3 LARGE-SCALE FLM MACHINES

Scaled-up FLM machines are necessary in several industries, to enable large seamless parts, for example in the automotive industry in order to build complete dashboards in one part, particularly when the limited surface quality of FLM parts is not especially important, because they will require mandatory coating or surface treatment anyway.

To analyze the requirements and find potential solutions for the development of large-scale FLM machines, the process itself, as well as all possible problems, must be defined in detail. This is necessary because simply increasing the well-functioning small versions to larger versions without considering design problems does not deliver satisfactory results. Increased performance is required in all relevant areas, such as material, machine size and software. The most important requirements and solutions of large-scale FLM printers are identified in the following section.

The standard sized Mendel-Prusa and the large-scale Stratasys Fortus 900 can be chosen as examples to analyze these problems, as comparing their build chamber size and accuracy provides an idea of the challenges encountered when producing larger machines. The Mendel-Prusa is a typical home-use FLM machine that comes with a build chamber of 200 x 200 x 120 mm³ and tolerances of +/- 0.3 mm. In contrast to this the professional Stratasys printer Fortus 900 has a build chamber size of 914 x 610 x 914 mm³ and enables tolerances of +/- 0.1 mm [7]. Simply increasing the size of the Mendel-Prusa printer will further deteriorate tolerances, which is not acceptable for industrial applications.

Currently, the Fortus is the professional machine with the largest build chamber. Existing individually made large-scale printers can reach build chamber sizes of over 1 m³, for example the large-scale printer BZT M5 with 1500 x 500 x 250mm³ build chamber produced by BZT and Material4Print.

3.1 Material

Established producers such as Stratasys or 3D Systems supply only a limited choice of materials for large-scale machines. The most commonly offered and applied thermoplastic polymers for these machines are ABS, PC and thermoplastic polyurethane (TPU). Contrary to the home printer market, the variety of colors is quite limited in the professional sector. This limitation can currently only be resolved by building an individual large-scale printer, because only an independent software system allows programming with parameter sets outside the pre-sets.

Another considerable problem with large-scale printers is the warping effect, which also exists on smaller machines, albeit on a lesser level. Warping is a particular thermal bending effect of the part as shown in fig. 3, and results from the thermal shrinkage of thermoplastic material during the cooling phase.

Figure 3: Part displaying warping effect.

While the upper layers are being built up, the lower layers are already cooling down and consequently shrink, because of the increased tension stresses inside the part, so the edges of the part bend up. This effect is particularly pronounced when using standard materials on large-scale machines, as the parts are much larger than standard ones, higher temperature differences develop between the layers, which subsequently causes major tension stresses and warping. Using a heated print bed and having a temperature control inside the build-envelope can reduce this effect. Another way to solve this problem is the use of fillers, which are thermally inactive, such as minerals, for instance the well-known ‘Laywoo-3D’ material, which is made out of 60% PLA and 40% wood fibers, has never shown any sign of warping.

The temperature control of the whole process including the build chamber, the nozzle and the print platform will have huge impact on the material properties. An overheated material will induce melting of the whole part, while fusing layers will become difficult if the temperature is too low.

3.2 Machine

Many large-scale machines are currently just scaled-up versions of standard machines. Examining these stretched machines, they often show unstable behaviors and insufficient rigidity, producing vibration, which causes problems in the part being produced. Machine concepts with printer heads attached to more rigid machine types show much better process results.

There are two main types of machines for large-scale processes: On the one hand there is a robotic arm with an attached printer head, which at first glance, seem to be acceptable regarding motion speed, accuracy and number of axes. More than three axes are not provided by standard software solutions. Therefore it is necessary to write a special postprocessor program for such machines. Furthermore, the high speed and accuracy of these

Large-Scale 3D Printers:

The Challenge of Outgrowing Do-It-Yourself

systems is largely unnecessary, as FLM structures are not usually created for the extremely small tolerances that such a system is able to provide. A potential advantage of applying robotic arms is the opportunity to increase the number of them to significantly increase productivity.

Portal frame type milling machines can be equipped with an attached printer head. A disadvantage of this solution is that these machines are not as fast and the functionality is limited compared to robotic solutions. But well-established standard post-processing software can be used, as the setup of these machines equals the Cartesian printer. The accuracy is normally good enough for all FLM printing processes. Another positive aspect is that a milling machine can be used for milling after changing the tool and removing the print bed.

Besides the machine setup, the printer head has a considerable influence on the accuracy, visual and haptic appearance of the product as well as on the process speed. The printer head is normally built out of three parts: the nozzle, the heater and the thermal insulator. The nozzle diameter determines the layer height, the heater is necessary to melt the thermoplastic filament and the thermal insulator is required to insulate the heater and the material supply system, as shown in fig. 4.

Figure 4: Printer head (hot end).

The choice of nozzle depends on the desired process result. If it is necessary to produce a part very fast but high accuracy is not required, a nozzle with a large diameter is useful as it creates thicker layers. If a high surface quality or dimensional accuracy is needed, it is more appropriate to use a nozzle with a smaller diameter, which will subsequently increase the process time.

Only two potential solutions are available as heating devices: a selective

wrapped around the whole heating device. The latter optimizes the melting process by equalizing the heat distribution in the thermoplastic material and enables a much faster heating-up time. The insulation must prevent heat transfer from the heater and internally works in a similar way to a hydraulic cylinder, by using the filament as a type of piston.

The material supply and feeding device also has an influence on the quality of the printing process, the two methods being direct drive and so-called Bowden extruders. In direct drive extruders, the material supply motor is positioned directly on top of the insulation, which makes it easy to control the parameters with standard control software. With Bowden extruders, the motor for the material supply is mounted outside of the build envelope and machine frame, and the material is fed through a polytetraflouroethylene (PTFE) tube. The use of only one of these feeding systems for a large-scale machine causes problems as the material support may be interrupted due to the long travel for the printer head. A combination of both systems may solve this problem.

Another major influencing factor on material and part quality is the temperature inside the machine envelope. Most materials, particularly ABS, require a heated printer bed to aid adherence between bed and part. The part is connected to the bed only by adhesion forces. Internal stresses caused by temperature gradients inside the build chamber can cause part detachment. To prevent process failure, caused by cold air from outside, and to get a more even temperature distribution, it is beneficial to build a closed build chamber. Additionally, a controlled active heater for the build chamber can be used to achieve constant temperatures and to consequently solve the problem of uncontrolled part shrinkage.

Certain problems are not common with small-scale printers, but are very important to large-scale ones. An example of this is that material handling between the roll supplying the material and the printer head has to be guaranteed, and it must be ensured that the filament wire does not touch the part, or collide with any printer component. This can be solved by using a PTFE tube for carrying and protecting the material. All other components, for instance power cables and signal wires, must also be protected by tubes to avoid collision.

Furthermore, that the material remains inside a heated build chamber for a long time, before fusing in the printer head must also be taken into account.

Emerging thermal material expansion can be avoided by a metal covered PTFE tube, which must have a slightly larger diameter than the filament itself. Further to this, bridging of long travel distances is an issue: if the material roll is positioned in the center of the machine, and at the top of the machine, the distances to all corners is the shortest possible, this is an optimal point. Small machines do not have long travel problems. However, with large-scale machines, the distance from the center to the corners is much greater and forces the material to be drawn out of the printer head. To prevent this, a brake device or a special command in the control program is necessary. The same problem occurs in reverse when the material is

Large-Scale 3D Printers:

The Challenge of Outgrowing Do-It-Yourself

pressed through the printer head and forms little droplets at the tip of the nozzle, which will cause decreased part quality. The solutions presented above can also solve this problem.

Another challenge encountered by small printers, is thermal expansion of the heated printer bed and its extensive influence on the whole process. There are two areas of concern: Between the heater and the printer bed and between the printer bed and the part. The first problem can be minimized by using a thermally inactive material as a printer bed or by including heating elements inside the printer bed. The second problem is far more difficult to solve, because the challenge is that separate printer beds have to be used for each printing material, each having nearly the same coefficient of thermal expansion as the printing material. This causes a problem when using different materials for the support and the part, which may each have different thermal expansion coefficients. To overcome this difficulty, most of the larger machines use a ‘raft’ structure as the first layer. This structure can compensate for the tension between the materials and the heated bed.

The problem of thermal expansion has to be allowed for in the design and construction of all machine parts. One solution might be the use of an expansion joint between the machine parts.

3.3 Software

In general the same software application can be used for standard FLM machines and for large-scale printers, but professional solutions often cannot be used for printers of other sizes because the user cannot change the embedded settings. In contrast, open source solutions open up the opportunity to change every parameter, but they are often not capable of processing large data files sufficiently and may crash while slicing the part.

The build and travel direction, and the process-parameters that are set in the machine postprocessor controller, also have significant influence on the mechanical properties of the part. Additionally the different part filling (infill) strategies can have a considerable influence on the strength and the process time, too. Finally, in order to solve all the problems mentioned above, the best approach is to program an individual software solution, customized to the systems being used.

3.4 Post-processing

After finishing the print process, the parts generally need to be reworked.

With FLM, this is necessary due to the often unavoidable use of support structures. These structures are mainly to support the main geometry where there are gaps and free hanging structures. They can be made out of the same material as the main part, or a cheaper and easier to remove material.

Some support structures are water-soluble, some are soluble in solvents, and some have to be removed mechanically. After removing the support structures, the parts must be cleaned of residue and they are then ready for

If a higher surface quality is needed, the parts can be reworked using traditional methods. Additionally, ABS has a particular reworking method to increase surface quality, if technical dimensional stability is not the main aim it can also be treated with acetone to raise the surface quality.