*2.1. Description of Selected Popular 3D Printing Methods*

The SLA method (i.e., stereolithography) was the first of the developed rapid prototyping methods. The company 3D Systems patented this method in 1986 in the United States. In 1987, it began to manufacture machines using the technique of stereolithography [12]. The SLA method is based on layered polymerization of epoxy or acrylic resin with a laser beam. It is necessary to build structures that support the model, which take the form of thin rods and are usually removed mechanically [13]. The process begins by building a model in a 3D CAD system, which is then converted to the STL format. The prepared material in the form of a liquid resin is placed in the tub of the device. Before each layer

is hardened, the scraper smooths the sheets of liquid and removes air bubbles. The laser beam scans the areas that show the current cross-section of the created element, which causes polymerization. Then, the working plate is lowered by the thickness of the layer. The whole process is repeated until the final geometry of the manufactured item is obtained. The finished element must be cleaned of unbound resin. This is done by rinsing the product in isopropanol or acetone. Then, the supporting structures must be removed mechanically. The last step is supplementary UV irradiation so that the polymerization process is completed in the entire volume of the model [14]. The element prepared in this way can be subjected to additional finishing:


Various types of resins are the materials used in the SLA method. Standard resin is mainly used to create concept models and prototypes [15,16]. ABS resin is used for elements that must exhibit high strength and significant elongation. The products that are to be elastically compliant are made of elastic resin. There are also resins such as high-temperature, foundry, or medical [17]. When designing a layout for rapid prototyping machines using the SLA method, it is necessary to take into account that apart from the printer itself, other devices are also used. The entire instrumentation also includes:


The full name of the SLS method is selective laser sintering. This was developed and patented by the founders of one of the first 3D printing companies—Desk Top Manufacturing Corporation [18]. The SLS method is based on the layered solidification of materials that are used in the form of a powder. The individual layers are joined by a laser beam that affects the powder surface. The whole process begins with loading a 3D spatial CAD model, which is then converted by the software to the STL format, thanks to which the model is divided into individual layers, then control instructions for the machine are generated [19]. The prepared plastic powder is applied to the work area with a scraper or a roller. There, it is heated and then laser sintered. The areas that show the current cross-section of the model are scanned by the laser beam, which causes their fusion. Then, the working plate is lowered by the thickness of the layer and another dose of powder is applied. The whole process is repeated until the final geometry of the manufactured item is obtained [20]. The final product, which we receive after printing, should be cleaned of any residual powder. Additionally, depending on the requirements and needs, it can be subject to finishing [21]:


The materials used in the SLS method are plastic powders. The most popular is the PA12 polyamide due to its high flexibility and high mechanical properties. Other materials used in this method include PA12 polyamide with the addition of glass balls, thanks to which the manufactured elements will show greater strength and stiffness to compression as well as slightly increased thermal resistance of the material and alumide (a mixture of polyamide powder and aluminum filings). The SLS method allows one to produce a finished product using one device, although other machines and devices are also necessary for the entire process, which should be taken into account when building the layout plan. Powder material must be properly prepared before it can be used in the process and its remains after printing can be reused after prior preparation. The elements of the instrumentation equipment include:


Another increasingly popular technique of 3D printing is the SLM method—Selective Laser Melting. This technology is protected by several patents [22,23]. Using this method, elements from metal powders are produced. However, they exhibit lower strength and durability than parts made with traditional shaping techniques. The SLM method ensures repeatability of mechanical properties, which allows the use of parts produced in this way as elements of machines and devices. The SLM method consists in melting metal powder with a metal beam. The process begins with applying a layer of powder and then levelling it. Selected areas are melted by the laser beam. The next step in the process is to lower the working plate by the layer thickness and apply the powder layer again. The whole process is repeated until the finished model is obtained. The produced model should be cleaned of unmelted powder and most often subjected to finishing treatments, which include:


The materials most often used in the SLM method include stainless steels, pure titanium, and its alloys as well as low-melting, zinc, copper, tool steel, silicon carbide, or aluminum oxide alloys. In machines used for SLM printing, it is necessary to use a protective gas in the working chamber and the type depends on the powder that will be used to print the element. The melting parameters of the powder must be selected in such a way that the overheating of the area that is irradiated with the laser is as little as possible, since too much heating could distort the resulting product [12]. When designing the layout plan, one has to take into account that in the SLM method, apart from the main machine that produces the finished model, other devices are also necessary and without them, the entire process could not take place. Such elements include, for example:


To test the methods of optimizing the placement of 3D printing devices, we planned to choose one of the several available methods. The selection was based on the popularity of the methods and their adequacy to the problem being solved.
