Abstract:
A method allows production of integral frame structures having at least three structural portions separated circumferentially about a central axis and at least one hollow region radially inward thereof. Each of plural mold sections fits between structural potions, and a portion of the mold interior includes a wedge shaped portion which allows the mold sections to slide past the structural portions when molding is complete.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a manufacturing method and article produced thereby, and more particularly to a method of manufacturing a three-dimensional structure having hollow portions and structures produced in accordance with the method.  
         [0002]     In order to produce an integrally molded product, various molding techniques have been conventionally employed. Blow molding offers one way in which a hollow product is produced. In accordance with such approach, a “bubble” of plastic is expanded into a shaped mold cavity, thereby assuming a hollow form corresponding to the interior topography of the cavity walls. While effective for producing structures having a relatively thin wall, such as bottles and the like, blow molding cannot be used to produce structures having substantial thickness. The process is further limited to the extent that if openings in the hollow structure were desired, such as provided to form an open frame structure, an additional fabrication step would be required.  
         [0003]     Rotational molding (roto-molding), a process which involves rotation of a mold to deposit molding material on an interior of a mold has been used to produce various hollow structures, but is also limited with regard to what materials can be used in the molding process, and generally involves relatively long cycle times. As with blow molding, finishing of the molded articles, particularly those with openings defining a frame structure, generally requires additional steps after molding.  
         [0004]     Notwithstanding the above approaches, it has remained difficult heretofore to produce a structure having hollow inner portions which is integrally molded as a single part, particularly utilizing injection and compression molding techniques.  
         [0005]     It would therefore be desirable to provide a method of molding structures having hollow interior portions which is applicable to injection molded and compression molding processes.  
         [0006]     Accordingly, it is an object of the invention to provide a molding method for hollow articles of integral unitary structure which overcomes the drawbacks of the prior art, and which is applicable to injection and compression molding techniques.  
         [0007]     It is a further object of the invention to provide an article of unitary structure which includes a hollow portion inward of a portion of the structure comprising the article.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with these and other objects of the invention, there is provided a method which permits the molding of three-dimensional articles which include at least one hollow region disposed radially inward of structure comprising the article.  
         [0009]     Briefly stated, the method includes providing a mold for producing an article having at least three structural portions separated circumferentially from one another about a central axis of the article, the mold having a number of mold sections corresponding to the number of structural portions to be produced thereby. The article further includes at least one hollow region radially inward of at least one of the structural portions, the hollow region being formed during molding of the article by engagement of respective mold sections which serve to form the particular structural portion(s) having the hollow region inward thereof. The shape of the mold cavity corresponding to the structural portion associated with the hollow region allows the corresponding mold sections to be withdrawn after molding is complete, by permitting clearance, of that portion of each of the mold sections which mutually engages with the other to form the hollow region, past the structural portion which is radially outward of the hollow region, and which would otherwise impede movement of the mold section outwardly of the article. This is accomplished by providing a portion of the mold interior (and hence the article produced thereby), with a wedge portion having a wedge-shaped profile which faces radially inward towards the hollow region. Such wedge-shaped profile of the wedge portion is provided with an angle no larger than an angle described between the axes along which the respective mold sections are withdrawn after molding of the article, thereby allowing the structure of the mold which forms the hollow region of the article to slide past the structural portion of the article disposed radially outward thereof.  
         [0010]     The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of an example of an article having four circumferentially-spaced structural portions disposed radially outward of an inner hollow region which can be produced by the method according to the invention;  
         [0012]      FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 ;  
         [0013]      FIG. 3  is a cross-sectional schematic view of a mold for use in carrying out the method in accordance with the invention for producing the article example of  FIG. 1 ;  
         [0014]      FIG. 4  is a cross-sectional schematic view of a mold for use in carrying out the method in accordance with the invention for producing an article having a structural form roughly analogous with that of  FIGS. 1 and 2 , but having three rather than four structural portions disposed outwardly of a hollow inner region;  
         [0015]      FIG. 5  is another example of an article which can be produced by the method in accordance with the invention; and  
         [0016]      FIG. 6  is a perspective view of yet another example of an article which can be produced by the method in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     Referring now to  FIGS. 1 and 2 , an example of an article which can be produced according to the method of the invention is generally designated  10 . Article  10  includes four circumferentially-spaced structural portions  11 , and a hollow region  12  inward thereof. Structural portions  11  in the depicted example each meet at common poles aligned with a central axis A L , however, the inventive method is not, by any means, limited to such arrangement. Each of the structural portions  11  includes a wedge portion  13  having a wedge-shaped profile which faces radially inward towards the hollow region  12 . The purpose of this wedge portion  13 , which lies at the heart of the invention, will be explained in detail below, with reference to description of the molding process used to form the depicted example of  FIGS. 1 and 2 , as well as a vast array of other structural articles which include hollow portions inward of solid structural portions of the article.  
         [0018]     Turning now to  FIG. 3 , a mold for producing the example of  FIGS. 1 and 2 , generally designated  20 , is depicted for illustrative purposes only, and it will be understood that the principles employed with respect thereto can be applied by analogy to any other mold used to produce any other article of different configuration and shape. Mold  20  includes four mold sections  20   a,    20   b,    20   c,    20   d,  corresponding in number to the number of structural portions  11  of article  10 . Each of the mold sections  20   a,    20   b,    20   c,    20   d  mutually engages an adjacent one thereof and adjacent pairs include cavity portions  21  which collectively define a mold cavity portion for producing a corresponding structural portion  11  of article  10 .  
         [0019]     The mold sections  20   a,    20   b,    20   c,    20   d  are moved inwardly and outwardly along respective molding axes A 1 , A 2 , A 3 , A 4  during mold cycling. As seen in the depicted example of  FIG. 3 , portions  22  of the mold sections  20   a,    20   b,    20   c,    20   d  inward of the cavity portions  21  mutually engage during molding to prevent flow of molding material therebetween for formation of the hollow region  12 . When molding is complete, the mold sections  20   a,    20   b,    20   c,    20   d  are moved outwardly along molding axes A 1 , A 2 , A 3 , A 4  to release the article  10  form the mold  20 . Cavity portions  21  each includes a wall  23  which, collectively with another wall  23  of a cavity portion  21  of an adjacent one of the mold sections  20   a,    20   b,    20   c,    20   d,  corresponds to the wedge-shaped profile  13  of the structural portion  11  of the article  10 . As shown, the walls  23  of adjacent ones of cavity portions  21  are advantageously arranged at an angle to one another which is less than or approximately equal to an angle between the respective ones of the molding axes A 1 , A 2 , A 3 , A 4 . This permits the portions  22  of the mold sections used to form the hollow region  12  to be withdrawn past the structural portion  11  of the formed article  10  to allow removal of the article  10  from the mold  20 .  
         [0020]     It is noted that when the article  10  is molded from a resilient material which demonstrates some “give,” the angle between the walls  23  of adjacent ones of cavity portions  21  may be a little larger than the respective ones of the molding axes A 1 , A 2 , A 3 , A 4 , since the structural portions will yield even though presenting an undercut. It is further noted that, although shown as defined by planar intersections, the wedge portions  13  can comprise curved surfaces as long as there is sufficient clearance to remove the mold sections  20   a,    20   b,    20   c,    20   d  past the structural portions  11 .  
         [0021]     Turning now to  FIG. 4 , another mold, designated  30 , is directed to production of an article  10 ′ which includes three structural portions  11 ′. In the depicted example, for simplicity of disclosure, the structural portions  11 ′ of article  10 ′ are circumferentially spaced by equal angular intervals. Each of the mold sections  30   a,    30   b,    30   c  move along respective molding axes A′ 1 , A′ 2 , A′ 3  which are arranged at an angle θ 1  to one another, where θ 1  is equal to 120°. Therefore, in accordance with the invention, each of the structural portions  11 ′ is provided with a wedge portion  13 ′ presenting a wedge-shaped profile describing an angle θ 2  which is less than or approximately equal to θ 1  (assuming the material of the structural portion can not be appreciably compressed).  
         [0022]     While the aforementioned and described articles have a generally spherical shape, the invention is not limited to such configurations. For example, the article of  FIG. 5 , designated  40 , can be produced by the method in accordance with the invention, provided the structural portions  11 ″ are provided with wedge portions  13 ″.  
         [0023]     The invention is also suitable for use in producing more complex structures, as shown for example in  FIG. 6 , and which employs the principles described above, i.e., the provision of wedge portions on the various structural portions radially outward of a hollow region.  
         [0024]     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.