You need not pay any customs duty on importation of materials for job work. Only thing is that the materials should be supplied on free of cost by supplier for job work against execution of export order.
The relevant notification No. 32/97 customs may be referred for details
The main condition is that the resultant product and imported goods if any should be re-exported to the supplier or any other person which the supply may specify within six months from the date of import or within such extended period that may allow by customs.
Further, the FOB value of export should be more than 10% of CIF value of imported goods.
Job work should be undertaken in accordance with the procedure set out in Custms (Import of Goods at Concessional rate of duty for Manufacture of excisable goods) Rules 2016.