Abstract:
The invention relates to an injection molding device ( 1 ) having an elongated nozzle ( 3 ) and a valve pin ( 11 ) coaxially in a channel of the nozzle. The valve pin can be displaced by means of an actuating cylinder ( 15 ) and is guided through a bush ( 13 ) at the upper end ( 12 ) of the nozzle. Cooling means ( 14, 18 ) are associated with said bush ( 13 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an injection moulding device comprising a mould body having a cavity, an elongated nozzle seated in the cavity, a valve pin coaxially in a channel of the nozzle and actuating means connected to the valve pin for axially displacing the valve pin in the nozzle, the valve pin exiting the channel via a bore at the upper end of the nozzle and being guided by a bush in said bore. 
     DETAILED DESCRIPTION OF THE RELATED ART 
     A valve gated injection moulding device of the above-mentioned type is known from EP-A-0 836 925 in the name of the applicant. By axial displacement of the valve pin, the gate of the nozzle is opened and closed to control the flow of molten thermoplastic material into a mould cavity. The valve pin head projects via a bore in the manifold body from the upper part of the nozzle and is seated in a sliding bush, which is axially displaced by means of a lever arm. The lever arm is connected to a hydraulic cylinder that is placed at a radially offset location from the valve pin. Leakage of molten thermoplastic material along the bore in the manifold body is prevented by a relatively long bore and by a narrow gap between the wall of the bore and the needle, wherein a gap width of less than 20 micrometers is required. Such narrow gaps are difficult to manufacture and may cause metal-to-metal wear (fretting, corrosion) of the needle against the bush. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a valve gated injection moulding device, having a valve pin which can be guided in a bush without leakage, and without the risk of prohibitive wear. 
     Hereto the injection moulding device according to the invention is characterized in that cooling means are associated with said bush. 
     By providing a cooled bush, the viscosity of the molten thermoplastic material in the gap between the needle and the bush may be increased. Hence, leakage may be prevented, even if the gap width is relatively large. Furthermore, the solidified thermoplastic material forms a lubricating material between the bush and the valve pin, such that wear of the needle is prevented. 
     In a preferred embodiment, the bush projects above a surface of the mould body, the cooling means comprising a cooling plate, spaced away from the mould body, and associated with the upper end of the bush. The cooling plate may form the base on which the actuating cylinder of the valve pin is placed, to provide a thermal insulation, such that the seals and the pressure medium of the cylinder are not exposed to the high temperatures of the manifold. In this way a very compact and leakage free actuating mechanism of the valve pin is obtained. The cooling plate may be supported on the manifold via a refractive spacer member. The bush may be seated in a cavity in the mould body at a distance from the sidewalls of the cavity, the bush having a lower shoulder, a clamping ring being screwed into the cavity and engages with the shoulder on the bush, an inner wall of the clamping ring being spaced away from the sidewall of the bush. In this way, heat transfer from the heated manifold, which may have a temperature of for instance 350° C. to the cool cylinder is via the bush minimised. Hereby the internal seals of the cylinder are not exposed to high temperatures, such that the service life of the seals is increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of an injection moulding device according to the present invention will, by way of example, be described in detail with reference to the accompanying drawing. In the drawing: 
     FIG. 1 shows a cross-sectional view of an in injection moulding device according to the invention, 
     FIG. 2 shows a cross-sectional view of the actuating cylinder of the injection moulding device of FIG. 1, 
     FIG. 3 shows an exploded view of the cylinder of FIG. 2, and 
     FIG. 4 shows an end position indicator for the cylinder, comprising a flow meter in the hydraulic ducts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an injection moulding device  1 , comprising a manifold  2  having a transverse duct  2  for supplying molten thermoplastic material to a vertical nozzle, or hot runner  3 . In the manifold  2 , the thermoplastic material is supplied at a temperature which may amount to over 350° C. at a pressure of for instance 200 bar. The nozzle  3  is connected to the manifold  2  and is seated in a well  4  of the mould body  5 . The nozzle  3  is separated from the mould body  5 , which may have a temperature of for instance 40° C. by an air gap, surrounding the nozzle  3 . The nozzle  3  is surrounded by a heating element  7 , to keep the temperature of the thermoplastic material above the melting point. The molten thermoplastic material exits the nozzle  3  via a gate  8  to enter into a mould cavity  9 . The gate  8  is opened and closed by a valve pin  11 , which can slide up and down in the nozzle  3 . The valve pin  11  exits at the upper end  12  of the nozzle  3 , a bore in the manifold  2 , and is guided by a bush  13 . The valve pin passes through a cooling plate  14  and is attached to a hydraulic cylinder  15  that is located in line with the valve pin  11 . The bush  13  is clampingly pressed into a central bore  17  of the cooling plate, or is connected via a sliding fitment to be in good heat conducting contact with the cooling plate  14 . The cooling plate  14  comprises a generally circular cooling channel  18 , in which a cooling medium, such as water, is circulated. The relatively cool bush  13  is seated in a cavity  21  in the manifold  2  and is only supported with a relatively small surface area of the end part  19  onto the hot manifold  2 , such that heat transfer from the manifold to the bush  13 , and hence to the cylinder  15 , is minimised. 
     The bush  13  is connected to the manifold  2  by a clamping ring  22 , which is screwed into the cavity  21 . 
     The clamping ring  22  comprises a shoulder  23  that engages a shoulder  24  of the bush  13 . The inner sidewall  25  of the clamping ring  22  is spaced from the bush  13 , such that an insulative air gap is formed between the bush and the clamping ring. By using a relatively long bush  13 , which is connected to the cooling plate  14 , the viscosity of the molten thermoplastic material at the position of the bush  13  is increased. Hereby leakage of molten thermoplastic material from the manifold  2  can be prevented, even at a relatively large play of the bush  13  around the valve pin  11 , such as 20 micrometer or more. Furthermore, the (partly) solidified thermoplastic material in the gap between the bush  13  and the valve pin  11  has a lubricating effect, so that metal-to-metal wear of the valve pin  11  against the metal bush material is prevented. 
     FIG. 2 shows a cross-sectional view of the cylinder  15 , including a cylinder housing  30  in which a piston  33  can be displaced between an upper chamber part  31  and a lower chamber part  32 . The piston  33  comprises a piston head  34  and a stem  35 . The stem  35  comprises a T-shaped recess  36  in which the valve pin head  37  is seated in a rotationally fixed orientation. 
     The cylinder housing  30  comprises two bores  38 , 39 , one on either side of the piston head  34 , connected to fluid ducts. The stem  35  of the piston  33  is seated in a bore of a height adjustment ring  40 , which is screwed into the lower part  32  of the cylinder housing  30 . By axially displacing the ring  40  within the housing  30 , the stroke of the piston  33  can be varied within a range of several mm. Flexible seals  41 ,  42 , for instance made of PTFE, prevent leakage of the hydraulic oil from the chamber  31 . The height adjustment ring  40  can be accessed via an elongated slot  43  in the cylinder housing  30 , through which a tool can be inserted into several recesses  44  in the circumferential wall of the height adjustment ring  40 , in order to rotate the ring. The ring  40  is locked at its proper height via a locking bolt  45 . 
     The valve pin  11  enters through a central bore  47  in the cooling plate  14  into the cylinder housing  30 , which is releasably connected to the cooling plate  14 , which in turn is connected to the manifolds via two locating pins  48  and two bolts  48 ′. 
     The piston  33  can slide axially within the housing  30  along a locator pin  50 , which maintains the piston in its desired angular orientation. The walls of the piston head  34  form a non-liquid tight connection with the inner wall of the cylinder housing, so that some hydraulic oil may leak from the high pressure side of the piston head  34  to the low pressure side, when the piston is not in either its upper or its lower end position. Hereby degradation of the hydraulic oil is prevented, and an automatic venting for removal of air from the cylinder  15  is achieved. Furthermore, the small oil flow from the high pressure side to the low pressure side of the piston head  34 , which occurs while the piston is travelling from one end position to the other end position, can be used to obtain an indication of the piston position. By placing the cylinder  15  onto the manifold  2 , via the cooling plate  14 , the seals  41 , 42  are protected from the high temperatures of the manifold. The construction of the cylinder  15  is very compact, and can easily be accommodated in the limited space available in injection moulding systems, in which a small distance between several nozzles is desired. 
     As can be seen from FIG. 3, the cylinder housing  30  can be easily detached from the cooling plate  14  by two locating pins  48  and tow bolts  48 ′, which project through refractive spacer members  52  via which the cooling plate  14  is supported on the manifold. After detaching the housing  30 , the piston  33  can be lifted from the height adjustment ring  40 . Next, the piston is moved parallel to the cooling plate  14 , such that the valve pin head exits the T-shaped axial slot  36  in the piston stem  35 , in the radial direction. Instead of a T-shaped slot in the piston stem  35 , the valve pin head may be seated in a groove in the stem  35  and be connected by pins or bolts. In this way, the cylinder  15  can be disconnected from the valve pin  11  without removal of the valve pin  11  from the nozzle  3 . Because the valve pin can remain seated within the nozzle, no cleaning of the valve pin/slide bush is required which would be the case if the valve pin were to be removed from the nozzle. Furthermore, removal of the valve pin from the nozzle would require heating of the nozzle prior to removal, which is not necessary for the valve pin connection, according to the present invention. 
     As can be seen in FIG. 4, the piston head  34  of the piston  33  comprises a bore  55 , via which hydraulic oil can pass when the piston head is not engaged with the cylinder housing  30  in its upper end position or with the height adjustment ring  40  in its lower end position. In the end positions of the piston, the bore  55  is blocked such that no oil can pass through. In the hydraulic ducts  56 , 57 , a flow meter  58 , 59  is comprised, generating a displacement signal, indicating when the piston is not in one of its end positions. The displacement signals can be transmitted to a processing and/or display device  62  for indicating the end position of the valve pin  11 . The processing device  62  can be used for automatic control of several valve pins in the injection moulding apparatus, for instance for sequentially filling a larger mould cavity from several nozzles. The flow meters  58 ,  59  as well as the processing/display device  62  can be placed outside the high temperature area of the injection moulding device. When the height adjustment ring  40  is moved to vary the stroke of the piston  33 , the stopping surface of the ring  40  closing off the bore  55  is moved, such that the end position of the valve pin is indicated properly at all times. Existing injection moulding apparatus having a hydraulically or pneumatically driven valve pin can be provided with a position indicator according to the present invention in a simple manner by providing a bore  55  in the piston  33 , and incorporating flow meters  58 , 59  in the ducts  56 , 57 .