Abstract:
Method comprises:  
     injecting a melt of thermoplastic material at a temperature higher than the glass transition temperature (T g ) of the thermoplastic material in a molding cavity defined by a two-piece mold inserts; and  
     after cooling and disassembling of the two piece mold insert, recovering the molded article;  
     wherein, each of two the pieces of the mold insert comprises at least one portion partly defining the mold cavity which is made of a material or a combination of materials, other than glass, the thermal diffusivity α of which fulfills the condition:  
     1&lt;α/α g &lt;11  
     where αg is the thermal diffusivity of a borosilicate crown glass and is equal to 6.20 E−7 m 2  s −1 , said portion being at least 1 mm thick.  
     Application for injection molding of weld line free minus lenses.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a method and a mold for injection molding of thermoplastic material articles, especially polycarbonate (PC) articles and the use of such a method and mold to obtain injection molded optical lenses and in particular minus lenses free of weld lines.  
           [0002]    Typically, thermoplastic optical lenses are made by injection molding of a thermoplastic material in a molding cavity defined by the internal surface of a two-piece mold insert.  
           [0003]    In injection molding of thermoplastic materials, weld lines are formed whenever advancing melt fronts meet, which may originate from multiple gate filling to leading to head-on impingement of separate flow fronts or splitting and converging of flow fronts due to presence of insert. Weld lines are important not only because they form visual defects such as lines, notches or color changes on the surface of the molded part, but also because they generally represent weak regions due to the difference in molecular orientation across the weld regions.  
           [0004]    In the case of injection molding of optical lenses, a thermoplastic material melt, at a temperature above the glass transition temperature (T g ) of the thermoplastic material, is injected in the mold cavity. The flow fronts then wrap around the edge of the mold cavity and then rejoin at the end of the cavity, forming a weld line. This problem of weld line formation occurs predominantly when molding minus lenses.  
           [0005]    The strength of the weld line may be characterized by a weld line strength factor, which is defined as the ratio of the composite strength in the presence of a weld line to that of the same composite without weld lines.  
           [0006]    Generally speaking, the weld line strength factor can be enhanced by increasing injection pressure, speed, and melt temperature or by locating weld lines closer to a gate to get enough packing. Also, providing venting at the weld region helps as the resistance to packing flow is reduced. However, none of these practices can eliminate weld lines completely. In ophthalmic lens application, presence of weld lines is simply unacceptable due to the resulting cosmetic defects and sometimes power variations.  
           [0007]    Several kinds of mold inserts are known in the art for molding optical lenses.  
           [0008]    One kind of mold insert which is used is a glass mold insert.  
           [0009]    Other kinds of mold inserts are also described in the prior art.  
           [0010]    Thus, U.S. Pat. No. 4,793,953 describes a die comprising a base material of Beryllium-Copper alloy having high heat transfer rate with a thick plating film of chromium or nickel giving desirable surface properties. The film is however very thin (up to about 380 micrometers). In this document it is indicated that optical mold made of electroformed metal (usually nickel) of thickness up to about 0.4 cm are known in the art and are preferably used instead of glass molds because their surfaces have an increased transfer rate, which is indicated as desirable for use in high pressure thermoplastic molding processes. It is also mentioned, that there have been previous attempts to combine, in a two-piece mold insert, metal materials of dissimilarly high and low thermal conductivities, to produce optical lens injection molds for thermoplastic materials. Reference is made to U.S. Pat. No. 2,292,917 which discloses a two piece mold insert with a steel die joined with a back body formed of a material having a thermal conductivity considerably higher than that of the material of the dies.  
           [0011]    U.S. Pat. No. 4,364,378 describes a two-piece mold insert with a conductive plug made of a material of lower thermal conductivity than the remainder of the insert and allowing obtention of a higher heat transfer rate at the peripheral portion of the mold than in the center of the mold.  
           [0012]    Document JP 07266343 describes the use of a metal mold with an inner surface coated with a thermal insulation layer of a polymer having a thermal conductivity of 0.002 cal/cm.s. ° C., and having a thickness of 0.01 to 2 mm, this thermal insulation layer having fine rugged surface. Due to the fine rugged surface, this mold is not suitable for molding optical lenses.  
           [0013]    Nevertheless, the weld line formation problem still remains when using these prior art molds.  
           [0014]    U.S. Pat. No. 4,364,878 describes an apparatus and a method for molding precisely shaped articles such as ophthalmic lenses, and in particular minus lenses, from synthetic resin. In order to solve the weld line problem, it is proposed in U.S. Pat. No. 4,364,878 a method which comprises moving the insert pieces to their relatively remote position to form a cavity of a volume greater than that of the finished lens, injecting into the cavity at a relatively low pressure a parison mass of the resin equal to the mass required to form the finished product and applying pressure to the insert pieces to bring their optical surfaces into engagement with the mass to coin the mass to fill completely the space between the insert pieces, and controllably cooling the viscous mass while under pressure so that all parts thereof cool to the glass transition temperature at substantially the same time. This process, which necessitates moving the insert pieces is complex and requires a sophisticated molding machine.  
           [0015]    U.S. Pat. No. 4,560,342 also discloses a method for molding weld line free concave lenses using a metallic insert. The above goal is reached according to U.S. Pat. No. 4,560,342 by firstly feeding the molten resin material through a feeding mouth into an intermediate thin portion of the mold cavity, while permitting hot water to circulate to maintain a high mold temperature and thereafter moving the feeding mouth to a thick portion of the mold cavity, and continuously feeding with molten resin. The implementation of such a method necessitates a complex device, in particular for moving the molten resin feeding mouth.  
         SUMMARY OF THE INVENTION  
         [0016]    One object of the present invention is to provide a method and mold for molding thermoplastic articles, such as ophthalmic lenses and in particular minus ophthalmic lenses, free of weld line.  
           [0017]    An other object of the present invention is to provide a method and a mold as defined above which result in shorter molding cycles.  
           [0018]    In general, the invention contemplates a method for injection molding a weld line free thermoplastic material article which comprises:  
           [0019]    injecting a melt of a thermoplastic material at a temperature higher than the glass transition temperature (T g ) of the thermoplastic material in a molding cavity defined by a two piece mold insert, and  
           [0020]    after cooling and disassembling of the two piece mold insert, recovering the molded article,  
           [0021]    wherein each of the two-pieces of the mold insert comprises at least one portion partly defining said mold cavity which is made of a material or a combination of materials, other than glass the thermal diffusivity α of which fulfills the condition  
           1&lt;α/α g &lt;11  
           [0022]    where α g  is the thermal diffusivity of a borosilicate crown glass and is equal to 6.20E-7m 2  s − 1, said portion having a thickness of at least 1 mm.  
           [0023]    The invention also contemplates a two-piece mold insert for use in a mold for molding of a weld line free thermoplastic material article, wherein at least a portion of said insert is as defined above in connection with the injection molding method.  
           [0024]    Preferably, the material constituting the portion of the pieces of the insert is a metallic material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    In the accompanying drawings to which reference is made in the instant specification and in which like reference characters are used to indicate like parts in the various views:  
         [0026]    [0026]FIG. 1 is a schematic representation of one embodiment of a two-piece mold insert according to the invention; and  
         [0027]    [0027]FIG. 2 is a schematic representation of another embodiment of a two-piece mold insert according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Referring to FIG. 1, there is represented one embodiment of a two-piece mold insert  1  suitable for carrying out the injection molding method according to the present invention.  
         [0029]    As is classical, the insert  1  comprises two separable pieces  2 ,  2 ′. Each of the pieces  2 ,  2 ′ of the insert comprises a base or substrate  3 ,  3 ′ to which is bonded a layer  4 ,  4 ′, the internal surface of which partly defines a mold cavity  6 . When the two pieces  2 ,  2 ′ of the insert  1  are assembled, the internal surfaces of layers  4 ,  4 ′ define the mold cavity  6 .  
         [0030]    As is known, the thermal diffusivity α of a material is defined as the ratio of heat dissipation and energy storage of the material, and is expressed by formula:  
         α=K/ρC p    
         [0031]    wherein K(W/m.K) is the thermal conductivity, ρ (kg/m 3 ) is the density and C p  (J/kg.K) is the specific heat of the material.  
         [0032]    According to the invention layers  4  and  4 ′ are made of a material which as a thermal diffusivity α such as α/α g  is superior to 1 and inferior to 11, where α g  is the thermal diffusivity of a borosilicate crown glass, equal to 6.20 E7(m 2  s−1).  
         [0033]    Preferably, the α/α g  ratio is inferior to 10 and more preferably ranges from 2 to 6. The preferred material for layers  4  and  4 ′ are Ti-alloys or Ni-alloys.  
         [0034]    Among the Ti-alloys there may be cited Ti-6Al-4V, Ti-8Al-1Mo-1V, Ti-4Al-10Cr-14V and Ti-2Al-5Zr-11Sn. The preferred titanium alloys are Ti-6Al-4V and Ti-8Al-1 Mo-1V.  
         [0035]    Among the Ni-alloys suitable for making the layers  4  and  4 ′ there may be cited the alloys commercialized under the trade-names of Hastelloy®, Inconel® and Invar®. The thermal diffusivity as well as the α/α g  ratio of some of these alloys and other materials are reported in table I below:  
                                                                   TABLE I                           Thermal properties of insert materials                            α               ρ(kg/cm 3 )   Cp(J/kg-K)   K(W/m-K)   (m 2 s −1 )   α/α g                          glass, borosilicate crown   2520   669.4   1.046   6.20E-07   1       beryllium copper (Cu bal/0.38-0.55 Be)   8750   397.5   220.08   6.33E-05   102.045       beryllium copper (Cu bal./1.7-1.9 Be)   8260   397.5   108.78   3.31E-05   53.430       chromium   7160   447.7   87.864   2.74E-05   44.204       nickel alloy hastelloy r-235   8220   439.3   9.2048    2.55-06   4.111       nickel alloy invar (hot-rolled or frgd)   8000   514.6   10.878   2.64E-06   4.261       steel, stainless 420   7800   460   24.9   6.94E-06   11.193       titanium   4500   523   20.92   8.89E-06   14.335       titanium alloy (Ti-6Al-4V)   4450   523   7.1128   3.06E-06   4.929       titanium alloy (Ti-8Al-1Mo-1V)   4370   543.9   5.8576   2.46E-06   3.974       titanium alloy (Ti-4Al-14V-10Cr)   5100   527.2   7.9496   2.96E-06   4.768       titanium alloy imi 679 (Ti-2Al-5Zr-   4840   481.2   6.6944   2.87E-06   4.635       11Sn)                  
 
         [0036]    The thickness of layers  4  and  4 ′ is generally of at least 1 mm, preferably at least 2 mm and more preferably at least 3 mm.  
         [0037]    As shown in FIG. 1 the two piece inserts of the invention can be a composite insert in which each of the two pieces  2 ,  2 ′ comprises a base (or substrate)  3 ,  3 ′ bonded via an adhesive layer (not shown) to a layer  4 ,  4 ′ as defined above; the surfaces of the layers  4 ,  4 ′ form the optical molding surface defining the mold cavity  6 . The bases  3 ,  3 ′ may be made of any suitable material. The materials mentioned in table I above are suitable for forming the bases  3 ,  3 ′ of the insert pieces  2 ,  2 ′. Among the preferred material for the bases  3 ,  3 ′, there may be cited steel and alloys such as beryllium-copper alloys. The bases  3 ,  3 ′ can also be composite bases including layers of different materials, in particular a metallic layer such as a steel or a glass layer. Usually the material of the bases  3 ,  3 ′ of the composite insert has a thermal diffusivity α b  such that the ratio α b /(α g  is higher than 1, and more preferably the base material will have a thermal diffusivity α b  higher than that of the material of the layers 4, 4′.  
         [0038]    Layers  4 , and  4 ′ may be bonded to bases  3 ,  3 ′ using any appropriate adhesive, but preferably an epoxide adhesive.  
         [0039]    In general, the whole thickness of the composite inserts will range from 3 to 5 cm.  
         [0040]    In FIG. 2 there is represented schematically another embodiment of a composite two-piece insert  1  according to the invention. This composite two-piece insert  1  is similar to the composite insert shown in FIG. 1 except that an intermediate layer  7 ,  7 ′ of borosilicate crown glass having a thickness of 1 mm or more is disposed between the bases  3 ,  3 ′, for exemple made of steel and the layers  4 ,  4 ′ made of Ti or Ni-alloy having a thermal diffusivity satisfying the above condition. Intermediate layer  7 ,  7 ′ and the layers  4 ,  4 ′ are respectively bonded to the bases  3 ,  3 ′and the intermediate layers  7 ,  7 ′ through adhesive layers (not shown), such as epoxide adhesive layers. The glass of the intermediate layer  7 ,  7 ′ can be borosilicate crown glass.  
         [0041]    Of course the two pieces of the insert may also be monolithic. In this case, the material constituting the whole insert piece will be a single material having a diffusivity α verifying the above condition.  
         [0042]    As is well known in the art the surfaces of the insert pieces  2 ,  2 ′defining the mold cavity  6  may be coated with one or several thin coatings for enhancing the surface properties and the optical quality of the insert pieces  2 ,  2 ′, such as for example a thin chromium or titanium coating for example CrN, TiN, TiC.  
         [0043]    Generally such additional property enhancing coating has a thickness of less than 100 μm, preferably less than 10 μm and more preferably less than 1 μm.  
         [0044]    Due to the choice of the materials constituting the pieces of the insert (composite or monolithic), the temperature at the interface between the thermoplastic material melt and the insert whithin the mold cavity is maintained above T g  of the thermoplastic material during at least one second and preferably during 2 to 5 seconds, after the thermoplastic material melt came into contact with the surface of the insert.  
         [0045]    As previously indicated any thermoplastic material can be used with the process of the invention, however the preferred thermoplastic material is polycarbonate.  
         [0046]    The process of the invention is particularly suitable for injection molding ophthalmic lenses and in particular minus ophthalmic lenses such as minus single vision ophthalmic lenses.  
         [0047]    As mentioned previously, the method of the invention allows to obtain lenses having good optical performances, in particular no weld lines, short injection cycles and there is no breakage of the mold inserts contrary to monolithic glass mold inserts.  
         [0048]    The following examples illustrate the present invention  
       Example 1  
       [0049]    Two sets of −2.00/0.00 and two sets of −4.00/0.00 titanium-alloy inserts were made. The titanium alloy used was Ti-6Al-4V. Each piece of the two-piece inserts was 4.445 cm (1.75 inch) long. These inserts were used for molding 1.5 mm thick, 76 mm round, −2.00/0.00 and −4.00/0.00 polycarbonate lenses using a 110 ton Roboshot apparatus equipped with a two-cavity, cold runner mold. The corresponding processing conditions are listed in tables II and III below. Due to the higher aspect ratio of the −4.00/0.00 lenses, pre-injection (simulated coining) instead of straight injection is required. With Ti-6Al4V inserts, the cycle time for molding −2.00/0.00 lenses is about 150 seconds which is 30 seconds shorter than the over 3 minutes cycle time in production using glass inserts. Similarly, the 4.00/0.00 Ti-alloy inserts achieved a 25% reduction in cycle time (180 second compared to the over 4 minutes using glass inserts).  
         [0050]    Coating of the resulting lenses with a silicone hard coat shows that the lenses were weld line free.  
         [0051]    Properties of the resulting lenses are given in tables IV and V.  
                             TABLE II                       Process conditions for injection molding −2.00/00 lenses       with Ti-Al6-V4 inserts                                    Melt Temperature (° F.)   600° F.           (° C.)   315° C.           Mold Temperature (° F.)   (275)           (° C.)   (135)           Injection Steps   3.81 cm/s (1.5 ips)                      
 
         [0052]    [0052]                             TABLE III                       Process conditions for injection −4.00/0.00 lenses       with Ti-Al6-V4 inserts                                    Melt Temperature (° F.)   600° F.           (° C.)   315° C.           Mold Temperature (° F.)   (275)           (° C.)   (135)           Injection Steps   3.556 cm/s (1.4 ips)                        
         [0053]    Polycarbonate was a General Electric Company production grade PC resin dried at 121° C. (250° F.) for 4 hours.  
                                                                         TABLEAU IV                       QA inspection report for −2.00/00 lenses molded with Ti-Al6-V4 inserts                                    nom. power   1     2     3                 −2.00-0   −2.00-0   −2.00-0           actual power   −1.97-0   −1.99-0   −1.99-0           0.08           0.12           thickness (mm)   1.67   1.66   1.67           1.3-1.7           1.3-1.8                cosmetics   NO WELD LINE                coating:   OK   OK   OK           drip: 3 mm                      
 
         [0054]    [0054]                                                                         TABLEAU V                       QA inspection report for −4.00/00 lenses molded with Ti-Al6-V4 inserts                                    nom. power     1   2     3                 −4.00-0   −4.00-0   −4.00-0           actual power   −4.11-0   −4.07-0   −4.10-0           0.08           0.12           thickness   1.61   1.63   1.60           1.3-1.7           1.3-1.8                cosmetics   NO WELD LINES                coating of a   SS   OK   OK           silicone hard           coating