Plastic injection moulding machine with automatic shut-off valve

Abstract: A plastic extruder comprising a rotating helical screw within a cylindrical shell (10). An automatic shut-off valve (24) is mounted at the forward end of the helical screw. The valve includes a valve body (34) and a poppet (36) which is axially movable and is rotatable relative to the valve body. A rearwardly extending axial bore (48) is provided at the forward end of the screw and the valve body is secured to the screw within said bore. The poppet has an axial stem (60) which extends through the valve body into the axial bore. A compression spring (52) is mounted on the rearward end of the stem to urge the poppet against the valve body and maintain the valve in a normally closed position. Flow of plastic through the valve upon rotation of the screw causes the valve to open. However, the valve automatically closes upon cessation of screw rotation to prevent back-flow of plastic during the injection step.

Description: Background of the invention This invention relates to a plastic extruder and more particularly this invention relates to an automatic shut-off valve for controlling flow of molten plastic through a plastic extruder. DE-A-25 24 746 discloses an extruder having an automatic shut-off valve consisting of several parts which are seperately connected to the helical screw of the extruder. Thus, the shut-off valve cannot be assembled and disassembled as a unit with respect to the helical screw. Furthermore, the poppet of the known shut-off valve is forced to rotate at the same speed as the helical screw of the extruder so that supplemental frictional forces are created. Summary of the invention The plastic extruder of this invention comprises a rotating helical screw within a cylindrical shell of the type which is used to feed molten plastic to a high pressure injection molding apparatus. The plastic extruder and the injection molding apparatus operate sequentially. First, the plastic extruder, by rotation of the helical screw, forces molten plastic through the injection molding apparatus. As the molten resin is pumped through the valve of this invention, volume is displaced within the cylinder which forces the helical screw to retract. When the screw is retracted to a pre-set position, screw rotation is stopped. The screw and valve apparatus are then ready for the next step in the injection molding process which is the injection of the molten resin into the awaiting mold. At this point, the pressures are much higher than during the extrusion step. The valve of the present invention is located near to discharge port of the extruder. The valve is normally closed but is forced open during the extrusion step to allow molten plastic to flow from the extruder to the injection molder. The valve automatically closes following the extrusion step to prevent back-flow of plastic through the extruder during the high pressure injection molding operation. The automatic shut-off valve is mounted at the forward end of the helical screw. The valve comprises a unitary or integral valve body all of which is axially stationary relative to the screw and a poppet which is axially movable relative to the screw. The clearance between facing surfaces of the valve body and the poppet is adjustable by axial movement of the poppet and this clearance comprises the valve opening. The outside diameter of the valve body is in close proximity to the inside wall of the cylinder, allowing rotation and retraction of the valve body relative to the cylinder but preventing plastic from by-passing the valve Openings are provided in the valve body to allow passage of molten plastic through the valve. The continuance or interruption of flow of molten plastic through the extruder is determined by the opening or closing of the valve, which is determined by movement of the poppet towards or away from the valve body. The core of the helical screw is provided with an axial bore at its forward end. A rearward projection of the valve body is secured within the axial bore so that the entire valve body is integral with, moves axially with and rotates with the helical screw. The movable poppet has a rearwardly extending axial stem which extends through a longitudinal bore in the valve body and thence into said axial bore. A compression spring is mounted around the rear of the axial stem out of contact with the molten plastic in the extruder to operate cooperatively with respect to the poppet whereby the spring tends to maintain the poppet in a retracted position so that the facing surface of the poppet and the valve seat on the valve body are normally in contact with each other, whereby the valve is normally closed. Rotation of the helix during the extrusion step and resultant flow of molten plastic in a forward direction creates a pressure in the valve which pushes against the compression of the spring and opens the valve. However, when the rotation of the helix stops so that the injection molding step can proceed, the valve closes automatically under the action of the spring to prevent back flow of molten plastic due to the forward movement of the screw and valve. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more clearly understood by reference to the attached drawings in which Figure 1 is a cross-sectional view of an extruder injection apparatus of this invention including the automatic shut-off valve; Figure 2 is a partial cross-sectional view taken through section 2-2 of Figure 1 showing the valve in the open position; Figure 3 is a partial cross-sectional view taken through section 2-2 of Figure 1 showing the valve in the closed position; and Figure 4 is a cross-sectional view of the valve taken through the section 4-4 of Figure 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 presents a cross-sectional view of an extruder injection apparatus of this invention. The injection apparatus includes a cylinder 10 enclosing a rotating screw 12. Screw 12 comprises core 14 around which helical flight 16 is coiled to define channel sections 18. Plastic pellets are fed to the rear of the injection apparatus through inlet port 20 and the apparatus is heated by heaters 22 so that the pellets become melted during transit within cylinder 10. This operation and apparatus is described more fully in U.S. Patent 4,752,136. The automatic-shut non-return valve of the invention is located at the forward end of rotating screw 12 and is indicated generally at 24 in Figure 1. A cross-sectional view of the valve taken through section 2-2 of Figure 1 is shown in Figures 2 and 3. The non-return valve is shown in the open position in Figure 2 with molten plastic flow through the valve indicated by flow lines 26 and 28, which represent regions of a common annular flow stream. The molten plastic stream discharges from cylinder 10 through forward discharge port 30 and passes to a conventional high pressure injection mold apparatus indicated schematically at 32. The auto-shut valve is shown in the closed position in Figure 3. The valve is in the closed position during the high pressure injection mold step. The auto-shut valve comprises body 34 and valve poppet 36 which is axially movable relative to stationary body 34. Poppet 36 has a frusto-conical valve face defined as tapered surface 38 which faces a complementary frusto-conical valve seat defined as tapered surface 40 on body 34. When valve face 38 and valve seat 40 are separated, as shown in Figure 2, the valve is open as indicated by annular channel 42 which allows the flow of annular plastic stream 26, 28. When surfaces 38 and 40 abut against each other, as shown in Figure 3, channel 42 disappears and the valve is shut so that plastic flow is prevented. Cylindrical collar 44 of valve body 34 is in close proximity to cylinder 10 so that molten plastic exiting from helical screw 12 cannot by-pass valve 24, whereby the path through channel 42 is the only path for molten plastic to reach discharge opening 30. Valve body 34 is provided with rearwardly extending axial projection 46 having outer threads. Core 14 of screw 12 is provided at its forward end with complementary rearwardly extending axial bore 48 having internal threads for receiving rearwardly extending projection 46 in threaded engagement therewith, with the rearwardmost portion of the bore 48 remaining unoccupied by projection 46. The rearward end of projection 46 is provided with forwardly extending bore or recess 50 containing coiled spring 52. Spring 52 is maintained under compression by means of thrust ball bearing 54 having forward race 56 and rearward race 58 which is wedged between coil 50 and retainer pin 64. Any other retainer, such as a nut, can be used in place of pin 64. Poppet 36 is provided with rearwardly extending rod-like axial stem 60 which extends through and in fluid tight engagement with longitudinal axial bore 62 in valve projection 46. Spring 52 and bearing 54 are mounted around the rearward end of stem 60 and are retained on stem 60 by means of pin 64 which extends through stem 60. Pin 64 forces forward race 56 of thrust bearing 54 to abut against rearward end abutment surface 66 of projection 46 when the valve is in the extreme open position shown in Figure 2 but spring 52 forces bearing 54 away from abutment surface 66 when the valve is in the closed position shown in Figure 3 where frusto-conical surfaces 38 and 40 abut against each other. Figure 4 is a cross-sectional view of section 4-4 of Figure 3 and shows inwardly directed passageways 68 in stationary body 34 providing access to annular channel 70 which in turn leads to annular channel 42, thereby providing the flow path for molten plastic indicated by dashed lines 26 and 28 shown in Figure 2. The apparatus operates as follows. Plastic pellets are passed to cylinder 10 through inlet port 20 and rotation of screw 12 forces the pellets forwardly in cylinder 10. During transit, the pellets become molten due to the hear input from heaters 22 and through frictional heat. The stream of molten plastic flows forwardly in valley or channel sections 18 of screw 12 until reaching collar 44 whereat the stream is forced inwardly through openings 68 to passageway 70, as indicated by dashed lines 26 and 28. Spring 52 normally holds poppet 36 in a retracted position, as shown in Figure 3, wherein surfaces 38 and 40 abut against each other so that passageway 42 disappears. However, the pressure of molten plastic generated by rotating screw 12 forces poppet 36 forwardly against the tension in spring 52. The forward movement of poppet 36 due to the pressure of plastic flow causes movable surface 38 of poppet 36 to be separated from stationary seating surface 40 to define passageway 42, as shown in Figure 2, so that molten plastic can pass outwardly and fill the cavity area in front of the screw/valve assembly. As soon as the screw has retracted to a preset position the rotation of the screw ceases. Instantaneously, the fluid (polymer) pressure starts to equalize within the cylinder and the spring forces the poppet to retract allowing surfaces 38 and 40 to abut and form a seal. The seal remains in effect during the injection step. Figure 2 shows that during the extrusion step the forward axial motion of poppet 36 is limited by the abutment of thrust bearing 54 against surface 66, as shown in Figure 2. During the time that the plastic is flowing forwardly through the valve, poppet 36 is forced open as shown in Figure 2. The load compresses spring 52 so that thrust bearing 54 abuts against surface 66 and pin 64. The tendency is for poppet 36 to rotate at a rate independent of screw 12. Valve 34 and the thrust bearing 54 allows poppet 36 to rotate freely and without any surfaces abrading one another. It is a feature of this invention that spring 52 and thrust bearing 54 are secluded and protected from contact with flowing plastic. This protection is accomplished by maintaining all contact surfaces between core 14 and rearward projection 46 fluid tight and also by maintaining the contact surfaces between rod 60 and rearward projection 46 fluid tight. Thereby, molten plastic cannot reach bore 48 or recess 50 containing bearing 54 and spring 66, respectively. It should be noted that the functioning of the device is such that the interface between longitudinal bore 62 and rod 60 is not subject to the high pressures existing at orifice 30 during the injection cycle because the valve is closed during that process stage. It is subject only to the somewhat lower pressures involved in the extrusion portion of the cycle. Pressures in front of the valve during the injection stage can be up to 60 times greater than pressures in the cylinder during the extrusion stage. Because the valve of this invention is closed during the injection stage, the valve constitutes an important barrier protecting the interface between longitudinal bore 62 and rod 60 against subjection to the high plastic pressures in front of the valve during the injection stage. Thereby, the valve of this invention provides an important barrier against spring 52 and thrust bearing 54 being subject to leakage of plastic into their compartment.

Claims: A plastic extruder comprising a helical screw (12) in a cylinder (10), said screw having a core (14) with a helical flight (16) on the surface of said core, an inlet (20) for feeding plastic into said cylinder and a discharge port (30) at the forward end of said cylinder for the discharge of molten plastic, an automatic shut-off valve (24) at the forward end of said helical screw, said valve having a valve body (34) and a poppet (36) at the forward end of said valve body, said poppet being axially movable relative to said valve body, a facing surface (38) on said poppet and a complementary seating surface (40) on said valve body defining a valve opening (42), an axial bore (48) in said core (14), said valve body being secured to said helical screw (12) so that said entire valve body (34) is integral with and rotates with said helical screw (12), a rearwardly extending axial stem (60) on said poppet (36), a longitudinal bore (62) in said valve body, a compression spring (52) mounted around the rear of said stem (60) and out of contact with molten plastic, said compression spring (52) operating cooperatively with respect to the poppet (36) to maintain the poppet in a normally retracted position so that said facing and seating surfaces (38, 40) are normally in contact with each other and the valve (24) is normally closed, opening means (68) in said valve body (34) for passing molten plastic into said valve body and towards said poppet (36) so that rotation of the screw (16) and flow of molten plastic towards the poppet (36) exerts pressure against the poppet to force the poppet away from the facing surface (40) of the valve body (34) thereby opening the valve (24), characterized in that said axial bore (48) is restricted to the forward end of said core (14) of said helical screw (12), that said valve body (34) has a rearward projection (46) extending into said axial bore (48) of said core (14) and secured therein, and that said stem (60) of said poppet (36) extends through said longitudinal bore (62) of said valve body (34) into said axial bore (48) of said core (14), said poppet (36) being rotatable relative to said valve body (34). The apparatus of claim 1, wherein said axial bore (48) in the forward end of said core is threadedly engaged with the rearwardly extending axial projection (46) of said valve body (34). The apparatus of claim 1 or 2, wherein the rearwardly extending axial projection (46) of said valve body (34) comprises a forwardly extending axial bore (50), and wherein said poppet stem (60) extends through said longitudinal axial bore (62) and said forwardly extending bore (50) before terminating within said rearwardly extending bore (48) within the core. The apparatus of claim 3, wherein said compression spring (52) mounted around the rear of said poppet stem (60) is located in said forwardly extending bore (50), a thrust bearing (54) is mounted on said stem (60) rearwardly of said compression spring (52), and a retaining means (64) on said stem is mounted rearwardly of said thrust bearing to hold said thrust bearing and said compression spring on said stem. The apparatus of any of claims 1 to 4, wherein the opening means (68) in said valve body extends to a flow channel (70) through said valve body (34) to the forward end of said valve (24), said flow channel (70) including the space (42) between said facing conical surface (38) of said poppet (36) and said conical seating surface (40) of said valve body (34) created by the force of molten plastic flowing through the valve.

Inventors: DURINA MICHAEL F, DURINA, MICHAEL F.
Applicants: SPIREX CORP, SPIREX CORPORATION
Application year: 1992
Main classification: B29C45
IPC codes: B29C45