND OF THE INVENTION
A frequent problem encountered when coating low-temperature substrates, such as wood, with coating powder is non-uniformity of coating in areas of the substrate which are difficult to coat, such as the edges and corners of kitchen cabinet doors. A frequently observed defect at edges, corners and other surface discontinuities is a rough, non-uniform finish. Surface moisture on wood provides a mechanism for the electrostatic coating of the otherwise non-conductive substrate, particularly when the substrate is grounded. The Faraday cage effect, the self limiting coating thickness and the ability of the powder to follow electrostatic field lines and wrap around edges, all typical electrostatic coating effects, are all found when wood is electrostatically coated at room temperature. Preheating of the wood to the powder fusing temperature defeats the self limiting mechanism so that thicknesses of 5 mils or more are achieved. Some improvement in Faraday penetration is also seen and the wrap around phenomenon is unchanged.
When a wooden substrate, e.g., medium density fiberboard or MDF, is machined into a complex part such as a cabinet door, a groove routed within the field of the door's front surface tends to exhibit problems of incomplete coverage that may be associated with the Faraday effect. Other machined areas of the door which seemingly should be easy to coat electrostatically, such as the outside edges which make up the perimeter of the door, also pose problems which make complete coverage very difficult. Because the thinned areas of the machined door surface tend to lose heat from the pre-heated door more rapidly than the areas of original thickness, the outside edges and corners become the most difficult to coat properly.
For example, the front side (or A side) of a cabinet door made from one piece of medium density fiberboard typically is routed along its perimeter to help give the visual appearance of a traditional “5 piece” natural wood cabinet door. The profile of this perimeter routing (the classical Ogee, for example) depends upon the selection of the proper router bit but it always results in a declining step shape, a portion of which is thinner than the original edge. Coating these thinner portions first from the front side before much heat is lost would be the logical approach but the result is unsatisfactory.
SUMMARY OF THE INVENTION
The invention is generally directed to heat-sensitive substrates having edges, corners, or other surface discontinuities. The heat-sensitive substrates are exemplified by certain plastics and fibrous lignocellulosic materials whether derived from trees or other plants and whether such material is in its natural state or its fibers have been separated, felted and/or compressed. Thus, lignocellulosic material includes hardboard, medium and high density fiber board, particle board, oriented strand board, cardboard, and paper. High- or medium-density fiberboard is generally substantially more dense adjacent to the surfaces than in interior regions. Typically, in the pre-pressed fiber preparation, three fiber layers are provided with outer layers more dense than the core layer, thereby providing surface toughness while reducing weight at the core. When such fiberboard is cut into workpieces, such as for kitchen cabinet doors, the less dense core layer is exposed. Rough, non-uniform finishes result when the coating soaks into the less dense areas.
The method of this invention is particularly adapted to the electrostatic powder coating of workpieces having complex parts such as sharp edges around the perimeter of the piece or grooves within the field thereof such as for cabinet doors. Workpieces having low density parts or flat stock having a profiled edge such as table tops, desk tops, and shelving are also amenable to this coating method.
Accordingly, it is a general object of the present invention to provide an electrostatic method for completely coating edges and corners of a heat-sensitive workpiece with a thermosettable or thermoplastic coating powder and fusing or fuse/curing the powder to form a film having a substantially uniform thickness.
This and other objects of the invention which will become apparent from the following description and drawings are accomplished by a method comprising the steps of preheating a heat-sensitive workpiece having a front side (the A side) and a back side (the B side) to achieve a surface temperature of from about 230° to about 260° F., said front side in turn having a field portion and edge portions bordering the field portion, electrostatically spraying a coating powder onto the B side first and allowing the powder to wrap around the edge portions to cover them and a portion of the A side, then electrostatically spraying a coating powder onto the A side, fusing the powder to form a film, and curing a thermosetting film or cooling a thermoplastic film.
Coating of a routed groove in the field of a cabinet door, for example, before the back side is coated may be carried out by any satisfactory method known to the coating art but an electrostatic spraying of a coating powder onto the heated front side with a gun having a pinpoint nozzle, such as is available from ITW GEMA, is preferred. The coating path follows the groove with limited overspray providing less than 50% coverage of the adjacent flat area.
As was said above, moisture is advantageous for electrostatic application of coating powder in that it enables the otherwise non-conductive material to transfer sufficient electrical charge for efficient electrostatic coating powder application. The moisture content of the wood or other heat-sensitive lignocellulosic substrate for the purposes of this invention is from about 3 to about 10 wt %. The moisture on the surface of a plastic substrate for the purposes of this invention is from about 0 to about 1% by weight.
For the purposes of this invention, the term “powder coated” means electrostatically powder coated and it applies to both the fused and non-fused powder and to a cured powder coating as the context requires.
REFERENCES:
patent: 5721052 (1998-02-01), Muthiah et al.
patent: 933 140 (1999-04-01), None
“Powder Coating: The Complete Finisher's Handbook”, pp. 88-98, First Edition, Ed by Nicholas P. Liberto, 1994.
Horinka Paul R.
Mill Gregory R.
Schlegel Grant E.
Morton International Inc.
Parker Fred J.
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