## 1 Introduction
The correct temperature in the process and the fact that the material is in a semi-molten state make the layers stick together, thus making it possible to obtain a solid 3D product [12]. Transparent, black, white, or colored materials can be used to make a product in the FFF method. Products made using the FFF method are characterized by one color depending on the spool of material used; however, modern devices often have two heads that work in parallel, allowing the production of products with different colors and properties [1]. Products obtained by the FFF method resemble products obtained by plastic injection molding processes in shape and structure. However, it is important to keep in mind the right choice of parameters, such as the orientation of making the product, layer thickness, or process temperature. The correct orientation has a direct impact on the possibility of making the product with or without process supports (as the material must be overlapped) and on the process time [1]. The properties of the surface obtained by the FFF process depend on the above-mentioned parameters, but as in any additive manufacturing process, there is a stepping effect. In some cases, it is necessary to make support to hold the manufactured product. After the process is completed, manual processing is necessary, which involves the removal of previously made process supports [1]. To achieve better quality of the product, it is possible to reduce the thickness of the layer; however, such a solution is associated with a longer process time, which can last up to several days [6]. Analysis of the state of the art has shown that several additive manufacturing methods have been developed, each with specific limitations as well as different quality of the products and the materials that are used. Due to the nature of additive manufacturing processes, there is a stepping effect in the manufactured products, resulting from the build-up of layers on top of each other. The thickness of the layers results from the method adopted or the choice of parameters of the additive process. When manufacturing products using the FFF method, the parameters used have a direct impact on the dimensional and shape accuracy as well as on the strength properties of the manufactured products. Despite many studies on the additive manufacturing process parameters, their implementation in industrial and research practice is limited due to the numerous distortions affecting the obtained dependencies to a degree that is difficult to predict. Current trends in the development of the FFF technique mainly focus on overcoming its fundamental limitations, which are shortcomings in the strength of models, their dimensional accuracy, and their morphology. Researchers are working intensively to expand the knowledge of the strength properties of models by determining, among other things:  
* The effect of changing the values of various process parameters [13; 14]. * The possibility of supporting the process with vibration to increase the strength of models and surface quality [15]. * The role of temperature in the formation of defects [16; 17]. * The influence of structural features of products (cell size and wall thickness [18]). * The influence of model operating conditions (e.g., cryogenic [19]). * The influence of building material [20; 21; 22; 23; 24]. Especially in the latter area, significant potential for improving the FFF process is evident. There are two main developments in this regard. The first concerns the reinforcement of the building material with additives in the form of fibers [20; 21] or with additives that improve crystallization, interlayer bonding, and mechanical performance of polylactide parts [21]. The second focuses on the possibility of using recycled polylactic acid filaments to build models using the FFF method [23; 24]. The paper [20] describes the possibility of using fibrous reinforcement to strengthen the thermoplastic matrix to produce structurally complex, small-sized parts using the FFF method.