Optical sheet manufacture method and optical sheet
A manufacture method of an optical sheet made of a transparent thermoplastic resin sheet on whose surface a regular geometric design working is performed, the optical sheet manufacture method comprising steps of performing a geometric design working on the resin sheet at a temperature not lower than a glass transition temperature of the resin sheet, by using a metal endless working belt formed with a geometric design on a surface thereof, rapidly cooling the resin sheet on which the geometric design working is performed to a temperature lower than the glass transition temperature, and detaching the rapidly cooled resin sheet from the metal endless working belt.
1. A manufacture method of an optical sheet made of a transparent thermoplastic resin sheet as a surface thereof a regular geometric design working is performed, the optical sheet manufacture method comprising:
performing a geometric design working on the resin sheet at a temperature not lower than a glass transition temperature of the resin sheet, by using a metal endless working belt formed with a geometric design on a surface thereof, wherein the temperature not lower than a glass transition temperature is not lower than Tg+60° C. and not higher than Tg+90° C., where Tg (° C.) is the glass transition temperature of the resin sheet;
rapidly cooling the resin sheet on which the geometric design working is performed to a temperature lower than the glass transition temperature, wherein the resin sheet is cooled at a cooling speed not slower than 5° C./sec and not higher than 40° C./sec; and
detaching the rapidly cooled resin sheet from the metal endless working belt,
wherein the resin sheet is made of transparent crystalline resin selected from the group consisting of PET, PEN, and a mixture or copolymer of PET and PEN and has a crystallization rate of 20% or less when the resin sheet is detached from the metal endless working belt.
2. The optical sheet manufacture method according to claim 1 , wherein an increase of a crystallization rate of the resin sheet is 5% or less before and after performing the optical sheet manufacture method.
3. The optical sheet manufacture method according to claim 1 , wherein a plurality of resin sheets are inputted, and the plurality of resin sheets are thermally bonded and integrated while a shape is transferred by the metal endless working belt.
4. The optical sheet manufacture method according to claim 1 , wherein a total thickness of the resin sheet is 500 μm or thinner.
5. The optical sheet manufacture method according to claim 1 , wherein a ratio of a height of a transferred portion to a total thickness of the resin sheet is 90% or less.
6. The optical sheet manufacture method according to claim 1 , wherein the geometric design transferred to the resin sheet is an emboss shape.
7. The optical sheet manufacture method according to claim 6 , wherein the emboss shape transferred to the resin sheet is a prism shape.
8. The optical sheet manufacture method according to claim 7 , wherein the prism shape transferred to the resin sheet is an isosceles triangle having an apex angle of 90°.
9. The optical sheet manufacture method according to claim 8 , wherein a transfer rate of the prism shape to the resin sheet is 98% or higher.
10. The optical sheet manufacture method according to claim 1 , wherein the metal endless working belt is wound around a heating roll set to a temperature higher than the glass transition temperature of the resin sheet and around a cooling roll set to a temperature lower than the glass transition temperature of the resin sheet, and transferred in synchronization with rotation of the heating roll and the cooling roll.
11. The optical sheet manufacture method according to claim 10 , wherein;
the resin sheet is processed between a nip roll disposed in a manner it faces the heating roll and the metal endless working belt; and
a nip line pressure between the metal endless working belt and the nip roll is not lower than 5 kg/cm and not higher than 30 kg/cm.
12. The optical sheet manufacture method according to claim 11 , wherein;
an endless belt is wound around the nip roll and around an opposing roll facing the cooling roll; and
the resin sheet is transferred with being sandwiched and held between the metal endless working belt and the endless belt.
13. The optical sheet manufacture method according to claim 12 , wherein while a shape is transferred to the resin sheet by the metal endless working belt, a shape is also transferred to an opposite side of the resin sheet by a geometric shape formed on a surface of the endless belt.
14. The optical sheet manufacture method according to claim 1 , wherein the geometric design worked on the surface of the resin sheet has at least one corner.
15. The optical sheet manufacture method according to claim 10 , wherein;
a roll temperature of the heating roll for transferring the resin sheet is set to be high in a central portion compared with the roll temperature in opposite end portions; and
a roll temperature of the cooling roll for transferring the resin sheet is set low in a central portion compared with the roll temperature in opposite end portions.
16. The optical sheet manufacture method according to claim 10 , wherein the resin sheet is processed between a nip roll disposed in a manner it faces the heating roll and the metal endless working belt, and wherein the nip roll has a cooling mechanism.
17. The optical sheet manufacture method according to claim 10 , wherein the resin sheet is processed between a nip roll disposed in a manner it faces the heating roll and the metal endless working belt, wherein while a shape is transferred to the resin sheet by the metal endless working belt, a shape is also transferred to an opposite side of the resin sheet by an emboss shape formed on a circumferential surface of the nip roll.