EnvisionTec Perfactory Mini microstereolithography machine

A modified EnvisionTec Perfactory Mini1 was used to fabricate flow cells directly from 3D CAD files designed in SolidWorks 2009. This machine uses MSL, a type of additive layer manufacture (ALM) discussed in chapter 3, to build 3D objects by sequentially depositing successive layers. Figure 4.1 shows a schematic of the Pefactory machine whilst figure 4.2 shows how the EnvisionTec machine fits into the work scheme for creating flow cells.

108 Figure 4.1: Schematic of EnvisionTec Perfactory MSL machine. 1: Removable build platform onto which completed parts are attached. 2: Tilting resin tray. 3: Z-stage driven by a lead screw. Computer is connected to the network for transfer of build jobs.

Parts fabricated on the EnvisionTec Perfactory are built onto the build platform, figure 4.1 (1). The build platform consists of a glass block and metal rails that facillitate attachement to the Z-stage, figure 4.1 (3) of the Perfactory machine. The liquid resin from which parts are cured is held in the resin tray, figure 4.1 (2). The resin tray of the EnvisionTec Perfactory Mini consists of a glass base onto which a thin (~2 mm) layer of transparant silicone rubber is attached. The silicone top surface is also treated with an agent to aid detachment of each layer. Walls around the resin tray hold in the liquid resin. The volume of resin can be varied depending on the task at hand.

The projector in the EnvisionTec Perfactory has been raised closer to the build platform to reduce the pixel size so that the minimum feature size can also be reduced. A modified version of the EnvisionTec Perfactory Mini firmware and software was kindly

109 provided by EnvisionTec. The resin tray, shown in figure 4.1, has a tilting mechanism. Thus, when a layer of resin is cured, one end of the resin tray is pulled down, away from the build platform. This tilting causes the part, including the newly formed layer, to peel from the resin tray. The peeling process is employed to reduce delamination of the new layer. Layer delamination is discussed in section 4.1.8.

4.1.1. EnvisionTec Perfactory workflow

The workflow for the EnvisionTec Perfactory is described in figure 4.2. STL files created in a CAD package, such as SolidWorks, are ‘sliced’ into layers by EnvisionTec RP software, compiled into ‘job’ files and transferred to the machine. On running a job file from the machine, the job file is unpacked and the build is executed. Once complete, parts are taken off the build platform using a scalpel or sharp knife. The part is then transferred to a 250 mL beaker and washed with acetone and/or isopropanol. Swilling of the beaker can then be employed if necessary, to desorb the uncured resin from the solid part. Acetone is a more aggressive solvent than isopropanol and will dissolve cured resin slowly. For this reason either acetone washing is employed sparingly or isopropanol is used. Difficult to clean parts can be soaked in isopropanol and continuously mixed using the horizontal rocker. High pressure air can also be used to clear uncured resin from tubes or holes where appropriate. Finally, the part is post-cured by flashing in a UV flasher box (see section 4.1.8

110 Figure 4.2: Workflow schematic of making parts with the EnvisionTec Perfactory Mini machine.

4.1.2. EnvisionTec Perfactory build material

The EnvisionTec Perfactory uses blue light (mainly between 250 and 550 nm) to cure photosensitive liquid resins into solid polymers. Although a variety of compatible resins are available, only one resin was used here, R11. R11 is a liquid resin composed of a di-acrylate monomer, tri-, penta- and hexa-acrylate crosslinkers, a free radical photoinitiator and a dye to control light penetration. The photoinitiator causes a radial reaction between acrylate groups in the mixture. Any of the acrylate group-containing species in the mixture can be covalently bound to any other. The result is a highly complex, disordered polymer. The dye limits the depth through the resin that the light can penetrate such that after around 25 µm the reaction is effectively prevented. The reaction is exothermic and polymerisation can

111 also be initiated by heating, so the dye also prevents a runaway reaction that could cure the entire of the available resin and damage the resin tray.

Several other materials are available for use with the Perfactory system2. These materials are formulated to include material properties for specific applications: Photosilver resin has high temperature resistance suitable for making moulds for vulcanising rubber (~130°C). PIC and WIC resins can be removed easily from an encasement by heating (‘burning out’) making these resins suitable for the production of moulds used in the jewellery industry. eShell resins are opaque and are formulated in a variety of colours for the manufacture of discrete hearing aids. NanoCure resin contains suspended nanoparticles that provide high stiffness and temperature resistance as well as being hard wearing.

4.1.3. Optical characterisation of R11 resin

R11 is formulated in several colours as a result of the dye molecule used, R11, red, blue, clear and rose. To determine the absorption profile of different colours of R11 resins available, samples of each resin were prepared. Sheets of each resin 0.5 ± 0.01 mm were cut from blocks of cured resin. Sheets were then polished using fine grain sandpaper followed by polishing with Wenol (Reckitt Benckiser, DE). Sheets were then trimmed to 9 mm wide pieces to fit into the sample holders (Hellmet spectrophotometer optical calibration filter holders). All pieces were tested in a Cary 100 Bio spectrophotometer (Agilent, UK) and the results presented in chapter 6.

4.1.4. EnvisionTec Perfactory capability

The maximum build envelope of the EnvisionTec Perfactory is 28 x 21 x 250 mm which is defined by the projected area and the maximum travel of the Z-stage. The resolution of the projector in the EnvisionTec Perfactory is 1400 x 1050 pixels which means that each pixel is

112 20 µm square. Theoretically, the minimum feature size is the same as the pixel size. In practice, the minimum feature size is around 100 micrometres for experienced users in good conditions. Furthermore, a wall is more likely to build successfully than a tower of the same diameter as the wall width. Single towers are not attached to the rest of the part by a sufficiently large surface area leading to delamination during the peeling step.

The key parameters affecting minimum feature size are exposure time, projector brightness and resin condition. Exposure times from 3.7 to 9.5 seconds will result in successful builds. Delamination increases with shorter exposure times. The same exposure and peel settings must be used throughout the part to get an even finish with minimal delamination. Projector brightness settings from 560 to 620 will result in successful builds. The effect of projector brightness on builds is very similar to exposure time. Resin condition deteriorates through use or time. The best builds (minimal delamination, smallest features) are made with new resin. Build failure results in significant deterioration of resin condition, but successful builds will also result in slow deterioration of the resin. Poor resin condition is defined by a high viscosity of relative to new resin and the presence of lumps which accumulate from failed and delaminated builds. Resin can be filtered through a 1 mm2 steel mesh in a Buchner funnel attached to a vacuum pump (Caper 2D, Charles Austin Pumps, UK) to remove the larger lumps. Smaller lumps are soft enough, however, to pass through the filter. Filtered resin significantly improves the build quality. Filtering is only performed once per batch of resin as the accumulation of small, unfilterable lumps renders the resins unable to produce good quality builds.

4.1.5. Burn-in range settings

A cured layer of R11 resin will make a conformal bond with the glass surface of the build platform and the silicone rubber of the resin tray. To ensure that the nascent part is securely fastened to the build platform rather than the resin tray, the first several layers

113 (~400 µm) are built with ‘burn-in’ settings. Burn-in settings comprise a longer exposure time (9.5 seconds) and a slower peel speed (800 µm/s) relative the standard range.

4.1.6. Build range settings

The remaining layers of the part are built with standard build settings; exposure time of 3.7 seconds and a peel speed of 1200 µm/s. These are sufficient to cure layers and allow parts to be built rapidly.

Thin parts tend to warp after they are removed from the machine. The warping process is most likely a result of the change in build parameters between the burn-in and the build ranges. Warping can be reduced by clamping thin parts flat during the post curing process. Warping can also be reduced by building the build range with the same settings as the burn-in range. Using the same settings for the burn-in and the build ranges is also used in taller parts where a consistent finish is required.

4.1.7. WYKO build characterisation

To characterise parameters such as surface roughness and layer thickness a Wyko (Microprecision Instruments, UK) optical profiling system was used. The Wyko uses interferometry to vertically scan a surface returning a 3D representation of the surface which can then be analysed. Analysis was carried out using Gwyddion (version 2.25-1); an open source scanning probe data analysis software3.

4.1.8. Post curing

Parts are post cured in the EnvisionTec flasher box (Otoflash, EnvisionTec, UK) after building. The flasher box has two metal halide tubular arc lamps which emit flashes of bright white light at ~10 Hz. EnvisionTec recommend at least 3000 flashes per part. The parts become notably hot during flashing so parts are flashed 1000 times, turned, left for a minute and flashed for another 1000 times. The heating of a part is probably at least in part

114 due to the exothermic reaction of the R11 monomer as it reacts. Flashing is necessary to ensure that all the uncured monomer that is trapped within the part is fully reacted. Uncured monomer can have a significant impact on the properties of finished parts. Post curing is necessary in order to maximise the strength of R11 parts and to prevent inhibition of the PDMS curing reaction. Post curing also reduces, but does not eliminate, warping in thin parts.