The present invention concerns a method and device for making a three-dimensional object from a hardenable liquid medium, whereby the object is built up layer per layer by each time providing a layer of liquid medium on a support and/or an already formed part of the object and by subsequently hardening it according to a specific pattern before a subsequent layer is formed in the same manner.

Such methods are mainly used for making prototypes and such and are therefore usually referred to as rapid prototyping processes. The most frequently used process is stereolithography, whereby the liquid medium or resin of each layer is hardened by means of electromagnetic beams, for example one or several laser beams, which are computer-controlled. Thanks to the beams, the surface of the medium hardens according to a specific pattern, namely an x-y section of the object to be made.

Such techniques are described among others in US-U- 4.575.330 and EP-A-0.348.061.

According to the known methods, a layer of liquid resin is provided by immersing the support with an already formed part of the object in a bath of liquid resin, providing it with more than one layer thickness, so that an overspill of resin flows on the support or the product. Subsequently, the already formed part of the

object is lifted up to exactly one layer thickness beneath the resin level. Due to the viscous character of the resin, the overspill of resin which is situated on the already formed part will not immediately flow out into an even layer with the required thickness. Said overspill is then scraped off with a scraper, but afterwards a waiting time is usually required to obtain an even surface.

It is clear that these known methods are very time- consuming. Especially when building up small objects, whereby only little time is required for the hardening, the required time for applying the layers of liquid resin determines the length of the entire process, which thus is very slow.

Moreover, the known methods cause quite some problems when applying layers on certain geometries of objects, such as for example hollow or thin-walled objects. The scraper is often incapable of removing sufficient resin from the already formed part of the object in this case, so that after a number of layers, the scraper strikes against a hardened layer.

The invention aims a method for making a three- dimensional object from a hardenable liquid medium which does not have the above-mentioned and other disadvantages and which thus is relatively fast.

This aim is reached according to the invention by means of a method for making a three-dimensional object from a hardenable liquid medium, whereby the object is built up layer per layer by each time providing a layer of liquid medium on a support and/or an already formed part of the

object and by subsequently hardening it according to a specific pattern before a subsequent layer is formed in the same manner, whereby moreover the successive layers of liquid medium are provided in the form of a stable curtain which is moved in a relative manner in relation to a support or an already hardened layer.

Preferably, the curtain is formed by pressing liquid medium through a slot under a pressure between 5 and 20 bar.

The curtain can be formed by means of a casting bob which is erected on top of a vessel containing liquid medium in which the support or the already formed part of the object is erected such that the surface upon which a layer must be provided practically coincides with the surface of the medium contained in the vessel.

The invention also concerns a device which is particularly suitable for the application of the method according to the invention.

The invention thus concerns a device for making a three- dimensional object from a hardenable liquid medium, which device contains a vessel filled with a hardenable liquid medium and controllable means situated on top of this vessel to make a layer of liquid medium harden according to a specific pattern, and which is characterized in that it contains a casting bob which is erected on top of the vessel and is provided with a slot at the bottom, and means to move said casting bob in a relative manner over the surface of the medium contained in the vessel.

Preferably, at least two guiding edges are provided on

the casting bob which extend downward near the ends of the slot so as to determine the shape of the curtain's edges.

Also between said guiding edges can be provided guiding elements on the casting bob which point downward and which guide the curtain.

In order to better explain the characteristics of the invention, the following preferred embodiments of a method and device for making a three-dimensional object from a hardenable liquid medium is described as an example only without being limitative in any way, with reference to the accompanying drawings, in which:

figure 1 is a schematic representation of a device according to the invention in a firεt stage of the application of a liquid medium layer on a part of an already formed object; figures 2 to 4 show a part of the device of figure 1, but in three successive stages of the application of the layer; figure 5 shows a section according to line V-V in figure 2; figure 6 schematically shows a variant of the device according to the invention; figure 7 shows a view in perspective of a practical embodiment of the part of the device according to figures 2 to 4; figures 8 to 12 show longitudinal sections of four embodiments of the casting bob of the device of figure 7, during the application of liquid medium.

According to the invention, an object 1 is built up by

applying a selectively hardenable liquid medium 2 layer after layer, for example by applying a resin and by hardening it according to a specific pattern, whereby each layer is applied by casting liquid medium in the form of a stable curtain 3.

As is represented in the figures 1 and 5, which schematically represent a device for applying the method, this curtain 3 is cast by means of a casting bob 4 which can be moved horizontally to and fro, for example by means of a piston mechanism 5, over a vessel 6 which is open at the top, which is filled with liquid medium 2 and in which the object 1 is formed.

The casting bob 4 is stationary in the height, but the vessel 6 contains a movable part, which in the example consists of the bottom 7, to move the already formed part of the object 1 upward, layer per layer.

The device contains means 8 which for example consist of a piston mechanism to move the bottom 7 step by step.

On the bottom 7 is also provided a platform 9 and upon it a supporting frame 10 which is usually formed of hardened medium in the same manner as the object 1. The bottom 7, the platform 9 and the supporting frame 10 together form a support for the object 1 to be formed.

The casting bob 4 contains an internal pressure distribution chamber 11 in which opens a slot 12 which opens into the bottom side of the casting bob 4.

The device further contains a reservoir 13 for liquid medium. The casting bob 4 is connected to this reservoir

13 by means of a pipe 14 in which is mounted a pump 15.

The top side of the vessel 6 is surrounded by a receptacle 16 which is connected to the reservoir 13 situated beneath it via a discharge pipe 17.

In order to harden the selectively hardenable medium layer per layer, a movable laser beam source 18 is mounted in a known manner above the vessel 6, which is moved by means that are not represented in figures 1 to 5 and that are controlled by a control unit 19 with built-in computer, such that the laser beam source 18 can describe a specific pattern and can also clear the space in front of the passage of the casting bob 4.

There where a laser beam touches the surface of the liquid medium, this medium will harden.

Figure 1 represents an already formed and hardened part of the object in the vessel 6. The top surface of this part is situated at the height of the surface of the medium in the vessel 6.

In order to form a subsequent layer, the bottom 7 of the vessel 6 is moved downward according to the invention over a distance which is equal to the thickness of the new layer to be formed, as is schematically represented in figure 2. Before, during or after said movement, the casting bob 4 is moved from one side to the other, while a liquid medium is pumped through the pump 15 from the reservoir 13 via the pipe 14 in the pressure distribution chamber 11 and flows out through the slot 12 in the form of a stable curtain 3, as is represented in figure 3.

The width of the curtain 3 and the speed at which the casting bob 3 is moved over the vessel 6 is selected such that the curtain 3 meets the following stability criterion:

p.u ² t = 2T

The medium is hereby pressed out of the slot 12 under a pressure of for example between 5 and 20 bar and with a minimum volume flow per unit width Q/b which is equal to the square root of 2T.t/p.

For both equations: p = the specific gravity of the liquid medium (kg/m ³ ) u = the speed of the liquid in the curtain (m/s) t = the width of the curtain (m) T = the surface tension of the liquid medium (N/m) .

The to and fro movement of the casting bob 4 is bigger than the dimensions of the vessel 6 in the direction of movement, whereas the slot 12 in the casting bob 4 is longer than the dimensions of the vessel 6 in the longitudinal direction of the slot 12, so that the curtain 3 does not only apply a new layer on the already hardened layer of the object 1, but also on the surrounding surface of the medium 2 in the vessel 6.

Medium which is spilled over the vessel 6 is collected in the receptacle 16 and flows back to the reservoir 13.

Figures 3 and 4 show the situation during and after the application of the new layer respectively.

In the final stage, the laser beam source 18 is moved according to a specific pattern over the new layer by the control unit 19, such that this layer hardens according to this pattern.

This process was or is repeated for all layers, whereby the direction of movement of the casting bob 4 is reversed in between two successive layers, and whereby it is moved back into its initial position over the vessel 6.

Once the object 1 is completed, it is taken out of the vessel 6 and the supporting frame 10 is removed.

The supporting character of the medium 2 in the vessel 6 can be enhanced by cooling the medium, for example by blowing cooling air over the vessel 6 by means of a fan 20.

The stability of the curtain 3 can be improved by heating the medium 2 which is cast by the casting bob 4, so that among others air bells are avoided in the curtain. This can be done by erecting heating means 21, for example an electrical resistance, in the reservoir 13.

The control unit 19 does not only control the above- mentioned means to move the laser beam source 18, but also the means 8 to move the bottom 7 and the piston mechanism 5 which moves the casting bob 4 and possibly the pump 15.

After one or several layers have been applied, the thickness of the applied layer is measured, and, depending on the measured thickness, the quantity of

medium which the casting bob 4 applies on the preceding layer during a movement, and thus the thickness of the new layer, is adjusted if necessary. This is possible by influencing the pump 15 and thus by changing the delivery or by controlling the piston mechanism 5 and thus by adjusting the speed of the casting bob 4.

On the basis of measurements of an applied layer and by influencing the laser beam source 18 and its movement, the hardening parameters and the hardening depth of a new layer can also be adjusted if required, such that successive layers adhere strongly to one another.

The above-described working method does not only allow to manufacture the object 1 relatively fast, it also makes it possible to build the supporting frame 10 in a more simple manner, since no lateral forces are exerted on the already formed part of the object 1 by a scraper or such.

Instead of pumping medium 2 from a separate reservoir 13 to the casting bob 4 , this medium can also be pumped out of the vessel 6 itself, as is represented in figure 6.

In this case, the bottom 7 of the vessel 6 cannot be moved, but the platform 9 is mounted on a lift 22. The means 8 to move the bottom 7 are replaced in this case by means 23 to move the lift 22 in the height.

A new layer of the object 1 is formed in the above- described manner, but the movement of the already formed part of the object over the thickness of a layer before a new layer is cast by the casting bob 4, is induced by lowering the lift 22 instead of the bottom 7.

This embodiment of the method offers the same advantages as the above-described embodiment.

In which manner the device and in particular the part thereof with which a layer of the liquid medium 2 can be cast are built in practice is represented in figures 7 to 12, in which elements which coincide with the above- mentioned elements are indicated with corresponding figures of reference.

The casting bob 4 is composed of two parts 24 and 25 which are fixed to one another by means of a sealing 26, whereby the sealing extends along the edges, except on the bottom side of the casting head, so that the slot 12 is formed on the bottom side between the parts 24 and 25. The pressure distribution chamber 11 is formed of a cavity in the part 25 and inside the sealing 26. The part 25 is provided with an entry 27 in its wall, opening into the cavity, onto which the pipe 14 is connected.

The vessel 6 is cylindrical, whereas the receptacle 16 has the shape of a dish.

In order to prevent that during the movement of the casting bob 4, when the curtain 3 suddenly makes contact with the vessel 6, the stability of said curtain 3 is threatened, a horizontal auxiliary panel 28 iε erected against or near the top edge of the vessel 6, above the receptacle.

In order to improve the stability of the curtain 3, two downward hanging guiding edges 29 for the guiding of the curtain 3 are fixed to the casting bob 4, as is represented in detail in the figures 8 to 12.

Without these guiding edges 29, due to the surface tension of the liquid medium 2, the curtain 3 will be immediately constricted when it leaves the slot 12, as is represented by means of a dashed line in figure 8.

Thanks to the guiding edges 29 on or near the edges of the curtain 3, such constriction is prevented. Thanks to the adhesion of the medium to the guiding edges 29, the surface tension is neutralized and thus is obtained an even curtain.

These guiding edges 29, which thus guide the edges of the curtain 3, can be situated at the ends of the slot 12, as is represented in figures 7 and 8.

Preferably however, the guiding edges 29 are provided at a short distance from the ends of the slot 12 so as to prevent the curtain 3 from coming loose. In this case, it is easier for the guiding edges 29 to be moistened by the outflowing medium, and the adhesion between the medium and the guiding edges 29 will be better.

In order to further improve the stability of the curtain 3, the lower ends of the guiding edges 29 can be folded inward, as is represented in figure 10.

Another possibility to improve the stability of the curtain 3 consists in fixing one or several extra guiding slips 30 between the guiding edges 29 which point downward in the curtain 3, as is represented in figure

11.

The best stability is obtained with guiding edges 29 which practically have the form of the natural curtain 3,

i.e. the curtain 3 in case no guiding edges 29 are present, and which thus bend towards one another, as is represented in figure 12.

The present invention is by no means limited to the above-described embodiments as represented in the accompanying drawings; on the contrary, such a method and device for making a three-dimensional object can be made in all sorts of variants while still remaining within the scope of the invention.

In particular, the invention is not limited to stereolithography. It can also be applied to other generally called rapid prototyping processes.

The liquid medium must not necessarily be supplied to the casting bob by means of a pump. It can for example also be supplied by means of compressed air.