1. Field of the Invention
The present invention is directed to a food press having a platen that moves up and down to press doughballs on a conveyor, and more particularly to a food press for tortillas.
2. Description of Related Art
In making food products, such as tortillas, doughballs are typically flattened by a press unit. The doughballs are generally moved on a conveyor belt to and through the press unit. When the doughballs are properly positioned in the press unit, an upper pressing platen is moved downwardly with an appropriate force and spacing from a lower pressing platen to press the dough balls to a desired thickness and shape.
Typically, one or both of the pressing platens are heated for parbaking the dough balls to maintain a flattened condition and proper texture. Following pressing, the upper pressing platen is raised and the conveyor belt is advanced to move the flattened dough balls from the press and to advance new dough balls to the press for repeating the cycle.
In a typical prior art system, the upper platen has a pressing/heating plate, a backing structure (which is typically an insulation plate, and a supporting framework) and a Teflon platen cover in some way attached to the pressing/heating plate. Unless precision is achieved in the flatness of the pressing surface, the pressed dough does not have a uniform circumference. To achieve the necessary tolerances, thin shims are placed between the pressing/heating plate and the insulation plate. Since the plates are bolted to a frame, the task of placing, removing, and testing the shims is very time consuming. One must typically wait a period of time to cool down the parts before attempting to perform a “shimming”.
Regarding the Teflon platen cover: In spite of the fact that tension is applied to pull the cover taught over the surface of the upper platen, when the top platen elevates each cycle, small gaps of air separate the platen cover from the heat platen. This condition produces wide platen cover surface temperatures variations that result in inconsistent diameters of pressed product.
Regarding the pressing/heating plate, if the pressing plate is too thin, the process of bolting the plates warps the pressing plate, resulting in uneven heat transfer. A thicker pressing plate is therefore necessary to avoid warping. A thicker pressing plate, however, also results in problems with heat transfer when separated from the heating plate by shims.
Other prior art systems use thin pressing plates held by edge clamps to hold the backing plate to the heating plate; however, these systems have bad contact across the width resulting in inconsistent heat transfer and poor roundness and size control.
Further regarding Teflon platen covers, platen covers rapidly wear out under the heat and pressure conditions of the press. Even when not fully worn out, the platen cover can develop defective areas or holes. Replacing the platen cover is costly and time consuming, and requires that the press stop operation, causing lost production time. Moreover, when the platen is heated, replacement of the cover requires either that the press stop operation until the platen cools down or that employees risk being burned.
In an attempt to remedy the shortcomings of the prior art, U.S. Pat. No. 5,649,473, the entire contents of which are incorporated herein by reference, teaches a platen cover belt on spools positioned over the pressing plate. Once a portion of the cover belt is compromised, the cover belt can be advanced on the spools so that a new portion of the cover belt can be used. However, this system does not alleviate the need for placing shims between the bolted backing plate and heating plate. Moreover, the belt adds complexity and expense to the system.
Prior art also has the problem that when dough sticks to the Teflon platen cover it pulls against the bottom belt. This creates small air gaps between the platen cover and the Teflon platen cover belt that results in uneven temperature on the Teflon Platen cover belt. Uneven temperature, even in a properly shimmed press can produce shape and diameter irregularities. The precision shimming that is required to produce the best consistency changes depending upon the array of dough balls a producer wants to make. Smaller desired diameters are higher in number per cycle, while larger diameters are fewer in number per cycle. The result is a need to shim for each array that a producer wants to use if the best quality is desired. With many producers making array changes multiple times per day, the cost of re-shimming each array is quite high; so, either the quality is inconsistent with high productivity, or productivity is low with high quality.
What is needed is a better platen system that allows for fast placement of shims, easy removal and replacement of the pressing surface, improved heat transfer characteristics, and close and uniform contact between the plates and shims.