Thousands of National Union of Metalworkers of South Africa members are expected to strike in more than 450 companies in the plastic sector on Monday over wages and benefits, the union said.
Numsa said these workers had, until 2016, enjoyed the same rights and benefits as those in engineering, but the introduction of a national minimum wage by government had given employers "the courage to undermine these hard won benefits".
"Employers now want to reduce and in some cases remove these progressive benefits," it said.
"Our demand is simple. We are fighting for workers in the plastics sector to retain the wages and benefits which they fought so hard for."
It said its members would picket at companies in KwaZulu-Natal and Johannesburg. In addition, workers would hand over a memorandum of demands to employers in the sector on Tuesday.