Plastics for Additive Manufacturing E-Book

Andreas Fischer

• Herausgeber: Carl Hanser Verlag GmbH & Co. KG
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 776

Veröffentlichungsjahr: 2025

Beschreibung

This practical book is aimed at companies that want to integrate thermoplastic-based 3D printing into their manufacturing processes and need to build up know-how in the selection of suitable plastics. It presents the thermoplastics relevant for additive manufacturing and their process-specific features as well as a wide range of applications. The book covers the following topics: - Functional principle and build-up strategy of thermoplastic-based additive manufacturing processes, in particular fused layer modeling (FLM) and selective laser sintering (SLS) - Classification and formulation of thermoplastic polymer materials - Mechanical, thermal, and chemical properties of thermoplastics that must be considered for use in 3D printing - Material, design, and process specifics of thermoplastics for successful processing - Support materials, multi-material processes, fiber-reinforced components - Bio-based or biodegradable thermoplastics, recycling of thermoplastics - Recommendations for occupational safety, environmental protection, and product safety Numerous practical examples showing recommended combinations of plastics and additive manufacturing methods make this book an indispensable guide for all those who use 3D printing and need sound knowledge of materials selection for this purpose.

Leseprobe

Andreas Fischer

Dirk Achten

Martin Launhardt

Plastics for Additive Manufacturing

Properties, Processing, and Applications of Thermoplastics

Print-ISBN:        978-1-56990-229-5 E-Book-ISBN:   978-1-56990-284-4

All information, procedures, and illustrations contained in this work have been compiled to the best of our knowledge and is believed to be true and accurate at the time of going to press. Nevertheless, errors and omissions are possible. Neither the authors, editors, nor publisher assume any responsibility for possible consequences of such errors or omissions. The information contained in this work is not associated with any obligation or guarantee of any kind. The authors, editors, and publisher accept no responsibility and do not assume any liability, consequential or otherwise, arising in any way from the use of this information – or any part thereof. Neither do the authors, editors, and publisher guarantee that the described processes, etc., are free of third party intellectual property rights. The reproduction of common names, trade names, product names, etc., in this work, even without special identification, does not justify the assumption that such names are to be considered free in the sense of trademark and brand protection legislation and may therefore be used by anyone.

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© 2025 Carl Hanser Verlag GmbH & Co. KG, Munich Vilshofener Straße 10 | 81679 Munich | [e-mail]: Dr. Mark Smith Production Management: Eberl & Koesel Studio, Kempten Cover concept: Marc Müller-Bremer, www.rebranding.de, München Cover design: Max Kostopoulos Cover picture: © Leonie Fensterle Typesetting: le-tex publishing services GmbH, Leipzig Printed and bound by Elanders Waiblingen GmbH, Waiblingen

Contents

1 Introduction to Thermoplastic-based Additive Manufacturing

1.1 Additive Manufacturing Building Strategy

1.2 Process Groups of Additive Manufacturing

1.2.1 Sintering and Extrusion

1.2.2 Binding Technology and Laminating

1.3 Functional Principle of Fused Deposition Modeling

1.3.1 Additive Extrusion with Filament

1.3.2 Characteristics of Fused Deposition Modeling

1.3.2.1 Surfaces

1.3.2.2 Support Structures and Building Chamber

1.4 Functional Principle of Fused Layer Modeling

1.4.1 Material Feed

1.4.2 Characteristics of Fused Layer Modeling

1.4.2.1 Support Structures and Building Chamber

1.4.2.2 Robot-based Fused Layer Modeling

1.5 Functional Principle of Selective Laser Sintering

1.6 Hybrid Additive Manufacturing with Thermoplastics

1.6.1 Fiber-reinforced Additive Components

1.6.2 Atomic Diffusion Additive Manufacturing

2 Polymers in Additive Manufacturing

2.1 History of Artificial Polymers (Plastics)

2.1.1 Homopolymers

2.1.2 Copolymers

2.1.3 Organic and Inorganic Polymers

2.2 Polymer Chemistry

2.3 Polymers as Materials

2.3.1 Thermoplastics

2.3.2 Duromers

2.3.3 Elastomers (Rubbers)

2.3.4 Thermoplastic Elastomers (TPEs)

2.4 Polymer Manufacturing Process

2.4.1 Chain-growth Reactions

2.4.2 Step-growth Reactions

2.4.2.1 Polyaddition Reactions

2.4.2.2 Polycondensation Reactions

2.4.3 Biological Polymerization

2.5 Physical Properties of Polymers

2.5.1 Viscoelasticity

2.5.2 Dynamic Mechanical Analysis (DMA)

2.5.2.1 Networking Density

2.5.2.2 Entropy Elasticity

2.5.2.3 Relaxation Times and Temperature

2.5.2.4 Observable Relaxations

2.5.2.5 Free Volume

2.5.3 Calorimetric Measurement Methods

2.5.4 Melt Viscosity

2.5.5 Melt Volume Flow Rate

2.5.6 Heat Transfer

2.5.7 Fluid Mechanics

2.5.8 Thermal Decomposition Temperature

2.5.9 Hardness Measurement

2.5.10 Tensile Test

2.5.10.1 Heat Tinting in Tensile Tests on Plastics

2.5.10.2 Aging Tests

2.5.10.3 Temperature-dependent Mechanical Testing (Heat Resistance)

2.5.10.4 Other Destructive Test Methods

2.5.10.5 Destructive Testing of Additively Manufactured Components

2.6 Thermoplastics

2.6.1 Classification of Thermoplastics

2.6.2 Semi-crystalline Thermoplastics

2.6.3 Glassy Thermoplastics

2.6.4 Thermoplastic Vulcanizates

2.6.5 Thermoplastic Polymer Blends

2.6.6 High-performance Plastics

2.6.6.1 Manufacturing Processes for High-performance Thermoplastics

2.6.6.2 Stereochemistry of Polymers

2.6.6.3 Mechanical Properties

2.6.6.4 Thermal Stability

2.6.6.5 Crystallinity

2.6.6.6 Solution Properties

2.6.6.7 Applications

2.6.6.8 Additive Manufacturing with High-performance Thermoplastics

2.6.7 Engineering Plastics

2.6.7.1 Polyamide (PA)

2.6.7.2 Polyethylene Terephthalate (PET)

2.6.7.3 Polybutylene Terephthalate (PBT)

2.6.7.4 Polyoxymethylene (POM)

2.6.7.5 Polycarbonate (PC)

2.6.7.6 Polylactic Acid (PLA)

2.6.7.7 Thermoplastic Elastomers (TPEs)

2.6.8 Standard Plastics

2.6.8.1 Polyethylene (PE)

2.6.8.2 Polypropylene (PP)

2.6.8.3 Polyvinyl Chloride (PVC)

2.6.8.4 Polystyrene (PS)

2.6.8.5 Acrylonitrile-butadiene-styrene (ABS) Copolymers

2.6.8.6 Polymethyl Methacrylate (PMMA)

2.7 Processing Methods for Thermoplastics

2.7.1 Injection Molding

2.7.2 Extrusion

2.7.3 Thermoforming

2.7.4 Extrusion

2.8 Formulation of Thermoplastics

2.8.1 Color Setting

2.8.2 Flame Protection

2.8.3 Stabilizers

2.8.4 Processing Aids

2.8.5 Fillers

2.8.5.1 Particle-filled Plastics

2.8.5.2 Carbon Black

2.8.5.3 Chalk and Talc

2.8.5.4 Mica

2.8.5.5 Particle-reinforced Thermoplastics in Additive Manufacturing

2.8.6 Fibrous Reinforcing Fillers

2.8.6.1 Fiber-reinforced Thermoplastics in Additive Manufacturing

2.8.6.2 Glass Fibers

2.8.6.3 Carbon Fibers

2.8.6.4 Ceramic Fibers

2.8.6.5 Natural Fibers

2.8.6.6 Polymer Fibers

2.8.6.7 Thermal Stability and Weathering Resistance of Plastic Fiber Composites

2.9 Weather-resistant Thermoplastics

2.10 Electrically Conductive/Insulating Thermoplastics

2.10.1 Electrically Passive Insulating Plastics

2.10.2 Electrically Active Conductive Plastics

2.10.3 Conductive Plastics in Additive Manufacturing

2.11 Fire-retardant Thermoplastics

2.11.1 Fire-retardant Thermoplastics in Additive Manufacturing

2.12 Foamed Thermoplastics

2.12.1 Foaming Process

2.12.2 Foam Extrusion

2.12.3 Thermoplastic Foam Casting

2.12.4 MuCell Process

2.12.5 Foamed Thermoplastics in Additive Manufacturing

2.13 Thermoplastics With Food Contact

2.13.1 Additive Manufacturing of Food Contact Products Based on Thermoplastics

2.14 Bioplastics and Biocompatibility/Degradability

2.15 Thermoplastics in Medical Applications

2.15.1 Use of Additive Manufacturing with Thermoplastics in Medical Applications

2.15.2 Organs From the 3D Printer

3 Special Features of Thermoplastic Processing in Additive Manufacturing

3.1 Extrusion-based Processes (Material Extrusion, MEX)

3.1.1 Filament Extruder

3.1.2 Pellet Extruder

3.1.3 Thermoplastic Properties

3.1.3.1 Melting Temperature

3.1.3.2 Heat Capacity

3.1.3.3 Melt Viscosity/Melt Volume Flow Rate

3.1.3.4 Solidification Temperature/Solidification Kinetics

3.1.3.5 Shrinkage

3.1.4 Process Conditions

3.1.4.1 Nozzle Temperature

3.1.4.2 Nozzle Geometry

3.1.4.3 Volume Discharge Speed

3.1.4.4 Building Space Heating

3.1.4.5 Building Platform Heating

3.1.5 Printing Problems

3.1.5.1 Adhesion to the Part Platform

3.1.5.2 Interlayer Adhesion

3.1.5.3 Threading (Stringing) and Oozing

3.1.5.4 Delay (Warping)

3.1.5.5 Under-extrusion

3.1.5.6 Over-extrusion

3.1.6 Process Simulation

3.1.6.1 Input Shaping

3.1.6.2 Pressure Advance System

3.1.6.3 Klipper

3.1.7 Support Materials

3.1.8 Post-processing Methods

3.1.8.1 Removal of Support Material

3.1.8.2 Surface Treatment

3.1.8.3 Chemical Functionalization

3.1.8.4 Consolidation (Thermal/Chemical)

3.1.8.5 Part Properties

3.1.9 Software

3.1.10 3D Printing Materials Recycling

3.1.11 3D Printing Materials Ageing

3.1.11.1 Thermal Stress

3.1.11.2 Mechanical Stress

3.1.12 Requirements for the Production Environment

3.1.12.1 Machine Selection

3.1.12.2 Workspace

3.1.12.3 Occupational Safety

3.1.12.4 Particulate Matter/Volatile Organic Compounds (VOCs)

3.1.13 Quality Assurance

3.1.14 Standardization

3.2 Selective Laser Sintering (SLS) of Plastics

3.2.1 Available Materials

3.2.1.1 Powder Manufacturing Process

3.2.1.2 Powder Testing Methods

3.2.1.3 Thermal Aging

3.2.2 Thermal Process Control

3.2.3 Exposure Strategy and Parameters

3.2.4 Production Systems and Process Technology

3.2.5 Build-job Preparation (Pre-processing)

3.2.5.1 CAD Part Design

3.2.5.2 Design Recommendations for SLS parts

3.2.5.3 Slicing

3.2.5.4 Recommendations for Build-job Layout

3.2.6 Part Characteristics

3.2.7 General Quality Assurance for SLS Parts

3.2.7.1 Shape Deviation

3.2.7.2 Mechanical Properties

3.2.7.3 Typical Part- and Process Failures

3.2.8 Profitability Analysis

3.2.9 Post-processing Procedures

3.2.9.1 Unpacking a Build-job

3.2.9.2 Part De-powdering

3.2.9.3 Smoothing

3.2.9.4 Dye Coloring

3.2.10 Materials Management

3.2.11 Industrial Safety

3.2.12 Sample Applications

4 Additive Building Materials and Applications in the Thermoplastic Sector

4.1 Additive Building Materials and Value Chains

4.1.1 Extrusion-based Processes (MEX)

4.1.1.1 Building Materials

4.1.1.2 Value Chains (Thermoplastic, Compounding, Filament)

4.1.2 Powder-based Processes (SLS, HSS)

4.1.2.1 Building Materials

4.1.2.2 Value Chains From Printers and (Certified) Materials

4.2 Products and Applications

4.2.1 Prototypes and Sample Components

4.2.1.1 Ventilation Grille

4.2.1.2 Large-format AM in the Design Process

4.2.2 Procedure with Dead-mold Casting

4.2.2.1 Soluble FDM Molds

4.2.2.2 Sustainable Disposable Coffee Cups

4.2.2.3 Hybrid AM Aircraft Seats

4.2.3 Functional Components

4.2.3.1 Silent Underwater Propulsion

4.2.3.2 Integration of Optical Fibers (Clear Cast Integration)

4.2.4 Series Components

4.2.4.1 Functional Tool Housing

4.2.5 Furniture

4.2.5.1 Small Furniture and Lamps

4.2.5.2 Street Furniture

4.2.5.3 Increasing the Usability of IKEA Products for People with Disabilities

4.2.5.4 AM as a Circular Manufacturing Model

4.2.6 Spare Parts and Accessories

4.2.6.1 Spare Parts for Segway X2

4.2.6.2 Miele Accessories

4.2.7 Transportation

4.2.7.1 Vehicle Body via FDM

4.2.7.2 Vehicle Frame and Body using Big-area AM

4.2.7.3 Electric Motorcycle

4.2.7.4 Jet-powered Drone

4.2.7.5 Camper Concept

4.2.8 Medical Applications

4.2.8.1 Valves for Breathing Masks

4.2.8.2 Implants Made of High-performance Plastic

4.2.8.3 Person-specific Prostheses and Orthoses

4.2.9 Sports Equipment and Fashion

4.2.9.1 Sustainable Surfboard

4.2.9.2 Force-absorbing Bicycle Helmet

4.2.9.3 Hygiene Warning System

4.2.9.4 Polyamide Spectacle Frames

4.2.9.5 Personalized Handbags

4.2.10 Manufacturing Systems Engineering

1Introduction to Thermoplastic-based Additive Manufacturing

The term additive manufacturing (AM) refers to manufacturing processes that produce three-dimensional components layer by layer by means of material application and accumulation. At times, the term rapid prototyping (RP) is also used for this type of component production, while the colloquial term “3D printing” is used.

When comparing the terms rapid prototyping and additive manufacturing, it becomes clear how the processes have developed over time, particularly with regard to their range of applications. In the initial phase of digital AM, the first systems used data sets to enable the direct – and, compared to conventional manufacturing, rapid – production of prototypes or sample parts. By using AM, product developments can be improved or shortened. Through the improvement and new development of processes and systems, the proportion of directly produced components and products using AM increased. This development also initiated the necessary development or adaptation of new materials for the various AM processes. In the early 1990s, the first commercial thermoplastic-based additive systems in the extrusion and sintering process cluster came onto the market.

In this chapter, we describe the basic strategy on which AM processes are based and on which process clusters are founded. We will then describe the basic principles of fused deposition modeling (FDM), fused layer modeling (FLM), and selective laser sintering (SLS) will be explained, before explaining the concept of hybrid additive manufacturing with thermoplastics.

1.1Additive Manufacturing Building Strategy

The building strategy of components by means of additive processes can be explained by the analog manufacturing strategy of contour relief maps patented in 1892 by J. E. Blanther. The terrain models built up with this manufacturing strategy consisted of wax plates, which were cut to size based on the respective contour line of the terrain and then placed on top of each other and positioned in a further step. The thickness of the wax plates resulted in the so-called layer thickness of the additively built model. In order to obtain a coherent wax component from the individual contoured wax plates, they were joined together using targeted heat. With a relatively large connection between the individual plates or layers, it was also possible to ensure layer adhesion corresponding to the material and the resulting stability. Accordingly, the basic idea for component building in AM is the layering and bonding of filled contours. This can be done in a manual process that is analogous to that for contour relief maps, or digitally and automatically. In the latter case, the component to be manufactured is available in a digital form, from which the layers with the respective contours are then derived. These layers have a specific constant thickness and follow a geometric axis through the component. After the virtual layers are generated, they are converted into control commands for the specific AM system. The additive system, in turn, generates the contoured layers in a specific order and their connection using specific materials.

Physically, especially for components with overhangs of more than 45°, there is a need to protect them against bending or breaking with a support structure. These support structures are generated digitally in advance in some additive processes (as are the virtual layers), depending on the geometry and the process. Depending on the process, these contours are in turn converted into control commands. Depending on the additive process used, these support structures lead to higher material consumption and longer manufacturing times, as well as to the need for subsequent removal of the support structure from the component.

Due to the layered structure of the additive processes, perceptible steps form on the surface in the build direction. The shape and visibility of these steps depends on the layer thickness. The thinner the respective layers can be built, the lower their perceptibility. At the same time, however, a thinner layer thickness also leads to longer manufacturing times and thus higher manufacturing costs per component. The thinnest layer thicknesses are achieved with additive processes whose materials are in a liquid or gaseous state. However, special processes in which the material is in powder form, such as laser micro-sintering, can also achieve layer thicknesses of 15 μm down to 1 μm.

The surfaces of additively produced components that are not characterized by the step effect are characterized by process-specific principles. In fused deposition modeling (FDM) and fused layer modeling (FLM) processes, there are melting nozzles for thermoplastic material with a corresponding nozzle diameter. In contrast, in selective laser sintering (SLS), the surfaces are imprinted by a pulsed laser, which locally melts thermoplastic powder. Laminated object manufacturing (LOM), selective adhesive and hot press process (SAHP) and layer milling process (LMP) processes are an exception with regard to both step formation and process-specific surface embossing.

In principle, the step formation in the processes can be eliminated by 5-axis machining. Furthermore, the surfaces that are not affected by layering are characterized by the surface of the film or sheet material used and not by the process. Additive processes, which primarily process thermoplastic materials, are anchored in the solid initial state (Figure 1.1). The thermoplastic to be processed can be present here as wire, round cord (filament), powder, or in film or sheet form.

Figure 1.1Subdivision of additive processes according to the initial state of the material (© Leonie Fensterle, adapted from [Sehrt 2016])

1.2Process Groups of Additive Manufacturing

For historical reasons, various terms have become established for layer-building processes. As a result of technological development and thus the expanded application possibilities of additive systems, the following terms have evolved over time and are still used specifically today:

       Rapid prototyping (RP): Manufacturing of sample or prototype parts

       Rapid tooling (RT): Manufacturing of tools for primary forming and forming production processes

       Additive manufacturing (AM): Production of components and small batches as marketable products.

In addition to the classification according to DIN 8580, additive systems can also be subdivided on the basis of the condition of the starting material, the material of the component produced, the use in the product development process, or on the basis of the process principles. The differentiation according to process principles is summarized in Figure 1.2. The reduction was made according to the criteria “plastic as a material for component production” and “establishment of the systems on the market”.

Figure 1.2Classification summary of AM processes according to process groups and principles (© Leonie Fensterle, based on [Fischer 2020])

1.2.1Sintering and Extrusion

The sintering and extrusion process groups are based on thermoplastic materials (Figure 1.2). SLS was commercially launched by DTM in 1992. At that time, the systems were called Mod A and Mod B. In 2001, DTM was acquired by 3D Systems. At that time, 3D Systems specialized in stereolithography (SLA) of the UV curing process group and was the market leader in AM systems.

In the extrusion process group, the following two processes are included in Figure 1.2: FDM and FLM. Both processes use the principle of spraying molten polymers by means of a nozzle. The differences between the systems of the two processes are minor in places. FDM systems are often priced higher than fused layer modeling systems, some of which are also designed for private users. FDM systems also often have a more robust manufacturing process and a higher degree of automation. The term FDM was introduced by Stratasys, which launched its first additive system, the 3D Modeler, in 1992. Focusing on low-end systems in AM, Stratasys became the market leader in 2003, selling nearly half of all additive systems at that time and representing the largest contingent of additive systems installed. In principle, additive systems manufactured by Stratasys and belonging to the Extrusion group are listed under FDM. This also applies to systems from manufacturers such as MakerBot, which belong to Stratasys. FLM includes all systems that belong to the extrusion process group and are not manufactured by Stratasys or associated companies. This also includes many additive systems that, for example, are not commercially available and are still in the research or development stage.

1.2.2Binding Technology and Laminating

Two further thermoplastic-based additive processes are included in the process group: binder technology (multi-jet fusion, MJF) and lamination (laminated object manufacturing, LOM) (Figure 1.2).

Multi-jet fusion (MJF)

The parts that can be produced by MJF are similar to those produced by SLS and can currently be produced using polyamide (PA) 12 and thermoplastic polyurethane (TPU) 90A. Due to the nature of the process, layer building is faster than in Selective Laser Sintering, which, combined with lower or equal equipment costs, results in lower part costs. In the MJF process launched by HP, heat-conducting liquid, called fusing agent, is sprayed onto the active powder-based layer in the part area via a print head. Another liquid, the detailing agent, is applied in parallel to the contours of the component in the layer and is responsible for creating sharp edges. After the two liquids are sprayed on, energy is added to the respective layer via IR light. The fusing agent increases the energy by absorbing the thermoplastic powder, causing the material to fuse in the wetted area. The detailing agent counteracts this process as an insulator and limits the melting of the material (Figure 1.3).

In the current systems, 300 million drops of liquid are sprayed onto the powder layer every second with an accuracy of 21 μm. Thus, the system has about half the accuracy of multi-jet modeling (MJM) systems in the “UV curing” process group, which are among the most accurate additive systems. Both processes use print heads similar to those used in inkjet printers. As a result, the XY resolution of the MJF systems is specified as 1200 dpi. The use of liquid and its optimum introduction into the powder lead to uniform fusion and a relative dense component, which also largely eliminates the problem of layer adhesion in additively produced components. The high speed at which the layers are created is generated by a high number of print heads and the incorporation of energy input in the wetting process. According to HP, multi jet modeling can achieve 3 cm of part height per hour, which is three times the build rate of SLS.

Figure 1.3Functional principle of the MJF process from HP (© Leonie Fensterle, from [3Druck 2016])

A special feature of the process is the effect of sinking, which leads to a burr in the order of 0.1 mm on the end surface of the component in the Z-direction. This effect is caused by the introduction of the fluids. The installation space of the current HP systems is 380 × 380 × 28 mm. The post-processing of the components is similar to that of SLS, in that the components must be cleaned of residual powder after removal from the system. The surface can be smoothed, for example, via vibratory grinding. As with SLS, the components can also be colored. Compared to SLS and FDM, MJF is a young additive process. As of 2016, the first commercial systems were available under the name HP Multi Jet Fusion 4200. HP cooperated with the companies Nike, BMW, and Johnson & Johnson [3Faktur 2020] from 2014.

High-speed sintering (HSS)

Another inkjet-based additive process with thermoplastic materials is HSS from the company Voxeljet. HSS differs only to a limited extent from HP’s MJF process (Table 1.1). As with the HP system, an energy-absorbing liquid is applied to a powder bed via a print head and the energy is supplied via IR light. The printed powder bed areas are thereby fused by the energy input, with the remaining powder area remaining loose (this can be reintroduced into the manufacturing process after preparation). The main difference to MJF is that no second liquid (detailing agent) is required, as the temperature of both powder areas can be controlled independently of each other using two different IR light emitters with different wavelengths. In addition, the system from Voxeljet allows full access to the pressure parameters in order to optimize the production for the respective material and application. Thus, it is a so-called open system, which can also be used for AM with customer-specific materials or new material developments. Differences exist between the print heads used. HP’s MJF system uses bubble-jet print heads with a resolution of with a resolution of around 1200 dpi. In contrast, HSS uses piezo print heads with a resolution of around 360 dpi (Table 1.1). However, the resolution of the components is significantly influenced by the grain size of the material powder. This is around 55 μm for both systems. The piezo print heads of high-speed sintering can process oil-based, water-based, and solvent-based fluids. This option further increases flexibility in terms of material selection [Voxeljet 2022].

Table 1.1MJF and HSS in Comparison [Voxeljet 2022]

High-speed sintering (HSS)

Multi-jet fusion (MJF)

Applications

Material development and research, prototyping, production applications, service centers, manufacturing, and injection molding companies

Prototyping, service centers, and production applications

Installation space

VX200 HSS: 290 × 140 × 180 mm VX1000 HSS: 1000 × 540 × 400 mm

MJF 540/580: 322 × 190 × 248 mm MJF 4200: 380 × 285 × 380 mm MJF 5200: 380 × 294 × 380 mm

Speed

VX200 HSS: 580 cm3/h VX1000 HSS: 6500 cm3/h

MJF 540/580: 1817 cm3/h MJF 4200: 4115 cm3/h MJF 5200: 505 cm3/h

Resolution

360 dpi

1200 dpi

Materials

PA12, TPU, PP, PEBA, and EVA Open system

PA12, PA11, TPA, and TPU

Minimum layer thickness

80 μm

80 μm

Cooling times

14 h (depending on the headroom)

48 h without “fast cooling” (depending on material and grain size)

Laminated object manufacturing (LOM)

LOM, from the laminating process group, is similar to the process principle of J. E. Blanther’s contour relief maps (see Section 1.1). The process group is based on layer generation, in which semi-finished products like sheet or film materials are contoured by means of knives, milling cutters, hot wires, or lasers, and are joined and bonded simultaneously or downstream specifically depending on the material. Since this group of processes also includes separating and ablative processing steps, it was also referred to as a subtractive-additive process by Bernard and Taillander in 1998.

Another term used for the process group is layer laminate manufacturing (LLM). This term is due to the fact that the term “laminated object manufacturing” was created by the company Helisys, similar to “fused deposition modeling” by the company Stratasys. In contrast to Stratasys, Helisys was dissolved in 2000, at which point their production of LOM systems ceased. Companies such as Mcor Technologies Ltd, founded in 2005, reintroduced A4 paper-based systems to the market. The first digital LOM systems were launched by Helisys in 1991, making them commercially available a year before SLS and Fused Deposition Modeling systems. The Helisys system was based on special paper which was laminated on one side with a thermally activated adhesive. This coated paper was in roll form and contoured using a CO2 laser. The special paper was pulled over the build platform. A roller heated to 330 °C activates the adhesive and uses additional pressure to bond the current layer to the partially finished component. After this step, the CO2 laser contours the current and joined layer, which corresponds to the specific cross-sectional layer of the underlying CAD model. This procedure increases the positioning accuracy of the respective layers relative to each other, but also increases the waste volume. Furthermore, the laser cuts a constant frame from the paper, which also corresponds to the maximum XY dimensions of the build space. Through this frame, the current layer is detached from the roll material. Paper that is in this frame but does not belong to the component is cut into squares. The elements resulting from this can be more easily mechanically removed from the component after completion. The elements are comparable to FDM support structures. The build speed in the Z-direction can be increased by laser-processing a maximum of four layers at a time. However, this reduces the resolution of the component in the Z-direction, and the machined contours become more discolored due to the energy input of the laser.

LOM has been further developed in various directions. The materials that can be used are generally unlimited, as long as they are in sheet or film form and can be joined. Plastics, metals, ceramics, and wood can all be processed using LOM. It is also possible to produce multi-material components made of several materials. The material change can be done layer by layer or locally in the layer. Various classic separating processes can be combined with different joining processes such as gluing, welding, clamping, and ultrasonic welding. In places, LOM systems can also be categorized according to the principle of joining.

The Cirtes company offers LOM solutions under the name Stratoconception. For contouring, micro-CNC milling, laser machining, hot wire, or an oscillating knife are used. The system behind Stratoconception was developed by Claude Barlier, and a patent was filed in 1991. The special feature of Stratoconception is the subsequent positioning of the layers with pins or recesses. Subsequent positioning allows maximum utilization of the sheet material for the creation of the components. Another special feature is the ability to choose between 2-axis, 2½-axis, or 5-axis machining of the plates. In comparison, the layers are clearly visible in 2-axis machining, depending on the thickness of the plate. With 2½-axis machining, this visibility is reduced to the maximum. With the Stratoconception system, both small and large components can be produced. In the xy-direction, dimensions of 2000 × 3000 mm are possible, and in the z-direction, there are only physical or static limitations.

One of the main advantages of LOM is the higher process speed compared to other additive processes for large components with high volume and low complexity of the geometry. Furthermore, the technology of the equipment is easy to master and partially independent of the material used. By bonding the layers, stresses are reduced and the components are thus produced primarily without distortion. A problem is the different load capacity of the components in and across the layer direction, which occurs in particular when joining the layers with adhesives. Ultrasonic- or diffusion-welded metal components are an exception here and exhibit constant load-bearing capacity in all directions. Furthermore, geometries with cavities or cutouts that prevent the subsequent removal of segments that do not belong to the component pose a problem. One solution to this is to remove the respective segment part in the layer. However, this is not possible with every LOM system, and it slows down the process speed. Furthermore, the waste material produced, which can be a factor of 1:10, can offset the same or lower material price. If LOM is used with a consistent thermoplastic material, the resulting waste could be reformed back into sheet material, ameliorating the environmental and economic disadvantages.

1.3Functional Principle of Fused Deposition Modeling

As described in Section 1.2.1, the name fused deposition modeling (FDM)in the extrusion process group is closely associated with the company Stratasys. This is a US company that was originally founded in Minnesota and has since moved its headquarters to Rehovot in Israel; the headquarters in Eden Prairie was nevertheless retained. In addition to MakerBot in 2013, Stratasys acquired equipment manufacturer Objet Geometries in 2012. MakerBot also covers the consumer thermoplastic extrusion market with its low-cost systems. The MJM systems from Objet Geometries belong to the UV curing process group and are marketed by Stratasys under the name PolyJet. Also, the service area, which was covered by RedEye at Stratasys, has been expanded through the integration of Solid Concepts and Harvest Technologies. Stratasys’ various FDM systems differ in a number of aspects, including build volume, material bandwidth, support material, and additional requirements (such as washers to remove support structures or set-up location). Both the Stratasys and MakerBot systems specialize in processing thermoplastics, which come in specific-diameter round cord. This round cord is referred to as “filament” in the additive field and is produced via extrusion from thermoplastic pellets. Thermoplastic material is used in FDM both for the creation of the components and for the necessary support structures.

1.3.1Additive Extrusion with Filament

The systems in the extrusion process group are distinguished from the other groups primarily by their accessibility. No other additive systems can be introduced into companies in such a timely and cost-effective manner. Due to the comparatively inexpensive and easy-to-produce thermoplastic consumables, product concept tests in particular can be carried out without major financial and time outlay.

The thermoplastic filaments for the production of the components and necessary support structures are available with a round cord diameter of 1,75 mm. The filament is produced by extruding thermoplastic granules. In this continuous process, the thermoplastic materials are pressed through a die by means of a screw press. The granules are melted and homogenized by heat input and internal friction. The pressure built up at the same time causes the thermoplastic to be ejected through the forming die. The thermoplastic exiting through the die then solidifies in a water-cooled calibration. Additional application of vacuum presses the thermoplastic profile against the walls of the calibration, thus completing the molding process. The manufacturing tolerances during extrusion are in the range of ±0,05 mm. The extruded thermoplastic filament is then wound onto spools with a view to unobstructed and smooth delivery. These filament spools are fed to some FDM systems in encapsulated form or via adapters. This method of feeding filament, in addition to recording consumption, also provides protection from unwanted particles and moisture. Furthermore, such filament storage supports the automatic loading and unloading process widely used in FDM systems. In this process, the filament is conveyed by conveyor motors through a hose system to the actual melting head and automatically threaded into or out of it. After the thermoplastic filament has been automatically threaded in, a certain length of it is extruded through the melting nozzle into a container. This procedure also serves to convey particles or residual material out of the nozzle. After this short extrusion, the nozzle is moved over a rubber lip and metal brush, which removes or reduces adhering material and contaminants. This cleaning process is also carried out during the build process when switching between active nozzles, thereby increasing the quality of the components.

With the F123 systems from Stratasys, the so-called purge tower is additionally is generated. This represents an automatically generated and separate component that is used in addition to the nozzle cleaning. This system was adapted from FLM, where stationary nozzle cleaning areas are sometimes missing. The idea behind the purge tower is that, on the one hand, the nozzle is cleaned of particles and residual material and, at the same time, is filled with sufficient material before it applies material to the actual component again. The disadvantage of this additionally generated component is the higher consumption of thermoplastic material and the increase in manufacturing time for components.

In principle, color changes in components can only be generated layer by layer in FDM systems. For this purpose, the build process is specifically stopped after the completion of a certain layer and the active thermoplastic filament is automatically threaded out of the melting head. Depending on the system, this filament can then be removed together with the coil and the adapter. The stop can be scheduled in advance by setting a pause or can be performed manually on the system. After the new filament has been inserted, it is automatically conveyed to the melting head and threaded in. The build process then starts again with the new different colored filament in the next layer of the part to be produced. Residual particles in the nozzle create a smooth color transition in the component. The same system is also used when further filament is needed to complete a component. In this case, however, stopping the build process and unthreading is initiated automatically by the system.

With a few exceptions in MakerBot systems, FDM systems are equipped with two melting heads. In each case, one melting head is intended for the component thermoplastic. The second head is designed for the thermoplastic support material. On the F-Series FDM systems introduced at Stratasys in 2017, the two melt heads are easily interchangeable. This is also due to the polylactic acid (polylactide, PLA) material, which is new to Stratasys systems. The thermoplastic material is very common in the additive private sector as a consumable material and is produced from renewable raw materials. The use of PLA as a material in FDM can be traced back to systems from MakerBot, which primarily use this material for the building process. The cost of a kilogram of PLA is approximately 50 % less than the thermoplastic acrylonitrile-butadiene-styrene (ABS), which is the most commonly used material in FDM systems. Accordingly, many of the Stratasys systems are optimized to process ABS or similar thermoplastics. For fast and cost-effective concept and verification models, PLAs are provided on the F-Series. To process these on the F-Series, the melt head is changed to create the part because PLAs have lower processing temperatures than the other system-compatible thermoplastics. If this change of melt heads was not made, residual particles of the thermoplastics with higher processing temperatures could lead to faulty extrusion or clogging. In the case of PLAs, the support structure is provided with the same material and additional targeted cooling via a special second attachment, which replaces the exchangeable second melt head for the support material. The PLA support structure must be mechanically removed in a downstream step.

FDM systems are basically designed as a 3-axis system in a gantry arrangement (Figure 1.4). The resolution in the xy-direction is determined by the nozzle diameter of the melt units. In Stratasys systems, this diameter averages around 0,45 mm and is not variable. The resolution in the z-direction is generated by an axis and can be changed in some FDM systems via the control software. On average, this is around 0,25 mm. The thermoplastic filament is mechanically conveyed by pinions into the melting nozzles and extruded by permanent mechanical push conveying. The resulting thin extrusion beads are then are then applied in a targeted manner and in layers via axis movements to produce the component or necessary support structures. In the process, the warmer applied material bonds with the material already present. The layer is usually created by tracing the respective outer contour and filling this contour with a 45° hatch. If the extrusion beads are placed directly next to each other in this process, a fully filled component is produced. By deliberately offsetting the extrusion beads in the 45° hatching, hollow structures can be created in the components and the support structure. In the z-direction, the small cavities are closed by placing several fully filled layers on top of each other.

In the majority of FDM systems, thermoplastic-based building plates or films are used as the substrate base for the component to be produced. These are designed to ensure good adhesion of the component material and the support material, and are sometimes provided with an adhesion-promoting structure. The adhesion between substrate, support material, and component material prevent both the component from detaching during the build process and the deformation of the layers in the z-direction, either of which can lead to the abortion of the build process.

Figure 1.4Functional principle of the FDM process from Stratasys (© Leonie Fensterle, from [Fischer 2020])

At the beginning of the build process with the z-resolution of 0,25 mm, approximately nine layers of support material are created, which ensures optimum perpendicular alignment from the last support material layer to the melting nozzles. This support material substructure is generated wherever the first component layers or support structures are created in the further course of the building process. If the nozzle is not optimally aligned with the layer, this can lead to clogging of the nozzle on the one hand and loss of adhesion between the layers on the other. If these support material layers are created from dissolvable thermoplastic, the detachment of the component from the support substrate can take place automatically. The support material substructure in FDM is the result of the calibration process that takes place prior to the build process. During this process approximately nine points on the substrate base are captured using styluses.

1.3.2Characteristics of Fused Deposition Modeling

Components created via FDM are characterized by the thermoplastic ABS, which is often used in Stratasys systems, and the distinctive extrusion beads. Depending on the nozzle diameter and z-resolution used, the stacked extrusion beads are haptically perceptible and clearly visible. When filling the layers via the 45° hatching, the tracks are rotated by 90° in each layer. This strategy is intended to prevent too many extrusion beads from being deposited directly on top of each other, and thus leads to better layer adhesion within the components. As a result of the diameter of the extrusion beads and the 45° hatching, certain geometric features can lead to defects without material being deposited in the layers. These flaws can have negative effects on intended functionalities such as load-bearing capacity, tightness and surface quality.

1.3.2.1 Surfaces

The surfaces of components produced by additive systems exhibit a stair-step effect due to the layered structure, especially in the z-direction; an effect that is more or less pronounced. Additive systems that primarily process liquids have the lowest stairstep effect due to a high z-resolution and thus the smoothest surfaces in the z-direction. Surfaces in the xy-direction have in the majority of cases a lower and process-related resolution. With FDM, the z-resolution is generally higher than the resolution in the xy-direction. The following z-resolutions are provided (in different combinations) on the various Stratasys systems:

       0,127 mm

       0,178 mm

       0,254 mm

       0,330 mm.

The most common z-resolution is 0,254 mm, which is available for the majority of systems.

In FDM, components with visible surfaces should be created in such a way that these surfaces are largely embossed via the z-resolution. The possible surface finishes in FDM are much more effective for surfaces created with z-resolution. In parallel, however, possible increases in cost and build time due to the specific orientation of the part should also be evaluated. Furthermore, it should be noted that in particular the possible tensile load for components in the z-direction is lower than that in the xy-direction.

The extrusion beads typical of FDM are more clearly visible in the xy-direction and reflect the typical nozzle diameter of the systems. The nozzle diameter for Stratasys systems is approximately 0,45 mm. Defects can also occur, especially in the xy-direction, which result from the interaction of the nozzle diameter, the geometry of the part and the corresponding path planning (Figure 1.5). In some cases, it is physically impossible to place another extrusion bead between the component contour and the component filling. This ultimately results in a defect, which can occur both on the surface in the xy-direction and inside the component. The surfaces produced in this way can usually only be optimized with mechanical finishing steps to produce smooth surfaces.

Figure 1.5Here you can see the path planning of a layer with 45° hatching in FDM. This is the fully filled mode with voids inside the component. The green lines represent the component and the purple lines the necessary support material in the layer (© Andreas Fischer, from [Fischer 2021])

Chemical finishing operations such as solvent dissolving of the thermoplastic surface, as is done in the Stratasys “Finishing Touch Smoothing Station”, cannot produce the desired surface finish on these special surfaces. The solvent used in the Finishing Touch Smoothing Station penetrates a maximum of 32–63 mm deep and does not change the part surface by more than 0,023 mm. In the z-direction, the extrusion beads protrude by approximately 0,051 mm during FDM. The extrusion beads are basically smooth and glossy after deposition. Along the beads in the z-direction, this results in a smooth haptic impression. If the component is chemically post-processed, the surface remains glossy. If the components are mechanically reworked, such as by manual grinding, sandblasting, or vibratory finishing, the surface of the machined components becomes matt. If the components have sharp edges or fine details, both chemical and mechanical post-processing of the components can lead to negative results.

Another factor that can lead to negative effects both in the component and on the surface are waste products that form at the die during extrusion. These include burnt thermoplastic residues and parts of extrusion beads which, if not removed, may fall off the die when the layers are produced and thus be incorporated into the component. Burnt thermoplastic residues often lead to deformations on the surface of the components that are difficult to repair and thus to AM rejects (Figure 1.6). Residues of extrusion beads that have been incorporated into the component, can usually be easily removed mechanically, although minor defects of the surface can result. To counteract this specific problem of extrusion and thus increase the quality of the components, FDM systems have a mechanical nozzle cleaning unit. The cleaning unit consists of a metal brush with an upstream heat-resistant rubber lip. In Stratasys’ F123 systems, in addition to the mechanical nozzle cleaning, the purge tower is generated in parallel with the build process. In addition to cleaning, this separated object also ensures that the nozzle is optimally filled with material. Below the mechanical nozzle cleaning, the removed waste products and the extrusion beads, which are generated there in intermediate steps, are collected in a replaceable container. The extrusion beads generated in the intermediate steps are also used for optimum material filling of the nozzles. Each time the model material and the support material are changed, the nozzles are run over the mechanical cleaning combination with a rapid back-and-forth movement, and thus cleaned.

Figure 1.6FDM component with surface defects in the xy- as well as z-direction, caused by the inclusion of waste particles from the nozzle in the component (© Andreas Fischer, from [Fischer 2022])

After a layer has been created, in FDM the z-axis is lowered by the value of the z-resolution and the printing unit is moved to the cleaning unit. There, the last nozzle used, in this case the nozzle for generating the component, is cleaned. The nozzle for the support structure is prepared for the generation of the following layer. It is cleaned, extrudes briefly, and is cleaned again. Then, at the starting point defined by the path planning, the generation of the support structure begins, followed (in the same way) by the preparation of the nozzle for the component generation and the start of the layer generation of the component at the planned starting point. If the planned start and end points of the component layers lie exactly on top of each other several times, clearly visible material accumulations can occur on the surface (Figure 1.7). These seams often occur in components generated by rotational axes, especially if such a component is generated such that the rotational axis is perpendicular to the XY orientation of the system. The problem can basically be solved by changing the starting points in the path planning, but this intervention in the path planning is not possible with all Stratasys systems. However, the seams can be easily removed by mechanically reworking the components. In principle, the specific FDM surface in the z-direction is particularly suitable for increasing the grip of a component.

Figure 1.7Two-color cylindrical FDM component with seams on the surface (© Andreas Fischer, from [Fischer 2022])

1.3.2.2 Support Structures and Building Chamber

In FDM, the necessary support structures are generated via the second nozzle of the printing unit. A support structure is required to generate overhangs that exceed 45°. Here, the z-axis of the FDM system represents the zero angle. This support structure stabilizes part contours and prevents specific parts from deforming during layer generation, or falling off during the build process. On some Stratasys systems, there are three selectable strategies for calculating the support structure:

       Basic

       SMART

       Surround Support.

The “Basic” strategy represents necessary support structures, which are filled with a 45° hatching. This structure is particularly suitable for component segments to be supported, which represent a high volume and thus weight. Due to its extended volume and dense filling, the Basic strategy leads to increased support material consumption and longer manufacturing times. With “SMART”, the volume of the necessary support structures is reduced by about half of the basic strategy. In addition, the filling of the support structures is also halved, which leads to a reduction in support material and manufacturing time. The surround support encloses the entire component, and this is filled to the same density as in the “Basic” variant. This support structure is particularly suitable for filigree components, which are thus additionally protected by the structure. The consumption of support material and the manufacturing time are extremely pronounced with this strategy.

After completion of the build process, the support material must be removed from the component. This can be done mechanically or chemically. However, manual removal of the support material is possible for all geometric conditions. Support structures, which are located in cavities with little access, for example, can hardly be removed mechanically and must therefore remain in the component. However, this can be planned as a stabilizing function when designing the component. Furthermore, filigree segments of the component can also be affected during mechanical removal. The surround support strategy is not suitable for mechanical support structure removal (Figure 1.8).

Figure 1.8Surround support during path planning of a FDM component: The component is shown in red and the support structure in purple (© Andreas Fischer, from [Fischer 2021])

Depending on which thermoplastic is used for the component, the suitable thermoplastic support material can be removed mechanically or dissolved chemically. In the case of mechanically removable support material, it should be noted that cavities can sometimes be difficult to free from the support structure, and filigree component segments can be deformed or damaged by the removal of the structure. The chemically dissolvable thermoplastic support material is removed in a circulating liquid at a temperature of approx. 60 °C; care must be taken to ensure that the liquid is not too hot. Also it must be ensured that liquid can flow in and out of intended cavities through openings. If these openings are not optimally selected, the dissolution of the support structure may require longer processing times. Components with thin and large-area component segments can be easily deformed by the circulation and the influence of heat.

In principle, several components can be processed in the specially designed systems for chemical support structure removal (Figure 1.9). Components with sensitive geometric segments should be processed individually, since the components can damage each other, especially due to the circulation of the liquid. If the interior of the components was created by means of a hollow structure, these can fill with the liquid during the chemical removal of the support structure. Furthermore, the surrounded air in the hollow structures ensures that the components float to the surface of the liquid. As a result, parts of the support structure, which are thus not in proper contact with the liquid, cannot be dissolved. FDM components with hollow structures or cavities should, if possible, be positioned so that the liquid can completely cover the component. After dissolving the support material, the components are cleaned with clear water and then dried. In the case of components with cavities or hollow structure, any residual liquid should be drained off before cleaning with water. The contact surfaces of the components to the support structure have a greater tendency to show defects on the surface. In the case of mechanical removal, defects on the surface can additionally be created by the removal process.

Figure 1.9Recirculation tank of the HP Designjet 3D Removal System for the removal of support structures of FDM components made of ABS with maximum dimensions of 200 × 200 × 150 mm (© Andreas Fischer, from [Fischer 2021])

Combining ABS as the material for the component, Stratasys uses P400-SR thermoplastic support material. Fourier-transform IR spectrometry of the ABS support material shows the typical absorption bands of an acrylic copolymer based on methacrylic acid methyl ester (methyl methacrylate, MMA) and acrylic acid butyl ester (butyl methacrylate, BMA). The glass transition temperature is 103 °C (midpoint). Rheological analysis also showed that the support material is thermoplastic and that the viscosity does not decrease noticeably with time.

In the Stratasys HP systems, the SR30L support material was used in combination with ABS. IR shows the typical absorption bands of an acrylic thermoplastic based on polystyrene-co-butyl acrylate-co-acrylic acid for this material. The glass transition temperature is 124 °C (midpoint). The two Stratasys support materials can be dissolved using sodium hydroxide powder dissolved in water, by agitation and application of heat. The mixing ratio should be 950 g of the P400SC powder and 42 L of water. Removal of the support material has no detectable negative effects specifically on the ABS components. If large quantities of the support structure are removed using the chemical process, special storage and disposal of the resulting liquid may be required. The volume of the material to be disposed of can be drastically reduced by the targeted evaporation of the water content of the liquid.

Another special feature of the FDM systems are the build chambers. The size of the components that can be produced ranges from the former entry-level model of Stratasys (the Mojo) with 127 × 127 × 127 mm, to the top model (the Fortus 900mc) with 914 × 610 × 914 mm. Inbetween are the two “Idea”-series systems and the three “Design”-series with three systems. In the “Idea” series, parts can be produced on the μPrint SE with 203 × 152 × 152 mm and on the μprint SE Plus with 203 × 203 × 152 mm. In the “Design” series, components of 203 × 203 × 305 mm can be produced on the Dimension Elite and components of up to 253 × 254 × 305 mm on the Dimension 1200es or Fortus 250mc. The “Production” series incorporates Stratasys’ highest-performance FDM systems. Here, the size of the components that can be generated ranges from 355 × 254 × 254 mm on the Fortus 360mc to 914 × 610 × 914 mm on the Fortus 900mc. Inbetween are the Fortus 380mc with 355 × 305 × 305 mm, the Fortus 400mc with 406 × 355 × 406 mm, and the Fortus 450mc also with 406 × 355 × 406 mm. In terms of performance and purchase price, the current F123 series is positioned between the Fortus 250mc and the Fortus 380mc. The build chamber capacities here range from 254 × 254 × 254 mm for the F170, to 305 × 254 × 305 mm for the F270, and 356 × 254 × 356 mm for the F370. The “Idea”-series and “Design”-series systems in particular were widely adopted and are often still in use – despite discontinued service from Stratasys. Specialized companies such as iSQUARED AG from Lengwil in Switzerland continue to offer service and consumables for these systems.

All the build chambers of these FDM systems can be heated uniformly by fans to around 80 °C and are self-contained. This property is very beneficial in additive component production via thermoplastics, as it prevents thermally induced residual stresses in the component in particular. These residual stresses can lead to plastic deformation of the component, which in turn negatively affects adhesion to the substrate material and the application of further thermoplastic. Furthermore, the higher and more uniform temperature in the chamber ensures that the additive extrusion process is more effective, thus also reducing scrap during part creation. If, for example, errors occur in the material feed or in the printing unit during the creation of a component, the component continues to be kept at a constant specific temperature. Troubleshooting can then take place in parallel and the building process can be continued afterwards without any problems. When the production of a component is started, the build chamber door locks automatically and mechanically. This ensures that operating personnel are not endangered during part creation, and that the ingress of foreign bodies or particles that could negatively influence the process is prevented. In order to prevent errors when threading or removing the thermoplastic filament into or out of the printing unit, which is located in the build chamber, and also to protect the operating personnel at the same time, this process is solved by automatic conveying of the filament. The process of threading and removing the filament takes place in the temperature-controlled build chamber. The printing unit is positioned in the build chamber via the axes in such a way that bends through which the filament has to be conveyed are minimized. If the filament to be threaded is in front of the printing unit, it is placed again above the cleaning unit via the axis system. Once the processing temperature of the nozzle is reached, the filament is mechanically conveyed into it and an extrusion bead of a specific length is extruded into the waste container. The nozzle is then cleaned mechanically. When the filament is removed, the die is heated to the process temperature to break the adhesion between the cooled thermoplastic and the metal of the die. Once this is done, the filament is mechanically conveyed out of the printing unit and can then be pulled out through the feed channel.

1.4Functional Principle of Fused Layer Modeling

Fused layer modeling (FLM) belongs to the extrusion process group and uses the same process principle as described in Section 1.3. The emergence of FLM systems is due to the expiration of the maximum protection period of early Stratasys patents. This now made it possible for other manufacturers to use the process principle developed by Stratasys. Additionally, conducive to this development was the fact that the basic design of a system based on additive extrusion is simple and can be achieved with lower-cost components than other process principles.

The simplest variant here are the 3D printing pens such as the 3Doodler or the Play 3D Pens from Polaroid. The pens basically contain a mechanical conveyor for the thermoplastic material and a heatable nozzle. The nozzle diameter is around 0.4 mm, and the material is fed in by means of rods or filaments with a diameter of 1,75 mm. The thermoplastics acrylonitrile-butadiene-styrene (ABS) and polylactic acid (polylactide, PLA) are primarily used here. These 3D printing pens are very similar in function to a mechanical hot glue gun, which in turn was used as a source of inspiration in the early development phase of fused deposition modeling (FDM). In very simplified terms, additive extrusion is a hot glue gun that is moved in a targeted manner via an axis system or manipulators. In the case of 3D printing pens, the user’s locomotor system takes over the role of the manipulator. The focus of use for these devices is the creative hobby sector. In some cases, professional designers also use the systems to create sculptures – for example, Grace Du Prez and her team, who were commissioned by Japanese car manufacturer Nissan to model the Nissan Qashqai in black using a 3D printing pen on a scale of 1:1. In around 800 h of work, the 4400 mm long and 1600 mm high sculpture was created by the design team from around 14 kilometers of thermoplastic filament.

FLM is particularly characterized by the variation in the structure of the different systems. In addition to the number of melting nozzles per printing unit, worthy of mention are the manipulation system for the printing unit, the design of the building chamber, and the material feed method. The most widespread FLM system variant is the combination of one melting nozzle, a mechanical filament feeder, a three-axis gantry system, and an open building chamber (Figure 1.10).

Figure 1.10Functional principle of the FLM process with one melting nozzle (© Leonie Fensterle, from [Fischer 2020])

An important distinguishing feature of FLM from other additive processes is the possibility of using different thermoplastic materials in one component. A prerequisite for this, however, is that the materials adhere to each other. To create such multi-material components using FLM, it is possible to equip the system with several melting nozzles and respective material feeds, or to weld together pre-calculated lengths of different filaments to form a multi-material filament, which then ensures that the material change takes place at specific points in the component. FLM systems are equipped with a maximum of up to four melting nozzles. Additional melt nozzles are theoretically feasible depending on the type of manipulation system used for the printing unit. With four melting heads, three different materials and a suitable support material are possible. In addition to the adhesion between the materials for the component, the support material must be designed to provide adhesion to the component materials.

A very widespread feature of FLM is the possibility of using the material in different colors in order to produce multicolored components based on one material. It is possible to produce specific parts of a component in the required color. In addition to the system of multiple melting heads or the use of adapted and welded multicolor filaments, it is also possible to use special melting nozzles with multiple accesses for filament to produce multicolor components. These special nozzles have approximately three accesses and require coordinated conveying of the filaments.

In contrast to FDM, FLM systems with different designs are offered by various commercial manufacturers. The target group for these systems and the matching consumables are mainly private customers but also industrial customers. In addition to complete systems, kits are also widespread in the lower price segment. In FLM, the variability of the process and the large number of system manufacturers mean that new approaches and further developments of the underlying additive extrusion are also advanced at short intervals. These further developments can enable new industrial applications at the research level as a first step. In order to advance such applications, additive process parameters should be specifically optimized. These parameters are graded as follows:

1.       Reduction of the manufacturing time and thus elimination of the limited component size

2.       Increase in the number of materials that can be processed and their possible combinations in the component

3.       Increase of the surface quality

4.       In-line quality assurance of the component

5.       Integration into serial production.

The parameters (1) and (2) have been further developed by various systematics in FLM. Parameters (3) and (4) have not yet been solved acceptably for industrial purposes. Parameter (5) is partly possible through the use of industrial robots as a manipulation system for the printing unit (see Section 1.4.2.2).

1.4.1Material Feed

In FLM, the thermoplastic material can be fed via various methods to the die that applies the thermoplastic extrusion beads for the component or support structure. Depending on the method, the material is available in the following forms:

       Filament

       Foil

       Granules.

As with FDM, the most common method is to feed the thermoplastics in filament form. In addition to the filament diameter of 1,75 mm, diameters of 3 mm are also occasionally available. A larger filament diameter also results in a larger die diameter, with the consequence of a larger cross-section of the extrusion beads. As a result, the achievable resolution and the manufacturing time for components are reduced. The principle of reducing the manufacturing time for components by means of a larger diameter of the extrusion beads is used in particular for feeding the material in granular form. Depending on the material, the manufacturing costs for 1 kg of thermoplastic filament is around €13, with the conversion of the granules into filament accounting for around €8.