Abstract:
A pressure vessel includes a polyolefin extruded tube, a polyolefin flange overmolded onto the tube, and a cap engaged to the flange. In the overmolding process, a polyolefin material is overmolded unto a polyolefin substrate.

Description:
This application is a divisional of application U.S. Ser. No. 08/719,668 filed Sep. 25, 1996, now U.S. Pat. No. 5,916,647. 
    
    
     FIELD OF THE INVENTION 
     The instant invention is directed to a method of making a pressure vessel using an overmolding process in which a polyolefin is overmolded onto a polyolefin; and the pressure vessel made thereby. 
     BACKGROUND OF THE INVENTION 
     Pressure vessels made with polyolefin housings are known. See: “LIQUI-CEL® Extra-Flow 4″×28″ Membrane Contactor For Process Scale” 1994 Product Bulletin. The housings of these vessels, however, must be machined to create the flanges to which the end caps are secured. Machining of the housings is expensive. A less expensive method for the production of the housings is necessary. 
     The overmolding process is known. See: U.S. Pat. No. 5,409,655; French Patent No. 2,550,848; and Rohner, M. L. “Injection Weldable Nylons” Society of Plastics Engineers, Inc. Rochester, N.Y., Sep. 15-16, 1993. Typically, in an overmolding process, an elastomer is overmolded onto a substrate (the substrate may be made of metal or rigid plastic). In U.S. Pat. No. 5,409,655, KRAYTON®, a thermoplastic elastomer sold by Shell Oil Company of Houston, Tex., is overmolded onto NORYL®, a modified nylon material sold by General Electric Company of Fairfield, Conn. In French Patent No. 2,550,848, a thermoplastic elastomer (an elastomer based on polypropylene and an ethylene-propylene copolymer) is overmolded onto a polypropylene. In the Rohner article, injection welding grade nylons are overmolded onto nylons. 
     It is believed by some in the plastic&#39;s industry that overmolding of a polyolefin (i.e., the overmolded material) onto a polyolefin (i.e., the substrate or also referred to as an insert) is not possible because a melt interface between the two parts can not be formed without overheating and thereby distorting (e.g., warping or caving in) the substrate. Accordingly, there is a need for a process by which a polyolefin can be overmolded onto a polyolefin substrate. 
     SUMMARY OF THE INVENTION 
     A method for overmolding a polyolefin onto a polyolefin substrate and for making a pressure vessel includes the steps of: 
     providing a polyolefin substrate; 
     placing the substrate into a mold; 
     supporting the substrate to prevent distortion; 
     injecting an unfilled polyolefin into a mold and over a surface of the substrate; and 
     releasing an overmolded part from the mold. 
     A pressure vessel includes: a polyolefin extruded tube; a polyolefin flange overmolded onto said tube; and a cap engaged to said flange. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is a plan view of a pressure vessel with parts broken away. 
     FIG. 2 is a plan view of the extruded tube from which the housing of the vessel is formed with parts broken away. 
     FIG. 3 is a cross sectional view of the overmolded section that forms a part of the housing of the vessel. 
     FIG. 4 is a plan view of the housing with parts broken away. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, wherein like numerals indicate like elements, a pressure vessel  10  is shown in FIG.  1 . Pressure vessel  10  includes a housing  12 , an end cap  14 , and a clamp  16 . Housing  12  comprises an overmolded section  18  having a flange  20 , and an extruded tube  24 . 
     Pressure vessel  10  (i.e. housing  12  and preferably cap  14 ) is made from a polyolefin material. Clamp  16  may be made of a metal or plastic material. Overmolded section  18  is welded to extruded tube  24  along a weld line  22 . Welding of the overmolded section  18  to extruded tube  24  is accomplished by an overmolding process (described herein below). 
     Pressure vessel, as used herein, refers to an enclosure capable of withstanding pressures up to 120 psi gauge. These pressure vessels may be used in hollow fiber membrane contactors disclosed in U.S. Pat. Nos. 5,264,171 and 5,352,361, which are incorporated herein by reference. Polyolefin, as used herein, refers to a class or group name for thermoplastic polymers derived from simple olefins; these polyolefins-specifically exclude elastomers. The polyolefins will be discussed in greater detail below. 
     Referring to FIG. 2, the housing  12  is shown. Housing  12  is formed, in part, from an extruded tube  24 . Each end of extruded tube  24  has a machined section  26 . Machined section  26  is formed with a surface that facilitates a weld line formation between machined section  26  and overmolded section  18 . Machined section  26  is preferably formed by cutting, as on a lathe, but could be molded instead. The extruded tube  24  is made from an unfilled, (or neat) polyolefin material. Unfilled, as used herein, refers to the lack of or the substantial lack of a material that acts as a mold release agent or a polymer flow agent or an internal polymer lubricant. The polyolefin material may be either an extrusion grade or injection moldable grade polymer, but preferably is extrusion grade polymer. The polyolefin material should have a melt flow index (ASTM D1238) of less than 5 and a specific gravity (ASTM D792A-2) of about 0.8 or greater (the specific gravity being indicative of the materials strength). Preferably, the polyolefin material has a melt-flow index of less than 1 and a specific gravity of about 0.9 or greater. The polyolefin material may be any thermoplastic polymer, e.g. polypropylene, and maybe either a homopolymer or a copolymer. Suitable polyolefin materials include PRO-FAX 6523 or 7823 polypropylene resins available from Montell USA of Wilmington, Del. 
     Referring to FIG. 3, overmolded section  18  is illustrated. Overmolded section  18  comprises a flange  20 , a neck section  28 , and a female mating section  30 . Female mating section  30  is adapted for hermatically sealing engagement (e.g. welding) with machined section  26  of extruded tube  24  via weld line  22 . Overmolded section  18  is formed in a mold during the overmolding process. The mold is discussed in greater detail below. Overmolded section  18  is made from an unfilled (or neat) polyolefin material. Unfilled, as used herein, refers to the lack of or the substantial lack of a material that acts as a mold release agent or a polymer flow agent or an internal polymer lubricant. The polyolefin material may be either an extrusion grade or injection moldable grade polymer, but preferably is an extrusion grade polymer. The polyolefin material should have a melt flow index (ASTM D1238) of less than 5 and a specific gravity (ASTM D792A-2) of about 0.8 or greater (the specific gravity being indicative of the materials strength). Preferably, the polyolefin material has a melt flow index of less than 1 and a specific gravity of about 0.9 or greater. The polyolefin material may be any thermoplastic polymer, e.g. polypropylene, and maybe either a homopolymer or a copolymer. Suitable polyolefin materials include PRO-FAX 6523 or 7823 polypropylene resins available from Montell USA of Wilmington, Del. 
     Referring to FIG. 4, overmolded section  18  is shown in engagement with extruded tube  24  via weld line  22 . 
     In manufacture, extruded tube  24  is, preferably, machined to form machined sections  26 . The machined extruded tube  24  is inserted into a mold. The mold (not shown) is adapted to receive the machined section  26  of the extruded tube  24  and support the internal surface thereof. The mold is also adapted to form the overmolded section  18  via an injection technique. The fabrication of this multi-parted mold is within the skill of the art. With the multi-parted mold in place, a polyolefin material is injected, at the resin&#39;s suggested use temperature, into the mold, and the overmolded section  18  is formed thereby. At the same time, the overmolded section  18  is welded to the machined section  26  of the extruded tube  24 . Thereafter, the housing  12  (i.e. tube  24  with integrally form overmolded section  18 ) is released from the mold. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of their invention.