Polyorganosiloxane composition for molding, optical member, light source lens or cover, and molding method

A polyorganosiloxane composition for molding comprising:

• (A) a straight-chain polyorganosiloxane having two or more alkenyl groups bonded to silicon atoms, on average, in one molecule and having a viscosity at 25° C. of 10,000 to 1,000,000 mPa·s;
• (B) 30 to 80 mass % of a polyorganosiloxane relative to a total of the (A) component and this component (B), the polyorganosiloxane including a resin structure containing a monofunctional siloxane unit represented by a formula:
• R13SiO1/2 (in the formula, R1s independently represent an alkenyl group or a substituted or unsubstituted alkyl group), a bifunctional siloxane unit represented by a formula:
• R12SiO2/2 (in the formula, R1s are as in the above), and a tetrafunctional siloxane unit represented by a formula:
• SiO4/2, at a molar ratio of a:b:c, on average (where 0.3≦a≦0.6, 0≦b≦0.1, 0.4≦c≦0.7, and a relation of a+b+c=1 is satisfied), and having two or more alkenyl groups bonded to silicon atoms, on average, in one molecule;
• (C) an amount of a polyorganohydrogensiloxane having a hydrogen atom bonded to a silicon atom, an average degree of polymerization of 10 or more, a content of the hydrogen atom of 5.0 mmol/g or more and 11.0 mmol/g or less, and a weight decrease rate up to 140° C. by thermogravimetric analysis (TGA) of 2.0 wt % or less so that an amount of the hydrogen atom in this component is 1.0 to 3.0 mol relative to a total 1 mol of the alkenyl group in the (A) component and the alkenyl group in the (B) component;
• and
• (D) a catalytic amount of a hydrosilylation reaction catalyst.