Bend the product by application of a force.
or
Bend the product by applying a force.

As a test description I, however, think this one is even better:

Bending of product by application of a force.
Comments?

They all work. To my ear, 'Bend the product by applying a force' sounds best.

Without context, it is difficult to advise, but even sweeter sounding is:

'Bend the product by applying force' (not no 'a')

However, this does conjour up an image of bending it over your knee.

'Bending of product by application of a force.' looks like a paragraph title. Again, you could remove the word 'a' from this for a more native-English feel.

All are grammatically sound.