Patent Application
20090057938
The present invention generally relates to injection units. More specifically, the present invention relates to methods for regulating operating parameters to ensure melt quality.
The injection molding process typically comprises preparing a polymeric (or sometimes metal) material in an injection unit of a molding system, injecting the now-melted material under pressure into a closed and clamped mold, solidifying the material in its molded shape, opening the mold and ejecting the part before beginning the next cycle. The molding material typically is supplied to the injection unit from a hopper in the form of pellets or powder. The injection unit transforms the solid material into a molten material (sometimes called a “melt”), typically using a feed screw, which is then injected into a hot runner or other molding system under pressure from the feed screw or a plunger unit. A shut off valve assembly is typically provided to stop and start the flow of molten material from the barrel to the molding system.
In the plastic injection process, screw torque (i.e., the load on the screw), melt quality, recovery rate and throughput are target variables to be controlled. The temperature of the molten material plays an important role in controlling these variables. The energy to melt the material is provided by the barrel's heater bands that are distributed across the length of the barrel, and by screw rotational shear energy. It is relative easy to control the melt temperature by adjusting the heating of different zone heaters. Generally speaking, in prior art injection units, the operator attempts to maintain and stabilize the temperature of different zones of the barrel, and further attempts to stabilize screw rotation speed (in RPM) at its set value.
Efforts have been made to improve melt quality and other target variables. For example, U.S. Pat. No. 4,256,678 to Shigeru et al. teaches a method of and apparatus for controlling a plasticizing process of a resin of an in-line screw-type injection molding machine, a position of the screw is continuously detected in accordance with the movement thereof and a control function is determined by a back pressure of the screw which is compensated for by taking into consideration such as resin heating energy and shearing energy, which determine a temperature distribution of a resin to be injected. The operating condition, particularly the number of revolutions and the back pressure of the screw, is controlled on the basis of the screw position so as to make uniform the temperature distribution of the resin.
U.S. Pat. No. 4,851,170 to Shimizu et al. teaches an injection molding apparatus using a motor as a driving source, the injection speed and the injection pressure are controlled via a speed sensor, a pressure sensor and a closed loop control system, to provide higher accuracy and better operability during switching of the apparatus from an injection speed control phase to an injection pressure control phase and thereafter to a back pressure control phase.
U.S. Pat. No. 5,360,329 to Lemelson teaches an apparatus for molding permits a fluent molding material to be flowed into a mold cavity for shaping into a configuration defined by the mold walls. A master controller controls the transfer of heat with respect to the molding material, to control the temperature of the molding material in a predetermined way. A sensor measures the temperature of the molding material flowed into the cavity and produces feedback signals, which are compared to reference signals indicative of a desired molding material temperature. The apparatus generates a further control signal, which is applied to control the variables of the molding operation, including the temperature of the molding material and the flow rate.
U.S. Pat. No. 5,885,624 to Katsuta et al teaches an apparatus for a feed-back control of an injection molding machine, comprising a control target which operational conditions are different in accordance with operational purposes; and a control unit for subjecting said control target to a feed-back control, is characterized in that said control unit comprises a judgment function section for judging operational purposes of the control target, a condition setting section for setting operational conditions in accordance with the operational purposes and a switching section for switching the condition setting section through the judgment function section.
U.S. Pat. No. 5,997,778 to Bulgrin teaches an injection molding machine uses a summed, multi-term control law to control ram velocity during the injection stroke of a molding cycle to emulate a user set velocity profile. An automatic calibration method sets no load ram speeds to duplicate user set ram speeds. Finite impulse response filters produce open loop, no load control signals at advanced positions on the velocity profile to account for lag in system response. An adaptive, error term indicative of load disturbance, observed from a preceding cycle is added at the advanced travel position predicted by the finite impulse response filter to produce a predictive open loop, load compensated control signal. Finally, an auto tuned PID controller develops a real time, feedback load disturbance signal summed with the open loop control signal to produce a drive signal for the machine's proportioning valve.
U.S. Pat. No. 6,849,212 teaches an injection machine comprising: a heating barrel which heats a powder material, a binder, and a resin material into a molten resin; a screw mounted in the heating barrel to mix the resin material; and a motor which drives the screw in rotation. The injection machine according to the present invention further comprises a through-hole disposed on a side surface of the heating barrel; a pipe in which a solvent for adjusting a viscosity of the resin material is conducted, the pipe being connected to the through-hole; a filter disposed in the through-hole to prevent the resin material from leaking to the pipe; a valve disposed midway on a pipeline of the pipe; a reservoir disposed on an end of the pipeline of the pipe; a load-detecting part which detects a load value of the motor; a controlling part which sets a reference value with respect to a load of the motor; and a driving part which compares the detected load value with the reference value to drive the valve to carry out either one of supply or discharge of the solvent.
According to a first broad aspect of the present invention, there is provided a method for improving melt quality in an injection unit, comprising:
regulating operation of the injection unit in accordance with a reference value for at least one operating parameter;
measuring a present value of a load upon a motor operable to rotate an injection screw during operation of the injection unit;
comparing the present value of the load to a reference value for the load; and
if the present value of the load deviates from the reference value of the load by more than a predetermined amount, then adjusting the reference value of the at least one operating parameter.
According to a second broad aspect of the invention, there is provided an injection unit, operable to:
regulate its operation in accordance with a reference value for at least one operating parameter;
measure a present value of a load upon a motor operable to rotate an injection screw during the operation of the injection unit;
compare the present value of the load to a reference value for the load; and
if the present value of the load deviates from the reference value of the load by more than a predetermined amount, then adjust the reference value of the at least one operating parameter.
A better understanding of the non-limiting embodiments of the present invention (including alternatives and/or variations thereof) may be obtained with reference to the detailed description of the non-limiting embodiments of the present invention along with the following drawings, in which
The drawings are not necessarily to scale and are sometimes illustrated by phantom lines, diagrammatic representations and fragmentary views. In certain instances, details that are not necessary for an understanding of the embodiments or that render other details difficult to perceive may have been omitted.
Referring now to
Material (typically plastic or magnesium alloy pellets) is fed from a hopper 32, through a feed throat 34 into melt channel 30. The rotational movement of screw 24 plasticizes the material prior to it exiting through nozzle 28. Preferably, screw 24 may include a plurality of specialized zones. For example, a first zone might be adapted for conveying solid material from the hopper, a latter zone for compressing and plasticizing the material, and a final for mixing the now-molten material prior to exiting through nozzle 28. Screw 24 may also include weirs or channels to separate out unmelted material from the melted material for further processing. The implementation of screw 24 is not particularly limited and other adaptations will occur to those of skill in the art.
In addition to rotating, screw 24 is preferably operable to reciprocate back and forth to express the melted material out through nozzle 28 and pack the material within a mold (not shown). Preferably, an injection valve 36 is provided near the tip of screw 24 to prevent the reentry of material during the return motion of the screw.
The rotational movements of screw 24 is provided by a motor 44, which may be an electric motor, a hydraulic motor, or a combination thereof (the embodiment depicted in
A load sensor 50 is provided for motor 44 that measures the effort required to turn screw 24. Load sensor 50 provides an estimate of the average viscosity of the material within the barrel. Preferably, load sensor 50 is a torque sensor that measures the torque force generated by the motor in order to turn the screw, but other types of sensors could be used. For example, the load sensor could measure the current drawn by the (electric) motor, or it could simply measure the rotational speed (in RPM) of screw 24. Other types of load sensor 50 will occur to those of skill in the art.
Heater bands 46 are provided along a portion of the length of barrel 22 (though away from the feed throat 34) to assist in the melting of the material (in addition to the heat generated by the shearing action of screw 24) and then maintain the temperature of the molten material as it approaches the nozzle. Preferably, heater bands 46 are covered with insulation 48 to minimize heat loss). As is known to those of skill in the art, heater bands 46 typically cycle on and off for fractions of a second so that a 20% duty cycle might represent a low, standby power setting and a 100% duty cycle would be the maximum power cycle. Preferably, each heater band 46 is independently controlled. Thermocouples 58 are provided along the barrel to provide an indication of the material's temperature. Since the thermocouples 58 do not actually contact the material in melt channel 30, they provide only an estimate of its actual temperature.
A processor 40 receives data from various sensors (such as load sensor 50 and thermocouples 58) located within injection unit 20, and further controls the overall operation of injection unit 20, including the rotational and reciprocating movement of screw 24 (via motor 44 and piston 38), heater bands 46 and all related and auxiliary equipment. Processor 40 is preferably a general-purpose computer; however it could also include a plurality of microcontrollers and/or specialized processing units distributed around the various components of injection unit 20.
Processor 40 can be controlled through a Human-Machine Interface (HMI) 52, as the Polaris® control system provided by the Applicant. HMI 52 includes visual display units (either onsite or remotely by network) for an operator as well as input devices for the operator. Processor 40 is also connected to a database 54 either directly or remotely via a network. Database 54 logs the alarms and events, and historical operational data of injection unit 20. Database 54 further maintains saved process parameter and HMI configuration settings for injection unit 20. As is described in greater detail below, database 54 can also store material-specific configuration data such as the minimum value, maximum value and set point value for each operating parameter. While database 54 is depicted as a single data storage device, it is contemplated that database 54 could comprise multiple storage devices locally provided, and/or remotely connected via network.
Processor 40 regulates the operating condition of injection unit 20 using closed or open loop control systems. Processor 40 can include a hardware or software PID controller, or another type of closed or open loop controller. For example, processor 40 controls the duty cycles of each of the heater bands 46. For each thermocouple 58, processor 40 receives a minimum and a maximum temperature (TMIN and TMAX respectively). TMIN represents the minimum operating temperature of melt channel 30 in which the desired level of plasticizing occurs in the material. Below this level, melt quality or operation speed will be compromised to an unsatisfactory degree. TMAX represents the maximum operating temperature of melt channel 30 that can be achieved without risk of damaging the melt quality or parts of the injection unit 20. The values of TMIN and TMAX are dependent upon many factors, including the type and grade of material being plasticized, the final article being produced, the length and rotational speed of screw 24, and other environmental effects. TMIN and TMAX can be inputted by an operator through HMI 52, or through a lookup table in database 54, based upon the material being plasticized and the application.
During operation, if the type of material or grade of material (e.g. MFI, etc.) changes, the viscosity of the molten material will change and generate different load on screw 24. In prior art injection units, a closed loop control system would adjust the power output of the motor in order to change its torque and compensate for the increased screw load, and thereby maintain the RPM target parameter. The inventors have determined that if the processing condition is too challenging for the machine (i.e., the barrel temperature is too low), the closed loop control system cannot keep the rotational speed of screw 24 stable, and furthermore, causes the recovery rate and material throughput to shift or oscillate around the target parameter. To improve operation of the injection unit 20, the inventors monitor the load on screw 24 as a target parameter, and regulate at least one operating parameter, such as barrel temperature, injection back pressure or screw RPM to achieve the targeted load value.
Referring now to
Referring now to
Each thermocouple 58 transmits the currently-measured temperature (TPV) to processor 40. When the received TPV deviates from TSP by more than a predetermined amount, processor 40 will output a control signal (TMV) to adjusts the duty cycle of the relevant heater bands 46 so that the measured TPV approaches TSP. (Thermocouples 58 can expect values below TSP during warm-up or standby). Thus, a stable temperature can be achieved in melt channel 30.
An outer control loop 220 is further provided to regulate load on screw 24. Processor 40 receives a minimum load value (LMIN), a maximum load value (LMAX) and an predetermined load set point (LSP) for motor 44. As described earlier, the current load (LPV) is measured by load sensor 50, and typically measures the torque of screw 24. A high torque reading (relative to LSP) typically indicates that the viscosity of the material in the melt channel is too high, and a low torque reading typically indicates that the viscosity of the material is too low.
When the received LPV deviates from LSP by more than a predetermined amount, processor 40 will adjust its temperature set point (ΔTSP-MV). By changing TSP, the inner temperature control loop 210 will then output a control signal (TMV) to adjust the duty cycle of the relevant heater bands 46, as is described above, so that the measured TPV approaches the new ΔTSP.
Preferably, specific values of LSP, LMIN, LMAX and TSP, TMIN, TMAX are available for each different type, brand and grade of material run through injection unit 20. Ideally, these values are stored in database 54 and are provided by the material supplier, the manufacturer or a materials consulting firm. In this way, the machine operator is not required to have detailed knowledge of the material being processed by injection unit 20. Also preferably, these values can be updated over time as the performance of different components changes over time (given screw wear, damage to insulation on the heater bands, etc), and the updated values stored in database 54.
Referring now to
Pressure transducer 56 transmits the currently-measured back pressure (PPV) to processor 40. When the received PPV deviates from PSP by more than a predetermined amount, processor 40 will output a control signal (PMV) to adjust the pressure applied by piston 38 so that the measured PPV approaches PSP.
An outer load control loop 320 is further provided to regulate load on screw 24. Processor 40 receives a minimum load value (LMIN), a maximum load value (LMAX) and an ideal load set point (LSP) for motor 44. As described earlier, the current load (LPV) is measured by load sensor 50, and typically measures the torque of screw 24. A high torque reading (relative to LSP) typically indicates that the viscosity of the material in the melt channel is too high, and a low torque reading typically indicates that the viscosity of the material is too low.
When the received LPV deviates from LSP by more than a predetermined amount, processor 40 will adjust its pressure set point (ΔPSP). By changing PSP, the inner pressure control loop 210 will then output a control signal (PMV) to adjust the back pressure set value PSP, as is described above, so that the measured PPV approaches the new PSP.
As with temperature parameters, specific values of LSP, LMIN, LMAX and PSP, PMIN, PMAX are, preferably, available for each different type, brand and grade of material run through injection unit 20. Ideally, these values are stored in database 54 and are provided by the material supplier, the manufacturer or a materials consulting firm. In this way, the machine operator is not required to have detailed knowledge of the material being processed by injection unit 20. Also preferably, these values can be updated over time as the performance of different components changes over time (given screw wear, damage to insulation on the heater bands, etc), and the updated values stored in database 54.
Referring now to
RPM sensor 60 transmits the currently-measured RPM (RPV) to processor 40. When the received RPV deviates from RSP by more than a predetermined amount, processor 40 will output a control signal (RMV) to adjust the RPM outputted by motor 44 so that the measured RPV approaches RSP.
An outer load control loop 420 is further provided to regulate the load on screw 24. Processor 40 receives a minimum load value (LMIN), a maximum load value (LMAX) and an predetermined load set point (LSP) for motor 44. As described earlier, the current load (LPV) is measured by load sensor 50, and typically measures the torque of screw 24. A high torque reading (relative to LSP) typically indicates that the viscosity of the material in the melt channel is too high, and a low torque reading typically indicates that the viscosity of the material is too low.
When the received LPV deviates from LSP by more than a predetermined amount, processor 40 will adjust its RPM set point (ΔRSP). By changing RSP, the inner RPM control loop 410 will then output a control signal (RMV) to adjust the RPM set value RSP, as is described above, so that the measured RPV approaches the new RSP.
As with temperature parameters, specific values of LSP, LMIN, LMAX and RSP, RMIN, RMAX are, preferably, available for each different type, brand and grade of material run through injection unit 20. Ideally, these values are stored in database 54 and are provided by the material supplier, the manufacturer or a materials consulting firm. In this way, the machine operator is not required to have detailed knowledge of the material being processed by injection unit 20. Also preferably, these values can be updated over time as the performance of different components changes over time (given screw wear, damage to insulation on the heater bands, etc), and the updated values stored in database 54.
Referring now to
As with the previously-described embodiments, the values of TSP, PSP and RSP are adjusted by an outer loop that regulates the measured load value (LMV). Processor 40 determines which of the inner loops will be fine-tuned to correct the value of LMV. For example, processor 40 may first adjust the temperature set point via control loop 510. If additional adjustments are required, processor 40 may then adjust the back pressure set point via control loop 520. If additional adjustments are still required, processor 40 may then adjust the screw's rotational speed set point via control loop 530.
Alternatively, processor 40 may adjust several of the inner parameters simultaneously. The degree of the adjustment may be equal for some or all of the three parameters, or the degree of change may be weighted differently between control loops.
Non-limiting embodiments of the present invention may provide a control system for an injection unit having a better quality and higher throughput of molten material. Non-limiting embodiments of the present invention may provide a control system for an injection unit that reduces wear on the screw and the motor. Non-limiting embodiments of the present invention may provide a control system for an injection unit having a reduced force requirement for actuation. Non-limiting embodiments of the present invention may provide a control system for an injection unit that provides customized, material specific data for better performance.
The description of the non-limiting embodiments provides examples of the present invention, and these examples do not limit the scope of the present invention. It is understood that the scope of the present invention is limited by the claims. The concepts described above may be adapted for specific conditions and/or functions, and may be further extended to a variety of other applications that are within the scope of the present invention. Having thus described the non-limiting embodiments, it will be apparent that modifications and enhancements are possible without departing from the concepts as described. Therefore, what is to be protected by way of letters patent are limited only by the scope of the following claims.
