Abstract:
A method of releasing pressure in a non-metallic injection mold has a pressure relief valve positioned in the molten resin flow path for handling excess resin pressure. The pressure relief valve responds to excess resin pressure in the cavity by providing an alternate resin flow path for the pressurized excess resin.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. application Ser. No. 09/452,930, filed Dec. 2, 1999, by Radzio, et al., and entitled, “Injection Molding Apparatus For A Non-Metallic Injection Mold,” and U.S. application Ser. No. 09/453,760, filed Dec. 2, 1999, by Radzio, et al., and entitled, “Pressure Relief Valve For Non-Metallic Injection Molds,” now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of injection molding. More particularly, the invention concerns an injection molding method having a pressure relief valve that provides an alternate resin passage for excess molten resin under a predetermined pressure in a non-metallic injection mold. 
     BACKGROUND OF THE INVENTION 
     Cost reductions in low to mid volume injection molding operations has led to the use of one piece case epoxy molds as an alternative to more expensive steel or aluminum tooling. Several have developed techniques to produce production injection molded parts using these cast epoxy molds. The epoxy molds are cast as a single piece for each half of the mold as opposed to hard tooling that is machined out of steel or aluminum. The main limiting factor of the epoxy mold is its inability to withstand the normal cavity pressures created within a conventional mold. Epoxy molds will only tolerate 10%-15% of the pressure that a steel or aluminum mold will tolerate. But they still require adequate pressure during injection and packing to produce an acceptable part. If cavity pressure should spike during injection or pack, the epoxy mold will fail catastrophically. This means that one half, or in some cases both halves, of the mold will split into pieces, effectively destroying the mold. 
     It is our experience that epoxy injection molds for molding low to mid volume production parts need protection against catastrophic failure. The method and apparatus for protecting these molds needs to be simple, adjustable for different molding resins, specific to each mold so that the mold is protected in any molding machine, and the method and apparatus needs to reset itself automatically whenever it is actuated to protect the mold. Several known attempts have been made to resolve these problems unique to non-metallic molds but most were rejected because of cost and the problem of modifying all the different molding machines that these molds can run in. 
     In U.S. Pat. No. 5,350,288, Sep. 27, 1994, by Kimoto et al, entitled Injection Molding Unit, an injection molding unit is disclosed that uses a pressure relief valve in a conventional metallic (steel) mold. A major shortcoming of the aforementioned valve arrangement is that if the relief valve operates and releases excess melt pressure from the mold, the mold and/or relief valve assembly will have to be disassembled to remove the solidified plastic material. Moreover, the relief valve operates as a part of the injection molding machine without any obvious means of adjusting the pressure. 
     Therefore, a need persists in the art for an injection molding method having a non-metallic mold that resists catastrophic failure under high molten resin pressure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of releasing pressure in an injection molding apparatus having a non-metallic injection mold for molding low to medium volume production parts without catastrophic failures. 
     Another object of the invention is to provide a method using an adjustable pressure relief valve that can be set at a predetermined value for accommodating molten resins having different flow characteristics. 
     It is another object of the invention to provide a method using an alternate flow path for accommodating excess molten resin produced by excess resin pressure. 
     It is a feature of the invention that a method for releasing pressure uses a pressure relief valve associated with the molding apparatus for providing excess molten resin access to the alternate molten resin flow passage when the pressure in the molten resin passage is beyond a predetermined value. 
     To achieve one or more of the above objects, features and advantages of the invention, there is provided in one aspect of the invention, a method of releasing pressure in an injection mold having a non-metallic mold and a first, molten resin flow path for receiving a molten resin, comprising the steps of: 
     providing a pressure relief valve having an outwardly extending, axially movable, biased pin member; 
     forming a second molten resin flow path in an adjacent structure associated with said pressure relief valve for excess resin flow away from said non-metallic mold for use when pressure in said first molten resin flow path exceeds a predetermined value, said alternate molten resin flow path being configurable for fluid communications with said first molten resin flow path; 
     positioning said axially movable, biased pin member at a terminal end of the first molten resin flow path so as to block the flow of resin beyond said terminal end; and, 
     ejecting a molten resin through said molten resin flow path of said non-metallic mold such that any excess molten resin in said molten resin flow path creates excess pressure on said pin member of said pressure relief valve thereby causing said biased pin member to retract away from said terminal end of said first molten resin flow path and then open said excess molten resin to fluid communications with said alternate resin flow path. 
     The method of the invention, therefore, has numerous advantageous effects over currently existing developments, including: parts can be molded in an epoxy injection mold without the prospects of mold failure; the pressure relief valve is adjustable for different resins (for instance PC requires a heavier spring and less adjustment pre-load than HIPS); the pressure relief valve is specific to the mold, i.e., once it is set up properly and adjusted for a specific resin, it operates to protect the mold in any molding machine that one might use; the pressure relief valve resets itself automatically after it actuates; and, any part molded when the pressure relief valve actuates will eject from the mold with the resin from the escape channel attached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
     FIG. 1 is a cross-section view of a typical injection molding machine; 
     FIG. 2 is a cross section of the injection molding apparatus of the invention having the pressure relief valve in a molten resin blocking position; and 
     FIG. 3 is a cross-section of the injection molding apparatus of the invention having the pressure relief valve in a molten resin unblocking position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and in particular to FIGS. 1-3, an injection molding apparatus  10  is illustrated. Broadly defined, according to FIG. 1, injection molding apparatus  10  has an injection molding machine  12  for injecting molten resin  14 . Skilled artisans will appreciate that injection molding machine  12  has a platen  17  supporting a screw cylinder  16  having a tip  18 , a nozzle  20  at the tip  18  and a screw  22  advanceable in the screw cylinder  16  for injecting molten resin  14  from the nozzle  20 . 
     According to FIGS. 2-3, the injection molding apparatus  10  of the present invention has a non-metallic injection mold  24  for molding a part (not shown). Non-metallic injection mold  24  comprises a stationary cavity mold  26  and a movable core mold  28  forming a hollow or first molten resin flow path  30  therebetween for forming an injection molded product therein. Core mold  28  is movable by ejector pins  29  arranged in mold  24  for forceably separating the hollow  30  from the core mold  28 . Non-metallic injection mold  24  may include various materials such as thermoset materials as well as cast epoxy, stereo lithography urethane and silicone. In the preferred embodiment, the injection mold  24  is constructed of cast epoxy. 
     Referring to FIG. 2, hollow or first molten resin flow path  30  extends from the screw cylinder  16  to a terminal end  35  of the hollow  30 . A pressure relief valve  36  is located on the hollow or first molten resin flow path  30  at the terminal end  35  of the hollow  30 . Pressure relief valve  36  is adapted to release the molten resin  14  from the first molten resin flow path  30  when the pressure of the molten resin  14  exceeds a predetermined level or value, further discussed below. 
     In FIG. 2, more particularly, pressure relief valve  36  is shown in a first position blocking molten resin flow beyond the terminal end  35  of the first molten resin flow path  30 . In this position, molten resin  14  is retained in the hollow or first molten resin flow path  30  by the pressure of movable pin  42 . The pressure of the molten resin  14  in this position is generally less than a predetermined value determined by the strength of a spring bias  44  (described below) biasing movable pin  42 . 
     Referring to FIG. 3, pressure relief valve  36  is shown in a second position unblocking the excess molten resin  14  in the hollow or first molten resin flow path  30  thereby enabling the excess molten resin  14  to flow into a second molten resin flow path  38  beyond the first molten resin flow path  30 . In this configuration, the first molten resin flow path or hollow  30  is in fluid communications with the second molten resin flow path  38 . As depicted in FIGS. 2-3, the pressure relief valve  36  has preferably a cylindrically shaped body with a movable pin  42  arranged for axial movements in one end  45 . A spring bias  44  is disposed between the movable pin  42  and a base plate  46  that affixes the spring bias  44  under the movable pin  42  in a biasing relations. In the preferred embodiment, an adjustment screw  48  is arranged in the base plate  46  for applying the pre-load to the spring bias  44 . Preferably, the entire pressure relief valve  36  is cast into the movable core mold  28  of an epoxy injection mold so that the movable pin  42  extends through the parting line  54  to shut off the second molten resin flow path  38 . The novel and unobvious design of the second molten resin flow path  38  on the surface of the parting line  54  allows the vented molten resin  14  to come out of the mold  24  with the part and then allows the pressure relief valve  36  to reset automatically. Skilled artisans will appreciate that pressure relief valve  36  may also be cast in the stationary cavity mold  26  of the mold  24 . 
     Referring to FIGS. 2-3, pressure relief valve  36  for epoxy injection molds  24  is adjustable by one of two ways. First, the adjustment screw  48  may be adjusted to apply more or less resistance on spring bias  44 . Second, the spring bias  44  may be selected having a pre-selected strength, as discussed further below. Although either means of adjusting pressure relief valve  36  has advantages over the other, we generally prefer adjusting the adjustment screw  48  to affect the resistance of the spring bias  44  that governs the movements of movable pin  42 . 
     Referring again to FIGS. 2-3, pressure relief valve  36  is actuated directly by the pressurized, molten resin  14  in the cavity mold  26 . As indicated above, movable pin  42  in the pressure relief valve  36  shuts off the resin escape channel or second molten resin flow path  38  under a predetermined pressure and is held in place by the spring bias  44 . When the cavity pressure exceeds the predetermined pressure limit the movable pin  42  pushes back against the spring bias  44  and allows the excess resin and pressure to escape the mold  24  through the second molten resin flow path  38  or escape channel. Not only does this vent the molten resin  14  out of the hollow  30 , but it also reduces the pressure in the cavity mold  26  below the cavity mold failure point. After the part cures (cools) in the mold  24 , the mold  24  opens and the part is ejected automatically. The molten resin  14  that flowed into the escape channel or second molten resin flow path  38  is ejected along with the part and the movable pin  42  resets itself automatically, shutting off the second molten resin flow path  38 . 
     We have demonstrated that the test epoxy mold  24  can successfully vent excess molten resin pressure before the cavity mold  26  fails. We were able to accommodate different materials that require different pressures of cavity mold  26  by changing the spring bias  44  in the pressure relief valve  36  and/or adjusting the pre-load on the spring bias  44 , as described. 
     An example of the process for selecting the proper strength spring bias  44  and pre-load adjustment for the cavity mold  26  and molten resin  14  is illustrated in Table 1. According to Table 1, the amount of pressure in the cavity mold  26  appeared fairly constant as a function of the spring bias  44  between minimum and maximum settings. Thus, one can select the appropriate spring type to enable ejection molding in the mold  24  at a certain pressure. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 HOLD 
                   
                   
               
               
                   
                 SETTING 
                 PRESS? 
                   
                 CAVITY 
               
               
                 SPRING 
                 MIN/MAX 
                 YES/NO 
                 PIN TRAVEL 
                 PRESS. (psi) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Blue - LIGHT 
                 Min 
                 No 
                 0.060 
                   
               
               
                   
                 Half 
                 Yes 
                 0.028 
                 875 
               
               
                   
                 Max 
                 Yes 
                 0.018 
                 875 
               
               
                 Orange 
                 Min 
                 Yes 
                 0.050 
                 1750 
               
               
                   
                 Half 
                 Yes 
                 0.025 
                 1750 
               
               
                   
                 Max 
                 Yes 
                 0.015 
                 1750 
               
               
                 Blue - HEAVY 
                 Min 
                 Yes 
                 0.025 
                 2000 
               
               
                   
                 Half 
                 Yes 
                 0.012 
                 2000 
               
               
                   
                 Max 
                 Unsafe 
               
               
                   
                   
                 conditions 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 injection molding apparatus 
               
               
                 12 
                 injection molding machine 
               
               
                 14 
                 molten resin 
               
               
                 16 
                 screw cylinder 
               
               
                 17 
                 platen 
               
               
                 18 
                 tip 
               
               
                 20 
                 nozzle 
               
               
                 22 
                 screw 
               
               
                 24 
                 non-metallic injection mold 
               
               
                 26 
                 stationary cavity mold 
               
               
                 28 
                 movable core mold 
               
               
                 29 
                 ejector pins 
               
               
                 30 
                 hollow or first molten resin flow path 
               
               
                 35 
                 terminal end of the hollow 30 
               
               
                 36 
                 pressure relief valve 
               
               
                 38 
                 second molten resin flow path 
               
               
                 42 
                 movable pin 
               
               
                 44 
                 spring bias 
               
               
                 45 
                 one end of pressure relief valve 36 
               
               
                 46 
                 base plate 
               
               
                 48 
                 adjustment screw 
               
               
                 54 
                 parting line