The deposition of an organic coating by electrical deposition processes on electrically conductive articles has received much attention in recent years. Some of this attention has been focused upon the coating of large numbers of small objects. One attractive feature of such a coating system, which may be termed electropainting, electrocoating or the like, is the generally desirable uniformity of coating.
Such coating uniformity is often difficult to achieve on small objects, as for example, screws, since they have sharp edges and abrupt changes in surface configuration that do not lend themselves to achieving a uniform surface coating. In the coating of such small objects by electrodeposition there has been tried the conventional coating procedure of placing many such items in a wire basket or similar conductive container. It was early appreciated that such method involves coating of the container during coating of the small items contained therein. This results in disposing of the container after merely one use or of the economically unattractive removal of the coating from the container before re-use.
It was therefore suggested to provide electrical connection between the objects to be coated and the container with a disposable connector-conductor, e.g., a layer of steel wool on the container bottom. For example, in U.S. Defensive Publication No. T856,035, such concept has been suggested after a discussion of the above noted problems associated with simply using wire baskets as coating containers. The concept of employing steel wool simply supplants one disposable item coated during the electrodeposition operation for another disposable item.
Such disposable items can be minimized; in the coating of small objects, this has been disclosed in U.S. Pat. No. 3,575,832. It has likewise been discussed in such patent the desirability of employing electromagnetic support means. And, by such support means, the small objects, and disposable connector-conductors, can be carried through not only the coating but also the curing step, although simultaneous coating of a plurality of objects is not ostensibly a practical feature of such operation.
As has been mentioned hereinabove, the difficulty in obtaining a uniform coating for many small objects is associated with the surface configuration of such objects. These configurations present a non-smooth appearance, but this feature may enhance the small objects making contact through deposited, wet coatings to an underlying electrode. Regardless, although paint films that are deposited by the electrocoating process are "self-limiting", at this resulting thickness conductivity of the deposit is retained as long as the coating is wet. Thus, parts deposited on the coating will have impressed upon them sufficient flow of electrical energy to themselves become coated. The "self-limiting" feature provides that the underlying article no longer is sufficiently conductive to achieve a further deposition of paint on itself once a sufficient paint film thickness has been achieved.
Although both anodically depositable and cathodically depositable paints can be formulated, and are contemplated for use herein, commercial development has focused upon anodically depositable paints and such are often used herein for illustration although both types are contemplated.
Movement of small articles during the coating operation is encouraged to achieve efficient coating; but since foaming of such baths may become a problem, suppression of foam generation must be kept in mind. For example, in U.S. Pat. No. 3,616,392 there is disclosed an electrocoating process employing a series of two conveyors, each conveyor being in association with a plurality of hoppers. Loading of parts into hoppers of the first conveyor, and subsequently dumping from same into the hoppers of the second conveyor, ostensibly permits adequate movement of the parts to obtain desirable coating thereon. The angle of the second conveyor then permits the coated parts to be conveyed out of the coating bath for subsequent curing. Such operation is not however simplistic in nature, involving many moving parts, and necessitates a large tank and a considerable bulk of apparatus, when considered in association with the size of the objects to be coated. In regard to this, and other aspects of the foregoing discussion, reference can be had to "Products Finishing", February, 1973, pages 34-43.