CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application No. 18202252.5 filed on Oct. 24, 2018, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention is directed to a method for building a shaped body by stereolithographic solidification of building material by photopolymerization.

SUMMARY

In the method herein, building material is conveyed between a building platform and a transparent bottom plate and is exposed in a spatially selective manner through the bottom plate in order to solidify building material above the bottom plate within a predetermined contour;

the building platform is lifted relative to the bottom plate, and building material is replenished; and

these two steps are continued until the shaped body is built by spatially selectively solidified building material.

The layer-wise or continuous building processes of shaped bodies belong to the production processes which fall under the generic term generative manufacturing or additive manufacturing. This is understood as a class of methods in which a three dimensional object (shaped body) is directly built based on a digital model of the shaped body (for example a CAD model). For this purpose, in case of layer-wise building, the digital model of the shaped body is subdivided into a plurality of subsequent thin slices, wherein each slice has a contour defined by the model. The building process is performed by adding material layers, wherein each layer is processed such that for this layer the contour is generated which is specified by the model. Eventually the pile of slices lying on top of each other and being connected to each other forms the shaped body which in some methods is processed further, for example by heat application for debinding and sintering. The term “contour” is used in the context of the present invention in a general sense and is not limited to a simple, closed boundary line which encloses a simply connected area, but may also comprise several separate contour sections which enclose adjacent separate areas which together form the respective slice, or there may be outer and inner contour sections which define annular areas of the slice.

The present invention is directed to additive manufacturing of shaped bodies, wherein building material is spatially selectively exposed for solidifying it by photopolymerization. The building material may have a broad range of viscosities from being flowable to not flowable, and may include besides photopolymerizable monomer compounds fillers such as ceramic, glass ceramic or metal powder, and optionally dispersing media and other additives. The present invention is in particular directed to the manufacturing of ceramic and glass ceramic shaped bodies (so called green bodies), such as dental inlays, onlays, veneers, crowns, bridges and frameworks.

WO 2010/045950 A1 and corresponding U.S. 2011310370, U.S. 2014044824, U.S. Pat. Nos. 8,623,264, 9,067,359, which US published applications and patents are hereby incorporated by reference in entirety, are directed to a method for generating a shaped body, which method is directed in particular to the generation of dental restorations from ceramic slurries which comprise a liquid, photopolymerizable component and a filler of ceramic or glass ceramic powder distributed therein. In this prior art method the shaped body is built up successively by layers solidified on top of each other. In this method a building platform is held vertically moveable above a tank bottom which is at least in the building area transparent. Below the tank bottom an exposure unit is located. The building platform is first lowered into the slurry in the tank down to a level at which a layer having the desired layer thickness remains between the building platform and the tank bottom. Thereafter this layer is exposed within the contour prescribed for this layer by the digital model, and is thereby solidified. After lifting the building platform up slurry is replenished, for example using a doctor blade, and then the building platform is lowered again into the slurry, wherein the lowering is controlled in such a manner that the distance between the last solidified layer and the tank bottom defines a layer of building material having the desired thickness. The last two steps are repeated so many times until the shaped body has been built with the desired three dimensional shapes by the successive solidification of layers, for each layer within the contour prescribed by the digital model.

The exposure unit is configured for exposing a large number of picture elements which are arranged in an array in an exposure field. The exposure unit comprises for example a light source and a spatial light modulator. The active surface of the spatial light modulator comprises an array or a matrix of exposing elements, wherein each exposing element is associated with one picture element in the exposure field of the exposure unit. The spatial light modulator can for example be a so called micromirror device or digital mirror device (DMD). Such device is known to be a chip provided with a large number of micromirror actuators arranged in an array. Each micromirror actuator is individually and selectively controllable by a control unit to be selectively tiltable between an exposure position in which the micromirror reflects light from the light source to the associated picture element, and a dark position in which the micromirror deflects the light into an inactive area. By switching the micromirror back and forth using a duty cycle which is selectively defined for each exposing element the generated exposure intensity for the associated picture element can be set as average intensity determined by the duty cycle. The exposure unit equipped with a DMD is also referred to as DLP unit (digital light processing). Another example for a spatial light modulator comprises a liquid crystal display which is trans-illuminated by a light source and which is subdivided into a matrix arrangement of large number of individual exposing elements, each of which being assigned to a picture element.

After the shaped body has been built up the building platform can be taken out together with the shaped body. In many cases further treatment steps follow. In the method described above, the layer-wise polymerization of building material results in a green body. This is then exposed to high temperatures in order to remove the binder, in this case the photopolymer. This takes place at high temperatures by thermal decomposition and reactions which decompose the polymer into lower mass molecules which by diffusion move to the surface of the body and eventually escape as gases. Subsequent to this process of debinding a further temperature treatment can follow for further densification, wherein the remaining ceramic particles in the shaped body are sintered.

The known method has disadvantages because only flowable building materials of relatively low viscosity (<10 Pas) can be processed. A tank open at the top for receiving the building material with a correspondingly large building platform occupies a large footprint for the overall system. Furthermore, a tank open at the top holding the building material is useable rather in laboratory environments only, but is in any case not well suited for operation for example in dental practices because in such practices it has to be ensured that the personnel is dealing with a preferably “dry” and simple process in which the personnel ideally does not come in contact with unreacted building material (monomer).

The present invention aims in particular at the capability of being able to process building materials of high viscosity which are not flowable. Building materials which include a high amount of fillers, for example in form of ceramic powder, may have a very high viscosity and may be not flowable. For such materials the replenishment of building material to the building area in the tank below the building platform is problematic since, even if a doctor blade is used, failure-free supply of highly viscous building material on the tank bottom to the building area is technically complex.

A further disadvantage is that, when a tank with building material is used, changing between different building material compositions during the building process is very difficult, in any case highly complex since for this purpose a plurality of tanks with different building material compositions have to be moveable to be positioned under the building platform. Very difficult is the realization of gradual, continuous transitions between building material compositions, for example the realization of colour transitions in the shaped body.

It is an object of the present invention to define a method for building up of shaped bodies by successive solidification of building material by stereolithographic photopolymerization, which method may be performed with low space requirements and with operational safety such that it can also be safely performed in surroundings where personnel without specific training is present.

This object is achieved by the method having the features of the claims. Preferred embodiments of the method are set out in the subclaims. A device for carrying out the method is defined in the claims, and preferred embodiments of the device are set out in the subclaims.

According to the present invention the building platform is formed by a front face of a piston which is slideably guided in a cylinder. An open end face or end plane of the cylinder which is facing the front face of the piston is closed by the bottom plate such that piston, cylinder and bottom plate form a closed chamber inside the cylinder as a building space. Further building material is replenished by moving the piston away from the bottom plate and by simultaneously conveying building material through a supply line which leads to and opens out into the interior of the cylinder, to fill the extra space created by the movement of the piston away from the bottom plate with building material. Preferably, the supply line opens out into the interior of the cylinder adjacent to the bottom plate.

The method according to the invention has a number of advantages. The method can be carried out using a very compact device structure since there are no portions of a tank which are laterally extending beyond the actual building area. Instead only a cylinder is required in which the building platform is guided as a shiftable piston. The open end face of the cylinder facing the piston is closed by the bottom plate. The piston is moved away from the bottom plate such that the building space enclosed by cylinder, bottom plate and piston grows together with the growing shaped body during the building process.

A further advantage is that there is no free access to building material in an open tank but the building material is contained in a closed building space defined by the cylinder, the piston and the base plate such that there is no danger that operating personnel could come into contact with building material. In addition, the building material is used in a very efficient manner since only as much building material is replenished in the building space growing during the building process as is needed for the building height of the shaped body.

In principle the method can be performed continuously, i.e. the piston is continuously moved away from the bottom plate (at a speed at which the shaped body grows). Simultaneously building material is continuously replenished in order to keep the building space in the interior of the cylinder filled, and the exposure unit exposes continuously, namely in a manner comparable to a video with a rapid succession of video frames, such that the contour currently to be solidified is continuously adjusted according to the digital model of the shaped body.

Among the stereolithographic building methods, however, methods are more common which operate in a step- or layer-wise fashion. In a preferred embodiment the method of the present invention is carried out as a layer-wise building method. At the beginning of the building process a first layer of building material is formed between the piston and the bottom plate which are set at a distance of a desired layer thickness to each other, by filling up the gap between the bottom plate and the piston, and by exposing the layer created in this manner through the bottom plate in a spatially selective manner in order to solidify a layer area with a contour as prescribed for this layer by the digital model of the shaped body, and to fix the solidified area to the piston. Thereafter the piston is moved further away from the bottom plate by a layer thickness and building material is simultaneously replenished of to form a further layer between the last solidified layer and the bottom plate. The further layer is exposed through the bottom plate in a spatially selective manner in order to solidify a layer area within a contour as described for the respective layer by the digital model. The two last mentioned steps are repeated as often as necessary until the shaped body is built up by layer areas solidified on top of each other. During the building process a further layer of building material is formed by moving the piston a distance corresponding to the desired layer thickness further away from the bottom plate and by simultaneously replenishing building material by feeding it through the supply line in order to fill the growing volume resulting from the movement of the piston away from the bottom plate with building material.

Insofar as the bottom plate is designated as “transparent” in the present description, this is intended to mean that it is possible to carry out a spatially selective exposure through the bottom plate which means that the bottom plate is translucent and which does not exclude that a certain amount of the incoming radiation is absorbed in the bottom plate or scattered therein.

In a preferred embodiment the piston is, during the movement in the cylinder away from the bottom plate, simultaneously rotated about a rotational axis coinciding with the longitudinal axis of the cylinder such that points on the outer circumference of the piston perform a spiral or helical movement. This has the effect that during the movement of the piston away from the bottom plate this linear movement away from the bottom plate is superimposed on a movement component transverse to the bottom plate, which transverse moving component facilitates the separation from the bottom plate. In a layer-wise method this means that after solidifying a layer, when the piston is moved in a direction away from the bottom plate, the movement during lifting off the last solidified layer is not a movement strictly perpendicular to the bottom plate but, by simultaneously rotating the piston, includes also a movement component transverse to the bottom plate. This movement supports the separation of the last solidified layer from the bottom plate. Further preferred means for preventing adherence of the solidified layers to the bottom plate and thereby for supporting separation will be explained further below.

Preferably the movement of the piston is effected by a drive motor, wherein a rotating movement of the piston is at the same time translated into the linear movement of the piston away from the bottom plate, e.g. by a spindle drive. This may for example be accomplished by forming at least part of a piston rod as a threaded rod which is received in an internal thread which is stationary with respect to the cylinder, wherein the drive motors rotates the piston rod which, as a spindle, then at the same time effects the longitudinal movement and the rotating movement of the piston. The internal thread may for example be formed in the inner wall of central opening in an intermediate wall parallel to the bottom plate or in a cover of the cylinder, wherein beyond the intermediate wall the drive motor and further components for operation may be accommodated. The threaded rod may then be rotated in this internal thread in the cover by the drive motor such that by the spindle drive a linear movement of the piston in addition to the rotating movement is effected. In this case the component with the internal thread which is engaged by the external thread of the threaded rod has to be fixed relative to the cylinder during the building process so that the rotation of the threaded rod in the internal thread is translated into defined movement of the piston relative to cylinder. The fixation of this component should be releasable so that after completion of the building process and after release of the fixation the component with the internal thread can be taken out together with the threaded rod and the piston having the built shaped body hanging thereon, wherein the component with the internal thread (for example a cylinder cover) remains in engagement on the external thread of the threaded rod like a nut.

In a preferred embodiment the building material is held available in a cartridge which is connected to the supply line of the cylinder, wherein building material in the cartridge is conveyed through the supply line into the cylinder by driving a cartridge piston in a controlled manner to press building material out of the cartridge into the cylinder. The movement of the cartridge piston may be driven in a controlled manner, for example hydraulically, pneumatically or by step motor.

In a preferred embodiment a plurality of building material compositions is held available, and under the control of a control unit, selectively one or more of the building material compositions is or are conveyed into the cylinder. There may be for example a plurality of cartridges, each of which being connected to a supply line to the cylinder, wherein the control unit selectively drives one or more of the cartridges holding the respectively desired building material compositions to convey material into the cylinder. In principle it is also possible that a number of cartridges are connected to a collecting pipe which then leads as a supply line to the interior of the cylinder.

In a preferred embodiment, after the building process of a shaped body has been completed, unused or surplus building material (unpolymerized building material) is, by means of pressurized air, pressed out of the building space enclosed in the interior of the cylinder, preferably back into the associated cartridge.

In a preferred embodiment, after removal of the unused building material, cleaning liquid is directed into the cylinder in order to purge the shaped body of building material residues possibly still sticking on the shaped body.

An input port for pressurized air and cleaning liquid may be located in an upper section of the cylinder, wherein this input port is unblocked or opened by moving the piston past the area of the input port such that the input port then opens out into the interior of the cylinder. Not solidified building material, if there is only one building material component present, is pressed by the pressurized air back again into the respective cartridge. In case several building material compositions are present they are collectively disposed. For discharging the cleaning liquid a separate opening similar to the building material supply line is provided as an outlet directly above the bottom plate. In this manner the cleaning liquid could for example be circulated to flush the building space which leads to an improved cleaning effect of the shaped body.

Thereafter the shaped body can be subjected to further processing steps, for example debinding and sintering.

A device for carrying out the method according to the invention is characterized by

a cylinder, wherein one of the end planes of the cylinder is closed by a bottom plate which is at least in a central area transparent, and wherein in the cylinder a piston is guided to be displaceable by a drive motor in the longitudinal direction of the cylinder, such that the cylinder, the piston and the bottom plate define a closed building space having a variable volume in the interior of the cylinder,

a container holding building material and being connected to a supply line which opens into the interior of the cylinder adjacent to the inner surface of the bottom plate, and a conveying device for conveying building material from the container through the supply line into the interior of the cylinder,

an exposure unit which is configured for spatially selective exposure through the bottom plate, and

a control unit which is arranged to control the drive motor of the piston and the conveying device so as to fill the additional space created by the movement of the piston away from the bottom plate with building material, and which is arranged to control the exposure unit for spatially selective exposure, in order to solidify replenished building material above the bottom plate within a contour prescribed by the control unit.

Such a device can be constructed in a very compact manner by accommodating the exposure unit in a cylindrical lower housing section which forms an extension of the cylinder, whereas the suspension of the piston and the drive motor and the control unit may be accommodated in an upper housing section at the opposite end which forms an extension of the cylinder. As a container with building material a cartridge may be located laterally at the cylinder and may be connected to the supply line of the cylinder. In such a design the footprint needed for the device can be limited to the cross-sectional area of the cylinder.

The cylinder is a vertical cylinder, i.e. the cylinder axis and the shell surface are oriented perpendicularly to the end faces. Preferably the cylinder is a circular cylinder. It is noted that the geometrical definition of the cylinder in connection with the present invention relates to the inner space of the cylinder. Insofar as the wall thickness of the cylinder shell is constant also the external shape is cylindrical. In principle it is also contemplated that a cylindrical hollow space could be formed in an elongated cubic housing body or in any other prismshaped body or housing, wherein such a body would also be a “cylinder” in the sense of the invention, in which cylinder a piston is slidably guided.

In a preferred embodiment the piston is, on the side facing away from the bottom plate, connected to a piston rod having an external thread which engages an internal thread in a component which is fixed relative to the cylinder. The piston rod has a recess which extends centred along the longitudinal axis of the piston rod and which is not rotationally symmetric; the recess may for example be rectangular or in any other manner polygonal. In this recess a drive pin is slidably introduced, which drive pin has a cross-sectional shape complementary to the recess such that engagement with positive fit with respect to rotational movements of the drive pin in the recess is realized, whereas the drive pin is free to move in axial longitudinal direction in the recess. The drive pin can be driven by the drive motor to rotate about its longitudinal axis, whereby the piston rod and the piston are rotated such that the external thread of the piston rod is rotated in the internal thread of the fixed component in order to thereby move the piston relative to the bottom plate, wherein at the same time the drive pin is shifted in the recess of the piston rod in its longitudinal direction. The recess of the piston rod can for example have a cross-sectional shape like the recess of a recessed head screw (Allen Screw), and the drive pin may have a complementary hexagonal shape as a corresponding hexwrench (Allen Key). During movement of the piston away from the bottom plate which is accompanied by corresponding movement of the piston rod, the drive pin is shifted relative to the recess of the piston rod further into the recess.

In a preferred embodiment the container is formed by a cartridge and the conveying device is formed as a cartridge piston which can be driven to be shifted and which, when controlled by the control unit to be shifted, presses building material out of the cartridge, through the supply line and into the cylinder.

At the supply line preferably a valve is provided which is configured to be able to open or block the supply line, for example if another building material is to be used or if cleaning liquid is to be directed into the building space, to thereby prevent that building material in the cartridge is contaminated. When pressing out unused building material, the building material, as long it is not contaminated by another material, should be pressed through the same supply line back again into the cartridge to recycle it for the next building job.

In a preferred embodiment a plurality of cartridges is located distributed around the circumference of the cylinder, wherein each cartridge is connected to a supply line leading to the cylinder and is selectively controllable by the control unit in order to press building material out of the respective cartridge and into the cylinder. An additional discharge line may serve to discharge mixed building materials or cleaning liquid.

In a preferred embodiment the bottom plate is configured such that the surface facing the interior of the cylinder releases agents acting as inhibitors which prevent or minimize sticking of the solidifying layer to the surface of the bottom plate facing the interior of the cylinder.

US 20150360419, which US published application is hereby incorporated by reference in entirety, is directed to a stereolithography device comprising a tank, which tank is made of a material which has a certain oxygen permeability. Oxygen acts in this case as inhibitor which forms a reaction inert thin intermediate layer which reduces sticking of the solidifying layer to the tank surface. As oxygen permeable materials Teflon AF and fluoro-polymers are mentioned as examples.

In a preferred embodiment the bottom plate consists of a nanoporous material in which inhibitor agents are accumulated and/or through which inhibitor agents diffuse and may pass through the bottom plate such that they are released on the surface of the bottom plate and prevent sticking of the polymerizing building material during the solidification.

In a preferred embodiment a chamber is formed on the side of the bottom plate facing away from the interior of the cylinder such that this chamber may be filled with a transparent inhibitor agent, for example a gas. In a preferred embodiment excess pressure may be applied to the chamber filled with the inhibitor agent in order to press the inhibitor agent into and through the nanoporous material.

From DE 10 2013 215 040 A1 and corresponding U.S. 2015034007, which US published application is hereby incorporated by reference in entirety, are directed to a stereolithography device which comprises at a base a semipermeable film covering a hollow space into which inhibitors are supplied. The use of semipermeable films and the supply of inhibitors to prevent sticking after the photopolymerization are also described in U.S. Pat. No. 5,122,441 B1, which is hereby incorporated by reference in entirety.

Nanoporous materials through which inhibitors may diffuse, to prevent sticking, are likewise described in the already mentioned document DE 10 2013 215 040 A1, U.S. 2015034007 and in U.S. 2017/0151718 A1, which US published applications are hereby incorporated by reference in entirety.

In a preferred embodiment the surface of the bottom plate facing the interior of the cylinder is coated with a superamphiphobic layer which minimizes sticking of the solidifying layer to the bottom plate. Superamphiphobic surfaces have the properties that they are phobic or repellent for both water and for oils. Such layers are described for example in the Articles “Glas, das sick selbst reinigt—Superamphiphobe Beschichtungen”, Doris Vollmer, Max-Planck-Society, Yearbook 2013, and “Superamphiphobic Surfaces”, Zonglin Chu et al., Chemical Society Reviews, Issue 8, 2014.

