Around 150 of the Stirling-based company's 275 staff will walk out today and again on Saturday after no agreement was reached during a consultation period.
According to Unite, UCP, which prints and manufactures packaging for the drinks industry, increased employees' contracted hours by five hours per week for no extra pay.
The union said that the new contracts would come into effect from 29 August. Norman King, regional political officer for Unite, said staff were currently "up against it".
He said: "Talks could have taken place today but the company wanted both sets of strike action called off for that to happen. Our members were willing to do that if these new contracts were changed but that hasn't been possible.
"It means that staff will be expected to work longer for less money. Some staff will also be £27 a week worse off."
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