Method of molding free dried pulp display tray

Abstract

A method of molding a free dried pulp display tray including the formation of a bottom surface with an offset marginal portion extending therearound and along side and end wall portions of the tray to surroundingly frame the general expanse of the bottom which lies in a plane offset from that of the marginal portion to thereby minimize warpage and insure bottom flatness. The method includes the removal of formed tray from its form mold members while in wet condition and placing it on a drying surface and free drying said tray. The side or end walls of the tray also may be formed with offset rail portions which stabilize these walls and further insure a more perfect flat bottom and uniformity of shape.

Parent Case Text

This application is a continuation-in-part of my co-pending application Ser. No. 99,100, filed Dec. 17, 1970, now abandoned.

Description

The present invention relates to warpage control in molded pulp articles and more particularly to an improved method of producing free dried, molded display tray. One way of drying molded pulp articles in the manufacture of such articles is by form drying, a process in which the molded articles are dried in shaped heating dies which restrain the articles. This restraining action causes stresses to develop from the shrinking of the fibers, which, on drying, are relieved by fiber-to-fiber slippage, and which in turn results in a stress-free and warp-free article. It is also common to completely free dry the pulp articles. Free drying, however, results in pulp articles with considerable warpage and can be used only where a degree of warpage in the article can be tolerated.

When ground wood molded pulp food trays, including pie plates and the like, having a projected bottom area in the order of about 16 square inches to 80 square inches and square or circular, or rectangular or oval bottoms ranging from approximately 2 .times. 8 inches to 4 .times. 6 inches to 6 .times. 6 inches to 6 .times. 9 inches, or from approximately 5 inches to 10 inches in diameter, and and somewhat larger and somewhat smaller, are manufactured, they are usually free-dried; that is to say, they are not dried on a form but they may be dried on a foraminous tray which is in the path of a heated air or gas stream. Such a drying tray may be passed through either a horizontal or vertical tunnel, as disclosed, for example, in U.S. Pat. No. 2,941,655.

Such ground wood molded pulp free-dried food trays generally emerge from the drying chamber or tunnel with a bottom which is warped so that when one looks inside the food tray from above, the bottom appears to be convex and when one looks at the outside bottom of the tray, then the bottom appears to be concave. Such warpage in food trays is not desirable for various reasons including the fact that the food tray's capacity is cut down and its appearance may be so distorted as to make the product unmerchantable.

Surprisingly, it has been found that an offset in a particular area of the bottom of such trays, as will be hereinafter described, can materially strengthen the bottom of such trays, increase their capacity, and markedly improve their appearance. This result was unexpected, particularly because the amount of offset found essential to accomplish the desired results is in the order of the thickness of the bottom wall of the food tray or slightly greater than the thickness of the bottom wall which, in the case of the trays described, is about 0.050 inch - 0.080 inch, more or less.

This surprisingly small amount of offset unexpectedly, markedly and significantly improves the food tray both as to its food carrying capacity and as to its appearance which is important in connection with the merchantability of the product intended to be sold in such trays.

Related to the trays of the sizes, and of the character described, this offset has been found to give the results set forth above when it extends downwardly for the small distance recited from a point extending inwardly from the side walls of the containers, of the sizes described, for a distance of about 1/4 inch to 1/2 inch. These latter dimensions may vary somewhat depending upon the nature of the formation of the place of joining of the side walls with the bottom. If the radius of curvature between the side walls and the bottom is larger, then the offset portion may be set in more from the side walls. If the radius of curvature between the side walls and the bottom is smaller, then the offset portion should be set closer to the side walls. As stated, with the joining formation usually found in commercial molded ground wood pulp food trays, the downwardly extending offset of from 0.050 inch to 0.080 inch begins at a point inwardly of the side walls at a distance of from about 1/4 inch to 1/2 inch.

In the expanding field of automated produce packaging, it has become apparent that the packaging medium must be uniform in shape, free from warpage, and of particular importance the packaging medium must have a flat bottom so that the film may be pressed firmly against it for properly joining in the automated or hand wrapping equipment. Uniformity of shape and freedom from warpage is also important to facilitate denesting of the packaging medium from storage.

With the foregoing in mind, an object of the present invention is to provide a simple and economical method of producing a molded pulp display tray.

Another object of the present invention is to provide a method of producing a free dried, molded pulp display tray incorporating features which reduce or minimize warpage to insure a flat bottom.

Another object of the present invention is to provide a method of producing a free dried, molded pulp display tray incorporating features which insure uniformity in shape without warpage.

Another object of the present invention is to produce a tray with features to insure consistent denesting.

Other objects and the nature and advantage of the instant invention will be apparent from the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a bottom view in perspective of a tray with a warpage problem which the subject of this invention avoids;

FIG. 2 is a sectional view of the tray taken along section 2--2 of FIG. 1 as supported on a drying surface;

FIG. 3 is a bottom view in perspective of one embodiment of my invention in which a rib portion is formed in the bottom;

FIG. 4 is a sectional view of the tray taken along section 4--4 of FIG. 3 as supported on a drying surface;

FIG. 5 is a bottom view in perspective of a second embodiment of my invention in which a shoulder portion is formed in the bottom; and

FIG. 6 is a sectional view of the tray taken along section 6--6 of FIG. 5 as supported on a drying surface.

With reference to the drawings, FIG. 1 shows a free dried, molded pulp display tray 1 of the type which my invention seeks to improve upon and avoid certain problems thereof. Display tray 1 is formed with a pair of side walls 3 and a pair of end walls 4 and a bottom wall 5. At the intersection of side walls 3 and end walls 4, outwardly projecting lip portions 2 are provided for denesting purposes, as where a supply of trays are stacked and ready for use. Display trays of this type have a tendency to warp at the bottom wall 5, particularly if it is dried as illustrated in FIG. 2, and produce an undesirable concavity due to warpage. As shown in FIG. 2, display tray 1 is disposed upon a foraminous drying surface 40 and is free dried thereon with its bottom wall 5 down. Free drying is accomplished on foraminous drying surface 40 which may be preheated or heated during drying or both. Drying surface 40 may conveniently be in the form of a two or more spaced-apart support bars 41 integrally related to a plurality of spaced-apart supporting cross bars 42 forming openings 43 between which heated drying air may be directed against a display tray 1 to be free dried. As shown in FIG. 2, heated drying surface 40 is a small section of a much larger surface upon which a large number of formed trays may be dried. The bottom wall 5 of tray 1, as clearly illustrated in FIG. 2, when subjected to free drying has a tendency to warp or become concave from the bottom side thereof. In becoming concave during the drying process the bottom wall 5 does not dry uniformly as the portion which is raised the greatest distance away from the drying surface 40 stays wet longer than the portions immediately adjacent the drying surface 40. It is this warpage or concavity due to free drying which my invention seeks to avoid.

FIG. 3 shows one embodiment of my improved free-dried, molded pulp display tray 11 which comprises side walls 13, end walls 14 and bottom wall 15. Bottom wall 15 as illustrated is generally rectangular, but may be generally square. The physical dimensions of the area represented by bottom wall 15 is in the order of 16 square inches to 80 square inches with the sides thereof ranging from approximately 2 .times. 8 inches to 4 .times. 6 inches to 6 .times. 6 inches to 6 .times. 9 inches, or from approximately 5 to 10 inches in diameter, and somewhat larger and somewhat smaller. It is noted that side walls 13 and end walls 14 extend upwardly from bottom wall 15 at a slight outward inclination so that a plurality of trays may be nestably stacked. As best seen in FIG. 4, bottom wall 15 of tray 11, which is being dried on drying surface 40, remains flat and free from any concavity due to warpage such as is present in the bottom wall 5 of tray 1, as seen in FIG. 2. The flatness of the bottom wall 15 of tray 11 is maintained by the presence of an inwardly indented margin or rib portion 16 extending completely around the general expanse of the bottom wall 15, as if framing a picture. The amount of the offset of the indented margin or rib portion 16 is generally of the order of the thickness of the bottom wall 15 which in accordance with the present invention is about 0.050- 0.080 inch. Further, the offset of rib portion 16 extends laterally inwardly for a distance of about 1/4 inch to 1/2 inch from the side and/or end walls 13 and 14 depending upon the radius of curvature between the bottom wall 15 and the side or end walls 13 and 14. Because of the formation of indented margin or rib portion 16, bottom wall 15 is held down against drying surface 40 and subjected to dry heat currents directed thereagainst. With the indented margin or rib portion 16, bottom wall 15 has a tendency to dry with greater speed and uniformity during manufacture and to retain bottom flatness which provides a tray having better sealing in automated wrapping machines than other products of this type, in particular trays with rounded or warped bottoms. As previously mentioned, drying surface 40 may be preheated and/or heated by hot air currents. Outwardly projecting rails 12 may be formed on end walls 14 to stabilize the end walls and thereby provide additional assurance of flatness in the bottom wall 15 to enhance the sealing qualities thereof. Rails 12 also facilitate denesting of the trays.

FIG. 5 shows a further embodiment of my free dried, molded pulp display tray 21 which comprises side walls 23, end walls 24, and a bottom wall 25. Bottom wall 25, which like bottom wall 15 is generally rectangular, may also be generally square with the physical dimensions thereof in a range comparable to that of bottom wall 15. In FIG. 6, the bottom wall 25 of tray 21 is seen to be held down against drying surface 40 and thereby maintained flat and free from warpage or concavity at the bottom side thereof. The flatness of bottom wall 25 of tray 21 is maintained due to its being lowered or offset from the plane of margin or shoulder portion 26 which extends therearound to effect quicker drying as in the case of tray 11. The amount of the offset of the indented margin or shoulder portion 26 is within the range described with respect to bottom wall 15. With respect to bottom wall 25 of the instant embodiment then, shoulder portion 26 is offset from bottom wall 25 by about 0.050- 0.080 inch, the thickness of the material thereof with the offset extending laterally inwardly for a distance of about 1/4 inch to 1/2 inch from the side and/or end walls 23 and 24 depending upon the radius of curvature between the bottom wall 25 and the side or end walls 23 and 24. The bottom wall 25 with the indented margin or shoulder portion 26 also dries with greater uniformity to maintain its flatness and provide better sealing qualities. Outwardly projecting rails 22 are formed on end walls 24 with similar rails 27 on side walls 23 which stabilize the end walls 24 and side walls 23, respectively, to provide additional assurance of flatness in the bottom wall 25. Rails 22 and 27 also serve as denesting ledges. The side walls 23 and end walls 24, as in the previous embodiment, extend upwardly at a slight outward inclination.

The side rails 27 extend upwardly with a slight inward taper or lie in a generally vertical plane as seen in FIG. 6. The purpose of this feature is to facilitate automation by avoiding tray interference in certain machines, particularly where trays are in stacked relationship and puch one another. When the trays push each other, contact may be established at the lower edge of the ledge and occasionally irregular "flush" projections can form at the free edge of the tray and interfere with automation.

In carrying forth the convept of the present invention of making a molded pulp food display tray with a flat surface, which is free from warpage, in particular a flat bottom surface, a tray is formed from wet wood pulp and molded with a flat bottom surface generally rectangular or square configuration with a surface area in the order of 16 square inches to 60 square inches. The bottom surface is formed with side dimensions ranging from approximately 2 .times. 8 inches to 4 .times. 6 inches to 6 .times. 6 inches to 6 .times. 9 inches, or from approximately 5 to 10 inches in diameter, and somewhat larger and somewhat smaller. The tray is formed with the thickness thereof in the order of about 0.050- 0.080 inch. In molding the tray an indented margin either in the form of an offset rib or an offset shoulder is formed around the general expanse of the bottom surface. The rib or shoulder that is formed in the bottom surface is in the order of the thickness of the bottom wall, that is of about 0.050- 0.080 inch beginning at a point laterally inwardly of the side and/or end walls and extending downwardly. The rib or shoulder is thus formed with the bottom wall of the tray disposed downwardly therefrom. Also, the bottom wall of the tray is formed so that it is laterally inward from the side or end walls by a distance of about 1/4 inch to 1/2 inch depending upon the radius of curvature between the side or end walls and the bottom. With the tray formed as described by molding the molded pulp tray is then transferred to a drying support surface for free drying without a form die.

In the free drying of the molded pulp tray it is disposed bottom surface down on a foraminous drying tray and subjected to heated air or gas which pass through the openings in the foraminous drying support surface and dry the bottom surface of the molded pulp tray uniformly and thereby provide a dried flat bottom surface free from warpage.

Additionally, it is noted that according to this invention the structure for maintaining flatness in the bottom wall may take the form of straight side and/or end wall portions in the vicinity of the bottom wall with the edge of the indented margin surrounding the general expanse of the bottom wall being slightly bowed or the form of slightly bowed side and/or end wall portions in the vicinity of the bottom wall with the edge of the indented margin surrounding the general expanse of the bottom wall being straight.

Thus it is clear that by practicing the method disclosed herein it is possible to produce a molded pulp tray with a flat bottom surface which is free from warpage while requiring less production time since the articles need not be inserted into and removed from drying dies. Also, because of the offset formation around the bottom surface, it is held against the drying tray supporting surface so that drying is uniform and more rapidly accomplished to effect considerable savings in manufacturing expenses.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown in the drawings and described in the specification.

Claims

What is claimed is:

1. The method of making a free-dried, molded pulp tray with a plurality walls of which at least one of said walls is to have a generally central flat area surrounded by an offset or indented peripheral margin including the steps of preliminarily forming wet pulp in the shape of a tray having a plurality of walls, forming a generally central flat area in at least one of said walls and an offset or indented peripheral margin around the generally central flat area of said at least one of said walls and then finally drying said freshly formed wet and undried tray formation in a free-drying step without restraint such as would occur in form drying.

2. The method of making a free-dried, molded pulp tray with generally vertically extending end walls and side walls and a bottom wall having a generally central flat area surrounded by an offset or indented peripheral margin including the steps of preliminarily forming wet pulp in the shape of a tray having generally vertically extending end walls and side walls and a bottom wall having a generally central flat area surrounded by an offset or indented peripheral margin and then finally drying said freshly formed wet and undried tray formation in a free-drying step without restraint such as would occur in form drying.

3. The method of making a free-dried, molded pulp tray according to claim 2, wherein said offset or indented margin around said bottom wall and said bottom wall have a thickness of the order of about 0.050- 0.080 inch.

4. The method of making a free-dried, molded pulp tray according to claim 3, wherein the bottom wall is molded with a bottom surface area of the order of 16 to 80 square inches with side dimensions ranging from about 2 .times. 8 inches to 4 .times. 6 inches to 6 .times. 6 inches to 6 .times. 9 inches and from about 5 inches to 10 inches in diameter, and of a projected geometrical shape including rectangles, squares, circles and ovals.

5. The method of making a free-dried, molded pulp tray according to claim 4, wherein said offset or indented margin is molded in the form of a rib.

6. The method of making a free dried molded pulp tray according to claim 5, wherein said offset or indented margin is molded in the form of a shoulder.

7. The method of making a free dried, molded pulp tray according to claim 6, wherein said bottom wall is formed so that it is disposed laterally inwardly from the side and end walls of said tray by said margin by a distance of about from 1/4 inch to 1/2 inch.

8. The method of making a free dried, molded pulp tray according to claim 7, wherein said tray is formed in a forming die and upon being removed from the forming die is disposed bottom side down upon a foraminous support drying surface for free drying thereon without use of drying dies.

9. The method of making a free dried, molded pulp tray according to claim 8, wherein free drying of the formed tray is carried out with the application of heat thereto.

10. The method as recited in claim 9, wherein said drying surface is preheated.

11. The method as recited in claim 10 wherein heated drying air currents are directed upwardly through the underside of said drying surface against the bottom wall of the tray disposed thereon to thereby dry said tray.