CN112757642A

CN112757642A - Screw type feeding device, material extrusion system and method for 3D printing

Info

Publication number: CN112757642A
Application number: CN202010255780.XA
Authority: CN
Inventors: 黄卫东
Original assignee: Suzhou Meimeng Machinery Co ltd
Current assignee: Suzhou Meimeng Machinery Co ltd
Priority date: 2020-04-02
Filing date: 2020-04-02
Publication date: 2021-05-07
Also published as: WO2021196850A1

Abstract

The application provides a screw feeding device, a material extrusion system and a method for 3D printing. The screw type feeding device is used for conveying flowable materials for the extrusion opening, and the width of the extrusion opening is continuously changed in the material extrusion process, and the screw type feeding device comprises: a screw; a rotation controller configured to drive the screw to rotate; wherein the screw type feeding device is configured to satisfy Q_max/D²＜5cm/min，L/D≤5，Q_maxThe maximum flow rate of the flowable material delivered by the screw feeder is indicated, D the diameter of the screw and L the length of the screw. The technical scheme is favorable for improving the control precision of the screw type feeding device on the material flow rate when the material flow rate is dynamically changed in a large range.

Description

Screw type feeding device, material extrusion system and method for 3D printing

Technical Field

The application relates to the field of material extrusion, in particular to a screw type feeding device, a material extrusion system and a material extrusion method for 3D printing.

Background

Material Extrusion based 3D printing techniques, such as Fused Deposition Modeling (FDM) techniques, typically utilize a material delivery system to deliver Materials that are deposited layer by layer on a work platform to form a 3D article.

Disclosure of Invention

The application provides a screw feeding device, a material extrusion system and a method for 3D printing.

In a first aspect, a screw feeder for feeding a flowable material to an extrusion opening, the width of the extrusion opening continuously varying during extrusion of the material, the screw feeder comprising: a screw; a rotation controller configured to drive the screw to rotate; wherein the screw type feeding device is configured to satisfy Q_max/D²Less than 5cm/min, L/D is less than or equal to 5, wherein Q_maxRepresents the maximum flow rate of flowable material delivered by the screw feeder, D represents the diameter of the screw, and L represents the length of the screw.

In a second aspect, there is provided a material conveying system for 3D printing, comprising: an extrusion head; the screw feeder of the first aspect; a first control system configured to control a width of an extrusion port of the extrusion head to continuously vary within a preset range during 3D printing.

In a third aspect, a material conveying method for 3D printing is provided, including: in a 3D printing process, delivering a flowable material to an extrusion head using a screw feeder apparatus as described in the first aspect; and controlling the width of an extrusion port of the extrusion head to continuously change within a preset range.

Drawings

Fig. 1 is a schematic structural diagram of a material conveying system according to a first embodiment of the present application.

Fig. 2 is a schematic structural diagram of a material conveying system according to a second embodiment of the present application.

Fig. 3 is a schematic structural view of a first feeding device in a second embodiment.

Fig. 4 is a schematic structural view of a buffer container in the second embodiment.

Fig. 5 is a schematic diagram showing the correspondence between the liquid level of the buffer container and the motor load in the second embodiment.

Fig. 6 is a flowchart of a calibration method of the correspondence relationship between the liquid level of the buffer container and the motor load in the second embodiment.

Fig. 7 is a schematic structural view of a buffer container having a pressure or liquid level feedback function in the second embodiment.

Fig. 8 is a schematic structural view of a second feeding device in the second embodiment.

Fig. 9 is a schematic flow chart of a material conveying method according to a third embodiment of the present application.

Detailed Description

The first embodiment:

as shown in fig. 1, a material delivery system 10 for 3D printing includes a screw-type feed device 40, an extrusion head 50, and a first control system 60.

The screw feeder 40 is configured to receive the flowable material and deliver the flowable material to the extrusion head 50.

The extrusion head 50 has an extrusion opening 51 with a continuously adjustable width. The first control system 60 may be configured to control the width of the extrusion port 51 to be continuously varied within a preset range during the 3D printing process. For example, in some advanced 3D printing systems, the width of the extrusion opening is required to vary with the cross-sectional profile of the material fill area (or, the width of the extrusion opening is made to match the length of the cross-sectional profile of the material fill area) in order to achieve ultra-efficient printing. The design and control method of such an extrusion opening can be seen in WO2018/205149a1 (it is to be noted that the width of the extrusion opening in this application corresponds to the length of the discharge opening in this patent application, and the material filling area corresponds to the target filling area in this patent application, which may be part or all of the area of the layer to be printed).

The screw feeder 40 creates pressure at the extrusion port as it delivers material to the extrusion port. Due to this pressure, there is a problem of backflow, which means that the material flows in the opposite direction along the screw grooves, or leakage, which means that the material flows in the gap between the screw and the barrel in the opposite direction to the extrusion direction of the screw. Further, materials (especially polymeric materials) are subject to a shear thinning effect, i.e. the viscosity of the material varies with the shear rate, such that a non-linear relationship is present between the flow rate of the screw-based feed system and the rotational speed of the screw. The relationship between material flow rate and screw speed becomes very difficult to measure due to the presence of reverse flow, leakage flow and shear thinning effects.

Although the effects of counter-flow, leakage and shear-thinning are present in common plastic extrusion systems and 3D printing systems, they do not significantly affect such systems because they generally employ extrusion orifices of fixed width for extrusion of material at a constant flow rate.

Unlike the above-described system, the system shown in FIG. 1 requires that the width of the extrusion orifice 51 be varied continuously within a predetermined range, which means that the flow rate of material being conveyed by the material conveying system 10 needs to be dynamically varied within a certain range. In this system, the effects of counter-flow, leakage and shear thinning are particularly pronounced for precise control of material flow rates. The inventors have found that it is difficult to meet the requirements of such systems using conventional screw type feeding devices, and thus, the embodiments of the present application provide a screw type feeding device in the first place.

As shown in fig. 1, the screw feeder 40 includes a screw 41 and a rotation controller 42 (the rotation controller may include, for example, a motor 421 and may further include a reducer 422). The screw type feeding device 40 is configured to satisfy Q_max/D²Less than 5cm/min, L/D is less than or equal to 5, wherein Q_maxThe maximum flow rate of the flowable material delivered by the screw feeder is indicated, D the diameter of the screw and L the length of the screw.

In a typical screw design, the screw diameter D has a strong positive correlation with the flow rate of the material, i.e., the smaller the material flow rate, the smaller the corresponding screw diameter D is designed to be. Thus, in common screw designs, the screw length L is typically greater than 20D, and common screws are typically elongated rods.

Since the embodiments of the present application are intended to precisely control the flow rate of the material over a wide range using the screw, it is necessary to suppress the leakage flow. In order to effectively inhibit leakage flow, the screw rod type feeding device needs to be processed with high precision, so that the clearance between the screw rod and the machine barrel sleeved on the screw rod is very small, and the design requirement is very difficult to realize on a long and thin screw rod because the rigidity of the long and thin screw rod is very poor, and the friction between the screw rod and the machine barrel and even the clamping of the screw rod and the machine barrel are easily caused by the excessively small clearance. Therefore, the embodiment of the application adopts the screw with small L/D (L/D is less than or equal to 5), and the screw is characterized by having an overlarge screw diameter D relative to the length of the screw and has enough rigidity to meet the design requirement of small clearance between the screw and a cylinder.

On the basis of the large screw diameter D, if the rotation speed of the screw is too fast, the shear thinning effect of the material in the screw groove is very obvious, and in order to avoid the problem, the embodiment of the application controls the maximum flow rate of the material conveyed by the screw type feeding device 40 to meet the following conditions: q_max/D²Is less than 5 cm/min. Through the parameter design, the ultra-low screw rotating speed is actually adopted in the application to solve the problem that the flow speed of materials in the screw groove is too high under the diameter of a large screw to increase the shearThe problem of the effects of shear thinning.

An ultra-low screw speed can significantly reduce the shear rate of the material in the screw. Under the condition of ultra-low shear rate, the material can be regarded as Newtonian fluid (the material in the traditional material extrusion process is generally non-Newtonian fluid), which brings the following advantages: (1) the complexity of a flow rule caused by shear thinning can be ignored, and the material conveying flow is controlled according to an accurate mathematical equation; (2) the viscous flow resistance in the screw is obviously larger than that of non-Newtonian fluid, and the screw has obvious effect on obviously reducing leakage flow and reverse flow; (3) under the condition of the same screw rotating speed, the extrusion force is higher, and the material can be extruded from an extrusion port with a very small size in high-precision extrusion.

In conclusion, the ultra-large screw diameter D and the ultra-low screw rotating speed are beneficial to reducing the leakage flow and the shear thinning effect of the screw, thereby being beneficial to accurately controlling the material flow rate when the width of the extrusion opening is dynamically changed in a large range.

It is noted that L/D ≦ 5 provides a large screw diameter design relative to the screw length, based on which, in some embodiments, the gap δ between screw 41 and barrel 43 may be designed to be δ < 0.001D. As mentioned above, this clearance design is difficult to achieve on a conventional elongated screw, but is easier to achieve on an oversized diameter screw in the embodiments of the present application. Therefore, leakage flow can be effectively reduced through the design of the ultra-large screw diameter relative to the length of the screw and the design of the ultra-small gap between the screw and the machine barrel, so that the precise control of the material flow rate when the width of the extrusion opening is dynamically changed in a large range is facilitated.

Alternatively, in some embodiments, the helix angle of the screw may be varied

Set to a smaller range (e.g., the helix angle may be set

Is designed as

) To increase the extrusion pressure of the screw, which can further reduce the reverse flow. The specific reason is analyzed as follows.

The extrusion flow rate of a screw feeder can be calculated by the following formula:

in the formula: q represents the extrusion flow rate (or flow rate) of the material, D represents the outer diameter of the screw 41, H represents the groove depth,

denotes the helix angle, n denotes the screw speed, P denotes the extrusion pressure of the extrusion port 51, P_maxRepresents the maximum extrusion pressure of the screw 41, L represents the screw length, δ represents the clearance between the screw and the barrel, e represents the normal width of the screw flight, η represents the viscosity of the material in the screw channel, η₁Representing the viscosity of the material in the gap delta.

As can be seen from the formula (2), the maximum value P of the screw extrusion pressure_maxThe larger the ratio of counter-flow to positive flow, the less pronounced the counter-flow phenomenon. P_maxAnd helix angle

Inversely related, i.e. helix angle

The smaller, P_maxThe larger. Thus, by controlling the helix angle

The extrusion pressure of the screw is increased, and the influence of reverse flow is reduced. Reducing screw backflow facilitates precise control of material flow rate over a wide dynamic range of extrusion port widths.

Alternatively, in some embodiments, the leakage flow may be suppressed by appropriately increasing the flight width e. For example, the flight width e of the screw may be designed to satisfy:

it can be seen from the formula (3) that the larger the value of e is, the smaller the ratio of the leakage flow to the positive flow is, and the less obvious the leakage flow phenomenon is.

Alternatively, in certain embodiments, the shear rate γ of the screw to the flowable material within the flight of the screw may be controlled such that γ ≦ 500/s. It has been previously noted that the lower the shear rate, the less pronounced the shear thinning effect. Materials with low shear thinning effects can be considered newtonian fluids whose flow rates are easier to accurately model and control than non-linear, non-newtonian fluids. Detailed description see the foregoing, which is not detailed herein.

Combining the various factors described above, several examples of specific parameters of a screw feeder are given below in conjunction with table one.

Watch 1

The units of D, H, e, L and delta in the table I are cm, and rpm represents the rotation speed per minute.

The first embodiment is not particularly limited to the source of the flowable material of the screw feeder 40. For example, referring to fig. 1, a feed port 45 may be provided for the screw type feeding device 40, and the screw type feeding device 40 may obtain the material in a flowable state from the outside through the feed port 45. For another example, the screw feeder 40 may be added with other material processing devices to convert solid materials into flowable materials.

Optionally, in some embodiments, as shown in the figure1, the material delivery system 10 may further includeThe second control system 44,52 is configured to control the viscosity of the material in the screw 41 and/or the viscosity of the material at the extrusion opening 51 such that the viscosity of the material in the screw 41 is greater than the viscosity of the material at the extrusion opening 51.

See formulas (1) to (3), the higher the viscosity of the material in the screw, η and η₁The higher the value of (a). As can be seen from equations (1) to (3), η and η₁The higher the value of (a), the lower the ratio of the counter-flow and the leakage flow to the positive flow, and the less obvious the counter-flow and the leakage flow phenomena. In addition, the reverse flow and the leakage flow are generated due to the extrusion pressure P of the materials at the extrusion port 51, and the higher the extrusion pressure P is, the more obvious the reverse flow and the leakage flow are. Therefore, the material at the extrusion port 51 can be controlled to have a lower viscosity than the material in the screw 41, and the lower the viscosity of the material at the extrusion port 51, the lower the extrusion pressure of the material at the extrusion port 51 is, so that the backflow and leakage phenomena can be reduced. The difference setting of the material viscosity can reduce the backflow and leakage problems of the screw type feeding device 40 to a great extent, even reduce the backflow and leakage to a negligible degree compared with the positive flow, thus being beneficial to the screw type feeding device 40 to accurately control the metering output of the material. Furthermore, the inventors have found that reducing the extrusion pressure P of the extrusion port 51 can significantly reduce the power consumption of the screw feeder 40, thereby reducing the cost of the material delivery system.

The viscosity of the material can be controlled in various ways, which is not limited in the examples of the present application. For example, the viscosity of the material can be controlled by stirring, and the viscosity of the material can also be controlled by temperature. As shown in fig. 1, the second control system 44,52 may include a first heater 44 and a second heater 52. The first heater 44 is configured to heat the screw 41. The second heater 52 is configured to heat the extrusion port 51. The heating temperature of the second heater 52 is higher than the heating temperature of the first heater 44.

The heating temperature setting described above can be such that the temperature difference between the extrusion port 51 and the screw 41 is made small. Setting a lower temperature at screw 41 increases the viscosity of the material and setting a higher temperature at extrusion port 51 decreases the viscosity of the material at extrusion port 51.

In some embodiments, the temperature of screw 41 may be controlled by first heater 44 such that the temperature of screw 41 is close to the melting temperature of the material. For example, the heating temperature T1 of the first heater 44 may be controlled to T_fTo T_fIn the range of +30 ℃ wherein T_fIndicates the melting temperature of the material. In addition, the temperature at the extrusion port 51 may also be controlled by the second heater 52 so that the temperature at the extrusion port 51 approaches the decomposition temperature of the material. For example, the heating temperature T2 of the second heater 52 may be controlled to be (T)_f+50 ℃ to (T)_dIn the range of-30 ℃), wherein T_fDenotes the melting temperature, T, of the material_dIndicating the decomposition temperature of the material.

Of course, different materials of different types of materials have different temperature control ranges. For example, the material is polylactic acid, and the temperature T1 of the first heater is more than 155 ℃ and less than 185 ℃; and/or the temperature T2 of the second heater is more than 205 ℃ and less than 310 ℃; or the material is acrylonitrile-butadiene-styrene copolymer, and the temperature T1 of the first heater is higher than 170 ℃ and lower than 200 ℃; and/or the temperature T2 of the second heater is greater than 220 ℃ and less than 250 ℃; or the material is polycarbonate, and the temperature T1 of the first heater is more than 220 ℃ and less than 250 ℃; and/or the temperature T2 of the second heater is greater than 270 ℃ and less than 320 ℃; or the material is nylon-6, and the temperature T1 of the first heater is more than 215 ℃ and less than 245 ℃; and/or the temperature T2 of the second heater is greater than 265 ℃ and less than 280 ℃; or the material is polyphenylene sulfide, and the temperature T1 of the first heater is more than 280 ℃ and less than 310 ℃; and/or the temperature T2 of the second heater is greater than 330 ℃ and less than 470 ℃; or the material is polymethyl methacrylate, and the temperature T1 of the first heater is more than 160 ℃ and less than 190 ℃; and/or the temperature T2 of the second heater is greater than 210 ℃ and less than 240 ℃; or the material is polyether-ether-ketone, and the temperature T1 of the first heater is more than 334 ℃ and less than 364 ℃; and/or the temperature T2 of the second heater is greater than 384 ℃ and less than 490 ℃.

It has been described above that ultra-low speed screw feeders have a low shear thinning effect, so that the material can be viewed as a newtonian fluid. Compared with non-Newtonian fluid, the viscosity of Newtonian fluid has the characteristic of obvious temperature effect, namely the viscosity reduction effect caused by increasing the temperature at the extrusion opening 51 is obvious, so that the head pressure is obviously reduced, and therefore, the ultra-low speed screw type feeding device is matched with a temperature difference mode, and the counter flow and leakage flow phenomena in the system can be obviously reduced.

For example, for polylactic acid (PLA) materials commonly used for 3D printing, which is a typical non-newtonian fluid under high shear conditions in conventional screws, the temperature dependence of viscosity is low, but at shear rates below 100s^-1Under the condition, when the viscosity is increased from 170 ℃ to 240 ℃, the viscosity can be reduced by more than 200 times. If the working temperature of the screw is set to be 170 ℃, the temperature of the extrusion opening is set to be 240 ℃ and the thickness of the extrusion opening is 0.1mm, if the small part is printed according to the parameters in the table I, the reverse flow of the material conveying system 10 is lower than 6% of the minimum positive flow, the leakage flow is lower than 1% of the minimum positive flow, the influence of the leakage flow and the reverse flow on the material conveying in the screw is very weak and almost negligible, and at the moment, the flow rate of the material conveying system is basically and accurately determined by the rotating speed of the screw. For large parts printed according to the parameters in table one, the leakage flow and the reverse flow are much smaller than the minimum positive flow, and are only below 0.42 and 0.59 percent of the positive flow respectively, and the influence on material conveying can be completely ignored. Therefore, the scheme restrains leakage flow and reverse flow to the maximum extent, and the material conveying flow rate and the screw rotating speed have good linear relation.

Second embodiment:

the second embodiment is similar to the first embodiment, and the main differences are: the second embodiment defines a source of the flowable material provided by the screw feeder 40 (it should be noted that, in the second embodiment, the screw feeder 40 is referred to as a second feeder) to the extrusion head 50, and is provided by a buffer container. The following mainly describes differences of the second embodiment from the first embodiment, and the same parts can be referred to the first embodiment.

In the field of material extrusion, it may be appropriate to continuously complete the transition of material from a solid to a flowable state by a single feed device and to meter the output of the flowable material to the extrusion head if the material delivery system delivers the material at a fixed flow rate. However, with the development of material extrusion technology, especially 3D printing technology, more and more material conveying systems are required to meet the requirement of dynamic change of material conveying flow rate. The inventors have found that in such a system, the two processes can be difficult to coordinate if the conventional material conveying manner is still followed, i.e. the material state transition and the metered output of the material to the extrusion head are continuously performed by one feeding device. Taking a screw type feeding device as an example, it generally includes a feeding section, a melting section (or compression section) and a metering section. The feeding section is used for receiving solid materials, the melting section is used for converting the solid materials into a flowable state, and the metering section is used for quantitatively outputting the flowable materials to the extrusion head. If the flow rate of the material is dynamically varied, the rotational speed of the screw feeder may also be dynamically varied. In this way, the pressure, flow rate and temperature of the flowable material exiting the melting section can fluctuate. The metering section has difficulty delivering material accurately and quantitatively to the extrusion head, subject to fluctuations in the flowable material output by the melting section.

In order to solve the problems, the two tasks of state conversion and material metering output of the materials are distributed to two different feeding devices, and the materials are buffered between the two feeding devices so as to shield the influence of fluctuation of the output of the first feeding device on the second feeding device. This is explained in detail below with reference to fig. 2.

Fig. 2 is a material delivery system 10 provided in accordance with certain embodiments of the present application. The material conveying system 10 may be applied to the field of plastic extrusion, and may also be applied to the field of 3D printing, such as 3D printing technology based on material extrusion (Materials extrusion).

The material of the material conveyed by the material conveying system 10 is not specifically limited in the embodiment of the present application. In some embodiments, material delivery system 10 may be used to deliver plastic and any flowable and extrudable material in the form of a paste. In some embodiments, material delivery system 10 may be used to deliver metal paste materials (metal paste materials may be formed by adding a liquid binder to a metal powder), ceramic paste materials (ceramic paste materials may be formed by adding a liquid binder to a ceramic powder), organic high molecular weight polymeric materials, inorganic paste materials (e.g., cement, gypsum slurry, mud slurry, etc.). In some embodiments, the material being delivered by the material delivery system 10 may also be a creamy food product such as cream, chocolate, or the like. More specifically, in certain embodiments, the material delivery system 10 may be used to deliver materials formed of: polylactic acid (PLA), acrylonitrile-butadiene-styrene copolymer (ABS), Polycarbonate (PC), nylon-6 (PA6), polyphenylene sulfide (PPS), polymethyl methacrylate (PMMA), and polyether ether ketone (PEEK).

As shown in fig. 2, the material conveying system 10 includes a first feeding device 20, a buffer container 30, a second feeding device 40, and an extrusion head 50. The wide arrows in fig. 2 represent the direction of material flow (or material transport). As can be seen from the direction of the broad arrow, in the material conveying system 10, material can pass through the first feeding device 20, the buffer container 30, the second feeding device 40, and the extrusion head 50 in sequence.

The first feeding device 20 is configured to convert the solid material into a flowable material (which may also be referred to as a molten material). The buffer container 30 is configured to store the flowable material output by the first feeding device 20. The second feed device 40 is configured to deliver the flowable material in the buffer container 30 to the extrusion head 50 of the material delivery system 10.

Due to the existence of the buffer container 30, the difficulty of coordination of two stages of material state conversion and metering output is reduced, so that the stable metering output of the materials is favorably realized.

The first material feeding device 20 provides a source of material, and thus, the first material feeding device 20 may be referred to as a material feeding section of the material conveying system 10. Because the first material feeding device 20 can melt the solid material into the flowable material, the first material feeding device 20 can also be regarded as a melting section of the material conveying system 10.

In some embodiments, the first feed device 20 may also provide preliminary flow regulation of the flowable material and deliver the flowable material to the buffer container 30 according to a given flow requirement. The flow regulating range of the first feeding device 20 is not specifically limited, and can be set according to actual requirements. For example, a flow rate adjustable range of 5 to 20 times can be configured for the first feeding device 20. The flow rate delivered by the first feeding device 20 can be regulated according to a certain rule, for example, according to the volume or level of the material in the buffer container 30.

In some embodiments, the first feed device 20 may also provide preliminary temperature control to deliver flowable material of a given temperature to the buffer container 30.

The first feeding device 20 can be implemented in various ways, and the embodiment of the present application is not limited thereto. For example, the first feeding device 20 may be a screw type feeding device (or screw pump, or screw extruder), and may also be a pneumatic feeding device or a piston type feeding device.

In some embodiments, the outlet of the first feeding device 20 may be provided with a filter element (e.g., a sieve plate) to filter the solid material that may be present before the material enters the buffer container 30.

Taking a screw type feeding device as an example, as shown in fig. 3, the first feeding device 20 may include a feeding port 21, a screw 22, a discharging port 23, and a heater 26.

The feed port 21 may be used to receive material in a solid state. In some embodiments, the feed port 21 may be a hopper.

The screw 22 can be divided into three sections: a feed section 221, a melt section (or compression section) 222, and a metering section 223. The feeding section 221 can convey the solid material received by the feeding inlet 21 to the melting section 222. The melting section 222 may convert the solid material into a flowable material and then deliver the flowable material to the metering section 223. The metering section 223 can provide a rough metered output of the material.

The heater 26 can control the operating temperature of the screw to maintain the material in a flowable state. The heater 26 may be disposed in the region of the screw 22 corresponding to the melting section 222 and the metering section 223.

The screw 22 can be driven by a motor 27 to rotate, thereby conveying the material. A speed reducer 28 may be provided between the motor 27 and the screw device 22 to match the rotational speed between the motor 27 and the screw 22.

The sieve plate 29 is located at the discharge port 23 to filter out solid substances that may be contained in the material.

Referring back to fig. 2, the buffer container 30 can be regarded as a transition means between the first feeding device 20 and the second feeding device 40. The buffer container 30 can receive the flowable material from the first feeding device 20 and serve as a feed reservoir for the second feeding device 40 to meet the dynamic flow supply of the material required by the second feeding device 40.

The buffer vessel 30 can isolate possible adverse effects of fluctuations in the output of the first feed device 20 on the second feed device 40. The fluctuations output by the first feeding device 20 include at least one of the following: pressure fluctuations, flow fluctuations, and temperature fluctuations. The isolation of the buffer vessel 30 from fluctuations in the output of the first feed device 20 facilitates the ability of the second feed device 40 to provide precise flow rate control of the material.

Specifically, if the first feeding device 20 is directly connected to the second feeding device 40, high-precision coordinated flow rate control between the first feeding device 20 and the second feeding device 40 is required in order to achieve high-precision flow rate control at the extrusion outlet. However, when the material extrusion flow rate of the extrusion head 50 needs to be dynamically changed, the state of the material output by the first feeding device 20 is very unstable. In this case, it is difficult to achieve high-precision coordination of the first feeding device 20 and the second feeding device 40. The addition of the buffer container 30 between the first feeding device 20 and the second feeding device 40 can reduce the requirement for cooperative control between the first feeding device 20 and the second feeding device 40. The buffer container 30 serves as a buffer member to stabilize the fluctuation of the output of the first feeding device 20 and ensure that the material is supplied to the second feeding device 40 in a stable state.

In order to achieve as uniform a temperature and a mixing state as possible of the flowable material in the buffer container 30 and to release the gases entrained in the material there, as shown in fig. 4, a stirring device 32 can be provided for the buffer container 30.

The configuration of the stirring device 32 may be various. For example, a blade mixer or a drum mixer may be used. The structure and operation of the stirring device 32 will be described below by taking a blade stirrer as an example.

As shown in fig. 4, the stirring device 32 may include a stirring blade 322 and a servo motor 324. The motor 324 drives the stirring blade 322 to stir the flowable material stored in the buffer container 30, so as to stabilize fluctuation of the state (such as temperature or composition) of the material conveyed to the buffer container 30 by the first feeding device 20, ensure uniformity of the state of the material in the buffer container 30, and accelerate overflow of residual gas in the material.

In some embodiments, the shape of the mixing blades 322 and the inlet 34 (which may be connected to the first feeding device 20) and the outlet 36 (which may be connected to the second feeding device 40) of the buffer container 30 may be designed in a specific structure so that the material entering the buffer container 30 from the inlet 34 is discharged from the outlet 36 after a sufficiently long flow path inside the buffer container 30. Therefore, the material in a flowing state can better ensure the uniformity of the material state through enough flow. For example, the mixing blade 322 may be configured such that the flowable material enters the buffer container 30 through the inlet 34, spirals upward, and then flows down the center to the outlet 36.

In some embodiments, as shown in fig. 4, the buffer container 30 may also be provided with a temperature control device 38 to control the temperature of the flowable material stored in the buffer container 30. The temperature control device 38 may include a heating element, a temperature sensor and a thermostat (not shown in fig. 4). The heating element may be a metal sleeve containing a heating rod/flexible heating sheet that fits over the exterior of the housing 37 of the buffer container 30. The temperature sensor may be a high precision thermocouple to measure the temperature of the housing 37 and feed back the temperature measurement to the thermostat. The temperature controller can adopt a high-precision PID controller. The controller may control the output power of the heating member according to the temperature fed back from the temperature sensor, thereby controlling the temperature of the housing 37 at a set target value.

In certain embodiments, the material delivery system 10 may also include a feedback control device. The feedback control device may adjust the amount of the flowable material output by the first feeding device 20 (or the rate of the flowable material output by the first feeding device 20) according to the amount of the flowable material stored in the buffer container 30.

The volume of the buffer container 30 can be set according to the actual material transport requirements. For example, the buffer container 30 may be configured with a suitable volume to maintain the liquid level in the buffer container 30 within a reasonable range, thereby reducing the effect of the pressure generated by the liquid level difference on the subsequent material delivery. For example, the liquid level in the buffer container 30 may be maintained at: the amount of material in the buffer container 30 is sufficient to supply the demand of the extrusion opening when the extrusion opening is at maximum flow rate, and the liquid level in the buffer container 30 is not so high that the material in the buffer container 30 overflows when the extrusion opening is at minimum flow rate. When the buffer container 30 satisfies the above two conditions, the pressure difference generated by the height difference of the liquid level of the material stored in the buffer container is very small and can be almost ignored, and the pressure of the extrusion opening is not affected.

As one example, extrusion head 50 may be an extrusion orifice 51 having a width that is continuously adjustable. The control of the feedback control device may be such that the amount of flowable material stored in the buffer container 30 is not less than the amount of material required when the extrusion port 51 is used for material extrusion with the maximum width.

Three alternative feedback control schemes are given below.

Feedback control scheme 1: feedback control based on agitator motor load/torque

This feedback control scheme is explained in detail below.

The viscosity of the flowable materials (especially polymeric materials) delivered by the material delivery system is generally high. Therefore, as the liquid level rises, the stirring resistance of the stirring device 32 increases accordingly, resulting in an increase in the load/torque of the motor 324. Accordingly, the feedback control device can adjust the amount of material output by the first material feeding device 40 according to the load or torque of the motor 324. For example, a load signal representing a load of the motor or a torque signal representing a torque of the motor (the torque signal may be collected by a torque sensor) may be obtained and then fed back to the first feeding device 20 to control the feeding rate of the first feeding device 20. The load signal or the torque signal of the motor is a continuously changing electric signal, so that the feeding rate of the first feeding device 20 can be controlled by a PID algorithm, and the control method has higher control precision compared with a simple on-off control method. In addition, the advantage of this solution is that the stirring device 32 can be used to realize feedback control without providing additional feedback control device, thereby reducing the complexity of the system.

In order to realize feedback control according to the load/torque of the motor, a correspondence relationship between the liquid level in the buffer container 30 and the load/torque of the motor may be established first. Since the correspondence between the liquid level in the buffer container 30 and the load/torque of the motor varies depending on the temperature and the material/quality of the material, the correspondence between the liquid level in the buffer container 30 and the load/torque of the motor can be calibrated in advance. One possible way of calibration is given below.

As shown in fig. 5, the agitating blade 322 may be structured such that the blade width of the agitating blade 322 is abruptly increased at H3. When the liquid level in the buffer container 30 rises to H3, the load of the motor 324 increases rapidly. During calibration, the load at H3 can be automatically identified by a program, and then, according to the load at H3, a target value of feedback control (which can be set according to actual needs, for example, can be set to 80% of the load at H3) is determined, so as to realize the height control of the liquid level. In addition, a safety threshold of the load can be set according to the load size of the motor at H3, and when the liquid level is high enough to enable the load of the motor to exceed the safety threshold, the load is increased rapidly to the safety threshold so as to control the first feeding device 20 to stop feeding.

It will be appreciated that the greater the slope of the level-load curve, the greater the accuracy of level control. In addition, the slope of the liquid level-load curve is determined by the shape of the stirring blade 322 when the composition of the material, the temperature, and the stirring rotation speed are all determined. In order to increase the slope of the liquid level-load curve, the stirring blade 322 may have a structure gradually changing from wide to narrow. As shown in fig. 5, the width of the stirring blade 322 below H3 can be designed as an "inverted triangle" (or, in other embodiments, as an "inverted trapezoid"), which can increase the slope of the liquid level-load curve and improve the control accuracy.

An embodiment of the above calibration process is described in detail below with reference to fig. 6. As shown in fig. 6, the calibration process may include steps S610 to S660.

In step S610, the temperature of the buffer container is raised to a target temperature, and is subjected to thermostatic control.

The target temperature may be selected as the temperature of the buffer vessel at which it is actually operating. The value of the target temperature can be set according to actual needs as long as the material can be ensured to be in a flowable state.

In step S620, the rotation speed of the stirring motor is increased to the target rotation speed and is operated at a constant speed, and the measurement of the load of the motor is started.

At this time, since no flowable material is yet introduced into the buffer container, the measured load of the motor is the load of the motor during idling.

In step S630, the first feeding device is controlled to start feeding the buffer container.

At this time, the outlet of the buffer container needs to be closed, and the buffer container is forbidden to output the materials to the second feeding device.

In step S640, as the liquid level of the material rises, a p (t) curve representing the change of the load of the motor with time is recorded.

For example, when the liquid level of the material reaches H1, the load P of the motor starts to increase gradually with time t, and when the liquid level reaches H3, the width of the stirring blade suddenly increases, and the load P of the motor rapidly increases with time t. The information can be recorded for subsequent analysis.

In step S650, the first feeding device is controlled to stop feeding, and the p (t) curve is automatically analyzed.

For example, the second derivative dP of the P (t) curve may be calculated²/dt²The maximum of the second derivative corresponds to the load P of the motor at H3, and assuming that the material of the system is material 1, the maximum of the second derivative corresponds to the load P1 of material 1 in fig. 5.

In step S660, a target value and a safety threshold value of the feedback control are set according to the motor load at H3.

For example, the motor load P1 at H3 may be multiplied by the height factor of the liquid level to obtain the target value of the feedback control. The height factor can be set according to actual needs. For example, the height factor of the liquid level may be set to 80% of the total height of H1 to H3 (i.e., at H2 in fig. 5), 0.8 × P1 may be set as the target value of the feedback control, and the liquid level height may be controlled to H2 or less.

Further, in some embodiments, a safety threshold for the load may also be set. The safety threshold may be set according to actual needs, for example, the safety threshold may be designed to be 120% of the P1 load, that is, 1.2 × P1 is set as the safety threshold. When the motor load exceeds the threshold value, the first feeding device 20 is controlled to stop feeding.

If material is changed from material 1 to material 2, the above operation can be repeated, and the load P2 of material 2 at H3 is retrieved, and 0.8P 2 is used as the target value of the feedback control, and 1.2P 2 is used as the safety threshold.

After the calibration process is completed, the material conveying system can enter a working state, the first feeding device and the second feeding device are started, and the feeding speed of the first feeding device 20 is fed back and controlled in real time according to the load of the motor, so that the liquid level in the buffer container is controlled below H2.

Feedback control scheme 2: pressure based feedback control

As in fig. 7As shown in the left figure, a pressure sensor 33 may be installed at the bottom of the buffer container 30, and the pressure value fed back by the pressure sensor 33 is used as the basis for controlling the feeding rate of the first feeding device 20. Since the buffer container 30 is an open container, the pressure sensor 33 may be a relative pressure measurement type pressure sensor. The range of the pressure sensor can be determined according to the material of the material, the volume of the buffer container 30 and other factors. For example, assume that the material is PLA and the density ρ is 1.25 × 10³kg/m³When the liquid level h needs to be controlled to about 0.1m, the maximum pressure P ═ ρ g h ═ 1.25 × 10, which the pressure sensor 33 receives, is set to be about 0.1m³X 9.8 × 0.1 ═ 1225 Pa. In this case, the pressure sensor 33 having a range of kilopascal may be selected, the pressure resolution of the pressure sensor 33 may be ten pascal, that is, 1% of the range, and the rate of change of the pressure with the height of the liquid level is 12.25Pa/mm, thereby realizing height control in millimeter level. If the feedback control scheme is adopted, the corresponding relation between the pressure and the liquid level height can be established in advance, and the relation can be determined according to the density (or melt density) of the material in the flowing state.

Feedback control scheme 3: feedback control based on a level float

As shown in the right drawing of fig. 7, the float 39 can be floated on the material level, and the float 39 rises as the level rises. The float 39 is provided with a float rod which rises out of the upper lid of the buffer container 30. An electronic ruler 37 may be installed on the upper cover of the buffer container 30, and the liquid level height is converted into an electrical signal by detecting the up-down movement of the float rod, and the electrical signal is used as a feedback signal to control the feeding rate of the first feeding device 20.

Referring again to fig. 4, the buffer container 30 may be designed as a cylindrical container, or may be designed as a container of another shape. The inner surface of the housing 37 of the buffer container 30 may be made of a high-finish material, such as a metal material (e.g., stainless steel or copper) with a finish Ra of 0.4-0.8 um. The high finish of the inner surface reduces the adhesion of the material and prevents it from contaminating other materials in the buffer container 30 after pyrolysis. In some embodiments, the lower end of the buffer container 30 may be funnel-shaped, so as to avoid dead corners in the container and material residue.

The inlet 34 and the outlet 36 may each be connected to the first feeding device 20 and the second feeding device 40 using standard interfaces (e.g., snaps or flanges). The inlet 34 may be located at a position where the side wall of the buffer vessel 30 is biased towards the bottom to prevent the material from being entrained in the gas from flowing from a high position. The outlet 36 may be disposed on the bottom surface of the buffer container 30, so as to ensure that the material can completely flow out without any dead angle residue.

In some embodiments, a filter element 35 (e.g., a filter screen) may be provided at the outlet 36 to prevent impurities from entering the second feed device 40.

Due to the existence of the buffer container 30, the second feeding device 40 can directly convey the material in the buffer container 30 in a flowing state to the extrusion head 50 without switching the material state, which is beneficial to realizing high-precision metering output of the material. The second feed device 40 can therefore also be referred to as a precision metering feeder. Alternatively, the second material feeding device 40 may be considered to be a precision feed section of the material conveying system 10. In some embodiments, the second material feeding device 40 can provide precise real-time control of the material over a wide range of flow rates during the material conveying process. For example, the second feeding device 40 can precisely control parameters such as flow rate, temperature, etc. of the material.

In the material conveying system 10, the first feeding device 20, the buffer container 30, the second feeding device 40 and the extrusion head 50 can be regarded as four functional sections of the whole material conveying system 10, and each functional section can be precisely controlled in temperature according to the needs of the functional section. For example, the first feed device 20 may be controlled to a temperature suitable for the material to change from a solid state to a flowable (or molten) state. The buffer vessel 30 may control the temperature at a target value, which may depend on the operating temperature of the second feeding device 40, e.g. may be slightly higher than the operating temperature of the second feeding device 40. The second feed device 40 and the extrusion head 50 may be temperature controlled using the temperature differential control described above.

The second feeding device 40 (see fig. 8) may be a screw type feeding device as in the first embodiment, and will not be described in detail herein to avoid redundancy.

The third embodiment:

the first to second embodiments are device embodiments, and the third embodiment is a method embodiment. The description of the apparatus side and the description of the method side correspond to each other, and the overlapping description is appropriately omitted for the sake of brevity.

Fig. 9 is a schematic flow chart of a material conveying method provided by the third embodiment. Method S900 of fig. 9 may be performed by the aforementioned material delivery system 10. The method S900 includes steps S910 to S920.

In step S910, during 3D printing, a screw feeder is utilized to deliver flowable materials to an extrusion head. The screw type feeding device may employ the screw type feeding device 40 provided in the first to third embodiments.

In step S920, the width of the extrusion port of the extrusion head is controlled to continuously vary within a preset range during the 3D printing process.

Optionally, in some embodiments, the method S900 further includes: the viscosity of the material in the screw and/or the viscosity of the material at the extrusion opening are controlled such that the viscosity of the material in the screw is greater than the viscosity of the material at the extrusion opening.

Optionally, in some embodiments, the controlling the viscosity of the material in the screw and/or the viscosity of the material at the extrusion opening such that the viscosity of the material in the screw is greater than the viscosity of the material at the extrusion opening may include: heating the screw; heating the extrusion opening; wherein the heating temperature of the extrusion opening is higher than that of the screw.

Alternatively, in some embodiments, the screw heating temperature T1 ∈ (T)_f,T_f+30 ℃ wherein, T_fIndicates the melting temperature of the material.

Alternatively, in some embodiments, the heating temperature of the extrusion port T2 ∈ (T)_f+50℃,T_d-30 ℃), wherein, T_fDenotes the melting temperature, T, of the material_dIndicating the decomposition temperature of the material.

Optionally, in some embodiments, before step S910, the method S900 further includes: converting the solid material into a flowable material; storing the flowable materials output by the first feeding device by using a buffer container; and conveying the flowable materials stored in the buffer container to a screw type feeding device.

Optionally, in some embodiments, step S920 includes: the width of the extrusion opening is controlled to change along with the change of the section contour line of the material filling area.

In the above embodiments, all or part of the implementation may be realized by software, hardware, firmware or any other combination. When implemented in software, may be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When loaded and executed on a computer, cause the processes or functions described in accordance with the embodiments of the invention to occur, in whole or in part. The computer may be a general purpose computer, a special purpose computer, a network of computers, or other programmable device. The computer instructions may be stored on a computer readable storage medium or transmitted from one computer readable storage medium to another, for example, from one website, computer, server, or data center to another website, computer, server, or data center via wire (e.g., coaxial cable, fiber optic, Digital Subscriber Line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.). The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device, such as a server, a data center, etc., that incorporates one or more of the available media. The usable medium may be a magnetic medium (e.g., a floppy disk, a hard disk, a magnetic tape), an optical medium (e.g., a Digital Video Disk (DVD)), or a semiconductor medium (e.g., a Solid State Disk (SSD)), among others.

Those of ordinary skill in the art will appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware or combinations of computer software and electronic hardware. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the implementation. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present application.

In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus and method may be implemented in other ways. For example, the above-described apparatus embodiments are merely illustrative, and for example, the division of the units is only one logical division, and other divisions may be realized in practice, for example, a plurality of units or components may be combined or integrated into another system, or some features may be omitted, or not executed. In addition, the shown or discussed mutual coupling or direct coupling or communication connection may be an indirect coupling or communication connection through some interfaces, devices or units, and may be in an electrical, mechanical or other form.

The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment.

In addition, functional units in the embodiments of the present application may be integrated into one processing unit, or each unit may exist alone physically, or two or more units are integrated into one unit.

The above description is only for the specific embodiments of the present application, but the scope of the present application is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present application should be covered within the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A screw feeder for delivering flowable material to an extrusion opening having a width that continuously varies during extrusion of the material, the screw feeder comprising:

a screw;

a rotation controller configured to drive the screw to rotate;

wherein the screw type feeding device is configured to satisfy Q_max/D²＜5cm/min，L/D≤5，Q_maxRepresents the maximum flow rate of flowable material delivered by the screw feeder, D represents the diameter of the screw, and L represents the length of the screw.

2. The screw feeder of claim 1, further comprising:

a barrel, the screw located within the barrel, a gap δ < 0.001D between the screw and the barrel.

3. The screw feeder of claim 1 or 2, wherein the screw has a helix angle

4. The screw feeder of claim 1 or 2, wherein the screw has a flight width e that satisfies:

wherein,

indicating the helix angle of the screw.

5. The screw feeder of claim 1 or 2, wherein the screw has a shear rate γ ≦ 500/s for the flowable material in the screw channel.

6. A material conveying system for 3D printing, comprising:

an extrusion head;

the screw feeder of any one of claims 1-5;

a first control system configured to control a width of an extrusion port of the extrusion head to continuously vary within a preset range during 3D printing.

7. The material transport system of claim 6, further comprising:

a second control system configured to control a viscosity of the material in the screw and/or a viscosity of the material at the extrusion port such that the viscosity of the material in the screw is greater than the viscosity of the material at the extrusion port.

8. The material transport system of claim 7, wherein the second control system comprises a first heater configured to heat the screw and a second heater configured to heat the extrusion port, wherein a heating temperature of the second heater is higher than a heating temperature of the first heater.

9. The material transfer system of claim 8, wherein the first heater has a heating temperature T1 e (T1 e)_f,T_f+30 ℃ wherein, T_fRepresents the melting temperature of the material.

10. The material conveying system according to claim 8 or 9, characterized in that the heating temperature T2 e (T2 e) of the second heater_f+50℃,T_d-30 ℃), wherein, T_fDenotes the melting temperature, T, of the material_dRepresenting said materialThe decomposition temperature of (a).

11. A material conveying system according to any one of claims 6-9, characterised in that the material conveying system further comprises:

a first feed device configured to convert a solid material into the flowable material, the screw feed device being a second feed device of the material delivery system;

the buffer container is connected with the first feeding device and is configured to store the flowable materials output by the first feeding device and convey the flowable materials to the second feeding device.

12. The material transport system of any one of claims 6-9, wherein the first control system is configured to control the width of the extrusion opening to vary as a cross-sectional profile of the material fill area varies.

13. A material conveying method for 3D printing is characterized by comprising the following steps:

conveying flowable material to an extrusion head during 3D printing using a screw feeder according to any one of claims 1-5;

and controlling the width of an extrusion port of the extrusion head to continuously change within a preset range.

14. The material conveying method according to claim 13, further comprising:

controlling the viscosity of the material in the screw and/or the viscosity of the material at the extrusion opening such that the viscosity of the material in the screw is greater than the viscosity of the material at the extrusion opening.

15. The material conveying method of claim 14, wherein the controlling the viscosity of the material in the screw and/or the viscosity of the material at the extrusion port such that the viscosity of the material in the screw is greater than the viscosity of the material at the extrusion port comprises:

heating the screw;

heating the extrusion opening;

wherein the heating temperature of the extrusion opening is higher than the heating temperature of the screw.

16. Method for transporting materials according to claim 15, characterised in that the heating temperature of the screw is T1 e (T_f,T_f+30 ℃ wherein, T_fRepresents the melting temperature of the material.

17. Method for transporting material according to claim 15 or 16, characterised in that the heating temperature T2 e (T) of the extrusion opening is T2 e_f+50℃,T_d-30 ℃), wherein, T_fDenotes the melting temperature, T, of the material_dRepresents the decomposition temperature of the material.

18. The material conveying method of any one of claims 13-16, wherein prior to conveying the flowable material to the extrusion head using the screw feeder, the material conveying method further comprises:

converting the solid material into the flowable material;

storing the flowable materials output by the first feeding device by using a buffer container;

delivering the flowable material stored in the buffer container to the screw feeder.

19. The material conveying method according to any one of claims 13-16, wherein controlling the width of the extrusion opening of the extrusion head to vary continuously within a preset range comprises:

and controlling the width of the extrusion opening to change along with the change of the section contour line of the material filling area.