Air Products staff helped trained colleagues in China
The first of 200 factory staff being made redundant are working their shifts for the last time.
Fifty Air Products workers at Acrefair, Wrexham, will be made redundant, with the remaining workforce expected to leave by the end of the year.
The Unite union said most of the workers gathered for a "final photo" on Thursday ahead of a "very sad day".
The plant, which opened in 1950, is closing because production of gas plant equipment is being switched to China.
The first 50 workers begin their shift at 0730 BST and will clock off for the last time at 1300 BST.
The rest of the redundancies will be made in phases, with production expected to end by December.
Air Products said the decision to close Acrefair was made "in response to on-going cost pressures and the changing demand for our products which is shifting to other parts of the world, notably Asia".
The Unite Union has expressed anger at the closure, as many of the Acrefair staff helped train colleagues in China, who will now take over their jobs.
The union said it had sought a rescue package from the UK government, and workers were described as "disgusted and disappointed" that the factory could not be saved.
Unite spokesman Graham Rogers said: "It is a very sad day.
"And the annoying thing is that the company has won two big orders. Both of these orders will be completed in China.
"One of them could have kept Acrefair going for 18 months to two years - and then there could be an uplift in the economy, who knows?"
He added: "There were 194 staff on site on Thursday and the workforce gathered together for a final photo.
"It's very daunting and traumatic for them.
"I think here's about four or five who have been lucky and managed to get positions with other companies."
In a statement, Laurence O'Donnell, general manager at Acrefair, said: "It is with great sadness that a number of employees are leaving us today.
"This follows a decision to close our manufacturing facility in Acrefair, north Wales, which will cease production in mid December 2009.
"I'd like to acknowledge the whole Acrefair workforce for their unfaltering professionalism during this difficult period."