Introduction

The Additive Manufacturing (AM) is a sophisticated technology used for three-dimensional processing by joining materials, usually layer by layer. This process offers the benefit of creating any complex object starting from a 3D representation of its external surface. It goes with a short realization time, cost efficiency, and minimum need for tooling. A wide variety of materials can be used in this process, such as ceramics, polymers, metals, alloys, and composites [1, 2], which allow them to invade different industrial sectors including, aerospace [3], building [4,5,6], biomedical [7,8,9,10], education [11], and also to use it for personal needs, such as art and hobb ies.

Actually, the AM is the most usable manufacturing process in medical industry to combat the recent pandemic (Covid-19 virus) [12,13,14]. New AM applications have appeared to withstand this virus, such as manufacturing medical face masks.

In a few recent years, the range of AM feedstock has been enlarged significantly by adding new materials to it, such as nanoparticles and smart and biobased materials to expand and to strengthen its application domains by helping in the protection of environment [2, 15,16,17,18,19].

The AM includes a large number of processes. All they differ in the principle of operation, the layer deposition manner, and the materials used to produce parts. There are three different categories of AM processes — liquid-, solid-, and powder-based ones. Depending on these categories, the material transformation process ranges from polymerization, binding, to melting [20]. In Fig. 1, the AM processes are compared in terms of material and product characteristics [2, 21,22,23]. The cost and speed are among the primary considerations when choosing an adequate printing machine. The Fused Deposition Modelling (FDM) is the least expensive process, with a low expenditure of energy. For this reason, it is the main technique used nowadays, especially to improve the quality of FDM-printed products, including their mechanical and thermal characteristics.

Fig. 1.
figure 1

AM classification based on the Main machine characteristics (a) and esthetic features (b): 1 — build cost, 2 — build speed, 3 — dimensional accuracy, and 4 — surface quality.

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This article reflects the challenges regarding the parameter identification of printed products and the sensitivity analysis of the parameters affecting their quality the most. It also analyzes their main characteristics, including surface quality, porosity, and mechanical features.

Fused Deposition Modelling (FDM)

The FDM is an additive manufacturing process where thin filaments of a plastic material are used to build objects by adding their layers. It was patented by Scott Crump in 1988 [24] and commercialized by Stratasys in the USA. The FDM printing process begins with adjustment of the melting temperature. Then, the machine is fueled with a filament feedstock, which is pushed to the heated liquefier via a stepper motor to a pinch roller mechanism to be melted there. The different steps of the feeding process present multiple mechanical and thermal actions (tension, compression, and heating). The melted material obtained is then extruded selectively through a small print nozzle that traces the cross-sectional geometry of parts by roads or by beads given by the STL model (Fig. 2) [25,26,27].

Fig. 2.
figure 2

Fused deposition modelling (FDM) (a) and nozzle geometry (b): 1 — build platform, 2 — foam base, 3 — part, 4 — build material filament spool, 5 — support material filament spool, 6 — print nozle, 7 — feed pinch rollers, 8 — heated liquefier, 9 — section 1, 10 — section 2, and 11 — section 3.

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The nonlinearity of the melt properties generated during printing mainly depends on its temperature and shear rate. The feedstock feed rate is controlled in the liquefier in order to maintain the volumetric flow rate constant. The melt flow in the liquefier is described by the generalized Newtonian power law where τ is the shear stress, \( \dot{\upgamma} \) is the shear rate, and m and Φ are material constants.

The liquefier geometry is divided into different sections, as shown in Fig. 2 [28], and the temperature variation during and after the extrusion process is described by the equation

$$ \rho {c}_p\frac{\partial T}{\partial t}=\lambda \left(\frac{\partial^2T}{\partial {x}^2}+\frac{\partial^2T}{\partial {y}^2}+\frac{\partial^2T}{\partial {z}^2}\right), $$

where ρ , cp , and λ are the density, the specific heat capacity (at a constant pressure), and the coefficient of thermal conductivity, respectively.

Furthermore, the latent heat in the transition phase of polymers during the extrusion process also cannot be ignored. The enthalpy in this case is

$$ H=\int \rho C(T) dT, $$

where ρ is the density, C is the specific heat, and T is temperature

Parameter Analysis and Discussion

Many researchers have investigated the properties of parts made by the FDM and the process parameters characterizing the deposited layer geometries, which are presented in Figs. 3 and 4.

Fig. 3.
figure 3

Principal FDM process parameters.

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Fig. 4.
figure 4

FDM tool path parameters: 1 — raster, 2 — raster angle, 3 — raster width, 4 — perimeter, and 5 — air gap.

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Surface roughness and porosity

Among the critical properties of FDM products treated are their surface quality and dimensional accuracy [29]. According to the recent reports and results, the surface roughness is affected most by the layer thickness, followed by the raster width and building orientation of a part [30]. The latter parameter greatly affects the dimensional accuracy, followed by the layer thickness [31]. A small layer thickness, as well as a small raster width of filaments, improves the surface quality.

A void between deposited filaments, called “air gap”, which is a crucial parameter of the final part of FDM, has a greater impact on the dimensional accuracy than the surface roughness. There are three types of air gaps. A positive air gap means the presence of a void between roads, a zero gap means that there are no voids between them (they touch each other), and a negative one means that roads overlap. The porosity of a part is indispensable in the case of FDM-mode products, due to the manufacturing method based on road deposition next to each other. The use of negative air gap factors may decrease the porosity, which reduces dimensional errors, but impairs the surface quality of final parts [32]. Therefore, to achieve a minimum dimensional error and ensure a good surface quality, it is recommended to use the following combination: a small layer thickness, an average raster width, a zero air gap, and the 0°/90° orientation of rods [33]. Besides, some esthetic defects can be removed by postprocessing techniques, including mechanical ones, such as the milling, the CNC, the abrasive or vibratory bowl machining, or chemical techniques by applying vapor or a slight amount of acetone to dissolve the rough or porous parts in order to achieve the best final quality [34].

The effect of raster angle on the esthetic properties of final parts have also been investigated. It is found that the effect of this parameter depends on the material printed. Generally, the +45°/–45° angles have found to give the best surface quality and dimensional accuracy [35]. The aesthetic properties of final FDM products depend mainly on the layer thickness and printing orientation.

Mechanical properties

When printing functional parts, their crucial features are Young’s modulus, the ultimate tensile strength, yield strength, and ductility. These parameters influence the mechanical properties of FDM-made parts to different degrees.

It is worth mentioning that the main FDM parameter to obtain a functional part is the part fill that specifies the percentage of material [36,37,38]. This parameter should be fixed at high values, which are linked to negative values of the air gap parameter.

There exists an extensive range of FDM feedstock materials, of which acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) have been widely studied owing to their good mechanical properties. A literature survey shows that up to 2/3 of the studies reported concern exactly these two materials. It reveals that the building orientation and raster angle are the factors that influence the tensile properties of products, including Young’s modulus, the ultimate tensile strength, and ductility most significantly, followed by the layer thickness and other FDM parameters. Building ABS and PLA parts in the plane direction is the optimal direction to ensure the highest Young’s modulus [39,40,41], but the impact of raster angle is substantially different. The highest ultimate strength is obtained by printing at 45° and 0°/90° angles for PLA [40] and ABS [42, 43] samples, respectively. The layer thickness is also an important input parameter affecting their tensile properties. It has been shown that small layer thicknesses (less than 1 mm) are the most adequate to realize FDMmade parts with good mechanical characteristics [36, 39, 40, 44]. The interlayer and interraster bondings are among the weakest ones in the FDM process. Therefore, the raster width should be great to cover the full adhesion surface in order to ensure a high degree of cohesion in the part [40, 45].

The FDM feedstock is not restricted to ABS and PLA materials. There are several polymers that are also classified as stiff and strong FDM materials, e.g., polycarbonate (PC), polypropy lene (PP), PEEK, UTEM, and polyamide (PA).

For PC, PEEK, and PLA, the same raster angles and building orientation are recommended [38, 42]. PP, PA and UTEM 9085 are similar to ABS, for which the zero raster angle is the best to reach a high Young’s modulus and the ultimate tensile strength [46,47,48,49].

As for the fracture toughness, it is highest at the zero raster angle, which may increase the fracture toughness Kc to about 2 and 5 MPa·m1/2 for ABS [50,51,52] and PLA [44] respectively. The fracture toughness can also be improved by increasing the roughness degree of material mixture, because the grains present in the material tend to resist crack growth [53].

Impact of printing temperature

Much attention has been focused on finding the optimal tool path parameters and on clarifying their impact on the printing pattern and the 3D-printing object. But the relation between these printing parameters and the printing temperature is often neglected. One of the major concerns related to the fused deposition modelling process is to maintain an adequate printing temperature to ensure a smooth printing operation.

An inadequate extrusion temperature range can damage the product. Low temperatures can lead to a low layer adhesion (Fig. 5), peeling off of layers, and even nozzle clogging. At a high printing temperature, large thermal gradients can arise, and attempts to of maintain a constant layer height can alter the desirable geometry of parts [31, 54].

Fig. 5.
figure 5

Low adhesion along the junction between layers in a FDM-printed part [54].

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In all conditions, high or moderate temperatures are recommended. An increased temperature raises the average temperature between layers, and more time above Tg is spent, which improves the interlayer welding. Moreover, the raster angle and width become more adjusted and ensure a high adhesion in layers [55,56,57]. The layers are flattened, and the surface roughness of the whole part decreases.

The anisotropy of printed parts is better controllable at high printing temperatures. The voids in the mesostructure with a cross-linked filament arrangement is the principal factor increasing the porosity f [58,59,60]

A low temperature hinders the adequate flow deposition through the nozzle and may generate discontinuous rasters and decrease the cohesiveness between the deposited filaments. A high printing temperature decreases the porosity of printed parts by minimizing the thermal transfers of deposited rasters and maximizing the cooling time during the printing operation (Fig. 6) [61]. Guessasma [62] confirmed the existence of this phenomenon and proposed an approximate formula for the porosity f and the printing temperature Tp based on microtomography measurements, namely,

$$ f\left(\%\right)=19.2-0.06{T}_{\mathrm{p}}\left({}^{\circ}\mathrm{C}\right) $$
Fig. 6.
figure 6

Filament cohesiveness at various printing temperatures (a) and the apparent density ρ (b) and porosity f (c) as functions of printing temperature [64].

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This linear model shows that pores are not distributed arbitrarily in the printed part — they are concentrated at the intersection of printed rasters (Fig. 7).

Fig. 7.
figure 7

The porosity f of a 3D-printed PLA revealed by the X-ray microtomography: cross-sectional views (a) and porosity network arrangement (b) [62]: 1 — void, 2 — raster, 3 — layer, 4 — frame, 5 — width, 6 — length, 7 — porosity network, 8 — surface porosity, 9 — building direction, and 10 — PA filament.

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The printing temperature strongly correlates with mechanical properties, with anisotropy of the printed pattern being the main reason for its mechanical weakness. As already mentioned, a high printing temperature improves the interfilament welding and leads to a high adhesion between layers [61, 63, 64], which increases the elastic modulus and tensile strength. A summary of the results of literature studies is presented in Fig. 8, where it is shown that the mechanical properties of different FDM-made polymer composites depend on printing temperature [31, 38, 39, 42,43,44, 46, 48, 50,51,52, 54, 64,65,66,67,68,69,70,71,72,73]. Further investigations are needed to determine the optimal values of FDM process parameters by employing various design optimization methods and techniques [74,75,76,77,78].

Fig. 8.
figure 8

Uniaxial tensile strength (UTS) (a), Young’s modulus E (b), and fracture toughness Kc (c) as a functions of printing temperature Tp .

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Conclusion

The first part of this article was dedicated to an overview of the additive manufacturing and fused deposition processes. Various printing parameters and their influence on the quality of printed patterns were considered and recent results of temperature influence on their roughness and porosity during the FDM process were judged.

The crucial factors to be taken into account before building a part were examined, and it was found that the most significant ones were those related to tool path parameters, such as the layer thickness and part building orientation.

The influence of the principal printing parameter was described by a theoretical model characterizing the thermal behavior of parts during the printing process. Finally, the impact of printing temperature on the porosity and the mechanical behavior of printing polymers was summarized.

Nonetheless, there still are a few impediments which hinder one to obtain a product with excellent mechanical properties. The control of porosity of printed parts, which worsens their mechanical properties is still a problem. Voids in deposited trajectories and between layers reduce cohesion and, as result, small cracks may appear and propagate in parts. Therefore, fatigue studies are required to obtain a further insight into this phenomenon.