since they imply a great use of the human perception for their valuation. For instance, in some manufacturing environments the planners use an average production cost per hour perceived in the calculations of the unitary production costs, what makes these costs fuzzy. The costs of under time and overtime of the productive resources neither could be exact because the manpower can change or because mishaps or production shortcomings can take place. On the other hand, the demand backlog cost is fuzzy in its composition. This cost consists so much of the administrative cost of managing the backlog of orders as of the cost due to the loss of the clients. This type of cost is commonly estimated by using human experiences.
Along the years there have been many researches and applications aimed to model the uncertainty in production planning problems [9]. Uncertainty can be present as randomness, fuzziness and/or lack of knowledge or epistemic uncertainty (see [2]).
Here, we model a production planning problem under uncertainty with fuzzy constraints, taking into account the fuzziness in the demand, the available capacity and the aspiration level of costs and fuzzy coefficients, the lack of knowledge of the demand backlog costs and the required capacity, based on fuzzy linear programming.