This invention relates to the free drying in an oven of molded fiber trays used for the display and sale of food and other products and particularly the way to control the characteristic arch warpage of this product.
The exact reason for such warpage is not fully known but one speculation is that warpage has occurred from the intersection drying differential between the rib tops and bottoms. This arching may be pronounced in molded fiber products containing latticed bottom panels.
The warpage of molded fiber products has been a problem for many years since the generally used molded products for display and sale of food has had this drawback of warping during free drying. This problem is described in U.S. Pat. No. 2,704,493 of Randall, granted Mar. 22, 1955, and entitled "Molded Pulp Articles and Process of Manufacture". In that patent an attempt was made to cope with this problem of warpage by a method of molding, free drying, and then attempting to press the article back into shape to remove the warping due to free drying.
A later patent relating to warpage control in a display tray which refers specifically to food trays which are of interest further discusses the material of the molded pulp food trays and their warpage. This is U.S. Pat. No. 3,929,564 of Reifers, granted Dec. 30, 1975, and entitled "Method of Molding Free Dried Pulp Display Tray" which discloses the making of a free dried, molded pulp tray with an indented peripheral margin which is formed during the molding of the tray.
U.S. Pat. No. 1,900,427 of Clapp, granted Mar. 7, 1933, and entitled "Manufacture of Fibrous Articles" also recognized the warpage problem and mentioned that drying may be effected between heated dies under pressure to effect a compacting of the product which was discussed in U.S. Pat. No. 2,704,493 of Randall introduced above, as an expensive process as opposed to the less expensive method involving free drying. Therefore Clapp in U.S. Pat. No. 1,900,427, further stating that articles moulded from ordinary paper pulp such as, for example, ground-wood pulp generally tend to warp badly and lose their shapes, has attempted to overcome warpage by incorporating materials such as asbestos fiber into the paper pulp which material incidentally would now be prohibited for use where food products are involved.
Warpage of molded food trays of the fibrous material generaly used for the molding of such trays has been a recognized problem for many years but the present invention discloses a method which does not require a change in composition of the fibrous molded material nor the elimination of free drying.