阅读说明：本技术 一种基于uv激光固化粉末材料的3d打印成型装置及方法 (3D printing forming device and method based on UV laser curing powder material ) 是由 唐晖 王磊 郝兆洲 于 2021-08-02 设计创作，主要内容包括：本发明公开了一种基于UV激光固化的3D打印成型装置及方法,该3D打印成型装置包括成型腔、用于向成型腔分层铺粉进而形成粉层的铺粉系统、压电喷头打印执行系统和UV激光执行系统,所述的压电喷头打印执行系统用于形成打印数据,并基于打印数据采用压电喷射的方式向粉层喷射粘结剂；所述的UV激光执行系统用于向粉层发出UV激光,对粉层进行局部高温处理,使得粉层内带有粘结剂的区域直接固化。因此,采用基于UV激光固化的3D打印成型装置的3D打印成型方法能够防止粘接剂固化过程的渗透现象,进而避免影响成型零件的尺寸,其产能也较高,适用范围更广。(The invention discloses a 3D printing forming device and a method based on UV laser curing, wherein the 3D printing forming device comprises a forming cavity, a powder laying system, a piezoelectric nozzle printing execution system and a UV laser execution system, wherein the powder laying system is used for laying powder to the forming cavity in a layering manner to form a powder layer; the UV laser execution system is used for emitting UV laser to the powder layer and carrying out local high-temperature treatment on the powder layer, so that the area with the binder in the powder layer is directly cured. Therefore, the 3D printing forming method of the 3D printing forming device based on UV laser curing can prevent the permeation phenomenon in the adhesive curing process, further avoid influencing the size of the formed part, and is high in productivity and wide in application range.)

1. The utility model provides a 3D prints forming device based on UV laser curing powder material, its includes the shaping chamber and is used for spreading the powder and then form the shop's powder system on powder layer to shaping chamber layering, its characterized in that: it still includes:

the piezoelectric nozzle printing execution system is used for forming printing data and spraying the adhesive to the powder layer in a piezoelectric spraying mode based on the printing data;

and the UV laser execution system is used for emitting UV laser to the powder layer and carrying out local high-temperature treatment on the powder layer so that the area with the binder in the powder layer is directly cured.

2. The UV laser curable powder material based 3D printing and forming device according to claim 1, wherein: the piezoelectric nozzle printing execution system comprises:

the modeling unit is used for setting the layer height and carrying out layering processing according to the set layer height to obtain printing data;

the circuit unit is used for reading the printing data, converting the printing data into binary data and sending a control signal to the piezoelectric nozzle based on the binary data;

an ink supply unit for supplying an adhesive to the piezoelectric nozzle;

and the piezoelectric nozzle is used for spraying the adhesive provided by the ink supply unit onto the powder layer based on the control signal of the circuit unit.

3. The UV laser curable powder material based 3D printing and forming device according to claim 2, wherein: the modeling unit comprises:

the layering module is used for importing the printing files and carrying out layering processing on the printing files according to the set layer height to obtain two-dimensional files;

the outline module is used for storing the lines of the outer edge of the geometric figure in the two-dimensional file;

and the BMP module is used for converting each two-dimensional file into a BMP file according to the printing resolution.

4. The UV laser curable powder material based 3D printing and forming device according to claim 3, wherein: the circuit unit includes:

the data module is used for reading the BMP file formed by the modeling unit, converting the BMP file into multi-section binary data according to the spray hole arrangement of the piezoelectric nozzle and sending the binary data to the piezoelectric nozzle;

the waveform module is used for generating a driving waveform corresponding to the piezoelectric nozzle, sending the driving waveform to the piezoelectric nozzle and further driving the piezoelectric nozzle;

the data module and the waveform module are both connected to a nozzle adapter plate, the piezoelectric nozzle is also connected to the nozzle adapter plate, and the nozzle adapter plate is used for transmitting binary data and waveforms.

5. The UV laser curable powder material based 3D printing and forming device according to claim 2, wherein: the ink supply unit comprises a first-stage ink box module, a first-stage filtering module, a second-stage ink box module, a second-stage filtering module and an ink supply nozzle which are sequentially connected, a negative pressure module is further connected to the second-stage ink box module, and the ink supply nozzle is connected to a liquid flow channel of the piezoelectric nozzle.

6. The UV laser curable powder material based 3D printing and forming device according to claim 1, wherein: the UV laser execution system comprises:

a laser for emitting UV laser light;

the laser shaping light path is used for shaping the UV laser into a light curtain;

and the laser alignment structure is used for adjusting the irradiation direction of the light curtain.

7. The UV laser curable powder material based 3D printing and forming device according to claim 6, wherein: the wavelength of the UV laser is 365 nm-415 nm.

8. The UV laser curable powder material based 3D printing and forming device according to claim 6, wherein: the laser shaping light path comprises:

a cylindrical mirror for shaping the laser into a diffused linear beam;

the collimating mirror is used for adjusting the diffused linear light beams into parallel linear light beams;

and the focusing mirror is used for focusing the parallel linear light beams on the printing plane.

9. The UV laser curable powder material based 3D printing and forming device according to claim 1, wherein: a first movable beam and a second movable beam are arranged above the forming cavity, the powder paving system is arranged on the first movable beam, and the first movable beam moves from right to left in the powder paving process; the piezoelectric nozzle printing execution system and the UV laser execution system are arranged on the second moving beam, the UV laser execution system is arranged on the left side of the piezoelectric nozzle printing execution system, and the second moving beam moves from left to right in the process of spraying and curing the adhesive;

the molding cavity is provided with a lifting mechanism for controlling the height of the molding cavity.

10. A3D printing forming method based on UV laser curing powder materials adopts the 3D printing forming device based on UV laser curing powder materials of claim 1, and is characterized in that: which comprises the following steps:

1) a powder layer is paved in the forming cavity by a powder paving system;

2) the piezoelectric nozzle printing execution system forms printing data based on the set layer height, and sprays the adhesive to the powder layer in a piezoelectric spraying mode based on the printing data;

3) the UV laser execution system is started simultaneously along with the piezoelectric nozzle printing execution system, after the binder is sprayed, the UV laser execution system immediately sends UV laser to the powder layer, and the powder layer is subjected to local high-temperature treatment, so that the region with the binder in the powder layer is directly cured;

4) and judging whether the 3D printing is finished or not, if the 3D printing is not finished, lowering the forming cavity by a height of one layer height, returning to the step 1), and if the 3D printing is finished, finishing.

Technical Field

The invention relates to the technical field of 3D printing, in particular to a 3D printing forming device and method based on UV laser curing

Background

The existing 3D printing forming mode mainly comprises: fused Deposition Modeling (FDM), laser powder sintering (SLS), Stereolithography (SLA), three-dimensional printing-bonding modeling (3DP), and Layered Object Manufacturing (LOM). Each molding method corresponds to a respective core execution element, material, process.

In industrial production, the three-dimensional printing, bonding and forming method (3DP) is common, the core component is an ink-jet printing head, and the used materials are generally divided into 2 types, namely powder and liquid; the types of the powder are generally 1-2, and the powder is mixed by a mixing mechanism and then is conveyed into a forming cavity by a powder spreading mechanism; the types of the liquid are generally 1-2, the liquid is conveyed to the spray head by a set of ink supply system, and is sprayed onto the powder by the spray head according to printing data to bond and form the powder. For example, chinese patent CN107599384A discloses a high-efficiency 3DP powder spreading printing apparatus and a powder spreading printing method thereof, wherein the high-efficiency 3DP powder spreading printing apparatus includes two or more than two printing platforms arranged in sequence along the X-axis direction, and each printing platform is correspondingly provided with a rectangular printing area; each printing area shares one powder spreading device and one shared printing head which are arranged above each printing platform; the powder spreading direction of the powder spreading device is the X-axis direction consistent with the arrangement direction of the printing areas, and the printing direction of the printing head is the Y-axis direction perpendicular to the powder spreading direction and can move along the X-axis direction to fold back and forth above each printing area for sequential printing.

Although the 3DP powder-spreading printing equipment improves the printing efficiency, the spraying liquid used in the three-dimensional printing, bonding and forming method is generally adhesive, and the curing process of the adhesive in the powder has a penetration phenomenon, and the penetration can also change along with the change of the external temperature, which can affect the size of the formed part.

The core component of the laser powder sintering method (SLS) is a laser unit (including laser generation, transmission and optical positioning), the material is generally various metal powder, the metal powder is conveyed into a forming cavity by a powder laying mechanism, and laser is deflected to the corresponding position of the powder for hot melting according to an optical path made after parts are sliced. For example, chinese patent CN106003713B discloses an SLS scanning method and a 3D printing method, where the 3D printing method uses an SLS processing technique to perform 3D printing of batch parts, the method includes the following steps: and (3) a data processing stage: setting printing positions of a plurality of parts to be printed according to a mode of sequentially distributing from inside to outside along the direction of the spiral line, and carrying out data slicing processing according to the set printing positions; a scanning stage: scanning each printing position with laser according to the trend from inside to outside order of helix, carrying out the SLS sintering of the equal aspect of different parts on each printing position according to the order, through following the helix is circulated scanning many times and is in order to accomplish the printing of batch part.

Laser powder sintering (SLS) optimizes the quality of all parts in small volume production, but has lower throughput and smaller molding cavity size.

Disclosure of Invention

The invention aims to provide a 3D printing forming device and method based on a UV laser curing powder material, and aims to solve the problems that the size of a formed part is influenced by the penetration phenomenon in the curing process of an adhesive in a three-dimensional printing bonding forming method (3DP) and the productivity of a laser powder sintering method (SLS) is low.

In order to solve the problems, the invention adopts the following technical scheme:

the invention relates to a 3D printing forming device based on UV laser curing powder materials, which comprises a forming cavity and a powder laying system for laying powder to the forming cavity in a layered manner to form a powder layer, and further comprises:

the piezoelectric nozzle printing execution system is used for forming printing data and spraying the adhesive to the powder layer in a piezoelectric spraying mode based on the printing data;

and the UV laser execution system is used for emitting UV laser to the powder layer and carrying out local high-temperature treatment on the powder layer so that the area with the binder in the powder layer is directly cured.

Preferably, the piezojet printing execution system comprises:

the modeling unit is used for setting the layer height and carrying out layering processing according to the set layer height to obtain printing data;

the circuit unit is used for reading the printing data, converting the printing data into binary data and sending a control signal to the piezoelectric nozzle based on the binary data;

an ink supply unit for supplying an adhesive to the piezoelectric nozzle;

and the piezoelectric nozzle is used for spraying the adhesive provided by the ink supply unit onto the powder layer based on the control signal of the circuit unit.

Preferably, the modeling unit includes:

the layering module is used for importing the printing files and carrying out layering processing on the printing files according to the set layer height to obtain two-dimensional files;

the outline module is used for storing the lines of the outer edge of the geometric figure in the two-dimensional file;

and the BMP module is used for converting each two-dimensional file into a BMP file according to the printing resolution.

Preferably, the circuit unit includes:

the data module is used for reading the BMP file formed by the modeling unit, converting the BMP file into multi-section binary data according to the spray hole arrangement of the piezoelectric nozzle and sending the binary data to the piezoelectric nozzle;

the waveform module is used for generating a driving waveform corresponding to the piezoelectric nozzle, sending the driving waveform to the piezoelectric nozzle and further driving the piezoelectric nozzle;

the data module and the waveform module are both connected to a nozzle adapter plate, the piezoelectric nozzle is also connected to the nozzle adapter plate, and the nozzle adapter plate is used for transmitting binary data and waveforms.

Preferably, the ink supply unit comprises a first-stage ink box module, a first-stage filtering module, a second-stage ink box module, a second-stage filtering module and an ink supply nozzle which are connected in sequence, the second-stage ink box module is further connected with a negative pressure module, and the ink supply nozzle is connected to a liquid flow channel of the piezoelectric nozzle.

Preferably, the UV laser performing system includes:

a laser for emitting UV laser light;

the laser shaping light path is used for shaping the UV laser into a light curtain;

and the laser alignment structure is used for adjusting the irradiation direction of the light curtain.

Preferably, the wavelength of the UV laser is 365 nm-415 nm.

Preferably, the laser shaping optical path includes:

a cylindrical mirror for shaping the laser into a diffused linear beam;

the collimating mirror is used for adjusting the diffused linear light beams into parallel linear light beams;

and the focusing mirror is used for focusing the parallel linear light beams on the printing plane.

Preferably, a first movable beam and a second movable beam are arranged above the forming cavity, the powder paving system is installed on the first movable beam, and the first movable beam moves from right to left in the powder paving process; the piezoelectric nozzle printing execution system and the UV laser execution system are arranged on the second moving beam, the UV laser execution system is arranged on the left side of the piezoelectric nozzle printing execution system, and the second moving beam moves from left to right in the process of spraying and curing the adhesive;

the molding cavity is provided with a lifting mechanism for controlling the height of the molding cavity.

The invention also relates to a 3D printing and forming method based on the UV laser curing powder material, which adopts the 3D printing and forming device based on the UV laser curing powder material and comprises the following steps:

1) a powder layer is paved in the forming cavity by a powder paving system;

2) the piezoelectric nozzle printing execution system forms printing data based on the set layer height, and sprays the adhesive to the powder layer in a piezoelectric spraying mode based on the printing data;

3) the UV laser execution system is started simultaneously along with the piezoelectric nozzle printing execution system, after the binder is sprayed, the UV laser execution system immediately sends UV laser to the powder layer, and the powder layer is subjected to local high-temperature treatment, so that the region with the binder in the powder layer is directly cured;

4) and judging whether the 3D printing is finished or not, if the 3D printing is not finished, lowering the forming cavity by a height of one layer height, returning to the step 1), and if the 3D printing is finished, finishing.

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

1. the invention relates to a 3D printing forming device based on UV laser curing powder materials, which comprises a piezoelectric nozzle printing execution system and a UV laser execution system, wherein when the device is used, a bonding agent is provided for a laid powder layer through the piezoelectric nozzle printing execution system, and in the operation process of the piezoelectric nozzle printing execution system, the UV laser execution system operates along with the piezoelectric nozzle printing execution system and is used for emitting UV laser to the powder layer and carrying out local high-temperature treatment on the powder layer, so that the region with the bonding agent in the powder layer is instantly cured, the permeation phenomenon in the curing process of the bonding agent is prevented, and the size of a formed part is further prevented from being influenced.

2. The invention relates to a 3D printing and forming method based on UV laser curing powder materials, which is a printing and forming method adopting a laser powder sintering method (SLS) to assist a three-dimensional printing, bonding and forming method (3 DP); compared with a three-dimensional printing, bonding and forming method (3DP), the method has the advantages that the size of the formed part is more accurate; compared with a laser powder sintering method (SLS), the method has the advantages that the forming cavity is used in a three-dimensional printing, bonding and forming method, the forming cavity is large, the productivity is high, and the application range is wide.

Drawings

Fig. 1 is a schematic structural diagram of a 3D printing and molding device based on UV laser curing of powder materials;

FIG. 2 is a block diagram of a piezo jet printing system;

fig. 3 is a block diagram of a UV laser execution system.

In the figure: 1-forming cavity, 2-powder laying system, 3-piezoelectric nozzle printing execution system, 31-modeling unit, 311-layering module, 312-outline module, 313-BMP module, 32-circuit unit, 321-data module, 322-waveform module, 323-nozzle adapter plate, 33-ink supply unit, 331-first-stage ink box, 332-first-stage filtering module, 333-second-stage ink box, 334-second-stage filtering module, 335-ink supply nozzle, 336-negative pressure module, 34-voltage nozzle, 4-UV laser execution system, 41-UV laser, 42-laser shaping optical path, 43-laser alignment structure, 5-first movable beam, 6-second movable beam, 71-bottom plate, 72-lifting drive, 73-lifting plate.

Detailed Description

For further understanding of the present invention, the present invention will be described in detail with reference to examples, which are provided for illustration of the present invention but are not intended to limit the scope of the present invention.

Example one

Referring to the attached drawing 1, the 3D printing and forming device based on the UV laser curing powder material according to the present invention includes a forming cavity 1, a powder spreading system 2 for spreading powder to the forming cavity 1 in a layered manner to form a powder layer, a piezoelectric nozzle printing execution system 3, and a UV laser execution system 4; wherein, the molding cavity 1 and the powder spreading system 2 are the same as the conventional three-dimensional printing, bonding and 3D printing molding device, and the description is omitted in the embodiment.

Referring to fig. 1 and 2, the piezojet printing execution system 3 is used for forming printing data and spraying adhesive to a powder layer by adopting a piezoelectric spraying mode based on the printing data. The piezojet printing execution system 3 includes a modeling unit 31, a circuit unit 32, an ink supply unit 33, and piezojets 34. The modeling unit 31 is used for setting the layer height and performing layering processing according to the set layer height so as to obtain printing data; the circuit unit 32 is used for reading the printing data, converting the printing data into binary data and sending a control signal to the piezoelectric nozzle based on the binary data; the ink supply unit 33 is used for filtering the bonding agent and supplying the bonding agent to the piezoelectric nozzle; the piezojet 34 sprays the adhesive provided by the ink supply unit 33 onto the powder layer based on the control signal of the circuit unit 32.

Referring to fig. 2, the modeling unit includes a layer module 311, a contour module 312, and a BMP module 313. The model of the layering module 311 is WL-03NLCH, and is used for importing a print file and performing layering processing on the print file according to a set layer height to obtain a two-dimensional file; the outline module 312 adopts a structure disclosed in chinese patent CN 204328521U, and is used for storing the lines at the outer edge of the geometric figure in the two-dimensional file; the BMP module 313 is of a model GY-BMP280, and is configured to convert each two-dimensional file into a BMP file according to a printing resolution.

Referring to fig. 2, the circuit unit 32 includes a data module 321, a waveform module 322, and a nozzle adapter plate 323; the data module 321 adopts a data conversion module with the model number of TXB0108PWR TI, and is used for reading the BMP file formed by the modeling unit 31, converting the BMP file into multiple sections of binary data according to the arrangement of the orifices of the piezojet, and sending the binary data to the piezojet; the waveform module 321 adopts a waveform generating circuit with an ICL8038CCPD model, and is used for generating a driving waveform corresponding to the piezoelectric nozzle, sending the driving waveform to the piezoelectric nozzle, and further driving the piezoelectric nozzle 34; the data module 321 and the waveform module 322 are both connected to a nozzle adapter plate 324, the piezoelectric nozzle 34 is also connected to the nozzle adapter plate 324, and the nozzle adapter plate 323 is used for binary data and waveform transmission.

Referring to fig. 2, the ink supply unit 33 includes a primary ink cartridge module 331, a primary filtering module 332, a secondary ink cartridge module 333, a secondary filtering module 334, and an ink supply nozzle 335 connected in sequence, the secondary ink cartridge module 333 is further connected with a negative pressure module 336, and the ink supply nozzle 335 is connected to a liquid flow channel of the piezoelectric nozzle 34; the negative pressure module 336 makes the inside of the ink supply unit 33 form air pressure difference, so that the adhesive can be output from the primary filter module 332 and enter the ink supply nozzle 335; the adhesive is stored in a primary ink box module 331, the primary filter module 332 filters the adhesive for the first time and transfers the adhesive to a secondary ink box module 333, the secondary filter module 334 filters the adhesive for the second time and transfers the adhesive to an ink supply nozzle 335, and the ink supply nozzle 335 supplies the adhesive to the piezoelectric nozzle 34.

Referring to fig. 1 and 3, the UV laser execution system 4 is configured to emit UV laser to the powder layer, and perform local high-temperature treatment on the powder layer, so that a region with a binder in the powder layer is directly cured, and the size of a molded part is prevented from being affected by a less permeation phenomenon of the binder in a manner of shortening the curing time. The UV laser execution system 4 comprises a laser 41, a laser shaping light path 42 and a laser alignment structure 43; the laser 41 adopts a UV laser with the model of PF-20-UV-AG-LC07-A and is used for emitting UV laser with the wavelength of 365 nm-415 nm; the laser shaping optical path 42 includes a cylindrical mirror for shaping the laser light into diffused linear beams, a collimating mirror for adjusting the diffused linear beams into parallel linear beams, and a focusing mirror for focusing the parallel linear beams on a printing plane, and the laser shaping optical path 42 functions to shape the UV laser light into a light curtain; the laser alignment structure 43 is used to adjust the irradiation direction of the light curtain.

Referring to the attached drawing 1, a first moving beam 5 and a second moving beam 6 are arranged above the forming cavity 1, the initial position of the first moving beam 5 is located at the upper right of the forming cavity 1, and the initial position of the second moving beam 6 is located at the upper left of the forming cavity 1; the powder paving system 2 is arranged on the first movable beam 5, and the first movable beam 5 moves from right to left in the powder paving process; the piezoelectric nozzle printing execution system 3 and the UV laser execution system 4 are installed on the second moving beam 6, the UV laser execution system 4 is arranged on the left side of the piezoelectric nozzle printing execution system 3, and the second moving beam 6 moves from left to right in the process of adhesive spraying and curing. The forming cavity 1 is provided with a lifting mechanism for controlling the height of the forming cavity 1, the lifting mechanism comprises a bottom plate 71, a plurality of lifting drives 72 (motors, cylinders or oil cylinders) arranged above the bottom plate 71 and a lifting plate 73 arranged on the lifting mechanism 72, and the forming cavity 1 is fixed above the lifting plate 73.

Example two

The embodiment relates to a 3D printing and forming method based on a UV laser curing powder material, the method adopts a 3D printing and forming device based on the UV laser curing powder material, and the 3D printing and forming method based on the UV laser curing powder material comprises the following steps:

1) driving the first movable beam 5 to enable the powder laying system 2 to move from right to left, laying a layer of powder layer in the forming cavity 1 by the powder laying system 2, returning to an initial position through the first movable beam 5 after laying a layer of powder layer by the powder laying system 2, laying 3-5 layers in advance at a powder laying speed of 200 mm/s, and ensuring that the powder body on the forming cavity is flat;

2) the second movable beam 6 is driven to move from left to right, at this time, the piezoelectric nozzle printing execution system 3 forms printing data based on the set layer height, and sprays the adhesive to the powder layer by adopting a piezoelectric spraying mode based on the printing data, and the specific steps are as follows:

2.1) storing the adhesive in the primary ink box module 331, and delivering the adhesive (negative pressure of-2.4 KPA) to the voltage spray head 34 through the negative pressure module 336, so as to form a proper meniscus in the spray hole, thereby completing the preparation for filling the adhesive, namely, the primary filtering module 332 filters the adhesive for the first time and delivers the adhesive to the secondary ink box module 333, the secondary filtering module 334 filters the adhesive for the second time and delivers the adhesive to the ink supply spray head 335, and the ink supply spray head 335 provides the adhesive for the piezoelectric spray head 34;

2.2) the layering module 311 in the modeling unit 31 imports the print file and performs layering processing on the print file according to the set layer height to obtain a two-dimensional file; then, the lines of the outer edge of the geometric figure in the two-dimensional file are saved through a contour module; converting each two-dimensional file into a BMP file through a BMP module;

2.3) the data module 321 in the circuit unit 32 interprets the BMP file formed by the modeling unit 31, converts the BMP file into multiple sections of binary data according to the arrangement of the orifices of the piezojet, and sends the binary data to the piezojet; a waveform module 322 in the circuit unit 32 generates a driving waveform corresponding to the piezoelectric nozzle 34, the waveform parameter is 90-2-6-2, and the driving waveform is sent to the piezoelectric nozzle 34 to drive the piezoelectric nozzle 34, so as to selectively spray the adhesive on the layers;

3) UV laser execution system 4 follows piezojet and prints execution system 3 and start simultaneously, after spraying the binder, UV laser execution system 4 sends UV laser to the bisque at once, carries out local high temperature treatment to the bisque for the regional direct (in the twinkling of an eye) solidification that has the binder in the bisque, and then shortens the curing time of binder, avoids the binder infiltration, and its concrete step is:

3.1) the laser 41 emits UV laser with the wavelength of 365 nm-415 nm;

3.2) the laser shaping light path 42 shapes the UV laser into a light curtain, i.e. the cylindrical mirror in the laser shaping light path 42 shapes the laser into diffused linear beams, the collimating mirror in the laser shaping light path 42 adjusts the diffused linear beams into parallel linear beams, the focusing mirror in the laser shaping light path 42 focuses the parallel linear beams on a printing plane, and further shapes the UV laser into a light curtain with a specified irradiation direction;

3.3) adjusting the irradiation direction of the light curtain through the laser alignment structure 43 to make the light curtain fall on the powder layer.

4) After printing of the structure of one layer is finished, judging whether 3D printing is finished completely, if the 3D printing is not finished, descending the forming cavity by a height of one layer (0.3 mm), and returning to the step 1); and if the 3D printing is finished, ending.

The present invention has been described in detail with reference to the embodiments, but the description is only for the preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made within the scope of the present invention shall fall within the scope of the present invention.