Abstract:
Thermoplastic parts are made of predominantly single unit thickness but with marginal areas of increased thickness by molding two parts in a common mold having overlapping mold portions. Two thicknesses of thermoplastic material are molded together in the areas of increased thickness at opposite ends of the common mold and the parts are separated from one another by trimming such that each part has an area of increased solid wall thickness and an area of reduced thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. patent application Ser. No. 60/733,644, filed Nov. 4, 2005 and titled “Offset Mold Twinning”. 
    
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     This invention relates to thermoforming, and in particular, to a method of creating a thermoplastic part with a band of selectively increased wall thickness or double wall features along one or two edges of the part. 
     BACKGROUND OF THE INVENTION 
     There is a need on many differing types of plastic parts when only one area of the part has to have substantial wall thickness and the remainder of the part can be thinner. One example of this is the plastic lid on a trash dumpster. The main requirement of a lid on a dumpster is to keep rain and snow out and contain the trash. A lid can be made using very thin material molded with structural ribs that will satisfy both requirements. When such a lid is made of thin material, it typically fails by tearing around the pivot point where the lid is hinged to the container. The most typical method of hinging is by placing a steel rod through holes that are drilled in the ribs or knuckles that are formed in the plastic. The existing technology is that lids are now typically made of thicker material to toughen the hinge area, resulting in a higher cost. 
     A second example of this need for selectively increased wall thickness is a wall panel of a plastic shelter like a portable animal house for calves or any similar application. The upper portions of the wall are primarily for weather protection. Nearer to the bottom the walls are subject to considerable abuse and need to be stronger and thicker to facilitate fastening to the base, that is often times of another material like wood or solid plastic structural material. Additionally it may be necessary to have the wall material surround a majority of the base. Heretofore such a wall panel would be molded of thicker material in its entirety or constructed using a twinsheet thermoforming method, at additional cost. 
     There are numerous other applications also within this category. For example, such applications include liners in semi trailers where the bottom takes a majority of the abuse and the sidewalls of material handling containers to facilitate connecting the panels to the base. 
     Parts that are made using an injection, compression or casting molding process require an inner and outer mold that is completely filled with the plastic material. Any requisite additional thickness is accomplished by increasing the distance between the inner and outer mold halves and completely filling the space. Parts that are made by thermoforming or blow molding are limited by the starting stock or sheet thickness, typically with pressure or a vacuum on at least one side of the sheet or stock. The subject of this patent is to provide increased stock thickness when using the thermoforming or blow molding process. 
     Simple common thermoforming utilizes a single sheet of plastic that is heated then molded utilizing vacuum or pressure or both to draw the material into a one sided mold. Once formed, one side of the plastic is against the mold and the other side is open to the atmosphere for cooling. Typically this process is performed on a thermoforming machine of which there are many sizes and configurations. 
     There also exists a well-known thermoforming process called twin sheet molding. This processing method yields a finished part that is double walled, with inner and outer walls, with varying amounts of space between them. The two sheets of thermo plastic are married in the molding machine and typically fused together at particular areas in an invisible seam completely, or nearly so, around the perimeter and in various other areas advantageous to the application. Twinsheet thermoforming requires two sheets of plastic heated simultaneously and two molds mounted face to face in the molding machine. Typically twinsheeting is performed on a specially configured thermoforming machine of which there are many sizes. Any shape hollow part can be twinsheeted but the most typical application is large rectangular parts with only several inches or less separating the walls. Other processes are better suited for rounder more voluminous parts like tanks. The advantages of twinsheet thermoforming are numerous, the most important being the ability to make a hollow part that has considerable structural strength and the ability to have differing features on the top and bottom or opposite sides of the part. 
     Blow molding is a process that shares the same molding system as thermoforming, but the stock in the shape of a tube, called the parison, is extruded into a vertical standing open mold. When large flat parts are molded, a parison stretcher is employed. This mechanism essentially stretches the parison to extend beyond the edges of the mold. The extrusion process provides the heat for the blow molding process. When making a simple flat panel by blowmolding the mold is configured to provide two parts, face-to-face. 
     All three of the methods, simple and twinsheet thermoforming and blowmolding, require that the molded part be removed from the scrap material around its perimeter. This can be accomplished in many fashions but the preferable way when trimming parts is to rout them. 
     To enable the routing or other cutting of the parts from the scrap and to separate them, the molded parts must be shaped such that the router bit can separate the material but not cut through the primary part. Additionally, two primary parts as molded must have adequate distance between them to allow access for the trimming equipment. The preferable way to accomplish this is to have the material that creates a twinned band depart from the primary part at a 45 degree angle and then move at a steeper angle away until it becomes the primary part from the opposite cavity—thus providing the tool access space. The steeper angle minimizes the overall length of the tooling, saving cost of the raw material and the tooling. 
     SUMMARY OF THE INVENTION 
     The invention is a method of simultaneously molding multiple plastic parts and includes providing a first mold portion and providing a second mold portion facing the first mold portion. The first and second mold portions are offset such that at one or more edges of each of two of the molded parts to be produced by the mold portions an increased thickness area of each part is produced adjacent to a reduced thickness area of each part. Plastic material between the mold portions is molded by pressing two layers of plastic together in the increased thickness areas of each part with each layer forming the reduced thickness area of one of the parts so as to produce the two molded parts connected together by plastic material. The molded parts are separated from one another by cutting the plastic material connecting the parts together so that each molded part includes at least one area of increased thickness and at least one area of reduced thickness. 
     Objects and advantages of the invention will appear in the detailed description and claims which follow. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross sectional view of two mold portions molding two parts (dumpster lids) according to the present invention; 
         FIG. 1B  is an enlarged view of one end of the still connected dumpster lids of  FIG. 1A  which shows the area of increased thickness on one end of part  10 A; 
         FIG. 1C  is a cross sectional view similar to  FIG. 1B  of the still connected dumpster lids of  FIG. 1A  after being demolded and illustrating where the composite part could be cut to separate the two parts; 
         FIG. 2A  is a perspective view of a dumpster lid made according to the invention as illustrated in  FIG. 1A ; 
         FIG. 2B  is a detail cross-sectional view of the lid of  FIG. 2A ; 
         FIG. 3A  is a perspective view of the dumpster lid of  FIG. 2A  and showing a cutaway portion; 
         FIG. 3B  is a detail view of the cutaway portion of  FIG. 3A ; 
         FIG. 4A  is a perspective view of an animal shelter made using a method of the invention; and 
         FIG. 4B  is a cross-sectional detail view of the lower portion of the shelter of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a combination of single sheet and twinsheet thermoforming. The same method works with blowmolding. In the examples described, the parts that are formed are primarily single sheet with a band along one edge that needs to be thicker or twinsheet molded. 
     The present invention is practiced in one way by mounting two reconfigured single sheet style molds face to face in the thermoforming machine to form two parts. Each of the molds consists of two sections: the primary section that molds one surface of the complete part and the twin section that forms the second surface of the selectively twinned section of the opposite part. The molds are mounted with the primary sections opposite and offset the requisite amount to provide space for the twinning section to mold the twin section of the opposite part. The shape of the twinning section of the mold is such to provide the shape and thickness of the selective twinned portion of the part. 
     In particular, referring to  FIG. 1A , two mold halves  14 A,  14 B are arranged face to face in the thermoforming machine to form two dumpster lids  10 A,  10 B having respective hinge areas  12 A and  12 B. A finished dumpster lid  10  with hinge area  12  is illustrated in  FIGS. 2A and 3A . The two lids  10 A,  10 B are formed as a composite part but are separated after molding. Each mold half  14 A,  14 B includes a respective first mold portion corresponding to an outer surface of the lid to be formed by that mold half and a respective second mold portion corresponding to the double thickness band of the other finished lid. The length of each finished part is indicated by Finished Part A and Finished Part B. Each finished lid will include a portion having a single sheet (reduced) thickness and a portion having a double sheet (increased) thickness. In  FIG. 1A , the length of the band of double thickness, i.e., the selective twinning, is indicated for each part by BAND A and BAND B respectively. 
       FIG. 1B  shows an enlarged view of one end of the still connected dumpster lids of  FIG. 1A  and better illustrates the area of increased thickness on one end of what will become finished lid  10 A. For illustration purposes, the sheet material  11 A is delineated from the sheet material  11 B in the increased thickness portion. 
     Referring to  FIGS. 2A-2B  and  3 A- 3 B, in this example of dumpster lid  10 , the finished lid  10  might be 6′ long by 4′ wide—a typical real size of many different industry sizes. As shown in  FIG. 2A , in the finished lid  10 , hinge area  12  runs along one 4′ edge. Further, the finished lid  10  includes parallel ribs  16  running lengthwise, with ridges  17  between the ribs  16  extending widthwise. The hinge is created by drilling holes  18  ( FIG. 3A ) in the side surfaces of the ribs near one end of the lid so a steel hinge pin (not shown) can be inserted parallel to the width dimension. The end that will be hinged is slightly different in shape than the opposite end to accommodate the hinging motion. When this new offset twinsheet process is used, the area of the ribs  16  that are drilled for the hinge is molded double thick along the entire width of the lid and extending from the hinge end into the part for several inches, for example three inches. This will require the mold portions to be offset by the required three inches of overlap plus several more inches to accommodate stretching of the sheet and trimming requirements. In this example the required double thick area of 3 inches by 4 feet, the two sheets  11 A,  11 B of plastic are completely fused together with no space between the two walls. This is accomplished by having the two mold portions closely match with only the space between them equal to the combined thickness of the formed double sheets of plastic. Relief areas are provided to allow a place for excess material to flow to if necessary or if advantageous to the design—like the area around the hinge pin holes in this example. 
     As mentioned, the two lids  10 A,  10 B are formed as a composite part and are subsequently separated from each other. In particular, as indicated in the detailed view of  FIG. 1C , a router is shown to indicate the angled wall area where the lid  10 A could be separated from the lid  10 B. Other cut locations for separating the parts are also possible, with subsequent trimming to be performed if necessary. It should be understood that a similar cut would be made on the other end as well, with the cuts being made after the composite part is ejected from the molds. These cuts are made on the inner side of the double thickness area that is retained in the respective lid  10 A or  10 B. There would also be one or more cuts, indicated by  23 , made along the outside edge of the increased thickness portions to remove the scrap material  22 A and  22 B. Again, additional trimming may be necessary depending on the location of first cuts and the desired finished configuration of each part. 
     In the example of  FIGS. 4A and 4B , a shelter  50  is illustrated. In this case the selective double wall requirement results in a significant amount of the selectively twinned band area of increased thickness to have a space between two walls  58 ,  59  of plastic, in which a wood or composite base  60  is positioned and assembled to the plastic walls  58 ,  59  by a bolt or other fastener (not shown). The selectively twinned band, having a length indicated as BAND C, is formed in the same manner as described above, but trimmed at a different location as illustrated in  FIG. 4B . The entire shelter  50 , which might be 8-feet by 8-feet base dimensions and 6 feet high and consist of four wall panels  52 , one of which would be configured so as to provide an entry way  54 , and a roof  56 . To provide for easy assembly, and a strong mounting system to mount the walls to the structural base, the walls could have the bottom be double walled in a band of selective twinsheeting, with the band having a length of around a few inches to perhaps a foot or so, depending on the dimensions of the structural base member  60 . Unlike the completely fused band in the dumpster lid  10 , for this application a space or gap would be provided between the walls  58 ,  59  of the twin band equal to the width of the structural base member  60 , which may be for example wood or a composite material for structural rigidity. By having this lower portion of each wall capturing the base on both the outer and inner sides and cladding it from the top, the base is protected from the climate and animal waste. Also, if the side of the shelter  50  is impacted from either the inside or outside of the shelter, the force of the impact is born by the panel  52  rather than the fasteners. 
     To be cost effective, it is preferred that two identical or nearly so items be molded using the process of this invention. Typically both the parts would be molded using the same starting gauge material; the resulting area of selective twinsheeting would be identical on both parts and effectively double thick. The plastic sheet that is used will be larger than the size that would ordinarily be used to make the part by roughly the amount of selective twinsheeting. 
     Another variation of this process includes offsetting the two molds in two directions so that two adjacent edges of a part can be selectively twinsheeted. 
     It is anticipated that often an existing set of single sheet molds could be reconfigured and a twinning section added to utilize a method of the invention. 
     Preferred embodiments of the invention have been described in considerable detail. Many modifications and variations to these embodiments will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiments described herein.