Patent Publication Number: US-2009218730-A1

Title: Acrylon Plastics Inc.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rotational mold and a method of use of the rotational mold for forming a tank having a fill hole formed by the mold. 
     BACKGROUND 
     When forming tanks, it is common to rely on rotational molding in which starting material is placed within a mold which is heated and rotated such that the starting material takes the form of the mold to form a molded part. Upon removal from the mold, the molded part typically requires being subjected to the following post production work: i) mechanical removal of plastic material from the tank thereby creating a hole in the fill tube; and ii) removal of any debris from within the interior of the tank created by the first step. For a typical fuel tank, the time required to perform the foregoing could take between 20 minutes and 1.5 hours per tank. The duration of required trim/cleaning of the tank is dependent upon the specific tank design. The more intricate and complex the fuel tank design, the more time required to remove debris from the interior to ensure the tank is clean. If the tank is not clear of debris, this will cause a failure in the field resulting in an expensive service call to clear and correct the problem. In the application of fuel tanks, any debris in the tank causes the fuel pump to clog and shut down, thereby shutting down the vehicle preventing its use until such time as the tank is cleaned. 
     U.S. Pat. No. 5,415,316 belonging to Pemberton discloses a rotationally molded tank including a threaded neck opening formed by a mold including a main body portion for forming the main body of the tank and an inlet portion for forming the neck at an intended opening of the tank. Material is molded across the inlet opening during the rotational molding process subsequent to which a hole saw is required to form the opening in the neck of the tank. The additional step of sawing involves additional labour time and cost as well as contributing to the problems noted above with regard to debris within the interior of the tank which must be subsequently cleaned out. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a rotational mold arranged to form a molded part comprising a hollow body having a hollow interior and an externally threaded inlet tube having an opening therethrough in communication with the hollow interior of the hollow body and a peripheral rim about the opening, the mold comprising: 
     a main portion arranged to form the hollow body of the molded part; 
     an inlet tube portion arranged to form the inlet tube of the molded part, the inlet tube portion comprising:
         a collar having grooves formed about an inner surface thereof so as to be arranged to form external threads on the inlet tube of the molded part;   an annular end wall formed at one end of the collar so as to be arranged to form the peripheral rim about the opening of the molded part;   a shaft joined to the annular end wall and extending through the collar to define an annular gap between the inner surface of the collar and the shaft such that the shaft is arranged to form the opening in the inlet tube of the molded part; and   insulating material on the shaft spaced from the annular end wall, the insulating material being arranged to resist material of the molded part being formed thereon during rotational molding such that the opening is arranged to be substantially wholly formed by the shaft.       

     According to the present invention, it is possible to form a dimensional opening (a “fill hole”) within the threaded fill tube of a plastic fuel tank during the manufacturing process, such that any further work/modifications (i.e. “trimming”) is eliminated from having to be performed to the part subsequent to production when rotational molding. 
     By providing a rotational mold including an inlet tube portion with a shaft arranged to wholly form the opening in the finished part during the rotational molding process, no subsequent machining is required thus saving considerable time and cost as well as avoiding any potential problems associated with debris in a tank. 
     According to a second aspect of the present invention there is provided a method of forming a molded part comprising a hollow body having a hollow interior and an externally threaded inlet tube having an opening therethrough in communication with the hollow interior of the hollow body and a peripheral rim about the opening, the method comprising: 
     providing a rotational mold comprising a main portion and an inlet tube portion; 
     forming the hollow body in the main portion of the rotational mold; 
     forming the inlet tube in the inlet tube portion of the rotational mold by arranging the inlet tube portion to comprise a collar; 
     forming external threads on the inlet tube of the molded part by providing grooves about an inner surface of the collar; 
     forming a peripheral rim about the opening of the inlet tube by providing an annular end wall at one end of the collar; 
     forming the opening of the inlet tube by providing a shaft joined to the annular end wall which extends through the collar to define an annular gap between the inner surface of the collar and the shaft; 
     insulating the shaft at a location spaced from the annular end wall such that the opening in the inlet tube is wholly formed by the shaft; 
     filling the rotational mold with starting material; 
     heating and rotating the rotational mold. 
     The shaft is preferably insulated with Teflon™. 
     The annular gap defined between the shaft and the collar preferably has a suitable dimension such that the annular gap is arranged to be substantially filled with material during rotational molding. 
     The shaft is preferably concentric with the collar. 
     The shaft is preferably tapered such that a diameter of the shaft decreases from the annular end wall towards the insulating material. 
     When the insulating material spans an end of the shaft opposite the annular end wall, the insulating material and the shaft are preferably near in diameter to one another at a junction between the insulating material and the shaft. 
     The insulating material is preferably also tapered such that a diameter of the insulating material decreases with increasing distance from the annular end wall. 
     There may be provided an annular shoulder portion at one end of the collar opposite the annular end wall so as to be arranged to form a shoulder about the inlet tube of the molded part. In this instance, the insulating material is preferably positioned farther from the annular end wall than the annular shoulder portion such that the shaft is arranged to form an inner portion of the inlet tube of the molded part which projects into the hollow interior. 
     There may be provided a rotational molding vent for venting the molding in communication through the shaft. 
     In the accompanying illustrated embodiment, the main portion is arranged to form the hollow body of a fuel tank and the inlet tube portion is arranged to form the inlet tube of the fuel tank. 
     One embodiment of the invention will now be described in conjunction with the accompanying drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a prior art rotational mold of a tank including an inlet opening in which trimming is required subsequent to the molding process. 
         FIG. 2  is a sectional view along the line  2 - 2  of  FIG. 4  of the rotational mold according to the present invention. 
         FIG. 3  is a perspective view of the inlet tube portion of the rotational mold according to  FIG. 2 . 
         FIG. 4  is an interior end elevational view of the inlet tube portion of the mold. 
     
    
    
     In the drawings like characters of reference indicate corresponding parts in the different figures. 
     DETAILED DESCRIPTION 
     Referring to the accompanying figures there is illustrated a rotational mold generally indicated by reference numeral  10 . The mold  10  is particularly suited for forming a molded part  12  of the type including a hollow body  14  and an inlet tube  16  in communication with the hollow body for access into the interior thereof. In the illustrated embodiment the hollow body comprises a fuel tank and the inlet tube comprises a fill tube of the fuel tank. 
     The hollow body  14  of the molded part comprises walls which surround and fully enclose a hollow interior  18  of the part. The inlet tube  16  communicates through one of the walls surrounding the hollow interior of the main hollow body  14 . The inlet tube  16  is generally tubular projecting externally outward from the body. An external surface of the inlet tube is externally threaded for threadably receiving a cap thereon to enclose the body. An annular shoulder  20  is formed about the inlet tube at the base thereof where the inlet tube connects to the walls of the body in the form of an annular flat end face which lies perpendicular to an axis of the inlet tube. A free end of the inlet tube opposite the base thereof also defines an annular end face  22  which forms a peripheral rim about an opening  24  extending through the inlet tube in communication with the hollow interior of the main body. 
     The rotational mold  10  comprises a main portion  26  which is formed of plural sections  28  assembled together to form an enclosure of walls which surround and define the shape of the main body  14  of the molded part formed therein during rotational molding. An opening is provided in the main portion  26  which receives an inlet tube portion  30  of the mold. The inlet tube portion is arranged to form the inlet tube  16  of the molded part therein during the rotational molding process. The inlet tube generally comprises a collar  32  which is generally cylindrical in shape with an inner surface  34  thereof being arranged to form the outer surface of the tube  16 . The inner surface  34  is internally grooved by a helical pattern of grooves  36  which are arranged to form external threads on the inlet tube  16 . The inner surface  34  has a near constant interior diameter along a length of the collar in the axial direction thereof. 
     An interior end face  38  of the collar is generally flat and annular in shape so as to lie generally perpendicular to an axis of the collar. The end face includes an inner portion extending about a circumference of the collar which defines an annular shoulder portion  40  of the mold which is arranged to form the shoulder  20  on the molded part about the base of the inlet tube. The end face  38  of the inlet tube portion of the mold also includes an outer portion  41  extending circumferentially about the collar radially outward from the inner portion which is suitably arranged for abutment against a peripheral rim about the opening in the main portion  26  of the mold which receives the inlet tube portion of the mold therein. 
     The inlet tube portion of the mold also includes an annular end wall  42  which spans one end of the collar  32  opposite the annular shoulder portion  40 . The annular end wall  42  forms an end face of the inlet tube  16  as the end wall  42  spans radially inward from the inner surface of the collar towards the collar axis to lie substantially perpendicular to the axis of the collar. The flat end face formed by the end wall  42  on the molded part comprises the flat peripheral rim of the inlet tube which lies in a plane which is perpendicular to the inlet tube axis and which extends circumferentially about the opening  24  of the inlet tube. 
     The inlet tube portion of the mold further includes a shaft  44  mounted on the annular end wall  42  to extend axially through the collar  32  concentrically therewith. The shaft  44  is jointed at one end to the end wall to project inwardly therefrom beyond the end face  38  of the collar so as to project into the hollow interior of the main portion  26  of the mold. The shaft  44  defines an annular gap  46  between the inner surface of the collar and the outer diameter of the shaft to form the inlet tube of the molded part therebetween. The gap is arranged to have suitable dimensions so that the gap is fully filed with moldable material during the rotational molding process. The opening through the inlet tube in the finished molded part is formed by the shaft such that the outer diameter of the shaft corresponds to the inner diameter of the finished opening in the finished part after molding and cleaning operations have been completed. 
     The diameter of the shaft tapers so as to be reduced in diameter with increasing distance of the shaft from the annular end wall  42  upon which it is joined. The gap between the shaft and surrounding collar thus has increasing radial dimension from the end wall  42  towards the free inner end of the shaft within the interior of the main body for facilitating the flow of molding material into the gap during the rotational molding process. 
     A cap  48  is mounted at the inner end of the shaft opposite the annular end wall  42 . The shaft comprises Teflon which acts as a insulating material and a non-stick material to resist any molding material being deposited thereon during the rotational molding process. The cap  48  of insulating Teflon material is formed to be substantially continuous with the shaft to extend in the axial direction of the collar into the hollow interior of the main body from the end of the shaft. The cap  48  and the shaft  44  are near in diameter to one another at the junction therebetween. In the illustrated embodiment the cap is shown to be slightly larger in diameter than the narrowest diameter of the shaft at the junction thereof by a dimension which corresponds to less than the amount of shrinkage of the molded part during cooling after the rotational molding process so that the cap can still be released by sliding removal through the molded inlet tube formed by the shaft during the molding process. The cap  48  is similarly tapered so as to be reduced in diameter from a greatest diameter at the junction with the shaft to decrease with increasing distance from the end wall to a narrowest diameter at the free inner end thereof. The cap resists the formation of material during the molding process on the end of the shaft  44  so that the finished opening  24  of the finished part  12  is formed wholly by the shaft during the molding process without any subsequent machining being required. 
     The shaft extends through the collar from the end wall  42  in an axial direction so as to be longer than the collar, thus defining an inner portion of the shaft which projects inwardly into the hollow interior of the main body of the mold beyond the annular shoulder  20  formed by the end face  38 . The insulating material of the cap is thus positioned farther from the annular end wall  42  than the annular shoulder portion  38  of the mold such that the inner portion of the shaft is arranged to form an inner portion  49  of the inlet tube of the molded part which projects into the hollow interior of the main body. 
     The shaft  44 , the annular end wall  42  and the collar  32  are all formed integrally with one another so as to be continuous of like material, for example aluminium or other suitable material for forming a mold during a rotational molding process. For ease of manufacturing, in the illustrated embodiment the shaft is secured to the end wall be fasteners inserted through the exterior side of the end wall  42 . 
     The rotational mold is arranged to be vented through the shaft  44  and end wall upon which it is supported. Venting is provided by a rotational molding vent tube  50  formed integrally with the Teflon cap  48  at the end of the shaft. The vent tube is mounted concentrically through the shaft and the end wall by press fitting the tube into a bore through the shaft and end wall. The vent tube  50  is filled with conventional material used in rotational molding to prevent escape of molding material from the mold through the vent tube during rotational molding. 
     The exterior of the end wall  42  includes suitable protrusions  52  formed thereon which extend axially outward for abutment with a suitable clamping mechanism to hold the inlet tube portion of the mold spanning across the opening in the main portion of the mold during the molding process. 
     In use, a user first assembles the various portions of the mold together such that the inlet tube portion is clamped to span across the opening in the main portion. Starting material in the form of suitable plastic powder and the like is placed in the mold. The mold is heated and rotated in the conventional manner of rotational molding so that the plastic powder forms a layer adhering to the inner surfaces of the mold with the exception of the Teflon surfaces. The tapering dimension of the gap between the shaft and the collar of the inlet tube portion of the mold is suitably sized to be substantially filled with the molding material during the rotational molding process. Venting occurs through the vent tube  50  in the inlet tube portion of the mold. After the mold has sufficiently cooled, the inlet tube portion of the mold is first unscrewed for separation from the main portion of the mold. Separation of the main portion sections permits the finished part to be removed from the mold without any further machining modification being required to the opening in the inlet tube due to the configuration of the inlet portion of the mold. 
     According to the present invention a trim less “fill hole” in a plastic fuel tank can be created during the molding process. This is accomplished by fitting the internal cavity of a fuel tank mold with an aluminum and Teflon fixture in such a fashion as to accommodate same. The aluminum fixture is machined to accommodate the fuel cap thread design. In order for the thread design to be obtained during production and the trim less fill hole created, a tapered shaft of aluminum and Teflon is added to the aluminum fixture. 
     The tapering of the aluminum shaft allows the resin to flow freely during the manufacturing process and correctly set up the threads required to accept a fuel cap. In the absence of the shaft, the hole must be mechanically cut open and the tank cleaned of debris. Similarly, in the absence of a tapered shaft, i.e. say a straight shaft, the resin will not flow freely and the threads may not set up properly causing the tank to be scrapped. 
     The taper of the shaft should be greater than 5 degrees. The greater the degree of the taper, the more flow of plastic during fabrication, and thus the greater the likely hood of ensuring the threads set up correctly. However, the greater the taper the narrower the interior hole on the inside which must be considered in the application. 
     The use of Teflon in the application is to control the depth of the interior throat wall created. Plastic will not stick and form where the Teflon is. 
     The embodiment described herein relates to the creation of the fill hole of a fuel tank, however, it could be applied and used in the creation any hole in a tank so long as the interior cavity of the tank is able to accommodate the interior plastic wall created during the process. 
     The innovation specifically relates to the application and use of Paxon 7203 plastic resin “Cross Link”. The application to cross link resin is important, as unlike most other plastic resins, whereby the location of a hole can be obtained by thinning the plastic wall during production to such a point as it can be easily removed by simply manually pealing the section away by hand. Cross Link once formed, cannot be peeled off and must be mechanically removed. 
     The innovation will work with other plastic resin such as Polyethylene. 
     The size of the opening or tank is not relevant. The innovation can be applied to any hole. 
     An added advantage of the innovation is that the surface area obtains a smooth finished look. In some applications, additional work would be required to obtain same were it not for the innovation described herein. 
     Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.