CH626290A5 - Detection device for moulding faults in a plastics injection moulding machine - Google Patents

Description

The invention relates to a detection device for casting defects in a plastic injection molding machine according to the preamble of independent claim 1.

In plastic injection molding machines, a piston runs from the injection molding start position to the injection molding end position during the injection molding process. Is the injection molding start position e.g. P "and the injection molding end position PT in FIG. 1, the piston (not shown) runs from P" to PT and returns to P0 when the injection molding is finished. In the case of continuous operation under the same molding conditions, insofar as a normal injection molding process is effected, the relationship between the path and the time in an injection molding stroke is essentially constant while the piston is running from the injection molding start position to the injection molding end position. An example of this relationship is shown by a solid line A in FIG. 2. The time during which the piston passes through a point Pi a distance lj away from the injection molding start position P0 has a substantially constant value t, in continuous operation and the position of the piston, i.e. the injection molding end position PT at the time of the injection molding end t0 is essentially constant, which corresponds to the distance 10. If a defective pouring due to the mixing of foreign matter in the resin, an inappropriate resin temperature or the like is expected, the relationship between the travel of the piston and the time is not the solid line A in Fig. 2, but similar e.g. curves B and C. Curve B occurs when the piston runs fast for some reason and extends beyond the injection molding end position PT depending on the respective circumstances. Curve C occurs when the piston runs slowly and does not reach the injection molding end point PT at time t0. A faulty pouring can therefore be detected by paying attention to the relationship between the piston travel and time.

A conventional casting defect detector has a limit switch S0 which is turned on when the piston starts injection molding, a switch Slt which is turned on when the piston passes through a desired point Pj located between P0 and PT, and a switch SE which is turned on when the plunger is over PT by a distance 1E to detect when the plunger is over PT, with switches S0, St and SB in the circuitry shown in FIG. 3. A timer 1 actuates a contact 1 a, which is switched on when the timer is actuated. Relays 2, 3 and 4 each have normally open contacts 2a, 3a and 4a and a normally closed contact 3b of relay 3. A lamp L 35 indicates the passage of the piston through point Pj. In this conventional circuit, the switch S0 is first switched on when the piston begins to move and, at the same time, the timer 1 is excited when the movement begins. Since the timer 1 is set to a time txl which is somewhat 40 longer than the time t] during which the piston usually moves through the point Pt, the switch S, is set to "on" before the timer is actuated to turn on contact la when the casting is carried out evenly. Then the relay 3 is operated, where-45 is turned on by its contact 3a and holds it itself, while the contact 3b is opened and thereafter the relay is prevented from being operated even when the contact la of the timer is turned on.

In the event that the piston runs more slowly and the timer 50 is actuated before the switch S is switched on, the relay 2 is excited via the contact la of the timer and the normally closed contact 3b of the relay 3 is closed. Thereafter, the contact 2a is closed to actuate the relay 4, whereby the contact 4a is closed to hold it, and the relay 4 emits a casting error signal, whereby an alarm is switched on or an electromagnet is actuated which is on a Shooting device for screening defective products is arranged. Also in the case where the plunger does not stop at the sprayer end point Px-60 and reaches the point PE, the switch SE is closed to turn on the relay 4, whereby the same effects as mentioned above occur. In this conventional detector, however, it has been found that if the time that the piston passes through 65 point P is shorter than the preset time t ,, of the timer, it is normal and if the time is longer than the preset Time is, this is flawed. In particular in connection with FIG. 2, it was found that.

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if tlt is tx, the curves passing through a coordinate Yx or above, e.g. A and B are normal curves, and a curve below the coordinate Yx is an erroneous curve. Although curve A or a curve practically close to it are normal curves, curve B is an erroneous one. With the detection device described above, it is therefore impossible to detect errors caused by the piston moving too quickly.

In addition, a detection device for casting defects is known, in which the time measurement is started at the point in time when the liquid pressure of the piston to advance the piston of the injection molding machine has reached a certain level, the time is measured during which the piston runs to the injection molding end position, and If the measured time is outside the preset time range, an alarm for excessively fast or excessively slow is issued. Unlike the previously mentioned detection device, this detection device can also detect an excessively fast state. However, this device is designed so that the time measurement is started at the point in time when the liquid pressure has reached the certain level. There is therefore a risk of errors if the viscosity of the casting material e.g. is changed during the injection molding stroke. A more detailed explanation follows later. As shown in FIG. 4, the relationship between the time and pressure of the injection molding during the normal injection molding process is J, the relationship between the time and the path as K, and the time during which the piston is in the injection molding end position 10 after the specific pressure hs has been reached as the injection molding pressure, designated t21. In contrast to the foregoing, the relationship between time and pressure is denoted by L in the case where the viscosity of the casting material is low and the temperature of the metal mold is low, or the viscosity of the casting material is low and the filler opening for it Material in the mold is blocked by foreign matter, and the relationship between the time and the path in the above cases is denoted by M and the measured time is t31. In the detection device of the type described, the viscosity of the material is initially low and the piston runs quickly. The curve M is therefore above the curve K. If the temperature of the metal mold is low or the filling opening of the mold is blocked by foreign parts, as a result of which the piston runs slowly and therefore the curve M falls below the curve K, then the time during which the Injection molding pressure hs reached, delayed by Ati and the time during which the piston reaches the injection molding end position I0 is also delayed by / \ t2. If the amount of Ati and At2 is essentially the same, t21 becomes t31, which makes it impossible to detect a faulty casting.

The aim of the invention is to create a detection device for casting defects which can reliably detect defective casting by means of a plastic injection molding machine.

The detection device according to the invention is characterized by the features in the characterizing part of independent patent claim 1.

The aforementioned and further features and objects of the invention will become clearer from the following description of an exemplary embodiment in conjunction with the drawings, in which the same reference numerals designate the same components.

In detail shows:

Fig. 1 is a graph showing the positional relationship of the switch used to detect the position of the plunger or

Piston are provided in a conventional detection device for casting defects,

2 is a graph showing the relationship between the stroke and the time of the ram in the conventional embodiment;

3 shows a circuit of a conventional detection device for casting defects,

4 is a graph showing the relationship between the injection molding pressure and the time and between the path and the time in the conventional embodiment;

5 and 6, the positional relationships of switches, which are provided in one embodiment of the invention,

7 is a graph showing the relationship between the stroke and the time of the plunger or piston to explain the operation of this embodiment;

8 shows an electrical circuit of the detection device according to the invention for casting defects.

5 shows a front mold 34 fastened on a fixed plate 35. A movable mold 36 is arranged on a movable plate 37. The front mold 34 has a connecting channel 40 which is connected to a nozzle part 39 of an injection molding cylinder 38. A hopper 41 for a plastic material is provided in the middle part of the injection molding cylinder. One end of the plunger, plunger or piston 42 is molded onto a piston 43. This piston 43 is provided in a hydraulic cylinder 44, as a result of which the plunger 42 can be moved forwards and backwards in the injection molding cylinder 38. The plunger 42 can be rotated by a drive mechanism, such as an electric motor, not shown here. A ring 45 is attached to the central part of the plunger 42. Limit switches S0, Sls S2 actuated by the ring 45 are arranged along the stroke of the plunger 42. The positional relationship of the limit switches S0, Sj, S2 is shown in detail in FIG. 6. A point Px at a distance lx from P0 and another point P2 at a distance L from P0 are preset between the injection molding start point P0 and the injection molding end point PT of the plunger, and the switches Sj and S2 are provided so that they move from the open to the closed state be switched when the plunger passes through the respective points in the direction of the arrow shown. As described above, the switch S "is switched from the open to the closed state at the same time when the plunger starts the injection molding process. This switch S0 can be arranged at a point a certain distance before Pt from the injection molding start position of the plunger, so that it starts the time measurement after the start of the injection molding. In addition, the positions and the number of preset points for the time measurement can be selected in a desired manner depending on the casting conditions. A pulse oscillator 5 oscillates at a certain frequency. A pulse emitted by the pulse oscillator is fed to a counter 7 via a blocking gate 6, which is closed by ON signals from switches S0, S. A display type digitally shows a counter reading X1 of the counter 7. In the same way, a blocking gate 9 can be closed by an ON signal from the switch Sa, while a display 11 shows a counter reading X2 of a counter 10. An output pulse from the pulse oscillator 5 is given to the counter 10 via the blocking gate 9. Digital presetting devices 12, 13, 14 and 15 are also provided. The presetting device 12 sets the lower limit value D, the point in time at which the plunger passes the preset point Pt, the presetting device 13 the upper limit value of this point in time, the presetting device 14 the lower limit value, the point in time

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which the plunger runs through the preset point P2, and the presetting device 15 sets its upper limit value. Multiplexers 16 and 17 and a subtractor 18 are also provided. Upon receipt of a timing signal TM, the multiplexer 17 selectively and sequentially switches the values D-G, which are preset by the digital presetting devices 12 to 15, in order to pass them on to the subtractor 18, for example, and to repeat the previously described mode of operation. In the same way, when the preset values D or E of the digital presetting devices 12 or 13 are selected, the multiplexer 16 selects the counter reading X! of the counter 7 and passes it to the subtractor 18, while if the preset values F or G of the digital presetting devices 14 or 15 are selected, he selects the counter reading X2 of the counter 10 and gives it to the subtractor 18, as well as the effect described above repeated. The subtractor 18 subtracts the preset value of the digital presetting devices 12-15 from the counter reading of the counter 7 or 10, whereby it is synchronized with the timing signal. In particular, the subtractor 18 performs the subtractions of four types, including X, -D, Xt-E, X2-F and X2-G, sequentially, being synchronized with the timing signal. The order of subtractions of these four types is not necessarily limited to the one given above. A circuit 19 judges the subtraction result whether it is plus or minus and is synchronized in the same way with the timing signal TM. A demultiplexer 20 and latch circuits 21 to 24 are also provided. The demultiplexer 20 switches the respectively determined results of the subtractions of the four types, which were judged by the judging circuit 19, to the respective latch circuits corresponding to them. Therefore, when the subtractor 18 performs the calculation of Xt-D and it has been determined that the result is plus or minus, the judged result is held by the latch circuit 21. Thereafter, the subtractor 18 performs the calculation X, -E, and the result is judged whether it is plus or minus and held by the latch circuit 22. Likewise, the result of the calculation X2-F with respect to plus or minus is held by the latch circuit 23, while the judged result of the calculation from X2-G, plus or minus, is held by the latch circuit 24. These latches 21 to 24 give a "1" when the judged result is plus and "0" when the judged result is minus. If the assessed results change, the corresponding output signals also change. A NOT gate 25 inverts an output signal of the latch circuit 21 and an AND gate 26 receives an output signal from the NOT gate 25 and an output signal from the switch S, and outputs an output signal when the time during which the plunger passes the preset point P, is shorter than the lower limit value D. A blocking gate 27 outputs an output signal if the switch S, does not output an output signal and the latch circuit 22 outputs a "1", as well as if the time during which the Tappet runs through the preset point Pj, is longer than the upper limit E ^ Similarly, a NOT gate 28 reverses an output signal from the latch circuit 23 and an AND gate 29 receives an output signal from the NOT gate 28 and an output signal from the switch So and outputs an output signal when the time during which the plunger passes through the preset point P2 is shorter than the lower limit value F. 30 is a barrier gate; therefore if switch S2 has no output and the blocking circuit sends out a "1", there is an exit and if the time

to which the piston comes via the preset point P2 is longer than the upper limit value G, there is an output signal. An OR gate 31 is connected to a latch circuit 32 for holding an output signal from the 5 OR gate 31 and outputs a casting error signal W. A switch 33 can reset the latch circuit 32. This switch 33 can be a manually operated switch or a timer provided to control the injection molding machine so that a molding error signal from the latch circuit 32 is only given during a molding cycle during which an unusual condition occurs. The operation of this circuit will now be explained. First, the upper and lower limit values D, E. F, G for the preset points Pj, P2 are preset using the digital presetting devices 12 to 15 and then the injection molding machine is started. At the start of injection molding by the plunger, the switch S0 is closed and pulses are fed to the counters 7 and 10 via the blocking gates 6, 9 20. When receiving timer signals, the multiplexers 16, 17, the demultiplexer 20, the subtractor 18 and the plus-minus judging circuit 19 perform the following effects, which are the reading of the counter readings Xi or X2 of the counter 7 or 10, the reading of the preset values of the 25 digital presetting devices 12-15, the subtraction, the plus-minus assessment and the summary of the assessed results on the associated latch circuits 21 to 24. This described reading for summarizing is repeated as a continuation of the increase in the counter readings Xx and X2 with the preceding time. Since the switch Sx is open (S2 is usually also open) until the plunger passes through the preset point Pj, the AND gates 26, 29 do not output any signals. In addition, if the casting is carried out nor -35 times, the counter reading X1 does not exceed the upper limit value E, which is preset for Px, before the ram passes through the preset point Pj. The latch circuit 22 therefore outputs an output signal “0”. The locking circuit 24 also emits an output 40 signal of “0”. The blocking gates 27, 30 therefore do not emit any output signals. If the plunger passes through point P15, switch S1 is closed. As a result, the blocking gate 6 is closed and the counting process is interrupted by the counter 7, and the count is shown by the display 8 on-45. If at this point in time Xt is greater than the lower limit value D, an output signal “1” from the locking circuit 21 is reversed by the NOT element. Therefore, even if the switch Su is closed, the AND gate 26 does not output an output. In this case, if X is less than the upper limit value Ej, the locking circuit 22 emits an output signal “0” and the blocking gate 27 does not emit an output signal. Therefore, assuming that the time during which the plunger passes through the point Px is Xn when Xn remains within the range of 55 D Xu E, the AND gate 26 and the lock gate 27 do not give an output signal. In contrast, the latch circuit 21 outputs an output signal "0" which is reversed by the NOT gate when D> Xn. The AND gate 26 therefore emits an output 60 signal, which actuates the locking circuit 32 to emit a pouring error signal W. In addition, if Xn> E, the locking circuit 22 sends an output information “1” to the OR gate 31 before the switch St is turned on. Thus, the signal “1” causes the locking circuit 32 to send out a casting error signal W. Even if the plunger does not reach point P, u. shortly before this stops, the locking circuit 22 emits an output signal «1» and in the same way a casting error is

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Signal W emitted. In the case where the plunger passes through point Pj and thereafter passes through point P2, when the time during which the plunger passes through point P2 is X2o, a casting error signal W is not transmitted as long as F ^ X22 G . If F> X22 or X22> G, the AND gate 29 or blocking gate 30 emits an output signal to actuate the latch circuit 32 to emit a casting error signal W. As described above, according to the invention, a casting error signal W is emitted if the respective times during which the plunger passes through the preset points P ^ P2 are not within the ranges between the upper and lower limit values, which are in each case determined by the preset values Pt , P2 are given. This signal can therefore actuate an alarm or an electromagnet of a shooting device to separate out defective castings. In the case where the relationship between the stroke of the ram and the time is the same as that of the curve A, 5 shown in FIG. 7, a casting error signal W is not output. In the case of B, C or H, a casting error signal W is emitted. In the present exemplary embodiment, the times during which the plunger passes through the points Plf P2 are each indicated by the displays 8, 11. The display can therefore be used to assess the quality of the defective castings or the quality of the good castings. In addition, the upper and lower limit values can be selected in the desired manner by the digital presetting devices 12 to 15.

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2 sheets of drawings

Claims (4)

PATENT CLAIMS 1. Detection device for casting defects in a plastic injection molding machine, which injection molding machine has a metal mold into which plastic is to be introduced, and a plunger that can be moved back and forth in a cylinder connected to the metal mold, characterized by a first measuring device (5, 6 , 7, 16, 17, 18) for measuring the time during which the plunger (42) passes through a first point (Pa) selected in the injection molding stroke of the plunger (42) by means (12) for presetting the lower one Limit value of the permissible time range for this first point, by means (13) for presetting the upper limit value of this time range for this first point, by means (32) for emitting a pouring error signal if the values from the first measuring device (5, 6 , 7, 16, 17, 18) is outside this time range, by a second measuring device (5, 9, 10, 16, 17, 18) for measuring the time during which the plunger (42) passes a second point (P2) which differs from the first and which is preset in the injection molding stroke of the plunger (42), passes through a device (14) for presetting the lower limit value of the permissible time range for the second point (P2), through a device (15) for presetting the upper limit of this time range for the second point (P2), the device (32) emitting a pouring error signal when the second measuring device (5, 9, 10, 16, 17, 18 ) measured time lies outside this time range for the second point (P2). 2. Detection device according to claim 1, characterized in that the measurements of the times by the measuring devices during which the plunger (42) passes through the first and second points (Pj, P2) each begin at the same time, namely then when the plunger (42) leaves the injection molding starting point (P0). 3, characterized in that the first and second measuring devices (5, 6, 7, 9, 10, 16, 17, 18) have counter circuits (7, 10) and that the devices (12, 13, 14, 15) for presetting of the upper and lower limit values for the first and second points (P "Pj have digital switches. 5. Detection device according to one of claims 1 to 3. Detection device according to claim 1, characterized in that the measurements of the times by the measuring devices, during which the plunger (42) passes through the first and second points (P1; Po), are started, namely when the plunger (42 ) passes through a point that is advanced a certain distance from the injection molding start point (P "). 4. Detection device according to one of claims 1 to 4, characterized in that display devices (8, 11) are provided for the respective display of the times measured by the first and second measuring devices (5, 6 ... 17, 18).