TITLE

ELECTROSTATIC PRINTING METHOD AND ITS USE IN RAPID

PROTOTYPING

DESCRIPTION

Field of the Invention

The invention relates to a modified electrostatic powder printing technique and its use in a novel rapid prototyping method for constructing freeform objects by successive layer-by-layer deposition of powder material.

Background Art

Rapid prototyping is the most common name given to a host of related methods that are used to fabricate physical objects from computer models or other electronic data sources. These methods are unique in that they add and bond materials in layers to form objects. A number of methods of rapid prototyping have been proposed, all involving the layer-by-layer build up of three-dimensional articles. Examples of current rapid prototyping methods include stereolithography, fused deposition modelling (FDM) and laminated object manufacturing (LOM).

It has been proposed to use electrostatic printing techniques to deposit layers of powder in rapid prototyping and such methods have been explored. In such methods electrophotographic, magnetographic or ionographic techniques may be used to deposit the powder. After deposition the area or areas of the deposited powder that is intended to form part of the final three-dimensional object are bound to the immediately preceding layer using conventional techniques. For example, they may be joined by a selectively deposited adhesive material. Alternatively, the powder layers may be thermally bonded, for example by laser or infra-red sintering. After the application of very many layers the appropriate three-dimensional shape will be produced.

US-A-5088047 and US-B-6206672 both disclose rapid prototyping methods that use electrophotographic powder deposition to build objects layer-by-layer. US-A- 5088047 uses a conveyor (transfer) belt to transport layers of the electrophotographically printed powder from a photoconductor drum to the object being built on a build platform. The process requires two transfers of the printed image: one from the photoconductor drum to the conveyor belt, and the other from the conveyor belt to the object surface. The material being deposited is relatively loose powder which is not compacted and sintered until after it has been placed on the surface of the object being built. Moreover, at least the first of the two depositions takes place between two moving surfaces, namely the photoconductor drum surface and the surface of the intermediate conveyor belt. The speeds of those moving surfaces must be very accurately controlled to ensure there is no loss of register between the two surfaces, and no break up of the image being transferred before the final sintering step.

It has been suggested that the problems associated with the use of an intermediate conveyor belt in US-A-5088047 can be overcome by printing the multiple layers directly from the photoconductor drum onto the object being built on the build platform. There are, however, problems with this process also. The powder image on the photoconductor drum carries an electrostatic charge and is transferred to the top face of the object being built by creating a charge of opposite sign on that top face. This is a technique that is common to electrostatic printing. However, when non- conductive powders are used the field strength that may be generated at the top layer of the object being built falls sharply as the height of the object (the total print height) is increased. As a result, acceptable transfer of the powder image from the photoconductor drum to the top face of the image may not be achievable beyond a maximum limiting height. Therefore the potential height of the components being built is limited.

US 6066285 discloses a rapid prototyping process using electrophotographic printing wherein prior to the deposition of the immediately subsequent powder layer each deposited powder layer is charged with an opposite polarity to that of the subsequent

powder layer. This is intended to facilitate the deposition of each powder layer by providing a constant attractive force between the powder layer being deposited and the object being built. However, it has been found that charging deposited powder layers can be difficult, for example if the powder substantially consists of a non- conductive polymer. Furthermore, this method gives only a marginal improvement in the printing efficiency of substantially non-conductive powders after deposition of anything more than a few millimetres of object thickness

Another problem associated with the electrophotographic deposition techniques taught in US-A-5088047 and US-B-6206672 is that if the photoconductor drum comes into contact with a charged surface such as the top surface of the object being built, then it may pick up some charge from that surface. Afterwards, when the electrophotographic apparatus tries to charge the drum with a charge of the opposite sign, the charge picked up from the top surface of the object being built will serve to neutralise the opposite charge picked up by that particular point or area of the drum surface. As a result, later in the electrophotographic process those neutralised areas will pick up powder from the image developer when possibly it was not intended that they should do so. That has always been seen as a problem to be avoided in all electrostatic copying.

Although the above patents each disclose the use of electrophotographic printing, it is to be understood that the problems discussed would be equally applicable if any other electrostatic printing technique were used, for example magnetography or ionography.

The present invention seeks to avoid the above problems and provide a method of rapid prototyping which operates at higher speed and accuracy using simple, inexpensive and reliable apparatus. In seeking to derive a new and better method for rapid prototyping by electrostatic transfer the invention has also provided a new and modified process for copying an image onto a substrate by electrostatic powder deposition.

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The Invention

The present invention provides a method of copying an image onto a first carrier by electrostatic deposition and transfer, comprising: (a) providing a deposition of fine particle powder on a second carrier by:

(i) providing a clean working face of the second carrier; (ii) creating a latent electrostatic image on the working face of the second carrier;

(iii)converting the latent electrostatic image into a real image by transferring a charged layer of fine particle powder to the working face of the second carrier which carries the latent electrostatic image; and (b) transferring the real image to the first carrier by electrostatic transfer CHARACTERIZED IN THAT the transfer of the real image to the first carrier is effected by the second carrier becoming subject to an electrostatic charge of the same polarity as that of the charged layer of the fine particle powder, thereby repelling the charged fine powder from the second carrier and onto the first carrier.

The present invention provides an advantage over the prior art in instead of transferring the real image, which is initially established as a charged layer of the fine particle powder on the working face of the second carrier, by placing an oppositely charged first carrier adjacent the second carrier, the real image is transferred by subject the powder image to an electrostatic charge of the same polarity as that of the charged powder. That is, the transfer is affected by electrostatic repulsion rather than attraction. It has been found that this is a complete and efficient solution to the problems discussed above. By using the method of the present invention a wide variety of powder materials may be deposited onto a wide range of substrates. Furthermore, as the substrate need not need not be charged, the powder may be deposited upon conductive or non-conductive surfaces.

The method of the present invention may utilise electrophotographic (xerographic) techniques. For example, the second carrier may be a conventional photoconductor plate or drum and the latent electrostatic image may be created on the working face of

the photoconductor in a conventional manner. That is, first a uniform electrostatic charge would be distributed over the working face, for example by corona discharge. The latent image would then be generated by selectively discharging areas of the working face by exposure to light, typically a laser or a light emitting diode (LED). Alternatively, the present invention may utilise any other electrostatic printing technique. For example, the present invention may utilise magnetographic or ionographic techniques.

The method of the present invention may utilise an intermediate carrier, whereby immediately after step (a)(iii) the real image is transferred from the second carrier to an intermediate carrier by conventional means. The image may then be transferred from the intermediate carrier to the first carrier according to step (b) of the present invention. The intermediate carrier may be conveyor belt or an intermediate plate. If the intermediate carrier is a conveyor belt a roller upon which the belt is mounted may be electrostatically chargeable and be used to subject the powder image to an electrostatic charge during step (b).

The method of the present invention can be used to transfer the image onto an uncharged first carrier. Alternatively, the method may incorporate the techniques of previous electrostatic copying methods by causing the first carrier to carry an electrostatic charge of the opposite polarity to that of the charge layer of fine particle powder, so that the transfer is a combination of repulsion, from the charge applied to the second carrier, and attraction, from the charge on the first carrier.

Furthermore, it has been found that the powder transfer may be made significantly more efficient by applying a forced vibration to the first carrier at the instant of transfer. The vibration assists the transfer by creating localised forces and/or heat. Ultrasonic frequencies of vibration have been found to be highly effective.

The powder transfer may also be assisted by the first carrier and the carrier from which the image is transferred being brought into contact during or immediately after transfer and optionally pressing the two carriers lightly against one another.

The powder transfer may also be assisted by tackification of the surface onto which the real image is deposited. For example, said surface may be made sticky by the application of heat and/or an adhesive.

The powder transfer may also be assisted by the use of baffled electrodes to control the positioning of the powder on to the first carrier. For example, a pair of electrically charged plates may be provided above the top surface of the first carrier, so that as the image is transferred the electrically charged plates control its lateral movement and ensure accurate placement on the top surface of the first carrier.

The powder utilised in the present invention is preferably a fine particle powder. The method of the present invention has the potential to extend to the use of nano-size powders. The powder may be plastic, metallic, ceramic, a mixture of these, or any other suitable material. The powder may be coated with pigments to increase its sensitivity to certain wavelengths of electromagnetic radiation, for example infrared radiation.

As the method of the present invention utilises electrostatic repulsion to affect the transfer of the real image from the second to the first carrier, it is preferable that the powder used is not substantially electrically insulating, i.e. have a resistivity of more than 1 x 10 ¹⁶ ωm ^"1 . Similarly, highly conductive powders with a resisitivity of less than 1 x 10 ^"8 ωm ^"1 are not preferred. However, the suitability of non-conductive powders may be improved by the addition of conventional anti-static (conductive) agents in the range of 0.1 to 10% by weight, as is required. Similarly, the conductivity of highly conductive powders may be modified by the addition of a substantially insulating powder additive. Increasing the conductivity of non-conductive powders may assist with the dissipation of charge from the deposited layers during the construction of multi-layer objects, as discussed below.

The method of the present invention may be used to deposit powder layers (real images) that are homogeneous in composition or that consist of two or more separate

areas of different powder components. The method of the present invention may also be used to deposit powder layers that are functionally graded. That is, in an analogous manner to colour printing whereby a wide range of colours can be produced from only three primary coloured inks, combinations of two or more different powder materials can be used to impart differing material properties across a deposited powder layer. For example, two or more different metal and/or ceramic and/or polymer powders may be used. The material properties that may be varied across a single layer include colour, strength, conductivity and density. For example, it may be preferable to have powder layers that contain substantially conducting and non-conducting portions or have areas that absorb specific wavelengths of electromagnetic radiation and areas that do not. If the deposited powder is heterogeneous and consists of two or more areas of separate areas of different powder components and/or is functionally graded it may be preferable to selectively sinter the layer according to the varying powder compositions across the powder layer.

If used in rapid prototyping (as discussed below) each deposited layer may be a combination of build powder and support powder, the build powder being capable of being sintered, or otherwise bonded, to form the object being constructed, whereas the support powder may resist sintering so that it could be removed at a later stage. Alternatively, the support powder may be soluble in a liquid after sintering whilst the build powder is insoluble to the same liquid. Alternatively, the support powder may be rendered by brittle by the deposition or subsequent post deposition processing whilst the build powder is not.

The method of the present invention has been found to be of significant benefit in rapid prototyping whereby a three-dimensional object is built up through the deposition of a plurality of powder layers first on a build plate and subsequently upon one another. The first carrier of the present invention may be the build plate. As it is not necessary to charge the surface upon which the powder is deposited, the present invention may be used to produce three-dimensional objects of unlimited height.

After deposition upon the build plate each powder layer may be bound to the immediately preceding layer using conventional techniques. For example, they may be joined by an adhesive material that may be deposited over the whole layer or over selected areas of the layer. Alternatively, the sintering of the top powder layer may act to thermally bond the layer to the preceding layers.

Preferably after the real image has been transferred to the build plate it will be sintered by exposing it to suitable electromagnetic radiation. The electromagnetic radiation used for sintering may be any radiation, such as infrared or microwave or a combination thereof, that is used in conventional rapid prototyping methods. For example, a combination of conventional infra-red and laser sintering, as disclosed in GB 2422344, may be used. Alternatively, laser sintering alone may be used.

The sintering of a deposited powder layer may be uniform over the powder layer or may be selective in order to control the thermal processing of the deposited powder layer. For example, different areas of a real image may be exposed to different durations and/or intensities and/or wavelengths of electromagnetic radiation. Selective sintering may be particularly preferable if a deposited layer is heterogeneous and consists of two or more areas of separate areas of different powder components and/or is functionally graded. This may be achieved through computer controlled selective laser sintering using one or more lasers. If a plurality of lasers are used then it may be preferable that those lasers are of differing wavelengths such that the different areas of the real image may be exposed different wavelengths of radiation during the selective sintering. Alternatively, an array of miniature infrared heaters each using a reflector for focussing the radiation onto a small surface area may be used. The heaters of such an array may be selectively switched on for differing durations so as to selectively sinter the deposited powder layer.

It is to be understood that selective sintering of a selectively deposited powder layer may be achieved using the method of the present invention. However, it is equally possible that selectively deposited powder layers produced by conventional deposition

techniques, as discussed in the background art section above, may also be selectively sintered.

If each deposited powder layer is sintered it may be preferable that after sintering the temperature of the surface of the deposited layer is monitored by a remote device. This temperature information may be used to control the sintering of the immediately subsequent layer. For example, the intensity of electromagnetic radiation applied to the subsequent layer may be varied depending upon the temperature of the deposited layer. Suitable remote devices include CCD cameras and IR cameras.

After transfer to the first carrier according to the method of the present invention it is possible that the powder layer may retain some residual charge. This is particularly likely to occur if the surface upon which the powder layer is deposited is substantially non-conductive. If the method of the present invention is used for rapid prototyping, this residual charge may be detrimental to the effective deposition of any subsequent powder layers. Therefore it may be preferable that any residual charge of the top surface of the previously deposited powder layer is discharged before the deposition of the subsequent layer. Discharging the top surface of a deposited powder layer may be simply achieved using conventional techniques that will be known to a person skilled in the art, for example, through ac/dc static elimination systems or by the addition anti-static agents such as titanium dioxide.

If the method of the present invention is used for rapid prototyping, in order to enable the efficient and even deposition of each subsequent powder layer it is preferable that the surface of each deposited powder layer is as flat and smooth as is possible. To achieve this each deposited powder layer may be mechanically levelled and smoothed prior to the deposition of said subsequent layer. This may be done using rotating mechanical elements, for example, rotating non-sticking type rollers.

Furthermore, the surface topography of each deposited layer may be measured using a non-contact sensor. Examples of suitable non-contact sensors include laser triangulation and interrupted beam sensors. This information could then be used to

control the thickness of the immediately subsequent powder layer in order to compensate for any irregularities in the surface topology of the deposited layer.

Additionally or alternatively this information may be used to control any mechanical levelling of the top layer.

Drawings

Figure 1 is a schematic diagram of a first preferred embodiment of an apparatus for carrying out the method of the present invention and illustrates the sequential steps of electrophotographic image creation and transfer and subsequent sintering; Figure 2 is a schematic diagram of a second preferred embodiment of an apparatus for carrying out the method of the present invention;

Figure 3 shows a preferred embodiment of a rapid prototyping apparatus that is capable of carrying out the method of the present invention;

Figure 4 is a schematic diagram illustrates a rapid prototyping apparatus capable of carrying out the method of the present invention and that has multiple build platforms and a single print station and a single fusing station;

Figure 5 is a partial schematic diagram of an third preferred embodiment of an apparatus capable of carrying out the method of the present invention illustrating the use of baffled electrodes to divert powder from the second carrier onto the appropriate position on the first carrier.

Referring first to Figure 1 there is shown a first preferred embodiment of an apparatus for carrying out the invention of claim 1 whereby the second carrier is a photoconductor drum 1. The photoconductor drum 1 rotates and passes through successive workstations that carry out the steps of the method of claim 1. The schematic illustration of Figure 1 is common to many known electrophotographic transfer methods and to a preferred embodiment of the present invention in that it does not identify the nature of the transfer step (b). According to the present invention the transfer step is subjecting the surface of the photoconductor drum to an electrostatic field of the same polarity as that of the charged powder applied at workstation (a)(iii), so that the charged fine particle powder is repelled from the photoconductor drum 1 and onto the first carrier. That transfer can be assisted by

applying an electrostatic charge opposite in sign to that of the charged powder to the said first carrier, but the charging of the first carrier is not a necessary step according to the invention. At the transfer step (b) the transfer of the real image can be assisted by the forced vibration of the photoconductor carrier, preferably at ultrasonic frequencies.

Also illustrated in Figure 1 is a sintering step (c) to fix the image onto the carrier. This is achieved by exposing the transferred image to suitable electromagnetic radiation, for example a row of radiation transmitters 2.

The means for subjecting the photoconductor drum 1 to a field of the same polarity as that of the charged powder in order to repel the powder image at the transfer station (b) is preferably internal to the drum and adjacent to an inner surface of the drum at transfer station (b), whereas the latent image is applied to an external surface of the drum.

Figure 2 shows a second preferred embodiment of an apparatus for carrying out the invention of claim 1. The apparatus includes a photoconductor drum 1 , upon which a charged powder image may be created in a conventional manner. The drum 1 is rotated to deposit the charged powder image upon an insulating intermediate transfer belt 14, which is mounted on a roller 15 adjacent to the photoconductor drum 1 and on a electrostatically chargeable roller 16 at its other end. The belt 14 is rotated to carry the charged powder image from the photoconductor drum 1 to a build platform 3. As the powder image reaches the closest point of the belt 14 to the build platform 3 the electrostatically chargeable roller 16 subjects the powder image to an electrostatic charge that is of the same polarity as the charge of the powder and thereby repels it onto the build platform 3. In this manner the image from the photoconductor drum 1 is indirectly transferred onto the build platform 3.

It is found that the image transfer according to the method of the invention is rapid and efficient, and in many ways superior to that of conventional electrostatic printing in general and electrophotography in particular and is of particular use in rapid

prototyping. The method of the invention is of great benefit in the fabrication of three dimensional objects by a rapid prototyping method which involves the successive layer-by-layer deposition of powder material onto a build platform or onto the top of a previous image supported on a build platform. Preferred features of that rapid prototyping method of the invention are illustrated in Figures 3 to 5.

The apparatus 10 illustrated in Figure 3 includes a build platform 3 that is movably mounted upon a base 4, a print engine 5 with an image transfer roller 6 and a sintering mechanism 7. When in use the build platform 3 is the first carrier of the present invention. The print engine 5 carries out steps (a)(i) to (a)(iii) of the method of the present invention. Preferably the transfer roller 6 is an intermediate belt 14 according to the apparatus of Figure 2. If the transfer roller 6 is an intermediate belt 14 it will function in conjunction with a photoconductor drum 1 contained within the print engine 5 in order to deposit the real image upon the build platform 3 in the manner described above and illustrated in Figure 2. Alternatively, the transfer roller 6 may be a photoconductor drum according to the apparatus of Figure 1.

During the rapid prototyping process the build platform 3 is moved about the apparatus 10 in a cyclical manner in the direction indicated by the bold arrows. First, a real image powder layer 8 is deposited upon the build platform 3 by the transfer roller 6. The build platform 3 is then moved to the left and positioned under the sintering mechanism 7 which exposes the deposited powder 8 to electromagnetic radiation in order to sinter the powder. The sintering mechanism 7 may be a laser capable of selective laser sintering or may be an array of miniature infrared heaters each using a reflector for focussing the radiation onto a small surface area may be used. Alternatively, the sintering mechanism 7 may be any suitable heating device. The sintering may be selectively applied over the powder layer 8 or the powder layer 8 may be uniformly sintered.

After the deposited powder layer 8 has been suitably sintered the build platform 3 is moved back under the print engine 5 in the manner indicated by the bold arrows. Limit switches 9 prevent the platform 3 being moved beyond the end of the base 4and

being positioned too close to the print engine 5. However, it is to be understood that these are a back-up system and the movement of the build platform 3 will be primarily controlled by an external CPU (not shown). When the build platform 3 has been repositioned under the print engine 5 the transfer roller 6 and the print engine 5 may act to deposit another real image powder layer 8 upon any previously deposited powder layers 8. In this manner a freeform three-dimensional object may be built up.

Alternatively, instead of a single build platform 3 being cyclically moved between a print engine 5 and a sintering mechanism 7 as illustrated in Figure 3, an apparatus 10 may have multiple build platforms 3 as illustrated in Figure 4. The multiple build platforms 3 would be moved through the apparatus 10 in the direction indicated by the bold arrows. Multiple build platforms 3 may be preferred if it is required to cool each powder layer 8 after sintering 7 and before the subsequent layer 8 is deposited on top. Multiple build platforms 3 may also be provide an increase in the output of a single machine 10 by enabling multiple objects to be constructed concurrently. These objects may be identical or multiple different objects may be constructed.

Although not illustrated, it would also be possible for a single machine 10 to have multiple print engines 5 and/or sintering mechanisms 7. For example, there may be an equal number of print engines 5 and sintering mechanisms 7 positioned alternately around an apparatus 10 such that each build platform 3 is cycled around the apparatus and passes through a print engine 5 and immediately subsequently passes through a sintering mechanism 7.

As illustrated in Figure 5, an electrostatic plate 12 may be used to deposit the real image powder layer 8 upon the build platform 3. This may operate in substantially the same manner as the photoconductor drum 1 of the apparatus of Figure 1 except that the whole of each powder layer 8 will deposited at the same time. Baffled electrodes 13, in the form of fixed electrodes are mounted between the upper surface of the top powder layer 8a that has been deposited on the build platform 3 and the electrostatic plate 12. Another pair of fixed electrodes (not shown) could be positioned

orthogonally if appropriate. The control of the electrodes 13 enables the very accurate positioning of the transferred layers 8 onto the build platform 3.