Abstract:
A cylindrical switch has one or more passages which open onto a lateral cylindrical surface of the switch. The switch is capable of being tightly received in a cylindrical hole in a hot plate and of being selectively orientated so that the passages are angularly in line with or offset from two or more channels in the hot plate which open onto the hole in order to selectively permit, interrupt or divert the flow of molten plastics material between the aforesaid channels. The switch incorporates a circuit for a cooling fluid.

Description:
This application is a national stage application of co-pending PCT application PCT/IB2006/053131 filed on Sep. 6, 2006, which was published Under PCT Article 21(2) in English, and of Application No. TO2005A000611 filed in Italy on Sep. 9, 2005, which is incorporated by reference herein in its entirety. 
   This invention relates to a switch for the flow of molten plastics material in a hot plate for injection moulding. 
   In machines for injection moulding, the molten plastics material is distributed to various injection nozzles from one or more external injection outlets through a network of channels (known as hot channels) formed in a distribution plate (hot plate) which is mounted above the mould and connected to the injection nozzles. 
   For various reasons it is from time to time necessary to open or close a hot channel to interrupt or restore the flow of molten plastics material to a particular nozzle or group of nozzles, or to selectively divert the flow of plastics material towards particular nozzles, excluding others. In other cases, when a hot plate is designed to be connected to several injection openings it is from time to time necessary to selectively open and close particular hot channels to feed the nozzles through one injection opening instead of another. 
   For this purpose, in a hot plate a cylindrical switch which is inserted with a forced interference fit into a cylindrical hole into which two or more channels which are placed in communication or closed depending upon the angular position in which the switch is set. The switch has one or more internal or peripheral passages with corresponding openings located on the lateral cylindrical surface which, depending upon the angular position of the switch, line up with the channels to establish fluid communication between two channels, or are angularly offset to prevent the passage of plastics material from one channel to another. 
   A perfectly leaktight seal is needed between the switch and the hot plate. When it is considered that the molten plastics material in the hot channels is at pressures of the order of 1000-1500 bar, even minimum play (of the order of 2-3/100 mm) between the hot plate and the switch can give rise to the leakage of plastics material, which must absolutely be avoided. 
   When it is necessary to change or vary the connections between the channels opening into the hole, the mould must be removed from the moulding press, various electrical and hydraulic or pneumatic connections for operating the plugs for the injection nozzles must be disconnected, the heavy hot plate must be lifted using a hoist, and then the switch must be removed from the hot plate and then forcibly reinserted in the desired new angular position. Before the mould is reassembled, any interface surfaces which may have been soiled by plastics material must be cleaned again. 
   On average the abovementioned operations require almost two working days. While the machine is being reassembled it sometimes happens that some electrical and/or hydrodynamic connections are not correctly restored, as a result of which the injection machine downtime is further increased. In addition to this, it will be noted that the abovementioned operations require action by specialist labour, as a result of which the user must sometimes transfer the hot plate to the manufacturer. 
   BRIEF SUMMARY OF THE INVENTION 
   This invention relates to a switch for the flow of molten plastics material in a hot plate for injection moulding. 
   A primary object of the invention is to perform the abovementioned switching operations in a short time without having to dismantle the mould from the injection machine. 
   Another object of the invention is to provide a seal which is perfectly leaktight for the high pressure molten plastics which flows in the hot channels during the moulding operations. 
   These and other objects and advantages which will be better understood below are achieved through a switch having the characteristics specified in claim  1 . In accordance with another aspect of the invention these objects are achieved through a method according to claim  4 . Preferred embodiments of the invention are defined in the dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the invention will appear from the detailed description of an embodiment thereof, given by way of example and referring to the attached drawings, in which: 
       FIGS. 1 and 2  are a plan view from above and an exploded perspective view, respectively, of a hot plate with a switch according to the invention, two injection openings and 16 injector nozzles, 
       FIG. 3  is a view in vertical cross-section along the line III-III in  FIG. 1 , 
       FIG. 4  is a view in vertical cross-section similar to that in  FIG. 1 , but with the switch orientated in a different angular position, 
       FIG. 5  is a view in vertical cross-section of another embodiment of a switch according to the invention. 
   

   DESCRIPTION 
   With reference initially to  FIGS. 1 and 2 ,  10  indicates a distribution plate (or hot plate) which forms a network of channels (or hot channels)  11 - 17  to distribute the molten high pressure plastics material from two injection bushings  20 ,  21  to sixteen injection nozzles subdivided into four groups  31 ,  32 ,  33 ,  34 . The nozzles in groups  31  and  32  are fed with molten plastics material through branches  11 ,  12  of a hot channel  15 , while the nozzles in groups  33  and  34  are fed from branches  13 ,  14  of a hot channel  16 . Hot channels  15  and  16  can receive the molten plastics material from injection bushings  20  or  21  alternately, typically to inject two different plastics materials.  17  indicates a hot channel for feeding plastics material originating from injection bushing  21 . 
   Hot channels  15 ,  16  and  17  open onto a cylindrical hole ( 18  ( FIGS. 3 and 4 ) having a central axis which is vertical or perpendicular to hot plate  10 . In the embodiment illustrated in the drawings, hot channels  15 - 17  are arranged in a T-shape in a horizontal plane. 
   In all this description and the claims which follow the terms and expressions indicating positions and orientations are to be understood to refer to the main (horizontal) plane of the hot plate. Thus, the term “vertical” indicates a direction perpendicular to the hot plate, and the term “transverse” indicates a direction perpendicular to the x axis. 
   A switch  40  is inserted in hole  18  in the manner described herein after. By varying the angular position of switch  40  hot channels  15 ,  16  can be alternately connected to or disconnected from one or other of injection bushings  20 ,  21  depending upon which bushing it is desired to use and which group of nozzles it is desired to inject with. 
   The switch  40  has a generally cylindrical shape with a lateral cylindrical wall  41  which almost matches that of hole  18 , but of slightly greater diameter. 
   In the embodiment in  FIGS. 1-4  the switch  40  forms two passages: an inner passage  42  and a peripheral passage  43  in the form of a groove which extends along a limited arc of the circumference of the switch. 
   The inner passage  42 , in this embodiment a straight transverse passage, has opposing openings on the lateral cylindrical surface  41  and receives the plastics material delivered from above by bushing  20  through a central vertical passage  44 . When the switch  40  is in the angular position illustrated in  FIG. 4 , the inner transverse passage  42  is in line with the hot channels  15  and  16  to establish a fluid communication between these and the upper bushing  20 . In angular positions which are offset with respect to that in  FIG. 4  the opposite openings of the transverse channel  42  are closed off by the wall of hole  18 . When the switch is in the angular position illustrated in  FIG. 3 , the groove or peripheral passage places channel  17  in communication with one or both of channels  15 ,  16 . 
   Notwithstanding the fact that this invention was initially conceived for hot plates which can be connected to more than one injection bushing for moulding in a multiple moulding press (as in the example illustrated), the reference to this possible field of application should not in any way be interpreted as restricting the scope of the patent. The invention may also be applied to hot plates having a single injection bushing and any number of injection nozzles. Likewise, the switch may have different numbers and shapes of passages, for example a single internal straight transverse passage to connect to or disconnect from two hot channels located at 180° with respect to each other, or it may have a passage with T or Y-shaped branches, or yet other shapes, and may therefore act merely as a member for selectively closing off and/or diverting flows of plastics material from one length of channel to another. 
   In the embodiment illustrated in the drawings (see in particular  FIG. 5 ), the switch comprises a main cylindrical body  45  rigidly fixed to an upper plate  46  and a lower plate  47 . A coil cooling circuit  50  for the passage of a cooling fluid which is delivered through a delivery connection  51  and exits via an exit connection  52  connected to lower plate  47  is formed in main body  45 . The cooling circuit  50  comprises a succession of parallel vertical lengths  53  joined by upper lengths  54  and lower lengths (not illustrated) which are closed off respectively by terminal plates  46  and  47 . In the example in  FIGS. 1-4  it is the body of the bushing  20  which closes off the circuit  50  at the top. 
   The switch is dimensioned in such a way that the outside diameter of the cylindrical surface  41 , measured when cold, is slightly greater than the diameter of hole  18 . For example, if the hole diameter is 54.00 mm, the external diameter of the switch will be 54+ 5/100 mm. In this way there is cold radial interference between the switch  40  and the hole  18 . 
   In order to insert the switch in the hole, the hot plate  10  is heated by activating electrical resistors (not illustrated) incorporated in the plate itself to cause expansion of hole  18  such that the switch  40  can be easily inserted into it. For example, if the hot plate is heated to a temperature of approximately 250° C., the diameter of the hole becomes 54+ 17/100 mm. If the hot plate is then allowed to cool, the hole  18  contracts and the switch  40  remains firmly secured through radial interference. In other words the switch is shrunk-on into the hot plate. The radial interference between the switch  40  and the hole  18  is such as to ensure a seal which is leaktight for the high pressure molten plastics material which fills the hot channels when the injection machine is in operation. The switch remains immobilised in the same angular position even when the hot plate and the switch are heated during use. 
   When it is necessary to change the connections between the hot channels opening into hole  18 , a cooling fluid (for example water) is caused to circulate in circuit  50 . The plate  10  is instead kept hot. The switch contracts, acquires slight radial play with respect to the wall of the hole  18  and can be rotated about the X axis to a new desired angular position lining up or offsetting passages  42 ,  43  with respect to the hot channels  15 ,  16 ,  17  depending upon whether it is necessary to stop or restore or divert the flow between the channels, or feed the nozzles from one injection bushing or another. 
   Once the new desired angular position of the switch has been reached, the circulation of cooling fluid is interrupted and the switch expands thermally thus becoming immobilised through radial interference in the new angular position reached in the hole  18 . 
   The switch may be rotated either manually or through an actuator, for example a hydraulic or pneumatic or electrical actuator, acting on a terminal part of the switch. In the example in  FIGS. 1-4  a hydraulic actuator  6  is provided which causes a toothed sector  62  of one piece with the switch to rotate by means of a rack  61 . In the example in  FIG. 5 , the upper plate  46  has a portion in the form of a hexagonal nut  63  which can be used to rotate the switch with a spanner. 
   Experimental tests carried out by the Applicant have demonstrated that the stages of cooling and angular disengagement of the switch from the hot plate with rotation of the switch into the new angular position do not together require more than 90 seconds.