Calibration

In order to maintain build quality, it is necessary for regular calibration to take place. These can be divided into 2 categories, initial setup procedures and regular procedures. During initial setup, the Z-axis platform must be calibrated so the system knows where the resin tray is in relation to its upper stop point, which is found accurately using an integrated pressure sensor. At the same time, the resin tray support mechanism must be made parallel to the build platform. Failure to complete this can result in damage to the machine, as the platform may crash into the tray, or failure of builds, as the platform will not get close enough to the tray for the cured resin to stick to the platform.

The Z-axis initial calibration uses a “calibration tray”, which has the same thickness (2 mm) as a standard resin tray, but lacks the sidewalls that would interfere with the calibration process. The tray is shown in situ in Figure 4.10. The Z-axis platform, with build platform installed, is manually lowered until it is a short distance from the calibration tray, which has been covered

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with a piece of clean A4 paper. The paper is used to protect the calibration plate, as a 100 µm thick steel feeler gauge is pushed underneath the 4 corners of the build platform. If one or more corners are felt to be “stiffer” in gauge insertion than the others, the calibration tray holding mechanism can be altered in its orientation using the spring-loaded screws on each corner.

Figure 4.10 – EnvisionTEC Perfactory Mini Multi-Lens system with calibration plate inserted in place of the resin tray. Also pictured is the integrated light sensor.

This process carries on, normally with multiple adjustments, until the operator is reasonably certain that each corner is as close to the calibration tray as possible, and that therefore the tray and build platform are parallel. The paper is then removed, and the Z-axis platform is moved down 300 µm – 100 µm for the paper, 100 µm for the feeler gauge, and 100 µm to allow for operator error. The build platform is then flush against the calibration plate, and

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this value is taken as the “zero” point for the Z-axis. A further menu allows for the system “pre- pressure” to be set. At the start of the build, the platform lowers to the zero position, and then moves a set distance further, normally around 50 µm. This extra pressure, against the silicon- coated tray, creates and extra-thin layer of resin to be cured, which enhances the adhesion of subsequent layers to the build platform.

Once the position and orientation of the resin tray has been fixed, the projector must be focused. Due to the short distance between the projector lens and the resin tray (around 150 mm) and the high resolutions involved, even small changes in the position of the resin tray can cause the projector not to be focussed exactly onto the top of the resin tray, where each layer is cured. The calibration process however is somewhat crude. One operator must reach into the machine, and take hold of the focussing ring of the projector lens. A second operator loads the calibration plate, and places smoked paper across it. In the focussing calibration mode, a fine detail mask is projected, which the second operator views through an eye piece through the smoked paper. The first operator then moves the focussing ring back and forth, until the second finds the point at which the image is in focus, and the procedure finishes. As mentioned above, this system is rather crude, and could do with automation. However, it is possible for a skilled operator to reach a good level of accuracy.

Another initial setup calibration is the size of the build area. This can vary with the exact position of the resin tray and the projector focus post calibration. Again, this is measurement is taken in a slightly haphazard method, using the smoked paper and a pair of callipers. The projector emits across its whole projection area, which shows as a rectangle on the smoked paper. This can then be measured using the callipers in the X/Y axes. The measurements are entered into both the machine via its keypad, and into the Perfactory Configuration Center (see below), through which the further calibration option of the “cube job” is available. This job file builds cubes of a predetermined size in the corners of the build area, which can be measured in

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turn to check that both the size of the parts is accurate and the build area is rectangular. Defects in the shape of the build area can be caused by the projector not being parallel to the resin tray, which can be set up using a spirit level on both the calibration plate and the projector itself. The projector frame can then be altered using integrated screw threads until it is parallel with the resin tray.

There are two types of calibration that must take place every time the machine is switched on. Due to variations in the light output of the projector bulb over time, it is necessary to calibrate the projector brightness. This is carried out with the calibration plate inserted. The Perfactory Mini Multi-Lens system has a built-in light sensor, pictured in Figure 4.10, which is used to measure the light output of the projector. In the projector brightness calibration procedure, a round masked output is projected through the calibration plate, with the diameter of the light beam being slightly larger than that of the light sensor. The operator places the light sensor over the output beam, and the light level is read off the integrated liquid crystal display of the system. The light level can then be increased or decreased by the operator, in steps of around 5-10 mW/dm2. For R11, the recommended light level is around 600 to 620 mW/dm2, but tests found that a lower level of 580 mW/dm2 produced less “over-cure”, where the curing process in overhanging areas was too high. This thickened the overhanging structure, resulting in geometry defects.

Once the projector brightness has been set, the “grey mask” must be calibrated. As mentioned earlier, the DLP chips employed in the Perfactory Mini Multi-Lens projector have many micro-scale mirrors, which can be switched between “on” and “off” positions. This is a digital process, but the mirrors can be driven by pulse width modulation to produce shades of grey. It is found that different areas of the DLP chip can be more reflective than others, requiring a calibration step to ensure that the light level is consistent across the whole build area allowed by the projector.

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Much as with the projector brightness calibration procedure, a round masked beam is projected up through the calibration plate, which the operator once again places the light sensor over. The light level is measured, and the operator confirms a stable reading has been reached by pressing the machine’s “Enter” button of the machine. The next beam is then projected onto the build area, working from top left to bottom right, moving in rows left to right across the build area. A total of 48 individual readings are taken, which overlap to allow better calculation of the average light level. Once the procedure has been completed, the lowest and highest light levels are displayed, along with the percentage difference between them, which can be used to diagnose problems with the DLP chip. On average, a non-calibrated system will have a max/min difference of around 10-15%. The projector then outputs across the whole of the build area, allowing the operator to check the grey mask settings with the light sensor. An observable difference of 10% across the range is acceptable, although in practice a value of less than 5% is easily achievable.

4.8 Software

4.8.1 SolidWorks

SolidWorks is a 3D CAD package designed for applications such as automotive design. At a basic level, it works on the principle of “solids”, where 2D geometric shapes are extruded to form simple structures. Multiple planes can be created for the creation of more complex compound shapes, along with extruded cuts. Extruded cuts and bosses can be defined either simply by a 1D direction, or in shapes that are more complex by using a path drawn on a perpendicular plane. Shapes can be mirrored, or patterned across a surface, with the patterns either being defined linearly or over curved paths, defined either by edges on the component already created or using lines in separate drawings. There are built-in text tools, which allow embossed labelling, along with other finishing tools such as chamfer options. Importantly, the program allows the export of high-resolution STL files for use in the rapid manufacturing systems.

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The package also includes a number of more advanced features, such as the ability to build large working assemblies from multiple CAD structures. Relations, or “mates”, can be added between each part to define their interactions, and complex systems such as car engines can be linked together as they would be in real life. This allows an extra layer of prototyping and testing for large-scale applications, but is less useful in microengineering applications where most of the parts built are monolithic. SolidWorks also has the option for direct output of CAD data in a technical drawing format. These drawings can also be exported as DXF and DWG files (AutoCAD etc). Finally, the built in mechanical testing software, COSMOSWorks can be used in a number of configurations, including calculation of mechanical stress, fluid dynamics calculations and in areas such as heat transfer through a structure.