For choosing a self- tapping screw, the flexural modulus of the material should be known. Will the material be able to absorb the stress created or not? Soft material like plastic can expand and absorb stress compared to stiff materials like wood or metal. Thread- forming screws create more stress than thread-cutting screws. For any help call SME group experts, Tel. +44 (0) 1789 206600, free of cost.