Patent Description:
Wire frame printing is a three-dimensional printing technique. Wire frame printing can be used to fabricate entire three-dimensional objects, or as part of a multi-discipline three-dimensional printing process so as to fabricate sections of a scaffolding-like frame that provide support for a three-dimensionally printed object.

Wire frames are constructed by extruding and depositing sections of printing material. Typically, a section of printed material is linear. In order to construct a wire frame structure, a series of printed sections are deposited. The series of sections may be deposited in any order. One difference between wire frame three-dimensional printing and typical layer-by-layer three-dimensional printing is that during wire frame printing the sections of printed material can be extruded and solidified in three-dimensional space without relying on a previous layer supporting each printed section along its full length. That is, printed sections can be deposited in any three-dimensional orientation. This is in contrast to layer-by-layer printing, in which printing is constrained to building up layers in successive horizontal planes. During wire frame printing, sections of material can be printed from a previously printed point into a point in space, or between two previously printed points.

<FIG> depicts the stages of one exemplary wire frame printing process. Other wire frame printing processes exist. The exemplary wire frame printing process alternates between first and second printing stages. In a first stage, a contour layer <NUM> is printed. The contour layer <NUM> may be a horizontal layer. In a second stage, a zig-zag type pattern layer <NUM> is printed on top of the contour layer <NUM>. The zig-zag type pattern layer <NUM> may extend substantially vertically. The exemplary wire frame printing process then returns to the first stage, and prints a further contour layer <NUM> supported by the apexes of the zig-zag type pattern layer <NUM>. Wire frame printing can be performed by an articulated robot arm <NUM>, an example of which is shown in <FIG> printing a wire frame structure <NUM>.

Wire frame printing processes typically suffer from problems caused by the printhead colliding with previously printed material.

For example, <FIG> depicts a cross-sectional view of a typical wire frame printhead <NUM> having printed a vertical section <NUM> of the zig-zag type pattern layer <NUM>. Filament <NUM> is drawn <NUM> into the printhead <NUM>. The printhead heats the filament <NUM> using heater <NUM> so that it can be extruded and deposited. In order to form the zig-zag type pattern layer <NUM>, a vertical section of material <NUM> (also shown in <FIG>) is deposited. The printhead <NUM> then moves in direction 105a so as to print a section of material <NUM> (shown in <FIG>) in direction 105b. Printheads are typically configured to print section of material <NUM> by following direction 105b because this is the shortest and most direct route between points in the zig-zag type pattern layer <NUM>. However, in doing so, the apex <NUM> of previously printed filament <NUM> will collide with the printhead <NUM> - for example, at corner <NUM>. This is because the nozzle of the printhead <NUM> has a non-zero width.

Collisions of this type limit the accuracy and fidelity of typical wire frame printing techniques. For example, as shown in <FIG>, if a printhead <NUM> follows a typical printing path 105b, the resulting wire frame <NUM> is distorted. This is because the printhead <NUM> impinges on each of the apexes of the previously printed vertical sections of material of the zig-zag type pattern layer <NUM>. In this way, it is not possible to form sharp apexes - and so the accuracy and fidelity of the zig-zag type pattern layer <NUM> is compromised. This compromises the deposition of the next contour layer <NUM>, as the apexes of the zig-zag type pattern layer <NUM> are poorly defined. This problem is compounded the more layers a wire frame structure comprises.

There is a need for an improved method of wire frame printing.

<CIT> relates to fast fabrication (i.e., rapid prototyping) of 3D objects in the form of wireframe structures. <CIT> relates to a high-hole-structure and no-support-space 3D printing device and a printing method.

According to a first aspect of the present invention there is provided method of three-dimensional wire-frame printing wherein material is printed by a printhead configured to move relative to the material at a printhead speed and extrude material at an extruding speed, the method comprising: printing an initial portion of material from a starting point; pausing printing of the initial portion of material so as to form a first anchor point, such that a first direction is defined from the starting point to the first anchor point, and wherein the anchor point lies in a plane substantially perpendicular to the first direction such that the initial portion of material is on the printed side of the plane; resuming printing from the first anchor point by printing, from the first anchor point, two or more intermediate portions of material on the non-printed side of the plane; printing a final portion of material to a second anchor point on the printed side of the plane; wherein the printhead speed and the extruding speed are independently variable; the average printhead speed is greater than the average extruding speed for the period of time during which the intermediate and final portions of material are printed between the first and second anchor points; and a first intermediate portion is printed on the opposite side of the initial portion to the second anchor point.

The printhead and extruding speeds may be constant when printing the intermediate and final portions of material between the first and second anchor points.

The method may further comprise: pausing the extrusion of material between the first and second anchor points whilst the printhead continues moving.

The printing of the first portion of material may be paused for a predetermined period of time.

The method may further comprise monitoring the temperature of the first anchor point when printing of the initial portion of material is paused; and printing one or more intermediate portions of material on the non-printed side of the plane after determining that the temperature of the first anchor point is below a threshold.

A thermal camera may be used to monitor the temperature of the first anchor point.

The first direction may be substantially vertical.

The method may further comprise cooling the first anchor point using one or more fans whilst printing is paused.

The method may further comprise: printing subsequent portions of material on an anchor point starting from a position offset from the centre of that anchor point.

Pausing printing of the initial portion of material so as to form a first anchor point may further comprise printing one or more other portions of material, remote from the first anchor point, before returning to the first anchor point so as to resume printing from the first anchor point.

According to a second aspect of the present invention there is provided a three-dimensional printing apparatus wherein material is printed by a printhead (<NUM>) configured to move relative to the material at a printhead speed and extrude material at an extruding speed, and a programmable microcomputer configured to control the three-dimensional wire frame printing apparatus to perform any of the methods described herein.

According to a third aspect of the present invention there is provided a computer program comprising computer program code means to cause the three-dimensional wire frame printing apparatus described herein to perform any of the methods described herein when said program is run on a programmable microcomputer configured to control the three-dimensional wire frame printing apparatus.

Embodiments are described by way of example only.

Wire frame printing can be performed by an articulated robot arm <NUM>, an example of which is shown in <FIG> printing a wire frame structure <NUM>.

Articulated robot arm <NUM> comprises a base <NUM> and a series of joints <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and a printhead <NUM>. The series of joints may comprise any appropriate joints, such as revolute joints. The series of joints can be manipulated so as to control the position and orientation of the printhead <NUM>. The movement of articulated robot arm310 via said series of joints can be controlled in accordance with principles well-known to the person skilled in the art.

The articulated printing robot comprises a printhead <NUM>. As described herein, <FIG> shows a cross-sectional view of a typical wire frame printhead <NUM>. An exemplary wire frame printhead <NUM> may have approximate dimensions of <NUM> in height and <NUM> in width.

Filament <NUM> is drawn <NUM> into the printhead <NUM>. The filament may be a thermo-plastic polymer material, such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) or polyethylene terephthalate (PETG).

The printhead heats the filament <NUM> using heater <NUM> so that it can be extruded and deposited. Heating a thermo-plastic polymer filament reduces its viscosity. This enables the filament to be extruded and deposited by the printhead. The printing temperature used depends on the thermo-plastic polymer to be printed. By way of example, the printhead may heat the filament to temperatures in the range of <NUM> to <NUM>. As shown in <FIG>, the exit diameter of the nozzle of printhead <NUM> may be smaller than the entrance diameter so as to compress the filament <NUM> as it is drawn through the printhead <NUM>. This compression can aid in fully melting the filament.

In other examples, the printhead is fed by material in pellet form. The material in pellet form may be a thermo-plastic polymer material, such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) or polyethylene terephthalate (PETG). Material in pellet form can be drawn into the printhead and melted by the printhead in order to be deposited. The extrusion of thermo-plastic polymers provided in pellet form is well-understood by the skilled person.

The deposited section of material cools such that the printed material solidifies. One or more fans <NUM> can be provided on or above the printhead <NUM>. The fans <NUM> may be directed towards the printed material so as increase the airflow impinging on the printed material in order to aid its cooling. The airflow generated by the fans may be balanced so as to avoid distorting printed material. The fans may be in operation continually during printing. Alternatively, the fans <NUM> may operate intermittently. Operating the fans intermittently may increase energy efficiency.

The printhead <NUM> may be moved by an articulated printing robot arm, as described with reference to <FIG>. The printhead <NUM> is configured to move relative to the three-dimensional structure <NUM> at a printhead speed. The relative printhead speed may represent the distance covered by one part of the printhead, such as corner <NUM>, per unit of time. In another example, the printhead <NUM> remains stationary and the printing bed <NUM> is actuatable so as to move the three-dimensional structure <NUM> relative to the printhead <NUM>. The relative printhead speed may represent the distance covered by one part of the three-dimensional structure <NUM>, relative to the printhead <NUM>, per unit of time. In some examples, the printhead speed is variable. The relative printhead speed may be in the region of <NUM> to <NUM> millimetres per second. The relative printhead speed can be considered to be a printing parameter.

It is worth noting that the description uses the terms "relative printhead speed" and "printhead speed" interchangeably to refer to the same printing parameter.

As described herein, filament <NUM> is drawn as shown by arrow <NUM> into the printhead <NUM>. The filament may be in the region of one to three millimetres in diameter. The filament may be drawn into the printhead by any means well known to the skilled person. For example, the printhead may comprise a motor configured to drive a set of wheels which drag the filament into the printhead by frictional contact. Material is extruded from the printhead. As described herein, the filament may be heated such that it can be extruded from the printhead. The printhead <NUM> is configured to extrude material at an extruding speed. The extruding speed may represent the length of material extruded by the printhead per unit of time. In some examples, the extruding speed is variable. The extruding speed may be in the region of <NUM> to <NUM> millimetres per second. In this way, the printhead may extrude thermo-plastic polymer at a rate in the region of one kilogram per hour.

The length of individual sections of printed material between anchor points of the wire frame structure may be in the region of <NUM> to <NUM> millimetres. An anchor point may be defined as a point in the wire frame structure at which a plurality of printed sections of material meet. The reason for limiting the length of individual sections of material is so that the bending moment applied to any individual section of the wire frame structure does not exceed a tolerable threshold.

It is to be understood that other three-dimensional printing devices could perform the methods disclosed herein. That is, the method need not be performed by an articulated printing robot, as described with reference to <FIG>. For example, the methods described herein could be performed by a printhead configured to move in three dimensions via a set of three rails oriented in X, Y and Z directions.

<FIG> shows the printing of a vertical section of the zig-zag type pattern layer <NUM> of a wire frame. Printing begins at a starting point <NUM>. The starting point <NUM> may be an arbitrary point on a surface, a point on a previously printed contour layer <NUM>, <NUM>, or an apex of a previously printed zig-zag type pattern layer <NUM>.

From the starting point <NUM>, an initial portion of material is printed. The initial portion of material leads to a first anchor point <NUM>. The initial portion of material may be a linear section of material directly from the starting point <NUM> to the first anchor point <NUM>. Alternatively, the initial portion of material could be a non-linear section of material, or a plurality of sections of material.

A first direction <NUM> is defined from the starting point <NUM> to the first anchor point <NUM>. The first direction <NUM> may be vertical, or substantially vertical. Substantially vertical may include angles <NUM>° either side of vertical. Alternatively, the first direction <NUM> may be any other direction.

The anchor point <NUM> lies in a plane <NUM> perpendicular, or substantially perpendicular, to the first direction <NUM>. Substantially perpendicular may include angles <NUM>° either side of perpendicular. The initial portion of material is on the printed side <NUM> of the plane <NUM>. The other side of the plane <NUM> may be referred to as the non-printed side <NUM> of the plane <NUM>.

A second anchor point <NUM> is on the printed side <NUM> of the plane <NUM>. In order to print the zig-zag type pattern layer <NUM>, the printhead needs to extrude a section of material between the first anchor point <NUM> and the second anchor point <NUM>. However, as described herein with reference to <FIG>, <FIG>, <FIG>, if the printhead <NUM> follows a typical printing path directly between the first anchor point <NUM> and the second anchor point <NUM>, the printhead, in particular corner <NUM>, will collide with the first anchor point <NUM>.

<FIG> shows an exemplary printing path not in accordance with the invention. It is worth noting that <FIG> depicts a path through which the printhead moves - rather than the form of the printed material itself. The starting point <NUM>, first anchor point <NUM>, first direction <NUM>, plane <NUM>, printed side of the plane <NUM>, non-printed side of the plane <NUM>, and second anchor point <NUM> are as described with reference to <FIG>.

In accordance with the methods described herein, an initial portion of material is printed between the starting point <NUM> and the first anchor point <NUM>. The printing process then pauses at the first anchor point <NUM>. Pausing at the first anchor point <NUM> enables the initial portion of material to partially, or fully, solidify. Partially, or fully, solidifying increases the rigidity of initial portion of material. The increased rigidity of the initial portion of material enables the forces excreted during the printing method described herein to be tolerated.

The printing process may be paused for a pre-determined period of time. For example, the printing process may be paused for <NUM> to <NUM> seconds. In another example, a thermal camera can be mounted to the printhead. The thermal camera could be used to determine when the initial portion of material, or the first anchor point <NUM>, has cooled below a threshold temperature. For example, the printing process could be paused until it is determined that the first anchor point <NUM> has cooled to a temperature below the glass transition temperature (Tg) of the printing material. During this time the printhead may act as a temporary anchor which supports the initial portion of material whilst it fully or partially solidifies.

When printing is paused, the printhead may remain static at or close to the first anchor point <NUM> after the first anchor point <NUM> has been printed. In other examples, when printing has been paused at the first anchor point <NUM>, the printhead may print other sections of the wire frame structure before returning to the first anchor point <NUM>. Said other sections may be remote from the first anchor point <NUM>. That is, the printhead may jump to another anchor point in order to print sections of material. In order to jump to another anchor point, the printhead may first be held static at the first anchor point <NUM> for a pre-determined period of time. Said period of time may be <NUM> to <NUM> seconds. During this time the initial portion of material may solidify sufficiently such that when the printhead jumps to another location the initial portion of material separates from the heated filament inside the nozzle.

Printing is resumed from the first anchor point <NUM> by printing an intermediate portion of material in direction <NUM> on the non-printed side <NUM> of the plane <NUM>. By printing the intermediate portion of material on the non-printed side <NUM> of the plane <NUM>, a collision between the printhead <NUM> and the first anchor point <NUM> is avoided (this can be understood with reference to <FIG>). A final portion of material is then printed in direction <NUM> to the second anchor point <NUM>.

The intermediate portion of material may be a linear section of material. Alternatively, the initial portion of material could be a non-linear section of material. The final portion of material may be a linear section of material. Alternatively, the final portion of material could be a non-linear section of material. In some examples, the intermediate and final portions of material may be printed as one curved section of material.

In accordance with the method described with reference to <FIG>, the printhead may impinge on the intermediate portion of material whilst printing the final portion of material. However, this impingement may be beneficial, as it can be used to "flatten out" the angle between the intermediate and final portions of material. That is, the printhead may push on the angle between the intermediate and final portions of material so as to increase that angle. For example, the angle may be increased to <NUM>°. This is possible because the intermediate and final portions of printed material are not able to solidify in the time it takes to print both portions. In this way, although the section of material deposited between the first and second anchor points is extruded in two portions, the resulting section of material may solidify as one linear section of material.

As described herein, the printhead speed and the extruding speed may be independently variable. The average printhead speed may be greater than the average extruding speed for the period of time during which the intermediate and final portions of material are printed between the first anchor point <NUM> and second anchor point <NUM>. The purpose of these relative speeds is such that, in the time it takes the printhead to follow a printing path in directions <NUM> and <NUM> from the first anchor point <NUM>, a length of material substantially matching the distance between the first anchor point <NUM> and second anchor point <NUM> is extruded. This is because the printhead travels a greater distance than the direct distance between the first anchor point <NUM> and the second anchor point <NUM> - and it is not desirable to print more material than is required to bridge those points.

The relative speeds described herein can be achieved in many ways. One way of achieving these speeds is to print the intermediate and final portions of material using a constant printhead speed that is greater than the constant extruding speed used. Another way of achieving these speeds is to initially match the printhead speed and the extruding speed, and stop extruding material (e.g. by making the extruding speed equal to zero) prior to arriving at the second anchor point <NUM>. A third way of achieving these speeds is to accelerate or decelerate either of the printhead speed or the extruding speed whilst printing the intermediate and final portions of material - such that, overall, the average printhead speed is greater than the average extruding speed.

The result of achieving these relative speeds is that the angle between the intermediate and final portions of material is "dragged out" by the movement printhead. That is, the printhead applies a tension force to the intermediate and final portions of printed material. The intermediate and final portions of material are of relatively low rigidity, when compared to the initial portion of material that has been allowed time to partially, or fully, solidify during the pause in printing. The angle between the intermediate and final portions of printed material accumulates the highest concentration of stress caused by the applied tension force. Therefore, the applied tension force acts increase the angle between the intermediate and final portions of material. For example, the angle may be increased to <NUM>°. In this way, although the section of material deposited between the first and second anchor points is extruded in as intermediate and final portions, the resulting section of material may solidify as one linear section of material.

The first anchor point <NUM>, formed in accordance with the method described with reference to <FIG>, may form the apex of a triangular section of the zig-zag type pattern layer <NUM>.

<FIG> shows another exemplary printing path in accordance with the methods described herein. It is worth noting that <FIG> depicts a path through which the printhead moves - rather than the form of the printed material itself. The starting point <NUM>, first anchor point <NUM>, first direction <NUM>, plane <NUM>, printed side of the plane <NUM>, non-printed side of the plane <NUM>, and second anchor point <NUM> are as described with reference to <FIG>.

An initial portion of material is printed between the starting point <NUM> and the first anchor point <NUM>. As described herein, the printing process then pauses at the first anchor point <NUM>.

Printing is resumed by printing two intermediate portions of material in directions 207a and 207b on the non-printed side <NUM> of the plane <NUM>. By printing the two intermediate portions of material on the non-printed side <NUM> of the plane <NUM>, a collision between the printhead <NUM> and the first anchor point <NUM> is avoided (this can be understood with reference to <FIG>). A final portion of material is then printed in direction <NUM> to the second anchor point <NUM>.

In the example depicted in <FIG>, the final portion of material originates on the non-printed side <NUM> of the plane <NUM>. In other examples, the final portion of material may originate on the printed side <NUM> of the plane <NUM>.

The printhead speed relative to the extruding speed during the printing of the intermediate and final portions of material may be as described herein with reference to <FIG>.

<FIG> schematically shows an exemplary printing path 521a achieved by repeating the series of printing directions <NUM>, 207a, 207b, and <NUM> as described with reference to <FIG>. It is worth noting that <FIG> depicts a path through which the printhead moves - rather than the form of the printed material itself.

An intermediate point <NUM>, shown in <FIG> and formed in accordance with the method described with reference to <FIG>, may form the apex of a triangular section of the zig-zag type pattern layer <NUM>. This is because the first intermediate portion of material is printed in a direction 207a away from the second anchor point <NUM>. Therefore, the printhead <NUM> does not impinge on the first intermediate portion of material when printing the second intermediate portion of material in direction 207b (this can be understood with reference to <FIG>). Thus, the angle between the first intermediate portion of material and the second intermediate portion of material is not "flattened out" as described herein. Thus, the intermediate point <NUM> can become the apex. The angle between the second intermediate portion and the final portion of material, printed in direction <NUM>, is "flattened out" or "dragged out" as described herein.

<FIG> schematically shows the expected result 521b when printing using the exemplary printing path as described with reference to <FIG> and <FIG>. As shown in <FIG>, by performing the method for three-dimensional printing described herein it is possible to form sharp apexes - and so the accuracy and fidelity of the zig-zag type pattern layer <NUM> can be improved.

It is to be understood that any printing path about the first anchor point <NUM> within the scope of the claims could be taken by the printhead so as to achieve the advantages described herein. For example, there could be any number of intermediate portions.

An anchor point can be returned to during wire frame printing in order to print further sections of material. When returning to an anchor point, the printhead returns to an offset position. That is, a position offset from the centre of that anchor point. For example, the printhead may return to a vertically offset position. This done in order to avoid collisions between the printhead and the anchor point. This is because the dimensions of an anchor point increase each time it is printed. A counter may be used to maintain a count of how many times an anchor point is returned to. The distance from anchor point to which the printhead returns to can be calculated in dependence on the counter value.

It is to be understood that the methods of wire fame printing described herein may also be applied to the three-dimensional printing of ceramics or metals. For example, rather than a thermo-plastic polymer filament, the printhead may extrude a high viscosity ceramic slurry from a reservoir. Instead of heating a filament to reduce its viscosity, the ceramic slurry may have a component of a volatile liquid, which enables the ceramic slurry to flow sufficiently for extrusion. The volatile liquid may evaporate after printing such that the printed material solidifies.

The methods described herein can be implemented in software. The methods described herein could be performed by an articulated printing robot <NUM>, or any other three-dimensional printing device, executing code that causes the three-dimensional printing device to perform the methods. Examples of a computer-readable storage medium include a random-access memory (RAM), read-only memory (ROM), an optical disc, flash memory, hard disk memory, and other memory devices that may use magnetic, optical, and other techniques to store instructions or other data and that can be accessed by a machine.

Claim 1:
A method of three-dimensional wire frame printing wherein material is printed by a printhead (<NUM>) configured to move relative to the material at a printhead speed and extrude material at an extruding speed, the method comprising:
printing an initial portion of material from a starting point (<NUM>);
pausing printing of the initial portion of material so as to form a first anchor point (<NUM>), such that a first direction (<NUM>) is defined from the starting point (<NUM>) to the first anchor point (<NUM>), and wherein the anchor point (<NUM>) lies in a plane (<NUM>) substantially perpendicular to the first direction (<NUM>) such that the initial portion of material is on the printed side of the plane (<NUM>);
resuming printing from the first anchor point (<NUM>) by printing, from the first anchor point (<NUM>), two or more intermediate portions of material on the non-printed side of the plane (<NUM>); and
printing a final portion of material to a second anchor point (<NUM>) on the printed side of the plane (<NUM>);
wherein:
the printhead speed and the extruding speed are independently variable;
the average printhead speed is greater than the average extruding speed for the period of time during which the intermediate and final portions of material are printed between the first and second anchor points (<NUM>, <NUM>); and
a first intermediate portion is printed on the opposite side of the initial portion to the second anchor point (<NUM>).