CN110899482A

CN110899482A - Thermoplastic alloy 3D printing system based on joule heat

Info

Publication number: CN110899482A
Application number: CN201911287290.1A
Authority: CN
Inventors: 李波波; 任晓飞; 卢秉恒; 王磊; 周梦杨; 汤永凯; 施建旭
Original assignee: National Institute Corp of Additive Manufacturing Xian
Current assignee: National Institute Corp of Additive Manufacturing Xian
Priority date: 2019-12-14
Filing date: 2019-12-14
Publication date: 2020-03-24

Abstract

The invention discloses a thermoplastic alloy 3D printing system based on joule heat, wherein a wire feeding device and a hot roller are relatively fixed, a wire feeding port of the wire feeding device is positioned at one side of the hot roller, the hot roller and a forming substrate are respectively connected with a negative electrode and a positive electrode of a joule heat power supply, the wire feeding device and the forming substrate can move relatively, a printing wire material is conveyed to the forming substrate by the wire feeding device, then the wire to be printed is pressed onto the forming substrate through the hot roller pressing wheel, the hot roller pressing wheel and the forming substrate form a loop, and the conductive wire instantly softens the material at the contact end of the hot roller pressing wheel and the substrate, the device has simple structure, can realize the additive manufacturing of metal parts with high utilization rate, high quality, low cost and environmental protection, and is a metal additive manufacturing method integrating material, machinery, measurement and control technology and information processing into a whole, and the hot-rolling pinch roller rolls the wires to be printed at the melting moment to realize good compactness inside the formed sample piece.

Description

Thermoplastic alloy 3D printing system based on joule heat

Technical Field

The invention relates to the field of metal additive manufacturing, in particular to a thermoplastic alloy 3D printing system based on joule heat.

Background

The additive manufacturing technology is commonly called as a 3D printing technology, has the advantages of rapid and free manufacturing of a three-dimensional structure, and is widely applied to new product development and single-piece small-batch manufacturing, wherein metal direct molding is a difficult point and a hot point in the additive manufacturing technology.

The amorphous alloy has mechanical, physical and chemical properties obviously superior to those of the traditional crystalline alloy due to the unique amorphous structure, such as high strength, good wear resistance, corrosion resistance and the like, shows superplasticity in a supercooling liquid phase region, can be subjected to thermoplastic forming, and is also called as a thermoplastic alloy. The amorphous alloy has wide potential prospect in various fields such as aerospace, precision instruments, military chemical engineering and the like.

At present, the preparation method of the amorphous alloy part mainly comprises a thermoplastic forming method, a powder sintering method, a welding assembly method, an additive manufacturing method and the like. Thermoplastic forming methods (such as molding, blow molding, etc.) have difficulty in producing large-sized parts of complex shapes; although the latter three methods break through the problem of size limitation, the powder sintering method requires a die and a sintering furnace for preparation, and the forming process is relatively complicated; the performance difference between the welding part and the base body during welding and assembling limits the application range of the parts; in the forming process of additive manufacturing (such as selective laser melting of amorphous powder), the high-energy beam additive manufacturing method represented by laser has the problems of high equipment manufacturing cost, complex equipment, larger volume, radiation pollution, low forming speed under the same power and the like. Therefore, a novel amorphous alloy large-size complex part low-cost and high-efficiency forming method is very important.

For example, patent document CN108080638A discloses a laser 3D printing forming system and a forming method for amorphous alloy foil, and specifically discloses a laser cutting excess sample material of amorphous alloy foil, heating to a super-plastic state of a supercooled liquid phase region by using amorphous alloy, rolling by using a preheated roller, combining with the ultrasonic vibration effect, making the upper and lower layers of amorphous alloy foil generate interatomic contact, and rapidly cooling to form amorphous alloy parts with large size, complex shape and hollow structure. The method overcomes the limitation of the traditional amorphous alloy preparation method on the size and the shape of an alloy part, but the amorphous alloy 3D printing and forming system disclosed by the patent has a complex process, and the laser device needs larger energy, occupies larger volume and space and has high system cost.

For example, patent document cn109434112a.pdf discloses a spatial 3D printing system based on amorphous alloy superplastic welding, which is suitable for spatial environment, and the prepared non-gold alloy parts have insufficiently compact internal structure and may have layered warpage.

Disclosure of Invention

The invention aims to provide a thermoplastic alloy 3D printing system based on joule heat to overcome the defects of the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme:

a thermoplastic alloy 3D printing system based on joule heat comprises a joule heat power source, a wire feeding device, a hot roller and a forming substrate;

wire drive feed unit and hot-rolling roller position relatively fixed, wire drive feed unit's a mouth is located hot-rolling roller one side, and the hot-rolling roller is connected in joule heating power supply positive pole or negative pole, and the shaping base plate is connected in joule heating power supply negative pole or positive pole, and wire drive feed unit and shaping base plate can relative motion.

Further, wire drive feed unit is including sending a frame and setting up the silk dish that send in sending a frame, send silk dish one side to be equipped with and send a action wheel, send silk dish surface and send a action wheel surface contact, send a frame lower extreme to be equipped with the godet head, the silk mouth that goes out of godet head is close to the hot-rolling pinch roller.

Further, the hot-rolling pinch roller is including rolling the wheel body and rolling the wheel body frame, rolls the wheel body and is fixed in through the pivot and rolls on the wheel body frame, rolls wheel body frame one side and is equipped with the brush box, is equipped with the brush in the brush box, brush and rolling wheel body surface contact.

Furthermore, a spring is arranged between the electric brush and the electric brush box, and one side of the electric brush box of the grinding wheel body frame is provided with a power supply interface which is used for being electrically connected with a joule heating power supply.

Further, on rolling wheel body frame and being fixed in the removal frame through the pre-compaction telescopic link, roll wheel body frame and be fixed in pre-compaction telescopic link lower extreme, pre-compaction telescopic link upper end passes through bolted connection with the removal frame, and pre-compaction telescopic link length is adjustable, is equipped with the pressure spring between pre-compaction telescopic link and the removal frame.

Furthermore, the wire feeding disc is fixed on the wire feeding rack through the wire feeding adjusting frame, the wire feeding adjusting frame and the wire feeding disc are respectively located on two sides of the wire feeding adjusting frame, an adjusting rod hole is formed in the wire feeding adjusting frame, an adjusting rod is arranged in the adjusting rod hole, a pressure spring is sleeved on the adjusting rod, a driven wheel shaft is fixed to the end portion of the adjusting rod hole, the wire feeding disc is installed on the driven wheel shaft, and a sliding groove used for the driven wheel shaft to move is formed in the wire feeding rack.

Furthermore, a pressing wheel fixed on the wire feeding rack is further arranged on one side of the wire feeding disc, and the pressing wheel is fixed on the wire feeding rack through a pressing support.

Furthermore, a compression spring is arranged on the compression bracket, a blocking part is arranged on one side of the compression bracket, the compression bracket enables the compression wheel to be always in surface contact with the wire feeding disc under the action of the compression spring, and an adjusting bolt is arranged on the other side of the compression bracket and used for adjusting the pre-tightening force of the compression bracket.

Further, the wire feeding device, the hot rolling wheel and the forming substrate are all arranged in the sealed cavity.

Furthermore, the forming substrate is fixed on the moving platform, the upper end of the moving platform is provided with a radiator, the upper end of the radiator is fixed with a heating plate through a radiating support, and the forming substrate is fixed on the heating plate; the heating plate is connected with a heating power supply.

Compared with the prior art, the invention has the following beneficial technical effects:

the invention relates to a thermoplastic alloy 3D printing system based on joule heat, which utilizes a joule heat power supply, a wire feeding device, a hot roller pressing wheel and a forming substrate to form the joule heat printing system, the wire feeding device and the hot roller pressing wheel are relatively fixed, a wire feeding port of the wire feeding device is positioned at one side of the hot roller pressing wheel, the hot roller pressing wheel and the forming substrate are respectively connected with a negative electrode and a positive electrode of the joule heat power supply, the wire feeding device and the forming substrate can relatively move, a printing wire material is conveyed to the forming substrate by the wire feeding device, then the wire material to be printed is pressed onto the forming substrate by the hot roller pressing wheel, the hot roller pressing wheel and the forming substrate form a loop, a conductive wire material is instantly softened at the contact end of the hot roller pressing wheel and the substrate, and additive manufacturing is realized on thermoplastic alloy based on joule heat, the device has simple structure, and can realize additive manufacturing of metal parts with high utilization rate, the method is a metal additive manufacturing method integrating material, machinery, measurement and control technology and information processing, and the inside of a formed sample piece is good in compactness by rolling the melting moment of the wire to be printed through a hot roller, so that the combination between layers and a substrate is facilitated, and high-quality metal 3D forming is realized; the joule heating power supply does not need to be conducted through heat conduction, directly heats the material and the substrate, and has high energy utilization rate.

Further, wire drive feed unit is including sending a frame and setting up the silk dish that send in sending a frame, send silk dish one side to be equipped with and send a action wheel, send silk dish surface and send a action wheel surface contact, send a frame lower extreme to be equipped with the seal wire head, the play silk mouth of seal wire head is close to the hot-rolling pinch roller, simple structure, it is convenient to treat printing silk material to change.

Further, the hot-rolling pinch roller is including rolling the wheel body and rolling the wheel body frame, rolls the wheel body and is fixed in through the pivot and rolls on the wheel body frame, rolls wheel body frame one side and is equipped with the brush box, is equipped with the brush in the brush box, and the brush is with rolling wheel body surface contact, ensures that the printing in-process rolls the wheel body electrically conductive well.

Furthermore, a pressing wheel fixed on the wire feeding rack is further arranged on one side of the wire feeding disc, and the pressing wheel is fixed on the wire feeding rack through a pressing support, so that the loosening of the wires to be printed is prevented, and the printing stability is improved.

Furthermore, the forming substrate is fixed on the moving platform, the upper end of the moving platform is provided with a radiator, the upper end of the radiator is fixed with a heating plate through a radiating support, and the forming substrate is fixed on the heating plate; the heating plate is connected with a heating power supply, so that the temperature control of the formed substrate can be realized, and the problems of cracking, warping, deformation and the like caused by thermal stress due to overlarge temperature gradient in the 3D printing process of metal are solved.

Drawings

FIG. 1 is a schematic view of a wire feeding device according to the present invention.

FIG. 2 is a schematic view of the structure of the present invention.

FIG. 3 is a schematic view of a wire feeding adjusting bracket according to the present invention.

Fig. 4 is a cross-sectional view of the grinding wheel body frame.

Wherein, 1, joule heating power supply; 2. a wire feeder; 3. a hot roller pressing wheel; 4. molding a substrate; 5. a motion platform; 6. a heat sink; 7. heating plates; 8. a heating power supply; 9. a wire feeding frame; 10. a wire feeding disc; 11. a wire feeding driving wheel; 12. a pinch roller; 13. compressing the bracket; 14. a compression spring; 15. a wire feeding adjusting frame; 16. an adjusting rod hole; 17. a pressure spring; 18. a driven axle; 19. a thread guide head; 20. rolling the wheel body; 21. rolling the wheel body frame; 22. an electric brush; 23. a spring; 24. a power interface; 25. moving the frame; 26. prepressing the telescopic rod; 27. sealing the cavity; 28. an anti-oxidation device; 29. and (5) controlling the system.

Detailed Description

The invention is described in further detail below with reference to the accompanying drawings:

as shown in fig. 1 and fig. 2, a thermoplastic alloy 3D printing system based on joule heat comprises a joule heat power supply 1, a wire feeding device 2, a hot roller 3 and a forming substrate 4;

wire

drive feed unit

2 and 3 position relatively fixed of hot-rolling roller, wire drive feed unit 2 send a mouth to be located hot-rolling roller 3 one side, hot-rolling roller 3 connects in 1 positive pole or negative pole of joule heat power supply, and forming substrate 4 connects in 1 negative pole or positive pole of joule heat power supply, and wire drive feed unit 2 and forming substrate 4 can relative motion, and hot-rolling roller 3 can contact with 4 surperficial roll extrusion of forming substrate.

Specifically, the forming substrate 4 is fixed on a moving platform 5, a radiator 6 is arranged at the upper end of the moving platform 5, a heating plate 7 is fixed at the upper end of the radiator 6 through a radiating support, and the forming substrate 4 is fixed on the heating plate 7; the heating plate 7 is connected with a heating power supply 8; the heating device is used for auxiliary heating of the molding substrate 4, and improves the heating molding efficiency.

The wire feeding device 2 comprises a wire feeding rack 9 and a wire feeding disc 10 arranged on the wire feeding rack 9, a wire feeding driving wheel 11 is arranged on one side of the wire feeding disc 10, the surface of the wire feeding disc 10 is in surface contact with the wire feeding driving wheel 11, a wire guiding head 19 is arranged at the lower end of the wire feeding rack 9, and a wire outlet of the wire guiding head 19 is close to the hot rolling wheel 3.

As shown in fig. 4, the thermal roller 3 includes a rolling roller body 20 and a rolling roller body frame 21, the rolling roller body 20 is fixed on the rolling roller body frame 21 through a rotating shaft, an electric brush box is arranged on one side of the rolling roller body frame 21, an electric brush 22 is arranged in the electric brush box, the electric brush 22 is in surface contact with the rolling roller body 20, a spring 23 is arranged between the electric brush 22 and the electric brush box, and a power interface 24 is arranged on one side of the electric brush box of the rolling roller body frame 21 and used for being electrically connected with the joule heating power supply.

The rolling wheel body frame 21 and the wire feeding frame 9 are both fixed on the movable frame 25; roll wheel body frame 21 and be fixed in on the removal frame 25 through pre-compaction telescopic link 26, roll wheel body frame 21 and be fixed in pre-compaction telescopic link 26 lower extreme, pre-compaction telescopic link 26 upper end passes through bolted connection with removal frame 25, pre-compaction telescopic link 26 adjustable length, be equipped with the pressure spring between pre-compaction telescopic link 26 and the removal frame 25 for adjustment pre-compaction telescopic link 26 height and pre-compaction pretightning force.

As shown in fig. 3, the wire feeding disc 10 is fixed on the wire feeding frame 9 through a wire feeding adjusting frame 15, the wire feeding adjusting frame 15 is connected with the wire feeding frame 9 through a bolt, the wire feeding adjusting frame 15 and the wire feeding disc 10 are respectively located at two sides of the wire feeding adjusting frame 15, an adjusting rod hole 16 is formed in the wire feeding adjusting frame 15, an adjusting rod is arranged in the adjusting rod hole 16, a pressure spring 17 is sleeved on the adjusting rod, a driven wheel shaft 18 is fixed at the end of the adjusting rod hole 16, the wire feeding disc 10 is mounted on the driven wheel shaft 18, and a sliding groove for moving the driven wheel shaft 18 is formed in the wire; good contact between the wire feeding disc 10 and the wire feeding driving wheel 11 is achieved by the wire feeding frame 9.

The wire feeding device is characterized in that a pressing wheel 12 fixed on a wire feeding rack 9 is further arranged on one side of the wire feeding disc 10, the pressing wheel 12 is fixed on the wire feeding rack 9 through a pressing support 13, a pressing spring 14 is arranged on the pressing support 13, a blocking portion is arranged on one side of the pressing support 13, the pressing support 13 enables the pressing wheel 12 to be in surface contact with the wire feeding disc 10 all the time under the action of the pressing spring, and an adjusting bolt is arranged on the other side of the pressing support 13 and used for adjusting the pre-tightening force of the.

The wire feeding device 2, the hot rolling wheel 3 and the forming substrate 4 are all arranged in the sealed cavity 27, and a printing atmosphere is provided through the anti-oxidation device 28; the joule heating power source 1, the heating power source 8 and the wire feeder 2 are all controlled by a control system 29, which provides control parameters as well as printing atmosphere parameters.

The principle of the invention is that joule heat is generated after alloy materials are electrified, thermoplastic stacking forming of the materials is realized through the joule heat, the alloy materials are sent into a hot roller pinch roller through an automatic feeding system, the hot roller pinch roller is connected with a positive pole or a negative pole of a joule heat power supply through an electric brush, a printing substrate is connected with the positive pole or the negative pole of the joule heat power supply, a loop is formed between the electric brush and a forming substrate, and current is conducted to enable conductive wire materials to instantly soften the materials at the contact end of the hot roller pinch roller and the substrate, for example, amorphous materials also show superplasticity in a supercooling liquid phase area, thermoplastic forming can be realized, and accumulated metal parts can be formed on the substrate along with the movement and continuous delivery of the substrate, so.

The rolling of alloy materials is realized by the ball bearing between the rolling wheel and the rolling wheel, and the rolling pressure of different materials in different processes is adjusted by the pre-pressing telescopic rod. The roller wheel is in good contact with the electric brush through the spring 23, the roller wheel conducts electricity to the conductive wire through the electric brush, hot rolling forming is realized through the roller device, an electrode binding post is arranged on the electric brush and is used for connecting a Joule heating power supply, a Joule heating control power supply part provides main electric energy for hot forming of the conductive wire, the direct current, alternating current or pulse mode can be adopted mainly according to the hot forming process characteristics of different alloy materials, the voltage source or current source mode can also be used, Joule heating generated instantly by short circuits of the conductive wire and the substrate enables the alloy material to have viscoplasticity without conduction heating, the material generates heat internally, the heat efficiency is high, the energy is high, and the printing parameters are set according to different materials and different forming efficiencies; the conductive wire material adopts a metal belt or an alloy wire.

The heating power supply part of the molding substrate enables the printing area to realize a constant temperature environment, can reduce the warping and deformation of the printed article caused by stress, and improves the molding quality. The forming substrate heating power supply can adopt various heating and temperature control modes, such as resistance heating or induction heating, and the like, and a temperature sensor is required to be added to realize PID temperature control, different temperatures can be set according to different heat treatment processes of different metal materials, and a radiator is required to be added in consideration of the fact that a motion platform cannot bear high temperature for a long time to cause the misalignment of deformation and motion, and air cooling, water cooling, semiconductor refrigeration and the like can be selected according to the condition of failure. If the temperature required by the substrate is not high, a radiator can be omitted, or the substrate is required to be cooled and directly cooled to dissipate heat so as to realize the constant temperature environment of the substrate.

The conducting wire is guided and delivered by a wire feeding disc 10, a pressing wheel and a wire feeding driving wheel 11, the conducting wire disc is provided with printing materials on a driven wheel, but alloy materials are easy to loosen, so that the pressing wheel is needed to prevent the materials from loosening, and delivery failure is caused.

The anti-oxidation part can design atmosphere protection according to different metal materials, and comprises a gas purification system, a circulating system and a gas temperature control system, wherein the gas purification system, the circulating system and the gas temperature control system need to be provided with a water oxygen sensor, a pressure sensor, a temperature sensor, a gas tank, a gas pump and a gas valve for controlling the inlet and the outlet, and can also be provided with a vacuum pump according to the design of a vacuum closed cavity, and if large parts are formed, the anti-oxidation system can also be designed into a local atmosphere protection mode.

The energy input part, namely the power supply control part, the space motion control part, the printing head and feeding control part and the anti-oxidation part are all required to be connected to a computer overall control system of the printer, and are uniformly coordinated and controlled through special software according to the process characteristics of different metal materials.

The working steps of the thermoplastic alloy 3D printing system based on the joule heat are as follows:

1) loading a model, wherein the model slicing is to translate a 3D model described by a triangular patch into a G code instruction group which can be executed by the 3D printer through a specific algorithm, namely, converting the three-dimensional model in the format of star to G codes which can be used by the 3D printer;

2) before printing, firstly, installing the conductive wire material, starting an anti-oxidation system, and preheating the substrate to a certain temperature;

3) starting a motion system, leveling a platform, delivering a conductive wire material to the bottom of a hot rolling wheel, turning on a joule heating power supply, softening an alloy material through high-efficiency joule heating, depositing the alloy material to a high-temperature substrate through a hot rolling device to realize good metallurgical bonding of the substrate and a first layer, and then further performing hot-compression molding stacking to realize good bonding between layers, stacking layer by layer and stacking and forming parts;

4) in the printing process, the temperature measuring element detects the temperature of the hot roller and the temperature of the substrate and transmits a temperature signal to the temperature control system, and the temperature control system adjusts the heating power in real time by comparing the feedback temperature with a preset temperature value.

5) After the printing is formed, heat preservation treatment is continued for a period of time according to the heat treatment requirements of various materials, internal stress is eliminated, the printing is finished, and the workpiece is taken out.

Claims

1. A thermoplastic alloy 3D printing system based on joule heat is characterized by comprising a joule heat power source (1), a wire feeding device (2), a hot rolling wheel (3) and a forming substrate (4);

wire drive feed unit (2) and hot-rolling roller (3) position relatively fixed, the mouth that send of wire drive feed unit (2) is located hot-rolling roller (3) one side, hot-rolling roller (3) are connected in joule heat power supply (1) anodal or negative pole, shaping base plate (4) are connected in joule heat power supply (1) negative pole or anodal, wire drive feed unit (2) can relative motion with shaping base plate (4), hot-rolling roller (3) can be with shaping base plate (4) surface roll extrusion contact.

2. The joule heat based thermoplastic alloy 3D printing system according to claim 1, wherein the wire feeding device (2) comprises a wire feeding frame (9) and a wire feeding disc (10) arranged on the wire feeding frame (9), one side of the wire feeding disc (10) is provided with a wire feeding driving wheel (11), the surface of the wire feeding disc (11) is in surface contact with the wire feeding driving wheel (11), the lower end of the wire feeding frame (9) is provided with a wire guide head (19), and a wire outlet of the wire guide head (19) is close to the hot roller (3).

3. The joule heat based thermoplastic alloy 3D printing system according to claim 1, wherein the thermal roller (3) comprises a rolling roller body (20) and a rolling roller body frame (21), the rolling roller body (20) is fixed on the rolling roller body frame (21) through a rotating shaft, one side of the rolling roller body frame (21) is provided with an electric brush box, an electric brush (22) is arranged in the electric brush box, and the electric brush (22) is in surface contact with the rolling roller body (20).

4. A joule heat based thermoplastic alloy 3D printing system according to claim 3, characterized in that a spring (23) is provided between the brush (22) and the brush box, and a power interface (24) is provided on the side of the brush box of the grinding wheel body frame (21) for electrical connection with the joule heating power source (1).

5. The joule heat based thermoplastic alloy 3D printing system according to claim 3, wherein the rolling wheel body frame (21) is fixed to the movable frame (25) through a pre-pressing telescopic rod (26), the rolling wheel body frame (21) is fixed to the lower end of the pre-pressing telescopic rod (26), the upper end of the pre-pressing telescopic rod (26) is connected with the movable frame (25) through a bolt, the length of the pre-pressing telescopic rod (26) is adjustable, and a pressure spring is arranged between the pre-pressing telescopic rod (26) and the movable frame (25).

6. The joule heat based thermoplastic alloy 3D printing system according to claim 2, wherein the wire feeding disc (10) is fixed to the wire feeding frame (9) through a wire feeding adjusting frame (15), the wire feeding adjusting frame (15) and the wire feeding disc (10) are respectively located on two sides of the wire feeding adjusting frame (15), a adjusting rod hole (16) is formed in the wire feeding adjusting frame (15), an adjusting rod is arranged in the adjusting rod hole (16), a pressure spring (17) is sleeved on the adjusting rod, a driven wheel shaft (18) is fixed to an end of the adjusting rod hole (16), the wire feeding disc (10) is mounted on the driven wheel shaft (18), and a sliding groove for moving the driven wheel shaft (18) is formed in the wire feeding frame (9).

7. The joule heat based thermoplastic alloy 3D printing system according to claim 2, wherein the wire feeding disk (10) is further provided with a pressing wheel (12) fixed on the wire feeding rack (9), and the pressing wheel (12) is fixed on the wire feeding rack (9) through a pressing bracket (13).

8. The joule heat based thermoplastic alloy 3D printing system according to claim 7, wherein a pressing spring (14) is arranged on the pressing support (13), a blocking portion is arranged on one side of the pressing support (13), the pressing support (13) enables the pressing wheel (12) to be always in surface contact with the wire feeding disc (10) under the action of the pressing spring, and an adjusting bolt is arranged on the other side of the pressing support (13) and used for adjusting the pre-tightening force of the pressing support (13).

9. The joule heat based thermoplastic alloy 3D printing system according to claim 1, wherein the wire feeder (2), the hot roller (3), and the forming substrate (4) are all disposed within the sealed chamber (27).

10. The joule heat based thermoplastic alloy 3D printing system according to claim 1, wherein the forming substrate (4) is fixed on the moving platform (5), the upper end of the moving platform (5) is provided with a heat sink (6), the upper end of the heat sink (6) is fixed with the heating plate (7) through a heat dissipation bracket, and the forming substrate (4) is fixed on the heating plate (7); the heating plate (7) is connected with a heating power supply (8).