FIELD OF THE INVENTION

[0001] The invention relates to 3D printing and, in particular, to photo-solidification printers.

BACKGROUND OF THE INVENTION

[0002] Photo-solidification (which may also be known as Stereolithography, Photo- Solidification, Solid Free-Form Fabrication, Solid Imaging, Rapid Prototyping, Resin Printing, and 3D printing) is a form of additive manufacturing technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photopolymerization, a process by which light causes chains of molecules to link together, forming polymers.

[0003] One type of stereolithography is an additive manufacturing process that works by focusing an energy source on to a vat of photopolymer resin. With the help of computer aided manufacturing or computer aided design software (CAM/CAD), energy source is used to draw a pre-programmed design or shape on to the surface of the photopolymer vat. Because photopolymers are photosensitive, the resin is solidified and forms a single layer of the desired 3D object. This process is repeated for each layer of the design until the 3D object is complete.

[0004] Another type of stereolithography uses 'bottom-up' manufacturing. Such systems have an elevator platform which descends to a distance equal to the thickness of a single layer of the design (e.g. 0.05 mm to 0.15 mm) into the liquid photopolymer. Then portions of the liquid photopolymer between the object or platform and the vat base are cured to cause the liquid to solidify. A complete 3D object can be formed using this process.

Prior Art Review

[0005] US 2015/0290876 discloses a stereolithographic apparatus including a container 300 for containing liquid photosensitive resin; an imaging means 200 for displaying a contour of a two-dimensional image with a transparent region inside the contour; a light source 100 device for projecting light onto a surface of the liquid photosensitive resin 400 through the transparent region to cure the liquid photosensitive resin; wherein, the imaging means is disposed between the container and the light source device; the light source device is a area light source emitting substantially panel light.

[0006] US 2015/0137426 discloses an additive manufacturing device, comprising: a vessel for containing a material which is polymerisable on exposure to radiation; a build platform having a build surface, the build platform being mounted or mountable for movement relative to the vessel; and a programmable radiation module comprising an array of individually addressable radiation emitting or transmitting elements, the array being configurable to produce radiation having a predetermined pattern by selective activation of elements of the array.

SUMMARY OF THE INVENTION

[0007] In accordance with the present disclosure, there is provided an LCD assembly configured to emit light in the 375-395nm wavelength range for use in light curing of a resin in 3D printing, wherein the LCD assembly comprises:

a light source configured to emit light with a wavelength between 375 and 395 nm;

first and second polarizers with a crossed polarization axes; and

a liquid crystal layer positioned between the polarizers,

wherein the LCD assembly is configured such that when light from the source is passed through the first and second polarizers and the LCD, the emitted light has a maximum spectral intensity between 375-395nm.

[0008] At least one of the polarizers may comprise a wire grid. The polarizers may have a peak transmissivity (e.g. a global or local peak in transmissivity) in the 375-395nm wavelength range.

[0009] The liquid crystal layer may comprise a TFT substrate.

[0010] The liquid crystal layer may comprise a twisted nematic liquid crystal layer.

[0011] The light source may comprise one or more LEDs. The light source may comprise a laser. The light source may comprise a fluorescent lamp. The light source may comprise a gas-discharge lamp. [0012] The LCD assembly may comprise an electrode layer, the electrode layer configured to define pixels my selectively activating portions of the liquid crystal layer.

[0013] In accordance with the present disclosure there is provided a 3D printer comprising the LCD assembly as described herein.

[0014] In accordance with the present disclosure there is provided an apparatus for making a three-dimensional object by photo-solidification, comprising:

a vat configured to contain UV solidifiable resin;

a UV light source; and

a pixel array positioned at the base of the vat, the pixel array having a liquid crystal sandwiched between two UV-compatible polarizers, wherein each pixel corresponds to a portion of the liquid crystal which can be switched between:

a transmitting state which allows UV light from the UV light source to pass through both polarizers to solidify at least a portion of the UV solidifiable resin in contact with the vat base; and

a blocking state which prevents UV light passing between both polarizers.

[0015] The UV light source may comprise an array of LEDs.

[0016] The UV light source may comprise an array of LEDs, each LED having a corresponding parabolic reflector configured to collimate light from the respective LED through the pixel array.

[0017] The UV light source may comprise an array of LEDs, each LED having a corresponding lens configured to collimate light from the respective LED through the pixel array.

[0018] The UV light source may be configured to emit radiation between 350-395nm.

[0019] The two polarizers may be oriented at substantially 90° to each other.

[0020] The pixel array may comprise a thin-film-transistor liquid-crystal display.

[0021] The vat may comprise a UV opaque cover configured to prevent the exposure of the UV solidifiable resin to stray UV light. [0022] Each pixel is configured to be the same transparency to UV light as each neighbouring pixel when in the transmitting state.

[0023] In accordance with the present disclosure there is provided a method for making a three-dimensional object by photo-solidification, the method comprising:

placing UV solidifiable resin in a vat above a pixel array, the pixel array having a liquid crystal sandwiched between two UV-compatible polarizers, wherein each pixel corresponds to a portion of the liquid crystal;

illuminating a UV light source; and

switching one or more pixels between:

a transmitting state which allows UV light from the UV light source to pass through both polarizers to solidify at least a portion of the UV solidifiable resin in contact vat base; and

a blocking state which prevents UV light passing between both polarizers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Various objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. Similar reference numerals indicate similar components.

Figure 1 a-1 c are cross-sectional schematic views showing how a stereographic 3D printer builds and object.

Figure 2 is an exploded view of the layers which make up a LCD screen.

Figure 3 is a cross section view of an embodiment of the invention.

Figures 4a-9c are graphs showing various experimental results.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

[0025] As noted above, some progress has been made in using conventional LCD screens as to cure successive layers of the object being printed. Using a screen with pixels may allow the entire layer of be solidified simultaneously. In contrast, a laser systems typically require the light source to scan across the layer being printed. This may make scanning techniques more difficult to scale up.

[0026] The inventors have realized that a disadvantage associated with methods employing conventional LCD screens is that the polarizers that allow images to be displayed through the screens only allow certain wavelengths to pass through. That is, most light with wavelengths shorter than 395nm will be blocked. Another problem identified by the inventors is that the LCD layers may shift the wavelength of the light emitted by the light source so that the light which is impinging on the resin has a significantly different spectrum compared with that emitted from the light source. This can result in diminished control in how the resin is cured.

[0027] In particular, the inventors have realised that simply replacing the conventional visible light source of a 3D printer with a UV source may not be sufficient to ensure that UV light is being transmitted to the resin through the polarizers and liquid crystals.

[0028] It will be appreciated that there are many resins that are used for SLA (Stereolithography) printing. For example, there are many resins which are acrylate monomer/oligomer based or acrylate urethane oligomer based. Some resins may comprise a photo-initiator to begin polymerization upon UV exposure. The present invention may allow better activation of such photo-initiators and/or better curing of the resins themselves.

3D Printer

[0029] Figures 1 a-1 c shows a side cross section view of an embodiment of an apparatus 100 for making a three-dimensional object by photo-solidification. In this case the apparatus comprises: a vat 101 configured to contain solidifiable resin 190; a build plate 1 11 which can move towards and away from the release layer; and a solidification energy source 103 for selectively curing portions of the solidifiable resin.

[0030] In this case, the solidification energy source 103 comprises a LCD screen which comprises UV LED lights. It will be appreciated that other light sources (e.g. incandescent or fluorescent bulbs and/or Digital Light Processing (DLP) projectors may be used). The LCD screen 103 in this case comprises an array of pixels which are positioned close to the bottom of the vat. When a pixel is turned on, light is emitted by the LED which is transmitted by the LCD and solidifies the corresponding portion of the liquid resin between the release layer and the build plate or the previously cured object. A pixel may be turned off by turning off the LED and/or by making the LCD pixel opaque. The pixel size of an LCD may be less than 100 microns.

[0031] Figure 1a shows this situation when a layer of the resin has been cured onto a previously cured object 191 which is in the process of being printer.

[0032] In order to continue (or complete) printing, the newly solidified layer needs to be detached from the vat base. To do this, as shown in figure 1 b, the build plate is raised along an axis of translation away from the bottom of the vat and the solidification source. In this case, the build plate 11 1 is configured to move at least the thickness of one printed layer away from the bottom of the vat 101 (one printed layer may be between e.g. 0.05 mm to 0.15 mm thick). The uncured liquid resin 190 flows into the gap between the bottom of the printed object portion and the release layer.

[0033] Then the next layer of the object can be printed by selectively turning on pixels which cure portions of the liquid layer between the release layer and the printed object portion (as shown in figure 1c). This returns the apparatus to a situation similar to that of figure 1 a (with an additional layer added). By iteratively curing, releasing and moving the build plate, the 3D object can be built up in layers.

LCD panel

[0034] An LCD uses four basic layers to display an image. Figure 2 is an exploded view of the LCD layers. Layer 231 and 232 are polarizers arranged transversely (e.g. at 90 degrees) to each other. (A polarizer allows light waves that are in a specific orientation through and blocks the rest by absorption or reflection). In the absence of any intervening optics, transversely aligned polarizers (e.g. at 90 degrees) would normally block all the light through the screen. Layer 234 is a film that allows patterns of electrical current through. In this way, layer 234 defines the array of pixels of the LCD. These currents align the liquid crystals in layer 233.

[0035] When liquid crystals are aligned they can turn the wavelength of light so that they will pass through the second polarizer. [0036] Conventional LCDs use a Polaroid or H-sheet polarizer because they are inexpensive and good at transmitting visible light.

[0037] Figure 3 is a cross-section of the LCD assembly shown in figure 1 a. The LCD display of figure 3 comprises first and second polarizers 331 , 332 with a crossed polarization axes; and a liquid crystal layer 333 positioned between the polarizers. Associated with the liquid crystal layer 333 are electronics to turn the liquid crystal between transmitting and blocking states. In this case, the display also comprises a light source 330 which in this case comprises US LEDs configured to emit light in the 375- 395nm wavelength.

[0038] The polarizers 331 , 332 in this case are wire grid polarizers. Wire grid polarizers comprise a series of aligned conductors (e.g. metal) mounted on a transparent substrate (e.g. glass or other transparent sheet). The wire grid polarizers work by absorbing light waves aligned to the wires. That is, wire grid polarizers transmit radiation with an electric field vector perpendicular to the wire. In this case, the wire polarizers are configured to transmit light across a broad UV band including in the 375-395nm wavelengths range.

[0039] Wire grid polarizers may be more flexible as the range of wavelengths transmitted may be adjusted by adjusting the separation of the wires. For example, the closer together the wires are the shorter the wavelengths of light can pass through polarized. The wires may be manufactured on a scale of a few hundred nanometers.

[0040] It will be appreciated that other polarizers capable of UV transmissions and polarization may be used in conjunction with the present invention. The panel may be a monochrome liquid crystal panel with TFT substrate.

[0041] For example, certain types of H-sheet/Polaroid polarizer capable of transmitting 375-395nm light but the contrast ratio on these sheets are not currently as good as wire- grid polarizers. H-sheet polarizers are typically made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction. Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized parallel to the chains is absorbed by the sheet; light polarized perpendicularly to the chains is transmitted. [0042] Some polarizers capable of transmitting UV light that may not be suitable. For example, some polarizers use light birefringence or refraction. These may change the direction of light making the imaging (and associated curing of the resin) distorted.

Experiments

[0043] For these tests a conventional 1536x2048 monochrome display was used with part of the polarizers removed from one corner.

[0044] Two wire grid polarizers were positioned on the clear of the screen part (i.e. replacing the removed portion of the polarizers). The wire grid polarizers were arranged with the optic axes perpendicular to each other to block the largest amount of light. The two polarizers are 50mm x 50mm Moxtek™ UVT 240A polarizers. Moxtek™ UVT 240A polarizers are configured to transmit light across a broad UV band including in the 375- 395nm wavelengths range.

[0045] Four different LEDs were used for this test. The LEDs were rated at 405nm, 380nm, and two at 365nm. To differentiate the two 365nm LEDs, one will be labelled as the 365nm SMD (surface mounted device) and the second will just be the 365nm LED.

[0046] The setup uses Ocean Optics™ USB 2000+ spectrometer to measure the wavelengths passing through the LCD.

Test 1 : Polarizer Test

[0047] The first test included setting up the light with a 3.6V power supply and placing it under the LCD. The light through the polarizers (simultaneously through both the spatially separated wire grid and stock) are measured with a white screen and a black screen. Then the light is measured from the LED on its own (no lens) to get a baseline. This process is repeated for the 4 LEDs. Below is a summary of reference numerals labelling the data lines for each of the lines in figures 4a-7b.

, an

[0048] The 405nm LED results are shown in figures 4a and 4b. The wire grid polarizers let the white light through mostly unchanged while the stock polarizer shifted the spectrum a little from a peak at 400nm to a peak at 403nm seen in figure 8.

[0049] From figure 4a it is also important to note that the Moxtek polarizers let though more light on a black screen giving it a much lower contrast ratio. When we analyze the contrast through the spectrum we notice that the Moxtek polarizer has a much lower contrast in the 400nm range but the contrast crosses over by 390-395nm. This may be because the stock polarizers aren't letting any light through in the 390nm range resulting in no contrast. This can be seen in figure 4b.

[0050] The 380nm light (see figures 5a and 5b) was also basically unchanged by the Moxtek polarizers with a naked light at 383nm and through the polarizers at 384nm. The stock polarizers on the other hand push the peak light to 400nm, seen in figure 5a. Looking at the contrast ratio we see the stock polarizers preform better at longer wavelengths and marginally worse at lower wavelengths.

[0051] The "365nm LED" results (see figures 6a-6b) indicate that the naked light is around 380-381 nm (rather than 365 as specified) and such the intensity results shown in figure 5a are much the same as the 380nm test seen in figure 5a.

[0052] The contrast ratio for the "365 LED" shown in figure 6b is a little different from that of the 380nm case shown in figure 5b. The Moxtek polarizers preform much better with the stock polarizers until much higher wavelengths. This may be due to the different angle the light is hitting the screen of it could be due to the fact that this light is just weaker.

[0053] The "365nm SMD" LED had a peak closer to 365nm than the other "365nm LED" though still not at 365nm. The light is closer to 373nm. When the light passes through the LCD with Motex polarizers the spectrum is shifted to 380nm. However the black screen has a peak closer to 375. This may be the result of the liquid crystals having no effect on the low wavelength light meaning only the 380nm light is twisted on a white screen and when no light is twisted on a black screen the peak is back at 375nm rage. This is seen in figure 7a.

[0054] Figure 7b indicates that the contrast ratio of the Moxtek polarizers appears to become much better as the wavelengths get lower becoming about the same as the stock with wavelengths greater than 405nm.

[0055] It appears as though the Moxtek polarizers preform better than stock polarizers for light with wavelengths around 380nm and below. However, the contrast ratio decreases as the wavelengths get higher. This may be because the Moxtek polarizers are designed for UV light polarization and may suffer as the spectrum gets closer to the visible range when compared to polarizers for visible light. It also appears that light below 380nm is unable so pass through this setup it is not clear from test 1 if this is due to the polarizers or effects of the liquid crystals.

Test 2: Lens wavelength alteration

[0056] Since lenses may be used to focus or collimate the light for better data it is important to see if they are affecting the wavelength from the LED.

[0057] The lenses in this case are configured provide an even dispersion of light while maintaining rays as normal (e.g. perpendicular) to the LCD plane. In this case, each LED is fitted with its own simple lens. The exact angle of the lens can vary depending on the density of the led array and the distance of the array to the LCD panel. The LEDs project most of there light forward which mitigates the need for mirrors to redirect the light before the lens although mirrors could be used as well. It will be appreciated that the lens and/or mirrors may be configured to collimate light in an even distribution based on the particular light source used (e.g. LEDs, fluorescent lamp, gas-discharge lamp).

[0058] For this test the four LEDs had their wavelengths tested with and without the lens. The rest of the setup is largely the same as test 1.

[0059] Figures 8a-d show the results: figure 8a shows the results for 380nm LED; figure 8b shows the results for 365nm SMD; figure 8c shows the results for 365nm LED; and figure 8d shows the results for 405nm LED. In each case, the line labelled 888 followed by the figure letter (e.g. "a" for figure 8a) represents the light transmitted without the lens and the line labelled 889 followed by the figure letter (e.g. "a" for figure 8a) represents the light transmitted with the lens.

[0060] Figures 8a-d indicate that the lenses are changing the spectrum of the light moving the peak to longer wavelengths on all except the 405nm light. This means that the transmission of the lens is likely limited to wavelengths above 382-386nm.

[0061] For this test, it was found that the lenses are made of PMMA which starts absorbing light around the 385nm point. For the present technology the lens should be configured to transmit light in the 375-395nm wavelength range. Therefore, it may be preferable to use glass, plexiglas® G-UVT or Zeonex™ lenses (e.g. Cyclo Olefin Polymer (COP) optical polymer lenses).

Test 3: Separating the elements

[0062] This test involved testing the light transmission though the LCD polarizer and just air. This test will be done with both the 365nm lights and the 380nm. The 405nm was omitted as it seems to work properly through the LCD.

[0063] The setup is largely the same as the one used for test one but the polarizer will be setup on a pedestal roughly the same height of the LCD.

[0064] Figures 9a-c show the results: figure 9a shows the results for 380nm LED; figure 9b shows the results for 365nm LED; and figure 9c shows the results for 365nm SMD. In each case, the line labelled 990 followed by the figure letter (e.g. "a" for figure 8a) represents the light transmitted through the liquid crystal; the line labelled 991 followed by the figure letter represents the light transmitted on its own; and the line labelled 992 followed by the figure letter represents the light transmitted through the polarizer.

[0065] The 380nm light appears to be unchanged though any of the separate element from seen in figure 9a.

[0066] The shifts in the individual elements are small and not really close to the shifts we saw in test 1. The shifts here may be due to the ambient light interference in naked test or reflection on the white interior of the pedestal in the polarizer test.

Test 4: Inverse polarizer [0067] This test is designed to see if the liquid crystals in the LCD are affecting the different light wavelengths.

[0068] This setup is largely the same as test 1 except the polarizers are aligned (rather than arranged to be transverse) to display a negative of the image on the screen. With the polarizers set up this way if you display a black screen the crystals are unaligned and will not twist the light allowing it to be sent through the second polarizer. If a white screen is displayed the light through the first polarizer will be twisted and should be blocked by the second polarizer.

[0069] The results indicate that at all wavelengths only a portion of the light is affected by the LCD. It is also worth noting that the peaks are where we expect them to be from their naked spectrum. The other very apparent thing is that the contrast of the LCD at 400nm is much higher then that at 375nm and even at 380nm.

Conclusion

[0070] From test 1 we have concluded with the right polarizers it is possible to transmit down to 380nm light.

[0071] This test also shows that these polarizers would not be as effective with wavelengths larger than 395nm as the contrast ratio starts to suffer and lower than 380nm does not seem effective.

[0072] Test 2 shows that the PMMA lenses are not effective with light lower than 385nm.

[0073] Test 3 shows that the individual components of the setup are not causing any major shift in the spectrum.

[0074] Finally test 4 indicates that liquid crystals used in LCDs Have a diminishing effect on light having a wavelength lower then 380nm making the screen most effective with light peaks at 380nm.

[0075] Although the present invention has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the invention as understood by those skilled in the art.