GB2584702A

GB2584702A - Adjusting three-dimensional print parameters

Info

Publication number: GB2584702A
Application number: GB1908418.5A
Authority: GB
Inventors: Cam Daghan; Desyllas Michail
Original assignee: Ai Build Ltd
Current assignee: Ai Build Ltd
Priority date: 2019-06-12
Filing date: 2019-06-12
Publication date: 2020-12-16
Anticipated expiration: 2039-06-12
Also published as: GB2584702B; GB201908418D0

Abstract

A method of three-dimensionally printing an object by extruding a plurality of layers 1400 of material from a printhead, comprising: dividing a layer to be extruded into a plurality of sections; determining a local geometry of the object about the section; determining a printing parameter in dependence on the local geometry; extruding the section of the layer in accordance with the determined printing parameter. The printing parameters may comprise printhead speed and extruding speed which may be independently variable. The local geometry may be the degree of cantilever wherein the width of the extrusion profile increases with the degree of cantilever; this may be achieved by providing printhead speed lower than extruding speed, or a variable size nozzle. This allows printing of overhanging sections without sagging or slide off. The local geometry may be a degree of curvature wherein the printhead speed decreases when the degree of curvature increases. The local geometry may be a distance between one or more previously printed layers wherein the height of the material to be printed is determined. A three dimensional printing apparatus and computer program adapted to perform the method are further provided.

Description

ADJUSTING THREE-DIMENSIONAL PRINT PARAMETERS

BACKGROUND

The present invention relates to a method of three-dimensional printing. In particular, the present invention relates to an improved method of three-dimensionally printing cantilevered sections.

Three-dimensional printing can be performed by an articulated printing robot 310, as shown in Figure 1. A three-dimensional object can be printed by extruding and depositing a plurality of layers of material. A typical extrusion cross-section may have a width to height ratio of two to one. A plurality of layers are typically stacked, one on top of another, so as to form one or more walls that form the three-dimensional object. For example, Figure 2a shows a cross-sectional view of a vertical wall portion 140a comprising fifteen layers of deposited material. Figure 3a shows a cross-sectional view of two layers of vertical wall portion 140a. The gravitational force, depicted by vector 130a, experienced by layer 104b is directed into layer 104a. In this way, layer 104a supports layer 104b.

A cantilevered, or overhung, portion of a three-dimensionally printed object is a portion in which a direction drawn between the centres of a plurality of layers is not perpendicular to the ground plane. The ground plane is defined as the plane perpendicular to the direction in which gravity acts. Typically, the ground plane will be parallel to the surface on which the object is printed -although in some examples the surface on which the object is printed may be at other angles relative to the ground plane. For example, Figure 2b shows a cross-sectional view of a cantilevered wall portion 140b comprising fifteen layers of deposited material. Typically, three-dimensionally printing cantilevered portions can be problematic because each layer of material in the cantilevered portion is not directly on top of the layer of material below it. For example, Figure 3b shows a cross-sectional view of two layers in cantilevered portion 140b. The gravitational force, depicted by vector 130b, experienced by layer 104d is not directed into layer 104c. In this way, layer 104c may be unable to support layer 104c. This can lead to layer104d "sagging", as it is not adequately supported by layer 104c. In some examples, layer 140d may "slide off' of 104c -leading to a failure of the cantilevered wall portion. This problem is compounded when the number of layers in a cantilevered portion is increased, or even when a vertical portion is printed on top of a cantilevered portion, because of the cumulative mass of those additional layers.

There is a need for an improved method of three-dimensionally printing cantilevered portions.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of three-dimensionally printing an object by extruding a plurality of layers of material from a printhead, the method comprising: dividing a layer to be extruded into a plurality of sections; determining a local geometry of the object about the section of the layer to be extruded; determining a printing parameter for the section of the layer in dependence on the local geometry; extruding the section of the layer in accordance with the determined printing parameter.

Determining the printing parameter in dependence on the local geometry may comprise: determining a desired extrusion profile in dependence on the local geometry about the section of a layer to be extruded; and determining the printing parameter for the section of the layer in dependence on the desired extrusion profile.

The printhead may be configured to move relative to the object at a printhead speed and extrude material at an extruding speed, the printhead speed and the extruding speed being independently variable. The printing parameter may be a printhead speed and/or an extruding speed.

The local geometry about the section of the layer to be extruded may be defined by one or more previously extruded sections of layers of material and/or one or more future sections of layers of material to be extruded.

The local geometry may be a degree of cantilever.

The desired extrusion profile may be dependent on a function of the degree of cantilever such that the width of the desired extrusion profile increases when the degree of cantilever increases.

The width to height ratio of the desired extrusion profile may increase when the width of the desired extrusion profile increases.

Determining the printing parameter may comprise determining a printhead speed that is lower than the extruding speed when the width of the desired extrusion profile increases.

The printhead may comprise a printing nozzle having a variable cross-section, and the printing parameter may be the size and/or shape of said printing nozzle.

The local geometry may be a degree of cantilever, and the desired extrusion profile may be dependent on a function of the degree of cantilever, such that the width of the desired extrusion profile increases when the degree of cantilever increases.

The local geometry about the section of the layer to be extruded may be defined by the section to be extruded itself.

The local geometry may be a degree of curvature.

The printing parameter may be the printhead speed, and the printhead speed may be dependent on a function of the degree of curvature such that the printhead speed decreases when the degree of curvature increases.

The local geometry may be a distance between one or more previously printed layers and the section to be printed, and the printing parameter may be determined so as to determine the height of the section of material to be printed.

According to a second aspect of the present invention there is provided a three-dimensional printing apparatus configured to perform any of the methods described herein According to a third aspect of the present invention there is provided a non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a three-dimensional printing apparatus, cause the three-dimensional printing apparatus to perform any of the methods described herein.

According to a fourth aspect of the present invention there is provided a computer program comprising computer program code means adapted to perform any of the methods described herein when said program is run on a programmable microcomputer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an exemplary printing robot for three-dimensional printing.

Figure 2a shows a cross-sectional view of a vertical portion.

Figure 2b shows a cross-sectional view of a cantilevered portion.

Figure 3a shows a cross-sectional view of two layers in a vertical portion.

Figure 3b shows a cross-sectional view of two layers in a cantilevered portion.

Figure 4 shows a plan view of a curved portion.

Figures 5 shows a cross-sectional view of a printhead.

Figure 6 shows cross-sectional view defining a cantilever angle.

Figure 7 shows a cross-sectional view of a cantilevered portion printed in accordance with the methods described herein.

Figure 8 shows a cross-sectional view of two layers in a cantilevered portion printed in accordance with the methods described herein.

Figure 9 shows a wall comprising vertical and cantilevered portions printed in accordance with the methods described herein.

Figure 10 shows a plan view defining a curvature angle.

Figure 11 a shows a side view of plurality of layers having a fixed layer height.

Figure 11 b shows a side view of a plurality of layers having a layer height that is variable across layers.

Figure 11 c shows a side view of a plurality of layers having a layer height that is variable across sections of layers.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is presented by way of example to enable a person skilled in the art to make and use the invention. The present invention is not limited to the embodiments described herein and various modifications to the disclosed embodiments will be apparent to those skilled in the art. Embodiments are described by way of example only.

Three-dimensional printing can be performed by an articulated robot arm 310, an example of which is shown in Figure 1 printing a cantilevered three-dimensional structure 308. The cantilevered three-dimensional structure 308 is supported by a printing bed 311.

Articulated robot arm 310 comprises a base 300 and a series of joints 301, 302, 303, 304, 305, 306 and a printhead 101. 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 101. The movement of articulated robot arm 310 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 101. Figure 5 shows a cross-sectional view of a typical printhead 101 for three-dimensional printing. Filament 103 is drawn 102 into the printhead 101. The printhead heats the filament 103 using heater 115 so that it can be extruded and deposited. Figure 5 shows six deposited layers of material 140.

Material is extruded via a nozzle 108. In some examples, the nozzle 108 may have a variable cross-section. The size and/or shape of the nozzle 108 can be considered to be a printing parameter.

Filament 103 is drawn in direction 102 into the printhead 101. 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 103 using heater 115 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 190°C to 240°C. The printing temperature can be considered to be a printing parameter.

As shown in Figure 5, the exit diameter of the nozzle of printhead 101 may be smaller than the entrance diameter so as to compress the filament 103 as it is drawn through the printhead 101. 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 extruded material cools such that the printed material solidifies. One or more fans 307 can be provided on or above the printhead 101. The fans 307 may be directed towards the printed filament 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 deposited material. The fans may be in operation continually during printing. Alternatively, the fans 307 may operate intermittently. Operating the fans intermittently may increase energy efficiency.

The printhead 101 may be moved by an articulated printing robot arm, as described with reference to Figure 1. The printhead 101 is configured to move relative to the three-dimensional structure 308 at a printhead speed. The relative printhead speed may represent the distance covered by one part of the printhead, such as corner 106, per unit of time. In another example, the printhead 101 remains stationary and the printing bed 311 is actuatable so as to move the three-dimensional structure 308 relative to the printhead 101. The relative printhead speed may represent the distance covered by one part of the three-dimensional structure 308, relative to the printhead 101, per unit of time. In some examples, the printhead speed is variable. The relative printhead speed may be in the region of 10 to 30 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 103 is drawn as shown by arrow 102 into the printhead 101. The filament may be in the region of four 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 101 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 10 to 30 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 extruding speed can be considered to be a printing parameter.

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 Figure 1. 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.

As described herein with reference to Figures 2b and 3b, three-dimensionally printing cantilevered sections can be problematic. According to the methods described herein, one or more printing parameters can be adjusted in dependence on the local geometry so as to address the problems associated with printing cantilevered sections.

The printing parameters may be adjusted so as to achieve a desired extrusion profile for cantilevered sections. For example, the printing parameters may be adjusted so as to increase the width of the deposited material in a cantilevered portion. This can be understood with reference to Figure 7, which shows a cross-sectional view of a cantilevered portion 140c printed in accordance with the methods described herein. As can be understood by comparing Figure 2b with Figure 7, the width W2 of each layer in Figure 7 is greater than the width Wi of each layer in Figure 2b. The height Hi of each layer is the same in both Figure 2b and Figure 7. That is, according to the methods described herein, the width to height ratio of each layer may be increased for a cantilevered portion. In other examples, the height need not be kept the same. That is, the height and width of a layer may be increased in a cantilevered section.

Figure 8 shows a cross-sectional view of two layers in a cantilevered portion printed in accordance with the methods described herein. The gravitational force, depicted by vector 130c, experienced by layer 104f is directed into layer 104e. In this way, layer 104f supports layer 104e, and sagging and/failure between layers 104e and 104f may be prevented.

There are a number of ways in which the printing parameters may be adjusted so as to achieve the desired extrusion profile. In one example, the width to height ratio of each layer is increased by reducing the printhead speed relative to the extrusion speed. In another example, the extrusion speed may be increased relative to the printhead speed. That is, the printhead speed to extrusion speed ratio is decreased. This leads to a greater "distance" of material being extruded than the distance of travel of the printhead. This leads to the printhead nozzle compressing the material as it is being extruded. This is because the height of the layer may be limited by the distance between the previously printed layer and the nozzle of the printhead. Therefore, when the material is compressed as it is being extruded, the width of the layer increases. This results in an increased width to height ratio of the layer.

In another example, the printhead nozzle 108 has a variable cross-section. The width to height ratio of the layer may be increased by selecting an appropriate cross-section for the variable cross-section nozzle 108.

In yet another example, the printing temperature may be increased. By increasing the printing temperature, the viscosity of the extruded material can be reduced. When the viscosity of the extruded material is reduced, the layer may "collapse" under its own weight. That is, the profile of the extruded layer immediately after it has been deposited may sag, before solidifying with an increased width to height ratio.

The printing parameters required to achieve the desired extrusion profile may be determined in dependence on the local geometry according to the following method.

Each layer to be printed is divided into a plurality of sections. Each section can be of any length. Each section could be of equal length. For example, each section may be four millimetres in length. Alternatively, a plurality of sections may be different in length.

One or more printing parameters may be determined for each section of each layer to be printed. In some examples, the printing parameters may be different for each section In order to determine a printing parameter, the local geometry about a section of a layer to be extruded is determined.

Determining the local geometry may involve considering the geometry or relative position of a number of sections in the same layer either side of the section of the layer to be printed, or a number of sections in a number of layers adjacent to the layer to be printed. Said number may be three. In another example, one or more sections within the proximity of the section of the layer to be printed may be considered. For example, any layer or section of material within a certain distance from the section of material to be printed may be considered. Said distance may be five millimetres.

In one example, this may involve determining the position of the section of the layer to be printed relative to the position of a previously printed layer. The previously printed layer may be the layer on which the section of the layer to be printed will be supported. In another example, the position of the section to be printed may be determined relative to the position of one or more layers to be printed in the future. For example, the one or more layers to be printed in the future may be the layers that will be supported by the section of the layer to be printed.

The local geometry about a section of a layer to be extruded may be a degree of cantilever. The degree of cantilever may be defined by a cantilever angle. Figure 6 shows cross-sectional view defining a cantilever angle 110.

In Figure 6, the local geometry about the section of the layer to be printed is determined relative to the position of a previously printed layer 104. The cross-section depicted by Figure 6 may represent the cross-sectional view in a cross-sectional plane through the centre of the section to be printed. A substantially vertical direction 125, in the plane and perpendicular to the ground plane may be defined which intersects a point of a previously printed layer 104. A vector 126 from the point of the previous printed layer to the centre of the nozzle 108 of printhead 101 may be defined in the same cross-sectional plane. The cantilever angle 110 may be the angle between direction 125 and vector 126.

The printing parameters may depend on the cantilever angle 110. In one example, the printhead speed can be reduced whilst the extrusion speed is held constant in order to achieve a desired extrusion profile having a larger width to height ratio than a typical extrusion profile. A default printhead speed, Vd, may be set. For example, the default printhead speed made be 20 millimetres per second. The default printhead speed may be intended for printing sections with a cantilever angle 110 of zero -e.g. vertical portion 140a. The printhead speed used for a section of a layer to be printed may depend on a function of the cantilever angle 110. For example, if the cantilever angle is 8, the printhead speed may depend on Va cos(61). That is, as the cantilever angle increases from 0°, i.e. no cantilever, to a positive cantilever angle 110, the value of cos(6) will decrease. The results in the value of Va cos(0) reducing -and so the printhead speed for that section of the layer decreases. In other examples, there may be a linear dependence between the printhead speed and the cantilever angle.

Figure 9 shows a wall comprising vertical and cantilevered portions printed in accordance with the methods described herein. The differences in cross-sectional profile between the vertical and cantilevered portions depicted in Figure 9 are illustrative only, and are not to be taken to be to scale. With reference to Figure 9, it can be understood that the desired extrusion profile may vary for different layers. In fact, the desired extrusion profile may be different for different sections of the same layer. That is, in accordance with the methods described herein, a unique printing parameter can be calculated for each section. In other words, the cross-sectional profile of each section of printed material can be tailored to the requirements of its local geometry. This is instead of printing an entire three-dimensional structure with one cross-sectional profile for the printed material, Three-dimensionally printing curved sections can also be problematic. A curved portion of a three-dimensional object is a portion in which a wall formed by one or more layers is not linear. For example, Figure 4 shows a plan view of a curved portion 203 of a three-dimensional object. Typically, if a curved section is printed at the same printhead speed as a linear section, adjacent layers may delaminate. This is because the extruded material is not given enough time to conform to the curve, and so can "slip-off" of the previously printed layer. This problem is compounded at higher degrees of curvature, that is, when the curves to be printed are "tighter". According to the methods described herein, one or more printing parameters can be adjusted in dependence on the local geometry so as to address the problems associated with printing curved sections.

One or more printing parameters may be determined for each section of each layer to be printed. In some examples, the printing parameters may be different for each section.

In order to determine a printing parameter, the local geometry about a section of a layer to be extruded is determined. In one example, this may involve determining the curvature angle of the section of the layer.

Figure 10 shows a plan view defining one example of a curvature angle 210. In the example described with reference to Figure 10, curve 203a is a trace of curve 203 (shown in Figure 4). Points 205a and 205b define the two ends of a section to be printed. Tangents 206a and 206b to curve 203a at points 205a and 205b respectively are be defined. Lines 207a and 207b are drawn perpendicular to tangents 206a and 206b and intersecting points 205a and 205b respectively. The angle defined by the intersection of lines 207a and 207b is a curvature angle 210.

The printing parameters may depend on the curvature angle 210. In one example, the printhead speed can be reduced in order to allow the extruded material in a curved section long enough to conform to the curve. A default printhead speed, Vd, may be set. For example, the default printhead speed made be 20 millimetres per second. The default printhead speed may be intended for printing sections with a curvature angle 210 of zero -e.g. a linear layer. The printhead speed used for a section of a layer to be printed may depend on a function of the curvature angle 210. For example, if the curvature angle is 0, the printhead speed may depend on V cos(0). That is, as the curvature angle increases from 0°, i.e. no curvature, to a positive curvature angle 210, the value of cos(0) will decrease. The results in the value of Va cos(0) reducing -and so the printhead speed for that section of the layer decreases. In other examples, there may be a linear dependence between the printhead speed and the curvature angle.

In another example, one or more printing parameters may be varied in order to vary the height of each layer. The layer height is represented in Figures 2b and 7 by Hi. Conventional three-dimensional printing methods assume that each layer of a layer-wise deposited object is of the same height. For example, Figure 11 a shows a side view of a plurality of layers 500 having a fixed layer height. This means that an object to be printed can be divided into a plurality of horizontal planes, each plane being parallel to the ground plane. Printing of one or more sections can then progress in each of these planes. After one or more sections has been printed in one plane, the printhead can begin printing one or more sections in the next, adjacent, plane by moving an incremental distance in the vertical direction.

In accordance with the methods described herein, one or more printing parameters may be varied in order to vary the height of the section of a layer to be printed. This increases the freedom with which objects to be printed can be designed. This is because printing need not be confined to a series of parallel planes, as is typically the case. The methods described herein can therefore be applied in order to determine the desired layer height for a section of a layer to be printed in dependence on the local geometry about that section. In this case, the local geometry may be defined by a distance between the one or more previously printed layers and the section of the layer to be printed. The printing parameters for a section of a layer to be printed may depend on the determined local geometry. For example, in order to increase the layer height, the printhead speed may be reduced relative to the extrusion speed whilst the distance between the printhead and the previous layer is increased.

For example, Figure 11 b shows a side view of a plurality of layers 501 having a layer height that is variable across layers. In this example, the layer height of adjacent layers may be different, although printing may continue in a set of parallel planes. In another example, Figure 11c shows a side view of a plurality of layers 502 having a layer height that is variable across sections of layers. In this example, the layer height for each section of each layer may be different. In this way, the design of objects to be printed need not be confined to a series of parallel planes, as is typically the case.

It is to be understood that the methods of three-dimensional 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 310, 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.

The terms computer program code and computer readable instructions as used herein refer to any kind of executable code for processors, including code expressed in a machine language, an interpreted language or a scripting language. Executable code includes binary code, machine code, bytecode, code defining an integrated circuit (such as a hardware description language or netlist), and code expressed in a programming language code such as C, Java or OpenCL. Executable code may be, for example, any kind of software, firmware, script, module or library which, when suitably executed, processed, interpreted, compiled, executed at a virtual machine or other software environment, cause a processor of the three-dimensional printing device at which the executable code is supported to perform the tasks specified by the code.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS1. A method of three-dimensionally printing an object by extruding a plurality of layers of material from a printhead, the method comprising: dividing a layer to be extruded into a plurality of sections; determining a local geometry of the object about the section of the layer to be extruded; determining a printing parameter for the section of the layer in dependence on the local geometry; extruding the section of the layer in accordance with the determined printing parameter.
2. The method as claimed in any preceding claim, wherein determining the printing parameter in dependence on the local geometry comprises: determining a desired extrusion profile in dependence on the local geometry about the section of a layer to be extruded; and determining the printing parameter for the section of the layer in dependence on the desired extrusion profile.
3. The method as claimed in claim 1 or 2, the printhead being configured to move relative to the object at a printhead speed and extrude material at an extruding speed, the printhead speed and the extruding speed being independently variable, and wherein the printing parameter is a printhead speed and/or an extruding speed.
4. The method as claimed in claim 2 or claim 3, wherein the local geometry about the section of the layer to be extruded is defined by one or more previously extruded sections of layers of material and/or one or more future sections of layers of material to be extruded.
5. The method as claimed in any preceding claim, wherein the local geometry is a degree of cantilever.
6. The method as claimed in claim 5, wherein the desired extrusion profile is dependent on a function of the degree of cantilever such that the width of the desired extrusion profile increases when the degree of cantilever increases.
7. The method as claimed in claim 6, wherein the width to height ratio of the desired extrusion profile increases when the width of the desired extrusion profile increases.
8. The method as claimed in claims 6 or 7, when dependent on claim 3, wherein determining the printing parameter comprises determining a printhead speed that is lower than the extruding speed when the width of the desired extrusion profile increases.
9. The method as claimed in any of claims 2 to 8, wherein the printhead comprises a printing nozzle having a variable cross-section, and the printing parameter is the size and/or shape of said printing nozzle.
10. The method as claimed in claim 9, wherein the local geometry is a degree of cantilever, and wherein the desired extrusion profile is dependent on a function of the degree of cantilever, such that the width of the desired extrusion profile increases when the degree of cantilever increases.
11. The method as claimed in any preceding claim, wherein the local geometry about the section of the layer to be extruded is defined by the section to be extruded itself.
12. The method as claimed in any preceding claim, wherein the local geometry is a degree of curvature.
13. The method as claimed in claim 12, when dependent on claim 3, wherein the printing parameter is the printhead speed, and the printhead speed is dependent on a function of the degree of curvature such that the printhead speed decreases when the degree of curvature increases.
14. The method as claimed in any preceding claim, wherein the local geometry is a distance between one or more previously printed layers and the section to be printed, and the printing parameter is determined so as to determine the height of the section of material to be printed.
15. A three-dimensional printing apparatus configured to perform the method as claimed in any preceding claim.
16. A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a three-dimensional printing apparatus, cause the three-dimensional printing apparatus to perform the method as claimed in any of claims 1 to 14.
17. A computer program comprising computer program code means adapted to perform the method as claimed in any of claims 1 to 14 when said program is run on a programmable microcomputer.