Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a U.S. National Stage of International Application No. PCT/IB2009/007046, filed Oct. 5, 2009, designating the United States and claiming priority to Italian Application No. TO2008A000731, filed Oct. 6, 2008. 
     TECHNICAL FIELD 
     The present invention relates to a plastic foam molding machine material change method, and to a molding machine implementing such a method. 
     In the following description, the term “material change” is intended to mean changing a first material in a molding machine with a second material different from the first. More specifically, the second material may either differ completely from the first, or have the same composition as the first but differ as to one or more characteristics, such as weight and/or density and/or simply colour. 
     Purely by way of a non-limiting example, the following description refers to the most common case of changing, in a molding machine, a first coloured plastic foam material—normally a polymer foam, such as polypropylene, polyethylene, polystyrene, to which the following description refers purely by way of example—with a similar foam material of different colour. 
     BACKGROUND ART 
     In known molding machines, the plastic material is fed into a mold by a feed circuit connected to a bin storing plastic material in the form of pre-expanded granules of a given colour. The feed circuit normally comprises at least one reservoir, in which a batch of material drawn from the bin is maintained at a given pressure; and at least one, normally pneumatic, loader connected to the reservoir by a header to receive a quantity of material from the reservoir and feed it directly into the mold. 
     A change in product colour involves emptying the feed circuit, in particular the reservoir, of the previous batch of a first material; cleaning the feed circuit; and loading the reservoir with a batch of a different-coloured second material. 
     As described, for example, in DE 102004016756, the feed circuit is still normally cleaned by successively aspirating the first material from the reservoir and then from various points of the feed circuit downstream from the reservoir, to clear the machine of the first material batch. 
     This method has serious drawbacks, mainly due to the tendency of the pre-expanded granular material to charge electrostatically and adhere to the inside of the reservoir and the feed circuit conduits, so that successive suction cleaning fails to ensure all the material is removed. As a result, when the machine is turned on again, any granules of the previous colour left inside the feed circuit contaminate the colour the initial output of the machine, which must therefore be rejected. 
     The above considerations apply to DE-3900664, U.S. Pat. No. 5,961,734 and WO-2004/103522. 
     SUMMARY 
     It is an object of the present invention to provide a material change method for a coloured plastic foam molding machine, which is cheap and easy to implement and designed to eliminate the above drawbacks. 
     According to the present invention, there is provided, in one embodiment, a method of changing material in a plastic foam molding machine, comprising: a cleaning cycle to remove a first batch of a first material from a feed circuit interposed, in the machine, between a storage bin and a mold, before replacing the first batch with a second batch of a second material different from the first; the cleaning cycle comprising: a first step of emptying a reservoir of the feed circuit by suction to remove any material of the first batch still inside the reservoir; the method being characterized by further comprising: a second step of pushing any material of the first batch remaining along the feed circuit at least in part outside the feed circuit and at least in part into a header located at the bottom of the reservoir by injecting compressed air into the feed circuit, the header having at least one transverse ejector associated with a respective outlet end loader of the feed circuit; and a third step of emptying the header by suction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  shows a schematic, partial block diagram of a preferred embodiment of the machine according to the present invention; 
         FIG. 2  shows a larger-scale, partly sectioned front view of a detail of the  FIG. 1  machine; 
         FIG. 3  shows a section along line III-III in  FIG. 2 ; 
         FIG. 4  shows a larger-scale detail of  FIG. 3 ; 
         FIGS. 5 to 10  show schematic block diagrams of the  FIG. 1  machine in different operating configurations. 
     
    
    
     DETAILED DESCRIPTION 
     Number  1  in  FIG. 1  indicates as a whole a machine for molding coloured plastic foam material, such as polypropylene, polyethylene, polystyrene and their copolymers. 
     Machine  1  comprises a bin  2  for storing plastic material in the form of pre-expanded granules of given colour; a known mold  3  (therefore not described); and a feed circuit  4  interposed between bin  2  and mold  3  to feed measured amounts of plastic material into mold  3 . 
     More specifically, as shown in  FIGS. 1 ,  2  and  3 , feed circuit  4  comprises a loading reservoir  5  defined by a hollow body  6  coaxial with a longitudinal axis  7  and closed at the top by a lid  8 , and at the bottom by a hopper  9 . Reservoir  5  is connected to bin  2  by a feed pipe  10 , the outlet end of which is connected to an inlet  11  on lid  8  by a valve  12  for enabling or disabling material flow to reservoir  5 . Reservoir  5  is also connected to a three-inlet fitting  13  fitted to lid  8  and comprising a suction inlet  14  connected by a valve  15  and a pipe  16  to a suction pump  17 , which is activated, when loading reservoir  5 , to create a vacuum inside reservoir  5  and so draw a batch of material into reservoir  5  along pipe  10  and through the open valve  12 . 
     Facing inlet  14 , fitting  13  comprises an inlet  18  for compressed air, which is fed along a feed pipe  20  to reservoir  5  by a compressor  19  to bring reservoir  5 , already loaded with a batch of material, to a given operating pressure. Compressed-air flow into reservoir  5  is regulated by a valve  21  upstream from inlet  18  and close to the outlet end of pipe  20 . 
     Between inlets  14  and  18 , fitting  13  comprises a third inlet connected to a relief valve  22  for discharging the pressure inside reservoir  5 . 
     An intermediate portion of compressed-air feed pipe  20  to reservoir  5  has a branch pipe connected, via an open/close valve  23 , to a number of branches  24  (only one shown in  FIG. 1  for the sake of simplicity) for feeding compressed air into reservoir  5  through respective inlets  25 , which are formed in a lateral wall of body  6 , are equally spaced about axis  7 , and are located in a plane crosswise to axis  7  and a given distance from hopper  9 . 
     In a variation not shown, pipe  20  has two or more superimposed branch pipes with respective numbers of inlets  25  located in respective planes crosswise to and superimposed along axis  7 . 
     As shown in  FIGS. 1 to 4 , hopper  9  is funnel-shaped, tapering downwards, with a substantially rectangular outlet  26 , and a substantially circular inlet connected smoothly to the bottom end of body  6 . Hopper  9  is closed at the bottom by a block  27  housing a substantially cylindrical header  28 , which is coaxial with an axis  29  perpendicular to axis  7 , communicates with outlet  26  of hopper  9  through an opening  30  parallel to axis  29 , and has a number of lateral outlets  31 , each communicating directly with the inlet of a respective ejector  32  of a group of ejectors  32  carried by block  27  and aligned side by side in a direction parallel to axis  29 . 
     Header  28  is controlled by a valve  33  comprising a shutter  34  housed in header  28  and rotating about axis  29 ; and a pneumatic actuator  35  ( FIG. 2 ) fitted to hopper  9  and for moving shutter  34  between an open position ( FIG. 4 ) allowing material to flow freely to ejectors  32  through opening  30 , header  28 , and outlets  31 , and a closed position in which shutter  34  closes outlets  31  to cut off material flow from header  28  to ejectors  32 . 
     As shown in  FIGS. 1 to 4 , ejectors  32  are pneumatic, and each have inlets for compressed air, which is injected under the control of a valve  36  and intermittently with a given frequency to “lubricate” material flow through ejectors  32 . 
     Each ejector  32  is connected by a respective material feed pipe  37  to a respective loader  38  (only one loader  38  shown in the drawings for the sake of simplicity) defined by a tubular body, which, close to its rear end, has an inlet  39  connected to material feed pipe  37 , and, at its front end, has a nozzle  40  for feeding the incoming material from reservoir  5  directly into mold  3 . 
     Loader  38  also comprises a first compressed-air inlet  42  connected by a feed pipe  43  to compressor  19 . An intermediate portion of pipe  43  has a branch pipe fitted with a switch valve  44  connected by a pipe  45  to a second compressed-air inlet  46  close to inlet  42 , and by a pipe  47  to a third compressed-air inlet  48  located at the rear end of loader  38  and communicating with the end of a piston (not shown), which is housed inside loader  38  and slid, by the compressed air through inlet  48 , between a withdrawn position opening nozzle  40 , and a forward position closing nozzle  40 . 
     Feed circuit  4  also comprises a two-inlet cyclone separator  49 , a first inlet  50  of which is connected by a pipe  51  to a two-way valve  52  fitted to material feed pipe  10  to selectively connect reservoir  5  to bin  2  by pipe  10 , and to cyclone separator  49  by pipe  51  and an end portion of pipe  10 . 
     A second inlet  53  of cyclone separator  49  is connected by a pipe  54  to a first outlet of a two-way valve  55  for connecting pipe  54  to a pipe  56  and, via an open/close valve  57 , to an axial end of header  28 . 
     Feed circuit  4  also comprises an aspirator  58  connected by a pipe  59  to a second outlet of valve  55  to communicate, via pipe  56  and valve  57 , with header  28 . 
     Operation of machine  1  will now be described with reference to  FIGS. 5 to 10 , which show a cleaning cycle to which machine  1  is subjected after performing a given number of operating cycles (in known manner) using material of a given colour, and after being stopped to make a colour change. 
     The cleaning cycle comprises a first subcycle comprising five steps; and a second subcycle comprising an end step hereinafter referred to as STEP  6 . 
     In a preferred embodiment, the first subcycle is repeated two or more times before performing the second subcycle. 
     For the sake of clarity, in  FIGS. 5 to 10 , the “active” portions of feed circuit  4  and the relative travelling directions are indicated, for each step, by continuous lines and directional arrows. 
     STEP  1  ( FIG. 5 ): Suction of the batch of material in reservoir  5 . 
     At this step, valve  12  is opened intermittently; valve  52  is set to connect cyclone separator  49  to reservoir  5  by pipes  51  and  10 ; valve  23  is opened intermittently; and, finally, valve  33  is closed, valve  57  is opened, and valve  55  is set to connect header  28  to aspirator  58 . Once aspirator  58  is activated, the batch of material in reservoir  5  is sucked into aspirator  58  via header  28  and pipes  56  and  59 . At the same time, compressor  19  ( FIG. 1 ) is activated, and the compressed-air jets blown into reservoir  5  through inlets  25  create an airflow to assist expulsion of the material from reservoir  5 , and, in particular, dislodge into header  28  at least some of the granules of material which, charged electrostatically, would adhere to the inner wall of reservoir  5 ; and the open valve  12  allows airflow from cyclone separator  49  into reservoir  5  to prevent formation of a vacuum inside reservoir  5 . 
     STEP  2  ( FIG. 6 ): Compressed-air injection to remove residue from reservoir  5 . 
     At this step, valve  12  is closed to isolate reservoir  5  from cyclone separator  49 ; valve  23  is kept open intermittently, and valve  21  is opened intermittently to also allow compressed air into reservoir  5  through inlet  18  as well as inlets  25 ; valve  33  is kept closed, valve  57  is kept open, and valve  55  is set to connect header  28  to cyclone separator  49  by pipes  56  and  54 . 
     Airflow is thus created through reservoir  5 , along header  28 , and into cyclone separator  49 , and which takes with it any granules of material remaining inside reservoir  5  and header  28 . 
     STEP  3  ( FIG. 7 ): compressed-air injection to remove residue from the portion of feed circuit  4  feeding material to reservoir  5 . 
     At this step, valve  33  is kept closed; valve  57  is closed; valves  23  and  21  are kept open intermittently, and valve  12  is opened; and valve  52  is set to connect reservoir  5  to cyclone separator  49  by pipes  10  and  51 . 
     Airflow is thus created through reservoir  5 , out of reservoir  5  through material inlet  11 , along pipes  10  and  51 , and into cyclone separator  49 , taking with it any granules still left inside reservoir  5  and pipe  10 . 
     STEP  4  ( FIG. 8 ): compressed-air injection to remove residue from the portion of feed circuit  4  feeding material to loaders  38 . 
     At this step, valves  12  and  21  are closed; valve  57  is opened; valve  55  is set to connect header  28  to cyclone separator  49 ; valve  33  is opened to also connect header  28  to loaders  38  by respective ejectors  32  and pipes  37 ; and valves  44  are set so that, for each loader  38 , compressed air fed along pipe  43  flows into loader  38  through inlet  42 , and compressed air fed along pipe  47  flows into loader  38  through inlet  48  to move the piston (not shown) into the forward position closing nozzle  40 . 
     For each loader  38 , operation of compressor  19  ( FIG. 1 ) thus creates airflow through loader  38  via inlets  42  and  39 , along respective pipe  37 , and through respective ejector  32  into header  28 , where it joins with the airflow fed through inlets  25  into reservoir  5 , and flows out along pipes  56  and  54  into cyclone separator  49 . 
     Any granules left on the material feed portions of loaders  38 , pipes  37 , and ejectors  32  are thus removed. 
     STEP  5  ( FIG. 9 ): suction to remove residue from the portion of feed circuit  4  feeding material to loaders  38 . 
     This step differs from step  4  by setting valve  55  to connect header  28  to aspirator  58  as opposed to cyclone separator  49 , so that the blow-off airflow produced by compressor  19  ( FIG. 1 ) along pipes  20  and  43  flows along part of reservoir  5 , loaders  38 , pipes  37 , and ejectors  32  into header  28 , and is sucked out by aspirator  58 . 
     This step provides for removing from feed circuit  4  any remaining granules fed into header  28  at STEPS  1  to  4  described above. 
     STEP  6  ( FIG. 10 ): “no-load” air run of loaders  38 . 
     At this step, operation of loaders  38  is simulated by feeding them with air, which is fed through inlets  25  and, after first opening valve  21 , through inlet  18  into reservoir  5 , and flows into loaders  38  through left-open valve  33 , header  28 , ejectors  32  and respective pipes  37 , following the same path as the actual material. Valve  36  for feeding air through ejectors  32  is opened intermittently, and valves  44  are set so that, for each loader  38 , compressed air flows along pipes  43  and  45 ; and cut-off of air supply to inlet  48  moves the piston (not shown) back to the withdrawn position opening nozzle  40 . 
     The compressed air injected into loader  38  through inlets  42  and  46  creates a venturi effect inside loader  38 , which expels the air “batch” through nozzle  40 , thus removing any granules from the front area of loader  38  close to nozzle  40 . 
     In a variation not shown, the cleaning cycle comprises a further step (not shown) before or simultaneously with STEP  4  described above. 
     The further step comprises blowing an antistatic product into reservoir  5 , together with the air fed through inlets  25 , to counteract the tendency of the granules to adhere to the inner wall of reservoir  5 , and so assist dislodging the granules into header  28 .

Technology Category: b