Process for manufacturing thermoplastic sheet and apparatus therefor

In an apparatus and process for manufacturing a thermoplastic sheet or film having superior optical characteristics such as improved transparency, gloss and surface smoothness, a thermoplastic sheet is extruded, passed between a cast drum and an endless metallic belt while being compressed and subjected to a primary cooling by the belt and drum. The sheet or film is secondarily cooled in a water bath, after which it is dewatered and subjected to a thermal treatment at a temperature in the range of 40.degree. C. below, to 15.degree. C. above the thermal deformation temperature of the resin sheet to produce the improved sheet of the invention. In addition to producing a film or sheet with improved characteristics, the apparatus of the invention allows production of sheets or films at a rate up to about 10 m/min.

TECHNICAL FIELD 
The present invention relates to a process for manufacturing a 
thermoplastic sheet or film and an apparatus therefore, which may be 
optimally used in various industrial applications to provide a package for 
foodstuffs, stationery document files etc., and various packaging cases. 
BACKGROUND ART 
The thermoplastic sheet, such as, for example, a crystalline thermoplastic 
sheet (or a film) is used extensively in various industrial applications, 
by virtue of its superior characteristics such as heat resistance, various 
physical properties, chemical resistance and hinging characteristics. 
However, neither a process for making a crystalline thermoplastic sheet of 
improved transparency, gloss and surface flatness nor a technology for 
making such a sheet at a high-speed has been developed and available. 
Known processes for manufacturing the crystalline thermoplastic sheet 
include the air-knife process, the polishing-roll process, the 
water-cooling process and the like. 
In the air-knife process, it may be difficult to maintain a surface 
flatness since the film upon which an air is sprayed may be variable, 
which is caused by a step of tightly urging the film-like fused 
thermoplastic resin against the cast drum under an air pressure to be 
injected from the nozzle. Besides, one side of the sheet cannot be cooled 
rapidly because it is cooled by air. Consequently, the process has a 
drawback in that it may not produce a sheet of improved transparency, 
surface flatness and thickness precision etc. Urging forces by an air 
pressure has approximately a 3000 mm water column as its upper limit, and 
a production speed of approximately 20 m/minute is a maximum for producing 
a commercially valuable sheet or film, because the air is directed only at 
a single point. As a result, this air-knife process is mainly used to make 
a reduced thickness sheet of approximately 0.2-0.5 mm which would not need 
particular appearance characteristics. However, this process is not 
suitable to produce an increased thickness sheet of approximately 0.7 mm 
or more. 
The polishing-roll process is a method for producing a sheet by forcing a 
film-wise fused thermoplastic resin between a pair of metallic rolls. In 
this process, the sheet thickness and the surface configuration are 
imposed simultaneously at a point where a pair of rolls contact each 
other, and if a drawing-speed is reduced to an order of approximately 4 
m/min or less, a sheet of improved surface flatness may be produced. 
However, when the drawing speed is higher than approximately 6 m/min, a 
sheet-forming step may not follow such a fast rate, and still produce a 
commercially usable sheet. 
If the reduced thickness sheet of approximately 0.4 mm or less is made in 
accordance with the polishing-roll process, water's edge-like waves are 
generated to prevent production of commercially usable sheets. 
Many proposals have been made in the water-cooling process, and a typical 
example is found in Japanese Patent Application Unexamined Publication No. 
Hei.4-158022. In this sheet manufacturing process, a sheet-like fused 
crystalline thermoplastic resin is cooled and solidified with water, and 
then re-heated by heating rollers, to be followed by the step of forcing 
the sheet between a pair of metallic belts such that its appearance 
characteristics and its production rate may be improved. Though this 
process may provide a sheet with an increased transparency, it may 
substantially compromise its appearance characteristics, since stripe-like 
patterns may be formed laterally in the sheet when the action of the 
water's edge-like waves are imposed against the sheet while the fused 
resin is being cooled by water. The greater the sheet-withdrawal forces 
are, the more frequently such patterns may be formed. 
On the other hand, while a non-crystalline thermoplastic sheet has been 
manufactured using a similar process, a sheet of improved gloss, and 
surface roughness may not be successfully provided. Thus, an improvement 
in this aspect has been desired. 
DISCLOSE OBJECTS OF THE INVENTION 
A primary object of the present invention is to provide a process and an 
apparatus therefore which may solve the above-described problems and 
permit the manufacture of crystalline thermoplastic sheet or film of 
superior appearance characteristics in transparency, gloss and surface 
flatness etc., at a high-speed, without requiring a special sheet 
thickness. 
A second object of the present invention is to provide a process and an 
apparatus therefore which may permit the manufacture of non-crystalline 
thermoplastic sheets of superior gloss and surface flatness at a 
high-speed. 
SUMMARY OF THE INVENTION 
The process of the present invention for manufacturing the thermoplastic 
sheet includes the steps of introducing a fused thermoplastic resin which 
has been extruded in a sheet-like configuration from the T-die of a T-die 
extruder into a clearance between a cast drum and an endless belt which is 
spaced from and opposes against the surface of the drum in an arcuate 
configuration with a predetermined clearance formed therebetween, exposing 
the sheet-like fused resin to a primary cooling operation to be 
effectuated in the temperature range covering a point where the resin is 
thermally deformed and a point where it is softened, while forcing said 
sheet-like fused resin into a clearance between the cast drum and the 
endless metallic belt, then immediately introducing the sheet which has 
been subjected to this primary cooling operation into a cooling tank in 
which cooling water is held so that it may be cooled secondarily to its 
solidification temperature, removing water which has collected on the 
secondarily cooled sheet, and subsequently thermally treating the sheet 
from which remaining water has been removed at a temperature range between 
40.degree. C. lower than the thermally deforming point and 15.degree. C. 
higher than the above-described reference point. 
The the apparatus of the present invention for manufacturing the 
thermoplastic sheet includes a T-die of the T-die extruder in which the 
thermoplastic resin may be fused and the fused resin is extruded in a 
sheet-like configuration. Compressing and cooling means are arranged at 
the downstream side of the T-die, and are adapted to expose the sheet-like 
fused resin from the T-die to a primary cooling operation while 
compressing the resin in a clearance between a cast drum and an endless 
metallic belt in a predetermined spacing from and opposing relationship in 
an arcuate configuration with the drum surface. A cooling tank is arranged 
at the downstream side of the compressing and cooling means for 
secondarily cooling the sheet after its primary cooling step. Water 
removing means is arranged at the downstream side of the cooling tank for 
removing water off the sheet has been secondarily cooled, and thermally 
treating means which is arranged at the downstream side of the remaining 
water removing means for improving quality of the sheet from which water 
has been removed as above-described by applying heat. The metallic belt of 
the compressing and cooling means is pivotally supported in position by 
means of three first, second and third metallic rolls, and the first 
metallic roll is positioned above the cast roll in a place most adjacent 
to the T-die, such that an opposing space between the cast drum and the 
metallic drum may be formed in an arcuate configuration between the first 
metallic roll and the second metallic roll. 
In the meantime, the sheet of the present invention as described herein 
includes the film.

BEST MODE FOR CARRYING OUT THE INVENTION 
Embodiment 1! 
The polypropylene sheet was manufactured using the apparatus as shown in 
FIG. 1. 
The polypropylene resin used was a nitrogen polymer XF 1893 (a product name 
of polypropylene monomer available from Tisso Co., Ltd) with density 0.90 
g/m.sup.3, melt-flow rate 1.8 g/10 minutes, thermal deformation 
temperature 90.degree. C., and softening temperature of 130.degree. C. 
This resin was melted in the T-die extruder (the extruder cylinder 
diameter 100 mm, L/D=36, the die width 550 mm, and the die lip clearance 
1.2 mm) (components other than the T-die not shown), and was extruded 
through the T-die 1 in a sheet-like configuration. This sheet-like fused 
resin was then introduced into a clearance between the cast drum 3 and the 
endless metallic belt 4, and was exposed to a primary cooling operation 
while it was being compressed, and it was then immediately guided to the 
cooling tank to effectuate a secondary cooling operation. After remaining 
water was subsequently removed through the pinch-roll 11 and the air-blow 
device 13, the polypropylene sheet of 0.3 mm thickness was made by 
applying a thermal treatment procedure to heat metallic rolls 15, 16, 17 
and 18. 
The particulars and sheet manufacturing conditions were as described below; 
Metallic Belt: SUS 
Cast Drum 
Carbon Steel with Hard Chrome Plated Surface, Width: 650 mm, Surface 
Roughness: 0.3 .mu.m, 
Surface Temperature: 90.degree. C. 
First Metallic Roll 
Silicon Rubber of 65 Rubber Hardness Coated Over the Surface in Thickness 
10 mm. 
Arcuate Compressing and Cooling Section 
Length: 60 cm. 
Clearance between Cast Drum and Metallic Belt: 0.3 mm 
Sheet Withdrawal Speed 
10 m/min (Rotating speed of the cast drum and the travelling speed of the 
metallic belt were both 10 m/min.) 
Primary Sheet Cooling Temperature 110.degree. C. 
Water Tank 
Length: 4 m, 
Cooling Water Temperature: 10.degree. C. 
Air-Blow Device 
Blow Air Temperature: 80.degree. C. 
Thermally Treatment Means (Heating Metallic Roll Group) 
Material: Carbon Steel with Hard Chrome-plated Surface 
Upstreammost Roll 15 Temperature: 60.degree. C. 
Downstreammost Roll 18 Temperature: 50.degree. C. 
Intermediate Rolls 16, 17 Temperature Between Above-described Rolls: 
100.degree. C. 
Surface Roughness of these Metallic Rolls: 1.0 .mu.m 
The polypropylene sheet which was made as above-described had haze 
(transparency) 3.4%, gloss 114% and superior surface smoothness with no 
streaks, wrinkles and water's edge-like waves etc., and so it had superior 
optical characteristics and appearance, with a high Young's Module 
(Rigidity) of 94. 
Meanwhile, the methods for measuring the thermally deforming temperature 
and the softening temperature were as described below. 
Thermally Deforming Temperature: JIS K-7207 (Load 4.6 kgf/c) 
Softening Temperature: JIS K-6758 (Vicar Softening Point, Load 1.0 kgf) 
Melt Flow Rate: JIS K-6758 (230.degree. C., Load 2.16 kgf) 
Density: JIS K-6758 
Haze: ASTM D-1003 
Gloss: ASTM D-523 
Young's Module: ASTM D-882 
Surface Smoothness: Visual Inspection 
Streaks, Wrinkles, Water's Edges: Visual Inspection 
Embodiment 2! 
A polypropylene sheet of 0.5 mm in thickness was made using the same 
process as that of the Embodiment 1, except that the clearance between the 
cast drum and the metallic belt was selected to be 0.5 mm. 
This sheet had superior characteristics substantially comparable to those 
in the Embodiment 1. That is, the sheet had a superior surface smoothness, 
i.e., haze (transparency) 4.0% and a gloss 124%. Besides, since no 
streaks, wrinkles and water's edge-like surface etc., take place, optical 
characteristics and appearance were improved, providing a high Young's 
Module (rigidity) of 90. 
Embodiment 3! 
A polypropylene sheet of 1.0 mm thickness was produced in accordance with 
the same manufacturing process as that employed in Embodiment 1, except 
that a clearance between the cast drum and the metallic belt was preset to 
be 1.0 mm. 
This sheet also showed improved characteristics substantially the same with 
those in the Embodiment 1, that is, the sheet was superior in its surface 
smoothness, showing a haze (transparency) 9.5% and a gloss 127%. Since no 
streaks, wrinkles and water's edge-like wavy surface etc., take place, it 
had improved optical characteristics and appearances. It showed a high 
Young's Module (Rigidity) of 88. 
Embodiment 4! 
A polypropylene sheet of 0.3 mm thickness was produced in accordance with 
the same procedure as that used in the Embodiment 1, except that the cast 
drum, the first metallic drum and the thermal treating means were modified 
as below. 
Cast Drum: Emboss Pattern Formed On the Surface 
First Metallic Roll: No Surface Coating Layer 
Thermal Treatment Means (Metallic Rolls Group) 
Intermediate Rolls 16, 17 Surface Temperature: 80.degree. C. 
The embossed sheet displayed superior optical characteristics of haze 3.6%, 
gloss 121% and the Young's Module 85. 
Comparison I! 
A polypropylene sheet of 0.3 mm thickness was produced using the same 
manufacturing process as that used in a Embodiment 1, except that the 
metallic rolls in a thermal treating means, i.e., downstreammost and 
upstreammost metallic rolls were made to have equal surface temperatures, 
and the intermediate metallic roll had the surface temperature of 
130.degree. C. The sheet had the haze of 11.5%, gloss 86% and the Young's 
Module 90. The sheet was verified to be inferior to one as obtained in 
respective embodiments, showing a poor surface smoothness and wrinkles on 
the sheet surface. 
Possibility for Industrial Utilization 
In accordance with the present invention, a thermoplastic sheet or film may 
be provided which has superior optical characteristics such as 
transparency and gloss etc, appearance characteristics such as surface 
smoothness etc, and rigidity and the like, when a crystalline 
thermoplastic resin is used. On the other hand, when a non-crystalline 
thermoplastic resin is used, a thermoplastic sheet or film may be provided 
which has superior optical characteristics such as gloss etc, appearance 
characteristics such as surface smoothness etc, and rigidity and the like. 
Either of these sheets or films may be embossed as desired. Furthermore, 
it is also possible to make a high quality product with a reduced or 
increased thickness sheet or film. Additionally, the production speed can 
be made several times faster than that available in the prior process. The 
process and the device of the present invention may be also applied in the 
production of multi-layered sheet or laminated sheet. 
Consequently, the sheet or the film which may be obtained using the present 
invention may be usably utilized in packaging foodstuff, drugs, packaging 
case for cosmetics and household articles and industrial raw materials.