The bombs were made from gas cylinders and are linked to detonators.
The move was the idea of Gabriel Gawin, of the CFTC union compulsory liquidation.
The 168 workers hope for a buyer to take on the factory, maintain as many jobs as possible and guarantee a 50,000 euro bonus to those who are made redundant.
“These days if you are nice, nobody takes any notice of you,” said Rosa, who has worked at the DMI foundry for 30 years. “Until now we have behaved properly but people must take our threat seriously. Some people have reached saturation point.”
The foundry has suffered particularly from the downturn in the French car industry.
The move is the latest in a series of high-profile campaigns at sites which are threatened with closure.
Most are led by the CGT trade union.
On 27 February the French senate voted for a law which would grant legal amnesty to activists who vandalised property as part of industrial action.
The law has not yet completed its passage through parliament but many business leaders and politicians on the right of French politics are furious.
The bill was proposed by Communist senators and many Socialists voted in favour.
Under the rules activists would not be prosecuted for any material damage they caused but they would not be exempt for kidnapping or injuries to people.
Misappropriation of monies including union funds could also be subject to the amnesty.