GB1567791A

GB1567791A - Method of and apparatus for injection moulding

Info

Publication number: GB1567791A
Application number: GB34026/77A
Authority: GB
Original assignee: Demag Kunststofftechnik Zweigniederlassung der Demag AG
Current assignee: Demag Kunststofftechnik Zweigniederlassung der Demag AG
Priority date: 1976-08-13
Filing date: 1977-08-12
Publication date: 1980-05-21
Also published as: FR2361642B1; FR2361642A1; SE423348B; DE2636554A1; SE7709173L

Description

(543 METHOD OF AND APPARATUS FOR INJECTION MOULDING (71) We, DEMAG KUNSTSTOFFrECHNIK ZWEIGNIEKERLASSUNG DER DFMAG AETIEN- GESELLSCHAFr, a German Body Corporate of Renmeg 37, D-8500 Nurnberg, The Federal Republic of Germany, 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 performed, to be particularly described in and by the following statement: -- The present invention relates to a method of, and apparatus for injection moulding.

it is known that the viscosity of the plastics melt in the injection cylinder of a screw injection moulding machine significantly influences both the injection sequence and the quality of the moulding produced.

This applies in particular to fluctuations in viscosity and homogeneity as, for example, if the granules have been insufficiently fused. Hence, there have been various proposals for monitoring the viscosity or the homogeneity of the plastics melt in the injection cylinder of screw injection moulding machines before putting the machine into operation, or also during operation, in order to make corrections where necessary, in the sense of increasing or lowering the viscosity. However, all these known proposals have disadvantages which detract from the reliability and precision of the viscosity determination, especially during operation of the screw injection moulding machine.

A known measure for determining and monitoring the melt viscosity is to measure the temperature of the melt in the head space of the injection cylinder or in the nozzle. It is true that viscosity changes in the melt caused by temperature changes, can be detected in this way, but a change of viscosity due to changes in molecular weight when changing plastics batches is not detectable and the influence of temperature inhomogeneity in the material can only be detected if several temperature measurement points are provided. In addition, measuring the material temperature of a flowing plastics melt in the nozzle is difficult because of the errors resulting from friction and heat conduction and because of the errors caused by the slowness of response of the measuring elements.

A further possible method of determining the viscosity of the plastics melt is to monitor the hydraulic pressure in the injection cylinder in addition to the measurement of the temperature of the melt in the head space, or instead of this measurement.

However, the magnitude of the hydraulic pressure also does not provide any unambiguous information regarding the viscosity of the melt, since the parameters being compared, namely the speed of advance of the screw and the injection pressure, are not only affected by the melt viscosity but also by the friction, for example the friction between the screw and the cylinder, the bearing friction, the piston friction and the like, and this factor cannot be regarded as constant.

Determining the melt viscosity by measuring the pressure of the melt in the cylinder head, coupled, if appropriate, with a temperature measurement, in the main encounters difficulties concerned with the measurement technique since the point of measurement is in a high temperature region.

Furthermore, it is frequently difficult to incorporate the fitment, and the measurement itself is insufficiently reliable in operation.

All the methods in which measuring elements must be provided in the nozzle or in the head space of the injection cylinder suffer from the common disadvantage that it is difficult to avoid dead spaces and gaps.

These lead to material accumulation and material scorching during operation, so that on the one hand there is the danger that the quality of the mouldings being manufactured will suffer, whilst on the other hand, the gradual clogging of the dead spaces and gaps throws doubt on the accuracy of the measurement.

Finally, measuring the viscosity of the melt by means of ultrasonics is also known.

This method is, however, very expensive and evaluation is complicated and in most cases requires an additional pressure measurement in order to obtain absolute values.

According to the present invention, there is provided a method of injection moulding using a screw injection moulding machine having a back-flow seal, the method comprising monitoring the viscosity or homogeneity of the plastics melt by monitoring the closing time of the back flow seal during injection.

The invention also provides an injection moulding machine comprising: a screw-fed plastics-injection unit; a back-flow seal for the screw of the unit; and a device arranged to monitor the closing time of the backflow seal during injection to produce a signal representative of the viscosity or homogeneity of the plastics being injected.

The choice of the closing time of the backflow seal as a criterion for determining the viscosity of the plastics melt comes very close to direct viscosity measurement, because the rheological effect exerted by the plastics melt on the back-flow seal is similar to that which arises in an annular gap viscometer; accordingly, the viscosity directly affects the closing time of the backflow seal, in contrast to pressure measurement or temperature measurement in the melt.

The closing time of the back flow seal can be measured by relatively simple means, without it being necessary to provide measurement points, which would entail the disadvantages described above, in the head space or nozzle of the injection cylinder. It has been found that the closing time can be measured particularly advantageously because the method is technically very simple, in terms of the duration of the rise in the torque transmitted to the screw, during the injection sequence. This is because from the start of the injection sequence up to the complete closing of the back-flow seal, the action of the injection pressure also extends into the screw flights, so that in these regions an increased force is exerted on the screw webs because of the prevailing pressure gradient and this leads to a corresponding rise in the torque acting on the screw if the customary screw reverse-rotation safety device is used.

After the back-flow seal has been closed, the increased pressure in the screw flights again dissipates, so that the torque also falls. Accordingly, the closing time of the back-flow seal can be determined very accurately as the time interval between the start of the rise in the toraue and the time at which the torque reaches a maximum.

Since the torque acting on the screw can be measured relatively easily, this provides a very simple technique, from the point of view of measuring technique, for measuring the closing time. Above all, no additional measuring elements whatsoever are required in the interior of the cylinder, that is to say on the screw, on the backflow seal, on the head or on the nozzle.

Consequently, there is also no need for any changes whatsoever in this region, in the case of already existing screw injection moulding machines, so that it is easy to modify already existing screw injection moulding machines for carrying out the method.

The method can be used to carry out closed loop control of the viscosity and/or homogeneity of the plastics melt. This can be done purely qualitively by modifying the viscosity and homogeneity of the plastic melt by appropriate known measures, for example by changing the screw speed and/ or the screw-back pressure and/or an adjustable nozzle cross-section (shear gap) or other viscosity and/or homogeneity influencing parameters, to the point that fluctuations in the closing time of the backflow seal in successive injection sequences no longer exceed a permissible amount.

Ilowever, a more quantitative embodiment is also possible by predetermining, on the basis of preliminary experiments or the like, an intended value of the closing time, then measuring the deviations of the actual closing time from this intended value from one injection sequence to another and taking appropriate measures which increase or reduce the viscosity and/or homogeneity, whilst keeping the remaining parameters constant.

If a particular intended value of the closing time of the back-flow seal is known, the screw injection moulding machine can be controlled or regulated by automatically compensating deviations of the closing time from the intended value by appropriate control of the functions which influence the viscosity and/or homogeneity of the plastics melt. This can be done by means of a device for controlling the screw speed, the screw back-presure and/or an adjustable nozzle cross-section or other parameters which influence the viscosity, as a function of the measured closing time.

A further advantage of measuring the closing time of the back-flow seal is that, if the viscosity of the plastics melt is known, the functioning of the back-flow seal itself can also be monitored.

The invention will be further described with reference to the accompanying drawings, which illustrate an embodiment of the present invention. In the drawings: Figure 1 is a schematic longitudinal section through the injection cylinder of a screw injection moulding machine with a back-flow seal (RSP); Figure 2 is a diagram in which the restoring torque (Md) measurable on the screw shaft, the screw back-pressure (Phydr) and the screw stroke (S) are plotted as a function of time; Figure 3 is a purely schematic circuit diagram of a measuring and regulating circuit for regulating the viscosity of plastic melt contained in the injection cylinder; and Figure 4 shows a schematic block circuit diagram for the measuring circuit shown in Figure 3.

In the injection cylinder 1, a screw 2 can be displaced axially by an injection piston (not shown) and is subjected to hydraulic pressure, and can be caused to rotate by a drive motor (also not shown), via a gearbox. The injection piston and the drive motor are indicated symbolically by the arrows 3 and 4 respectively.

A back-flow seal, generally designated 6, is provided on the screw head, and comprises essentially an annular thrust ring 9 which can be caused to slide between a front stop face on the screw tip and a rear sealing face 8. The screw head possesses passages 10 through which plastics material from the screw 2 can enter the head space 11 of the injection cylinder 1, as soon as the rear sealing face 8 is released by the thrust ring 9 (see the drawn position of the thrust ring 9). If the screw 2 is pushed forwards by the injection piston 3, for the purpose of injecting the plastics melt from the head space 11 into a mould (not shown), the injection pressure and resistance of the melt which flows backwards through the passages 10 and the interior of the thrust ring 9 cause the thrust ring 9 to slide against the sealing face 8. When the thrust ring 9 rests on the sealing face 8, further back-flow of the plastic melt into the screw flights is prevented. This sequence, and the nature and design of back-flow seals of the type shown here are known and therefore do not require to be explained in more detail at this point.

The invention is based on the recognition of the fact that the time interval between the start of injection, that is to say the start of the forward movement of the screw 2, and the contact of the thrust ring 9 with the sealing face 8, that is, the socalled "closing time" of the back flow seal 6, deDends on the viscositv and/or homogeneity of the plastics melt present in the head space 11 and is a measure of these properties. In the illustrated embodiment, this dosing time is measured verv simnly in terms of the duration of the rise in the screw torcue. which commences at the start of the forward stroke of the screw 2. The duration of the rise of this restoring tornue is terminated bv the thrust ring 9 coming to rest on the sealing face 8; after a seal has been made. the torque ain drones.

The rise in torque can be measured at any point between the drive motor 4 and the screw 2 in front of the screw reverse-rotation safety device (which is not shown); in the illustrated embodiment, the torque measuring unit is indicated on the screw shaft. The measured value of the torque is converted to an analog voltage and fed to the input of a measuring circuit. The measurement of the duration of the rise in the torque starts at the beginning of the rise, which follows, with a brief delay, onto the signal "inject". As soon as the torque rise has reached its maximum, the measuring circuit provides an output signal proportional to the measured closing time.

The output signal is compared with a predetermined intended value of the closing time in a regulating circuit RK (Figure 3).

If the measured closing time deviates from the intended value, an increase or decrease of the screw speed, of the screw backpressure and/or of an adjustable nozzle cross-section (shear gap) or of other parameters which influence the viscosity is effected as a function of the magnitude of the deviation. This correspondingly increases or decreases the viscosity of the melt contained in the injection cylinder 1, until the measured closing time agrees with the intended closing time of the back-flow seal.

The intended closing time can be set by the regulating circuit RK and is determined by preliminary experiments for a particular plastics material and for certain injection parameters which must be kept constant (for example the injection speed and injection pressure).

Figure 2 illustrates the course of the hydraulic injection pressure acting on the injection piston 3 during the injection sequence, and also shows that this injection pressure only builds up to its full value after a certain delay measured from the time of the "inject" electrical signal, which is indicated by the dot-dash line. Relative to this signal, the rise in the screw torque is correspondingly delayed.

The block circuit diagram for the measuring and regulating circuit, shown in Figure 3, comprises as the measuring circuit, a measurement receiver for the screw torque Md and a measurement converter, which converts the closing time of the back-flow seal (tri") as determined from the duration of the rise of the torque, into a time dependent analog electrical parameter. This actual value of the closing time is fed into a comparator stage VS, into which the intended or set value of the closing time of the back-flow seal is also fed. The difference signal resulting from the comparison of these two values then passes to a servoamplifier RV, is there amplified and controls a correcting element by which, for example, the speed of the drive motor 4-of the screw 2 or some other suitable parameter is altered.

Figure 4 shows in detail the basic principle of the mode of operation of the measurement converter according to Figure 3.

The electrical signal of the measurement receiver Md passes to the input of an amplifier as a function of the screw torque, is amplified and is fed to a subtractor. The subtractor measures the torque-analog electrical parameter at certain intervals and determines the difference between the signals of successive time intervals. This difference in signal is fed to an AND-gate UG, to the second input of which is connected the output of a 1,000 Hz oscillator. If the difference signal from the subtractor is less or equal to zero, no signal is produced at the output of the AND-gate and neither a counter connected thereto is advanced nor is a parameter corresponding to the actual value passed to the regulating circuit RK.

If, however, the difference signal is nonzero and positive, signals of the frequency of the oscillator are applied to both inputs of the AND-gate, by means of which the counter is advanced or an analog signal representing the actual value of the parameter is passed on to the regulating circuit RK. The start and end of the increase in the torque can thus be determined with an accuracy of 1 thousandth of a second and the closing time can be determined correspondingly. The closing time can be read off directly from the counter as the number of thousandths of a second as represented by the number of cycles which have passed through the AND-gate while a non-zero positive difference signal has been applied to the input of the AND-gate.

WHAT WE CLAIM 1S: - 1. A method of injection moulding using a screw injection moulding machine having a back-flow seal, the method comprising monitoring the viscosity or homogeneity of the plastics melt by monitoring the closing time of the back flow seal during injection.

2. A method according to claim 1, wherein one or more operating parameters of the machine which affect the homogeneity or viscosity of the plastics melt are varied in dependence on the monitored value of the viscosity or homogeneity of the plastics melt.

3. A method according to claim 2, wherein the monitored value of the viscosity or homogeneity of the lastics melt is used to vary one or more of said parameters to compensate for variations in said monitored value and thereby to effect closed loop control of the viscosity and/or homogeneity of the plastics melt.

4. A method according to claim 1, 2 or 3, wherein the closing time of the back flow seal is monitored by measuring the duration of the increase in the screw torque during the injection.

5. An injection moulding machine com prising: --a screw feed plastics injection unit; a back-flow seal for the screw of the unit; and a device arranged to monitor the closing time of the back-flow seal during injection to produce a signal representative of the viscosity or homogeneity of the plastics being injected.

6. An apparatus according to claim 5, and including means responsive to said signal to vary one or more operating parameters of the apparatus in a manner to effect closed-loop control of the viscosity and/or homogeneity of plastics being injected.

7. An apparatus according to claim 6, wherein said means is operative to vary the screw speed and/or the screw back pressure and/or an adjustable nozzle cross-section in dependence on said closing time of the back flow seal as measured by said device.

8. Apparatus according to claim 5, 6, or 7 wherein the device comprises a torquemeasuring device for measuring torque transmitted to the screw of the injection unit.

9. A method of injection moulding substantially as hereinbefore described with reference to the accompanying drawings.

10. An injection moulding apparatus constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. An article when moulded using the method of any one of claims 1 to 4 or 9 or an apparatus according to any one of claims 5 to 8 or 10.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the speed of the drive motor 4-of the screw 2 or some other suitable parameter is altered. Figure 4 shows in detail the basic principle of the mode of operation of the measurement converter according to Figure 3. The electrical signal of the measurement receiver Md passes to the input of an amplifier as a function of the screw torque, is amplified and is fed to a subtractor. The subtractor measures the torque-analog electrical parameter at certain intervals and determines the difference between the signals of successive time intervals. This difference in signal is fed to an AND-gate UG, to the second input of which is connected the output of a 1,000 Hz oscillator. If the difference signal from the subtractor is less or equal to zero, no signal is produced at the output of the AND-gate and neither a counter connected thereto is advanced nor is a parameter corresponding to the actual value passed to the regulating circuit RK. If, however, the difference signal is nonzero and positive, signals of the frequency of the oscillator are applied to both inputs of the AND-gate, by means of which the counter is advanced or an analog signal representing the actual value of the parameter is passed on to the regulating circuit RK. The start and end of the increase in the torque can thus be determined with an accuracy of 1 thousandth of a second and the closing time can be determined correspondingly. The closing time can be read off directly from the counter as the number of thousandths of a second as represented by the number of cycles which have passed through the AND-gate while a non-zero positive difference signal has been applied to the input of the AND-gate. WHAT WE CLAIM 1S: -

1. A method of injection moulding using a screw injection moulding machine having a back-flow seal, the method comprising monitoring the viscosity or homogeneity of the plastics melt by monitoring the closing time of the back flow seal during injection.