Abstract:
A method and apparatus for improving the quality of molded parts in a molding system having a valve stem and a valve gate. The valve stem is movable between a fully retracted position where the gate is fully open to a fully forward position where the gate is fully closed and into an intermediate position where the gate remains closed but the valve stem is displaced from the gate so heat transfer through the valve stem and into the gate region is minimized.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates, generally, to an improved valve gate for an injection molding system, and more particularly, but not exclusively, the invention relates to means for controlling positioning of a valve stem in a valve gate to improve the efficiency of the molding operation. 
     2. Background Information 
     In an ideal situation, the valve stem of the valve gate is open during the fill and hold portion of the molding operation to allow proper filling of the mold and compensate for any shrinkage arising during hold. The stem would then be closed for cooling of the part in the mold, opening of the mold, removal of the part from the mold and the subsequent closure of the mold. The stem would only move to the open position just before the next fill cycle starts. However, this ideal operation can induce defects in the molded part due to heat transfer through contact of the heated valve stem with the cooled gate because of the steel-to-steel contact between the stem and the gate. To avoid these defects, it is common practice to maximize the time that the stem is held in the open position. This practice creates the risk that entrapped pressure in the hot runner channel may cause melt to be pushed in front of the open stem. This melt in front of the stem may become of different consistency, viscosity, and temperature compared to the melt upstream of the stem front face. If this inconsistent melt is then injected into the mold cavity, drool and other defects may occur. 
     Current injection molding systems have failed to satisfactorily resolve these conflicting requirements of long stem closure time to avoid drool and other related defects and a short stem closure time to avoid defects caused by undesirable heat transfer. 
     The present invention avoids this conflict by enabling the stem to be partially pulled back in such a way that it is not in contact with the cold gate when in the closed position. The front face of the valve stem is kept inside the nozzle tip land to avoid or at least reduce the likelihood of melt being pushed in front of the stem. Pulling back the stem slightly accomplishes the ideal function of minimizing heat leakage to the gate and encapsulating the melt in the hot runner. 
     U.S. Pat. No. 5,162,125 to Akselrud et al, patented Nov. 10, 1992, shows a molding device that uses a spring to close a mold gate. 
     U.S. Pat. No. 5,423,672 to Gordon, patented Jun. 13, 1995, shows a molding device for forming a disc with a hole therein. This patent shows a valve gated hot runner in which the valve stem is moved to an intermediate position by means of a dual piston combination. In the disc molding operation the valve stem is first moved forward to open the valve gate and allow resin to fill the mold cavity. Next the valve stem is partially retracted to block the melt flow and allow a lower part of the valve stem to form the hole in the disc. Finally, the valve stem is fully retracted to pull its hole forming section out of the molded part and to allow the molded part to be ejected conventionally from the core side of the mold. 
     U.S. Pat. No. 6,214,275 to Catoen et al, patented Apr. 10, 2001, shows a molding device that includes means to move a valve stem into an extended position to assist the ejection of molded parts and remove debris from the gate orifice and nub area of the melt channel. 
     U.S. Pat. No. 6,228,309 to Jones et al, patented May 8, 2001 shows a molding device that includes apparatus for moving a valve stem between a closed position, a partially open position and a fully open position. The partially open position enables restricted flow of melt to the mold cavity. 
     None of these references teach the concept of moving a valve stem into an intermediate position between an open and a closed position to minimize cooling of the end of the valve stem by the cooled gate insert. 
     SUMMARY OF INVENTION 
     The present invention provides an improved injection nozzle system and method for injection molding including a valve stem that is movable to an intermediate position to minimize cooling of the valve stem and prevent drooling of the melt into the gate insert and the egress of cooled melt back into the hot melt channel. 
     The present invention provides an injection nozzle for injection molding plastic resin from a source of molten resin to a mold cavity, which comprises: a mold cavity; an injection nozzle with a nozzle body and a nozzle tip and having an internal flow channel therein communicating with an injection orifice which in turn communicates with the mold cavity for transportation of molten resin to the mold cavity through an injection orifice. A valve stem is mounted in the injection nozzle and is movable between an open position retracted from the injection orifice permitting the flow of resin to the mold cavity, a closed position blocking the injection orifice and preventing flow of resin to the mold cavity, and an intermediate position between the open and closed position. Moving the stem to an intermediate position reduces the cooling effect of the mold cavity on the valve stem while the melt is solidifying in the mold cavity. 
     The present invention also provides a method for injection molding resin from a source of molten resin to a mold cavity which comprises: providing an injection nozzle with a nozzle body and a nozzle tip and having an internal flow channel therein communicating with an injection orifice which in turn communicates with a mold cavity; transporting molten resin from the internal flow channel to the mold cavity; mounting a valve stem in the injection nozzle; and moving the valve stem between an open position retracted from the injection orifice permitting flow of resin to the mold cavity, a closed position blocking the injection orifice and preventing flow of resin to the mold cavity, and an intermediate position between the open and closed positions to reduce heat transfer along the valve stem. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view through a nozzle assembly of a first embodiment of the present invention with the valve gate open and the valve stem retracted; 
         FIG. 2  is a sectional view similar to  FIG. 1  with the valve gate closed and the valve stem in the closed position; and 
         FIG. 3  is a sectional view similar to  FIG. 1  with the valve stem in the intermediate position. 
         FIG. 4  is a sectional view of the cylinder and valve pin assembly of the embodiment shown in  FIGS. 1 ,  2  and  3 . 
         FIG. 5  is a detailed sectional view of a portion of the valve bushing in the embodiment shown in  FIGS. 1 ,  2  and  3 . 
         FIG. 6  is a sectional view through a nozzle assembly of a second embodiment of the present invention with the valve gate open and the valve stem retracted; 
         FIG. 7  is a sectional view similar to  FIG. 6  with the valve gate closed and the valve stem in the closed position; and 
         FIG. 8  is a sectional view similar to  FIG. 6  with the valve stem in the intermediate position. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings that show a first preferred embodiment of the nozzle assembly of the present invention.  FIGS. 1-3  show the nozzle assembly in each of three valve stem positions.  FIG. 1  shows an injection nozzle including nozzle housing  12  and nozzle tip  14  secured thereto. The injection nozzle is located in mold manifold plate  16  and supporting manifold  18 . Mounted in manifold  18  is valve bushing  20  that contains pneumatic piston  22  that is attached to valve stem  26 . 
     Melt channel  28  in manifold  18  is connected through extension  10  of valve bushing  20  to central melt channel  30  in nozzle housing  12  which in turn leads to injection orifice or gate orifice  32  in gate insert  34 . Insulator  36  occupies the space between nozzle tip  14  and gate insert  34  and also contains a melt channel opening  38  therein. When the valve stem  26  is in the fully retracted position as shown in  FIG. 1 , resin can be injected through the melt channels to fill mold cavity  40  in a known fashion. In this embodiment the mold cavity has a gate nub in the gate orifice  32  so that when the mold cavity  40  and orifice  32  are filled with resin a molded part is formed having a nub. 
     Pneumatic piston  22  is operated by air pressure through air lines  44  and  46  from a source of compressed air (not shown) such that, by directing compressed air appropriately, valve stem  26  can be moved to one of two positions. In  FIG. 1 , piston  22  is fully retracted by compressed air flowing through line  44  causing the piston to move upward thereby fully retracting valve stem  26  within nozzle housing  12  and permitting resin to flow into the gate nub and mold cavity. 
       FIG. 2  shows valve stem  26  in the closed position protruding into the gate nub area. The valve stem  26  is moved to the closed position shown in  FIG. 2  by exhausting air from line  44  to permit piston  22  to move forward and introducing compressed air into line  46  to move piston  22  forward. 
       FIG. 3  shows valve stem  26  in the intermediate position shutting off resin flow to the filled mold cavity  40  and to the filled gate nub but out of contact with the cooled gate insert  34 . The valve stem  26  is moved to the intermediate position shown in  FIG. 3  by spring  19  that operates to retract piston  22  a limited amount when the pressure on either side of piston  22  is equalized. Spring  19  is compressed when piston  22  moves forward to close the valve opening as shown in  FIG. 1 . In this slightly retracted intermediate position cooling channels  50  in gate insert  34  cause resin in the mold cavity  40  and gate nub to solidify prior to opening the mold but do not cool the end of valve stem  26  because it has been retracted into the warm and heated nozzle tip  14 . For ease of illustration, the retraction into the intermediate position is exaggerated. In operation, the retraction to the intermediate position would be a few millimeters. 
     In operation, spring  19  is compressed when air pressure is applied to piston  22  to move valve stem  26  into the nozzle closed position. The spacer  21  contacts the upper surface  23  of the valve bushing  20  in the lower pneumatic chamber. The spacer  21  compresses the spring  19  and controls the extent of the compression of spring  19 . After the end of cool time, the pressure on both sides of the piston  22  is equalized by opening both lines  44  and  46  to atmospheric pressure. Equalization of the pressure enables the spring  19  to decompress and thereby retract the pin  26  a distance corresponding to the decompressed state of spring  19 . In a preferred embodiment, the spring  19  retracts the pin  26  about 2-3 mm. The pin  26  is kept in this retracted position out of contact with the cooled mold until the molded parts are ejected from the mold. After the mold has been opened, the parts ejected and the mold reclosed, the pin  26  is retracted by the application of pneumatic pressure through lines  44  to the underside of the piston  22  to open the nozzle and permit injection of the melt. When injection is complete and after hold, the piston  22  is activated to force pin  26  into the nozzle closed position and the cool and ejection cycle is repeated. 
     As shown in  FIG. 5 , the valve bushing  20  includes a boss  24  formed on its upper surface  23 . The spacer  21  on piston  22  is driven into contact with the boss  24  by the application of pressure through line  46 . When piston  22  is in this position, valve stem  26  closes the valve as shown in  FIG. 2 . A cutout portion  25  is formed within boss  24  to receive the spring  19 . The cutout portion  25  is dimensioned so that the spring  19  can be fully compressed within the cutout portion  25  and does not prevent the piston  22  from moving into full engagement with the boss  24 . Additionally, the spring  19  should only require minimal additional energy to be fully compressed by the piston  22  but sufficiently strong in compression that the spring  19  is capable of overcoming the frictional force created by the seals and sliding surfaces of the piston  22  and the valve stem  26  to move the valve stem  26  into the intermediate position shown in  FIG. 3 . In practice, we have found that Belleville washers work very well. 
     Obviously, other spring means could be used to move the valve stem. For example, a spiral spring could be placed between the outer cylindrical portion of the piston  22  and the base of the well of the valve bushing  20  as long as the spring did not interfere with the normal opening and closing of the valve gate. Also, compression of the spring by the air pressure would have to be avoided since such compression of the spring might lead to excess retraction of the valve stem so that the valve is open. Consequently, the preferred position of the spring is in a cutout portion  25  at the base of the well of the valve bushing  20  as shown in  FIGS. 1 to 3 . Of course, the cutout portion need not be on the base of the cylinder, it could be located within the base of piston  22  or spacer  21  and spring  19  compressed into such a cutout portion. 
     Referring to the drawings that show a second preferred embodiment of the nozzle assembly of the present invention,  FIGS. 6-8  show the nozzle assembly in each of three valve stem positions.  FIG. 6  shows injection nozzle  110  including nozzle housing  112  and nozzle tip  114  secured thereto. The injection nozzle  110  is located in mold manifold plate  116  and supporting manifold  118 . Mounted in manifold  118  is valve bushing  120  that contains two pneumatic pistons  122 ,  124  to which is attached valve stem  126 . 
     Melt channel  128  in manifold  118  is connected to central melt channel  130  in nozzle housing  112  which in turn leads to injection orifice or gate orifice  132  in gate insert  134 . Insulator  136  occupies the space between nozzle tip  114  and gate insert  134  and also contains a melt channel opening  138  therein. When the valve stem  126  is in the fully retracted position as shown in  FIG. 6  resin can be injected through the melt channels to fill mold cavity  140  in a known fashion. This mold cavity has a gate nub in the gate orifice  132  so that when the mold cavity  140  and gate orifice  132  are filled with resin a molded part is formed having a nub. 
     Pneumatic pistons  122 ,  124  are operated by air pressure through lines  144 ,  146  and  148  from a source of compressed air (not shown) such that by directing compressed air appropriately valve stem  126  can be moved to one of three positions. In  FIG. 6 , both pistons  122  and  124  are fully retracted by compressed air causing both of the pistons to move upward thereby fully retracting valve stem  126  within nozzle housing  112  and permitting resin to flow into the gate nub and mold cavity. Thus, in  FIG. 6 , compressed air is introduced into line  148  causing piston  124  to retract and is also introduced into line  144  causing piston  122  to retract. This fully retracts valve stem  126 . 
       FIG. 7  shows valve stem  126  in the closed position protruding into the gate nub area. The valve stem  126  is moved to the closed position shown in  FIG. 7  by exhausting air from lines  144  and  148  to permit piston  122  to move forward and introducing compressed air into line  146  to move both pistons  122  and  124  forward. 
       FIG. 8  shows valve stem  126  in the intermediate position shutting off resin flow to the filled mold cavity  140  and to the filled gate nub. The valve stem  126  is moved to the intermediate position shown in  FIG. 8  by introducing compressed air into line  144  to move piston  122  into its retracted position while also introducing air into line  146  to advance piston  124  slightly forward and thus move the end of valve stem  126  into orifice  132 . Piston  124  will contact piston  122  and not push it downward provided the projected area of piston  124  is less than the projected area of the underside of piston  122  since both are exposed to same air pressure. Cooling channels  150  in gate insert  134  cause resin in the mold cavity  140  and gate nub to solidify prior to opening the mold but do not cool the end of valve stem  126  because it has been retracted into the warm orifice  132 . 
     Thus, the present invention provides a simple and expeditious three position hot runner valve stem that can move the valve stem into an intermediate position between the gate and the mold cavity to reduce or prevent substantial cooling of the valve stem by the cooled mold cavity. 
     It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention.