An Overview of Material Extrusion Troubleshooting

Material extrusion (ME) systems offer end-users with a more affordable and accessible additive manufacturing (AM) technology compared to other processes in the market. ME is often used to quickly produce low-cost prototyping with the freedom of scalability where parts can be produced in different geometries, quantities and sizes. As the use of desktop ME machines has gained widespread adoption, this review paper discusses the key design strategies and considerations to produce high quality ME parts, as well as providing actional advice to aid end-users in quickly identifying and efficiently troubleshooting issues since current information is often fragmented and incomplete. The systemic issues and solutions concerning desktop ME processes discussed are not machine-specific, covering categories according to printer-associated, deposition-associated and print quality problems. The findings show that the majority of issues are associated with incorrect printer calibration and parameters, hardware, material, Computer Aided Design (CAD) model and/or slicing settings. A chart for an overview of ME troubleshooting is presented allowing designers and engineers to straightforwardly determine the possible contributing factors to a particular problem.


Introduction
Material extrusion (ME) is the second most popular form of additive manufacturing (AM) process with 844,800 and 499,500 searches on Google as well as Google Scholar, respectively, after Powder Bed Fusion that has been searched 578,000 and 26,820,000 times, respectively. The term 'ME' was commonly searched together with 'Fused Deposition Modelling' and 'Fused Filament Fabrication'. These three terms refer to the same AM process of additively building up material by selectively dispensing through a nozzle or orifice [1]. The technology involves the material from a spool of filament that is loaded into the printer, melted above its glass transition temperature (T g ) for amorphous polymers and above its melt temperature (T m ) for semi crystalline polymers to be selectively dispensed through the heated extrusion nozzle and deposited onto the build platform at a predetermined location (Figure 1) [2]. As the use of desktop ME machines has gained widespread adoption, this paper discusses several key design strategies and considerations to produce high quality ME parts and focuses on the current issues concerning the desktop ME process which is not machine specific. The purpose of this paper is to serve as a troubleshooting guide to determine and remedy common causes of problems and symptoms. This review summarizes the findings of many development engineers, researchers at equipment manufacturers, scientists, ME users and our personal experience available in the open literature. These findings were derived from working with desktop Cartesian printers that use a system of X-Y-Z coordinates to determine the location of the extrusion nozzle, but similar issues could also be encountered in Delta ME machines, Polar ME machines or extruders fitted to robotic arms.
Advantages and Disadvantages of ME ME processes allow a wide variety of materials with diverse characteristics and properties to be used, ranging from commodities, engineering, to high-performance thermoplastics, composites, and functional materials (Table 1). Examples of commercially available commodity thermoplastics used in ME include polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), styrene acrylonitrile (SAN), and polypropylene (PP). Examples of engineering polymers commercially available for ME include acrylonitrile styrene acrylate (ASA), polymethyl methacrylate (PMMA) polyamide 6 (PA6), polyamide 66 (PA66), polyamide 12 (PA12), polycarbonate (PC), thermoplastic polyurethane (TPU), polyethylene terephthalate with glycol (PETG), recycled PET (rPET), thermoplastic elastomer (TPE), polyoxymethylene copolymer (POM-C), thermoplastic copolyester elastomer (TPC), polyvinyl alcohol (PVA), polyhydroxyalkanoate (PHA), butenediol vinyl alcohol (BVOH), blended with PLA and other blends of polymers (Table 3) [3]. Examples of high-performance polymers available as filaments for ME include polyphenylsulfone (PPSF, PPS or PPSU), polyetherimide (PEI), polyamide imide (PAI), polyaryletherketone (PAEK), polyether ether ketone (PEEK), polyvinlydene floride (PVDF) [3]. Examples of composite filaments include Carbonyte (PA with carbon fibers), and other engineered polymers such as those with wood fibers, glass-filled fibers or particles, carbon fibers and magnetic particles like ferrites, as well as special polymer blends with ceramic, cement and metal for sintering applications [3,4]. As the use of desktop ME machines has gained widespread adoption, this paper discusses several key design strategies and considerations to produce high quality ME parts and focuses on the current issues concerning the desktop ME process which is not machine specific. The purpose of this paper is to serve as a troubleshooting guide to determine and remedy common causes of problems and symptoms. This review summarizes the findings of many development engineers, researchers at equipment manufacturers, scientists, ME users and our personal experience available in the open literature. These findings were derived from working with desktop Cartesian printers that use a system of X-Y-Z coordinates to determine the location of the extrusion nozzle, but similar issues could also be encountered in Delta ME machines, Polar ME machines or extruders fitted to robotic arms.
Advantages and Disadvantages of ME ME processes allow a wide variety of materials with diverse characteristics and properties to be used, ranging from commodities, engineering, to high-performance thermoplastics, composites, and functional materials (Table 1). Examples of commercially available commodity thermoplastics used in ME include polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), styrene acrylonitrile (SAN), and polypropylene (PP). Examples of engineering polymers commercially available for ME include acrylonitrile styrene acrylate (ASA), polymethyl methacrylate (PMMA) polyamide 6 (PA6), polyamide 66 (PA66), polyamide 12 (PA12), polycarbonate (PC), thermoplastic polyurethane (TPU), polyethylene terephthalate with glycol (PETG), recycled PET (rPET), thermoplastic elastomer (TPE), polyoxymethylene copolymer (POM-C), thermoplastic copolyester elastomer (TPC), polyvinyl alcohol (PVA), polyhydroxyalkanoate (PHA), butenediol vinyl alcohol (BVOH), blended with PLA and other blends of polymers (Table 3) [3]. Examples of high-performance polymers available as filaments for ME include polyphenylsulfone (PPSF, PPS or PPSU), polyetherimide (PEI), polyamide imide (PAI), polyaryletherketone (PAEK), polyether ether ketone (PEEK), polyvinlydene floride (PVDF) [3]. Examples of composite filaments include Carbonyte (PA with carbon fibers), and other engineered polymers such as those with wood fibers, glass-filled fibers or particles, carbon fibers and magnetic particles like ferrites, as well as special polymer blends with ceramic, cement and metal for sintering applications [3,4]. ME processes offer end-users desktop equipment at a lower price range as compared to other AM technologies in the market as well as at a professional and industrial level for higher precision and performance components fabrication (Table 2). An open-source ME printer offers end-users the freedom to utilize third party filaments that fit the machine parameters, while a closed system limits the use to only materials offered by the printer manufacturer [18]. ME is useful to produce quick and low-cost prototyping with great freedom of scalability where parts can be printed in different geometries, quantities and sizes quickly [35]. However, ME is less suitable for producing intricate or miniaturized parts due to the circular profile of the print nozzle where the minimum nozzle size is 0.2 mm. As a result, corners and edges will have a radius that is equal to the size of the machine nozzle. Layers from the ME process are printed as a round-ended rectangle, in which the joints and seams between each layer are often visible as small valleys ( Figure  2). These visible layers sometimes result in a stair-stepped effect and post-processing is required to achieve a smooth surface. Unlike the material properties achieved through subtractive or formative manufacturing, most ME parts are inherently anisotropic and not fully dense due to the nature of the layer-by-layer fusing of material. The bond and strength of the printed part are usually weaker along the plane of the layer interface [36]. ME parts may also encounter an issue with repeatability due to differential printer characteristics or inconsistency in printing conditions such as cooling or warping [2]. Explanation of the print layers from the ME process. Figure 2. Explanation of the print layers from the ME process.

General Strategies to Achieve High Quality Parts
The quality and performance of parts produced by ME are highly dependent on the build parameters used. These parameters can be grouped into temperature-related parameters, infill, adhesion to the build platform and the orientation which are discussed in the following sections.

Temperature
The quality of parts being produced using ME is highly dependent on the optimum nozzle temperature. If the correct temperature is used, high-quality prints with smoother finishing, better clarity of details, better overhangs, and better bridging can be achieved. The temperature of the build platform should be properly set according to the material used. It is recommended that the build platform temperature should be above the T g of the deposited material to enable sufficient Appl. Sci. 2020, 10, 4776 5 of 28 adhesion [37]. The build platform heats up the surrounding environment and influences the contact between the deposited and depositing strands, which can affect the interlayer adhesion that may influence the mechanical properties of the produced parts. Please note that high-performance polymers often require an extruder that is capable of functioning above 300 • C and a heated chamber to prevent warpage or distortion during the printing process [2]. The use of a cooling fan is recommended for some materials since it can improve the geometrical accuracy of the printed parts by holding the deposited strands in place. However, mechanical performance may sometimes be affected when cold air is drawn into the print area using fans [38]. Table 3 provides a reference guide for the optimal nozzle and build platform temperatures based on different materials [39].

Infill Pattern and Density
The right percentage of infill reduces the amount of print time and material usage, but this needs to be balanced with the overall strength of the part that is required. The infill pattern and infill density influence the mechanical performance of printed parts. In general, the mechanical performance of the printed part reduces when the infill density is lower. Slicing software such as Slic3r PE by Prusa Research offers end-users with up to 13 types of infill patterns as shown in Figure 3. The infill density can be set between a value of 0% and 100%, with 0% being hollow and 100% as being completely solid.
to be balanced with the overall strength of the part that is required. The infill pattern and infill density influence the mechanical performance of printed parts. In general, the mechanical performance of the printed part reduces when the infill density is lower. Slicing software such as Slic3r PE by Prusa Research offers end-users with up to 13 types of infill patterns as shown in Figure 3. The infill density can be set between a value of 0% and 100%, with 0% being hollow and 100% as being completely solid.

Rectilinear
Grid   The common infill percentage used is 20%, enough to support flat ceilings with most thermoplastics. A 40% infill is usually sufficient to provide good mechanical strength, while 80% can be used to generate a tighter filled model for higher strength by using more material. [20]. The higher infill density generally provides better mechanical properties, although the choice of infill pattern can also make a difference. For example, the tensile properties of ABS have been investigated on printed specimens with 100% rectilinear, honeycomb and concentric infill patterns and it was observed that the rectilinear patterns provide the best results with the strongest parts, followed by the concentric pattern and the honeycomb pattern. When the density is reduced, a better tensile strength was observed with the honeycomb pattern for ABS specimens [52]. Similar results have been reported for PLA with tensile specimens filled to 10%; at this low infill, the linear pattern had the highest maximum strength, followed by hexagonal, and diagonal patterns. However, the elongation at break of the linear and diagonal was not significantly different and the hexagonal infill was significantly lower [53]. For PMMA specimens, it was observed that the 3D honeycomb infill pattern outperforms the rectilinear and gyroid infill patterns for infill densities from 30% to 70% when referring to impact resistance [54]. Therefore, selecting the infill parameters depends on the application of the part.

Additional Structures
A key factor that can help improve the print quality of ME parts is to implement features such as raft, brim and skirt ( Figure 4). The use of rafts and brims help to stabilize the printed part with a small surface area or footprint to improve adhesion and to reduce warping and delamination. A raft is made up of a predetermined layer of material with an adjustable infill percentage. It is first printed with a specific layer depth and with an optimal separation distance between the sitting model to aid part removal when the object cools. It can also be used to compensate for small inaccuracies in the build platform or for warped bed surfaces, scratches or dents [55]. The disadvantage of using a raft is that the model may be difficult to remove if the settings are not properly chosen. The lower layers may also have a different finishing result as compared to parts produced from printing directly onto the platform. A raft also consumes slightly more material and thus generates additional waste. As a result, the brim feature is often preferred [55]. A skirt consists of several layers of an outline that is offset away from the model. It is mainly used to ready or prime the extruder, and to also enable a final check for bed levelling issues and the quality of the filament being extruded before the print process commences. A brim feature is similar to a skirt, but it has a connected offset that is outward from the perimeter of the model. The brim can be used to prime the extruder as well as to enlarge the bottom surface area and hold down the edges of the model to improve the build platform adhesion. In the case of warping, the corners of the model are less likely to curl up or deform because the brim is attached to it. This is particularly useful when using materials that have a large shrinkage factor such as ABS or with semi crystalline polymers (e.g., PA12). The brim width is determined by the number of skirts that are built around the model [56].
is made up of a predetermined layer of material with an adjustable infill percentage. It is first printed with a specific layer depth and with an optimal separation distance between the sitting model to aid part removal when the object cools. It can also be used to compensate for small inaccuracies in the build platform or for warped bed surfaces, scratches or dents [55]. The disadvantage of using a raft is that the model may be difficult to remove if the settings are not properly chosen. The lower layers may also have a different finishing result as compared to parts produced from printing directly onto the platform. A raft also consumes slightly more material and thus generates additional waste. As a result, the brim feature is often preferred [55]. A skirt consists of several layers of an outline that is offset away from the model. It is mainly used to ready or prime the extruder, and to also enable a final check for bed levelling issues and the quality of the filament being extruded before the print process commences. A brim feature is similar to a skirt, but it has a connected offset that is outward from the perimeter of the model. The brim can be used to prime the extruder as well as to enlarge the bottom surface area and hold down the edges of the model to improve the build platform adhesion. In the case of warping, the corners of the model are less likely to curl up or deform because the brim is attached to it. This is particularly useful when using materials that have a large shrinkage factor such as ABS or with semi crystalline polymers (e.g., PA12). The brim width is determined by the number of skirts that are built around the model [56].

Raft Brim Skirt
A raft is a horizontal latticework surface that sits under the print part with a specific distance away from the sides of the object.
A brim is a single layer flat area around the base of the model to prevent warping.
A skirt is a printed line that is offset around the object on the first layer.  An example of how a brim can be used to keep a part attached to the build platform during the printing of PA12 on a glass surface is shown in Figure 5. PA12 is a semi crystalline polymer which shrinks significantly when it cools down. The shrinkage is strong enough to cause the part detaching from the build platform when only using a skirt ( Figure 5A). If a brim is used, the part can be printed more successfully ( Figure 5B).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 29 An example of how a brim can be used to keep a part attached to the build platform during the printing of PA12 on a glass surface is shown in Figure 5. PA12 is a semi crystalline polymer which shrinks significantly when it cools down. The shrinkage is strong enough to cause the part detaching from the build platform when only using a skirt ( Figure 5A). If a brim is used, the part can be printed more successfully ( Figure 5B).

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the Appl. Sci. 2020, 10, 4776 8 of 28 lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Weaker Resultant Part Stronger Resultant Part
According to the general rule of thumb for ME, avoidance or minimal use of support structures is always recommended to reserve the best cosmetic surfaces. Strategies such as splitting a model, limiting the degree of overhang, minimizing overhangs and changing the build directions can avoid the use of a support structure which can help to reduce material usage, cost, increase the print speed, improve the strength and the final print quality of ME parts (Table 5) [59]. When a part alteration is not possible, an alternative way of eliminating overhanging features that require a large amount of

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Weaker Resultant Part Stronger Resultant Part
According to the general rule of thumb for ME, avoidance or minimal use of support structures is always recommended to reserve the best cosmetic surfaces. Strategies such as splitting a model, limiting the degree of overhang, minimizing overhangs and changing the build directions can avoid the use of a support structure which can help to reduce material usage, cost, increase the print speed, improve the strength and the final print quality of ME parts (Table 5) [59]. When a part alteration is not possible, an alternative way of eliminating overhanging features that require a large amount of

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Weaker Resultant Part Stronger Resultant Part
According to the general rule of thumb for ME, avoidance or minimal use of support structures is always recommended to reserve the best cosmetic surfaces. Strategies such as splitting a model, limiting the degree of overhang, minimizing overhangs and changing the build directions can avoid the use of a support structure which can help to reduce material usage, cost, increase the print speed, improve the strength and the final print quality of ME parts (Table 5) [59]. When a part alteration is not possible, an alternative way of eliminating overhanging features that require a large amount of

Build Orientation
Due to the anisotropic nature of the ME processes, it is crucial to position the CAD model at the right orientation to achieve the most optimal mechanical performance. Parts printed using ME processes have inherently weaker tensile properties when the load is applied perpendicular to the direction of the deposition of strands. For example, ABS specimens tested perpendicular to the layers had only 74% to 79% of the tensile strength of specimens tested along the layers [58]. This is due to the lack of continuous material paths and the stress concentration created by each layer creates weaknesses where cracks are likely to form. The tensile strength is better when the load is applied perpendicular to the deposition direction of the strands as shown in Table 4 [2]. Table 4. Build direction and its effects on part weakness [2].

Weaker Resultant Part Stronger Resultant Part
According to the general rule of thumb for ME, avoidance or minimal use of support structures is always recommended to reserve the best cosmetic surfaces. Strategies such as splitting a model, limiting the degree of overhang, minimizing overhangs and changing the build directions can avoid the use of a support structure which can help to reduce material usage, cost, increase the print speed, improve the strength and the final print quality of ME parts (Table 5) [59]. When a part alteration is not possible, an alternative way of eliminating overhanging features that require a large amount of According to the general rule of thumb for ME, avoidance or minimal use of support structures is always recommended to reserve the best cosmetic surfaces. Strategies such as splitting a model, limiting the degree of overhang, minimizing overhangs and changing the build directions can avoid the use of a support structure which can help to reduce material usage, cost, increase the print speed, improve the strength and the final print quality of ME parts (Table 5) [59]. When a part alteration is not possible, an alternative way of eliminating overhanging features that require a large amount of support is to split the design model into half. The sections can be later joined at the post-processing stage. Table 5. Strategies to avoid support structure.

Support Needed Support Not Needed
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 29 support is to split the design model into half. The sections can be later joined at the post-processing stage. Table 5. Strategies to avoid support structure.

Support Needed Support Not Needed
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 29 support is to split the design model into half. The sections can be later joined at the post-processing stage. Table 5. Strategies to avoid support structure.

Support Needed Support Not Needed
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 29 support is to split the design model into half. The sections can be later joined at the post-processing stage. Table 5. Strategies to avoid support structure.

Support Needed Support Not Needed
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 29 support is to split the design model into half. The sections can be later joined at the post-processing stage. Table 5. Strategies to avoid support structure.

Support Needed Support Not Needed
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60].
During the inherent process, ME has some build limitations and constraints on what can be printed. It is therefore important to refer to existing design rules to ensure a successful print. Table 6 summarizes the recommendations when printing key features using the ME process [2,60]. Table 6. Design rules for ME [2,60].
Designing Key Features for ME Printing Table 6. Cont.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Troubleshooting of ME Parts
This section discusses the most common problems encountered with the ME processes which have been categorized according to (3.1) printer-associated, (3.2) deposition-associated and (3.3) print quality issues (Table 7). This section also includes recommendations to overcome print failures and to improve print reliability. These results are derived using the Original Prusa i3 MK3 MR printer, Wanhao i3 duplicator and a Hage3D printer, as well as from existing literature [2,[61][62][63][64][65][66][67]. The recommendations are not printer-specific and can be applied to most Cartesian based ME machines.

Not Extruding at Start
The extruder is not extruding the material at the beginning of the print. This may be due to the extruder not being properly primed before the beginning of the print. It occurs when the nozzle height is set too low or too close to the build platform, the filament is stripped and/or having a clogged nozzle. Solutions include the following:

1.
Drawing a skirt to prime the extruder before starting the print.

3.
Check if there is a lot of plastic shavings caused by the drive gears (refer to 3.1.6: Grinding filament).

4.
Unclog the nozzle and check if there is any foreign debris or plug inside the nozzle. Clean the nozzle using a brass wire brush or replace with a new nozzle (refer to 3.1.4: Clogged nozzle).

Poor Adhesion of the First Layer
Having a successful first layer that adheres to the build platform is crucial to ensure a good print. The first layer of print that does not stick to the build platform may be due to an incorrect built platform levelling sequence, nozzle height set too high, first layer printing speed too high, and/or poor build platform adhesion due to the surface grease or residues from previous prints [37,68]. Solutions include the following:

1.
Recalibrating the built platform through active levelling or manual levelling. Run through the first layer calibration to adjust the Z-height to get the optimum distance between the nozzle and the first layer as shown in Figure 6 [63,67].

2.
Decrease the first layer print speed. If the first layer speed is set at 50%, it could mean that the first layer will print 50% slower than the rest of the sections.

3.
Disable the cooling fan for the first few layers. Ensure that the recommended bed temperature is used.

4.
Increase the nozzle temperature by 5 • C for better adhesion or as recommended in Table 3.

5.
Check the extrusion settings to ensure that the right amount of plastic is extruded. 6.
Wipe the build surface using 90% isopropyl alcohol (IPA) solution or Acetone to remove residues from previous prints and to keep the surface grease-free. If the problem continues, improve the surface adhesion such as with a PEI sheet, blue painter's tape for PLA, Kapton Tape for ABS or separate agent coatings such as glue stick or hair spray for PETG. Check if the material requires a special surface such as when using PP filaments [3,69]. 7.
Increasing the roughness of the surface of the build platform by sanding or creating cleated surfaces [37]. 8.
Printing an additional structure like a brim or a raft, as discussed in Section 2.3. The extruder is not extruding the material at the beginning of the print. This may be due to the extruder not being properly primed before the beginning of the print. It occurs when the nozzle height is set too low or too close to the build platform, the filament is stripped and/or having a clogged nozzle. Solutions include the following: 1. Drawing a skirt to prime the extruder before starting the print. 2. Readjust the first layer height by running through the first layer calibration [63,67] (refer to 3.1.2: (1) Incorrect build platform levelling). 3. Check if there is a lot of plastic shavings caused by the drive gears (refer to 3.1.6: Grinding filament). 4. Unclog the nozzle and check if there is any foreign debris or plug inside the nozzle. Clean the nozzle using a brass wire brush or replace with a new nozzle (refer to 3.1.4: Clogged nozzle).

Poor Adhesion of the First Layer
Having a successful first layer that adheres to the build platform is crucial to ensure a good print. The first layer of print that does not stick to the build platform may be due to an incorrect built platform levelling sequence, nozzle height set too high, first layer printing speed too high, and/or poor build platform adhesion due to the surface grease or residues from previous prints [37,68]. Solutions include the following: 1. Recalibrating the built platform through active levelling or manual levelling. Run through the first layer calibration to adjust the Z-height to get the optimum distance between the nozzle and the first layer as shown in Figure 6 [63,67]. 2. Decrease the first layer print speed. If the first layer speed is set at 50%, it could mean that the first layer will print 50% slower than the rest of the sections. 3. Disable the cooling fan for the first few layers. Ensure that the recommended bed temperature is used. 4. Increase the nozzle temperature by 5 °C for better adhesion or as recommended in Table 3. 5. Check the extrusion settings to ensure that the right amount of plastic is extruded.

Stops Extruding during Mid Print
If the printer stops extruding during the middle of a print, this could be a result of running out of filament, a snapped or stripped filament against the drive gear, mid-print clogged nozzle, an overheated extruder motor driver, and/or a file or CAD file or G-code error. Solutions include the following:

1.
Replacing a new spool of filament. Newer and more expensive printers have a position sensor that stops the printing job when no filament is detected.

2.
Replace the snapped filament and/or reduce the idler tension by loosening the filament if it is too tight or if there is a lot of plastic shavings or debris caused by the drive gears (refer to 3.1.6: Grinding filament).

3.
Remove the filament and run through a diagnostics test to confirm that fans are functioning properly. Ensure that the hot-end fan is attached properly. It should direct the air inside while spinning. Check that all wires are still connected to the correct place.

4.
Ensure that the printing temperature is correct for the loaded filament. Make minor temperature adjustments of ±5 • C. Alternatively, use a different filament to check if the clog is caused by the current filament that could be out of acceptable tolerance.

5.
Turn off the printer to allow the electronics to cool down and check the cooling fan. Overheated motors can cause the printer to shut down. 6.
Check the generated G-code, preview the slices using the slicing software. Repair any errors on the .STL File, or upload a new file if necessary.

Clogged Nozzle
A sign of clogged nozzle is that the filament is only extruding a little and does not curl nicely or do not extrude anything at all. It is usually caused by an incorrect print temperature that results in the material buildup, due to a malfunctioning fan or build-up of old materials, especially when dealing with composite materials. Solutions include the following: 1.
If the filament is not extruding at all, heat the nozzle to the recommended print temperature for the loaded filament. Push a small needle into the nozzle and repeat until the plug unclogs. Reload a new filament to observe if the filament can extrude smoothly.

2.
Perform a cold pull [70]. If the filament is pouring out a little or does not curl nicely, heat up the nozzle according to the material used (Table 3). Wait for 5 to 10 min and load the filament again.
The new filament should push out the stuck material from the nozzle.

3.
Check that there are no loose or broken wires from the main-board to the extruder assembly, check the transistors of the main-board. Replace the wires or the main-board if required [71].

4.
When the clog is due to material build-up, replace the nozzle with a new one; clean the old one with a suitable solvent.

Caked Nozzle or Extruder Blob
Caked or extruder blobs that occur within the first five minutes of the print could be due to the nozzle catching on a lifted piece of print. Solutions include the following:

1.
Monitoring the print for the first five to ten minutes before leaving the print to make sure that the first layer is sticking well onto the build platform.

2.
To remove the extruder blob, heat the nozzle according to the material used. This allows the block of plastic to soften up while carefully pulling out the blob. It is important to be extra cautious around the thermistor wires when doing this.

3.
Use a plier and a brass brush to thoroughly clean the nozzle.

Grinding Filament
Grinding or stripping of filament refers to excessive shaving of plastics, causing the gear teeth to have little material to grab onto (Figure 7). The contributing factors include if the retraction setting is set too fast, the printing temperature is too low, the printing speed is too high, overtightened extruder, dirty extruder gears, jammed filament tubes, and/or a clogged nozzle. Solutions include the following:

1.
Reducing the retraction speed by 50%. Excessive stress will be applied on the extruder if the retraction speed or the distance is set too high.

2.
Increase the nozzle temperature by 5 to 10 • C to reduce the viscosity and better flow of the material.

3.
Use a slower printing speed or decrease the default printing speed by 50%. This reduces the rotation of the extruder motor which may be the cause of the grinding issue.

4.
Make sure that the tension of the extruder idler wheel is correct as an over-tightened idler cannot push the filament towards the nozzle [72].

5.
Remove any plastics residue with pliers or using a pin. 6.
Open the extruder idler wheel to inspect if there is any debris in the tube that may inhibit the flow of filament. 7.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 29 1. Monitoring the print for the first five to ten minutes before leaving the print to make sure that the first layer is sticking well onto the build platform. 2. To remove the extruder blob, heat the nozzle according to the material used. This allows the block of plastic to soften up while carefully pulling out the blob. It is important to be extra cautious around the thermistor wires when doing this. 3. Use a plier and a brass brush to thoroughly clean the nozzle.

Grinding Filament
Grinding or stripping of filament refers to excessive shaving of plastics, causing the gear teeth to have little material to grab onto (Figure 7). The contributing factors include if the retraction setting is set too fast, the printing temperature is too low, the printing speed is too high, overtightened extruder, dirty extruder gears, jammed filament tubes, and/or a clogged nozzle. Solutions include the following: 1. Reducing the retraction speed by 50%. Excessive stress will be applied on the extruder if the retraction speed or the distance is set too high. 2. Increase the nozzle temperature by 5 to 10 °C to reduce the viscosity and better flow of the material. 3. Use a slower printing speed or decrease the default printing speed by 50%. This reduces the rotation of the extruder motor which may be the cause of the grinding issue. 4. Make sure that the tension of the extruder idler wheel is correct as an over-tightened idler cannot push the filament towards the nozzle [72]. 5. Remove any plastics residue with pliers or using a pin. 6. Open the extruder idler wheel to inspect if there is any debris in the tube that may inhibit the flow of filament. 7. Unclog the nozzle (refer to 3.1.4: Clogged nozzle).

Extrusion Away from Previous Layers: Spaghetti Monster
A spaghetti monster occurs when the model detaches from the build platform during midprinting and the deposited material stops sticking to the object (Figure 8). This could be a caused by poor build platform adhesion and/or a CAD file or G-code error. Solutions include the following:

Extrusion Away from Previous Layers: Spaghetti Monster
A spaghetti monster occurs when the model detaches from the build platform during mid-printing and the deposited material stops sticking to the object (Figure 8). This could be a caused by poor build platform adhesion and/or a CAD file or G-code error. Solutions include the following:

1.
Make sure that the first layer sticks entirely onto the build platform. Increase the bed temperature by 5 • C for better adhesion (refer to 3.1.2: Poor adhesion of the first layer).

2.
Detect any CAD file or object error to observe if there is any gaps or broken geometry in the .STL file. Check the generated G-code, preview the slices using the latest slicing software. Repair any errors on the .STL file if required.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 29 1. Make sure that the first layer sticks entirely onto the build platform. Increase the bed temperature by 5 °C for better adhesion (refer to 3.1.2: Poor adhesion of the first layer). 2. Detect any CAD file or object error to observe if there is any gaps or broken geometry in the .STL file. Check the generated G-code, preview the slices using the latest slicing software. Repair any errors on the .STL file if required.

Under-Extrusion
Under-extruded areas will show visible gaps. There will be material missing in or between the

Under-Extrusion
Under-extruded areas will show visible gaps. There will be material missing in or between the print layers. Under-extruded objects will be fragile and may easily break. This is usually due to hardware problems such as a loose extruder gear, dirty extruder gears, poor quality filament with diameter fluctuations, incorrect extrusion multiplier setting, and/or slicing errors if manual adjustments have been made. Solutions include the following: 1.

2.
Clear any plastics with the sharp corner of a plier or with a durable pin (refer to 3.1.6: (5-6) Dirty extruder gears).

3.
Poor quality filaments may have inconsistent diameter tolerances that often lead to a clogged nozzle. Use a caliper to measure the consistency of the filament diameter. The diameter difference should not exceed ± 0.05 mm. Replace the filament or use a better-quality filament if necessary. 4.
Tune the extrusion multiplier settings only if the under-extrusion is not caused by a hardware issue. Increasing the extrusion multiplier consequently increases the flow of the filament into the extruder. Decreasing the extrusion multiplier will also slow down the flow of the filament into the extruder.

5.
Check the generated G-code, preview the slices using the latest slicing software. Repair any errors in the .STL file and make adjustments in slice settings if necessary.

Over-Extrusion
The printer is extruding more plastic than the software expects, thereby forming print blobs. Over-extrusion affects both dimensional accuracy and the aesthetics of the printed object. It could be due to high extrusion multiplier settings, and/or the printing speed set too high. Please note that sometimes a bit of over extrusion can be beneficial for the mechanical properties of the material since some of the porosity can be filled by over-extruding [73]. However, over-extrusion should be done carefully so as not to affect the dimensional accuracy of the part. Solutions include the following:

2.
Reduce the filament flow rate.

Overheating
The print looks melted or deformed. Overheating may be a result of insufficient heat dissipation, printing temperature set too high, and/or printing speed set too fast. Solutions include the following:

1.
Increasing the power of the cooling fan to allow faster cooling of the plastic. Alternatively, try multiple prints per batch to allow longer cooling time for each model, while printing subsequent models in the same bed.

2.
If the cooling fan is on, reduce the printing temperature by 5 to 10 • C.

3.
Reduce the printing speed, especially for small models to allow each layer to cool down properly.

Small Features Not Printed
Most ME printers use a 0.4 mm nozzle which will not generate a solid perimeter for extremely thin features with dimensions smaller than 0.4 mm. It is best to avoid setting the dimensions of a CAD model to be less than the nozzle size. Redesign the part to have thicker features above the minimum printable dimension. Solutions include the following: 1.
Increasing the dimension of the part to be more than the nozzle size.

2.
Enable single wall extrusion by selecting detect thin wall.

3.
Lower the print speed while printing intricate details such as text.

4.
Install a nozzle with a smaller tip size if the size of the model cannot be modified and re-slice the model with the new nozzle diameter.

Inconsistent Extrusion
Inconsistent extrusion of the material may result in a bumpy surface that may affect the quality and dimensional accuracy of the printed object ( Figure 9). The contributing factors could be tangled or stuck filament, a semi-clogged nozzle, nozzle height set too low, incorrect extrusion width, poor quality filament, and/or having an overtightened extruder idler wheel. Solutions include the following: 1.
Making sure that the filament is feeding into the printer. There should not be too much resistance to stop the filament spool from moving freely and pulling into the printer (Refer to 3.1.6: Grinding filament).

4.
Make sure that the extrusion width is set at 100% or 150% greater than the default nozzle diameter of 0.4 mm. The printer will struggle to extrude any width less than the nozzle diameter (refer to 3.2.4: Small features not printed).

5.
Make sure that the filament is not degraded. Replace the filament that may be at fault and use a better-quality filament (refer to 3.2.1: (3) Poor quality filament with diameter fluctuations). 6.
Make sure that the tension of the extruder idler wheel is correct (refer to 3.1.6: (4) Overtightened extruder idler wheel.
1. Increasing the power of the cooling fan to allow faster cooling of the plastic. Alternatively, try multiple prints per batch to allow longer cooling time for each model, while printing subsequent models in the same bed. 2. If the cooling fan is on, reduce the printing temperature by 5 to 10 °C. 3. Reduce the printing speed, especially for small models to allow each layer to cool down properly.

Small Features Not Printed
Most ME printers use a 0.4 mm nozzle which will not generate a solid perimeter for extremely thin features with dimensions smaller than 0.4 mm. It is best to avoid setting the dimensions of a CAD model to be less than the nozzle size. Redesign the part to have thicker features above the minimum printable dimension. Solutions include the following: 1. Increasing the dimension of the part to be more than the nozzle size. 2. Enable single wall extrusion by selecting detect thin wall. 3. Lower the print speed while printing intricate details such as text. 4. Install a nozzle with a smaller tip size if the size of the model cannot be modified and re-slice the model with the new nozzle diameter.

Inconsistent Extrusion
Inconsistent extrusion of the material may result in a bumpy surface that may affect the quality and dimensional accuracy of the printed object ( Figure 9). The contributing factors could be tangled or stuck filament, a semi-clogged nozzle, nozzle height set too low, incorrect extrusion width, poor quality filament, and/or having an overtightened extruder idler wheel. Solutions include the following: 1. Making sure that the filament is feeding into the printer. There should not be too much resistance to stop the filament spool from moving freely and pulling into the printer (Refer to 3.1.6: Grinding filament). 2. Remove any foreign debris in the nozzle (refer to 3.1.4.: Clogged nozzle). 3. Recalibrate the build platform (refer to 3.1.2: (1) Incorrect built platform levelling). 4. Make sure that the extrusion width is set at 100% or 150% greater than the default nozzle diameter of 0.4 mm. The printer will struggle to extrude any width less than the nozzle diameter (refer to 3.2.4: Small features not printed). 5. Make sure that the filament is not degraded. Replace the filament that may be at fault and use a better-quality filament (refer to 3.

Dimensional Accuracy
The measurement of the printed object differs from the CAD model. For example, the holes may be in the wrong size; parts do not align or do not fit together. Lack of dimensional accuracy creates a fitting problem while assembling other parts of the model. Inaccurate or incorrect dimensions could be due to incorrect measurements in the CAD model, not adhering to design rules, layer height issues, issues with the belt movement or loose pulley, and/or material shrinkage. Solutions include the following:

1.
Check the physical and working unit of measurements of the CAD model especially when using the .STL file format. Ensure that correct and identical unit selection (mm or cm) is used for the CAD and slicing software.

2.
Set proper tolerances such as 0. A 1-mm reduction in size could be due to material shrinkage after cooling. Calculate the percentage of error and increase the scale of the print to compensate for the shrinkage.

Shifted Layers or Leaning Prints
Layer shifting is a result of improper axis movement, causing the extruder to misalign during mid-print ( Figure 10). This includes a movement being blocked on the axis, a loose pulley causing the layer shift, and/or due to a geometry that is difficult to print. Solutions include the following: 1.
Check the tightness of X and Y belts. When plucked, the belts should sound like a low bass note. The back of the X-carriage should not be loose. Check and tighten all screws.

2.
Ensure that there is no obstruction in the path of the bearing, or any possible waste stuck around the belt.

3.
Check the tightness of X and Y pulleys. Ensure that the pulley on the motor shaft is secure and the idler pulley can move freely on the opposite end.

4.
Design according to the guidelines and rules for ME to avoid geometry that is difficult to print, particularly for models with large overhangs that tend to warp during mid-print.
The measurement of the printed object differs from the CAD model. For example, the holes may be in the wrong size; parts do not align or do not fit together. Lack of dimensional accuracy creates a fitting problem while assembling other parts of the model. Inaccurate or incorrect dimensions could be due to incorrect measurements in the CAD model, not adhering to design rules, layer height issues, issues with the belt movement or loose pulley, and/or material shrinkage. Solutions include the following:  (3) Loose pulley). 5. A 1-mm reduction in size could be due to material shrinkage after cooling. Calculate the percentage of error and increase the scale of the print to compensate for the shrinkage.

Shifted Layers or Leaning Prints
Layer shifting is a result of improper axis movement, causing the extruder to misalign during mid-print ( Figure 10). This includes a movement being blocked on the axis, a loose pulley causing the layer shift, and/or due to a geometry that is difficult to print. Solutions include the following: 1. Check the tightness of X and Y belts. When plucked, the belts should sound like a low bass note.
The back of the X-carriage should not be loose. Check and tighten all screws. 2. Ensure that there is no obstruction in the path of the bearing, or any possible waste stuck around the belt.

Lower Parts Caving in
The lowest parts of the print appear to shrink before reaching the proper dimension due to insufficient heat dissipation and/or an incorrect platform temperature. Solutions include the following: 1.
Increase the cooling fan power to allow faster cooling of plastic (refer to 3.1.2: (3) Incorrect bed temperature or cooling settings; 3.2.3: (1) Insufficient heat dissipation).

2.
Adjust the right bed temperature for proper layer adhesion to the build platform and to prevent the print from shrinking before reaching the intended dimensions (Table 3).

Skipped Layers
Some layers are shown to be missing and visible gaps appear on the printed object. This could be due to under-extrusion because of an inconsistent diameter of the filament, extruder issues caused by the feeder wheel, misaligned or shifted rod, a worn bearing, and/or a semi-clogged nozzle. Solutions include the following: material. As the extruder starts to extrude, it will only push 0.8 mm of plastic into the nozzle. Adjust this setting until the defect no longer occurs when the extruder initially begins printing the perimeter [78]. 4. If blobs appear at the end of a perimeter, turn on the wipe before retracting feature (Slic3r PE) or coasting (Cura) to enable the nozzle to move to the next starting point while the extruder retracts. Adjust the value between 0.2 and 0.5 mm [78].

5.
Select a more suitable seam position. Set the start point of the perimeter loop, by choosing either Random, Nearest or Aligned positions [74,79].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 22 of 29 Blobs and zits are marks that appear on the outer shell surface of the part, at times within a specific location where the nozzle starts to deposit and return ( Figure 13). Examine the deposition to check if the blobs appear at the starting point or at the end of each layer. Solutions include the following: Figure 13. Blobs and zits marks on outer shell surface of the part.
1. Having proper retraction and coasting settings. 2. Selecting the best location for the deposition point. 3. Blobs that appear in the beginning could indicate that the extruder is priming too much plastic.
Use a higher retraction setting with an extra length when restarting the print. This option determines the retraction distance when the extruder is stopping as well as the priming distance used when the extruder restarts. Reduce the priming distance by adding a negative value for the extra length during restart. For example, if the retraction distance is 1.0 mm, and the extra length of restart distance is −0.2 mm, each time when the extruder stops, it retracts 1.0 mm of material. As the extruder starts to extrude, it will only push 0.8 mm of plastic into the nozzle. Adjust this setting until the defect no longer occurs when the extruder initially begins printing the perimeter [78]. 4. If blobs appear at the end of a perimeter, turn on the wipe before retracting feature (Slic3r PE) or coasting (Cura) to enable the nozzle to move to the next starting point while the extruder retracts. Adjust the value between 0.2 and 0.5 mm [78]. 5. Select a more suitable seam position. Set the start point of the perimeter loop, by choosing either Random, Nearest or Aligned positions [74,79].

Irregular Circles
Printed circles may appear misshapen, oval or lines that are not properly touching. Irregular circles may be a result of an alignment issue, different belt tensions, differing step values entered for the X and Y axes, and/or blobs affecting the shape. Solutions include the following: 1. Checking the unit measurements used in the slicing software to ensure all axes have the correct input. 2. Check the Z-scar to prevent blobs altering the circle shape (refer to 3.3.13: Blobs and zits). 3. Refer to 3.2.7: Shifted layers or leaning print, to counteract alignment issues.

Vibration and Ringing
Visual waves or rippling effect can sometimes be seen on the print surface, also known as echoes in print is usually caused by vibration due to an unsteady 3D printer, worn linear bearings, loose print heads, unsmooth axis movement, and/or print speed set too high. Solutions include the following: 1. Placing the printer on a sturdy surface to reduce vibration.

Irregular Circles
Printed circles may appear misshapen, oval or lines that are not properly touching. Irregular circles may be a result of an alignment issue, different belt tensions, differing step values entered for the X and Y axes, and/or blobs affecting the shape. Solutions include the following: Appl. Sci. 2020, 10, x FOR PEER REVIEW 25 of 29 Figure 14. The most common problems of ME processes and their causes. Figure 14. The most common problems of ME processes and their causes. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.