Material extrusion additive manufacturing

1.Principle & Definition:

Material Extrusion (Material Extrusion 3D Printing) refers to a process where material is continuously extruded in a controlled manner and stacked layer by layer to form a three-dimensional solid structure.
This technology is one of the most widely used 3D printing processes globally, and it can be further categorized by material form:

1.         FFF / FDM: Uses thermoplastic filaments.

2.         FGF: Uses granules as raw material.

3.         DIW: Uses high-viscosity pastes or fluid inks.

4.         Cementitious Extrusion: Uses mortar, cement, or geopolymers.

Fig. Material Extrusion 3D Printing. A spool of material (usually ABS or PLA) is fed by drive gears into a heated block, where the material is heated to a molten state. The molten material is then extruded through a nozzle and deposited onto the build platform or the underlying layer. Each layer of the model cools and solidifies after extrusion, forming the final solid model layer by layer [1].

1.     A.  Mechanism for General Filaments:

The material is gripped by a drive gear assembly and pushed at a constant rate into the guide tube and hot end. This mechanism can be a Bowden system or a Direct Drive system. In the Bowden system, the motor is located far from the hot end, feeding the filament through a long tube, which reduces moving load but results in greater response lag. In contrast, the Direct Drive system has the motor directly above the hot end, offering high precision and suitability for flexible materials.

Figure 2. Bowden system (left) & Direct Drive system (right) [2].

The hot end consists of a heating block, a nozzle, and a thermal insulation zone. After the material enters, it is heated above its glass transition temperature in the melting zone (typically 180 ~ 350°C), and then extruded with precise flow control by the extruder. Printing speed is jointly controlled by the extruder motor and the movement speed of the equipment mechanism, aiming to achieve maximum production efficiency while ensuring stable filament diameter and consistent layer thickness [3].

Figure 3. Hot End. This cross-section shows the various components of the hot end [4].

B.  Pellet Material Feeding Mechanism:

Pellet Extrusion is centered around the screw extrusion system. Common forms include single-screw systems, which have a simple structure and are suitable for pure materials, and twin-screw systems, which offer better mixing and dispersion capabilities, making them suitable for carbon and glass fiber composite materials. After passing through a filter screen and flow channel for pressure stabilization, the molten material is deposited onto the forming platform through a nozzle at a controlled flow rate. The nozzle diameter generally ranges from 0.8 to 4 mm and can be adjusted according to material viscosity and output requirements.

Figure 6. Schematic diagram of multi-feed extruder (a), co-rotating twin-screw extruder head (b), and twin-screw 3D printer (c) [10].

Pellets enter the barrel from the hopper, and the rotating screw generates thrust and shear heat, heating the material to a molten state. The barrel is generally divided into three sections: the solid conveying zone, the melting and mixing zone, and the metering and pressure-stabilizing zone. Through screw compression ratio and heater control, the material temperature and viscosity can be kept stable [8].

Figure 7. Pellet Extrusion. Schematic diagram of various types of fused granular fabrication extruder structures. (A) Extruder chamber directly fed by pellets; (B) Pellets melted in a piston/plunger-style chamber; (C) Pellets melted in a storage tank. [9]

2.     Motion Control Platform:

A.  Common Mechanisms for Small Equipment:

The relative motion between the extrusion module and the forming platform is achieved by a three-axis servo control system. Common structural systems include the following three types:

(1)    Cartesian type: The XYZ axes move independently. This is the simplest and clearest mechanical structure for Material extrusion and also the easiest to maintain. Therefore, it was the most common architecture for household 3D printers when 3D printing was widely adopted.

(2)    Delta type: The hot end position is controlled by three-arm linkages. Due to the lighter moving mass and simultaneous pulling by three-axis motors, it has the potential for high-speed printing.

(3)    CoreXY/H-bot: This type uses planar cross-belt drives for the extrusion module. The workpiece does not move in the XY plane within the equipment. Its structural design maximizes printing space utilization, making it the most common equipment drive structure today. In terms of power, it is divided into Hybrid enhanced power type and IDEX independent dual nozzle types. Therefore, this structure has the potential to withstand extremely high acceleration and forces and provides optimal printing stability for the workpiece. [5]

Figure 4. Motion Control System [6].

Cartesian type (left), Delta type (middle), CoreXY type (right)0

B.  Common Mechanisms for Large Equipment:

Pellet Extrusion typically uses a CNC three-axis platform or a six-axis robotic arm to control the extrusion path. The G-code generated by slicing software specifies the movement trajectory, speed, and extrusion rate. High-end equipment can be equipped with closed-loop control and thermal infrared monitoring to ensure consistent interlayer bonding and dimensional stability.

Figure 7. Main motion controls based on a gantry system (left) and a robotic arm system (right) [11].

3.     Forming Platform and Cooling System:

After extrusion, the material is deposited on a heated forming platform. Heating the baseplate enhances first-layer adhesion and prevents warping. Cooling fans control the material solidification rate, affecting interlayer adhesion and surface quality. High-temperature engineering materials (such as PC, PEEK) require an enclosed heated chamber to maintain thermal equilibrium [7].

Figure: Machine Cooling System

4.     Overview of the Forming Process:

(1)Generate 3D files: Download from open-source platforms or create your own with CAD software.

(2)Slicing software processing: The 3D model is processed by slicing software to generate G-code (equipment motion control code).

(3)Preheating stage: Nozzle and platform heating.

(4)Bed leveling: Adjust the relative height of the nozzle and the bed.

(5)Material first undergoes purge extrusion.

(6)Nozzle moves along the trajectory, stacking layer by layer.

(7)Cooling and solidification completed.

二、Advantages:

Material Extrusion (ME) technology, with its low cost and easy material availability, has become the most widespread form of additive manufacturing. Due to the relatively low price of equipment and consumables, users can enter the application with a very low threshold, enabling the technology to quickly expand from education and research to makers and small and medium-sized enterprises, promoting the widespread adoption of 3D printing [12].

The greatest appeal of this technology lies in the real-time connection between design and manufacturing. Users can transform digital models into physical prototypes in a short time, and through rapid printing àtesting àmodification cycles, effectively shortening development cycles and costs [13]. This highly efficient iterative process transforms product development from a linear stage to a dynamic verification process, significantly accelerating innovation.

Figure: By eliminating the mold-making step, designers can use rapid prototyping technology to experiment and explore product functionality and design [30].At the same time, ME offers a high degree of material and structural flexibility. By adjusting material types and printing parameters, components with different mechanical and aesthetic properties can be produced [8]. In metal applications, because the powder is encapsulated in a polymer matrix within the filament, process safety is significantly higher than traditional Powder Bed Fusion (PBF) systems, effectively avoiding dust dispersion and explosion risks [7].

Figure: 3D printing for small batch production [31].Furthermore, is particularly suitable for rapid prototyping and small batch production. It can produce multiple parts at once or integrate multiple structures, reducing subsequent assembly processes and thus improving overall production efficiency [14]. With advancements in multi-material extrusion technology, FFF can also combine rigid, flexible, and conductive materials in a single process to form composite structures with both mechanical and functional properties [3]. Moreover, the emergence of large FFF systems has broken through printing size limitations, allowing for the production of structural components several meters long, such as boat hulls, walls, and furniture, demonstrating its potential for applications in architecture and large-scale manufacturing [5]. Thus, FFF not only offers advantages in cost and flexibility but also possesses the characteristic of high open-source community resources, providing infinite expansion possibilities and gradually becoming one of the important technologies driving the popularization of digital manufacturing.

三、Common Features and Characteristics of Processed Products: process is its layer-by-layer stacking principle. While this layered construction offers high design flexibility, it also results in discernible layer lines and the stair-stepping effect. When the layer thickness is large or the surface inclination angle is small, noticeable layer lines and stepped contours appear on the part's surface, forming what is known as the stair-stepping effect. This phenomenon not only affects the aesthetic quality but also poses potential limitations in optical and airtight applications, often requiring improvement through reducing layer thickness, adjusting infill angles, or post-processing (such as polishing, coating, or vapor smoothing) [5].

Figure: General 3D printing process parameters (left figure); stair-stepping effect caused by layer-by-layer stacking (right figure) [32].

Mechanically, extruded material generally exhibits significant anisotropy. Since interlayer bonding primarily relies on thermal diffusion and recrystallization between molten lines, the fusion between layers is incomplete. This means that the strength perpendicular to the printing layers (Z-axis) is typically only 40–60% of the strength in the planar direction (XY-axis), leading to reduced tensile and impact resistance [13]. This "interlayer fragility" is one of the key factors affecting the mechanical reliability of FFF.

Figure: Schematic diagram illustrating how materials bond between layers (or interfaces) through thermal diffusion, melting, and recrystallization [33].

Figure: The strength of 3D printed parts depends on the relationship between the direction of force and the stacking direction [35].

In addition, the temperature gradient generated during the cooling of molten material can cause significant differential thermal shrinkage, leading to warping and insufficient adhesion. This phenomenon is particularly noticeable in semi-crystalline materials (such as ABS, PA, PP), often resulting in corner warping and dimensional errors. To suppress this effect, it is necessary to maintain thermal stability through heated build chambers, build plate adhesives, and appropriate temperature control strategies [8].

Another common defect is internal porosity and voids. When extruded lines do not fully fuse, or when gas bubbles remain within the material, micropores and voids form inside the part, leading to reduced density and shortened fatigue life. Increasing extrusion temperature, controlling flow stability, and optimizing path overlap can effectively reduce porosity [3].

Figure: Under-extrusion voids [34]. (a) Schematic diagram of the formation of under-extrusion voids. The area marked with ▲ represents the under-extrusion void formed between the outer perimeter and the internal infill lines; (b) Schematic diagram of a typical rectangular FFF layer; (c) SEM micrograph of the top surface of an FFF part with under-extrusion voids (s); (d) Cross-sectional image of the specimen, with red arrows pointing to under-extrusion voids.

Overall, the surface roughness and dimensional accuracy of FFF parts are generally limited by layer thickness control, nozzle precision, and material shrinkage behavior. The surface roughness typically falls within the Ra 10–25 μm range, which is higher than that of photopolymerization (SLA) and powder bed fusion (SLS) processes [5]. Therefore, this technology is more suitable for applications requiring medium-to-low precision parts, functional prototypes, or those that undergo post-processing.

IV.Public Law Categories:

1.   Fused Filament Fabrication:

A.  Definition and Description:

Fused Filament Fabrication (FFF) is an additive manufacturing technology that primarily uses thermoplastic polymers.

Its basic principle is to feed polymer filament at a controlled speed into a heated nozzle, where it melts at high temperatures and is extruded through the nozzle's small opening. The material is deposited layer by layer onto a build platform according to a computer-generated path. After cooling and solidifying, the layers are stacked to form a three-dimensional object. According to the ASTM F2792 standard, FFF is categorized as "an additive manufacturing technology that builds objects by selectively depositing molten material."

B.  Available Materials:

(1)     Categories of usable materials

(a)  Commodity Thermoplastics

PLA (Polylactic Acid): Easy to print, low shrinkage, environmentally friendly, suitable for educational and display models.

ABS (Acrylonitrile Butadiene Styrene): Good toughness, high heat resistance, commonly used for mechanical parts and casings.

PETG (Polyethylene Terephthalate Glycol-modified): Combines the toughness of ABS and the stability of PLA, suitable for transparent and outdoor applications.

(b)  Engineering Thermoplastics

PC (Polycarbonate): High rigidity, high heat resistance (>120°C), suitable for structural and optical parts.

PA (Nylon, PA6 / PA12): Good toughness, wear resistance, used for mechanical parts and connectors.

TPU (Thermoplastic Polyurethane): High elasticity, commonly found in shock-absorbing pads, shoe soles, and medical protective gear.

(c)  Functional & High-Performance Materials

PEEK (Polyether Ether Ketone): High strength, heat resistance up to 300°C, excellent chemical resistance, used in aerospace and automotive industries.

PEI / ULTEM (Polyetherimide): Flame-retardant and electrically stable, commonly used in electronics and aerospace components.

PPS (Polyphenylene Sulfide): Excellent chemical and heat resistance, suitable for high-temperature fluid systems and industrial applications.

(d)  Composite Materials:

CF / GF Short Fiber Composite Materials: Carbon fiber or glass fiber filled materials provide high rigidity and dimensional stability, used in automotive, racing, and fixtures.

CF Long Fiber Composite Materials: (Markforged's exclusive patented technology)

(e)  Conductive, Antistatic and Metal-Filled Materials

Conductive carbon black / copper powder: Can be used for sensor housings and electrostatic protection components.

Metal fillers: After printing, debinding and sintering can produce

pure metal parts, which is the core of Metal FFF or Bound Metal Printing applications.

(f)   Biomaterials and Medical-Grade Materials

PLA, PCL (Polycaprolactone): Biocompatible and biodegradable, used in tissue engineering and medical aids.

PETG-Med, PEEK-Med: Used for surgical guides, prosthetics, and implant prototype production.

C.  Application Examples and Applications:

(1)      Nervous System – Kinematics Dress

    The "Kinematics Dress" created by the American design team Nervous System using FFF technology is a one-piece foldable garment. It consists of thousands of interconnected joints forming a movable structural mesh, allowing the entire garment to be as soft as fabric yet maintain its three-dimensional shape. This work demonstrates the potential of FFF in complex geometry generation, flexible joints, and digital fashion, and is permanently collected by the Museum of Modern Art (MoMA) in New York, becoming a classic case of art and manufacturing combined.

(2)      Prusa Research – “Printed Farm Tools” Program

    The Czech Prusa Research community launched the "Printed Farm Tools" program in 2020, using open-source FFF printers to create farm tools, medical equipment, and educational aids. This project demonstrates the social value of desktop FFF systems in resource-limited areas, allowing manufacturing capabilities to be brought to educational and agricultural settings through easy maintenance, low-cost materials, and community-shared designs, becoming a model for open-source hardware applications in social innovation.

(3)      NASA – PEEK & PEI Components for CubeSat (2022, USA)

    In 2022, NASA adopted high-temperature FFF technology in its CubeSat project to print PEEK and PEI (Ultem) structural parts for satellite electronic module casings and structural brackets. These materials possess high temperature resistance and strength, enabling operation in the vacuum environment of space. This case demonstrates FFF technology's breakthrough from desktop prototyping to the manufacture of aerospace-grade functional components, establishing its practical value in space applications.

2.   Pellet Extrusion (Fused Granulate Fabrication) Pellet Extrusion

A.  Definition and Explanation:

Pellet Extrusion, also known as Fused Granulate Fabrication (FGF) or Pellet-based Material Extrusion, is a type of material extrusion additive manufacturing. Its basic principle is similar to FFF, differing in that the material form is changed from "filament" to "plastic pellets."

In this process, pelletized thermoplastic polymers are fed directly into a heated barrel via a hopper, melted by a screw, and extruded through a nozzle. The material is then built layer by layer according to a CAD model. This technology is derived from the combination of traditional injection molding and extrusion mechanisms, featuring high output rates, low material costs, and large-scale printing capabilities. ([3],[5],[8])

B.  Available Materials:

(1)  Commodity Thermoplastics

PLA (Polylactic Acid): Easy to process, low cost, biodegradable.

ABS (Acrylonitrile Butadiene Styrene): Good toughness, high heat resistance.

PP (Polypropylene): Good chemical stability, lightweight.

HIPS (High Impact Polystyrene): Often used as support material.

(2)  Engineering Thermoplastics

PA (Nylon): Wear-resistant, fatigue-resistant, suitable for gears and connectors.

PC (Polycarbonate): High rigidity and impact resistance, suitable for transparent structures and heat-resistant applications.

PETG (Polyethylene Terephthalate Glycol): Combines toughness and stability, commonly used for industrial prototypes and display models.

TPU (Thermoplastic Polyurethane): High elasticity and wear resistance, can print flexible or shock-absorbing structures.

(3)  High-Performance and Composite Materials

CF/PA, CF/PP, GF/ABS: Composite materials containing carbon fiber or glass fiber fillers, which can significantly enhance rigidity and dimensional stability.

PEEK, PEI, and PPS: High-temperature and high-strength materials with chemical and heat resistance (>250°C).

Short/long fiber reinforced materials: Can be uniformly mixed using a twin-screw extruder to achieve structural-level part strength.

(4)  Recycled and Sustainable Polymers

Recycled PET, recycled PP, recycled PLA, and other recycled plastics.

Ocean plastic recycled pellets are used in eco-friendly design works. This application contributes to sustainable manufacturing and circular economy practices.

(5)  Metal / Ceramic-Filled Pellets

By dispersing metal or ceramic powder in a polymer matrix, they can be printed as "Metal-Polymer Feedstock" or "Ceramic Green Body".

C.  Notable Works and Applications:

(1)  Local Motors – Strati Car (2014, USA)

In collaboration with ORNL (Oak Ridge National Laboratory), Local Motors of the USA used the BAAM (Big Area Additive Manufacturing) system to print the world's first driveable electric car, the Strati. The car body is primarily made of carbon fiber reinforced ABS pellets and took only 44 hours to form. This project became a milestone for the first successful application of FGF technology in automotive structural manufacturing, demonstrating that large-scale pellet extrusion can be used for functional part production [17].

(2)  Branch Technology – C-FAB Facade (2019, USA)

Branch Technology from the USA developed the C-FAB (Cellular Fabrication) system, which combines industrial robotic arms with screw extruders to print large lattice-style building facades using FGF technology. Its hollow honeycomb structure offers both strength and lightweight properties and is used for building facades, public installations, and art walls. This work demonstrates the innovative application of FGF in the manufacturing of complex curved surfaces and freeform architecture [18].

(3)  The New Raw – Print Your City (2019, Greece)

The Print Your City project, spearheaded by Greek design team The New Raw, uses recycled plastic granules as raw material and the FGF system to print urban furniture such as benches, planters, and trash cans. Each piece recycles approximately 8 kg of waste plastic, combining aesthetics, environmental protection, and civic participation. The project has become a symbol of the circular economy and sustainable design, demonstrating the practical contributions of FGF technology to social and environmental issues [19].

3.   Direct ink writing

A.        Definition and explanation:

Direct Ink Writing is an extrusion-based additive manufacturing technique. Its principle is to extrude high-viscosity inks with flowability and shear-thinning properties through a nozzle using pneumatic, piston, or screw systems, depositing them layer by layer along a computer-controlled path to form a three-dimensional structure.

The ink composition can include polymers, ceramics, metals, or bio-based materials, which are shaped through thermal curing, UV curing, chemical cross-linking, or solvent evaporation. The Direct Ink Writing (DIW) technology is characterized by its ability to simultaneously control material rheological behavior and curing rate, allowing for the creation of multi-material, gradient structures and functional composites ([20],[21]).

B.        Materials that can be produced and their applications

(1)   Polymeric Inks

Composition and properties:

    Polymeric inks typically consist of thermosetting or photocurable resins, solvents, and additives (thickeners, crosslinkers). Typical materials include solutions or UV curable resins. These inks exhibit shear-thinning properties, making them easy to flow through the nozzle while rapidly recovering viscosity after deposition to maintain their shape.

Curing methods: Thermal curing, UV light curing, solvent evaporation

Application fields: Microfluidic chips, flexible electronics,mechanically flexible structures, and soft robotics

(2)   Ceramic Inks

Composition and Properties:

    Composed of ceramic powders (e.g., , , , ) dispersed in a high-viscosity solution (e.g., or solution) to form colloidal inks. These inks must have the characteristic of "high concentration (>50 vol%) yet still flowable" to ensure high density and low shrinkage after forming.

Curing Methods: Solvent evaporation and drying, followed by high-temperature sintering

Applications: Structural ceramic components, porous scaffold materials, high-temperature thermal insulation structures, optical components

(3)   Metallic Inks

Composition and Properties:

Metallic inks are composed of metal nanoparticles ( ) or metal oxide precursors dissolved in organic solvents. They possess viscoelastic and conductive properties, allowing for deposition at room temperature, followed by annealing or reduction to form conductive structures.

Curing Methods: Thermal decomposition, chemical reduction, photocuring

Applications: Printed circuits, RF antennas, electrodes, sensors, flexible wearable devices

(4)   Composite Inks

Composition and Properties:

    Two or more materials are mixed (e.g., polymer matrix + conductive fillers, ceramic powder, or carbon nanotubes) to form functional composite inks. For example:

CNT/Graphene + PDMS → conductive elastomer

HA (Hydroxyapatite) + gelatin → biomedical composite scaffold

Curing Methods: Depending on the composition, multi-stage crosslinking is possible (e.g., photocuring followed by thermal curing).

Applications: Energy storage components, sensors and actuators, integrated structural functions

(5)   Bioinks

Composition and Properties :

    Composed of hydrogels and cell suspensions, with good biocompatibility and biodegradability. Common materials include GelMA, sodium alginate, and collagen. The ink must be extruded without damaging cells.

Curing Methods : Chemical crosslinking, temperature-induced gelation

Applications : Tissue engineering, organs-on-a-chip, regenerative medicine

(6)   Cementitious and Clay-based Inks

Composition and Properties :

    Composed of cement, water glass, lime mortar, or clay particles, typically containing natural fibers or thickeners to stabilize rheology. Allows for low-cost, large-scale, and sustainable architectural structures.

Curing Methods : Hydration reaction, drying and hardening

Applications : Freeform architectural structures, renewable building materials, and eco-friendly construction

C.        Notable Works and Applications:

(1)      Harvard SEAS – 3D Printed Microbattery (2013, USA)

DIW printing of interdigitated Li-ion electrodes with a resolution of ≈ 30 μm, demonstrating the fabrication of functional energy devices at the microscale [22].

(2)      Transforming The Electronic Industry Using 3D Printing

DIW co-extrusion of conductive silver ink and flexible polymer to create flexible electronic circuits and wearable devices [23].

(3)      Harvard Wyss Institute – Organ-on-a-Chip (2016, USA)

Research teams from Harvard University's School of Engineering and Applied Sciences (SEAS) and the Wyss Institute published this research in 2016, using DIW technology to fabricate heart-on-a-chip devices with built-in sensors. The chips are printed from multiple layers of flexible material and conductive ink, capable of simultaneously replicating myocardial contraction and real-time monitoring of mechanical signals, demonstrating the innovative application of DIW in biomedical engineering and organ simulation [24].

4.   Cementitious Extrusion Mortar Printing

A.   Definition and Explanation:

Cementitious Extrusion is a large-scale additive manufacturing technology that builds structures layer by layer using a screw or piston-based extrusion system with cement, mortar, or other mineral binders as the main body. This technology is typically classified as a Material Extrusion AM process, operating similarly to FFF, but the ink (or slurry) is a highly viscous, self-supporting "wet concrete." After printing, no formwork is needed, and walls, structures, or decorative components can be formed directly [26].

B.   Printable Materials

Geopolymer

Pumice,

Fly ash,

Alkaline activator

Low carbon, high heat resistance

Polymerization and solidification

Sustainable building materials

Fiber-reinforced mortar

Cement mortar

PVA/ PP/

Basalt fiber

Improved ductility

Hydration reaction

Building walls

Structural reinforcement

Gypsum or Limestone

Gypsum powder,

Water,

Stabilizer

Low shrinkage, quick-drying

Crystallization and solidification

Interior decoration

Artistic modeling

Eco-soil-based

Clay, water,

Fiber

Naturally Degradable

Drying and Hardening

Eco-architecture

Sustainable Living Units

C.   Notable Works and Applications:

(1)      WASP – TECLA House

The TECLA sustainable home, completed by the Italian WASP team in collaboration with architect Mario Cucinella, uses clay and natural fibers as the main printing materials. Large-scale extrusion is performed synchronously by dual-arm robots. The entire house is built entirely from local materials, demonstrating the potential of mortar printing in ecological and sustainable architecture [27].

(2)      ICON – Vulcan II Housing System

The Vulcan II system developed by American company ICON uses its proprietary Lavacrete™ cement composite material to print an entire house in just 24 hours. This technology has been successfully used in low-cost housing projects in the United States and Mexico, marking a pioneering case of commercial construction printing [28].

(3)      COBOD – BOD2 Construction Printer

The BOD2 gantry construction printer launched by Danish company COBOD has a large construction capability (up to 10×10×3 m) and has been applied in the construction of residential buildings, schools, and wind turbine foundations in multiple countries. It is currently the most widely used cement-based large-scale printing system globally [29].

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