Safety at the Rheinmetall Denel Munitions plant in Macassar, Somerset West, has been called into question after an explosion on the premises on Sunday night.
Trade union UASA said it was "anxious about the safety of those employed by the state-owned weapons manufacturer".
"This reckless lack of concern for the well-being of workers is completely unacceptable," spokesperson Abigail Moyo said.
UASA wants proof that qualified explosion and other managers are placed at all Denel divisions.
Moyo said: "The safety of workers cannot be jeopardised by plants that are operating without qualified managers to oversee processes."
"We trust that the company will conduct a thorough investigation into the events leading up to the explosion and expect an update as a matter of urgency. UASA needs to be able to ensure its members that they are operating in a safe environment, as is their right as employees."
The explosion is the second in three years. In an explosion in September 2018, eight factory workers died.
A public inquiry into that explosion is under way.
FF Plus leader Dr Pieter Groenewald said the explosion "raises alarming questions about operational safety and continuing production at the plant".
"It is time for Denel to play open cards about the plant's safety. The safety problems at the plant are no longer just a ticking time bomb. Two explosions serve as proof of that," Groenewald said.
He added that when handling and working with explosives, stringent safety measures must be in place in accordance with the Explosives Act.
"If these measures in terms of the law and the statutory licences - that restrict, for instance, the quantity of explosives that may be stored at a particular point – are strictly adhered to, then the chances of an accident occurring, such as the one at Macassar, and the probability of losing lives are significantly reduced," Groenewald said.
Meanwhile, the families of the eight people who died in the 2018 blast have called on the Department of Labour to suspend the company's explosives licence.
In a letter written by the law firm that represents the families, they threatened to take the company to the SA Human Rights Commission and Public Protector if their licence was not revoked.
No casualties were reported in Sunday's blast, and the ensuing fire was contained to the N86 magazine building.