*2.2. Printing Material and Strategy*

Among the different additive manufacturing processes, Fused Filament Fabrication (FFF) and Vat Photopolymerization (VPAM) are widely used in the field of rapid prototyping. In this work, both printing methods are investigated. FFF is an additive manufacturing process, where a molten polymer filament is extruded through a nozzle onto a build platform, layer by layer. In VPAM, a liquid resin is selectively cured through light activated polymerization by use of a UV light. Low costs and high manufacturing speeds are the core of rapid prototyping, so for the purpose of this study, the most commonly available materials were selected for both FFF and VPAM. PLA was used for the tools produced by FFF; this material is the most commonly used in FFF, it is biodegradable and produced from renewable resources such as corn starch and sugar cane. Furthermore, it is characterized by relatively low melting temperature and low shrinkage, reducing internal residual stresses [9]. The resin selected for VPAM is an acrylate-based photopolymer with phosphine oxide photo initiator. This photopolymer is affordable and is characterized by a great trade-off between market price and mechanical and printing properties. It was selected for its high hardness after curing compared to similar resins: 83 shore D hardness according to its datasheet. Some mechanical and physical properties of the materials that that are used in this work are shown in Table 1.

**Table 1.** Typical mechanical and physical properties of materials that are used in this work.


The final strength and quality of the 3D-printed tools are directly influenced by various printing parameters [1,10–13]. Klimyuk et al. [1] showed how layer thickness, wall thickness and infill density have a major impact on the final quality of the printed part. The

authors of this work conducted an investigation on the influence of printing orientation, layer thickness, wall thickness and infill density in order to select the optimal parameters for printing the tools. In the case of VPAM, the effect of the exposure time was also studied. The selected printing parameters are shown in Table 2.

The printing orientation is an important parameter due to the anisotropy of the properties of the printed part, which is generated by its layer structure [9]. The printing orientation was chosen to be in the out of plane direction in Figure 2, because parts printed in this direction withstand compressive forces during metal forming better by having the layers perpendicular to vertical forces. This minimizes the risk of delamination between layers.

The selected layer thickness is 0.1 mm. In FFF, thinner layers lead to improved adhesion of layers to one another and denser parts since the heat from the nozzle, being closer to the previous layer, helps the layers bond together. Kuznetsov et al. [9] showed that by decreasing the layer thickness, it is possible to decrease the generation of voids, which negatively affect layer adhesion and, therefore, tool strength. The layer thickness is also an important parameter for VPAM, where a smaller layer thickness ensures complete UV penetration through the layer and a higher degree of polymerization [11].

The shell thickness was fixed to 2.10 mm (double of the default wall thickness of 1.05 mm). Aslani et al. [12] investigated the influence of the shell thickness on the geometrical accuracy of the part showing higher accuracy when a double wall thickness was used.

The infill density is another important parameter. The higher the infill density, the larger the amount of material that the component is comprised of, and the greater the bonding areas between layers. The larger bonding area leads to an adhesion gain among the layers, reducing the likelihood of delamination and fracture and promoting the strength of the component. An infill density of 50% was selected for FFF as a balance between printing time/material volume and component strength. The infill density was set to 100% for the VPAM tools in order to ensure complete polymerization and to maximize the bonding between layers, reducing the risk of delamination.

The exposure time in VPAM directly affects the light penetration depth, which is the length by which the UV light is able to penetrate and cure the photopolymer. The light penetration depth needs to be equal to, or more than, the layer thickness in order to assure bonding between the layers. The optimal exposure time, for the specific resin used in this work, was found to be 30 s for the burn in layers and 3.5 s for the rest of the layers.


**Table 2.** Printing process parameters for FFF and VPAM.
