阅读说明：本技术 一种3d打印系统及方法 (3D printing system and method ) 是由 夏春光 贺晓宁 于 2021-07-21 设计创作，主要内容包括：本申请涉及一种3D打印系统,包括：膜,用于供打印样本依附而在光学引擎的照射下固化；膜分离器,膜分离器能够沿膜的上表面移动,膜分离器包括：外壳,外壳朝向膜一侧具有第二空腔；第一辊子,可旋转的安装在第二空腔的一侧；第二辊子,可旋转的安装在第二空腔的另一侧；真空源,与外壳连接,用于使第二空腔内产生负压。本发明的有益效果是：膜分离器能够有效将膜从打印样本上分离,过程中不会对膜有损伤,分离效果良好,避免了传统方法设置有氧抑制层带来的误差。(The application relates to a 3D printing system, including: a film for adhering the print sample to be cured under irradiation of the optical engine; a membrane separator movable along an upper surface of the membrane, the membrane separator comprising: the side of the shell facing the membrane is provided with a second cavity; a first roller rotatably installed at one side of the second cavity; a second roller rotatably installed at the other side of the second cavity; and the vacuum source is connected with the shell and is used for generating negative pressure in the second cavity. The invention has the beneficial effects that: the membrane separator can effectively separate the membrane from the printed sample, the membrane cannot be damaged in the process, the separation effect is good, and errors caused by the fact that an oxygen inhibition layer is arranged in the traditional method are avoided.)

1. A3D printing system, comprising:

a film (110) for the print sample to adhere to for curing under illumination by the optical engine (102);

a membrane separator (120), the membrane separator (120) being movable along an upper surface of the membrane (110), the membrane separator (120) comprising:

a housing (122), the housing (122) having a second cavity (122 b) on a side facing the membrane (110);

a first roller (126) rotatably installed at one side of the second cavity (122 b);

a second roller (128) rotatably installed at the other side of the second cavity (122 b);

a vacuum source connected to the housing (122) for generating a negative pressure within the second cavity (122 b).

2. The 3D printing system according to claim 1, wherein the material of the first roller (126) and the second roller (128) is a metallic material, a ceramic material or a combination of metallic and ceramic material, and the surface of the first roller (126) and/or the second roller (128) is coated with a protective layer.

3. The 3D printing system according to claim 1, wherein the first roller (126) and the second roller (128) are both mounted in the second cavity (122 b) through bearings (126 c), and the distance between the inner wall of the second cavity (122 b) and the outer walls of the first roller (126) and the second roller (128) is 50 [ mu ] m-100 [ mu ] m.

4. The 3D printing system of claim 1, further comprising a buffer chamber (124) disposed within the housing (122), the buffer chamber (124) connected on one side to the vacuum source by a vacuum port connection (130) and on another side in communication with the second cavity (122 b).

5. The 3D printing system according to claim 4, wherein the buffer chamber (124) comprises a third cavity (241 a), a plurality of apertures (124 b), the third cavity (241 a) being adjacent to the second cavity (122 b) and communicating through the plurality of apertures (124 b), the plurality of apertures (124 b) being aligned along an axial direction of the first roller (126), and the apertures (124 b) being aligned with a middle of the second cavity (122 b).

6. The 3D printing system according to any of claims 1-5, further comprising:

a resin tank (108) for storing printing consumables;

an optical engine (102) for projecting an image to cure the printing consumables;

and a sample bearing table (112) which is arranged in the resin groove (108) and is connected with a multi-axis driving mechanism.

7. The 3D printing system of claim 6, further comprising: a control computer (106) for controlling the multi-axis drive mechanism, the optical engine (102), the membrane separator (120), membrane (110) activity.

8. The 3D printing system according to claim 6, wherein a lens (104) is provided below the optical engine (102).

9. A3D printing method is characterized by comprising the following steps:

printing a layer of resin on the lower surface of the film;

providing a membrane separator (120), the membrane separator (120) comprising:

a housing (122), the housing (122) having a second cavity (122 b) on a side facing the membrane (110);

a vacuum source connected to the housing (122) for generating a negative pressure within the second cavity (122 b);

the vacuum source is activated and the membrane separator (120) is then moved along the upper surface of the membrane (110) to progressively separate the membrane (110) from the printed layer of resin.

10. The 3D printing method according to claim 9, wherein the membrane separator (120) further comprises a first roller (126) rotatably mounted at one side of the second cavity (122 b);

and a second roller (128) rotatably installed at the other side of the second cavity (122 b).

Technical Field

The invention relates to a 3D printing technology, in particular to a 3D printing system and a method convenient for separating a membrane.

Background

Stereolithography was originally recognized as a rapid prototyping technique and refers to a range of techniques used to create true scale models of a production part directly from Computer Aided Design (CAD) in a rapid (faster than before) manner. Since its concept was created and disclosed in us patent 4575330, stereolithography has not only helped engineers greatly, but has also enabled visualization of complex three-dimensional part geometries, detection of errors in prototype schematics, testing of critical components, and verification of theoretical designs at a lower cost and in a shorter time frame, among many other functions.

Over the past decades, the advent of micro-stereolithography (μ SL) has been fueled by continued improvements in the field of micro-electro-mechanical systems (MEMS), a technique that inherits the basic principles of traditional stereolithography, but has very high spatial resolution. K Ikuta and K Hirowataris were described above in "true three-dimensional micromachining using stereolithography and metal forming" at the 6 th IEEE mems seminar in 1993. The resolution of the mu SL is further improved to be less than 200 nm with the aid of single-photon polymerization and two-photon polymerization technologies. S, Maruo and k, Ikuta in appl. phys. lett., vol. 76, 2000 "three-dimensional microfabrication using single-photon crystals", j. MEMS in vol. 7, pp. 411 "near infrared light polymerization for two-photon absorption for two-dimensional microfabrication", and s, Kawata, h.b. Sun, t. Tanaka, and k. Takada in Nature, vol. 412, pp. 697, 2001 "filler Features for Functional micro devices" all express the above viewpoints.

The speed of microlithography SL is greatly increased as projection microlithography (P. mu. m on Design, Test and Microlithography of MEMs/MOEMs) is developed by Bertsch et al, Microsystem Technologies, pp. 42-47, 1997, "Microstereolithography using a Liquid Crystal Display as Dynamic Mask-Generator," herein and by Beluze et al, Proceedings of SPIE, v3680, n2, pp. 808, 1999, "Microstereolithography: A New Process to Build Complex 3D Objections, Symposi. mu. m on Design, Test and Microlithography, MS/MOEMs. At the heart of this technology is a high-resolution spatial light modulator, which can be a Liquid Crystal Display (LCD) panel or a Digital Light Processing (DLP) panel, both of which are available from the microdisplay industry.

Although the projection micro-stereolithography (P μ SL) technique has been successful in providing fast manufacturing speeds with good resolution, further improvements are still needed. There are three types of resin layer definition methods in projection micro-stereolithography (P μ SL): the first uses a free surface, where the layer thickness is defined by the distance between the resin free surface and the sample stage. Due to the slow viscous movement of the resin, it takes more than half an hour to determine a resin layer of 10 μm thickness and a viscosity of 50 cPs when the print area is larger than 1cm x 1 cm. The second and third methods use transparent films or hard windows. The materials of the films and rigid windows in some printing methods are gas permeable, typically oxygen permeable, such as PDMS or Teflon AF, so that the gas permeable material forms a photopolymerization inhibiting layer or so-called "dead zone" in CLIPS technology. The film did not stick to the printed sample due to the oxygen inhibited layer. However, the thickness of the suppression layer is 20 to 50 microns, which may be a large source of dimensional error in precision 3D printing, as tolerance requirements in printing may be at the same or even smaller level. On the other hand, the flow resistance due to the higher viscosity of the resin significantly reduces the printing speed, especially for dense parts without internal channel connections, due to the thicker oxygen barrier layer. It can be seen that in precision 3D printing, the provision of an oxygen inhibited layer to facilitate separation of the membrane can introduce certain errors.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the 3D printing system and method for facilitating the separation of the membrane are provided for solving the problem that certain errors are caused by the fact that the oxygen inhibition layer is arranged to facilitate the separation of the membrane in the prior art.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a 3D printing system, comprising:

a film for adhering the print sample to be cured under irradiation of the optical engine;

a membrane separator movable along an upper surface of the membrane, the membrane separator comprising:

a housing having a second cavity on a side of the housing facing the membrane;

a first roller rotatably installed at one side of the second cavity;

a second roller rotatably installed at the other side of the second cavity;

a vacuum source connected to the housing for generating a negative pressure within the second cavity.

Preferably, in the 3D printing system of the present invention, the material of the first roller and the second roller is a metal material, a ceramic material, or a metal and ceramic combined material, and the surface of the first roller and/or the second roller is coated with a protective layer.

Preferably, in the 3D printing system of the present invention, the first roller and the second roller are both mounted in the second cavity through bearings, and a distance between an inner wall of the second cavity and outer walls of the first roller and the second roller is 50 μm to 100 μm.

Preferably, the 3D printing system of the present invention further includes a buffer chamber disposed in the housing, one side of the buffer chamber is connected to the vacuum source through a vacuum port connection, and the other side is communicated with the second cavity.

Preferably, in the 3D printing system of the present invention, the buffer chamber includes a third cavity and a plurality of openings, the third cavity is adjacent to the second cavity and is communicated with the second cavity through the plurality of openings, the plurality of openings are arranged along the axial direction of the first roller, and the openings are aligned with the middle of the second cavity.

Preferably, the 3D printing system of the present invention further comprises:

the resin tank is used for storing printing consumables;

an optical engine for projecting an image to cure the printing supplies;

and the sample bearing table is arranged in the resin tank and is connected with a multi-axis driving mechanism.

Preferably, the 3D printing system of the present invention further comprises: a control computer for controlling the multi-axis drive mechanism, the optical engine, the membrane separator, and the membrane activity.

Preferably, in the 3D printing system of the present invention, a lens is disposed below the optical engine.

A 3D printing method, comprising the steps of:

printing a layer of resin on the lower surface of the film;

providing a membrane separator comprising:

a housing having a second cavity on a side facing the membrane;

a vacuum source connected to the housing for generating a negative pressure within the second cavity;

the vacuum source is activated and the membrane separator is then moved along the upper surface of the membrane to progressively separate the membrane from the printed layer of resin.

Preferably, in the 3D printing method of the present invention, the film separator further includes a first roller rotatably installed at one side of the second cavity;

and a second roller rotatably installed at the other side of the second cavity.

The invention has the beneficial effects that:

the membrane separator can effectively separate the membrane from the printed sample, the membrane cannot be damaged in the process, the separation effect is good, and errors caused by the fact that an oxygen inhibition layer is arranged in the traditional method are avoided.

Drawings

The technical solution of the present application is further explained below with reference to the drawings and the embodiments.

FIG. 1 is a schematic diagram of a 3D printing system provided by the present invention;

fig. 2 is a schematic structural view of a membrane separator of the 3D printing system of the present embodiment;

FIG. 3 is a schematic side view of the structure of the membrane separator of the present embodiment;

FIG. 4 is a schematic view of the principle of membrane separation of the present embodiment;

fig. 5 is a flowchart of a 3D printing method of the present embodiment;

fig. 6 is a schematic diagram of a 3D printing system process state of the 3D printing method of the present embodiment.

The reference numbers in the figures are:

100 printing system

102 optical engine

104 lens

106 control computer

108 resin tank

108a first cavity

108b resin

110 film

112 sample carrying platform

112b second cavity

120 membrane separator

122 outer casing

122a inner surface

122b second cavity

122c curved surface portion

122e first space

124 buffer chamber

124a third cavity

124b orifice

126 first roller

126c bearing

126d small gap

128 second roller

130 vacuum port connection

140 print the sample.

Detailed Description

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.

In the description of the present application, it is to be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in the orientation or positional relationship indicated in the drawings for convenience in describing the present application and for simplicity in description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed in a particular orientation, and be operated in a particular manner, and are not to be considered limiting of the scope of the present application. Furthermore, the terms "first", "second", etc. are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first," "second," etc. may explicitly or implicitly include one or more of that feature. In the description of the invention, the meaning of "a plurality" is two or more unless otherwise specified.

In the description of the present application, it is to be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present application can be understood by those of ordinary skill in the art through specific situations.

The technical solutions of the present application will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

Examples

The present embodiments provide a more reliable 3D printing system and method that can separate the film from the printed sample at a faster rate during the P μ SL process. The method of the present embodiment is not limited to P μ SL, but is also effective for any other printing system that uses a film to assist 3D printing. In one non-limiting embodiment, the method of this example uses two-roll membrane separation in combination with the application of transparent membranes and vacuum. This method not only gently separates the film from the printed sample in the P μ SL system, but also inserts a layer of printing material between the film and the printed sample. At the same time, ambient dust and dirt contaminating the film will be drawn away due to the vacuum operation to protect the film from damage and maintain its optical clarity. The printing material in the present invention is typically a resin, such as a photocurable resin or mixture thereof with solid particles commonly used in the industry to print and cure build-up layers in 3D printing operations.

The roll membrane separator described in this example has at least one roll, typically made of metal or ceramic, with a 50 μm to 100 μm thick silicon or rubber surface coating. In this embodiment, the membrane separator has two parallel rollers (first roller 126, second roller 128) of 5mm diameter and 7.5mm apart. Generally, the smaller the diameter of the roll, the better. The gap between the first roller 126 and the second roller 128 is typically between 1.5 and 2 times the diameter of the rollers to produce sufficient film deflection to separate the samples.

The 3D printing system of the present embodiment includes an optical engine, such as a DLP or LCD having a light source; laser beams for projection micro-stereolithography or with turning mirrors for Stereolithography (SLA); a lens for defining a magnification of a pixel size of the printing surface; a two-roll membrane separator on top of the membrane to separate the membrane from the printed sample; the three precision platforms with the precision of 1 mu m are used for controlling the movement of the printing substrate; to support a print substrate to print a sample in X, Y and the Z direction or to print an optical projection system; and a resin groove under the film. The system is arranged relative to the base plate of the substrate (e.g. sample holder or sample) such that the lens is located between the surface of the sample holder 112 and the optical engine 102.

Preferably, in this embodiment, the membrane 110 is made of durable PFA (perfluoroalkoxyalkane) or FEP (fluorinated ethylene propylene). The film thickness is 50 μm or 100 μm and may be repeatedly deformed during the roll film separation scanning. Therefore, the resistance of the material to deformation is of paramount importance. Other materials may also be used, such as gas permeable Teflon AF (Teflon AF) from Dupont, whose photo-polymeric oxygen inhibition may further reduce the separating force.

Referring to fig. 1-4, a 3D printing system 100 is disclosed that includes an optical engine 102, a lens 104, a control computer 106, a resin tank 108, a membrane 110, a sample holder 112, and a membrane separator 120.

The optical engine 102 is electrically connected to communicate with a control computer 106 and a lens 104. The control computer 106 includes a processor (not shown) and a memory (not shown) that may be coupled to the processor. The memory stores instructions that, when executed by the processor, may transmit an image to the optical engine 102 and then emit from the lens 104. As described above, the optical engine 102 may be a Digital Light Processing (DLP) projector, a Liquid Crystal Display (LCD) with a light source, or the like. In an embodiment, the optical engine 102 may include a laser, which may include a turning mirror or the like. It will be appreciated that the lens 104 determines the magnification of the pixel size on the print surface.

The resin tank 108 may be any container capable of holding resin or other substances used in the stereolithography process. In this way, the resin tank 108 defines a first cavity 108a in the interior thereof for holding the resin 108b therein.

The film 110 may be an optically clear film and may be formed of a durable Perfluoroalkoxyalkane (PFA) or Fluorinated Ethylene Propylene (FEP), although other suitable materials are also contemplated. The film 110 has a thickness of 50 μm to 100 μm and may be formed of an elastic material. In this way, the film 110 undergoes repeated deformation during use, and therefore, deformation durability (over 10k cycles) is a key property of the material. In an embodiment, the film 110 may be formed of breathable teflon AF manufactured by dupont, which may further reduce the separating force from the cured resin due to oxygen inhibition of photopolymerization.

The sample holder 112 is a platform translatably supported within the first cavity 108a of the resin tank 108. In this way, the sample stage 112 translates in the Z-direction toward and away from the lens 104 (e.g., the vertical direction). The platform supports the 3D printed sample 140 because the resin 108b in the resin tank 108 is polymerized and cured by the image from the optical engine 102 and lens 104. Specifically, after each layer of the printed sample 140 is formed, the sample holder 112 is translated away from the lens 104, causing a portion of the amount of fresh resin 108b to flow between the finished layer of the printed sample 140 and the film 110. The sample holder 112 may be formed of any suitable material that can be used in a stereolithography process and may be of any suitable profile, such as circular, square, rectangular, etc.

The membrane separator 120 (fig. 2-4) includes a housing 122, a buffer chamber 124, a first roller 126, a second roller 128, and a vacuum port connection 130. The housing 122 preferably has a generally rectangular profile, but may also be oval, square, circular, etc. The housing 122 includes an inner surface 122a that defines a second cavity 122b inside the housing 122. The inner surface 122a of the housing also defines a pair of curved portions 122c that match the outer profile of the first roller 126 and the second roller 128.

The buffer chamber 124 is disposed inside the housing 122 and defines a generally open third cavity 124a therein. A plurality of orifices 124b extend through the housing 122 to communicate the third cavity 124a of the buffer chamber 124 with the second cavity 122b of the housing. In this manner, the plurality of apertures 124b provide for even distribution of vacuum throughout the length of the housing when vacuum is applied to the housing 122. In the present embodiment, the buffer chamber 124 restricts the flow of air between the buffer chamber 124 and the second cavity 122b of the housing 122 in an array of 10 × 500 μm orifices 124 b. It will be appreciated that the plurality of apertures 124b increases the pressure drop through the housing 122, resulting in a more uniform air flow across the surfaces of the first and second rollers 126, 128 along the length of the membrane separator 120, thereby achieving better pressure uniformity.

The first roller 126 and the second roller 128 are substantially similar, and therefore only the first roller 126 will be described in detail below. Preferably, the first roller 126 is made of metal or ceramic with a 50 μm to 100 μm thick silicone or rubber surface coating (not shown), or any suitable material may be used, or no cost savings may be made and the first roller may be provided with no surface coating. Specifically, the metal or ceramic is much harder than the film 110, and thus the first roller 126 may cause damage to the surface of the film 110 and thus reduce optical transparency, e.g., light transmittance, of the film 110. Thus, the coating may protect the film 110 from scratching by the hard metal or ceramic of the first roller 126, while helping to seal the gas at the point of contact between the first roller 126 and the film 110. The protective layer of the first roller 126 may be a radially stretched tube or formed during a coating process such as dip coating or vapor deposition. The protective coating can significantly increase the static coefficient of friction between the first roller 126 and the film 110 by up to 10 times.

The first roller 126 is rotatably supported within the second cavity 122b of the housing by a pair of bearings 126c (fig. 2), the bearings 126c being disposed within a portion of the inner surface 122a of the housing. The pair of bearings 126c may be ball bearings, roller bearings, bushings, oil bearings, or the like. In this embodiment, the bearing 126c is selected to have a diameter of 5 mm. A pair of bearings 126c ensure that the first roller 126 rolls only on the film 110 without slipping, and the bearings 126c and roller surface coating cooperate to avoid scratching the film 110.

The metal or ceramic core material maintains the rigidity of the first roller 126 and maintains a small gap of 50 μm to 100 μm between the first roller 126 and the inner surface 122a of the outer casing 122 when the first roller 126 rolls on the film 110 under vacuum. In this manner, the first roller 126 is rotatably supported within the second cavity 112b of the housing 122, forming a small gap 126d (fig. 2 and 4) between the outer surface of the first roller 126 and the inner surface 122a of the housing 122. In the present embodiment, it is preferable that the small gap 126d minimize the amount of air flowing between the outer surface of the first roller 126 and the inner surface 122a of the casing 122 on the premise that the first roller 126 can freely rotate within the casing.

Specifically, the small gap 126d impedes airflow between the inner surface 122a of the housing and the outer surface of the first roller 126 and creates a pressure drop between the top and bottom surfaces of the film 110. The first roller 126 and the second roller 128 cooperate to form a gap of 7.5mm, typically 1.5 to 2 times the diameter of the rollers. As shown in FIG. 4, the first space 122e between the first roller 126 and the second roller 128 causes a pressure drop of 100 to 200Pa between the top surface and the bottom surface of the film 110, and causes the film 110 to deform by bouncing the film 110 upward by 150 to 200 μm, thereby peeling the film 110 off the printed sample 140. As the first roller 126 and the second roller 128 move over the film 110, the film 110 gradually separates from one side of the printed sample 140 to the other.

Parametrically, a first roller 126 and a second roller 128 of 5mm diameter, 12.5mm apart and 104mm long can be used to cover a 100mm by 100mm print area. For a fixed membrane deflection, typically 200 microns, the spacing between the rollers will determine the pressure drop and thus the air flow rate during membrane separation. A larger spacing requires less flow to produce the same amount of deflection. By taking into account the form factor of the film separation, in the present embodiment, the distance between the axial centers of the first roller 126 and the second roller 128 is 12.5mm, typically 2.5 to 3 times the roller diameter; the width of the first space 122e is 7.5mm, typically 1.5 to 2 times the roll diameter.

Pressure is related to flow rate as shown by the bernoulli equation:

ρv²+2P=Const

here, ρ is density, v is flow velocity, and P is pressure. This equation shows that the pressure is lower when the mach number along the flow streamlines is lower in the higher velocity region. The bottom of the membrane 110 is subjected to a uniform atmospheric pressure, but the pressure at the top of the membrane 110 depends on the local flow rate of the air. Therefore, it is important to have a uniform flow velocity along the membrane separator 120, in embodiments where the membrane separator 120 is 104mm long. According to the invention, the third cavity 124a of the membrane separator 120 is connected to a vacuum source (not shown) by a vacuum port connection 130 having a diameter of 5 mm.

Referring to fig. 1-6, there is provided a method of printing a 3D sample including generating a 3D model in a control computer 106, and then segmenting the generated 3D model into a sequence of images, wherein each image represents a layer (typically 5-20 μm thick) of the generated 3D model (S100). The control computer 106 sends an image to the optical engine 102 and the image is projected through the lens 104 onto the bottom surface (which may be a wet surface) of the film 110 (as shown in step S102). The bright areas of the projected image on the resin immediately below the film 110 are polymerized and cured while the dark areas remain liquid (as shown in step S104). After one layer is cured, the membrane separator 120 is parked away from the print sample 140 and a vacuum environment is created to hold the membrane 110 against the first roller 126 and the second roller 128 (as shown in step S106). Thereafter, the sample holder 112 is moved downward (i.e., away from the lens 104) by 0.5mm to 2mm to create space for the resin 108b to flow under the film 110 behind the first roller 126 during the film separation process (as shown in step S108). The membrane separator 120 translates from one side of the membrane 110 to the other at a speed of 5 to 20mm/S, the speed of the membrane separator being determined by the air flow rate between the membrane and the printed sample, while leaving a layer of fresh resin 108b between the membrane 110 and the printed sample 140 (as shown in step S110). The speed should be slow enough to allow fresh resin to fill the gap after membrane separation. Next, the sample holder 112 is moved upwards (i.e. in the direction of the lens 104) to define the thickness of the next layer, and the membrane separator 120 is returned to the original position (as shown in step S112). When the film 110 is flattened by film tension or other coating techniques, such as the roll-to-roll film coating technique of Boston Micro Fabrication, the above process is repeated and the next layer of image is projected on the film 110 (as shown in step S114). The above steps are repeated until a complete printed sample 140 is replicated in the resin tank 108.

While several embodiments of the invention are illustrated in the drawings, the invention is not intended to be limited thereto, since the scope of the invention is as broad as the art will allow and this specification should be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments.

In the drawings and the foregoing description, terms such as front, back, up, down, top, bottom, and the like, and terms of similar orientation, have been used for convenience of description only and are not intended to limit the present invention. In the description above, well-known functions or constructions are not described in detail to avoid obscuring the invention in unnecessary detail.