They have threatened industrial action against builders they claim are directly importing cheap glass from China.
Thirty-seven workers at CSR Viridian's glass facility in the Melbourne suburb of Clayton were made redundant this week.
The job cuts represent about 10 per cent of the plant's workforce.
The Construction, Forestry, Mining and Energy Union said local managers had partly blamed the jobs cuts on reduced business as a result of cheap Chinese imports undercutting the company's sales.
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