Workers at the Hoover electrical goods factory in Merthyr Tydfil, south Wales, are on strike in a dispute over pay.
Workers appeal for support from passing motorists in Merthyr
Members of the Amicus union are taking part in the 24-hour action, which is expected to hit production of washing machines and tumble driers.
Around 60 workers formed picket lines from 0600 GMT.
An Amicus spokesman said it was unacceptable that Hoover was freezing pay this year.
But Hoover, which is owned by Italian electrical goods firm Candy, said it had offered two deals to improve pay, based on a profit-related bonus.
"The company is disappointed that its Merthyr manufacturing employees have decided to take industrial action," said a spokesman for the firm.
"The result of this action is that both employees and the company will suffer."
Norman Southwell of Amicus says pay rises based on bonuses are unreliable
The firm added it had suffered heavy trade losses in recent years due to international competition.
Regional Amicus officer Norman Southwell claimed that pay rises in the last two years had been swallowed up by increased pension contributions.
Mr Southwell said plans for another strike next month have been drawn up if the disagreement was not resolved.
He said: "This year, there's no increase on basic rates, although there's a discussion about a bonus payment, but that's unreliable when you look at the last three year period. Our members need a real increase."
Assembly line worker Louise Bowen added: "We can't achieve the bonus because the parts don't come in on time on a regular basis, it's all put down to us that we don't achieve our numbers at the end of the day."