Apparatus for controlling thickness of film formed by melt extrusion

Abstract: An apparatus for controlling the thickness of a film molded by a melt extruding method is provided with a plurality of cooling means for cooling the vicinity of a die lip close to a plurality of heating means (5) and with many grooved holes in which the heating means (5) are embedded and provided with air holes (11) at the both ends thereof, whereby the temperature can be freely adjusted to the high and low set values in a short period of time, by which the time loss and material loss can be reduced.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus for controlling the thickness of a synthetic resin film in which upon the formation of a film by a melt extrusion method, ununiformity in thicknees of the film is eliminated and controlled so as to make the thickness of the film uniform or the thickness of the film can be locally controlled as required. Related Backgound Art Upon the formation of a film by discharging molten resin from a die lip and gradually thinning the thickness of the film, it is well known in the art that in order to eliminate the ununiformity in thickness of the film, temperature distribution of the lip is varied to thermally control the same. As an example of the above, there has been proposed an apparatus in which many heaters are provided around the lip, the heating power of each of the heaters is varied to locally adjust the heating amount to the resin, by which fluidity and extensibility of the resin is changed to adjust the ununiformity in thickness of the film. The concrete contents thereof are described in detail in Japanese Patent Applications Nos 63-143694 and 63-143695 which have been filed by the applicant of the present application. In a film molding die used in the execution of the above mentioned method, the heating power of each of the electric heaters provided around the lip is varied in accordance with the degree of the ununiformity in thickness of a finished film to eliminate the ununiformity in thickness of the film or to adjust the film thickness. However, there is a tendency that heating-up with the use of the electric heaters is attained easier than cooling-down. That is, the film thickness should be adjusted and controlled such that in order to make the film thickness thinner, the temperature of a part of the die lip which forms the thick part of the film is increased, while in order to make the film thickness thicker, the temperature of a part of the die lip which forms a thin part of the film is decreased. In the above mentioned case, in order to rapidly increase the temperature of the die lip, a large amount of power can be applied to in order to rapidly increase the temperature of the die lip, the heaters. However, what can be done is to turn off the heaters to minimize the heating amount thereof at the best. Therefore, there is a difference between the increase in temperature of the die lip and the decrease in temperature of the die lip in responsiveness and easiness. In addition, the heat capacity of the film molding die is great, so that simply through natural cooling, it takes much time to lower the temperature of the die lip to make the film thickness uniform and a time lag in response to heat leads to an industrial loss. Further, even when only a heater to be controlled is completely turned off, in a case that other heaters around the heater concerned are turned on under the normal control, it takes much time for the heater to be controlled to cool down to a predetermined set temperature influenced by the temperature values of the other heaters therearound. In a case that the temperature is automatically adjusted with relative temperature to make the film thickness uniform, the increase in temperature is atteined easier than the decrease in temperature, so that the set temperature will be gradually increased and hence the temperature of the entire die will be also increased, sometimes beyond the appropriate temperature range for molding, which causes a failure to mold the film in the worst case. In the EP 0 180 571, it is disclosed an extruder arrangement comprising heating means and cooling means arranged arround the forming surface. These heating and cooling means are controlled by a control unit comprising a micro-processor in view of measures of thickness made on the extruded tube. With such an arrangement one does not assume a good control of the thickness. The FR 2 221 264, discloses an extruder arrangement comprising an annular die divided in thermally insulated sections by divisions. Each section comprises two ducts containing heating means and pipes in which coolant is distributed, disposed along said ducts. A temperature measuring mean is disposed in each section between two units comprising an heating mean and its cooling mean. With such an apparatus, one can assume a better control of the thickness of the film molded but the part of the die lip which temperature is controlled by one temperature measuring element is too large, about a sixth of said die lip, to permit a good control of the thickness. SUMMARY OF THE INVENTION According to the present invention, in a film molding die of the type in which heating powers of a plurality of heating means provided in the vicinity of a die lip are individually controlled to change temperature distribution around an edge of said lip, thereby controlling the thickness of a film, an apparatus for controlling the thickness of a film molded by a melt extruding method, comprising said heating means, cooling means and temperature measuring means for measuring temperature in the vicinity of said die lip, said heating and cooling means being controlled by the temperature measuring means, characterized in that said temperature measuring means are provided with one-to-one correspondence with said heating means. Air is vented to the air holes pierced in the die, by which the inside of the die lip is forcedly cooled, so that the responsiveness to the control to lower the temperature can be improved and stable constant temperature control can be realized. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view showing a quarter part of a die lip according to one embodiment of the present invention; Fig. 2 is a longitudinal sectional view showing the relation between heating element and temperature measuring (thermometric) element in fig. 1; Fig. 3 is a schematic diagram showing the structure of a film thickness controllable melt extruding type film molding apparatus to which the present invention is applied such as an inflation film molding apparatus or the like; Fig. 4 is a sectional view taken along the line IV-IV in Fig. 3; Fig. 5 is a longitudinal sectional view showing the essential part of a T die according to another embodiment of the invention; Fig. 6 is a cross sectional view taken along the line VI-VI in Fig. 5; Fig. 7 is a plan view showing the essential part of an inflation film molding die according to a further embodiment of the invention; Fig. 8 is an enlarged longitudinal sectional view taken along the line VIII-VIII in Fig. 7; Fig. 9 is an enlarged sectional view of a hole in which a heating element is to be embedded; and Fig. 10 is an enlarged cross sectional view of another hole in which a heating element is to be embedded. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Next, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. Figs. 1 and 2 show one embodiment of the present invention. Figs. 3 and 4 are schematic diagrams of a film thickness controllable melt extruding type film molding apparatus to which the present invention is applied such as an inflation film molding apparatus or the like. First, the structure of an inflation film molding apparatus which is capable of adjusting the film thickness will be briefly described with reference to figs. 3 and 4. In the drawings, (1) denotes an extruder; (2), a film molding die; (3), a film which is pulled out of the die (2); (4), a nip roll for pulling out the film (3); (5), a plurality of heating elements which are provided around a die lip (2a) in paires close to the die lip (2a) of the die (2); and (6), a plurality of temperature measuring (thermometric) elements adapted to measure the temperature of the vicinity of the plurality of heating elements. (7) denotes an arithmetic unit adapted to control the heating power from each of the heating elements (5) as required. The arithmetic unit (7) is provided on the periphery thereof with a film thickness measuring unit (8) adapted to measure the thickness of a finished film (3a) prepared by cooling the film (3) down to a predetermined temperature and then folding the film by the nip roll (4) into a flat state and to input data on the measurement to the arithmetic unit (7), a monitor unit (9) adapted to display predetermined data in the arithmetic unit (7) or to display predetermined indication, and a key unit (10) adapted to input predetermined function codes and numeric data into the arithmetic unit (7). The heating powers of the heating elements (5) can be individually controlled. In the embodiment, ceramic sheath type electric heaters are used as the heating elements. The heating elements (5) are provided on the outer periphery of the annular die lip (2a) densely close to one another. Fig. 1 is a detailed plan view of a quarter part of the die lip (2a) according to one embodiment of the present invention. The temperature measuring elements (6) are provided between the heating elements (5) which are provided in pairs to measure the temperature of the vicinity of the individual heating elements (5). Fig. 2 is a longitudinal sectional view along the axis of the die (2) showing the relation between the heating elements (5) and the temperature measuring elements (6) in the vicinity of the die lip (2a) in Fig. 1. A mandrel (2c) is fitted into the center of a die body (2b) of the die. Lip adjusting rings (2e) are fitted on the outer periphery of an upper end (2d) of the mandrel spaced from one another. An air ring (not shown) for air cooling the film (3) (which is referred to as bubbles in case of the inflation molding) pulled out of the die lip (2a) is provided on an outside upper part of the lip adjusting ring (2e). In this connection, it is to be noted that in the inflation molding, although there are provided inside mandrels for generating the internal pressure of the bubbles, bubble guide for guiding the bubbles along the outer periphery and the like, these elements are not directly related to the present invention, so that the figuring and description thereof are omitted. The lip adjusting ring (2e) is aligned with the mandrel (2c) by an adjusting screw (2f) to uniformly adjust the intervals between the die lips (2a) over the full arc of the mandrel. The individual heating elements (5) are embedded in the lip adjusting rings (2e) close to the die lips (2e) of the lip adjusting rings (2e) along the axis of the die (2). The temperature measuring elements (6) are also embedded in the lip adjusting rings (2e) between the paired heating elements (5). As the temperature measuring element (6), a thermo-electric couple, a platinum resistor or any other thermo-sensitive element may be employed. In this embodiment, the thermo-electric couple is employed. An air hole (11) which is coaxial with the paired heating elements (5) is pierced directly under the temperature measuring elements (6) between the paired heating elements (5). The upper end (11a) of the air hole (11) terminates directly under the temperature measuring element (6). The lower end of the air hole (11) is opened sideward in a position which is lower than the lower end of the heating elements (5) and at the lower root of an upper flange (2g) of the lip adjusting ring (2e). The upper end (11a) of the air hole (11) communicates with an air hole (12) which is pierced so as to extend from the outer peripheral surface of the upper flange (2g) to the air hole (11) at right angles and an air pipe (13) is connected to the air hole (12). The air pipe (13) is also connected to an accumulator (15) for accumulating therein air compressed by a compressor (14) through an adjusting valve (16). The adjusting valve (16) adjusts the amount of air flowing from the accumulator (15) into the air holes (11) and (12). The air (17) fed into the air holes (11) and (12) comes into contact with heated walls of the air holes (11) and (12) to act as a coolant for cooling the die. A side portion of the heating element (5) is exposed within the air hole (11), so that the heating element (5) is also cooled with the air (17) passing through the air hole (11). As a result, the temperature around the die lip (2a) rapidly responds to the heating power of the heating element (5), so that if a predetermined target temperature is set to the temperature measuring element (6), the temperature of the die lip (2a) will be rapidly fixed under the control of the heating power of the heating element (5). In addition, according to the present invention, in order to change the temperature of the die lip (2a), it is also possible to change the degree of cooling of the air hole (11). For example, the adjusting valve (16) may be constituted by an automatic controlling valve to electrically control the amount of air flowing into the air hole (12) and to perform feedback control on the temperature measuring element (6) so as to maintain the predetermined target temperature. As an alternative, a blast amount controllable blast fan may be connected to each of the air holes (12) such that the amount of blast can be fed back and controlled based on the measured value by the temperature measuring element (6). Further, according to the present invention, both the amount of heating by the heating element (5) and the amount of air through the air-hole (11) can be controlled to adjust the thickness of the film. Other Embodiments The embodiment in which the configuration of the lip formed by the inflation film molding is annular has been described above. In addition, the present invention is also applicable to a T die (20) by which a film is shaped into a sheet form as shown in Figs. 5 and 6. A die lip (21) of the T die (20) is constructed such that the thickness of resin discharged from a manifold (25) can be variably controlled by a fixed lip bar (23) fixed to a die main body (22) and a movable lip bar (24) which is fine-adjustably provided on the die main body (22). The T die (20) can restrict the thickness of the film by the same thickness adjusting method as that shown in Figs. 3 and 4. In this case, a heating element (26) which is the same as the heating element (5) in Fig. 3 is embedded in either the fixed lip bar (23) or the movable lip bar (24) to control the temperature distribution of the die lip (21) in the width direction. In the embodiment shown in Figs. 5 and 6, many heating elements (26) are embedded in the fixed lip bars (23) at regular intervals in the direction of the width of the film. An air hole (27), the axis thereof being in parallel with a direction in which the film is pulled out, is provided between the adjacent paired heating elements (26). The lower part of the air hole (27) extends down to a position in which it comes near the die lip (21). An air hole (28), the axis thereof being in almost parallel with the axis of the air hole (27), is provided in the fixed lip bar (23) near an outside surface (23a) of the bar outside of the air hole (27). The lower ends of the air holes (27) and (28) are communicated with each other through an air hole (29) which is horizontally pierced from the outer surface (23a) of the bar (23). The outer surface (23a) of the horizontal air hole (29) is closed by a plug (30). An air pipe (31) is coupled to the upper end of the outer air hole (28). A temperature measuring element (32) is embedded in a somewhat outer position in the inner air hole to measure a mean temperature between the two heating elements (26) and (26). Thus, cold air (33) is fed from the upper end of the outer air hole (28) through the air pipe (31) as a coolant. The air (33) passes through the air holes (27), (28) and (29) to locally cool down one side surface of the die lip (21). The air discharged from the upper end of the air hole (27) blows upward along a side surface of the die and hence has no influence on the stream of films (34) which are pulled out downward from the die lip (21). In the above mentioned embodiment, a communication section (35) between the air holes (27) and (29) is bent at an acute angle in a position which is the closest to the die lip (21). As mentioned above, a portion of the section at which air current changes its orientation at an acute angle exhibits great heat exchanging effect, so that a portion of the die lip (21) is locally well-cooled. In case of the above mentioned T die, the cooling means can be readily provided on the both fixed lip bar (23) and movable lip bar (24) which are provided on the both sides of the die lip (21) corresponding to the both surfaces of the film. In this connection, it is to be noted that even when a lip for molding an inflation film is annular in configuration, a cooling means is sometimes provided on the lip side at the upper end of the mandrel. It is also possible to provide the cooling means on the both sides of the die lip corresponding to the both surfaces of the film as required. Figs. 7 to 10 show other embodiments in which the present invention is applied to the inflation film molding apparatus which is the same as that shown Figs. 1 and 2. In this connection, it is to be noted that in Figs. 7 to 10, like numerals denote like parts as in Figs. 1 and 2 hence the description thereof is omitted. Fig. 7 is a detailed plan view showing a part of the die lip according to another embodiment of the present invention. Many heating elements (5a) and temperature measuring elements (6a) are provided around the die lip (2a) in pairs in a vertical direction. As shown in Fig. 8, the heating element (5a) is embedded in a hole (36) so provided as to come near the die lip (2a) and an opening (37) of the hole (36) is closed by a screw cap (38). The screw cap (38) is provided therein with a hole (39) passing therethrough in a direction orthogonal to the axis thereof and the temperature measuring element (6a) passes through the hole (39) in the screw cap (38) over the heating element (5a) and comes into abutment against the lip adjusting ring (2e) in a portion which is the closest to the wall of the die lip (2a). As shown in Fig. 9, many grooves (40) are provided along the axis of the hole (36) in the circumferential surface of the hole (36) in which the heating element (5a) is to be embedded. As shown in Fig. 8, air holes (41) and (42) are pierced at the upper and lower ends of the hole (36) from the outer surface of the lip adjusting ring (2e) and the air pipe (13) is connected to the upper air hole (41). Therefore, air which is fed from the air pipe (13) into the hole (36) in which the heating element (5a) is embedded passes through the grooves (40) and is discharged to the outside through the lower air hole (42). When the air passes through the grooves (40), the air comes into contact with the outer periphery of the heating element (5a) and with the lip adjusting ring (2e) to cool down the both. In particular, the outer periphery of the heating element (5a) is uniformly cooled, so that no partial variation of heating temperature is induced and hence stable heating can be attained. Fig. 10 shows another embodiment of the groove (40) in the hole (36) in which the heating element (5a) is embedded shown in Fig. 9. The bottom of the groove (40) shown Fig. 9 is square, while the bottom of a groove (40a) shown in Fig. 10 is semicircular. In the embodiments shown in Figs. 7 to 10, the heating elements (5a) and the temperature measuring elements (6a) are provided with one-to-one correspondence, so that the lip temperature within a local narrow range can be accurately controlled, and the entire of one heating element (5a) is cooled to improve the responsiveness to the controlled temperature. In the above mentioned embodiments, air is used as the coolant of the cooling means. As an alternative, gas other than air, or water or other liquid, or a mist of gas and liquid may be employed. As has been described above, according to the present invention, the temperature can be freely adjusted to the high and low set values with almost the same ease. In addition, the temperature can be ajusted to the set value in a short period of time, so that time loss and material loss can be reduced. The present invention has been described in relation to the preferred embodiments thereof. However, the present invention is not limited thereto and various other changes and modifications may be made within the scope of the appended claims.

Claims: Apparatus for controlling the thickness of a film molded by a melt extruding method for use in a film molding die of the type in which heating powers of a plurality of heating means (5a) provided in the vicinity of a die lip (2a) are individually controlled to change temperature distribution around an edge of said lip, thereby controlling the thickness of a film, said apparatus, comprising said heating means (5a), cooling means and temperature measuring means (6a) for measuring temperature in the vicinity of said die lip (2a), said heating and cooling means being controlled by the temperature measuring means, characterized in that said temperature measuring means (6a) are provided with one-to-one correspondence with said heating means (5a). Apparatus according to claim 1, wherein the heating element (5a) is embedded in a hole (36) coming near the die lip (2a) and having an opening (37) closed by a screw cap (38), and said screw cap (38) is provided with a hole (39) passing therethrough in which the temperature measuring element (6a) passes to come close to the die lip (2a). Apparatus according to claim 2, wherein many grooves (40, 40a) are provided along the axis in the circumferential surface of said hole (36) in which the heating element (5a) is embedded. Apparatus according to claim 3, wherein the bottom of the grooves (40) is square. Apparatus according to claim 3, wherein the bottom of the grooves (40a) is semicircular. Apparatus according to any one of claims 1 to 5, wherein said cooling means cools the vicinity of the die lip (2a) by using gas as a coolant. Apparatus according to any one of claims 1 to 5, wherein said cooling means cools the vicinity of the die lip (2a) by using liquid as a coolant. Apparatus according to any one of claims 1 to 5, wherein said cooling means cools the vicinity of the die lip (2a) by using a mist of gas and liquid as a coolant.

Inventors: SHINMOTO JITSUMI, SHINMOTO, JITSUMI
Applicants: TOMI MACHINERY MANUFACTURING C, TOMI MACHINERY MANUFACTURING CO., LTD.
Application year: 1990
Main classification: B29C47
IPC codes: B29C47, B29C55, B29L7