AU687777B2 - Control of injection moulding machine - Google Patents

Description

AUSTRALX A Patents Act 1990 COMPLETE SPECIFICATION STA~NDARD PATENT Applicant(s): MOLDFLOW PTY. LTD.

A.C.N. 005 647 496 Invention Title: CONTROL OF INJECTION MOULDING MACHINE 0 00S

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The following statement is a full description of this invention, including the best method of performing it known to me/us: mmnm w I I -2o* CONTROL OF INJECTION MOULDING MACHINE Technical field The present invention relates to injection moulding of materials, especially of plastics, and relates particularly to a system and method for optimising operation of an injection moulding machine. Even more particularly, the present invention relates to detecting the hot melt front in a mould, especially by the use of optical or acoustical methods and devices, including sensors.

Description of the Prior Art An injection moulding machine typically has a barrel and ram arrangement which forces a plastic melt into a die or mould through a nozzle to a runner and gate system by displacement of the ram within the barrel, so to produce an article in accordance with the particular shape of the mould.

3 The procedure of moulding an item from a die involves three basic steps. These steps are the filling phase, during which the plastic melt flows into a cavity of the die, the pressurisation phase and finally the compensation phase. The pressurisation phase accounts for the compressibility of the plastic in its molten state, while the compensation phase accounts for the volumetric change in the transition between a molten and solid state of the plastic.

Design criteria of the injection moulding process often require that the time available to inject the plastic melt into the die is very short, perhaps fractions of a second, which necessitates the need for close control over the operation of the ram of the injection moulding machine.

15 Further, during the filling phase, the ram is required to move at high speed to obtain the required plastic flow velocity, whereas, during the pressurisation phase, the ram must move slowly to maintain the required plastic pressure.

During the transition between the filling and 20 pressurisation phases excessive pressure spikes can occur, which can affect the quality of the moulding or the moulded article being produced in the die.

In this specification, reference to the moulding of a single item is referred to as a shot. A single shot 25 comprises the execution of a number of different but sequenced :teps constituting the overall moulding process, and such steps are repeated for each individual shot. One or more individual articles can b& produced from the one shot depending on the nature and size of the mould.

In the past, designers of conventional injection moulding machines have used sophisticated process controllers to regulate the speed or extent of displacement of the ram of the machine according to a predetermined profile or characteristic which has been inputted to the process controller. This technique most often relies on the operator of the machine having to continually correct the ram input characteristic by means of a trial and error 4 process at least during the initial operation of the moulding machine. By such a technique, when one moulding is made, it is then inspected, and appropriate or bestguess changes are then made based on the inspection and comparison with what should be made by the mould. Such a practice is inefficient, cumbersome and wasteful of both time and materials as well as producing items that may not be entirely in accordance with the designer's specification.

Conventional techniques have been developed largely around control of the ram and all rely on the design variable (which is often guessed) being a property of the ram dynamics rather than considering the behaviour of the plastic melt itself. Therein lies one particular 15 disadvantage with the prior art, in that the actual position of the plastic flow front as it enters a die is largely unknown. Therefore, in areas where slow injection ram speed is required (such as in the gate region), the length of slow speed operation must be made long in order 20 to ensure correct filling of the die to obtain a good quality and accurate moulding. This can result in the plastic loosing temperature such that the melt becomes less flowable with subsequent increase in the residual stresses within the article being moulded, even to the extent that 25 the mould may not be completely filled due to the melt prematurely solidifying within the die or mould.

The present applicant has developed computer design programs which simulate and optimise the injection moulding cavity, runner and gate design, as well as the flow of plastics into the mould to ensure good quality mouldings are produced. These programs are sold in many countries, and are identified by the trade marks: MOLDFLOW/FLOW or MF/FLOW belonging to the present applicant.

The above indicated programs take into consideration the rheological, thermal and physical properties, such as for example, pressure-volumetemperature relation, compressibility and the like of the plastic materials. The design program has recourse to the filling pattern of the plastic as it flows into a cavity as a function of temperature, pressure, shear rate, stress and other factors relevant to the process of injection moulding. The design programs therefore predict optimised moulding conditions in terms of plastic melt and mould temperatures, and the filling speed of the cavity. In particular these programs can be used to predict an optimised or reference flow characteristic for injection of the plastic at any point within a mould cavity as a function of time or of ram displacement.

The flow characteristic is to be understood as :being the progress of a plastic into a die as a function of 15 time, or alternatively, on a displacement contour basis throughout the die. The characteristic can be either on the basis of the actual progress of the flow front (hot melt front) though the die, or the required plastic flow pressure at a given static point in the die, such as at the eeeo S 20 gate.

Even with the use of the present applicant's :i design software, which most accurately allows for correct mould cavity, runner and gate design as well as melt and mould temperature and the reference plastic flow 25 characteristic, conventional injection moulding machines still require continuous adjustment on a trial and experimental basis to correct for variations in the moulding conditions.

One way of obtaining information about the hot melt front is by using pressure sensors in the mould.

However, the use of pressure sensors suffers from one or more disadvantages which include the presence of high background noise which makes it difficult to accurately determine signal peaks representing the hot melt front from other noise related variations in the signal trace produced by the pressure sensors. The present invention seeks to overcome one or more of these disadvantages by using 6 sensors which are more discriminating of the presence of the actual hot melt front than previously used sensors.

Objects and Disclosure of the Invention It is an object of the present invention to provide a system and method for operating an injection moulding machine wherein the position of the hot melt front is detected in the mould or cavity by the use of an optical or acoustic system or optical or acoustic device and the information relating to the position of the hot melt front is then used to optimise the parameters of operation of the injection moulding machine and to optimise the method for making articles so that the articles meet all of their design and performance criteria.

It is to be noted that the use of the terms "optical" and "optical sensors" in the present specification is not meant to be restricted to visible light detectors or sensors sensitive to visible light, but 20 rather the term "optical" includes within its scope other wavelengths of the radiation spectrum, such as for example goinfra-red, ultraviolet or other regions without limitation.

Therefore, according to the present invention there is provided a method for optimising operation of an S 25 injection moulding machine for the production of moulded plastic articles, the machine having a ram and data of processing means for providing said optimisation, the 0 method comprising the step of: inputting to the data processing means a 30 reference plastic flow characteristic of the molten plastic material from which the article or articles are to be moulded; analysing by the data processing means a measured plastic flow characteristic, obtained by the use of an optical or acoustic sensor means or sensing system to measure the position of the hot melt front of the molten plastic material, from an immediately preceding shot of the H;\Sigrid\teep\patent\59288-94.doc 8/12/97 I

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7 injection moulding machine to determine distinct changes in the gradient of the measured characteristic compared to the gradient of the reference characteristic; comparing by the data processing means changes in gradient between the gradient of the reference characteristic and the gradient of the measured characteristic as a function of time or ram displacement i: order to obtain an adjustment value; adjusting the injection moulding machine in accordance with the adjustment value so obtained; and performing a next shot of the injection moulding machine based on the adjusted value to more accurately control operation of the injection moulding machine.

The invention further provides an injection moulding machine for production of moulded plastic articles, the machine having a ram for forcing molten plastic material from a reservoir into a die and the ram being under control of control means, the injection 20 moulding machine further comprising: an optical or acoustic sensor means or system for "measuring the actual plastic flow characteristic associated ":with the position of the hot melt front of the molten plastic material from any one shot; and data processing means having inputs corresponding to at least the said measured characteristic and a reference characteristic for the article or articles to be characteristic obtained by the use of an optical or acoustic sensor means or sensing system from the immediately preceding shot in order to determine distinct changes in the gradient of the measured characteristic and the reference characteristic and comparing the distinct changes in gradient between the said reference characteristic and the said measured characteristic as a function of time or ram displacement in order to generate an error function, deriving a ram displacement H:\SigriKeep\patents\59288-9 4 .doc 8/12/97 *P 111 7a characteristic from the said reference characteristic, compensating the said ram displacement characteristic by the error function to produce a compensated ram displacement characteristic and providing said compensated ram displacement characteristic to the control means for use in a subsequent shot, thereby providing accurate control of the injection moulding machine.

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8 ram dhaiaeatmaentt '=rmtrsi-to the cnt-oeli mazamarnno The invention further provides an injection moulding machine for production of moulded plastic articles, the machine having a ram for forcing molten plastic material from a reservoir into a die and the ram being under control of control means, the injection moulding machine further comprising: an optical or acoustic sensor means or an optical or acoustic sensing means or system or device for measuring the actual plastic flow characteristic, including the hot melt front of the plastics material, from any one shot; and p.

data processing means having inputted at least the said actual characteristic and a reference 15 characteristic for the article or articles to be moulded and, in operation, comparing the said actual characteristic from the immediately preceding shot with the said reference characteristic to generate a compensated plastic flow characteristic, deriving a new ram displacement 20 characteristic from the said compensated characteristic and providing said new ram displacement characteristic to the control means for use in a subsequent shot.

Typically, the optical or acoustic sensor or sensing means includes one or more of fibre optics (b) 25 laser, or acoustic sensors or detectors. More typically, the optical detector or sensor has a light emitting or producing portion and a light receiving or reflectance measuring portion.

Brief Description of the Drawings In order that the invention can be more clearly explained, embodiments will be described with reference to the acompanying drawings, in which:- Figure 1 is a cross-sectional view partially schematic of a simple injection moulding machine; Figure 2 is a block diagram of a system ,,RA constructed in accordance with the present invention; bl L- IL -9 Figure 3 is a sectional view along line A to A of Figure 1.

Description of Preferred Embodiments A system constructed in accordance with the present invention operates upon the prediction of the plastic flow progress in a die during the moulding process, and automatically regulates production of moulded articles taking into account system dynamics, and particularly control over ram velocity. One of the measured parameters for controlling the operation of the injection moulding o .**machine is to accurately locate in the mould, die, or cavity the position of the hot melt front of the plastics material at any given time or to be able to monitor the movement of the hot melt front over time as it flows into S 15 the mould, die or cavity. In the present invention this is accomplished by an optical or acoustic sensor or detector.

In the present specification, it is to be noted that the use of terms such as up, upper, lower, base and Ce..

the like are relative to the normal in use orientation of the injection moulding machine.

Figure 1 shows a sectional view of a simple ooo.

•o "injection moulding machine 10. Hopper 11, into which solid plastic material is fed, usually in pellet form, is mounted on the upwards-facing surface of the cylindrical body 12 oo.. 25 when in its normal orientation. The cylindrical body 12 forms a barrel chamber in which is located a concentric ram 13 having a ring check valve located at its distal end and screw 14. A plastic melt 16 occupies the forward extent of the chamber between the ram 13 and the machine nozzle (or barrel end adaptor) 17. The walls of body 12 are temperature controlled so a. to maintain the plastic in a molten state. Heating elements to achieve the melt are not shown.

The body of the injection moulding machine is attached to a mould die structure 19, essentially comprising two sections forming the mould cavity 20. In

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10 one example, one of the sections 19a of the mould die structure is fixed whereas the other of the sections 19b is movable or replaceable. The nozzle 17 is connected to the mould cavity 20 by means of a sprue, runners and gates, all of which can be of varying and determinate dimensions. In the present example, a simple representation of such a system is shown as a single sprue/runner/gate combination 18. However, the exact arrangement will be described in more detail later. As with cylindrical body 12, the die structure 19 is maintained at a specified temperature by heater elements, which again are not shown.

s* The injection moulding process involves the steps of auguring the desired amount of plastic material into the barrel chamber 12 using the screw 14 so as to form the melt 15 16. Once the melt 16 and die 19 have achieved the desired temperature, the moulding process can commence. The temperatures of the melt 16 and the die 19 play a key role in determining the quality and performance of the injection moulded product.

20 Figure 2 shows a block diagram which embodies an example of the invention. The injection moulding hachine denoted as 10 in Figure 2 corresponds to the like-numbered item in Figure 1.

The injection moulding machine 10 has connected 0. 25 to it a processor controller 22, which outputs various signals such as those corresponding to melt temperature, cavity temperature, and ram displacement. The process controller 22 also has control over the sequencing and hydraulic operations of the injection moulding machine, particularly the ram 13.

The looped connection to the process controller 22 from the injection moulding machine 10 represents closed loop control o':er the ram displacement.

Sensors 24 detect mould and melt temperatures, as well as measuring the flow pressure of nozzle 17 with time by virtue of a pressure sensor (not shown) mounted thereat.

This is the preferred location for the sensor since it then 11 does not need to be changed with each new die. The sensor could be any commercially available type with the requisite characteristics. It may also be preferable to provide other optical or acoustic sensors in the mould die structure 19 such as for example to form part of or be located in the wall of cavity With particular reference to Figure 3, one preferred form of the sensors used in the present invention is to provide a suitable aperture in the runner portion located between sprue portion 32 and gate 34 at one end or peripheral portion of cavity 36 at the end of the fill. In the aperture 38 of runner 30 is located a bundle of optical 9.

fibres shown as a block 40. Some of the optical fibres are used to project light into the runner 30 and others of 15 the optical fibres are used to detect any reflected light.

9* As the hot melt front passes aperture 38 it will cause changes in the amount of reflected light detected by the optical fibres. By measuring and comparing the amount of reflected light in different circumstances correspondj.ng to 20 different positions of the hot melt front, information can be obtained about the exact position and configuration of the hot melt front from which information can then be .o gained to optimise the control of the injection moulding machine to produce moulded articles to the correct 25 specification and properties in accordance with the design criteria of the articles.

A modification of the above described arrangement, useful in other embodiments of the present invention, involves the use of detecting infra-red radiation from the hot melt rather than detecting reflected light. Infra-red radiation is present in the cavity of the mould. The amount of infra-red radiation is dependent on the temperature of the mould, so that prior to the mould being filled with plastic material, a background level of radiation can be detected by suitably selected fibre optics. As the hot melt front progresses through the mould as it is being filled, more infra-red radiation is L I 12 produced. The increase in radiation can be detected by the infra-red radiation sensitive fibre optics which can then be used to provide information about the hot melt front.

In this embodiment the fibre optic is selected to suit the wavelength of infra-red radiation.

One or more sensors may be used if required depending on circumstances. The sensors may be different or the same.

All these sensors are connectc to a data processing device 26, which could be a datalogger or some other larger microprocessor based machine. The data processing device also has an input device 27 connected to it, which provides a means by which a reference plastic flow characteristic as determined by the FLOW software is 15 inputted. Suitable embodiments of the input device 27 include optical media storage, or a floppy disk or hard disk drive usually operated in conjunction with a keyboard.

Alternatively, the input device 27 could represent a networked computer system. Indeed, all the elements 22, 26 20 and 27 could be the one data processing means such as a personal computer.

The data processing device 26 outputs a sign?.l to process controller 22, which represents the desired ram displacement characteristic with time. Other signals are 5 passed to the process controller 2,2 including temperature settings for the melt 16 and die 19.

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

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Claims (28)

1. A method for optimising operation of an injection moulding machine for the production of moulded plastic articles, the machine having a ram and data processing means for providing said optimisation, the method comprising the step of: inputting to the data processing means a reference plastic flow characteristic of the molten plastic material from which the article or articles are to be moulded; analysing by the data processing means a measured plastic flow characteristic, obtained by the use of an optical or acoustic sensor means or sensing system to measure the position of the hot melt front of the molten plastic material, from an immediately preceding shot of the injection moulding machine to determine distinct changes in the gradient of the measured characteristic compared to the gradient of the reference characteristic; 20 comparing by the data processing means changes in gradient between the gradient of the reference characteristic and the gradient of the measured *g characteristic as a function of time or ram displacement in order to obtain an adjustment value; 25 adjusting the injection moulding machine in accordance with the adjustment value so obtained; and performing a next shot of the injection moulding machine blnsed on the adjusted value to more accurately control operation of the injection moulding machine.

2. A method for optimising operation of an injection moulding machine for production of moulded plastic articles according to claim 1 in which the machine has a ram for forcing molten plastic material from a reservoir into a die, the method further comprising the steps of: deriving by the data processing means a ram displacement characteristic from the said reference e nRAd YH;\Sigrid\Keep\patents\59 2 B 8 94 .doc 8/12/S- A2Z ^j 14 characteristic; performing a shot; measuring by an optical or acoustic sensor means or system the measured plastic flow characteristic from one or more preceding shots; analysing by the data processing means the measured characteristics from preceding shots to determine distinct changes in gradient thereof; comparing by the processing means changes in gradient between the said reference characteristic and said measured characteristic as a function of time or displacement to generate an error function; deriving by the data processing means a compensated ram displacement characteristic from the error function for the next shot; and performing the next shot based on the ram displacement characteristic.

3. A method as claimed in claim 1 or claim 2, in 20 which the step of comparing is conducted only with the measured characteristic of the immediately preceding shot.

4. A method according to claim 2 or claim 3 in which a 0 a compensated plastic flow characteristic for the next shot is generated by the data processing means on the basis of the comparison of the changes in the gradient; deriving by the data processing means a ram displacement characteristic from the said compensated characteristic; and ease*: 30 performing a next shot based on the ram :o displacement characteristic.

A method as claimed in any preceding claim in which the step of comparing comprises identifying points of the reference characteristic gradient corresponding to a maximum change of gradient in the reference characteristic, performing a numerical analysis to identify the points of 7i\ H:\Sigrid\geep\pBtenta\59288-94. doc 8/12/97 15 the measured characteristic gradient corresponding to a maximum change of gradient in the said measured characteristic which correspond to said point of the reference characteristic gradient; rnd determining differences in time or ram displacement with respect to a common origin.

6. A method as claimed in claim 5, in which the said reference characteristic and said measured characteristic are measures of pressure or relate to pressure of the plastic melt.

7. A method as claimed in claim 6, in which all of the said characteristics are in respect of a derivative with respect to time or a function of ram displacement.

8. A method as claimed in any one of claims 2 to 7 in which the sensor means is an optical sensor means. 20

9. A method as claimed in claim 8 in which the optical sensor mears is a fibre optic sensor, a laser sensor, or an infra-red detecting sensor.

10. A method as claimed in any preceding claim in 25 which the sensor means is located in or at the nozzle of the injection moulding machine. S*

11. A method as claimed in any one of claims 1 to 9 in which the sensor means is located in the die, mould or 30 cavity. 0 0

12. A method as claimed in any preceding claim in which the sensor means is located in the wall of the die, mould or cavity.

13. A method as claimed in any preceding claim in which the sensor means is located in an aperture in a H:\sigrid\Keep\patents\59288-94.doc 8/12/97 ro 16 runner portion located between a sprue portion and a gate portion.

14. A method as claimed in claim 13 in which a bundle of optical fibres are located in the runner such that some of the fibres are used to project light into the runner and others of the optical fibres are used to detect reflected light.

15. An injection moulding machine for productionz of moulded plastic articles, the machine having a ram for forcing molten plastic material from a reservoir into a die and the ram being under control of control means, the injection moulding machine further comprising: an optical or acoustic sensor means or system for measuring the actual plastic flow characteristic associated with the position of the hot melt front of the molten plastic material from any one shot; and data processing means having inputs corresponding 20 to at least the said measured characteristic and a reference characteristic for the article or articles to be ,moulded, and in operation analysing the said measured characteristic obtained by the use of an optical or acoustic sensor means or sensing system from the 25 immediately preceding shot in order to determine distinct changes in the gradient of the measured characteristic and S. the reference characteristic and comparing the distinct changes in gradient between the said reference characteristic and the said measured characteristic as a 30 function of time or ram displacement in order to generate an error function, deriving a ram displacement characteristic from the said reference characteristic, compensating the said ram displacement characteristic by the error function to produce a compensated ram displacement characteristic and providing said compensated ram displacement characteristic to the control means for use in a subsequent shot, thereby providing accurate H\Sigrid\Keep\patents\59288-94.doc 8/12/97 I4, Sr- 17 control of the injection moulding machine.

16. An injection moulding machine according to claim in which a compensated plastic flow characteristic is generated to derive a new ram displacement characteristic from the said compensated characteristic and in which the said new ram displacement characteristic is provided to the control means for use in a subsequent shot.

17. An injection moulding machine according to claim or 16 in which the step of comparing is conducted only with the measured characteristic of an immediately preceding shot to the subsequent shot.

18. An injection moulding machine according to any one of claims 15 to 17 in which the said reference characteristic and said measured characteristic are characteristics of pressure of the plastic melt. 20

19. An injection moulding machine according to any one of claims 15 to 18 in which all of the said 0@ characteristics are in respect of a derivative with respect to time or a function of ram displacement. 25

20. An injection moulding machine according to any one of claims 15 to 19 in which the oensor means is an optical sensor means.

21. An injection moulding machine as claimed in claim 30 20 in which the optical sensor means is a fibre optic sensor, a laser sensor, or an infra-red detecting sensor.

22. An injection moulding machine as claimed in any one of claims 15 to 21 in which the sensor means is located in or at the nozzle of the injection moulding machine.

23. An injection moulding machlie as claimed in any iHt\Slgrid\Keep\patents\5928B-9 4 .doc 8/12/97 m. tv] 18 one of claims 15 to 21 in which the sensor means is located in the die, mould or cavity.

24. An injection moulding machine as one of claims 15 to 21 in which the sensor in the wall of the die, mould or cavity.

An injection moulding machine as one of claims 15 to 21 in which the sensor in an aperture in a runner portion located portion and a gate portion. claimed in any means is located claimed in any means is located between a sprue a. a a. as a a. a a a ar as a a a. .4 44 a,. 4 a.o 4r a. a Sa

26. An injection moulding machine as claimed in claim in which a bundle of optical fibres are located in the runner such that some of the fibres are used to project light into the runner and others of the optical fibres are used to detect reflected light.

27. A method substantially as described herein with 20 reference to and as illustrated in the accompanying drawings.

28. An injection moulding machine substantially as described herein with reference to and as illustrated in 25 the accompanying drawings. Dated this 8 th day of December, 1997 MOLDFLOW PTY LTD 30 By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia H:\Sigrid\Xeep\patents\59288-94.doc 8/12/9) 4 ABSTRACT A method and machine for optimising operation of an injection moulding machine for the production of moulded plastic .articles, the machine having data processing means for processing parameters for providing said optimisation, wherein one or more of the parameters needed to effect said optimisation is obtained by a method comprising the steps of: providing an optical and/or acoustic means comprising an optical sensor or optical sensoring system or means or an acoustic device at one or more strategic locations in the mould for detecting the position of a hot melt front of the plastics material, detecting the position of the hot melt front of the plastics material in the mould in use by the optical and/or acoustical means and using the information obtained from the sensor or sensing to optimise the operation of the injection moulding machine to produce the moulded plastic articles. *.G ooo•