components give too high a crosslink density and hence produces too rigid a foam further the polyol components cannot give a uniform cross linked structure and hence cannot produce a normal foam as the foregoing isocyanate there may be used an aromatic isocyanate such as toluene diisocyanate tdi pure diphenylmethane diisocyanate p mdi polymeric mdi naphthalene diisocyanate ndi 3 3 39 ditolylene 4 4 39 diisocyanate todi xylene diisocyanate xdi and tetramethylxylene diisocyanate tmxdi an aliphatic isocyanate such as 1 6 hexamethylene diisocyanate hdi isophorone diisocyanate ipdi lysine diisocyanate ldi xylene diisocyanate hydride hydrogenated xdi and cyclohexane diisocyanate chdi or modification product thereof particularly preferred among these isocyanates are tdi mdi and modification product thereof in the present invention the isocyanate index percentage of the ratio of the equivalent of isocyanate to the hydroxyl value of polyol is defined to a rancfe of from 110 to 150 if the isocyanate index is raised so high that the reaction can proceed thoroughly the content of unreacted monomers initiator of polyol adducts and oligomers in the resulting foam can be minimized preventing the unreacted monomers from being eluted with an ink during the use of the ink reservoir and hence inhibiting the reduction of the surface tension of the ink if the isocyanate index falls below the lower limit much oligomers are contained in the resulting foam besides the unreacted monomers these oligomers and unreacted monomers can be eluted with the ink during.