Yes, formic acid can be harmful to humans, depending on the concentration. I don't know what concentration of formic acid you use to make rubber sheets, but you should know this since you are involved (or about to become involved) in rubber production. Concentrated formic acid is dangerous.
Read the label and information sheet that comes with the formic acid carefully, contact the manufacturer if necessary, and do some research about the safety of the particular concentration of acid that you need to use.
Anyone who plans to make rubber should investigate all the details, requirements, and safety rules before they buy, accept or use rubber-making equipment. Whether you are making rubber as a business or as a hobby, it's very important to know how to perform all the steps safely and to know about any potential dangers.