## 2 PolyJet Technology  
Before designing the test plate for analysing repeatability and reproducibility, it is important to understand how the PolyJet process works. The company Objet Geometries developed the PolyJet (nozzle-network) technique in 2000 [26] (Figure 1).  
Figure 1: PolyJet technology [27] (printing head photo by [PERSON]).  
The network of nozzles moves in the direction of \(x\) and \(y\) axes and prints a layer of photosensitive polymeric material of 16 \(\upmu\)m thickness. In each layer, polymerization takes place under the influence of UV light, immediately after printing, forming a completely cured product, with no need for second curing. Two different materials are used for the model and for the support. After the completion of the first layer, the working platform lowers for the thickness of the next layer. Once the product is produced, the support is removed by water or manually [26; 28; 29].  
Polylet technology belongs to the material jetting procedures according to the ASTM F42:2010 standard. The material in a liquid state solidifies through a process of photopolymerization, also known as photo-curing. Photopolymerization is based on using monomers/oligomers in a liquid state that can be cured/photopolymerized upon exposure to a light source of a specific wavelength. Photoinitiators are needed to convert photolytic energy into the reactive species (radical or cation) which can drive the chain growth via radical or cation mechanism. Typically photoinitiators with high molar extinction coefficients at short wavelength (mostly UV \(<\) 400 nm) are used to initiate the photochemical reaction. Materials that are used in Polylet are acrylates that need a radical method for polymerization. In the process, photoinitiators convert light energy into chemical energy by forming free radicals upon UV exposure.  
Vero material photopolymers that were used in this experiment are generally composed of a binder, monomers (10-40%) and photoinitiators (50-80%). PY-GC/MS (pyrolysis) results proved that it is a composite material, i.e., a monofunctional polymer based on acrylic acid, styrene tetramer with benzophenone-based binders, phenolic antioxidants and organophosphorus oxides [30].  
Liquid resin is heated from 30-70 \({}^{\circ}\)C to achieve optimal viscosity for printing [31; 32]. The advantages of the Polylet technology are: high quality of the product, high precision (very thin layers), high surface quality, possibility of producing small and thin walls and details, usage in offices, and fast production. Additionally, no second curing is needed and it is possible to mix FullCure materials which allow achieving of different geometries, mechanical properties and colours [27; 33; 34].  
It must be noted that the surfaces of the finished product can be matte and glossy. In a glossy setting, the support material is added only when it is structurally required (i.e., for overhangs), so surfaces not in direct contact with support will have a glossy finish, while supported areas will be matte. In the matte setting, a thin layer of support material is added around the whole part, regardless of orientation or structural requirements. This way, all surfaces have a matte finish [35].  
As the material is sensitive to light, properties of the products can change over time.  
The first few layers are made of just supporting structure so the product can be easily and without damage removed from the platform when finished. The support material is injected from nozzles located on the printer head. Support (gel-like material) is dissolvable material that can be removed after printing using pressurized water (water jetting) or solvents (caustic soda and sodium metasilicate), by immersion in an ultrasonic bath or manually [33; 35].  
Polylet technology allows the producers and engineers to shorten production cycles and lead-times for launching the product on the market. It is used in the automotive industry, electronics, in the production of toys, footwear, consumer goods, and in the production of jewellery [33].