LISBON - A vote to bring in the machinists union is in dispute at the town's third-largest employer.

Knight-Celotex, a maker of eco-friendly fiberboard, has challenged the April 30 vote by its workers to join a chapter of the International Association of Machinists. According to papers filed with the National Labor Relations Board, the vote was 36 for union representation and 34 against. Three of the votes are being challenged on the basis of worker eligibility by the company, said Robert Redbord, deputy regional attorney for the NLRB in Boston.

A hearing date has been set for June 17. The union must get 50 percent of the vote plus one to win.

"Obviously the outcome depends on how those three challenged votes fall," said Redbord.

The company has also filed an objection with the NLRB regarding union conduct in advance of the vote. If the NLRB hearing officer decides in favor of the union regarding the contested votes, the board will then take up the company's other complaint regarding union organizing. If the officer decides in favor of the company on the first complaint, the second objection becomes moot.

John Carr, a spokesman for the union, said he believes the organizing effort was conducted appropriately. He said workers at the Lisbon Falls plant were inspired by the contract that machinists negotiated at Bath Iron Works.

Workers represented by S6 of the IAM's northeast district were able to get 3.5 to 4 percent wage increases yearly for four years and increased pension contributions without increasing employee percentages for medical insurance.

"They saw what those people were able to negotiate in a tough economy ... that can spark interest," said Carr. He said the Lisbon Falls workers were looking for protection of wages, benefits and health care coverage from the union.

Redbord said once the hearing is closed, each side has 10 days to file additional information and then the hearing officer renders a decision - typically within two or three weeks.

Knight-Celotex makes specialty composite fiberboard with high-insulating and sound-proofing qualities. It has production facilities in Sunbury, Pa., Marrero, La., and Danville, Va., as well as Lisbon Falls. Calls placed to the local company, its attorney in the NLRB case and its headquarters in Chicago were not returned.

The company employs roughly 100 people, according to the town's economic development office.