1. Field of the Invention
This invention relates to a method of making an improved die set for forming ceramic tiles and the tiles formed thereby, which die set is of the type which includes a punch having the profile of the tile edge, and with a coating of synthetic resin or rubber bonded thereto.
2. Description of the Prior Art
The use of a die set for the forming of ceramic tiles in which one or more sides of the tiles are shaped in the so-called "Bullnose" or "Step Edge" manner is described; which means that the edge, viewed in cross section, has its profile described by an arc that connects with the upper face plane and the lower back plane of the tile, and whose radius develops through an angle of about 90.degree., in the case of the "Bullnose", or an arc whose radius is developed through an angle notably greater than 90.degree., in the case of the "Step Edge". The shaped side as configured is viewed as the front face of the tile, and the tile is typically selected to form a step or structural similar edging element.
Shaped tiles are disclosed in the publication ANSI A137-1-1988, published by The Tile Council of America, Inc., P.O. Box 326, Princeton, N.J. 08542-0326,
The U.S. Pat. Nos. 4,522,768 to Roscrow; 4,350,486 to Croseck, et al., No. 5,330,346 to Scardovi, and the Japanese Patent No. 56-124237 describe various structures, but do not disclose my invention.
The die set incorporates two punches, one upper and one lower, and a matrix. The ceramic material is held in place by the matrix and the lower punch, and is compacted as the upper punch is driven towards the lower punch to compress the material in the cavity described by the matrix. The two punches shape the ceramic material into a tile in manner which forms the front face of the tile, including the shaped edge (or edges), and respectively, the ulterior face (so-called "trademark side") of the tile.
A technical problem present in prior art die sets is that as the upper and lower punches are mated together to form the ceramic tile, the metal of the edge of the upper punch, and the coating on the portion of the lower punch which forms the desired shaped edge side, approach each other to closely to be exactly controlled. In fact the large pressing action needed to form the ceramic tile tends to create a tangential force that forces the edge of the upper punch closer to the shaped lateral portion of the lower punch. It has been suggested that the lower punch cutout be coated with a layer of material to reduce the tendency of the tile residue to stick to the punch. However, the end point of the coating which was vulcanized onto the lower punch is very easily damaged by the "paring" action of the edge of the upper punch, which results from its close proximity.
In order to overcome this drawback existing technology has often chosen to create a lower punch from which a certain amount of coating is intentionally left off the lateral portion of the punch, and the metal is left exposed to form the desired shape. This action creates another disadvantage, that is to say that the aesthetic appearance of the surface of the tile is not uniform and is more coarse where the coating is present than where the polished metal is in contact with the surface of the tile being formed. Such non-uniformity in finished facial quality is evident for glazed products as well as that of unglazed tile. Furthermore, where the coating is lacking the material residues stick, and the necessity of cleaning becomes relatively frequent and consequently greater machine downtime occurs.
The present invention provides for completely coating the lower punch area which forms the tile edge with a layer of synthetic resin or of rubber, which has the property of being substantially self cleaning during operation. The coating is effective in preventing any rapid build up of unwanted material residues which would rapidly adhere to the uncoated punch. Accordingly the machine does not have to be stopped and residues from the tile material on the punch cleaned, and therefore no production time is lost.