High Efficient Casting Machine for Biaxially Oriented Film Manufacturing Plant

Transcription

1 107 High Efficient Casting Machine for Biaxially Oriented Film Manufacturing Plant Hideo Kometani Hidetoshi Kitajima Hiroshi Tsuji Takuya Goto Masahiro Yoshizawa (MHI) has made incessant efforts to improve the performance of biaxially oriented film manufacturing plants in order to comply with the demand of high productivity of the biaxially oriented films used in various fields, and has achieved the highest operating speed on commercial production (width 8.3 m x speed 450 m/min) in the world. MHI has developed a high-efficiency cooling roll with a new-type structure equipped with straight circular channel as a process of improving the performance of the above-mentioned plant and applied the roll to the casting machine. The adoption of the high-efficiency cooling roll (heat transfer coefficient: 1.5 times) reduces the cooling roll diameter of a 450 m/min-class machine from conventional mm to mm, needing less space and improving the operability at the start-up because of the down-sizing of the machine. Further, the adoption of straight circular channel enables cleaning of the cooling channel, making a drastic improvement in its maintainability. 1. Introduction With the increase of multi-layer films and with the new resin materials increasingly used, the plastic films used in various fields including industrial products and items for every day use, etc. have come to be applied more and more in wider fields. MHI supplies the manufacturing plants to a number of countries in the world for a global- scale, high-level production output of biaxially oriented film among various other films under increasingly high demand. Fig. 1 shows the production process of the biaxially oriented film manufacturing plant. The polymer uniformly melted by the extruder is extruded in flat sheet from the die. The extruded molten polymer is then cooled to solidification in the casting machine and stretched in the direction of film line by the longitudinal machine before finally being stretched in the direction of film width by the transverse machine. After going through the process, the polymer is subjected to surface treatment, etc. in the take-off unit before being wound up in the winder. In order to meet with the customer s need of higher productivity, MHI has developed mass producing machine by improving the width and speed of biaxially oriented film manufacturing plant, paying consideration to the operability, maintainability and so on. MHI has developed new machines one after another. The operating speed on commercial production of the latest MHI biaxially oriented film manufacturing plant is shown in Fig.2. With the performance of the component machines of the above-mentioned plant improved, a world-top operating speed on commercial production level (width 8.3m x speed 450 m/min) has been achieved. This paper describes the new structure of the casting machine, the main component of the biaxially oriented film manufacturing plant, in terms of the high performance and improved operability and maintainability. 2. Need for casting machine development The casting machine for biaxially oriented film manufacturing plant is shown in Fig. 3. The casting machine Filter Die Thickness gauge Preheating Heat setting Thickness gauge Material supply Extruder Casting machine Longitudinal machine Transverse machine Takeoff unit Winder Fig. 1 Production process of biaxially oriented film manufacturing plant The molten polymer extruded from the die is cooled to solidification by casting machine before being stretched by longitudinal and transverse machines and wound up.

2 108 Operating speed on commercial production (m/min) Film width 8.3 m (BOPP* 1 ) (150) *1: Biaxially oriented film manufacturing plant for polypropylene (450) Fig. 2 Operating speed on commercial production of biaxially oriented film manufacturing plant World-top operating speed on commercial production (width 8.3 m x speed 450 mm/min) has been achieved. Air knife Die 1st bath Cooling roll 2nd bath Fig. 3 Casting machine The molten polymer extruded from the die is stuck to the cooling roll by means of the air knife before being cooled to solidification in 1st and 2nd water baths is composed of 1st water bath and 2nd water bath equipped with cooling roll. The molten polymer extruded from the die in flat sheet is stuck to the cooling roll by air knife before being cooled to solidification in 1st and 2nd water baths. Fig. 4 shows the sectional photographs of the sheets molded by the casting machine. The figure shows the photographs of the surface not-facing the roll subjected to air cooling and to water cooling. Compared with when subjected to air cooling, the surface not-facing the roll gets rapidly cooled when subjected to water cooling and has the crystal size reduced and the crystallization suppressed. The crystalline state of the sheet during casting process depends on the cooling characteristics and inflicts greater effect on the stretchability in the after process and on the strength, transparency, etc. of the production film (1) (2). We have so far explained the cooling characteristics of the surface not-facing the roll and the crystalline state. However, the performance of the cooling roll also has a great effect on the crystalline state of the sheet, so that the cooling roll has to be highly efficient in order to improve the product quality. Further, the conventional 450 m/min-class machine had to be large in size because of the cooling roll diameter becoming mm due to high speed. Consequently, Crystal size: large Surface not-facing the roll (air cooling) Surface facing the roll the cooling roll with smaller diameter and needing less space in addition to the improvement in operability at the start-up (particularly at the threading) was earnestly demanded. In order to meet with such demand or need, MHI have developed a new structure to improve efficiency (by reducing the roll diameter) and maintainability of the cooling roll. The structure and performance of the high-efficiency cooling roll are described below. 3. Structure of high-efficiency cooling roll Fig. 5 shows the structure of the new type cooling roll compared with the conventional type. In the case of conventional cooling roll, the cooling water is led into the spiral rectangular channel installed inside the external, with the external designed independent of the cooling channel, causing the water pressure to apply throughout the inner surface of the external. This causes the roll diameter to become large and eventually the external had to be thick in order to secure sufficient strength, leading consequently to the deterioration of the heat transfer coefficient. In the new type cooling roll, the straight circular channel is embedded in the external. The solid (integral) structure of the external and the cooling channel ensures sufficient strength and, in addition, improves the cooling efficiency through proximity of the cooling channel and the roll surface. Features are described below. 4. Features of high-efficiency cooling roll Crystal size: small Surface not-facing the roll (water cooling) Surface facing the roll Fig. 4 Sectional photographs of casting sheet The crystalline state of the sheet depends on the cooling characteristics, with the growth of crystal on the surface not-facing the roll when subjected to air cooling. 4.1 Strength characteristics Supposing the water pressure inside the cooling channel to be 0.5 MPa, the results of roll deformation analysis due to roll diameter and water pressure for conventional and new type cooling rolls are summed up in Fig. 6, with the figures in the parentheses indicating the external thickness ratio of each roll diameter against 0.8 mm

3 109 Conventional type External Spiral rectangular channel Independent structure from the external Internal Section facing the roll Straight circular channel Integral structure with the external External Section facing the roll New type Fig. 5 Cooling roll structure The new type cooling roll is designed to have the straight circular channel embedded in the external instead of the spiral rectangular channel of a conventional cooling roll. Indication zone Deformation Conventional type 0.2 Water pressure inside cooling channel: 0.5 MPa Deformation [x 500] Deformation 0.126mm Deformation (mm) 0.1 : Analytical result of conventional type : Analytical result of new type (3.0) (1.0) (1.5) (1.8) (2.3) New type Deformation [x 500] Roll diameter (mm) Fig. 6 Cooling roll diameter and deformation Indicates analytical results of roll deformation under water pressure 0.5 MPa, with the external member between the straight circular channels in the new type cooling roll working as the strength member to suppress the deformation. the external thickness of the mm roll diameter of the conventional cooling roll. The figure also shows the analytical examples of the external deformation both for conventional type (roll diameter: mm) and new type (roll diameter: mm). In the conventional type, the independent structure of external and cooling channel causes the water pressure to apply throughout the entire inner surface of the external restrained by the side plate, resulting in convex deformation to occur at the center against the roll edge section. This causes the roll diameter to increase, so that the deformation has to be suppressed by making the external thicker as shown in the figures in brackets. In the case of the new type cooling roll (with roll diameter: mm), the solid structure of external and cooling channel allows the external member between straight circular channel to become strong enough to suppress the deformation caused by water pressure. Consequently, the deformation in new type ( mm) is extremely small at 0.7 m against the deformation mm in conventional type ( mm). Thus the problem of structural strength in conventional type (needing thicker external because of the larger cooling roll diameter) has been improved and the deformation of external due

4 Heat transfer coefficient ratio (1.0) (1.5) (1.8) (2.3) : Analytical result of conventional type : Analytical result of new type : Test result of new type (3.0) Sheet External Straight circular channel (cooling channel) ( o C) Temperature distribution on sheet and cooling roll (analytical example) Roll diameter (mm) Fig. 7 Cooling roll diameter and heat transfer coefficient ratio The heat transfer coefficient in the conventional type gets deteriorated because of the large roll diameter, while the heat transfer coefficient in the new type remains constant, irrespective of the roll diameter. to water pressure has been eliminated in the new type cooling roll. This has contributed to a drastic improvement in durability and reliability. 4.2 Heat transfer characteristics The roll diameters and heat transfer coefficient ratio of the conventional and new type cooling rolls are summed up in Fig. 7. The heat transfer coefficient ratio indicates the ratio of the heat transfer coefficient for each roll diameter against the heat transfer coefficient for roll diameter: mm in a conventional type cooling roll, with the figures in the parentheses indicating the external thickness ratio for each roll diameter against the external thickness for roll diameter: mm in a conventional type cooling roll. As is evident from the above example, the roll diameter in a conventional type cooling roll gets increased, causing the external to become thick. This in turn causes the heat transfer coefficient to get deteriorated as shown in Fig. 7. The external for roll diameter: mm becomes 3 times thicker than for roll diameter: mm, causing the heat transfer coefficient to reduce to 2/3. In the case of new type cooling roll, the distance between cooling channel and roll surface can be kept constant, ensuring high heat transfer coefficient, irrespective of the roll diameter size. The figure shows the test results for roll diameters: mm and mm of the new type cooling roll, indicating the obtained heat transfer coefficient to be equivalent to the heat transfer coefficient for the roll diameter mm of a conventional type. It should be noted here that in the case of the new structure applying straight circular channel it is important to carry out uniform cooling of sheets in the roll circumferential direction (between circular channels). In the above-mentioned figures an example of non-steady heat conduction analysis of the sheet and cooling roll in the new type cooling roll is shown. As is clear in the figure, the roll surface temperature and sheet temperature in the new type roll are kept uniform in the roll circumferential direction (between circular channels) through optimization of the shape and arrangement of straight circular channels. 4.3 Maintainability The external and side plate of a conventional type cooling roll are of welded structure, making it necessary to clean off the scale, etc. adhered to the cooling channel through chemical cleaning, which calls for complicated handling and long-term stoppage of the line. Further, the cleanliness degree cannot be confirmed after the cleaning, so that the improvement in maintainability of cooling roll has been a long demanded need. The photo in Fig. 8 shows the appearance of the newtype high-efficiency cooling roll (with roll diameter: mm), while Fig. 9 shows the maintenance structure of the new type. The side plate is equipped with detachable plugs corresponding to the straight circular channels. As shown in Fig. 9, these plugs can be removed at the maintenance for cleaning by brush and the cleanliness degree after the cleaning can also be easily confirmed. With the adoption of straight circular channels, the new-type cooling roll allows cleaning of the cooling channels, leading to drastic improvement in maintainability. 5. Up-to-date specifications for casting machine The specifications for casting machine applying the high-efficiency cooling roll currently in operation are

5 111 Straight circular channel Roll side face with plug removed Straight circular channel Plug Roll external Fig. 9 Maintenance structure of high-efficiency cooling roll The plugs installed to the straight circular channels can be removed at the maintenance to allow cleaning of the cooling channel. Table 1 Specifications for new type casting machine For biaxially oriented film manufacturing plant (width 8.3 m X speed 450 m/min) Polymer Polypropylene Cooling capacity Sheet thickness 0.6 to 3.0 mm Machine speed max. 110 m/min Cooling roll diameter Cooling roll width mm Figure in the parenthesis is for conventional type roll diameter. given in Tabl able 1. The latest wide and high-speed line (width 8.3 m x speed 450 m/min) applies the high-efficiency cooling roll with heat transfer coefficient 1.5 times higher than that of the conventional type, and has the cooling roll diameter reduced from mm to mm, needing less space. This has also contributed to reducing roll diameter and the size of the whole machine and to improving the operability at the start-up (threading). conventional mm to mm and needs less space. Further, the downsizing of the machine contributes to the improvement in operability at the start-up. (4) The adoption of straight circular channel enables the cleaning of the cooling channel, leading to drastic improvement in maintainability. In addition to the newly developed casting machine introduced in this paper, MHI is determined to develop machines to meet with the customers needs for improvements in production of the biaxially oriented film manufacturing plant, in operability and maintainability. References (1) Tsutsui, Y. et al., Study on Biaxially Oriented Film Forming Technique, Mitsubishi Heavy Industries Technical Review Vol.23 No.2 (1986) pp (2) Kometani, H. et al., Fundamental Research on Crystallization in Plastic Sheet Molding, Mitsubishi Juko Giho Vol.28 No.5 (1991) pp Conclusion Summed up below is the casting machine, a major component of the biaxially oriented film manufacturing plant. (1) A high-efficiency cooling roll with new structure and equipped with straight circular channel has been developed to take the place of the conventional cooling roll equipped with spiral rectangular channel. (2) The newly developed high-efficiency cooling roll eliminates the external deformation caused by water pressure in the conventional cooling roll, with the reliability in terms of durability drastically improved. (3) The high-efficiency cooling roll (with 1.5 times higher heat transfer coefficient) reduces the cooling roll diameter of a 450 m/min-class machine from Hideo Kometani Nagoya Research & Development Center, Technical Headquarters Hiroshi Tsuji Masahiro Yoshizawa Hidetoshi Kitajima Nagoya Research & Development Center, Technical Headquarters Takuya Goto