Method of producing undercut tubular plastic articles

Abstract

Undercut tubular synthetic plastic articles, such as fittings, are formed in the mold cavity of an injection molding machine on a two-piece core a tubular component of which is surrounded by the undercut portion of the article and a first smaller-diameter portion at one side of the undercut portion. A cylindrical component of the core slidably guides the tubular component and is surrounded by a second smaller-diameter portion of the article at the other side of the undercut portion. When the injection of plasticized material is completed, the housing of the mold is opened to expose the undercut portion and the first smaller-diameter portion of the article on the core, the first smaller-diameter portion and the undercut portion are heated to or maintained at a temperature at which they can undergo elastic deformation, and the tubular component is moved axially of the article and of the cylindrical component so that the first smaller-diameter portion of the article is expanded by an external ring-shaped projection which forms part of the tubular component and was surrounded by the undercut portion. The first smaller-diameter portion of the article thereupon contracts so that its inner diameter is again less than the inner diameter of the undercut portion. The article is allowed to harden prior to complete separation from the core, and the first smaller-diameter portion is held against excessive contraction during hardening of the article.

What is claimed is: 
     
       1. A method of making a tubular pipe fitting of the type having a longitudinally extending axis and having a predetermined wall thickness throughout and wherein two outer circumferentially extending annular portions having first inner diameters flanking an intermediate circumferentially extending annular portion having a larger second inner diameter defining a circumferentially extending annular groove for receiving a gasket, packing or the like, the difference between the radii of said outer annular portions and the radius of said intermediate annular portion being within a range of more than to at least equal to said thickness, comprising the steps of introducing plasticized synthetic plastic material into a mold whose cavity has a configuration matching that of the fitting, the mold being shiftable between an open position to facilitate removal of the fitting and a closed position to form the cavity and facilitate formation of the fitting, the mold cavity being defined by a core having a first core part for cooperating in forming one of the outer annular portions and a second core part for cooperating in forming the other of the annular portions together with the intermediate annular portion, the core parts being axially shiftable relative to one another between a first position at which the core parts cooperate in forming the mold cavity and second position at which the second core part is adapted to be stripped from the intermediate annular portion of the fitting, the mold cavity also being defined by a first outer mold part radially shiftable between a first position at which it cooperates in forming part of the cavity for one of the outer annular portions and part of the second intermediate annular portion and second radial position away from the fitting to permit stripping of the core, the mold cavity also being defined by a second outer mold part shiftable axially between a first position at which it cooperates in forming part of the cavity for the other of the outer annular portions and the remainder of the intermediate annular portion and a second axial position away from the fitting to permit stripping of the core, the mold cavity surrounds the core including a larger portion with an outer diameter corresponding to said second inner diameter and two smaller portions flanking said larger portion and having outer diameters corresponding to said first diameters whereby the introduced material is converted into said fitting; exposing the exterior of the fitting by shifting the first and second outer mold parts to their respective second position, at least in the region of said intermediate portion and one of said outer portions; maintaining the latter one outer portion of the exposed fitting in a state in which it can undergo elastic deformation; and moving the fitting and the second core part relative to each other in a direction to force the larger portion of the core through said latter one outer portion of the fitting whereby this one outer portion expands during the passage of said larger portion therethrough and thereupon contracts so that its inner diameter is again less than the inner diameter of said intermediate portion. 
     
     
       2. A method as defined in claim 1, wherein said introducing step includes injecting said plastic material into the cavity. 
     
     
       3. A method as defined in claim 1, wherein said plastic material is a thermoplastic substance and said maintaining step includes maintaining the temperature of said one outer portion within a predetermined range. 
     
     
       4. A method as defined in claim 1, further comprising the first additional step of causing the material of the entire article to set and the second additional step of thereupon completely withdrawing the core from the article. 
     
     
       5. A method as defined in claim 4, further comprising the step of internally supporting at least said one outer portion of the article in the course of said first additional step. 
     
     
       6. A method as defined in claim 1, wherein said maintaining step includes maintaining said one outer portion of the article at a temperature which exceeds the temperature of the other outer portion of the article. 
     
     
       7. A method as defined in claim 1, further comprising the step of establishing a layer of gaseous fluid between said core and at least a portion of the surrounding article in the course of said moving step. 
     
     
       8. A method as defined in claim 7, wherein said gaseous fluid is compressed air. .Iadd. 9. A method for forming a hollow thermoplastic article, having an enlarged bell end with an inner annular groove therein, and a body portion having at least a longitudinal section thereof of smaller inner diameter than the diameter of said inner annular groove of said bell end, which comprises the steps of: introducing molten plastic into a hollow article forming mold means including a cavity means and core means which cavity and core means together, in closed position, define said bell end and said body portion, said core means being movable, with respect to said cavity means, from a closed position to at least one partially retracted position and thence to a fully retracted position clear of said cavity means and said cavity means being openable to enable said article to be ejected from said cavity means;   cooling the molten plastic of at least a first part of said body portion, while said mold means is in closed position, to a first temperature range within which said plastic forming said first part of said body portion is substantially rigid and non-deformable and,   cooling the molten plastic forming said bell end, while said mold means is in closed position, to a second temperature range in which said plastic forming said bell end remains resiliently deformable, said second temperature range being substantially higher than said first temperature range;   in sequence, partially retracting said core means to thereby remove said core means defining said bell end with some elastic deformation of said bell end while the plastic forming said bell end lies within said second higher temperature range and is still readily deformable and said first part of said body portion lies within said first lower temperature range and then, fully retracting said core means; and   ejecting said formed thermoplastic article from said cavity means before said bell end has cooled to a substantially rigid condition to allow said bell end to recover to its undeformed shape. .Iaddend. .Iadd. 10. The method of claim 9 which includes the step of opening said cavity means just prior to said partial retraction of said core means. .Iaddend..Iadd. 11. The method of claim 9 wherein said plastic material is PVC. .Iaddend. .Iadd. 12. The method of claim 9 wherein cooling fluid is passed through that portion of the core means defining said first part of said body portion of said hollow article to cool said first part of said body portion to temperatures within said first temperature range at the time of partial retraction of said core means. .Iaddend..Iadd. 13. A method for forming a thermoplastic article having an enlarged bell end with an inner groove and a body portion having at least a section thereof of smaller inner diameter than the diameter of said inner groove, which comprises the steps of:   heating thermoplastic material to a temperature where said material is resiliently deformable;   molding said material to form said article by disposing the heated thermoplastic material around a first and second molding means, said first molding means forming said bell end of said hollow and said second molding means forming said body portion of said hollow;   cooling said second molding means to cause the thermoplastic material adjacent to said second molding means to cool to a temperature range where said thermoplastic material is substantially rigid and non-deformable;   cooling said first molding means to cause said thermoplastic material adjacent to said first molding means to be cooled to a second temperature range in which said thermoplastic material remains resiliently deformable, said second temperature range being substantially higher than said first temperature range;   withdrawing said first molding means causing elastic deformation of said bell end; and   withdrawing said second molding means and allowing said bell end to recover to its undeformed shape before said bell end has cooled to a substantially rigid condition. .Iaddend.