Abstract:
A device for feeding an extrusion head for plastic material includes a main extruder connected to a main die for extruding a continuous main flow of material, and an auxiliary feed mechanism associated with the main die and selectively controlled to cover the outer exit flow of the main die with a surface layer. The auxiliary feed mechanism continuously conducts two auxiliary flows, a first flow forming the surface layer and the second flow being mixed with the main flow. An invertor valve permits selective exchanging of the two auxiliary flows such that the second flow can cover the main flow and the first flow can be mixed with the main flow.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The subject of the present invention is a device for feeding an extrusion head for plastic material. 
     Devices of this kind, already known, are used especially for fabricating by extrusion elongated products of plastic material presenting a colored surface layer. 
     It is usual to use two method for that: 
     1. Master-batches of colors are added to the mass of plastic material, 
     2. An additional extruder is used, which deposits a colored surface skin around the extruded shape. 
     The second method has the advantage of saving on the coloring agents since only a thin layer of the extruded product is colored. 
     The object of the present invention is to improve the feed devices already known, in order to apply the second method described above by achieving an instantaneous change of color without waste of material. 
     To this end, the device according to the invention is characterized in that is comprises a main extruder connected to a main die and auxiliary feed means associated with the main die and capable of being selectively controlled so as to cover the exit flow of the main die with a surface layer formed of successive segments, the appearance of which differs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the subject of the invention will be described below, by way of example, referring to the appended drawing, in which: 
     FIG. 1 is a schematic top plan view showing an extrusion installation equipped with the device according to the invention, 
     FIG. 2 is a sectional view on a horizontal plane showing on a larger scale the extrusion head and its different feed orifices, 
     FIG. 3 is a sectional view along the line III--III of FIG. 2, and 
     FIG. 4 is a perspective view, partially cut away, of a length of a section of plastic material fabricated by the device of FIGS. 1 to 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The installation depicted in FIG. 1 is intended for the continuous production of a section of plastic material, for example, a bar, or a tube, the cross-section of which may have any shape. It comprises a main extruder 1 provided with a feed hopper 2 and driven by a motor 3 having a reduction gear 4. Mounted at the end of the cylinder of this extruder 1 is an extrusion head 5, the details of which are visible in FIG. 2. On the other hand, this extrusion head 5 is connected to an auxiliary twinscrew extruded 6 provided at its rearward end with two feed hoppers 7 and 8. As is seen in FIG. 2, the auxiliary extruder 6 comprises two parallel cylindrical bores 9 and 10, in each of which a screw 11 or 12 rotates. These screws are driven by a single speed-reduction gear 13 from a single motor 14, ensuring that the two screws constantly rotate at the same speed. As the dimensions of the screws are the same, the flows of plastic material formed in the two cylinders 9 and 10 are of the same rate and reach the exit orifices 15 and 16 of the extruder 6 in the same state of fluidity and compression. 
     FIG. 2 further shows the structure of the extrusion head 5. Mounted in the cylindrical die body 17 are a main die 18, supported by a die holder 19 and, in front of the die body 17, an auxiliary die 20 supported by a die holder 21 and fixed by a nut 22, the die holder being itself secured to the body 17 by a nut 23. The die 18 is fed at the rear of the extrusion head through the die holder 19 in a conventional manner. Thus, a direct connection of the die holder to the outlet of the cylinder of the extruder 1 is provided for. 
     The auxiliary extruder 6 is connected to the extrusion head 5 by ducts which form two paths, these pass through a multiple-way valve 24, the body 25 of which is fixed to the end of the body of the extruder 6 in such a way as to be able to feed the head 5 laterally. The multiple-way valve 24 comprises a cylindrical valve needle 26. At one end, this valve needle is provided with a prolongation of square cross-section (FIG. 3) permitting it to be rotated about its axis through an angle of 120° in order to effect the conversion of the colors, as will be seen below. One of the two inlets 27 and 28 made in the body 25 ends facing a radial bore 29 passing through the valve needle 26 up to its axis, and the other facing a groove 30 which extends over 120° along the circumference of the valve needle 26. In the position depicted in FIG. 2, the groove 30 causes the inlet 28 to communicate with an outlet 31 which ends, through the die body 17, in the outside surface of the die holder 21 and thence, via a distribution duct 44, feeds the conical space comprised between the dies 20 and 18. By this route, the plastic material forced back by the screw 12 forms a tubular film on the surface of the elongated product which is shaped in the die 18 with the material supplied by the main extruder. While the valve 24 is in the position of FIG. 2, the material forced back by the screw 11 passes through the orifice 15 and into the duct 27. It arrives, as is seen in FIG. 3, through an axial duct 32 of the valve needle 26, in an outlet duct 33 which is connected to a helical channel 34 made in the body 17 in such a way as to encircle the die 18 and, via the groove 37, the receptacle 35, and a radial duct 36, to arrive in the inlet orifice 42 of the main die 18. This material is therefore mixed with the material supplied by the main extruder, and as a result of its being projected in the form of a jet into the receptacle 35 from the groove 37, a rather rapid mixture of these two materials takes place. 
     It will be realized that the functions of the two auxiliary extruder screws 11 and 12 may be reversed by rotating the valve needle 26 about its axis clockwise, as viewed in FIG. 2 through an angle of 120°. In this case, the inlet duct 27 is then connected directly to the outlet duct 31, while the inlet duct 28 is connected via the radial bore 29 and the axial channel 32 to the duct 33, to the helical channel 34, and to the groove 37. 
     The use of the device described and the advantage of this device follows from FIG. 4, which depicts in cut-away perspective a length of section, for example, a cylindrical rod of plastic material, fabricated with the device described. It will be understood that each of the hoppers 7 and 8 of the auxiliary extruder 6 is provided with a volumetric metering apparatus 45, 46, permitting a certain quantity of coloring agent to be inserted in this hopper, with the plastic material which feeds it, the two colorant distributors containing materials of different colors, for example, red and blue. It is therefore seen that if the valve needle 26 is in the position of FIG. 2, and if the cylinder 10 is fed by a plastic material provided with a batch of red colorant, the front portion 3a of the cylindrical section produced by the installation will be provided with a red surface film 39. However, the installation will permit changing the coloring of the surface film 39 in the following segment 38b by a simple manipulation of the valve 24. It will be necessary beforehand to start up the colorant-metering apparatus 45 of the cylinder 9 in order that the hitherto virgin plastic material which is led into the receptacle 35 via the ducts 29, 32, and 33 starts to be colored blue, as depicted at 40 in FIG. 4. At the moment when the desired red length of the elongated section has been produced, it will suffice to switch over the valve 24. Immediately, the blue material supplied in the inlet duct 27 will be directed toward the auxiliary die 20 and will give the surface film 39 a blue coloring, which will appear on the section, forming the segment 38b. The mass of red plastic material which the screw 12 forces into the duct 28 will then be led via the by-pass 34 into the receptacle 35 and will form a colored zone 41 in the thickness of the section. After the reversal of the colors, the introduction of the coloring agent into the hopper 8 by the metering apparatus 46 can be stopped, so that the cylinder 10 will then carry virgin plastic material, similar to that which enters the die 18 through the orifice 42 from the main extruder 1. 
     The system described lends itself to the production of any section of plastic material produced continuously. It applies, for example, to the production of tubes, but equally to the production of insulating layers covering an electric conductor, to the production of sheaths on cables, etc. The device permits, for example, producing a certain length X km of elongated section covered with a surface layer of one color during a given time, changing the visible color instantly, and producing a length Y km of the same product covered with a surface layer of another color. The extruder continuously works virgin material. One of the two cylinders of the auxiliary extruder continuously works on the virgin material, the other on the colored material. The passing from one color to the other is achieved by starting up the metering apparatus containing the coloring agent having the required color, then, at the required moment, by manipulating the valve 24. 
     The starting up of the volumetric metering apparatus can be achieved automatically by electronic means, such as a preset counter. Of course, the colors may be alternated, or sections successively having lengths of several different colors may be produced.