More than 160 manufacturing workers struck for two days last week at specialist component maker Trelleborg in Leicester.
The Unite union members are angry at a new pay offer.
Workers would be paid just 2p an hour more at the same time as losing their annual bonus.
This means they will lose an average of £561 a year even before inflation is taken into account.
Unite regional officer Lakhy Mahal said, “The message is we are staying firm. We are not going to accept that insult of an offer.”