The irregular packing problems are cutting and packing problems, in which smaller irregular pieces (which we call items) should be packaged entirely in one large piece (which we call a plate), obeying non-overlapping constraints and minimizing the dimensions of the plate. To ensure non-overlapping, we make use of separation lines, that is, lines that separate one item from another. We present nonlinear programming models for problems of packing regular and irregular items that rotate freely. The items can be circles, convex and nonconvex polygons. The main advantage of the models is their simplicity, because they use only basic geometry concepts. We use the nonlinear programming algorithm IPOPT (an algorithm of interior points type), which is part of COIN-OR, to solve the problems. Computational tests were performed using known instances of the literature and the results were compared with results presented in the literature, obtained with other methodologies that also use free rotations, showing that our models are competitive. We also propose the use of separating parabola to avoid items overlaping in the models, which could provide greater computational eficiency as well as solutions with better quality.