FILAMENT FOR EXTRUSION-BASED ADDITIVE MANUFACTURING SYSTEM

Abstract

A filament is fed to an extrusion head. The filament has a semi-crystalline polymeric reinforcement portion and a polymeric matrix portion. The reinforcement and matrix portions run continuously along a length of the filament. The reinforcement portion has a higher melting point and a higher crystallinity than the matrix portion. The temperature of the filament is raised in the extrusion head above the melting point of the matrix portion but below the melting point of the reinforcement portion so that the matrix portion of the filament melts within the extrusion head, thereby forming a partially molten filament within the extrusion head. The partially molten filament is extruded from the extrusion head onto a substrate, the reinforcement portion of the partially molten filament remaining in a semi-crystalline state as it is extruded from the extrusion head. Relative movement is generated between the extrusion head and the substrate as the partially molten filament is extruded onto the substrate in order to form an extruded line on the substrate. The matrix portion of the extruded line solidifies after the extruded line has been formed on the substrate.

Claims

1. A filament for use in an extrusion-based additive manufacturing method, the filament comprising: a semi-crystalline polymeric reinforcement portion which runs continuously along a length of the filament; and a solid polymeric matrix portion which runs continuously along a length of the filament, wherein the reinforcement portion has a higher melting point and a higher crystallinity than the matrix portion.

2. The filament of claim 1 wherein the reinforcement portion and the matrix portion are intertwined with each other so that they both follow tortuous paths along the length of the filament.

3. The filament of claim 2 wherein the reinforcement portion and the matrix portion are twisted together so that they both follow helical paths along the length of the filament.

4. The filament of claim 1 wherein the reinforcement portion comprises a ferroelectric polymer.

5. The filament of claim 1 wherein the reinforcement portion comprises polyvinylidene fluoride.

6. The filament of claim 1 wherein the matrix portion is formed from the same polymer as the reinforcement portion, with different molecular weights.

7. The filament of claim 1 wherein a polymer forming the reinforcement portion has a higher molecular weight than a polymer forming the matrix portion.

8. The filament of claim 1 wherein the matrix portion has a melting point which is below 180° C.

9. The filament of claim 1 wherein the reinforcement portion has a melting point which is below 200° C.

10. The filament of claim 1 wherein a difference in melting points between the reinforcement portion and the matrix portion is less than 15° C.

11. The filament of claim 10 wherein a difference in melting points between the reinforcement portion and the matrix portion is greater than 2° C. and less than 15° C.

12. The filament of claim 10 wherein a difference in melting points between the reinforcement portion and the matrix portion is greater than 5° C. and less than 15° C.

13. The filament of claim 1 wherein a difference in melting points between the reinforcement portion and the matrix portion is less than 10° C.

14. The filament of claim 13 wherein a difference in melting points between the reinforcement portion and the matrix portion is greater than 2° C. and less than 10° C.

15. The filament of claim 13 wherein a difference in melting points between the reinforcement portion and the matrix portion is greater than 5° C. and less than 10° C.

16. The filament of claim 1 wherein the reinforcement portion occupies more than 60% of a volume of the filament.

17. The filament of claim 1 wherein the reinforcement portion occupies less than 91% of a volume of the filament.

18. The filament of claim 1 wherein the reinforcement portion comprises a plurality of fibres.

19. The filament of claim 1 wherein the reinforcement portion has a yield strength which is greater than 500 MPa.

20. The filament of claim 1 wherein the reinforcement portion comprises at least one reinforcement fibre in which more than 50% of the fibre by volume comprises crystals aligned within 1° of being parallel with a length of the fibre.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0033] FIG. 1 is a longitudinal cross-sectional view of a filament with the section taken along a line B-B in FIG. 2;

[0034] FIG. 2 is a transverse cross-sectional view of the filament with the section taken along a line A-A in FIG. 1;

[0035] FIG. 3 is a transverse cross-sectional view of an alternative filament;

[0036] FIG. 4 is a longitudinal cross-sectional view of a twisted filament;

[0037] FIG. 5 is a schematic view of apparatus for manufacturing an object by an extrusion-based additive manufacturing method;

[0038] FIGS. 6-10 show an object being manufactured using the apparatus of FIG. 5;

[0039] FIG. 11 is a schematic view of alternative apparatus for manufacturing a piezoelectric object by an extrusion-based additive manufacturing method;

[0040] FIG. 12 shows an object being manufactured using the apparatus of FIG. 11;

[0041] FIG. 13 is a sectional view of an object manufactured by the method of FIGS. 6-10 viewed in section transverse to the extruded lines;

[0042] FIG. 14 shows a uniformly poled object manufactured by the method of FIG. 12 viewed in section along the length of the extruded lines; and

[0043] FIG. 15 shows a non-uniformly poled object manufactured by the method of FIG. 12 viewed in section along the length of the extruded lines.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0044] FIGS. 1 and 2 show a filament for use in an extrusion-based additive manufacturing method according to a first aspect of the invention. The filament comprises a thermoplastic semi-crystalline polymeric reinforcement portion (or core) 1 which occupies a central axis la of the filament and is surrounded by a thermoplastic amorphous polymeric matrix portion (or sheath) 2. Both portions 1,2 run continuously along the length of the filament.

[0045] The core 1 occupies about 40-60% of the volume of the filament, including the geometric centre 1a of the cross-sectional area of the filament, the rest of the volume being occupied by the sheath 2. The filament has a maximum outer diameter D which is less than 2 mm and more preferably less than 1 mm.

[0046] The core 1 is manufactured by spinning and drawing a polymer under tension to form one or more reinforcement fibres with crystallites aligned with the length of the fibre(s). The core 1 may consist of a single one of such fibres only, or it may comprise a plurality of such fibres.

[0047] The sheath 2 is formed and bonded to the core 1 by the following process. The core 1 is pulled through a heated ring along with a number of amorphous fibres (or tows of fibres). As they are pulled through the heated ring, the amorphous fibres/tows melt and coalesce to form an annular sheath around the core and then cool and solidify to become bonded to the core.

[0048] In the case of FIG. 2 the matrix portion 2 comprises a continuous annular sheath layer with no gaps which completely surrounds the core 1. In the case of FIG. 3 the matrix portion comprises a set of separate axially extending fibres or tows 2a separated by gaps. The fibres/tows 2a are bonded to the core 1 in a similar manner to the annular sheath 2 but do not coalesce with each other as they pass through the heated ring.

[0049] FIG. 4 shows a filament 10 according to a further aspect of the invention. The filament comprises a spun fibre semi-crystalline reinforcement portion 11 which is twisted with a spun fibre amorphous matrix portion 12 so that both fibres 11,12 follow helical paths running continuously along the length of the filament. The fibres 11,12 are twisted by turning a bobbin. The fibres 11,12 may or may not be bonded together.

[0050] The portions 11,12 occupies about the same volume of the filament 10. The filament 10 has a maximum outer diameter D which is less than 2 mm and more preferably less than 1 mm.

[0051] The polymer chains and crystallites in the reinforcement portion 1,11 are aligned with the length of the fibre(s) which form it. So in the case of FIG. 1 the polymer chains and crystallites are parallel with the length of the filament whereas in FIG. 4 they follow a helical path.

[0052] Suitable materials for the reinforcement portion 1,11 are polyethylene (PE), High Density polyethylene (HDPE), Ultra High Density polyethylene (UHDPE), Acrylonitrile butadiene styrene (ABS), Polypropylene (PP), Polydimethyl siloxane (PDMS), Polyoxymethylene (POM), Polyethylene terephthalate (PET), Polyetheretherketone (PEEK), Polyamide (PA), Polysulphone (PS), Polyphenylene sulphide (PPS), Polyphenylsulfone (PPSF), Polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF).

[0053] Dyneema (R) 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%.

[0054] The matrix portion 2,2a,12 is typically formed from the same polymer as the reinforcement portion 1,11, optionally with different molecular weights. Where the molecular weights are different, then preferably the reinforcement portion has the higher molecular weight (for instance between 2,000,000 and 6,000,000 in the case of UHDPE). The reinforcement portion 1,11 has a higher crystallinity than the matrix portion 2,2a,12. This higher crystallinity results in a higher melting point.

[0055] Typically the fibres of the reinforcement portion 1,11 and the matrix portion 2,2a,12 are both formed by drawing the fibre under tension from a polymer melt. However the crystallinity of the fibres of the reinforcement portion 1,11 is enhanced compared with the amorphous fibres of the matrix portion 2,2a,12 by using a slower cooling rate, a higher drawing rate and/or a polymer with a higher molecular weight.

[0056] Apparatus for manufacturing an object by an extrusion-based additive manufacturing method using a filament 3 similar to those shown in FIGS. 1-4 is shown in FIG. 5. The apparatus comprises an extrusion head 4 having a channel 4a with an extrusion outlet 4b; a pair of drive wheels 8; a heater 6 arranged to raise the temperature of material within the channel 4a; a heated build plate 5; and an XY drive motor 7 arranged to cause relative movement in the XY plane between the extrusion head and the build plate—in this case by moving the extrusion head 4. A Z drive motor 9 can move the build plate 5 up or down in the Z direction as the part is built.

[0057] The outlet 4b has a maximum diameter greater than 0.1 and less than 1 mm. More preferably the maximum diameter is greater than 0.25 mm and less than 0.5 mm. The outlet 4b has a smaller diameter than the filament but a greater diameter than the reinforcement portion.

[0058] A controller 20 controls the heater 6, drive wheels 8 and drive motors 7,9 in order to manufacture a part in accordance with a Computer Aided Design (CAD) model of the part in a store 21 by following the process shown in FIGS. 6-10. Note that FIGS. 6-10 omit certain parts of the apparatus of FIG. 5 for purposes of clarity.

[0059] First, the drive wheels 8 are driven to feed the filament 3 through the channel 4a and the motors 7,9 are driven to move the extrusion head into a desired position above the build plate 5 as shown in FIG. 6. The heater 6 is operated to raise the temperature of the filament in the extrusion head above the melting point of the matrix portion but below the melting point of the reinforcement portion so that the matrix portion of the filament melts within the extrusion head, thereby forming a partially molten filament 3a within the extrusion head. The partially molten filament 3a is then extruded from the extrusion head onto the build plate 5 and the XY motor 7 is operated to deposit an extruded line 3b as shown in FIG. 7. Only the matrix portion of the filament melts within the extrusion head and the reinforcement portion of the partially molten filament 3a remains in a semi-crystalline state as it is extruded from the outlet 4b of the extrusion head. The matrix portion of the extruded line 3b solidifies when it cools after it has been laid down on the build plate 5.

[0060] The relative XY movement between the extrusion head 4 and the build plate 5 is sufficiently fast relative to the feed rate of the drive wheels 8 that the reinforcement portion is in tension as the extruded line 3b is deposited. The polymer chains and crystallites in the reinforcement portion are oriented with the extruded line 3b —either lying parallel with the length of the extruded line 3b in the case of the filament of FIG. 1 or lying in a helix with the axis of the helix lying along the length of the extruded line 3b in the case of the filament of FIG. 4.

[0061] Next, the heater 6 is operated to temporarily raise the temperature of the filament in the extrusion head 4 above the melting point of the reinforcement portion after the extruded line 3b has been formed on the substrate, thereby forming a break in the continuous reinforcement portion. At the same time the Z drive motor 9 is operated to lower the build plate 5 and effectively “cut” the filament to form an end 3c of the extruded line as shown in FIG. 8.

[0062] Next, the heater 6 is operated to lower the temperature of the filament in the extrusion head back below the melting point of the reinforcement portion to enable a further extruded line 3d to be formed as shown in FIG. 9. In the case of FIG. 9 the second line 3d is deposited on top of the first line 3b with which it fuses, although it may be formed next to (and fuse with) the line 3b in the same XY plane if required.

[0063] Next, the heater 6 is operated to temporarily raising the temperature of the filament in the extrusion head 4 above the melting point of the reinforcement portion after the extruded line 3d has been formed, thereby forming a break in the continuous reinforcement portion. At the same time the Z drive motor 9 is operated to lower the build plate 5 and effectively “cut” the filament to form an end 3e of the extruded line as shown in FIG. 10.

[0064] This process is then repeated a number of times as required to manufacture a part in accordance with the CAD model.

[0065] The length of time of the heat pulse which “cuts” the filament at the end of each line will depend on a number of factors, mainly the thermal mass of the extrusion head, but it will typically be of the order of 0.1 to 10 s.

[0066] In the case of a filament where the reinforcement portion 1 comprises a collection of fibres with inter-fibre gaps, then as the matrix portion melts in the extrusion head the melted material impregnates these inter-fibre gaps. In such a case the twisted filament of FIG. 4 is preferred due to the more intimate engagement between the reinforcement portion 1 and the matrix portion 2 which makes such impregnation easier.

[0067] In the case of a filament where the reinforcement portion 1 comprises a single fibre, then no such impregnation is necessary within the extrusion head. In such a case the filaments of FIGS. 1-3 (in which the reinforcement portion runs parallel with the length of the filament and is at least partially surrounded by the matrix portion) are preferred because they make it more easy for matrix material to flow between and bond together adjacent reinforcement fibres after they have been extruded, filling the gaps between the reinforcement fibres in adjacent extruded lines.

[0068] Alternative apparatus for manufacturing an object by an extrusion-based additive manufacturing method is shown in FIG. 11. Most components of the apparatus are the same as those shown in FIG. 5 and are given the same reference numbers. A coil of electrically conducting wire 30 surrounds the channel in the extrusion head and can be selectively energized by the controller 20 to apply an electromagnetic field 31 to material within the channel as shown in FIG. 12

[0069] The apparatus of FIG. 11 is used with a filament 3f in which the reinforcement portion comprises a ferroelectric semi-crystalline polymer such as polyvinylidene fluoride and the matrix portion comprises an amorphous (and hence non-ferroelectric) polymer such as polyvinylidene fluoride.

[0070] First, the drive wheels 8 are driven to feed the filament 3f through the channel and the motors 7,9 are driven to move the extrusion head into a desired position above the build plate 5. Next, the heater 6 is operated to raise the temperature of the filament in the extrusion head above the melting point of the matrix portion but below the melting point of the reinforcement portion so that the matrix portion of the filament melts within the extrusion head, thereby forming a partially molten filament within the extrusion head. At the same time the coil 30 is energised to apply an electromagnetic field 31 to the filament within the extrusion head. This causes the ferroelectric polymer within the extrusion head to become poled.

[0071] The partially molten filament is then extruded from the extrusion head onto the build plate 5 and the XY motor 7 is operated to deposit an extruded line 3g as shown in FIG. 12 in which the ferroelectric polymer is poled as indicated schematically by arrows 32. The rest of the process is identical to the process described above with reference to FIGS. 6-10.

[0072] Leaving the crystal structure of the reinforcement portion intact and under tension whilst being poled by the coil 30 enables the extruded line to have ferroelectric properties.

[0073] An object manufactured by the method of FIGS. 6-10 is shown in cross-section in FIG. 13, the section being taken transverse to the length of the extruded lines. The object comprising a stack of four layers 41-44 each containing a plurality of extruded lines. Each extruded line comprises a semi-crystalline polymeric reinforcement portion 41a,42a etc and a solid thermoplastic polymeric matrix portion surrounding the reinforcement portion. Each reinforcement portion 41a,42a etc. runs continuously along the length of a respective one of the extruded lines and has a higher melting point and a higher crystallinity than its respective matrix portion. The matrix portions of the extruded lines are fused together to form a matrix phase 45 which extends continuously throughout the object and bonds together the reinforcement portions. Each layer 41-44 has a different number of extruded lines.

[0074] In the example of FIG. 13 all lines in all layers are parallel, but in an alternative embodiment (not shown) the lines may extend in different directions in the manner of a composite layup with some layers oriented with their lines at 0°, others at+/−45° and others at 90°.

[0075] An object manufactured by the method of FIG. 12 is shown in cross-section in FIG. 14, the section being taken in this case along the length of the extruded lines. The object comprising a stack of four layers 51-54 each containing a plurality of extruded lines. Each extruded line comprises a semi-crystalline polymeric reinforcement portion 51a,52a etc and a solid thermoplastic polymeric matrix portion surrounding the reinforcement portion. Each reinforcement portion runs continuously along the length of a respective one of the extruded lines. The matrix portions of the extruded lines are fused together to form a matrix phase 55 which extends continuously throughout the object and bonds together the reinforcement portions. The lengths of the extruded lines differ between the layers.

[0076] In the case of FIG. 14 the coil 30 has been energized permanently during manufacture of the part, with the direction of current in the coil alternating between lines so that the object is poled uniformly, that is with all parallel lines being poled in the same direction.

[0077] FIG. 15 shows an alternative in which the coil 30 has been turned on and off to pole some layers but not others. In the example of FIG. 15 the part has a number of poled layers 52,54 interleaved with un-poled layers 51,53. Also the layers 52,54 are oriented with their lines extending parallel with each other but poled in opposite directions.

[0078] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.