Inflation sealing structure and sealing method of forming cabin for additive manufacturing

Abstract

The invention relates to a forming cabin inflation sealing structure and method for additive manufacturing. The sealing structure comprises a forming cabin, a working cylinder and a sealing assembly, the working cylinder is fixed, the forming cabin is movable, the connecting position of the forming cabin and the working cylinder is sealed by the sealing assembly, and the building space is a sealed building space; the sealing assembly is installed at the bottom of the working cylinder and comprises a sealing ring and a pneumatic control module, a sealing cavity is formed in the sealing ring and connected with the pneumatic control module through an air pipe, and the pneumatic control module inflates or exhausts air into the sealing cavity so as to control expansion and contraction of the sealing ring. By the adoption of the sealing structure, the cylinder does not need to be jacked, or the requirement for the jacking force of the jacked cylinder due to the sealing requirement can be reduced, and then the problems that the cylinder moves in the jacking process, and rigidity is lost can be reduced or eliminated.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive manufacturing, and in particular relates to an inflation sealing structure and a sealing method for an additively manufactured molding cabin. Background Art

[0002] The current mainstream construction space in the additive manufacturing industry consists of two parts: one is the molding cabin, and the other is the cylinder that holds the material powder. The two are combined to form a sealed printing space to ensure an inert gas atmosphere during the construction process. For current small-scale equipment, the cylinder and the working chamber are fixedly connected, so static sealing is generally used. However, as the market gradually increases the demand for large-size equipment, the size of the cylinder is getting larger and larger, and the weight is also greatly increased. In order to achieve the convenience and safety of cleaning the powder after printing, the cylinder needs to be movable, which leads to the detachable structural seal between the molding cabin and the cylinder.

[0003] The sealing principle of the existing separable structural seal is as follows: Figure 1 As shown in (a), a lifting mechanism is installed under the cylinder body 2 to lift the cylinder body 2, and the cylinder body 2 is abutted against the bottom of the molding chamber 1 to achieve sealing; when the cylinder body 2 needs to be cleaned, as shown in Figures (b) to (d) in sequence, the lifting mechanism lowers the cylinder body 2, and moves to the right to the bottom of the powder cleaning chamber on the right side of the molding chamber 1, and then the lifting mechanism lifts the cylinder body 2 again to make it abut against the bottom of the powder cleaning chamber to achieve sealing, thereby performing the powder cleaning operation. The sealing method using the lifting cylinder body requires that the cylinder body be lifted at the bottom of the molding chamber to abut against the bottom of the cylinder body in order to achieve the sealing effect, and the lifting force needs to be increased in order to improve the sealing effect. The sealing method has the following problems:

[0004] As the size of the cylinder becomes larger and larger, the weight of the cylinder (including its own weight + total weight of the powder + mass of the formed workpiece) also becomes larger and larger. Not only does the bearing transmission mechanism required for the cylinder during the lifting process become more complicated, but more importantly, during the lifting process, the cylinder has a vertical up and down motion trajectory along the Z axis, and due to dynamic bearing or lack of rigidity during lifting, the cylinder will move in the horizontal X and Y directions during the printing process, which will seriously affect the accuracy of the parts in the X and Y directions during the layered printing process. Summary of the invention

[0005] In order to solve the problems existing in the background technology, the present invention provides an inflatable sealing structure and sealing method for an additively manufactured molding chamber, which can reduce the lifting effect on the cylinder body required for sealing, thereby reducing the movement of the cylinder body during the lifting process, improving the quality of printing, and ensuring the sealing effect.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] On the one hand, the present invention provides an inflatable sealing structure of a molding cabin for additive manufacturing, comprising a molding cabin, a cylinder body and a sealing assembly, wherein the cylinder body is fixed, the molding cabin is movable, the molding cabin and the cylinder body are connected to form a building space, and the position where the molding cabin and the cylinder body are connected is sealed by the sealing assembly;

[0008] The sealing assembly is installed at the bottom of the cylinder body, and includes a sealing ring and a pneumatic control module. A sealing cavity is formed inside the sealing ring, and the sealing cavity is connected to the pneumatic control module through an air pipe. The pneumatic control module inflates or exhausts air into the sealing cavity to control the expansion and contraction of the sealing ring. When the sealing ring expands, the molding cabin and the cylinder body are sealed. When the sealing ring contracts, the molding cabin and the cylinder body are separated.

[0009] Furthermore, an annular groove is formed at the bottom of the cylinder body, and the sealing ring is installed in the annular groove.

[0010] Furthermore, the bottom of the sealing ring is a sealing lip, and the top of the sealing ring is fixedly installed in the groove. The expansion and contraction of the sealing ring is achieved by the expansion and contraction of the sealing lip. When the sealing lip expands outward, it abuts against the top of the molding chamber. When the sealing ring contracts, the molding chamber and the cylinder body are separated.

[0011] Furthermore, the pneumatic control module includes a vacuum generator assembly and an air intake pressure maintaining assembly, wherein the vacuum generator assembly is used to control the pneumatic control module to inflate air into the sealed cavity, and the air intake pressure maintaining assembly is used to control the pneumatic control module to draw a vacuum into the sealed cavity.

[0012] Furthermore, the pneumatic control module includes a pressure detection component. A pressure sensor is installed at the top of the molding chamber at a position where the top contacts the sealing ring, for detecting the internal pressure value of the sealing ring. The pressure sensor is electrically connected to the pressure detection component.

[0013] Furthermore, the pressure sensor is installed at the bottom of the sealing ring. When the sealing ring expands, the pressure sensor abuts against the top of the molding chamber to generate a pressure signal. The pressure detection device is electrically connected to the pressure detection assembly.

[0014] Furthermore, the pressure sensor is a thin film pressure sensor.

[0015] Furthermore, the sealing ring is a silicone or rubber sealing ring.

[0016] On the other hand, a method for inflating and sealing a molding chamber of additive manufacturing, based on the sealing structure, comprises the steps of:

[0017] Moving the molding chamber to a position opposite to the cylinder body;

[0018] Controlling the air intake and pressure maintaining component of the pneumatic control component to inflate air into the sealed cavity, so that the sealing ring expands to seal the connection position between the molding cabin and the cylinder body, and the construction space becomes a sealed construction space;

[0019] The additive manufacturing equipment performs construction printing or powder cleaning in the sealed construction space. After the construction printing or powder cleaning is completed, the vacuum generator component of the pneumatic control component is controlled to extract air from the sealed cavity to form a vacuum in the sealed cavity, the sealing ring shrinks, and the molding cabin and the cylinder body are separated;

[0020] The molding chamber can be removed from the bottom of the cylinder.

[0021] Furthermore, the method further comprises the steps of:

[0022] When the sealing ring is inflated or exhausted, the sealing ring expands or contracts, and the pressure sensor detects the pressure signal in real time and feeds the pressure signal back to the pneumatic control module. The pneumatic control module controls the operation of the air intake pressure maintaining component and the vacuum generator component according to the pressure signal.

[0023] The present invention adopts the above technical solution, which has the following advantages:

[0024] (1) The present invention adopts a pneumatic sealing method, which eliminates the need for lifting the cylinder or can reduce the need for lifting force on the cylinder due to sealing requirements, thereby reducing or eliminating the problems of cylinder movement and lack of rigidity during the lifting process, thereby improving sealing and printing quality;

[0025] (2) In the present invention, the sealing assembly is installed on the working cylinder. Compared with installing the sealing assembly on the molding chamber, this avoids the need for the sealing pipeline and the pneumatic control module to move with the molding chamber during the movement of the molding chamber. The structure is simple and compact, and the operation is more convenient.

[0026] (3) The present invention also provides a pressure sensor, which can detect the pressure condition inside the sealing ring, and further monitor and feedback the sealing effect. At the same time, the pneumatic control can perform feedback adjustment according to the result of the pressure detection. The whole process can be automated, thereby improving the sealing reliability and sealing efficiency and reducing the sealing cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Throughout the accompanying drawings, the same reference numerals are used to represent the same components. In the accompanying drawings:

[0028] Figure 1 It is the principle diagram of the cylinder powder cleaning or construction operation of the existing large-scale additive manufacturing equipment;

[0029] Figure 2 It is a structural schematic diagram of the inflatable sealing structure of the additively manufactured molding cabin provided by the present invention;

[0030] Figure 3 It is a schematic diagram of the structure of the sealing ring;

[0031] Figure 4 It is a structural diagram of the pneumatic control module;

[0032] Figure 5 This is a state view of the sealing ring expanding to connect the powder cleaning cylinder and the molding cabin;

[0033] Figure 6 This is a state view where the sealing ring shrinks to separate the molding chamber and the powder cleaning cylinder;

[0034] Figure 7 This is the state view of the molding cabin moving to the bottom of the working cylinder;

[0035] Figure 8 This is a view of the state where the sealing ring expands to connect the working cylinder and the molding chamber.

[0036] The symbols in the accompanying drawings are as follows:

[0037] 1-working cylinder, 2-molding cabin, 3-sealing ring, 4-molding cabin body, 5-working cylinder body, 6-pneumatic control module, 7-trachea, 8-pressure sensor, 10-powder cleaning cylinder, 3-1-sealing ring in inflated state, 3-2-sealing ring in exhaust state, 6-1-vacuum generator assembly, 6-2-air intake pressure maintaining assembly, 6-3-pressure detection assembly DETAILED DESCRIPTION

[0038] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0039] like Figures 1 to 4As shown, the present invention provides an inflatable sealing structure for an additively manufactured molding chamber, the main purpose of which is to solve the problem that as the size of the molding chamber cylinder body 4 of large or super-large equipment becomes larger and larger, the weight of the molding chamber cylinder body 4 (including its own weight + total weight of powder + mass of the molded workpiece) also becomes larger and larger. The existing method of sealing the molding chamber cylinder body 4 and the working cylinder cylinder body 5 by lifting the molding chamber cylinder body 4 not only makes the load-bearing transmission mechanism required for the molding chamber cylinder body 4 in the lifting process more complicated; more importantly, during the lifting process, the molding chamber cylinder body 4 has a vertical up and down motion trajectory along the Z-axis direction, and due to dynamic load-bearing or lack of rigidity during lifting, the cylinder body will move in the horizontal X and Y directions during the printing process, thereby seriously affecting the accuracy of the parts in the X and Y directions during the layered printing process. Technical problems such as.

[0040] The inflatable sealing structure of the additively manufactured molding cabin includes a molding cabin 2, a working cylinder 1 and a sealing assembly. The working cylinder 1 is fixed, and the molding cabin 2 is movable. The molding cabin 2 and the working cylinder 1 are connected to form a construction space, and the position where the molding cabin 1 and the working cylinder 1 are connected is sealed by the sealing assembly.

[0041] The sealing assembly is installed at the bottom of the working cylinder 1, and includes a sealing ring 3 and a pneumatic control module 6. A sealed cavity is formed inside the sealing ring 3, and the sealed cavity is connected to the pneumatic control module 6 through an air pipe 7. The pneumatic control module 6 inflates or exhausts air into the sealed cavity to control the expansion and contraction of the sealing ring 3. When the sealing ring 3 expands, the molding cabin 2 and the working cylinder 1 are sealed and connected. When the sealing ring 3 contracts, the molding cabin 2 and the working cylinder 1 are separated.

[0042] The present invention adopts a pneumatic sealing method, which eliminates the need to lift the molding cabin cylinder body 4 or reduces the need for a lifting force to lift the molding cabin cylinder body 4 due to sealing requirements, thereby reducing or eliminating the problems of movement and lack of rigidity of the molding cabin cylinder body 4 during the lifting process; in addition, installing the sealing component on the working cylinder 1, compared to installing the sealing component on the molding cabin 2, avoids the need for the sealing pipeline and the pneumatic control module 6 to move with the molding cabin 2 as a whole during the movement of the molding cabin 2, and has a simple and compact structure and is more convenient to operate.

[0043] The bottom of the working cylinder 1 is provided with an annular groove, and the sealing ring 3 is installed in the annular groove. The cross-sections of the working cylinder 1 and the molding chamber 2 can be square or circular, so the annular groove can be circular or square ring.

[0044] Preferably, the bottom of the sealing ring 3 is a sealing lip, and the top of the sealing ring 3 is fixedly installed in the groove. The expansion and contraction of the sealing ring 3 are realized by the expansion and contraction of the sealing lip. When the sealing lip expands outward, it abuts against the top of the molding cabin 2. When the sealing ring 3 contracts, the molding cabin 2 and the working cylinder 1 are separated. The top of the sealing ring 3 is adapted to the cross-section of the groove. When the groove is a square groove, the top of the sealing ring 3 is square. The top of the sealing ring 3 is fixed to the inner wall of the groove, and the bottom of the sealing ring is an arc that can shrink inward and expand outward.

[0045] The pneumatic control module 6 includes a vacuum generator component 6-1 and an air intake pressure maintaining component 6-2. The vacuum generator component 6-1 is used to control the pneumatic control module 6 to inflate air into the sealed cavity, and the air intake pressure maintaining component 6-2 is used to control the pneumatic control module 6 to draw a vacuum into the sealed cavity.

[0046] When construction printing is required, the molding chamber 2 is moved to a position opposite to the working cylinder 1; the air intake and pressure maintaining component 6-2 is controlled by the pneumatic control component 6 to inflate the sealed cavity, so that the connection position between the molding chamber 2 and the working cylinder 1 is sealed by the sealing component, and the construction space becomes a sealed construction space; the additive manufacturing equipment performs construction printing in the sealed construction space, and after the construction printing is completed, the pneumatic control component 6 controls the vacuum generator component 6-1 to extract air from the sealed cavity to form a vacuum in the sealed cavity, the sealing ring 3 shrinks, and the molding chamber 2 and the working cylinder 1 are separated; the working cylinder 1 can be removed from the bottom of the molding chamber 2.

[0047] Compared with the existing method of sealing and building a space by lifting the molding chamber 2 until it abuts against the bottom of the working cylinder 1, in the present invention, the sealed connection between the working cylinder and the molding chamber can be achieved through the contraction or expansion of the sealing lip, without the need to lift the molding chamber 2 until it abuts against the bottom of the working cylinder 1.

[0048] The pneumatic control module 6 includes a pressure detection component 6-3. A pressure sensor 8 is installed at the top of the molding chamber 2 where it contacts the sealing ring 3. The pressure sensor 8 is used to detect the internal pressure value of the sealing ring 3. The pressure sensor 8 is electrically connected to the pressure detection component 6-3.

[0049] As another embodiment, the pressure sensor 8 is installed at the bottom of the sealing ring 3. When the sealing ring 3 expands, the pressure sensor 8 abuts against the top of the molding chamber 2 to generate a pressure signal. The pressure sensor 8 is electrically connected to the pressure detection component 6-3.

[0050] When the sealing ring 3 is inflated, the sealing ring 3 expands, the pressure sensor 8 detects the pressure signal, and feeds the pressure signal back to the pneumatic control module 6, and the pneumatic control module 6 controls the air intake pressure maintaining component 6-2 and the vacuum generator component 6-1 to operate according to the pressure signal. The present invention is provided with a pressure sensor 8, which can detect the pressure condition in the sealing ring 3, and then monitor the sealing effect. At the same time, the pneumatic control can perform feedback adjustment according to the result of the pressure detection, and the whole process can be automated, which improves the reliability and efficiency of the sealing and reduces the cost of the sealing.

[0051] It should be noted that the sealing structure can also be installed at the bottom of the powder cleaning cylinder 10. The working cylinder 1 and the powder cleaning cylinder 10 are collectively referred to as a cylinder body. When it is necessary to clean the powder of the printed part, the molding cabin is moved to the bottom of the powder cleaning cylinder, and the sealing ring is inflated. The sealing lip expands downward to seal the molding cabin and the powder cleaning cylinder, and then the powder cleaning action can be performed.

[0052] The pressure sensor 8 is preferably a thin film pressure sensor.

[0053] The sealing ring 3 is preferably a silicone or rubber sealing ring.

[0054] Another embodiment of the present invention further provides an inflation sealing method for an additively manufactured molding chamber, based on the sealing structure, comprising the steps of:

[0055] S1, moving the molding chamber 2 to a position opposite to the working cylinder 1 or the powder cleaning cylinder 10;

[0056] S2, controlling the air intake and pressure maintaining assembly of the pneumatic control module 6 to inflate air into the sealed cavity, so that the sealing ring expands to seal the connection position between the molding cabin and the working cylinder or the powder cleaning cylinder 10, and the construction space becomes a sealed construction space;

[0057] S3, the additive manufacturing equipment performs construction printing or powder cleaning in the sealed construction space. After the construction printing or powder cleaning is completed, the vacuum generator component of the pneumatic control component is controlled to extract air from the sealed cavity to form a vacuum in the sealed cavity, the sealing ring shrinks, and the molding chamber and the working cylinder are separated;

[0058] S4. The molding cabin can be removed from the bottom of the working cylinder or the powder cleaning cylinder 10.

[0059] When air is inflated into the sealing ring 3, the sealing ring 3 expands, the pressure sensor 8 detects a pressure signal, and feeds the pressure signal back to the pneumatic control module 6. The pneumatic control module 6 controls the operation of the air intake pressure maintaining component 6-2 and the vacuum generator component 6-1 according to the pressure signal.

[0060] As a specific example, Figures 5 to 8 As shown, the sealing method comprises the steps of:

[0061] S1, move the molding cabin 2 to the bottom of the powder cleaning cylinder 10 and align it with the powder cleaning cylinder 10, such as Figure 5 As shown;

[0062] S2, control the air intake pressure maintaining component 6-2 to inflate the sealing ring 3 through the air pipe 7, so that the sealing ring 3 is inflated, and at the same time the pressure detection component 6-3 performs pressure maintenance monitoring, controls the film pressure sensor 8 to monitor the pressure value on the inflatable sealing ring 3 in real time, and feeds back to the control system, the control system determines the real-time state of the silicone rubber inflatable sealing ring 3, when the detected pressure value reaches the pre-trial threshold, the workpiece and metal powder in the molding cabin 2 can be cleaned, such as Figure 5 As shown;

[0063] S3, after cleaning the workpiece and metal powder in the molding chamber 2 and taking out the workpiece, replace the new processing substrate, the control system controls the vacuum generator component 6-1 in the pneumatic control module 6 to exhaust the inflatable sealing ring 3 through the air pipe, and the sealing ring 3 is exhausted until it has no contact with the upper top surface of the molding chamber 2. The system determines the real-time status of the sealing ring 3 and the movable status of the molding chamber 2 through the real-time feedback pressure value of the thin film pressure sensor 8 and the pressure detection component 6-3. Figure 6 As shown;

[0064] S4, the molding chamber 2 moves to the bottom of the working cylinder 1. After the system determines that it is in place, Figure 7 As shown;

[0065] S5, the control system controls the inflation and pressure-maintaining component in the pneumatic control module 6 on the working cylinder 1 to inflate the sealing ring 3 through the air pipe 7, so that the sealing ring 3 is inflated, and the pressure detection component monitors the pressure and feeds back to the system. At the same time, the film pressure sensor monitors the pressure value on the silicone rubber inflatable sealing ring in real time, feeds back to the system, and determines the real-time state of the silicone rubber inflatable sealing ring. After confirmation, the molding chamber can now carry out subsequent laser processing work, such as Figure 8 shown.

[0066] The additively manufactured molding cabin inflation sealing structure provided by the present invention designs a dynamic sealing structure between the two components under the premise that the molding cabin 2 and the working cylinder 1 do not move, and adopts the method of inflating and expanding the hollow silicone rubber sealing ring to seal. The sealing structure has a compact structure and a temperature resistance of more than 220°. An external air source is used to control the expansion and contraction of the silicone rubber sealing ring 3 through pneumatic components and related control methods to achieve the function of sealing or disengagement.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An inflatable sealing structure for an additively manufactured molding cabin, comprising a molding cabin, a cylinder body and a sealing assembly, wherein the cylinder body is fixed and the molding cabin is movable, the molding cabin and the cylinder body are connected to form a building space, and the position where the molding cabin and the working cylinder are connected is sealed by the sealing assembly, characterized in that: The sealing assembly is installed at the bottom of the cylinder body and includes a sealing ring and a pneumatic control module. A sealing cavity is formed inside the sealing ring. The sealing cavity is connected to the pneumatic control module through an air pipe. The pneumatic control module inflates or exhausts air into the sealing cavity to control the expansion and contraction of the sealing ring. When the sealing ring expands, the molding chamber and the cylinder body are sealed and connected. When the sealing ring contracts, the molding chamber and the cylinder body are separated.

2. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 1, characterized in that: An annular groove is provided at the bottom of the cylinder body, and the sealing ring is installed in the annular groove.

3. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 2, characterized in that: The bottom of the sealing ring is a sealing lip, and the top of the sealing ring is fixedly installed in the groove. The expansion and contraction of the sealing ring is achieved by the expansion and contraction of the sealing lip. When the sealing lip expands outward, it abuts against the top of the molding chamber. When the sealing ring contracts, the molding chamber and the cylinder body are separated.

4. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 1, characterized in that: The pneumatic control module includes a vacuum generator assembly and an air intake pressure maintaining assembly. The vacuum generator assembly is used to control the pneumatic control module to inflate air into the sealed cavity, and the air intake pressure maintaining assembly is used to control the pneumatic control module to draw a vacuum into the sealed cavity.

5. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 1, characterized in that: The pneumatic control module includes a pressure detection component. A pressure sensor is installed at the top of the molding chamber at a position in contact with the sealing ring for detecting the internal pressure value of the sealing ring. The pressure sensor is electrically connected to the pressure detection component.

6. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 1, characterized in that: The pressure sensor is installed at the bottom of the sealing ring. When the sealing ring expands, the pressure sensor abuts against the top of the molding chamber to generate a pressure signal. The pressure detection device is electrically connected to the pressure detection component.

7. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 5 or 6, characterized in that: The pressure sensor is a thin film pressure sensor.

8. The inflatable sealing structure of the additive manufacturing molding cabin according to claim 5 or 6, characterized in that: The sealing ring is a silicone or rubber sealing ring.

9. A method for inflating and sealing a molding chamber in additive manufacturing, based on the sealing structure according to any one of claims 1 to 8, characterized in that: Includes steps: Moving the molding chamber to a position opposite to the cylinder body; Controlling the air intake and pressure maintaining component of the pneumatic control component to inflate air into the sealed cavity, so that the sealing ring expands to seal the connection position between the molding cabin and the cylinder body, and the construction space becomes a sealed construction space; The additive manufacturing equipment performs construction printing or powder cleaning in the sealed construction space. After the construction printing or powder cleaning is completed, the vacuum generator component of the pneumatic control component is controlled to extract air from the sealed cavity to form a vacuum in the sealed cavity, the sealing ring shrinks, and the molding cabin and the cylinder body are separated; The molding cabin can be removed from the bottom of the cylinder body or the powder cleaning cylinder.

10. The method for inflating and sealing a molding chamber for additive manufacturing according to claim 9, characterized in that: Also includes the steps: When air is inflated or exhausted into the sealing ring, the sealing ring expands or contracts, and the pressure sensor detects the pressure signal in real time and feeds the pressure signal back to the pneumatic control module. The pneumatic control module controls the action of the air intake pressure maintaining assembly and the vacuum generator assembly according to the pressure signal.

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