GB1566315A

GB1566315A - Controlling the quality of injection moulding

Info

Publication number: GB1566315A
Application number: GB4191976A
Authority: GB
Original assignee: Umformtechnik Herbert Warnke VEB
Current assignee: Umformtechnik Herbert Warnke VEB
Priority date: 1975-10-08
Filing date: 1976-10-08
Publication date: 1980-04-30
Also published as: FR2327072A1; DD124026A1; DE2639938A1

Description

(54) CONTROLLING THE QUALITY OF INJECTION MOULDINGS (71) We, VEB KOMBINAT UNFARMTECHNIK "HERBERT WARNKE" ERFURT, of 1, Schuerbarner Strasse, 501 Erfurt, German Democratic Republic, a Corporation organised under the laws of the German Democratic Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be described in and by the following statement: This invention relates to a method of pro ducing mouldings from mouldable materials in injection moulding machines, particularly from melts of plastics material which are injected with the aid of pressure media into a tool defining the contour of the moulding to be produced, the moulding then receiving its final shape through the controllable or servo-controllable action of pressure.

The utilisation of parts made of mouldable materials, particularly plastics materials, as technical parts necessitates increasingly stringent demands on moulding tolerances, which cannot be achieved with the known conventional injection moulding machine, or can be achieved only with unacceptable rejection rates. This is due to the fact that fluctuations of processing conditions, on the one hand due to the injection moulding machine itself and on the other hand due to fluctuations of properties of the material, may lead to impermissible variations of the properties of the mouldings.

It is known that the pressure in the tool is a factor substantially influencing the proper ties of the moulding, since it plays a part in determining the properties of the moulding in the shaping phase of the injection mould ing process.

In addition, methods and apparatuses are known which on the basis of this fact keep the pressure in the mould constant in order to obtain constant properties of the moulding.

In these known methods and apparatuses the tool pressure is measured and the maximum tool pressure occurring is kept constant in the cycle or from one cycle to another.

Another known apparatus not only controls the maximum tool pressure, but controls the chronological pressure pattern in the pressure application phase in accordance with a predetermined pressure curve.

In addition to the above-described apparatuses or processes for measuring and maintaining a constant value of the maximum tool pressure or tool pressure curve, other processes and apparatuses are known which during the injection phase measure deviations in the performance of the process by means of values characteristic of the plastics melt, and base thereon the control or maintenance of a constant value of the tool pressure.

One known apparatus measures at the commencement of the injection process a so-called process index which characterizes the condition of the plastics melt and knowledge of which permits predetermination of the maximum tool pressure to be expected, so that correction of tool pressure can be initiated within the cycle itself. If nevertheless there is a deviation from the predetermined maximum tool pressure, the correction factor is varied for the next cycle.

Another apparatus has already been proposed in which a time index, measured from the moment when the maximum tool pressure is reached to the moment when a predetermined position of the screw is reached, is used as controlled variable. If this characteristic value deviates from the prescribed desired value, the time of action of the maximum tool pressure is varied analogously.

In addition, an apparatus has already been proposed in which in dependence on a predetermined time range - measured from the moment when the apparatus is switched over to the pressure phase until the front end position of the screw is reached - adjusts a vari able (for example applied pressure) influencing the compaction of the plastics composition in the mould cavity. In this arrangement, if the aforesaid time range should deviate from a predetermined desired value the pressure application phase, for example, is varied so as to reduce this deviation.

Moreover, a method and an apparatus are known which controls the pressure application phase with the aid of a process computer which takes into account the thermodynamic correlation p-v-T in relation to the properties of the moulding. In accordance with the p-v-T diagram for the plastics material to be processed the pressure application phase is controlled in such a manner that a constant target magnitude (shrinkage or specific volume of the moulding) is achieved. In accordance with a measured deviation of the course of the process the computer determines a new p-T curve in the pressure application phase, in order to achieve once again the predetermined target magnitude and thus to produce parts having constant moulding properties.

These known methods and apparatuses have the disadvantage that although by maintaining a constant tool pressure it is possible to achieve an improvement of the tolerances of the properties of the moulding, nevertheless in accordance with the thermodynamic correlation (p-v-T) the desired constant properties of the moulding are not achieved. It is true that constant properties of the mouldings can be achieved by the use of a process computer, but this entails considerable investment for the owner of the machine and is economically unacceptable for the control of a single injection moulding machine.

The aim of the invention consists in eliminating the abovementioned shortcomings of known apparatuses or methods and producing mouldings from plastics materials in injection moulding machines with constant or substantially constant properties of the mouldings, thus achieving a substantial improvement of quality of the mouldings produced at an economically acceptable cost.

It has been found that between the specific volume of the plastics melt which is to be injected into the tool and the speed of compression achieved during the compression phase there is functional relationship which can be used for measuring and therefore characterising the specific volume of the plastics melt during the injection operation.

The invention consists in a method of injection moulding in which a mould able material is injected by an injecting device into an injection moulding tool defining the contour of the moulding to be produced, the moulding material being subjected to a controllable or servo-controllable pressure by the injecting device whereby to achieve its final shape, and wherein by means of a measured quantity obtained for the specific volume during the compression phase of the injected material, a variable influencing the specific volume of the moulding material is controlled, and wherein on the occurrence of a measured deviation of the specific volume from a predetermined desired value, the said influencing variable is automatically varied in such a way as to reduce this deviation.

The speed of compression may be measured and used as the said measured quantity.

Moreover, the time for pressure increases between two programmable pressure values may be measured and used as said measured quantity. These two pressure values may be a selectable starting pressure and the maximum tool pressure, the pressure increase time being measured in the linear range of the pressure curve during the compression phase.

Preferably, the influencing variable is the pressure applied from the injecting device.

For the stable regulation of the specific volume and for maintenance of the required pressure, the desired value of pressure to be applied from the injecting device is predetermined with an adjustable tolerance range, and only on the exceeding of the tolerance range is there an automatic variation of the said influencing variable so as to reduce this deviation.

Correction of the influencing variable is preferably effected for the next cycle and may be programmable so as to be applied in stages in respect of its magnitude.

In addition to using the said measured quantity for regulation of the specific volume, it is also possible to use the said measured quantity as a measured quantity for an over-riding control device, for example a process computer.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example one embodiment thereof, and in which; Fig. 1 is a graph showing the basic relationship between speed of compression K, specific volume v, and pressure p; Figure 2 is a graph showing a basic for the resentation of the measured variable for the specific volume, as a functional relationship between pressure p and time t; Figure 3 shows the tolerance and correction range for the controlled variables: pres sure and cushion volume of moulding composition; and Figure 4 is a block circuit diagram for the arrangement for carrying out the process.

During the compaction of the plastics material injected into an injection moulding tool cavity, a functional connection exists, as shown in Figure 1, between the speed of compression, the specific volume, and the pressure prevailing in the tool cavity. If the original specific volume 1 should change to a greater specific volume, for example through a rise in temperature of the plasticised material, the original speed of compression 4 at the given pressure 3 will likewise change to a higher speed of compression 5. If it is desired to restore the original specific volume 1 and thus the same density of the mouldings, it will consequently be necessary for the injection pressure 3 to be changed to a higher injection pressure 6. In the event of the specific volume becoming smaller, corresponding reciprocal requirements will apply.

According to Figure 2, the speed of compression 4 is characterised by the pressure mcrease during time 7 measured from a starting injection pressure 8 up to the maximum tool pressure 3, during the compression phrase. If for a given injection pressure, a higher speed of compression 5 should occur, for example as the result of a changed specific volume of the plastics melt, there will correspondingly be a shorter time 9 before the maximum tool pressure 3 is reached, this variation between the time 7 and the time 9 being proportional to the changed specific volume. For the purpose of correcting this changed specific volume 2, the maximum tool pressure 3 is therefore changed in the direction of a higher tool pressure 6 (as shown in Figures 1 and 2), in order to restore the original specific volume 1 and accordingly the original time 7. By measurement of the time taken for a given maximum tool pressure to be reached, it is thus made poss ible on the one hand to apply measurement techniques to a variation of the specific volume of the plastics melt and to the effects of correspondmg corrections of regulating variable, while on the other hand this measured magnitude is used as input variable for a control device for maintaining a constant specific volume or density of the moulding.

In order to ensure the necessary variation of the maximum pressure in the tool (for example from 3 to 6 in Fig. 2) the control device should include means of maintaining pressure requirements, which is for example achieved by means of a known device for maintaining a constant cushion of moulding composition upstream of the screw tip in the pressure application phase. For stable control the desired value of the controlled variable (designated 10 in Fig. 3), namely the time for pressure in the tool to increase to a given maximum is predetermined within an adjustable tolerance range 11, in accordance with Figure 3, and only after this tolerance range 11 has been exceeded is a correction dependent thereon, with the correct sign, of the regulating variable 12 - namely the tool injection pressure effected in order to maintain the controlled variable 10 within the predetermined tolerance range 11. If for example the said pressure increase time should fall below the tolerance range 11, the correction for the next cycle will be made by raising the maximum tool pressure, the magnitude of the correction value 12 being programmable in stages 13. If the tolerance range 11 should be exceeded, the procedure is correspondingly reversed. The same also applies to the control of the cushion of moulding composition for the purpose of maintaining pressure requirements in the tool.

The arrangement for carrying out the method will be explained in accordance with Figure 4. Here the injection moulding machine 14 is started up until the required operation point is reached by way of the program input of the machine control 15. In addition, measuring systems for pressure in the tool 16 and screw piston travel 17 are provided in the injection moulding machine 14. The actual value of the measuring signal 16 is fed to a switch point-dependent time contour 18 for the pressure increase time 22 taken between pressure 19 and pressure 21, the desired values for the starting pressure 19 being taken from the input of the control unit 20 and for the maximum tool pressure 21 from the machine control 15.

After the required operation point has been reached, the actual values of the control variable - pressure increase time 22 can be stored by way of gate 24 as desired values 25 and 26 in the input of control unit 20. The desired values including the tolerance range for the pressure increase time 27 between starting pressure and maximum pressure and the cushion volume 28 of moulding composition are thereafter supplied by the unit 20 and fed simultaneously with the instantaneous actual value of the two variables 22 and 23 to comparator 29, which determines the direction of the required correction for both variables in dependence on the deviation occurring. The correction signals 35, 36 for the two regulating variables, maximum tool pressure 33 and condition of the plastics melt 34, are formed in correcting unit 30 from the direction signals 31, 32 and from the correction factors, which are programmable in stages in the input 20, for the said regulating variables; these correction signals are fed to the machine control 15 for correction of the desired values - maximum tool pressure and condition of the plastics melt - for the following cycle.

WHAT WE CLAIM IS: 1. A method of injection moulding in which a mouldable material is injected by an injecting device into an injection moulding tool defining the contour of the moulding to be produced, the moulding material being subjected to a controllable or servocontrollable pressure by the injecting device whereby to achieve its final shape, and wherein by means of a measured quantity

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pressure prevailing in the tool cavity. If the original specific volume 1 should change to a greater specific volume, for example through a rise in temperature of the plasticised material, the original speed of compression 4 at the given pressure 3 will likewise change to a higher speed of compression 5. If it is desired to restore the original specific volume 1 and thus the same density of the mouldings, it will consequently be necessary for the injection pressure 3 to be changed to a higher injection pressure 6. In the event of the specific volume becoming smaller, corresponding reciprocal requirements will apply. According to Figure 2, the speed of compression 4 is characterised by the pressure mcrease during time 7 measured from a starting injection pressure 8 up to the maximum tool pressure 3, during the compression phrase. If for a given injection pressure, a higher speed of compression 5 should occur, for example as the result of a changed specific volume of the plastics melt, there will correspondingly be a shorter time 9 before the maximum tool pressure 3 is reached, this variation between the time 7 and the time 9 being proportional to the changed specific volume. For the purpose of correcting this changed specific volume 2, the maximum tool pressure 3 is therefore changed in the direction of a higher tool pressure 6 (as shown in Figures 1 and 2), in order to restore the original specific volume 1 and accordingly the original time 7. By measurement of the time taken for a given maximum tool pressure to be reached, it is thus made poss ible on the one hand to apply measurement techniques to a variation of the specific volume of the plastics melt and to the effects of correspondmg corrections of regulating variable, while on the other hand this measured magnitude is used as input variable for a control device for maintaining a constant specific volume or density of the moulding. In order to ensure the necessary variation of the maximum pressure in the tool (for example from 3 to 6 in Fig. 2) the control device should include means of maintaining pressure requirements, which is for example achieved by means of a known device for maintaining a constant cushion of moulding composition upstream of the screw tip in the pressure application phase. For stable control the desired value of the controlled variable (designated 10 in Fig. 3), namely the time for pressure in the tool to increase to a given maximum is predetermined within an adjustable tolerance range 11, in accordance with Figure 3, and only after this tolerance range 11 has been exceeded is a correction dependent thereon, with the correct sign, of the regulating variable 12 - namely the tool injection pressure effected in order to maintain the controlled variable 10 within the predetermined tolerance range 11. If for example the said pressure increase time should fall below the tolerance range 11, the correction for the next cycle will be made by raising the maximum tool pressure, the magnitude of the correction value 12 being programmable in stages 13. If the tolerance range 11 should be exceeded, the procedure is correspondingly reversed. The same also applies to the control of the cushion of moulding composition for the purpose of maintaining pressure requirements in the tool. The arrangement for carrying out the method will be explained in accordance with Figure 4. Here the injection moulding machine 14 is started up until the required operation point is reached by way of the program input of the machine control 15. In addition, measuring systems for pressure in the tool 16 and screw piston travel 17 are provided in the injection moulding machine 14. The actual value of the measuring signal 16 is fed to a switch point-dependent time contour 18 for the pressure increase time 22 taken between pressure 19 and pressure 21, the desired values for the starting pressure 19 being taken from the input of the control unit 20 and for the maximum tool pressure 21 from the machine control 15. After the required operation point has been reached, the actual values of the control variable - pressure increase time 22 can be stored by way of gate 24 as desired values 25 and 26 in the input of control unit 20. The desired values including the tolerance range for the pressure increase time 27 between starting pressure and maximum pressure and the cushion volume 28 of moulding composition are thereafter supplied by the unit 20 and fed simultaneously with the instantaneous actual value of the two variables 22 and 23 to comparator 29, which determines the direction of the required correction for both variables in dependence on the deviation occurring. The correction signals 35, 36 for the two regulating variables, maximum tool pressure 33 and condition of the plastics melt 34, are formed in correcting unit 30 from the direction signals 31, 32 and from the correction factors, which are programmable in stages in the input 20, for the said regulating variables; these correction signals are fed to the machine control 15 for correction of the desired values - maximum tool pressure and condition of the plastics melt - for the following cycle. WHAT WE CLAIM IS:

1. A method of injection moulding in which a mouldable material is injected by an injecting device into an injection moulding tool defining the contour of the moulding to be produced, the moulding material being subjected to a controllable or servocontrollable pressure by the injecting device whereby to achieve its final shape, and wherein by means of a measured quantity

obtained for the specific volume during the compression phase of the injected material, a variable influencing the specific volume of the moulding material is controlled, and wherein on the occurrence of a measured deviation of the specific volume from a predetermined desired value, the said influencing variable is automatically varied in such a way as to reduce this deviation.

2. A method as claimed in claim 1, wherein the speed of compression is meas ured and is used as the said measured quantity.

3. A method as claimed in claim 2, wherein the time for pressure increase between two programmable pressure valves is measured and is used as said measured quantity.

4. A method as claimed in claim 3, wherein the two pressure valuves are a selectable starting pressure and the maximum tool pressure.

5. A method as claimed in any one of claims 1 to 4, wherein the pressure increase time is measured in the linear range of the pressure curve during the compression phase.

6. A method as claimed in any one of claims 1 to 5, wherein the influencing variable is the pressure applied from the injecting device.

7. A method as claimed in claim 6, wherein for regulation of the specific volume and the maintenance of pressure requirements, the desired values of pressure to be applied from the injecting device is predetermined with an adjustable tolerance range, only when the tolerance range is exceeded is there an automatic variation of the said influencing variable so as to reduce this deviation.

8. A method as claimed in any one of claims 1 to 7, wherein the correction of the influencing variable is effected for the next following cycle.

9. A method as claimed in claim 1 to 8, wherein the correction value for the influencing variable is programmable so as to be applied in stages.

10. A method as claimed in any one of claims 1 to 9, wherein the measured quantity for the specific volume serves as measured quantity for an everriding control device, for example a process computer.

11. A method for producing mouldings substantially as hereindescribed with reference to and as shown in the accompanying drawings.

12. Moulding whenever produced by the method claimed in any of the preceding claims.