JPS6254654B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of controlling an extrusion rate to keep the output rate of an extruder constant over time.

FIG. 1 shows a conventional method for controlling the extrusion rate. That is, method A in Fig. 1 measures the pressure at the tip of the extruder and controls the rotation of the screw drive motor 1 to keep this pressure constant (the pressure at the tip of the extruder is controlled by the rotation of the motor). (feedback method) is generally widely adopted.

In addition, method B in Fig. 1 measures the thickness and weight of the semi-finished product or product after it comes out of the die 2 and controls the rotation of the screw drive motor 1 in order to keep the thickness and weight constant. (a method in which the thickness and weight of the product are fed back to the rotational speed of the motor), which is also a method widely used in extrusion molding.

In addition, 3 in Fig. 1 is a hopper for supplying raw material resin;
4 is a raw resin, 5 is a speed reducer, 6 is a heater that heats the cylinder, 7 is a cylinder, 8 is a screw that plasticizes the resin, 9 is a resin pressure detector, 10 is an extruded resin, 11, 11' are controls ( It is a calculation) device.

Now, in method A, the pressure at the tip of the extruder can be kept constant, but when the melt viscosity of the resin changes, Q≒aP _D /η... [a: constant determined by the shape of the die, η : viscosity, P _D : extruder tip pressure (≈die pressure), Q: extrusion rate] The extrusion rate cannot be constant (for example, even if the extruder tip pressure is controlled to be constant). This happens when it is difficult to keep the mixing ratio of the raw materials constant, such as when the raw materials are a blend of several types of resins or when the raw materials include recycled raw materials.

Although method B is an effective control method in terms of eliminating long-term fluctuations in extrusion rate, it has the following two major drawbacks. That is, in order to continuously and accurately measure the thickness and weight of a product without damaging the product, expensive equipment is required.

Thickness and weight are usually measured after the product has cooled down, that is, after a certain period of time has elapsed after it has come out of the die, so there is a time lag between the thickness and weight and the screw rotation speed, which is a control element. can be,
Short-term fluctuations cannot be eliminated.

In an extrusion molding machine, keeping the extrusion amount constant is
This is an important point in terms of product quality. At the same time, the extruder is required to have low resin temperature, high extrusion rate, etc. Most of these requirements are mainly achieved by the functions of the screw, but it is becoming difficult to achieve all of them with the screw. That is, in order to meet the demands for low resin temperature and high extrusion rate, the groove depth of the screw metering section must be made deep, but if this is done, the measurability of the extrusion rate becomes poor. Note that current extruder screws tend to do this because they aim for a high throughput, but if there is non-uniformity in the raw material composition, problems such as fluctuations in the extrusion rate often occur.

In addition, this high quality (low resin temperature, small fluctuation in extrusion amount)
In order to meet the demand for high production volume, methods have been adopted in which the extrusion rate is kept constant by means other than the metering properties of the screw, but each of these methods has problems in terms of cost or technology. There were many.

The present invention aims to provide a method for controlling the extrusion rate with low equipment cost and good controllability.

The extrusion amount Q is expressed by the above-mentioned formula when treated as a Newtonian fluid. That is, Q=P _D /η...Even when the molten resin is treated as a non-Newtonian fluid, the relational expression between P, η, and Q can be derived. Therefore, the present invention is characterized in that both P _D and η are measured, Q is calculated from the calculated equations, and Q is controlled to be constant. Another feature of the present invention is that the viscosity η is determined from the pressure waveform generated by the screw using a pressure detector attached to the cylinder at the tip of the screw.

If we organize the above points, we can see Figure 22 (conventional A
23 (conventional method B), and FIG. 24 (method of the present invention).

According to the present invention, even if the composition of the raw material changes, the temperature of the resin at the exit of the extruder changes due to changes in the cylinder temperature setting, changes in the screw tip pressure (due to screen clogging, etc.), and changes in the screw rotation speed. However, the extrusion amount can be controlled to a target value by detecting the resin pressure and viscosity at the die inlet and calculating the extrusion amount.

In addition, regarding the viscosity in Fig. 24, the same resin flows in the screw and the die, and the resin temperature in both is almost the same, so the detection of the resin temperature and the handling of the resin temperature in calculations are as follows: You can also turn it off (this does not significantly affect accuracy).

An embodiment of the present invention will be described below with reference to the drawings.The outline of the system is as shown in FIG. 24. FIG. 2 illustrates the control system of the present invention, and the same parts as in FIG. 1 are designated by the same reference numerals.

Now, in FIG. 2, the present invention uses a resin pressure detector 9 in front of the inlet of the die 2 after exiting the extruder, and a resin pressure gauge 12 attached to the cylinder 7 at the tip of the screw 8 to measure the pressure inside the screw 8. Detects pressure and calculates viscosity. The extrusion amount is calculated from the resin pressure before the inlet of the die 2 and its viscosity, and the extrusion amount is adjusted so that the calculated extrusion amount matches the set target extrusion amount.
This relates to a method for controlling the rotation speed of the motor 1 that drives the screw 8. Also 1
3 is an arithmetic and control unit.

In addition, calculation of viscosity from the pressure inside the screw 8,
In addition, the calculation formula for calculating the extrusion amount from the inlet pressure of die 2 and its calculated viscosity may be based on the assumption that the molten resin passing through is a Newtonian fluid, a non-Newtonian solidifying fluid, or based on the flow of the resin. Although it differs somewhat depending on whether or not the accompanying heat generation and heat conduction are taken into account, here we will explain it using a calculation formula that is simply a Newtonian fluid and ignores heat generation and heat conduction. Furthermore, detection of the resin pressure at the die inlet is generally widely performed, and its explanation will be omitted here.

The formula for calculating the extrusion amount from the resin pressure P at the die inlet and the viscosity η (which varies depending on the raw material composition, shear rate, and resin temperature) is as shown in the above formula. Further, as an example of the present invention, there is a method in which a viscosity characteristic equation is stored as a function of shear rate and resin temperature based on a previously predicted raw material composition as described below.

Next, we will discuss how to detect the pressure of the resin attached to the cylinder at the tip of the screw, and how to calculate the viscosity from the detected pressure (hereinafter, we will assume that everything is a Newtonian fluid and ignore heat transfer). In the present invention, the following three embodiments are shown as methods for this purpose.

First, as a first example, a full flight screw 8 (number of flights 1) which is a general shape of a screw
3), the pressure waveform recorded by the pressure detector 12 attached to the cylinder 7 is as shown in FIG. It increases and decreases with each rotation. Furthermore, Fig. 3 is a cross-sectional view showing the state in which a resin pressure gauge is attached to the cylinder at the tip of the screw.
The figure is a developed sectional view of FIG. 3, and FIG. 5 is a resin pressure waveform diagram measured at the tip of the screw.

Now, when treated as a Newtonian fluid, the fifth
The pressure difference when the waveform shown in the figure is shown can be expressed by the following formula.

In the formula, N indicates the screw rotation speed. Letters indicating other shapes are shown in FIGS. 3 and 4. Also
Vb is the circumferential speed of the screw, and Vbj is the Z of that circumferential speed.
Indicates the directional component. However, this formula is for the case of a general full-flight screw (one flight thread). At this time, the contact length d of the screw groove in the inner peripheral direction of the cylinder in a cross section perpendicular to the axis of the screw is as follows: d=W/sin θb=πD-e/sin θb.
Therefore, from the formula, by measuring the resin pressure waveform at the full flight part at the tip of the screw where the liquid phase resin is conveyed, the viscosity of the resin passing through this part can be calculated.

Next, as another method for determining the viscosity (second example), the shape of the tip of the screw through which the liquid phase resin passes is made into a shallow groove shape without a lead as shown in Fig. 8, and one revolution of the screw in this part is made. There is a method to obtain it by measuring the resin pressure waveform generated at each time. In this case, the relational expression between the pressure waveform and viscosity as shown in FIG. 7 can be simplified as ΔP/d=ΔP/l=6ηπDN/H ² .

In addition, with the usual method of determining viscosity using a resin pressure wave meter for a full-flight screw-shaped part, the groove depth cannot be made too shallow in order to obtain a certain extrusion amount, but in the case of this method, the viscosity can be Since it is possible to make the groove depth of the portion devised for determination shallow, there is an advantage that ΔP/d can be measured in an enlarged form. Here, FIG. 9 is a sectional view from X to X in FIG.
Figure 0 is an external development view of the screw in the X-X portion in Figure 8, and Figures 11, 12, and 13 are cross-sectional views of B-B, C-C, and D-D in Figure 10, respectively. .

Next, to explain in detail why the viscosity η is determined from the pressure, as shown in Figure 8, the pressure measurement part has a U-shaped groove (half circumference length in the figure) at the tip of the screw that opens in the rotational direction. It is being That is, both sides have banks and shallow grooves with banks at the end (there are no banks at the entrances A to A).

When the screw part is filled with molten resin and the screw rotates, since the molten resin has a high viscosity, the molten resin is pushed into the U-shaped groove by the shear resistance generated between the cylinder and the bottom of the groove. The pressure increases deeper into the groove. When this is detected by the resin pressure gauge 12a, it will be as shown in Fig. 7, where the pressure P ₁ is the pressure when the pressure gauge is at the deepest part of the U-shaped groove, just before reaching the edge of the bank, and the pressure P ₂ is the pressure This is the time between the meter position passing through the U-shaped groove and reaching the U-shaped groove entrances A to A.

The above equation is a mathematical expression of this relationship, and if the operating conditions and ΔP are known, the viscosity η can be determined. Although the structures of the first and second embodiments are different, the principle of viscosity measurement is the same as that described above.

Furthermore, the screw and cylinder structure of the first and second embodiments are changed (Fig. 14), that is, the screw 8 has a cylindrical shape without a groove, and the groove member 15 has a groove 14 cut on the cylinder side. When installed, the pressure gauge 12 attached to the cylinder in this part
The resin pressure indicated by b and 12c is no longer the same as that of screw 1.
The sawtooth waveform for each rotation is not drawn, but the pressure waveform is as shown in Figure 6. (Figure 6 shows the resin pressure when the extruder tip has the screw and cylinder structure shown in Figure 14. (temporal change diagram). Note that the pressure difference between the two points 12b and 12c in the section perpendicular to the screw axis shown in FIG. 15 corresponds to ΔP in the equation. Also, l corresponds to d.

Therefore, in the present case, ΔP/d=P ₁ -P ₂ /l=6ηπDN/H ² ..., and the calculation for determining the viscosity can be simplified (the calculation device can be simplified). The shear rate γ can be simply given as follows, where h is the gap between the cylinder and the screw.

Also, to explain the viscosity η ₀ obtained from the pressure waveform at the tip of the screw and the viscosity η used when calculating the amount of resin extruded from the die, the viscosity of the molten resin is determined by the shear rate γ 〓 which is the measurement condition. , temperature T
Therefore, the viscosity measured by the screw tip device must be converted to the resin viscosity at the pressure detector 9 provided for calculating the flow rate to the die.

Regarding the viscosity characteristics of the molten resin, the relationship between the shear rate γ and the temperature T is investigated in advance using a viscosity measuring device such as a capillary rheometer (this is used as the base data for the viscosity characteristics).

Figure 20 shows that, for example, the viscosity characteristics measured with a capillary rheometer and the viscosity measured with a screw tip device differ even at the same temperature and shear rate due to differences in measurement methods (changes in raw material composition).
This paper describes a means for correcting the viscosity characteristics of a capillary rheometer using actual measurements of the screw tip device.

Figure 21 shows the case where the temperature at the viscosity measuring device at the tip of the screw is assumed to be the same as the temperature at the pressure detector 9 position at the inlet of the die. This is a method in which the properties are measured by changing the viscosity, and the actually measured viscosity at a certain shear rate at the tip of the screw is approximated to the corresponding temperature curve of a capillary rheometer to determine the viscosity at any shear rate.

Now, the viscosity obtained by calculating from the pressure waveform at the tip of the screw is, as is generally well known as a viscosity characteristic, one point of viscosity η expressed as a function of shear rate γ〓 and temperature T (at a certain shear rate , viscosity at a certain temperature). The viscosity η of this one point
The relationship between ₀ and the viscosity η calculated by the extrusion amount from the die is that the same resin flows out, and the tendency is the same.

That is, if the viscosity determined from the internal pressure of the screw decreases, the viscosity flowing inside the die also decreases. In this case, assuming that the rate of decrease in both is equal, the viscosity corrected for the change in viscosity due to the change in screw rotation speed, that is, the change in shear rate of the viscosity detection section, is defined as η′,
It is sufficient to control P ₀ /η′ to be constant. That is, it can be corrected using the relational expression η'= ^Kγ〓n =K(πDN/h) ⁿ , etc. (K: function of temperature (constant in this case), n is constant).

However, in order to further improve accuracy, it is possible to measure the expected viscosity of the raw material composition in advance using a separate physical property measuring instrument, store this value, and compare this viscosity with the calculated viscosity at one point. ,
There is a method of estimating the viscosity of the extruded resin as a function of shear rate and temperature. In addition,
Regarding the handling of viscosity as a function of temperature, it can be ignored since the same resin passes through the screw and the die, and the resin temperature during both passages is the same.

Next, as the viscosity characteristic formula, 2 when using the following formula
Let's discuss two examples.

logη＝a ₀ +a ₁ logγ〓＋a ₁₁ (logγ〓) ² +a ₂ T＋a ₂₂ T ² +a ₁₂ Tlogγ〓 ... (a ₀ , a ₁ , a ₁₁ , a ₂ , a ₂₂ , a ₁₂ are constants) ○I If the above equation is known in advance from the viscosity measured with another physical property measuring instrument, then by substituting the viscosity η ₀ , γ〓 ₀ , T ₂ (measured resin temperature) determined from the resin pressure inside the screw, logη ₀ can be calculated. =a′ ₀ +a ₁ logγ〓 ₀ +a ₁₁ (logγ〓 ₀ ) ² +a ₂ T ₂ + a ₂₂ T ₂ ² +a ₁₂ T ₂ logγ〓 ... Find a′ ₀ from (a ₁ , a ₁₁ , a ₂ , a ₂₂ , a ₁₂ are constants of the memorized viscosity characteristic equation), substitute a′ ₀ in place of a ₀ to create the viscosity characteristic equation, and use this as the viscosity when calculating the amount of extrusion from the die. do.

○B If temperature is ignored, η is added to the above equation.
₀ , γ〓 Substitute ₀ and find the value of T. Substituting this obtained value of T, T′, a″ ₀ = a ₀ + a ₂ T′, a″ ₁ = a ₁ + a ₁₂ T′, and logη = a″ ₀
+a″ ₁ logγ〓+a ₁₁ (logγ〓) ² ... is used as the viscosity characteristic formula, and this is used as the viscosity when calculating the amount of extrusion from the die.

Using the above examples of ○a and ○b as an explanation aid, the 20th
The viscosity characteristic curves are shown in Fig. 21. In addition, Fig. 20 detects the resin temperature and estimates the viscosity of the extruded resin so that the rate of change in viscosity with respect to shear rate and temperature is equal to the viscosity measured in advance with another measuring device. Show how. Also the 21st
The figure shows a method for estimating the viscosity of the extruded resin so that the viscosity, measured in advance with another measuring device, changes at a rate equivalent to the shear rate.

[Brief explanation of the drawing]

Fig. 1 is a system diagram explaining a method of controlling the extrusion rate in a conventional extruder, Fig. 2 is a system diagram explaining a method of controlling the extrusion rate in an extruder showing an embodiment of the present invention, and Fig. 3 is a system diagram of the present invention. 4 is a developed view of FIG. 3, FIG. 5 is a resin pressure waveform diagram measured at the screw tip, and FIG. 6 is a cross-sectional view of the screw tip in the first embodiment of the invention. FIG. 7 is a diagram of the temporal change in resin pressure due to the screw in FIG. 8. FIG. Figure 9 is a cross-sectional view from X to X in Figure 8, and Figure 10 is a cross-sectional view from X to X in Figure 8.
External developed view of the screw in the X section, Figure 11, Figure 1
Figures 2 and 13 are B~B, C~ in Figure 10, respectively.
14 is a sectional view of the tip of the cylinder and screw in the third embodiment of the present invention; FIG. 15 is a sectional view of Y-Y in FIG. 14;
The figure is a developed view of the cylinder side groove in the Y-Y cross section of Figure 14, and Figures 17, 18, and 19 are
B-B, C-C and D-D sectional views of Figure 6, No. 20
and FIG. 21 are diagrams showing viscosity characteristic curves, respectively.
FIG. 22 is a block diagram of the conventional method A, FIG. 23 is a block diagram of the conventional method B, and FIG. 24 is a block diagram showing one embodiment of the method of the present invention. Explanation of the main parts of the diagram: 1... Screw drive motor, 2... Die, 3... Hopper, 4... Raw resin, 6... Heater, 7... Cylinder, 8... Screw, 9... Resin Pressure detector, 10... Extruded resin, 12... Resin pressure gauge, 13... Arithmetic control device.

Claims (1)

[Claims] 1 Calculate the amount of extrusion passing through the die from the resin pressure at the die inlet and the viscosity calculated from the pressure measured with the resin pressure detector attached to the cylinder at the tip of the screw, and make sure that this calculated amount of extrusion matches the set amount of extrusion. In addition to controlling the rotation speed of the screw so that it is equal,
When calculating the viscosity, the viscosity determined in advance with another physical property measuring device is memorized, and this is compared with the viscosity determined from the resin pressure at the tip of the screw to calculate the amount of extrusion through the die. A method for controlling an extrusion amount, characterized by estimating a viscosity to be used at a time.