Extrusion-based additive manufacturing system and method

A method of manufacturing an object is disclosed. Material is extruded from an extrusion head onto a tool, the extrusion head having an extrusion axis along which the material flows as it exits the extrusion head. Relative movement is generated between the extrusion head and the tool as the material is extruded so that the material is deposited as a series of layers, wherein the material cures on or after deposition so that the layers are fused together. The extrusion and the relative movement are controlled so that the series of layers are shaped in accordance with a stored three-dimensional model of the object. At least some of the layers are non-planar layers.

RELATED APPLICATIONS

The present application is a National Phase of International Application Number PCT/GB2014/052256, filed Jul. 24, 2014, which claims priority from Great Britain Application Number 1314030.6, filed Aug. 6, 2013.

FIELD OF THE INVENTION

The present invention relates to an extrusion-based additive manufacturing system, and a method of manufacturing an object.

BACKGROUND OF THE INVENTION

An extrusion-based additive manufacturing system is described in WO2012/037329. The system uses a filament as consumable feedstock. The filament has a core portion and a shell portion with different peak crystallization temperatures. Both the core and the shell portions are melted in an extrusion head, and after they have been deposited the portion with the higher crystallization temperature crystallizes before the other portion.

SUMMARY OF THE INVENTION

The present invention provides a method of manufacturing an object, the method comprising: extruding material from an extrusion head onto a tool, the extrusion head having an extrusion axis along which the material flows as it exits the extrusion head; generating a relative movement between the extrusion head and the tool as the material is deposited so that the material is deposited as a series of layers, wherein the material cures on or after deposition so that the layers are fused together, and controlling the extrusion of the material and the relative movement so that the series of layers are shaped in accordance with a stored three-dimensional model of the object. At least some of the layers are non-planar layers, and the relative movement during the deposition of each non-planar layer includes at least an element of rotation which causes an orientation between the extrusion axis and the tool to change at the same time as the material is extruded.

Forming some of the layers as non-planar layers enables the object to have improved structural properties. For instance the non-planar layers can be designed to follow lines of stress (such as hoop stress) in the object.

The extrusion axis meets the non-planar layer at a tilt angle which may remain constant or may change as the non-planar layer is deposited. Causing an orientation between the extrusion axis and the tool to change at the same time as the material is deposited enables this tilt angle to be controlled—for instance so that it does not change. An advantage of maintaining a tilt angle which remains constant as the non-planar layer is deposited is that it maintains a stable flow dynamic from the extrusion head to the non-planar layer.

Typically the relative movement during the deposition of each non-planar layer causes the orientation between the extrusion axis and the tool to change in accordance with a change in angle of the non-planar layer relative to the tool. In other words, the orientation is changed during deposition to follow the shape of the non-planar layer.

The material may be a thermoplastic material which cures by cooling, a thermosetting material which is cured by heating, or a material which is cured by some other mechanism (such as by photocuring or reacting with a chemical curing agent).

The extrudate which is extruded from the extrusion head and forms the object may be homogenous, but more typically it has a heterogenous structure. For instance the extrudate may comprise a reinforcement portion and a matrix portion which both run continuously along a length of the extrudate, wherein the reinforcement portion and the matrix portion have different material properties. The reinforcement portion and the matrix portion may for instance have a different melting point or a different crystallinity.

The tool may remain attached to the object, but more preferably the object and the tool are separated after the object has been manufactured, for instance by dissolving the tool. The tool may be dissolved by the action of a liquid dissolving agent, or by heating the tool so it melts and can be removed by a scraping tool or other mechanical method.

Typically at least some of the layers have a different size and/or a different shape in accordance with the stored three-dimensional model of the object.

The method may further comprise manufacturing the tool prior to manufacturing the object, the tool being manufactured by: extruding a tool material from an extrusion head onto a build member (such as a build plate); generating a relative movement between the extrusion head and the build member as the tool material is extruded onto the build member plate so that the tool material is deposited as a series of tool layers, wherein the tool material cures on or after deposition so that the tool layers are fused together, and controlling the extrusion of the tool material and the relative movement so that the series of tool layers are shaped in accordance with a stored three-dimensional model of the tool.

The extrusion head which manufactures the tool may be the same as the extrusion head which forms the object. Alternatively the tool and the object may be manufactured by different respective extrusion heads.

Typically each tool layer is substantially planar, in contrast with the object in which at least some of the layers are non-planar. In this case the tool and the object are preferably manufactured by different extrusion heads—the tool extrusion head being able to translate but not rotate.

The tool material may have different material properties to the material forming the object. For instance the tool material may be homogenous and the material forming the object may be heterogenous. In this case the tool and the object are preferably manufactured by different extrusion heads—it being easier to switch between different extrusion heads than to change the material being fed to a single extrusion head.

The relative rotation may be generated by rotating the extrusion head without rotating the tool, by rotating the tool without rotating the extrusion tool, or by rotating both. The relative motion of the extrusion head and the tool during deposition of the non-planar layers may be a pure orbital rotation (that is a rotation about a single point with no relative translation) but more typically it is a compound motion comprising a mixture of rotation and translation.

A further aspect of the invention provides a system for manufacturing an object by an extrusion-based additive manufacturing method, the system comprising: an extrusion head having a channel with an extrusion outlet; a build member; a feed mechanism for feeding material into the channel of the extrusion head so that the material is extruded from the extrusion outlet; a drive system arranged to cause relative translation between the extrusion head and the build member along three axes, and relative rotation between the extrusion head and the build plate about at least two axes; a memory (typically a computer memory) for storing a three-dimensional model of the object; and a controller programmed to operate the feed mechanism and the drive system in order to manufacture the object on the build plate by extrusion-based additive manufacturing in accordance with the three-dimensional model of the object stored in the memory, wherein the controller is programmed to cause the drive system to generate at least an element of relative rotation between the extrusion head and the build member at the same time that the feed mechanism causes the material to be extruded from the extrusion outlet.

Each layer of the object may be manufactured with a single extruded line or road only, for instance following a serpentine pattern. However more preferably each layer is manufactured with multiple extruded lines.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Apparatus for manufacturing an object by an extrusion-based additive manufacturing method is shown inFIG. 1. The apparatus comprises a tool extrusion head1a, an object extrusion head1band a build plate7(which may be heated).

A 3-axis drive system8ais arranged to move the tool extrusion head1aalong three axes relative to the build plate7. The drive system8ais capable of translating the tool extrusion head1aback and forth in the X and Z directions (in the plane ofFIG. 1) as well as in the Y direction (in and out of the plane ofFIG. 1).

A 5-axis drive system8bis arranged to move the object extrusion head1balong and about five axes relative to the build plate7. The drive system8bis capable of translating the object extrusion head back and forth in the X and Z directions (in the plane ofFIG. 1) as well as in the Y direction (in and out of the plane ofFIG. 1). The drive system8bis also capable of rotating the object extrusion head1bclockwise and anti-clockwise about axes of rotation parallel to the X or Y axis, in order to change its orientation (that is, its angle of tilt) relative to the build plate7. To facilitate manipulation in addition to X, Y and X movements the head is mounted on a two-axis gimbal to allow this rotation. A suitable 5-axis drive system is an industrial robot such as the Kuka KR240 R3200 PA (KR QUANTEC) available from Kuka Roboter GmBH. Alternatively a higher accuracy (+/−0.05 mm) robot may be used such as the Kuka KR R2900 EXTRA HA (KR QUANTEC) also available from Kuka Roboter GmBH. The latter is a 6-axis robot rather than a 5-axis robot.

In this embodiment of the invention the build plate7remains stationary and all movement is performed by the extrusion heads1a,b. However it will be appreciated that in alternative embodiments some or all of the relative movement between the extrusion heads and the build plate7may be achieved by moving the build plate instead. For instance the build plate7could be translated in Z so the 5-axis drive system8bis replaced by a 4-axis drive system and the 3-axis drive system8ais replaced by a two-axis drive system.

The detailed construction of the extrusion heads1a,bis shown inFIGS. 2 and 3respectively. Each extrusion head has a chamber2a,bwith an extrusion outlet3a,b; a pair of motorized fibre drive rollers4a,b; and a chamber heater5a,b. The object extrusion head1balso has a pair of matrix feed rollers30, a fibre heater6; and a matrix heater34.

A controller35controls the heaters5a,5b,6,34, the motorized rollers4a,4b,30and the drive systems8a,bin order to manufacture an object in accordance with a Computer Aided Design (CAD) model of the object in a computer memory36by following the process shown inFIGS. 4-14.

First, the tool extrusion head1ais used to manufacture a tool on the build plate by the process ofFIGS. 4-6. The drive wheels4aare driven to feed a single-part filament30into the chamber2aand the three-axis tool extrusion head drive system8amoves the tool extrusion head1ainto a desired position above the build plate7. The heater5ais operated to raise the temperature of the filament30in the extrusion head above its melting point so that the filament melts within the extrusion head. The melted material is then extruded from the extrusion head onto the build plate7and the system8ais operated to deposit an extruded line31as shown inFIG. 4. The extruded line31solidifies when it cools after it has been laid down on the build plate7.

FIG. 5is a plan view of the build plate showing the extruded line31from above. When the line31has been extruded, the filament drive wheels4aare stopped, the extrusion head1ais translated in X and Y to the next position, and a second extruded line32is laid down as shown inFIG. 5. This process is then repeated to lay down a series of extruded lines as shown inFIG. 5, each forming part of the same planar layer and each fusing with an adjacent line or lines. In the example ofFIG. 5the planar layer is formed by a series of individual extruded lines31,32etc, but alternatively the layer may be formed from a single line by continuously extruding material as the head1atraverses the build plate in a spiral or raster pattern for example.

The process ofFIG. 5is then repeated to form a stack of planar layers as shown inFIG. 6, each layer fusing with a previously deposited layer and some of the layers have a different shape and/or size in accordance with the CAD model to build up a three-dimensional mandrel or tool40.

Next, the tool extrusion head1ais moved away and the object extrusion head1bis used to manufacture an object on the tool40by the process ofFIGS. 7-12. Before these process steps are described, the construction of the object extrusion head1bwill be described in more detail with reference toFIG. 3. The extrusion head1bhas a cylindrical body10with an opening at its upper end and a conical part at its lower end leading to the extrusion outlet3b. A plug11carried on a base12is received in the upper end of the body10. The base12is attached to the upper rim of the cylindrical body10. A fibre feed tube passes through the base12and the plug11and extends into the chamber2b. The interior of the tube provides a cylindrical fibre feed channel with an inlet at its upper (proximal) end and an outlet in the chamber2bat its lower (distal) end. The fibre feed tube has a continuous upper (proximal) portion15which passes through the plug11into the chamber and is a continuous cylinder with no holes, and a discontinuous lower (distal) portion16fully inside the chamber which has a helical lattice structure with openings distributed along its length.

The drive rollers4bfeed a reinforcement fibre20into the chamber2via the fibre feed channel at a velocity V1m/s. The diameter of the reinforcement fibre20is typically of the order of 0.08 mm to 0.6 mm, with the drive rollers4bbeing spaced apart as required to provide a fibre feed channel with an equivalent diameter.

The reinforcement fibre20is manufactured by spinning and drawing a polymer under tension to form one or more filaments with crystallites aligned with the length of the fibre. The reinforcement fibre20may consist of a single one of such filaments, or it may comprise a plurality of such filaments. The polymer chains and crystallites in the reinforcement fibre20are aligned with its length.

Dyneema® is one example of a suitable UHDPE fibre which can provide a yield strength greater than 2 GPa and preferably greater than 2.4 GPa, a crystallinity by weight which is greater than 80% and preferably greater than 85%, and has polymer chains with a parallel orientation greater than 90% or more preferably greater than 95%.

A matrix feed tube37is mounted to the body10towards the upper end of the chamber2b. The interior of the matrix feed tube37provides a cylindrical matrix feed channel with an inlet31at its outer (distal) end and an outlet32in the side of the body10at its inner (proximal) end. A pair of motorized matrix feed rollers30are arranged to feed a matrix fibre (not shown) into the matrix feed channel. Alternatively the matrix material could be fed into the matrix feed channel in the form of a powder. The tube37carries a matrix heater34which melts the matrix fibre in the tube37to transform it into liquid matrix material. The liquid matrix material then flows into the chamber2bthrough the outlet32at a velocity V2m/s controlled by the rotation rate of the rollers30.

The matrix feed tube37is oriented at right angles to the fibre feed tube15,16but may also be oriented so that the matrix is fed downwardly into the chamber at an acute angle to the reinforcement fibre if desired.

The matrix material forming the matrix fibre is typically the same polymer as the material forming the reinforcement fibre20, optionally with different molecular weights. Where the molecular weights are different, then preferably the reinforcement fibre material has the higher molecular weight (for instance between 2,000,000 and 6,000,000 in the case of UHDPE). The reinforcement fibre20also has a higher crystallinity than the matrix fibre33. This higher crystallinity results in a higher melting point.

Typically the fibres are both formed by drawing the fibre under tension from a polymer melt. However the crystallinity of the reinforcement fibre20is enhanced compared with the matrix fibre by using a slower cooling rate, a higher drawing rate and/or a polymer with a higher molecular weight.

First, the fibre drive rollers4bare driven to feed the reinforcement fibre20into the chamber and through the extrusion outlet3b. The inwardly tapering shape of the lower part of the chamber2bassists in guiding the fibre20towards the extrusion outlet3b. The drive system8bis driven to move the extrusion head1binto a desired position. The matrix heater34is turned on to melt the matrix fibre in the tube37and transform it into liquid matrix material. The matrix drive rollers30are then operated to feed the liquid matrix material into the chamber2b.

The liquid matrix material wets the upper portion of the reinforcement fibre20in the fibre feed tube16via the lateral holes in the lattice structure, as well as contacting the lower portion of the reinforcement fibre20between the outlet of the fibre feed channel and the extrusion outlet3b.

The fibre feed rollers4band matrix feed rollers30are then driven simultaneously to extrude a coated fibre50from the extrusion outlet3bas shown inFIG. 8, the coated fibre50comprising the reinforcement fibre20with a coating of the liquid matrix material.

The diameters of the fibre20and the extrusion outlet3bare selected to provide an extrudate (that is, the coated fibre50) in which the fibre20occupies a volume greater than 30% of the extrudate and preferably a volume in the range of 40-60% of the extrudate.

The reinforcement fibre20may be relatively rigid so it can be “pushed” through the chamber by the fibre driver rollers4b, moving in and out of the chamber at the same velocity V1relative to the chamber. Alternatively the reinforcement fibre20may be pulled into the chamber by the viscous drag forces created by the action of the flowing liquid matrix material on the reinforcement fibre in the chamber.

The reinforcement fibre20does not change in cross-section as it passes through the chamber, so the extruded coated fibre50has a cross-sectional area transverse to its length (defined by the area of the extrusion outlet3b) which is greater than that of the fibre20entering the chamber.

The matrix feed channel on the other hand has a diameter D2of the order of 3 mm which is much greater than the diameter D1of the extrusion outlet3b. Consequently the cross-sectional area A2of the matrix feed channel (and the solid matrix fibre being fed into it) is greater than the cross-sectional area A1of the extrusion outlet (and the coated fibre50being extruded from it). The area A2is also greater than the cross-sectional area of the matrix coating of the coated fibre50. Consequently the liquid matrix material has a relatively slow velocity V2relative to the chamber as it flows into the chamber at the inlet32into the chamber, but it is extruded out of the extrusion outlet3bwith the coated fibre50at a higher velocity V1.

The large diameter D2of the matrix feed channel (and the solid matrix fibre being fed into it) means that the solid matrix fibre has sufficient buckling strength to allow it to be driven by the matrix feed rollers30into the matrix feed channel with sufficient force to apply a positive pressure. This positive pressure elevates the pressure of the liquid matrix material in the extrusion chamber, and can be controlled by appropriate operation of the rollers30. The elevated pressure in the extrusion chamber provides two benefits. Firstly it assists the wetting of the reinforcement fibre20by the liquid matrix material. Secondly, it reduces the likelihood of defects in the coating of the extruded coated fibre50.

As the coated fibre50is extruded, the drive system8bis operated to cause relative movement between the extrusion outlet3band the build plate7and tool40as the coated fibre50is extruded from the extrusion outlet, depositing a first extruded line50(also known as a “road”) onto the tool as shown inFIG. 8. Only the matrix coating is molten and the reinforcement fibre20remains in a rigid semi-crystalline state as it passes through the chamber and out of the extrusion outlet3b. The matrix coating of the coated fibre solidifies when it cools after it has been deposited. Optionally cooling fans (not shown) may be positioned near the outlet3bto cool the extrudate more quickly.

The pair of heaters5b,6are independently controllable by the controller35. As the coated fibre is extruded, both heaters5b,6are operated to heat the chamber and prevent the matrix material in the chamber from solidifying. However during extrusion the temperature in the chamber is kept below the melting point of the reinforcement fibre20so it remains rigid.

When a break is required in the extruded line50, then the fibre heater6is operated to temporarily raise the temperature of the reinforcement fibre in the lower part of the extrusion head1babove its melting point, thereby forming a break in the continuous reinforcement fibre. At the same time the drive system8bis operated to move the extrusion head1baway from the tool40and effectively “cut” the coated fibre to form an end22of the extruded line50.

Next, the fibre heater6is turned down to lower the temperature in the lower part of the extrusion chamber back below the melting point of the reinforcement fibre20to enable a further line51to be extruded as shown inFIG. 9.

A small amount of amorphous material is extruded out of the chamber2bbetween the cut lines50and51. This material can deposited at the edge of the part and machined away after the whole part has been formed. The number of cuts22in a given part is minimised in order to minimise the quantity of such amorphous material.

The length of time of the heat pulse which “cuts” the coated fibre at the end of each line will depend on a number of factors, mainly the thermal mass of the extrusion head1b, but it will typically be of the order of 0.1 to 10 s.

The matrix coating of the extruded coated fibre51fuses with the coating of the previously extruded coated fibre50and solidifies after it has done so. In the case ofFIG. 9the second line51is deposited alongside the first line50with which it fuses. The matrix material flows between and bonds together adjacent reinforcement fibres after they have been extruded, filling the gaps between the reinforcement fibres in adjacent extruded layers.

Next, the fibre heater6is operated again to temporarily raising the temperature of the fibre in the extrusion head above its melting point after the second line51has been extruded, thereby forming a break. At the same time the drive system8bis operated to move the head1baway from the tool and effectively “cut” the fibre to form an end of the extruded line51.

This process is then repeated a number of times as required to manufacture a first layer55of the object in accordance with the CAD model in the memory as shown in the plan view ofFIG. 9. Although each individual line50,51etc. in the layer55lies in a single plane (parallel with the XZ plane ofFIG. 11) the layer55as a whole is non-planar—in this case the layer55has a domed shape.FIG. 10shows an alternative arrangement for the first layer in which each individual line follows a radial path extending away from the centre, the lines overlapping at the centre.

The next layer70is then formed over the first layer55as shown inFIGS. 11 and 12in accordance with the CAD model in the memory. As shown inFIG. 12the extruded lines in the second layer70are formed with each line following a two-dimensional curve which lies in a plane parallel with the YZ plane (in and out of the plane ofFIG. 11) rather than the XZ plane (the plane ofFIG. 11). Alternatively the extruded lines in the second layer70may follow two-dimensional curves which lie in any other plane, or they may follow three-dimensional curves (that is, curves which do not lie in a single plane). The second layer70has a slightly increased size compared to the first layer55(this difference in size being exaggerated inFIG. 12).

The process is then repeated further until the entire object has been built with a series of dome-shaped layers similar to the layers of an onion. The object may be, for example, a helmet.FIG. 14shows the object80as only two layers for ease of illustration but in practice there will be a much larger number of layers.

The rotation60of the head1bshown inFIG. 8causes an orientation or tilt angle between the extrusion axis62and the tool40and build plate7to change at the same time as the material is deposited. Thus when the head1bis at the start of the line50as inFIG. 7it is oriented with its extrusion axis62at a tilt angle of about +45° relative to a nominal tool axis64. As the line50is extruded this tilt angle changes continuously from about +45° inFIG. 7to about −45° inFIG. 8. The orientation between the extrusion axis62and the tool axis64changes in accordance with a change in angle of the non-planar layer relative to the tool axis64, thus maintaining a constant orthogonal extrusion angle between the extrusion axis62and the line being deposited.FIG. 11illustrates the orthogonal extrusion angle64between the extrusion axis62and a line63tangent to the layer being deposited where the extrusion axis62meets the layer.

In the simplified example ofFIG. 8the extrusion head follows a pure orbital rotation about a single point in the tool to form a line50with constant curvature, but if a more complex shape is required then the motion of the head1bwill also include an element of translation in X, Y and/or Z so the curvature of the line varies across the layer.

This change of orientation of the head1brelative to the tool axis64during extrusion provides a number of advantages:it enables a single line50to be deposited not only on the upper side of the tool40but also on its undersideit enables each line50of each layer to follow lines of stress in the object, such as circumferential hoop stress. Since the extrudate is stiffer and stronger along its length this provides greater resistance to circumferential hoop stress than if the domed shape of the object was built up in a conventional manner by forming a stack of planar layers (like the tool40)the constant extrusion angle64between the extrusion axis62and the line being deposited provides the advantage that it maintains a stable flow dynamic from the extrusion head to the line being deposited. A 90° extrusion angle64is shown inFIG. 11, and this may be preferred since it enables the angle of the extrusion head to remain constant when a planar layer is being deposited. In an alternative embodiment the extrusion angle64between the extrusion axis62and the line being deposited may be an oblique angle. In this case the extrusion axis may be tilted forward in the direction of motion of the extrusion head so that there is a constant acute angle between the extrusion axis and a direction of the relative movement between the extrusion head and the tool (and equivalently a constant obtuse angle between the extrusion axis and the extruded line).

After the object has been fully formed, the tool40carrying the object80is immersed in a bath of solvent85as shown inFIG. 13. The tool40dissolves away, leaving only the object80as shown inFIG. 14which can then be removed from the bath. A suitable material for the tool is Stratasys SR20/SR30 available from Stratasys Ltd, and a suitable solvent is “WaterWorks” solution also available from Stratasys Ltd. Alternatively the tool40may be left intact in the final product.