Mold and a method of hot-forming a thermoplastic lens
View Patent ↗The mold comprises two dies mounted in a sheath to slide along the axis of the sheath and each presenting a forming face facing the other. The two dies and the sheath are provided with intrinsic heat transfer means. A parison or preform of said thermoplastic material is placed between the two dies and the assembly is enclosed in the sheath; the dies and the sheath are heated by their intrinsic heat transfer means up to a forming temperature; The dies are moved towards each other to shape the material by plastic deformation until a predetermined relative position is reached corresponding to the thickness desired for the lens; the dies and the sheath are cooled by their intrinsic heat transfer means down to an unmolding temperature; and the finished lens is extracted from the mold.
1. A method of hot-forming a thermoplastic lens, the method comprising the steps of:
providing a mold comprising two dies mounted in a sheath to slide along the axis of the sheath, and each presenting a transverse forming face facing the other, the two dies and the sheath being provided with intrinsic and both-way heat transfer means;
placing a parison or preform of said thermoplastic material between the two dies and enclosing the assembly in the sheath;
heating the dies and the sheath by their intrinsic heat transfer means up to a forming temperature;
moving the dies towards each other to shape the material by plastic deformation until a predetermined relative position is reached corresponding to the thickness desired for the lens;
cooling the dies and the sheath by their intrinsic heat transfer means down to an unmolding temperature; and
extracting the finished lens from the mold.
2. A forming method according to claim 1 , in which the forming temperature is greater than or equal to the vitreous transition temperature of the thermoplastic material used.
3. A forming method according to claim 2 , in which the forming temperature exceeds the vitreous transition temperature of the thermoplastic material used by an amount lying in the range 30° F. to 120° F.
4. A forming method according to claim 3 , in which the forming temperature exceeds the vitreous transition temperature of the thermoplastic material used by an amount of about 45° F.
5. A forming method according to claim 1 , in which the unmolding temperature is significantly below the vitreous transition temperature.
6. A forming method according to claim 5 , in which the unmolding temperature presents with respect to the vitreous transition temperature a difference lying in the range 20° F. to 50° F.
7. A forming method according to claim 6 , in which the unmolding temperature presents with respect to the vitreous transition temperature a difference of about 35° F.
8. A forming method according to claim 1 , using a mold in which the interstitial gap defined by the forming faces of the two dies co-operates with the annular recess to form a sealed inside volume which is connected to the outside via at least one suction channel passing through the sheath, including, before and/or simultaneously with the dies being moved towards each other in order to shape the material by plastic deformation, a step of establishing a relative vacuum in the sealed inside volume of the mold by means of the suction channel.
9. A forming method according to claim 8 , in which the relative vacuum is lower than 0.25 bar.
10. A forming method according to claim 9 , in which the relative vacuum lies in the range 0 to 0.1 bar.
11. A forming method according to claim 1 , and using a mold including a trimming ring slidably mounted in the sheath, including, after the material has been fully shaped by moving the dies towards each other, a step of actuating the trimming ring to cut off the peripheral flash formed by the surplus material projecting sideways from the dies.
12. A forming method according to claim 1 , including a step of pre-coating said parison or preform with any single or multi layer coating before being placed between the two dies.
13. A forming method according to claim 1 , including, before heating the dies and the sheath, a step of disposing a thermoplastic film between the parison or preform and at least one of the two dies, said thermoplastic film being applied and self-welded to the corresponding face of the lens during the shaping step.
14. A forming method according to claim 13 , including a step of coating the film with any single or multi layer coating.
15. A forming method according to claim 13 , including a step of providing a photochromic film.
16. A forming method according to claim 13 , including a step of providing a polarized film.
17. A forming method according to claim 13 , including a step of providing a tinted and/or colored film.
18. A forming method according to claim 1 , including a step of disposing a single or multi layer coating on at least one of the two dies before heating them, this coating being then hot-transferred on the corresponding face of the lens during the shaping step.
19. A method of hot-forming a thermoplastic lens, the method comprising the steps of:
providing a hot-forming mold comprising two opposing dies mounted in a sheath that slide along an axis of the sheath, each of the two dies presenting a transverse forming face facing the other, the two dies and the sheath each having a respective intrinsic and both-way heat transfer device;
placing a preformed thermoplastic material between the two dies and then enclosing the material in the sheath;
then, heating the two dies and the sheath by the respective intrinsic heat transfer device up to a forming temperature;
moving the dies towards each other to shape the material by plastic deformation until a predetermined relative position is reached corresponding to the thickness desired for a finished lens;
cooling the dies and the sheath by the respective intrinsic heat transfer device down to an unmolding temperature; and
extracting the finished lens from the mold.