Abstract: There are provided a plastic molded optical element and a molding mold therefor capable of preventing flash generation as well as making the handling of the molded optical element easy by forming a projection on the main body of an optical element, corresponding to the matching portions (parting planes) of the molding members of the molding mold or die members for molding the optical element. A thin projection is formed on the main body of the optical element in a thickness required for securing a required strength in relation to the matching portions (parting planes) of the molding members or die member for molding the optical element.

Abstract: A fluoroelastomer is disclosed which comprises vinylidene fluoride units and hexafluoropropylene units, optionally together with tetrafluoroethylene units, and wherein the amount of tetrafluoroethylene units is 35 to 0% by weight and the weight ratio of vinylidene fluoride units to hexafluoropropylene units is in the range of 80:20 to 40:60. The fluoroelastomer has a relatively low limiting viscosity number and a specific molecular weight distribution. The fluoroelastomer has excellent processability, e.g., excellent roll processability, mold release characteristics and metal-adhesion properties, and is capable of providing a vulcanized rubber having excellent mechanical strength and compression set resistance. The vulcanized rubber is advantageous in the manufacture of O-rings, oil seals, packings, gaskets, diaphragm materials and the like.

Abstract: The fluorine-containing elastomer of the present invention is a novel elastomer comprising vinylidene fluoride units and, hexafluoropropylene units, optionally with tetrafluoroethylene units, and wherein the fluorine content of the elastomer is 63.5 to 71% by weight, the content of fractions having molecular weights of 50,000 or less (M.sub.5) in the elastomer is 5% by weight or less, the content of fractions having molecular weights of 1,000,000 or more (M.sub.100) in the elastomer is 10 to 35% by weight, and the limiting viscosity number (ml/g) of the elastomer is in the range of from 100 to less than 250. The fluorine-containing elaastomer of the present invention is excellent in resistance to heat and chemicals and is not only extremely excellent in compression set resistance properties and mechanical properties but also has a satisfactory workability. Further, by vulcanization of the elastomer, there can be provided excellent shaped articles such as rings, gaskets, seals, diaphragms and the like.