Processing

Following will illustrate the CeraFab 7500-system provided by Lithoz GmbH, with its main features, components and the operating software.

3.2.2.1 Lithoz GmbH CeraFab 7500 - system

The Lithoz CeraFab 7500 – 28 system (Figure 3) was the machine used for the production of HA scaffolds at Lithoz GmbH.

Figure 3. CeraFab 7500 system [1].

Building envelope (X,Y,Z)

~ 76 mm x 43 mm x 150 mm

Pixel 1920 x 1080

Resolution 40 µm (635 dpi)

Layer thickness

25 – 100 µm

Light Source Light emitting diodes (LEDs) Manufacturing

speed up to 10 mm per hour Data type .stl (binary)

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The CeraFab 7500 system uses the Digital Light Processing (DLP) based stereolithography (SLA) method, previously described in Chapter 2.

This system is among the most popular for the manufacture of complex 3D ceramic structures.

State-of-the-art industrial electronics, as well as the opportunity to control the manufacturing of the part in real time, guarantee absolute precision of the printed object (in the micrometer range) combined with short production times.

The Cerafab system allows different types of ceramic powder (e.g. Alumina, Silica, TCP,

Magnesia, Porcelain and HA) to be homogeneously dispersed within the organic matrix containing reactive monomers, solvent and photoinitiator.

The setup is a constrained surface DLP (digital mirror device) system with an LED radiation source in the blue visible light.

The light engine utilizes powerful LEDs as a light source and a DMD (digital mirror device) chip as a dynamic mask with a resolution of 1920 × 1080 pixels and a pixel size of 40 × 40 μm [2].

The DMD - chip of CeraFab 7500 guarantees a selective photopolymerization of the layer; the resolution in the x/y plane is 40 µm and a single layer thickness that can vary between 25 - 100 µm, depending on the manufacturer's choice.

Layer-by-layer production involves the addition of fresh slurry every time a layer is produced.

Figure 4 illustrates how the light source radiates from below the rotating vat uniformly filled with slurry.

When the photoinitiator is hit by external light, it forms free radicals which start the polymerization by reacting with the monomers contained in the mixture.. The chain reaction forms the desired matrix of monomers that will bind the ceramic particles together in the desired pattern.

Compared to other methods of vat photopolymerization, the parts produced are no longer immersed in the slurry and this not only reduces the amount of material required but also the risk of introducing defects during construction [3].

Figure 4. Functional principle of the ceramic DLP-system by Lithoz GmbH; the arrows indicate the directions along which the building platform and the vat move [1].

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Figure 5 illustrates the machine’s workspace and its main components:

Figure 5. Workspace of the CeraFab 7500 – 28.

The workspace of CeraFab 7500-282 is composed of:

- Rotation Vat: contains the liquid slurry and is coated with new material after each new layer is formed. The vat consists of a layer of transparent glass interposed with a layer of silicon. The lower layer of the vat is irradiated by the light source.

It is worth noting that the newly created layer does not remain attached to the surface of the vat, but that all layers are firmly fixed to each other and attached to the building platform.

To reduce the forces of interaction between the scaffold under construction and the surface of the vat, the latter is coated with a hydrophobic silicone film. In addition, after

polymerization of the layer, one side of the vat is slowly lowered before the other, in order to facilitate the detachment of the new layer from the surface.

- Building Platform: is placed above the vat and forms the basis for the construction of the structure. It consists of a metal body and a glass plate and can only move along the z axis.

To improve the adhesion between the building platform and the scaffold under construction, the glass platform has been coated with an FDM Adhesion Foil.

- Cartridge: contains the slurry. Through a controlled dosing system, new liquid slurry is added to the vat each time the next layer is formed.

Vat Cartridge Wiper

Blade Building Platform

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- Wiper blade: its task is to evenly distribute the new slurry added to the vat after the construction of a layer. When adjusting the position of the wiper blade, the height of the slurry on the vat may be modified; the initial slurry thickness is one of the two parameters which must be checked before initiating the printing process.

3.2.2.2 CeraFab Hardware Control (HC)

CeraFab Hardware Control (HC) is the user interface for direct interaction with the Cerafab 7500 machine. Through the machine's touch screen, the operator can start printing, check the process status, the number of layers being printed and the time required to complete the job, move the building platform and check the general machine status [1].

The HC control panel provides access to the software directory of the machine to which the fab - file previously created with CeraFab DP has been transferred.

After selecting and confirming the working file, this is loaded and the slicing starts. During slicing the parts are cut into individual layers, according to the predefined layer thickness.

Two mandatory checks, described below, must be performed from the control panel before starting to print, as the accurate assembly of the vat, the building platform and the wiper blade are not sufficient to guarantee a good final result of the product printing; it is thus necessary to:

1. Ensure that the slurry evenly covers the vat’s surface by sing a rubber spatula to pour the slurry, taking care not to rub against the vat. Then, through the control panel, set the continuous rotation of the vat to about 1 minute so that the material is evenly distributed on the surface.

Finally, measure the initial level of liquid slurry present in the vat and if the height does not reflect the desired level, change the position of the wiper blade.

In the present work, the height of the slurry in the vat was 175 µm.

2. Make sure that vat and building platform are parallel to each other.

This alignment is essential for the success of the manufacturing run and the quality of the fabricated parts.

Once the fab-file has been loaded and all aspects of the machine have been checked, printing may begin. The process starts with a complete rotation of the vat, then the building platform lowers towards the vat until there is space left which is equal to the height you wish to achieve for a single layer. The next step involves the selective exposure of the slurry to visible blue light with consequent polymerization of the desired part. To facilitate the separation between the newly formed layer and the surface of the vat, the latter is slowly tilted downwards and at the same time the building platform is raised as much as the height of a single layer. The sequence of the process is then repeated, as the next layer is formed, and so on until the entire job is completed.

The Table 3 shows all the various types of tests conducted during the Lithoz period to characterize the LithaBone 480E slurry and the printing work to obtain the final porous scaffolds. All tests will be explained in detail below.

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Table 3. Tests/ Jobs conducted in Lithoz GmbH.

Tests/ Jobs Chapter Geometry of printed samples

Aim

Stability Tray Test

3.3. 7 compact bars (2.5x2x25 mm) 4 cuboids

2 porous open - cell cylinders

3 - point bending strength test, density test and evaluation of the feasibility of printing scaffolds with controlled porosity.

Viscosity test 3.3.1. Uncurred LithaBone 480E slurry

Rehological behaviour of LithaBone 480E slurry.

Grindometer test

3.3.2. Uncurred LithaBone 480E slurry

Check the degree of dispersion of ceramic particles into the LithaBone 480E.

Shrinkage factor 3.3.5. 18 compact bars (2.5x2x25 mm)

Evaluate the shrinkage factor in x-y direction and z direction.

Design

specification for printing

3.4. Different sample geometries use pre-existing fab-files

Check of:

- Wall thickness - Aspect Ratio - Overhangs

- Minimal Feature and overpolymerization

Non-porous scaffolds

3.5.1. 3 different sets of

compact cylinders Evaluate the possibility of printing non-porous scaffolds.

Cubic porous scaffolds

3.5.2. Cubic scaffold (4.72x5.15x5.0 mm)

The aim of the thesis: obtaining bone-like bioceramic scaffolds using µ-CT with DLP process.

Cylindrical porous scaffolds

3.5.2. Cylindrical scaffold

(5x5x10 mm) The aim of the thesis: obtaining bone-like bioceramic scaffolds with aspect ratio 2:1 using µ-CT with DLP process.