2 STATE OF TECHNOLOGY 2.1 Process

Fused Layer Modeling, which was invented by Scott Crump and developed by the American company ‘Stratasys’ under the name Fused Deposition Modeling (FDM), is one of the processes which use a solid wire-shaped raw material (filament). To build the part, the filament is melted in a heated nozzle and laid onto a platform (print bed), that may or may not be heated. In order to do this, the print head travels back and forth repeatedly, and places the melted filament on the print bed until the entire part contour and filling are completed. Then the print head moves to the next layer and repeats this process until the part is completely generated, as shown in figure 1. The plastic filament is unwound from a nearby roll [3]. The surface quality of parts produced in this way is usually lower than that of other processes, caused by the thickness of the filament and the so-called staircase effect.

The FLM process is not very complex and the entire equipment is therefore available at low cost [3]. With certain part geometries, such as overhangs,

removed afterwards in an additional post-processing step. This can be done manually, by the use of chemicals, or water (wash away) depending on the support material employed.

Figure 1: Schematic diagram of Fused Layer Modeling.

2.2 Material

The material for FLM processes is regularly extruded into a wire form, which can be described as a monofilament. These materials should have amorphous characteristics because of the wide process temperature range of up to of 280°C maximum. The settings for crystalline and semi-crystalline thermoplastics are much more difficult to control and the low viscosity of these materials makes the process even more complicated.

The materials provided primarily consist of thermoplastic materials such as polylactide (PLA), acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acetate (ASA) and polysulfone (PSU). PLA, which is widely used, is produced from renewable raw materials, such as corn, including lactic acid molecules and is biodegradable in industrial composting plants [4]. Visually, PLA and ABS plastics cannot easily be distinguished from each other, but they differ in their mechanical and thermal properties. During the FLM process they exhibit different adhesion to the print bed. To remain firmly on the print bed, ABS requires a higher temperature, while PLA may also be printed on an unheated platform [5]. PLA has lower temperature resistance, whereas ABS requires a higher print temperature and shows higher thermal shrinkage [3].

Wood or plaster particles can be used as filling materials to obtain organic component composites. For extremely resistant parts there is a polyamide 6

Large-Scale 3D Printers:

The Challenge of Outgrowing Do-It-Yourself

(PA 6) filament, which has particular advantages in the automotive industry, for example in gears or bearings [6].

The most commonly used materials for technical applications are acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polylactide (PLA), polyphenylsulfone (PPSU) and polyetherimide (PEI). An environmentally friendly polyvinyl alcohol (PVA) filament can be used as a support structure, which easily dissolves in water after printing, and can be safely disposed of with waste water [3].

The material can be provided in so-called filament rolls, with different filament diameters, i.e. 3 mm, 2.8 mm and 1.75 mm. Filaments for FLM must provide a round cross section, the diameter should have a maximum tolerance of +/- 0.15 mm and they have to be free of vacuoles to guarantee stable processing. The storage temperature of ABS should be close to 70°C, with a maximum humidity of 5%. Additives and fillers should be used to minimize thermal decomposition. While filament material is produced in an extrusion process, the special ‘Fuse Stick Deposition’ sticks, which are used in some FLM equipment, are produced by Exjection® molding.

2.3 FLM machines Classification

Current FLM machines can be classified according to three categories. The low-price category includes all machines below €10,000, including most of the well-known home printers, many of the machine producers, material material providers are currently active in this area of the market, all of which apply cartridge systems to facilitate material handling. Thus every manufacturer produces material that can only be used with their proprietary machines. The postprocessor software is normally included in these systems and the user can choose only very few parameter settings. For research activities, such machines are unattractive, but they ensure reproducibility and process stability for production purposes under predefined conditions.

Machines costing more than €50,000 include most large-scale machines and belong to the high-price, high-quality and special sizes field.

Machine setup

The setup of the machine has a considerable influence on quality and printing time. The mass, size and motion control of the machine has a major effect on the possible speed, build size and accuracy of the whole process.

Figure 2: Printer motion concepts.

Fig. 2 depicts the most common approaches to building FLM printers today, displaying from left to right: Cartesian FLM Printer, Delta FLM Printer, Polar FLM Printer and a SCARA FLM Printer. The Cartesian principle resembles a portal milling machine and is the most commonly used concept. It is also the structure that is used by professional, large-scale machines from Stratasys (Fortus). The Delta printer uses the parallelogram kinematics principle, and is an advantageous option for parts which are small in x and y direction, but large in z direction. The head can also be programmed to give a slightly sloped position. The concept of the polar printer is a mixture of the Cartesian and the Delta printer, the z axis and the x axis work like a portal machine, whereas the y axis is realized with a rotating print bed. It is used for round small-diameter parts with bigger z extension. The SCARA (Selective Compliance Assembly Robot Arm) uses robotic technology for the x and y axes, only the z axis is similar to the Cartesian kinematic. There are also approaches using robotic arms such as those used for welding robots.

Fraunhofer IPA has built such a machine for testing purposes.

2.4 Software

As always with additive manufacturing, a 3D CAD model is required. Most CAD-Software solutions can extract the STL format, which is a quasi-standard format for data transfer from CAD to 3D printer. The next step is to convert the STL data into a machine format, which most often uses G-code.

Using established companies’ standard software, the required postprocessor software is included and the machine control software performs the conversion automatically. When using open source or individual machine control software, the application of a special postprocessor software solution for 3D printers is necessary, for example Slic3r or Simplify3d are available.

Such software systems, which are used to set all printing parameters, influence the process considerably.

Large-Scale 3D Printers:

The Challenge of Outgrowing Do-It-Yourself