Abstract:
The present invention has an object to prevent an unstable weighing due to the plunger sliding resistance, by controlling the plunger retraction movement for the weighing in the prepla type injection molding machine dividing into speed and pressure The method consists in setting sequentially a plastic resin supply start position and a forced retraction release position thereof in the way of the plunger retraction stroke referring to a weighing start position, for weighing plastic resin by moving a plunger  10  from a weighing start position after injection and pressure-keeping to a retraction position of weighing stop; starting to supply the forward portion of the injection cylinder with plastic resin, when the plunger attains the supply start position from the weighing start position, by forced retracting the plunger up to the forced retraction release position thereof by speed control; and stabilizing the weighing by changing the speed control to the pressure control once the plunger  10  attains the forced retraction release position.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention concerns a weighing method for the prepla type injection molding machine comprising both of a plasticating apparatus and an injection apparatus.  
           [0003]    2. Detailed Description of the Prior Art  
           [0004]    In the prepla type injection molding machine, plastic resin by screw rotation is supplied to the injection cylinder forward portion from the plastication cylinder through a resin path, plastic resin is weighed by accumulating in the injection cylinder forward portion by retracting the plunger at the weighing start position by resin pressure, the plunger is advanced by blocking the resin path after the weighing, and plastic resin is injected and charged into the die from an injection cylinder end nozzle.  
           [0005]    The plunger after this injection and filling is in the most advanced position thereof, but the plunger retracts slightly to the place where the pressure is mitigated by resin counterforce generated by decompression after the completion of injection pressure-keeping, and almost at the same time, the resin path is opened to start supplying plastic resin.  
           [0006]    The plunger retraction weighing only by resin pressure makes the density of plastic resin weighed there unstable; therefore, a fixed backpressure is applied to the plunger, and the plastic resin supply is stopped, when the plunger attains a predetermined weighing stop position under this backpressure control.  
           [0007]    For the aforementioned screw, there are both of a type wherein resin is plasticized by rotating at a fixed position and, at the same time, plastic resin is supplied to the injection cylinder by discharge pressure, and a inline screw method where plastic resin is accumulated in the screw forward portion by retracting under resin pressure all the way rotating, and supplied by forward movement, and in either of them, plastic resin supply starts by opening the resin path.  
           [0008]    In such method of the prior art, the resin weighing (weighing time, weighing density, resin temperature, or other) may become irregular, making often difficult to obtain a good product continuously. It is supposed that, according to resins, a minute change of sliding resistance or others of the whole apparatus retracted by the plunger sliding resistance or resin supplied from the screw provokes backpressure variation, and changes the plastic resin feed state by screw rotation or forward movement.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention devised to solve the problem of weighing discrepancy of the aforementioned prior art has an object to provide a weighing method for the prepla type injection molding machine that can cancel the influence due to the plunger sliding resistance and stabilize the weighing, by controlling the plunger retraction movement for the weighing dividing into speed and pressure.  
           [0010]    The present invention according to the aforementioned object, is a weighing method for a prepla type injection molding machine, comprising both of a plasticating cylinder with an internal screw and an injection cylinder with an internal plunger, for supplying the injection cylinder forward portion with plastic resin by screw revolution or advance-movement, and injecting plastic resin by advance of the plunger after blocking the resin passage, comprising the steps of:  
           [0011]    setting sequentially a plastic resin supply start position and a forced retraction release position thereof in the way of the plunger retraction stroke referring to a weighing start position, for weighing plastic resin by moving the aforementioned plunger from a weighing start position after injection and pressure-keeping to a retraction position of weighing stop, starting to supply the forward portion of the injection cylinder with plastic resin, when the plunger attains the supply start position from the weighing start position, by forced retracting the plunger up to the forced retraction release position thereof by speed control, and stabilizing the weighing by changing the speed control to the pressure control once the plunger attains the forced retraction release position.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic longitudinal cross section illustration of the prepla type injection molding machine, object of the present invention;  
         [0013]    [0013]FIG. 2 is also a schematic longitudinal cross section illustration of the prepla type injection molding machine of another embodiment;  
         [0014]    [0014]FIG. 3 illustrates the stroke of the weighing method of the present invention;  
         [0015]    [0015]FIG. 4 shows the product weight comparison of the resin weighing by the present invention (A) and the resin weighing by the prior art (B); and  
         [0016]    [0016]FIG. 5 shows the comparison of plunger speed, plunger position and pressure waveform in the resin weighing by the present invention (A) and the resin weighing by the prior art (B). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    In the drawing,  1  is a injection apparatus, and  2  is a plastication apparatus juxtaposed on the injection apparatus, and both apparatuses communicate through a resin path  3  provided over the extremity portion.  
         [0018]    Said injection apparatus  1  has an inner plunger for injection  10  capable of advancing and retracting, and further, comprises an injection cylinder  12  provided with a nozzle  11  at the extremity, a hydraulic cylinder  13  provided behind this injection cylinder, and a holder member  14  integral with the forward portion of this hydraulic cylinder  13  and engaging and holding the rear end portion of the injection cylinder  12 .  
         [0019]    Said plunger  10  comprises a head portion  10   a  at the extremity, and a shaft portion  10  formed smaller in diameter that the head portion  10   a,  and the shaft portion rear end is coupled with the extremity portion of the piston  15  of the hydraulic cylinder  13  in the rear portion of said holder member  14 . A clearance if formed between an inner peripheral wall of the injection cylinder  12 , by the small diameter shaft portion  10   b,  and resin entered into the clearance is fed sequentially backward when the plunger retracts, and discharged outside through a rear opening  16 .  
         [0020]    Said plastication apparatus  2  has rotatively a screw for plastication  20 , comprises a plastication cylinder  22  having a discharge path  21  to the extremity, a holder portion  23  at the rear end portion for holding this plastication cylinder  22 , a hydraulic cylinder  24  for screw movement coupled to the rear end of the holding cylinder  23  and a motor for screw rotation  25 , couples said screw  20  rear end with the piston  26  of this hydraulic cylinder  22  inside said holding cylinder  23 , and at the same time, couples with a driving shaft  27  of a motor  25  and a piston  26  slidably in the axial direction, allowing to rotate the crew  20  with the piston  26 .  
         [0021]    Said resin path  3  comprises an inflow path  17  disposed above advance of the plunger limit position of the extremity portion of the injection cylinder  12 , and a piping provided slant across said discharge path  21  of the plastication apparatus  2 , and plastic resin by the screw  20  rotation is supplied into the extremity portion of the injection cylinder  12  through the resin path  3 .  
         [0022]    The injection machine of the embodiment shown in FIG. 1 forms a valve seat  28  at the inner periphery of the plastication cylinder  22  extremity connecting with said discharge path  21 , and forms the screw extremity portion  29  into a mushroom shaped valve disk, to open the valve by the advance-movement of said piston  26  for weighing, and to close the valve by retraction movement for other than weighing.  
         [0023]    The injection machine of the embodiment shown in FIG. 2 has a structure to plasticate resin by the screw  20  rotation, all the way accumulating the same in the screw forward portion, and retracting the screw  20  by resin pressure, and the supply the injection cylinder  12  with plastic resin, by advance-movement of the screw, and the inflow path  17  shown in FIG. 1 is provided with an opening/closing valve  18 , and the weighed resin is injected and charged by blocking the resin path  3  by this opening/closing valve  18 .  
         [0024]    Plasticated resin weighing in these injection machines is performed by mobbing said plunger  10  from the weighing start position after injection pressure-keepinging to the retraction stop position after weighing stop.  
         [0025]    This weighing is performed by, first as shown in FIG. 3, setting sequentially a plastic resin supply start position and a forced retraction release position thereof in the way of the plunger retraction stroke referring to the weighing start position, and forced retracting the plunger  10  from the weighing start position to the forced retraction release position, by the piston  15  of said hydraulic cylinder  13 . This forced retraction is performed by speed control.  
         [0026]    It is set so that the plunger  10  retracts from the forced retraction release position to the weighing stop position by supply resin pressure and the backpressure control can be performed by changing the control from speed to pressure. In case of decompressing after weighing completion, the retraction stroke for decompression from the weighing stop position is set.  
         [0027]    Under such setting, the plunger  10  is decompressed after the injection pressure-keeping, and thereafter, when the plunger  10  attains said supply start position by forced retraction by the function of said hydraulic cylinder  13  piston  15 , in the embodiment of FIG. 1, the screw  20  advances by the advance-movement of said hydraulic cylinder  24  piston  26  and the valve closed by the screw extremity portion  29  during injection filling is changed over to the valve open, and the screw  20  rotates with the piston  26  according to said motor  25  start, to plasticate resin and supply the injection cylinder  12  with plastic resin.  
         [0028]    In the embodiment shown in FIG. 2, the resin path  3  blocked by said opening/closing valve  18  is released and plastic resin and accumulated in the screw forward portion is supplied to the injection cylinder  12  by the screw  20  forward movement.  
         [0029]    In both of the aforementioned embodiments, resin is supplied to the injection cylinder  12  by the plastication apparatus  2  side discharge pressure or extrusion force, and plastic resin is charged in the forward portion of the injection cylinder  12  increasing the capacity thereof by force retraction of the plunger, with a inflow speed slightly delayed than the forced retraction speed.  
         [0030]    Consequently, there is no more supply amount discrepancy due to plunger sliding resistance, because the weighing filling resin pressure does not act directly as plunger  10  retraction force as in the prior art.  
         [0031]    In addition, plastic resin is not supplied until the plunger  10  retracts to the supply start position, the plunger forward portion cylinder capacity is increased so far after the retraction of set distance, and the plunger forward portion residual resin is in rough state; therefore, even if the resin path  3  resin pressure increases before supply start according to the valve opening, the plunger  10  movement in the beginning of weighing does not become instable under the influence thereof.  
         [0032]    For instance, in the embodiment of FIG. 1, residual resin in the forward portion of the screw extremity  20  is pressed and the resin pressure increases in the resin path  3 , because resin is plasticized and supplied by the screw  20  rotation, after having separated the screw extremity portion  29  from the disk seat  28  to open the valve by advancing the screw  20 ; however, as the plunger forward portion cylinder capacity being increased, the resin pressure is necessarily lowered and does not act as plunger  10  retraction force, and the plunger  10  retracts with a constant speed, thereby preventing the plunger  10  unstable movement which, otherwise, tends to occur in the beginning of weighing.  
         [0033]    In the embodiment of FIG. 2, the backpressure during plastication increases the resin pressure in the resin path  3  and affects the plunger  10  according to the opening of the opening/closing valve  18  before supply start; however, as the plunger forward portion cylinder capacity being increased as in case of FIG. 1, the resin pressure is lowered and does not act as plunger  10  retraction force, and the plunger  10  retracts with a constant speed, thereby preventing the plunger  10  unstable movement which, otherwise, tends to occur in the beginning of weighing, also in the embodiment of FIG. 2.  
         [0034]    Further, even when sliding resistance is generated by the plunger  10 , its influence is eliminated by the forced retraction, thereby keeping constant the plunger  10  retraction speed in the forced retraction area. Therefore, plastic resin feed state change due to plunger sliding resistance supposed to be a factor of weighing discrepancy is improved, and the weighing is always performed without supply amount.  
         [0035]    Next, when the plunger  10  attains the forced retraction release position, the retraction control changes from speed to pressure, and thereafter, in the resin pressure retraction area, the plunger  10  control becomes pressure control. In this area, plastic resin being weighed under the backpressure by the plunger  10 , the plastic resin density is kept constant up to the weighing stop, even if resin supply density becomes irregular in the forced retraction area depending on the plastication apparatus  2  side pressure.  
         [0036]    When the plunger  10  retracts to the weighing stop position, resin supply from the plastication cylinder  22  is stopped. In the embodiment of FIG. 1, the screw  20  rotation stops to stop the plastication, and following this, the screw  20  retraction movement the valve is opened by the screw extremity portion  29 , to close the resin path  3 . In the embodiment of FIG. 2, said opening/closing valve  18  closes to shut the resin path  3  and, thereafter, the screw  20  rotates to plasticate the resin.  
         [0037]    In any of these embodiments, resin backflow to the plastication apparatus  2  side is prevented during weighed resin injection and filling, by blocking the resin path  3  after the weighing stop, and excess resin suction by decompression after the weighing stop is prevented, thereby preventing weighed resin from becoming excessive or insufficient. Moreover, as necessary, the weighed resin injection and filling starts by the plunger  10  forward movement after the decompression.  
         [0038]    [0038]FIG. 4 shows the weighing state in the resin weighing by the present invention (A) and the resin weighing by the prior art (B) in respect of the product weight, and as it is obvious from the comparison of variation in both drawings (A), (B), the weighing discrepancy is improved in the present invention and stabilized than the conventional (B).  
         [0039]    [0039]FIG. 5 shows the plunger speed, plunger position and pressure waveform in the resin weighing by the present invention (A) and the resin weighing by the prior art (B) , and as it is obvious from the comparison of variation in both drawings (A), (B), the pressure variation is improved in the present invention and the discrepancy is smaller than the conventional (B), thereby keeping an extremely stable state.  
       EXAMPLE  
       [0040]    [0040]                                                       Molding machine   Embodiment of FIG. 1               (ES400/TM2E made by Nissei               Jushi Kogyo Co., Ltd.)           Material resin   LCP liquid crystal polymer           Product weight   about 6 g           Screw speed   260 rpm           Plunger retraction distance   18 mm           Forced retraction distance   10 mm           Plunger retraction speed   12 m/sec           Plunger backpressure   30 Kgf/cm2           Weighing speed (plunger   9 mm/sec           retraction speed by resin)