In the manufacture of intricate products, such as jewelry, it is common practice to make use of a rubber or silicon mold into which molten metal is poured, which mold is rotated to cause the metal to flow by centrifugal force into all parts of the mold cavity. A particularly desirable system for accomplishing this operation makes use of a generally disk-like mold which consists of upper and a lower halves, with mold cavities in each half facing one another across a parting plane consisting of facing surfaces. The mold cavities are positioned radially outward from the axis of the mold and feed passages lead from a central pouring passage outwardly to the cavities. During the molding operation, molten metal is poured into the central pouring passage and is forced by centrifugal force through the radial passages to the mold cavities.
As successive molding operations are carried out in a particular mold, the mating facing surfaces begin to wear, especially at the outer periphery of the cavities. When this happens, some of the molten metal may be forced from the mold cavities into the gap between the two mold halves caused by the wear. This results in an undesirable "flash" on the cast product. Since this defect in the product is unacceptable, it is necessary either to discard the flawed product or to perform an expensive hand operation to remove the flash, and the worn mold must be discarded.
Various methods have been used in the past in order to compensate for the wear at the mold separating surfaces and to prevent flashing. Since the mold is enclosed in a housing and clamped by a piston during the pouring and rotating operations, it is sometimes possible to insert a shim between the piston and the mold, so that the mold is compressed to close the gap that would otherwise exist due to the wear. One such shim consists of several layers of paper in the shape of rings. Since these paper rings are all of the same thickness, there is no greater pressure on the worn parts of the mold than there is on the rest of the mold, so that the shim has only limited effect in preventing flashing on the finished product. On occasion, attempts have been made to overcome the effect of wear by using a shim made of a foamed material, such as rubber or an elastomer. Here again, the pressure produced on the mold is the same all around its periphery, thus minimizing the effectiveness of the shim.
So far as the prior art is concerned, the CECERE, U.S. Pat. No. 3,280,434 discloses a mat which is positioned beneath the mold, but the mat is not inflatable, a primary feature of the present invention, and is not indicated as performing a shim function. The same is true of the material disclosed in the COREA, U.S. Pat. No. 4,350,481, which shows a lower plate underneath a mold half, but here again the plate is neither inflatable nor does it act as a shim. The ARONS, U.S. Pat. No. 4,294,792 does appear to address the problem of flash, but the concept of an inflatable shim is completely absent.
With regard to the concept of a hollow bladder to be used as a shim in a mold system, the BULL, U.S. Pat. No. 2,739,093 shows a method of producing a tufted article by thermal bonding in a criss-cross pattern. The PRISK, U.S. Pat. No. 3,829,342 shows a hollow bladder used as a pump. The ELKINS, U.S. Pat. No. 3,830,676 shows a device for cooling and heating of the human body or the like, in which two sheets of a waterproof material are sealed together in an intricate pattern to receive suitable cooling or heating fluid. The KIEVES, U.S. Pat. No. 4,917,646 illustrates a balloon formed from two sheets of flexible plastic bonded together and including an inflating valve. None of these structures would seem to show or suggest a solution to the problem of flash in a centrifugal molding system. These and other difficulties experienced with the prior art systems have been obviated in a novel manner by the present invention.
It is, therefore, a primary object of the invention to provide a centrifugal mold system in which flash in the finished molded product is substantially reduced.
Another object of this invention is the provision of a shim for use in a centrifugal mold system that will eliminate the formation of a gap and the production of flash as the mold becomes worn.
A further object of the present invention is the provision of a mold system having a compensating shim to eliminate any gap between the mold halves.
A still further object of the invention is the provision of a shim which automatically compensates for wear in a mold at the locations where the wear takes place.
It is a further object of the invention to provide a mold shim which is simple and rugged in construction, which can be inexpensively manufactured from readily available materials, and which is capable of a long life of useful service.
Another object of the invention is the provision of a shim for a centrifugal molding system, which shim will prolong the useful life of the mold.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.