An apparatus and a method for manufacturing plastic containers, where the plastic containers are produced from aplastic material such as recycled plastic. The plastic preforms are heated in a heating unit and expanded into plastic containers in an expansion device. An infrared absorption degree of the plastic material or of the plastic preforms produced from said plastic material is then examined at least once in the course of the production process. From this examination conclusions can be drawn with respect to the quality of the preforms and their behavior when they are being heated.