**2. The State of Research in the Application of AM Technology in Polish Manufacturing Companies**

Polish manufacturing enterprises make little use of additive technologies (2.4% according to GUS data). In 2017, 3D printing technology was most often used by enterprises in the Dolno´sl ˛askie (3.5%), Podkarpackie (3.2%) and Swi ˛ ´ etokrzyskie (3.1%) voivodships, yet least frequently so in the Lubelskie (1.0%) and Lubuskie voivodships (1.1%). In 2017, 3D printing technology was mainly used to create prototypes or models for personal use (1.8%), of which 10% were mainly for large enterprises. The least frequently used 3D printing was to create goods for sale, excluding prototypes or models (0.4%) and to create goods for use in production processes, excluding prototypes or models (0.5%). Spatial printing was mainly used by large enterprises employing 250 people or more.

Based on the analysis of the literature on the subject [5–29], the characteristics of AM technologies used have been broken down into separate dimensions:

	- (1) Production process of machinery parts and equipment made of plastic and metal, including high precision mechanical parts and sub-assemblies,
	- (2) Production process for functional prototypes and co-operating mechanisms,
	- (3) Production process in models used in strength tests, tests and modelling.
	- (1) Prototype and model production process,
	- (2) The production process for injection moulds, foundry moulds, highly precise metal structures and other elements with a complex geometry or requiring high mechanical properties, the production of which would often not be possible using foundry technologies,
	- (3) The process of the repair and regeneration of complex damaged metal parts.

The following AM technologies according to the defined dimensions were identified for further research (Table 1).


#### **Table 1.** Characteristics of additive manufacturing (AM) technology.

\* Technologies are also used in the automotive industry (spare parts).

In the processes carried out in AM technologies, the following technologies can be distinguished: technologies using a powder-filled platform, the powder bed technology, so-called, occurring in such technologies as Selective Laser Melting (SLM), Direct Metal Laser Sintering (DMLS), Electron Beam Melting (EBM) and also direct deposition technology, in such technologies as Electron Beam Additive Manufacturing (EBAM), Laser Engineering Net Shape (LENS) [30].

By evenly placing the powder on the working platform, the material feeding system has the advantage because the model does not need to have brackets generated. Moreover, the powder bed gives the opportunity to make models in the higher parts of the working chamber [27]. Selective powder micro-metallurgy (SLM) has many advantages related to accuracy of performance, versatility and the possibility of the complete elimination of the assembly phase, by sintering ready-made constructions. In the selective laser melting of powders of various metals, stainless steel powders are most often used. Unlike Selective Laser Sintering (SLS), the powder is re-melted through a laser beam, which significantly affects the tightness of the model and the lack of pores [27]. Metal additive manufacturing (MAM) is becoming a popular industrial production solution. Metal AM is characterised by two leading methods using metal powders, namely SLM/DLMS which is used mostly in the automotive industry and electron beam melting (EBM) [31,32]. The benefits of using additive manufacturing techniques are observed in the production of engine parts, fuel systems or turbines with a complex geometry and with defined aerodynamic properties. In addition, the technology of laser sintered metal powders enables the development of advanced and lightweight structures that combine high strength with a weight reduction of up to 60%. Even very complex elements, made of high-strength material, can easily be manufactured by means of additive manufacturing techniques, where the use of traditional processes, for the production of such parts, is impossible or very expensive [33]. Powder sintering methods are used to make injection moulds for mass production as also for the production of pressure casting moulds and other elements requiring high mechanical properties, the production of highly precise metal structures, making surgical instruments, passive and active implants, as well as for medical parts/devices, manufacturing elements complex shapes and structures that would not have been possible with casting technologies (DMLS).

FDM (fused deposition modelling) technology is used for rapid prototyping. In FDM technology, one of the nozzles of the printing device feeds the material for modelling, while the other feeds the material to produce supports, when necessary. Depending on the printing machine and the material used, the supports, produced in the final phase, can be broken off (BST series machines) or dissolved in water (SST series machines). Manufacturing supports is sometimes necessary due to the presence of hanging elements that affect the stability of the manufactured item. Molten thermoplastic layer extrusion methods are used for the production of plastic machinery parts and equipment, the production of prototypes of co-operating mechanisms and the production of models, used in strength tests, etc. [19].

Material application and UV hardening methods are used for the production of high-precision parts, mechanical components, the production of flexible elements, such as gaskets and washers and the production of high-quality prototypes over a short time.


#### *2.1. Technologies Based on the Photo-Initiated Polymerisation Method (SLA, DPL)*

Liquid photo-polymer is placed in a tank (ladle) and then selectively cured, layer by layer at source, emitting heat using a digital screen, LCD screen, UV radiation or laser beam, until the element is completely manufactured. In the case of Stereo-lithography (SLA), a laser beam is mainly used which is focussed on the surface of the tank, creating each layer of the desired 3D object by cross-linking or polymer degradation. Digital Light Processing (DLP) uses a digital projector screen, through which the entire layer of the element is cured simultaneously over the entire surface. This reduces the time it takes to manufacture a part [3,6,25].
