Patent Publication Number: US-7220073-B2

Title: Slide-type writing instrument with a dry prevention unit

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to writing instruments, and more particularly to a slide-type writing instrument with a dry prevention unit, which is constructed so that a nib which dispenses ink supplied from a cartridge is projected out only when the writing instrument is in use, and the nib is retracted into a body of the writing instrument to be sealed in the body when the writing instrument is not in use. 
   2. Description of the Related Art 
   Generally, writing instruments are typically classified into fixed-type writing instruments, rotary-type writing instruments, knock-type writing instruments, and slide-type writing instruments. The fixed-type writing instruments are designed so that a cartridge is fixed in a shaft and a cap is used to cover a nib. The rotary-type writing instruments are designed so that a part of a shaft rotates to make a part of a cartridge be projected out. The knock-type writing instruments are designed so that a cartridge is projected out by a spring when a part of a shaft is pressed. Further, the slide-type writing instruments are designed so that a cartridge slides to be retracted into and projected out of a shaft. 
   The slide-type writing instruments have an advantage in that it is unnecessary to open or close an additional cap. However, the slide-type writing instruments have a problem in that a nib hole is formed on an end of the writing instrument, so that such a slide-type structure may be limitedly applied to only non-volatile writing instruments, such as oil-based ink. 
   Thus, writing instruments having high volatility, for example, a marker pen, a correction pen, a roller ball pen, a highlight pen, etc., must have caps, although it is inconvenient to open or close the caps. The reason why the writing instruments having high volatility have the caps is that ink of the nibs is dried up when the nibs of the writing instruments are exposed to the air, thus shortening the life spans of the writing instruments. 
   In order to solve the problems, there have been efforts to develop a writing instrument which prevents ink of the writing instrument using liquid or semi-liquid ink, or volatile or non-volatile ink from being dried up, while protecting a nib of the writing instrument. 
   In Japanese Patent No. 1987-0012570, there is proposed a writing instrument titled ‘capless writing instrument with dry prevention unit’. The writing instrument disclosed in the document has a slide-type structure, and prevents a nib from being dried up while protecting the nib. When a user presses a push-button of the writing instrument once, a cover is opened to open a nib hole by a tensile force of an elastic cord. Further, when the user presses the push-button once more, the cover is closed, thus preventing ink from being dried up. 
   However, the writing instrument according to Japanese Patent No. 1987-0012570 has problems in that it uses the tensile force of the elastic cord, so that the opening or closing of the cover is not rapidly executed, and it has low durability, and further, a sealing effect of the cover to temporarily close a nib hole is very poor. 
   Furthermore, there is a proposed writing instrument in Korean U.M. Registration No. 172486, registered in 1999 and titled ‘slide-type writing instrument with a tip protective unit’. According to Korean U.M. Registration No. 172486, the writing instrument is provided with the tip protective unit to prevent a nib from being dried up. In this case, the tip protective unit seals the tip of the nib while the products are transported and marketed, thus preventing ink from being dried up. 
   However, the writing instrument according to Korean U.M. Registration No. 172486 has a problem in that the tip protective unit must be discarded during the use of the writing instrument, so that the writing instrument loses a dry prevention function. Further, the writing instrument may be limitedly applied to a ball-point pen. 
   Furthermore from Korean U.M. Registration No. 174279, which was registered in 1999, there is a known nib dry prevention unit. According to Korean U.M. Registration No. 174279, when a push-button of a slide-type writing instrument is pressed, the nib passes through a cut slit of a rubber packing to be exposed to the atmosphere, so that a user can write with the writing instrument. Meanwhile, when the push-button is released or is pressed once more, the nib returns to an original position thereof, and the cut slit is closed by elasticity of the rubber packing, thus preventing ink from being dried up. 
   However, the writing instrument according to Korean U.M. Registration No. 174279 has a problem in that plastic deformation of the cut slit may occur due to frequent use of the writing instrument. The writing instrument has another problem in that it is difficult to seal the push-button, which executes a sliding motion, so that sealing efficiency is low. 
   In Korean Patent Application No. 10-2000-65693 there is a proposed ‘writing instrument with inseparable elastic cap’. According to Korean Patent Appln. No. 10-2000-65693, the writing instrument is provided with an elastic cap. The elastic cap has, at a predetermined position thereof, a cut slit through which a nib passes. Further, a predetermined portion of the elastic cap, which is opposite to the cut slit, is in close contact with a guide groove of the writing instrument. Thus, when the cap moves backward, the nib is exposed to an outside so that a user writes with the instrument. Conversely, when the cap moves forward, the cap prevents the drying of ink. A middle portion of the writing instrument has the same shape as a body of a usual writing instrument. 
   However, the writing instrument according to Korean Patent Appln. No. 10-2000-65693 has a problem in that a portion around the cut slit may be stained with ink, because the nib is exposed to the outside through the cut slit. Further, since the cut slit is closed by a subsidiary unit, such as a rubber ring which may be easily elastically deformed, the nib is in direct contact with the cut slit and thereby is broken or damaged, and durability of the writing instrument is relatively low. The writing instrument is problematic in that the rubber ring is exposed to the outside of the cut slit, so that the rubber ring may be damaged when the rubber ring is in contact with an external object. Further, the cut slit may, be deformed due to frequent use of the writing instrument, so that efficiency of sealing the nib may be deteriorated. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a slide-type writing instrument having a dry prevention unit which is rapidly opened or closed in a direct transmission manner and is convenient to use, thus allowing an end of a cartridge, that is, a nib to be extended and exposed to an outside through a non-contact extension operation, while sealing a portion around the nib. Further, the slide-type writing instrument with the dry prevention unit prevents ink of the nib from being dried up without using any cap, and safely protects the nib. 
   Another object of the present invention is to provide a slide-type writing instrument with a dry prevention unit, in which an O-ring allows for a sliding motion of a nib extension part and accomplishes a sealing effect without the necessity of forming any slit on the nib extension part, thus having excellent sealing capacity. 
   In order to accomplish the above object, the present invention provides a slide-type writing instrument with a dry prevention unit, including a shaft to provide a body of the writing instrument, with a nib hole provided at a lower end of the shaft; a knock part inserted into an insert hole provided at an upper end of the shaft; a cartridge inserted into the shaft while being coupled at opposite ends thereof to the knock part and a nib, respectively; first and second springs coupled to the cartridge to elastically bias the cartridge in an axial direction of the cartridge; a link holder provided in the shaft and supported by the first spring; a link slidably inserted into the link holder and supported by the second spring, the link including a link projection to engage with a guide groove provided on a predetermined portion of the cartridge; and a spherical door having a pin slit to engage with a parallel pin of the link. 
   Further, in order to accomplish the above object, the present invention provides a slide-type writing instrument with a dry prevention unit, including a knock part to control a nib so that the nib is projected out of or retracted into a nib hole provided at an end of a shaft; a cartridge inserted into the shaft while being coupled at opposite ends thereof to the knock part and the nib, respectively; a spring provided in the shaft to elastically restore the cartridge to an original position thereof; an O-ring provided in the shaft to be supported by the spring; a link holder partially inserted into the O-ring; a link slidably coupled to the link holder, and having a plurality of elastic pieces to engage with a clutch part of the cartridge within an elastic range of the elastic pieces; and a spherical door having a pin slit to engage with a parallel pin of the link. 
   Furthermore, in order to accomplish the above object, the present invention provides a slide-type writing instrument with a dry prevention unit, including a knock part to control a nib so that the nib is projected out of or retracted into a nib hole provided at an end of a shaft; a cartridge inserted into the shaft while being coupled at opposite ends thereof to the knock part and the nib, respectively; a first spring provided in the shaft to elastically restore the cartridge to an original position thereof; an O-ring provided in the shaft to be supported by the first spring; a link holder provided in the shaft while an end of the link holder being axially inserted into the O-ring; a link supported in the link holder by a second spring, and having a plurality of elastic pieces to engage with a clutch part of the cartridge; and a spherical door having a pin slit to engage with a parallel pin of the link. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1   a  and  1   b  are perspective views to show an exterior of a slide-type writing instrument with a dry prevention unit, according to a first embodiment of the present invention; 
       FIG. 2  is an exploded perspective view to show an interior of the writing instrument of  FIG. 1   a;    
       FIGS. 3   a  and  3   b  show a shaft to provide a body of the writing instrument of  FIG. 2 , in which  FIG. 3   a  is a front view of the shaft and  FIG. 3   b  is a sectional view taken along the line A—A of  FIG. 3   a;    
       FIGS. 4   a  and  4   b  show a knock part included in the writing instrument of  FIG. 2 , in which  FIG. 4   a  is a front view of the knock part and  FIG. 4   b  is a sectional view taken along the line B—B of  FIG. 4   a;    
       FIGS. 5   a  through  5   d  show a cartridge to provide an ink reservoir of the writing instrument of  FIG. 2 , in which  FIG. 5   a  is a front view of the cartridge, and  FIGS. 5   b,    5   c,  and  5   d  are sectional views taken along the lines C—C, D—D, and E—E of  FIG. 5   a,  respectively; 
       FIG. 6   a  is a perspective view to show a rear part of a link holder included in the writing instrument of  FIG. 2 ; 
       FIGS. 6   b  through  6   d  show the link holder of  FIG. 6   a,  in which  FIGS. 6   b  and  6   c  are a front view and a left side view of the link holder of  FIG. 6   a,  respectively, and  FIG. 6   d  is a sectional view taken along the line F—F of  FIG. 6   c;    
       FIG. 7   a  is a perspective view to show a rear part of a link included in the writing instrument of  FIG. 2 ; 
       FIGS. 7   b  and  7   c  show the link of  FIG. 7   a,  in which  FIG. 7   b  is a side view of the link and  FIG. 7   c  is a sectional view taken along the line G—G of  FIG. 7   b;    
       FIGS. 7   d  and  7   e  show the link of  FIG. 7   a,  in which  FIG. 7   d  is a plan view of the link and  FIG. 7   e  is a sectional view taken along the line H—H of  FIG. 7   d;    
       FIG. 7   f  is a front view of the link of  FIG. 7   a;    
       FIG. 8   a  is a perspective view to show an interior of a spherical door included in the writing instrument of  FIG. 2 ; 
       FIGS. 8   b  and  8   c  show the spherical door of  FIG. 8   a,  in which  FIG. 8   b  is a side view of the door and  FIG. 8   c  is a sectional view taken along the line I—I of  FIG. 8   b;    
       FIGS. 8   d  through  8   f  show the spherical door of  FIG. 8   a,  in which  FIG. 8   d  is a plan view of the door and  FIGS. 8   e  and  8   f  are sectional views taken along the line J—J and K—K of  FIG. 8   d,  respectively; 
       FIG. 8   g  is a front view of the spherical door of  FIG. 8   a;    
       FIGS. 9   a  and  9   b  show the writing instrument of  FIG. 1   a,  in which  FIG. 9   a  is a side view of the writing instrument and  FIG. 9   b  is a sectional view taken along the line L—L of  FIG. 9   a  to illustrate an assembled state of the writing instrument; 
       FIGS. 10   a  through  10   c  are schematic perspective views to illustrate a forward operation of an internal operating module of the writing instrument shown in  FIG. 2 , except the shaft; 
       FIGS. 11   a  through  11   c  are detailed sectional views to illustrate the retraction and projection of a nib included in the writing instrument of  FIG. 2 ; 
       FIG. 12  is an exploded perspective view of a slide-type writing instrument with a dry prevention unit, according to a second embodiment of the present invention; 
       FIG. 13  is a sectional view of a cartridge included in the writing instrument of  FIG. 12 ; 
       FIG. 14  is a sectional view of an O-ring included in the writing instrument of  FIG. 12 ; 
       FIG. 15  is a sectional view of a link holder included in the writing instrument of  FIG. 12 ; 
       FIG. 16  is a sectional view of a link included in the writing instrument of  FIG. 12 ; 
       FIG. 17  is a sectional view of a spherical door included in the writing instrument of  FIG. 12 ; 
       FIGS. 18   a  through  18   m  are views to illustrate assembly and operations of the writing instrument of  FIG. 12 ; 
       FIG. 19  is an exploded perspective view of a slide-type writing instrument with a dry prevention unit, according to a third embodiment of the present invention; and 
       FIGS. 20   a  through  20   f  are views to illustrate assembly and operations of the writing instrument of  FIG. 19 , in which  FIGS. 20   a,    20   c,  and  20   e  are side views of the writing instrument,  FIG. 20   b  is a sectional view taken along the line S—S of  FIG. 20   a,    FIG. 20   d  is a sectional view taken along the line T—T of  FIG. 20   c,  and  FIG. 20   f  is a sectional view taken along the line U—U of  FIG. 20   e.    
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. 
   Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. 
   &lt;First Embodiment&gt; 
     FIGS. 1   a  and  1   b  show a writing instrument, according to a first embodiment of the present invention. 
   The writing instrument includes a shaft  10 , which provides a single body or a body divided into two parts. A dry prevention unit operated in a direct transmission manner is installed in the shaft  10 . 
   According to the present invention, the dry prevention unit means a unit that is operated in a direct transmission manner to selectively open or close a spherical door  90 . That is, when the writing instrument is not in use, the spherical door  90  closes a nib hole  11  of the shaft  10 . However, when a user manipulates a switch  20 , a force is applied to the knock part  30  and directly transmitted to the spherical door  90 , thus opening the spherical door  90 . 
   The nib hole  11  is formed at a cone-shaped lower end of the shaft  10 , while the knock part  30  is coupled to an upper end of the shaft  10 . 
   That is, the user holds the shaft  10  of the writing instrument by one hand, and then presses the knock part  30  provided at an upper end of the writing instrument, corresponding to a pressing part, by the user&#39;s thumb. Subsequently, the switch  20  is rotated by a predetermined rotating angle to be stopped by a stepped slot  13  which is provided at an inside end of a shaft guide slit  12 . 
   Meanwhile, when the user presses the knock part  30 , the spherical door  90  operated in conjunction with a cartridge  40  having the knock part  30  rotates at a rotating angle of +90° in an opening direction so that a passage of the spherical door  90  is aligned with a nib  41  to open the nib hole  11  of the shaft  10 . 
   Thereafter, the nib  41  is projected out of the nib hole  11  through the passage of the spherical door  90 , so that the user can write with the writing instrument. 
   Conversely, when the user rotates the switch  20  by a predetermined rotating angle in a reverse direction, the switch  20  is removed from the stepped slot  13  of the shaft guide slit  12 . Further, the nib  41  returns to an original position thereof, that is, is retracted into the shaft  10 , by first and second springs  60  and  65  provided in the shaft  10 . 
   Subsequently, the spherical door  90  rotates at a rotating angle of −90° in a closing direction by the first and second springs  60  and  65  so that the passage of the spherical door  90  is perpendicular to the nib  41 , and simultaneously, the nib hole  11  of the shaft  10  is closed. 
   The writing instrument according to the present invention will be described in the following with reference to  FIG. 2 , and  FIGS. 3 through 8   g.  In this case,  FIG. 2  is an exploded perspective view to show internal elements of the writing instrument of this invention, and  FIGS. 3 through 8   g  show the internal parts in detail. 
   The dry prevention unit of this invention means a unit that opens or closes the nib hole  11  of the shaft  10  which will be described in the following in detail. Further, several parts, including the switch  20 , the knock part  30 , the cartridge  40 , an O-ring  50 , the first and &#39;second springs  60  and  65 , a link holder  70 , a link  80 , and the spherical door  90 , are organically assembled with the shaft  10 , and the constructions and operations of the above-mentioned parts will be described in the following. 
   Referring to  FIGS. 2 ,  3   a  and  3   b,  the shaft  10  to provide the body of the writing instrument has a shape of a hollow pipe or tube. 
   The shaft  10  is made of plastics and manufactured through an injection molding process or a molding process. 
   Further, the shaft  10  has general characteristics of plastics, namely, elasticity or flexibility. 
   The nib hole  11  is formed at the cone-shaped lower end of the shaft  10 , while an insert hole  14  is formed at the upper end of the shaft  10  and has an inner diameter sufficient to accommodate the knock part  30  and the parts which will be described later. The knock part  30  is inserted into the insert hole  14 . 
   As described above, the shaft guide slit  12  and the stepped slot  13  are provided on predetermined portions of the shaft  10  in a thickness direction of the shaft  10 , to form a hook shape. 
   The stepped slot  13  includes a stop edge  13   a,  which acts as a stopper, and an inclined side  13   b,  which allows a smooth sliding motion of the switch  20 . 
   Preferably, a locking slit  15   a  is formed along a longitudinal direction of the shaft guide slit  12 . The locking slit  15   a  is provided at an edge of the insert hole  14 . Preferably, a mouth of the locking slit  15   a  is enlarged, thus allowing a coupling shaft  22  of the switch  20  to be smoothly coupled to the mouth. Further, in order to prevent unexpected removal of the coupling shaft  22 , a plurality of right-angled steps  15   b  are provided at junctions between the locking slit  15   a  and the shaft guide slit  12 . In this case, a width of the locking slit  15   a  is narrower than a width of the shaft guide slit  12 . 
   Due to the shaft guide slit  12  and the locking slit  15   a  having such constructions, the insert hole  14  can be slightly enlarged in a circumferential direction thereof. Further, the plurality of right-angled steps  15   b  can restrain the switch  20  so that the switch  20  is not easily removed from the shaft guide slit  12 . 
   In a detailed description, since a portion around the locking slit  15   a  is made of plastics, the switch  20  is easily fitted into the shaft guide slit  12  through the locking slit  15   a  which can be deformed within a predetermined elastic limit, and further, the locking slit  15   a  is elastically restored to an original shape thereof after the fitting of the switch  20  is completed. 
   Further, a rounded part  11   a  is provided in the cone-shaped lower end of the shaft  10 . The rounded part  11   a  allows the spherical door  90  coupled to the rounded part  11   a  to smoothly rotate, in addition to providing a stable support surface for the spherical door  90 . Preferably, a lubricant, such as grease, is very thinly coated on a surface of the rounded part  11   a.  Further, at least one annular door seat  11   b  is provided between the rounded part  11   a  and the nib hole  11   e.  In this case, the door seat  11   b  is in line contact with a surface of the spherical door  90 , thus sealing an interior of the shaft  10 , preventing foreign materials from entering the shaft  10 , and allowing the spherical door  90  to smoothly rotate. 
   In order to seal the interior of the shaft  10 , an annular projection  10   a  is provided on a predetermined portion of an inner circumference of the shaft  10  to be slightly projected toward a center of the shaft  10 , and has a diameter that is slightly smaller than an inner diameter of a main portion of the shaft  10 . The thickness of the annular projection  10   a  depends on a thickness of the O-ring  50 . That is, at a first stroke start position where the nib hole  11  of the shaft  10  is closed by the spherical door  90 , the O-ring  50  is in close contact with the annular projection  10   a,  thus sealing an interior of the shaft  10  between the nib hole  11  and the O-ring  50 . 
   Further, a straight-shaped shaft sliding groove  10   b  axially extends from an edge of the insert hole  14  of the shaft  10  to a predetermined position. The shaft sliding groove  10   b  guides a rectilinear movement of the cartridge  40  in a direction of the slit  12 , in addition to preventing the rotation of the cartridge  40 . 
   Preferably, the shaft  10  further includes a clip  19  which is shown in dotted lines in  FIG. 3   a.  The clip  19  is used to hold the writing instrument in a desired place, such as a pocket, to allow a user to easily carry the writing instrument. 
   As shown in  FIG. 2 , the switch  20  functions to control the projection and retraction of the writing instrument. 
   The switch  20  has a ring  21  fitted over a smaller diameter part of the knock part  30 . Preferably, the ring  21  of the switch  20  has an inner diameter which allows the ring  21  to be rotatably fitted over the smaller diameter part of the knock part  30 , and an outer diameter which allows the ring  21  to be rotatably fitted into the insert hole  14  of the shaft  10 . 
   The switch  20  also includes the coupling shaft  22 . The coupling shaft  22  is integrally projected from a surface of the ring  21  to have a predetermined height, for example, a height which is slightly greater than the thickness of a wall of the shaft  10 . Preferably, the coupling shaft  22  has a thickness which allows the coupling shaft  22  to freely slide along the shaft guide slit  12 . 
   Further, the switch  20  has a tap  23 , which is integrally provided at an upper end of the coupling shaft  22  and has a predetermined area. In this case, the tap  23  serves as a contact surface to allow a user to easily manipulate the switch  20 . The shape of the tap  23  may be selected out of a flat shape, a hemispherical shape, and a shape with a non-skid projection. 
     FIG. 4   a  is a front view of the knock part  30  of  FIG. 2 , and  FIG. 4   b  is a sectional view taken along the line B—B of  FIG. 4   a.    
   Referring to  FIGS. 2 ,  4   a  and  4   b,  the knock part  30  functions to transmit a pressing force of a user to the cartridge  40 . Such a knock part  30  has a shape of a tube which includes the smaller diameter part and the larger diameter part to form a step. An end of the smaller diameter part is opened, while an end of the larger diameter part, which is opposite to the end of the smaller diameter part, is closed. 
   A plurality of uneven grooves  31  are provided on an outer circumferential surface of the larger diameter part of the knock part  30 , thus allowing a user to easily grip the knock part  30 . Further, a land  34  is provided on an outer circumferential surface of the smaller diameter part of the knock part  30  to allow the ring  21  of the switch  20  to be rotatably coupled to the knock part  30 . A plurality of non-skid projections  32  are provided between the land  34  and the end of the smaller diameter part of the knock part  30 . 
   When the smaller diameter part of the knock part  30  is inserted into a cartridge hole  42 , the non-skid projections  32  of the knock part  30  are fitted into the cartridge hole  42  through a force-fit method, thus preventing the knock part  30  from being easily removed from the cartridge hole  42 . 
   Preferably, a plurality of ribs  33  are axially arranged on an inner surface of the knock part  30  to reinforce the knock part  30 . 
     FIG. 5   a  is a front view of the cartridge  40  of  FIG. 2 , and  FIGS. 5   b,    5   c,  and  5   d  are sectional views taken along the lines C—C, D—D, and E—E of  FIG. 5   a,  respectively. 
   Referring to  FIGS. 2 , and  5   a  through  5   d,  the cartridge  40  acts as an ink reservoir, and has a predetermined internal capacity. 
   The cartridge  40  has a shape of a hollow shaft with a surface tapered at a predetermined angle, for example, about 1˜5°. The cartridge  40  is opened at opposite ends thereof, and steps are formed on predetermined positions of the cartridge  40 . 
   That is, the cartridge  40  is formed to gradually reduce a diameter thereof in a direction from the cartridge hole  42  to the nib  41  while forming the steps. The cartridge  40  includes a tank part  43 , a coupling part  44 , and a nib extension part  45  that are integrated into a single structure. In this case, the tank part  43  has the largest diameter. The coupling part  44 , having a diameter smaller than the diameter of the tank part  43 , is provided under the tank part  43 . The nib extension part  45 , having a diameter smaller than the diameter of the coupling part  44 , is provided under the coupling part  44 . 
   A non-skid ring  43   a  is provided on an upper portion of an inner circumference of the tank part  43 , and is coupled to the non-skid projections  32  of the knock part  30  through the force-fit method. 
   Further, a plurality of support ribs  43   b  are provided on a middle portion of the inner circumference of the tank part  43  to support an ink tube (not shown) and increase durability of the tank part  43 . 
   A cartridge projection  43   c  is provided on an upper portion of the tank part  43  to slide along the shaft sliding groove  10   b  (see,  FIG. 3   b ). 
   The cartridge projection  43   c  is coupled to the shaft sliding groove  10   b  through a slide coupling method, thus guiding the rectilinear movement of the cartridge  40  in the direction of the slit  12 , and preventing the rotation of the cartridge  40 . 
   Although not shown in the drawings, the cartridge projection  43   c  of the cartridge  40  may comprise a plurality of cartridge projections  43   c,  and the shaft sliding groove  10   b  of the shaft  10  may comprise a plurality of shaft sliding grooves  10   b.    
   A plurality of ink feeding passages  46   a  are provided on an inner circumference of the nib extension part  45 . Further, a plurality of cartridge guide grooves  46  are axially provided on an outer circumference of the nib extension part  45  to be formed at diametrically opposite positions of the nib extension part  45 . 
   Each cartridge guide groove  46  engages with an associated link projection  86  (see,  FIGS. 7   a  through  7   f ) provided on an inner circumference of the link  80 . 
   Further, the nib  41  is secured to an end of the nib extension part  45 , and contents stored in the tank part  43 , such as ink, are fed from the tank part  43  to the nib  41  through a conventional ink feeding method adopted according to a kind of a writing instrument, for example, a feeding method using a capillary action, a feeding method using a pressure difference, a feeding method using a suction, etc. 
   As the nib  41  is used a tip for oil- or water-based ink, a correction fluid discharge tip, a tip for highlight pens, a tip for marker pens, and others, according to a kind of a writing instrument. It is possible to use a suitable ink feeding method according to the kind of the tip. 
   As shown in  FIG. 2 , the O-ring  50  slides along the inner circumference of the shaft  10  together with the cartridge  40 , while sealing a gap between the cartridge  40  and the shaft  10 . The O-ring  50  is fitted over the coupling part  44  of the cartridge  40 , and is made of a material, such as rubber, silicone, and soft plastics. The O-ring  50  has a grooved outer circumference  51  to maximize sealing performance, in addition to reducing friction between the cartridge  40  and the shaft  10  when the cartridge  40  slides. 
   Further, as shown in  FIG. 2 , the first spring  60  is fitted over the coupling part  44  of the cartridge  40 . The first spring  60  is supported by a side surface of the O-ring  50  which is fitted over the coupling part  44 . 
   In this case, the first spring  60  functions to axially bias either the link holder  70  or the cartridge  40  within a stroke distance of the cartridge  40  during the operation of the writing instrument. 
   For example, when the cartridge  40  and the knock part  30  move forward by a predetermined stroke distance, the first spring  60  is compressed. At this time, the first spring  60  tends to return from the compressed state to an extended state, thus generating an elastic restoring force. 
   In the present invention, the stroke distance of the cartridge  40  is equal to either a stroke distance of the knock part  30  or a length of the shaft guide slit  12 . 
   Further, as shown in  FIG. 2 , the second spring  65  is fitted over the nib extension part  45  of the cartridge  40 . The second spring  65  is supported by a stepped junction between the coupling part  44  and the nib extension part  45 . 
   In such a state, the second spring  65  functions to axially bias either the link  80  or the cartridge  40  during the operation of the writing instrument. 
   For example, when the cartridge  40  and the knock part  30  move forward, the second spring  65  is compressed for a short period. At this time, the second spring  65  biases the link  80 , thus allowing the spherical door  90  rotatably coupled to the link  80  to be smoothly and rapidly opened or closed. 
     FIG. 6   a  is a perspective view to show a rear part of the link holder  70  of  FIG. 2 , and  FIGS. 6   b  and  6   c  are a front view and a left side view of the link holder  70  of  FIG. 6   a,  respectively, and  FIG. 6   d  is a sectional view taken along the line F—F of  FIG. 6   c.    
   Referring to  FIG. 2  and  FIGS. 6   a  through  6   d,  the link holder  70  is fitted over the nib extension part  44 , and then is elastically supported by the first spring  60 , thus allowing the spherical door  90  to be in close contact with the rounded part  11   a  inside the nib hole  11  of the shaft  10 . 
   The link holder  70  has a shape of a hollow bushing which includes a front larger diameter part  71  and a rear smaller diameter part  72 . An end of the first spring  60  is fitted over the rear smaller diameter part  72  of the link holder  70 . Thus, the first spring  60  is supported by a stepped junction between the front larger diameter part  71  and the rear smaller diameter part  72 . 
   Further, a link mount hole  79  is formed at a center of the link holder  70 . A rounded part  70   a  is provided in the front of the link mount hole  79 . The rounded part  70   a  of the link holder  70  allows the spherical door  90  seated on the rounded part  70   a  to smoothly rotate, in addition to providing a stable support surface for the spherical door  90 . Further, another door seat (not shown) having the same function as the above-mentioned door seat  11   b  may be provided on the rounded part  70   a.    
   A stopper step  73  is inwardly projected from an inner circumference around the rounded part  70   a  by a predetermined height, thus forming a  -shaped cross-section. 
   The link  80  is axially inserted into the link mount hole  79  of the link holder  70 . 
   At this time, the shape of the cross-section of the stopper step  73  is similar to the shape   of the cross-section of a rear portion of the link  80 , thus allowing the link  80  to be easily inserted into the link mount hole  79 . 
   When a worker rotates only the link holder  70  by 90° after inserting the link  80  into the link mount hole  79 , the stopper step  73  of the link holder  70  rotates so that the cross-section thereof is changed from a  -shaped state to a  -shaped state. The rear portion of the link  80  maintains the  -shaped state. 
   In this way, the link  80  can be easily inserted into the link holder  70 , and further, the link  80  is stopped by the stopper step  73  of the link holder  70 , thus preventing the link  80  from being unexpectedly removed from the link holder  70 . 
   Particularly, a plurality of straight-shaped guide projections  74  are provided in the link mount hole  79  to extend from the stopper step  73  to an edge of the rear smaller diameter part  72 , thus minimizing friction between the link mount hole  79  and the link  80 , partially supporting the link  80 , minimizing the compression of air, and allowing the link  80  to slide along the link holder  70  while the center of the link  80  is aligned with the center of the link holder  70 . 
     FIGS. 7   a  through  7   f  are views to show the shape of the link  80 . 
   Referring to  FIG. 2  and  FIGS. 7   a  through  7   f,  the link  80  has a shape of a tube. That is, an insert hole  89  is axially formed to pass through the link  80 , so that the nib extension part  44  is inserted into the insert hole  89  of the link  80 . 
   The link  80  is inserted into the link holder  70  while being elastically supported by the second spring  65 . 
   Further, fan-shaped wings  81  are provided at diametrically opposite sides of the rear portion of the link  80 , thus forming the  -shaped cross-section. 
   A plurality of elastic slits  85  are provided on the link  80  to forwardly extend from the wings  81  to predetermined positions of the link  80 . 
   The elastic slits  85  minimize a weight of the link  80 , attenuate vertical vibration or impact during the operation of the writing instrument, and provide a clearance for the link  80  when the link  80  is elastically deformed. 
   Further, a parallel pin  82  is provided at a lower end of a front portion of the link  80  to be perpendicular to a central axis of the link  80 . In this case, the parallel pin  82  is slightly outwardly projected at opposite ends thereof. The opposite ends of the parallel pin  82  are chamfered to correspond to a rounded part of the spherical door  90 . 
   A plurality of link projections  86  are provided on an inner circumference of a rear portion of the insert hole  89 . The link projections  86  slide along the corresponding cartridge guide grooves  46  within a predetermined stroke distance, and serve as a locking unit when outer surfaces of the link projections  86  come into contact with inner surfaces of front portions of the cartridge guide grooves  46 . Thus, the link projections  86  allow the link  80  to axially slide along the link holder  70 , in addition to serving as a hook. 
   Further, when the parallel pin  82  is coupled to pin slits  92  of the spherical door  90 , the link  80  functions to rotate the spherical door  90  at a rotating angle of ±90° in a direct transmission manner. 
     FIGS. 8   a  through  8   g  are views to show the shape of the spherical door  90 . 
   Referring to  FIG. 2  and  FIGS. 8   a  through  8   g,  the spherical door  90  serves as a driven unit of the link  80 . As a result, the spherical door  90  acts as a door to open or close the nib hole  11 . 
   For a smooth operation, the door  90  has a roughly spherical shape. Further, a passage  91  is formed to pass through a center of the spherical door  90  while being opened at both sides of an outer circumference of the spherical door  90 . 
   The plurality of pin slits  92  are provided at eccentric positions of the spherical door  90  to receive the parallel pin  82  of the link  80 . 
   In this case, the pin slits  92  are formed at opposite sides of the passage  91  to be inclined at a predetermined angle. 
   The pin slits  92 , formed to be inclined at the eccentric positions of the spherical door  90 , receive the parallel pin  82 , thus functioning as a cam slit. That is, the pin slits  92  receiving the parallel pin  82  convert a link sliding force into a rotating force, which rotates the spherical door  90  within a predetermined angular range. 
   Further, a step  93  is provided on an inner circumference of a front portion of the passage  91 , thus limiting a stroke of the link  80 . 
   The assembly of the above-mentioned elements will be described in the following. 
     FIG. 9   a  is a side view of the writing instrument of  FIG. 1   a,  and  FIG. 9   b  is a sectional view taken along the line L—L of  FIG. 9   a  to illustrate an assembled state of the writing instrument. 
   An internal operating module  100  having the above-mentioned elements is provided in the shaft  10 . 
   In the internal operating module  100 , the knock part  30  is fitted into the cartridge  40 . 
   Particularly, the ring  21  of the switch  20  is rotatably installed between the knock part  30  and the cartridge  40 . 
   When a user desires to lock the switch  20 , the coupling shaft  22  of the switch  20  moves forward along the shaft guide slit  21 , together with the knock part  30  and the cartridge  40 . Subsequently, the coupling shaft  22  rotates at a junction between the shaft guide slit  12  and the stepped slot  13  at a rotating angle corresponding to a depth of the stepped slot  13 , and then is locked to the stepped slot  13 . 
   Meanwhile, when the user desires to unlock the switch  20 , the coupling shaft  22  of the switch  20  rotates at the same rotating angle as the locking angle but in a direction opposite to the locking direction. Thus, the coupling shaft  22  of the switch  20  is released, and then moves backward along the shaft guide slit  12  by the first and second springs  60  and  65 . 
   Referring to  FIGS. 10   a  through  10   c,  and  FIGS. 11   a  through  11   c,  the forward operation of the internal operating module  100  provided in the shaft  10  will be described in brief in the following. 
     FIGS. 10   a  and  11   a  show the state where the nib hole  11  is closed by the spherical door  90 , at the first stroke start position. That is, the passage  91  of the spherical door  90  is perpendicular to a central axis of the cartridge  40 . In this case, a part of the spherical door  90  is in line contact with the door seat  11   b  provided around the nib hole  11  of the shaft  10 , thus sealing the interior of the shaft  10 . 
   Further, the O-ring  50  is fitted over the coupling part  44  of the cartridge  40 . Since the O-ring  50  is in close contact with the annular projection  10   a  and the spherical door  90  is in close contact with the nib hole  11 , the interior space of the shaft  10  between the nib hole  11  and the O-ring  50  is sealed, thus preventing ink from being dried up. 
   At this time, the nib  41  is surrounded with the link  80 . The link projections  86  of the link  80  slidably engage with the corresponding cartridge guide grooves  46  of the nib extension part  45 . In this case, the outer surfaces of the link projections  86  are in contact with the inner surfaces of the front portions of the cartridge guide grooves  46 . 
   The first spring  60  acts as an elastic force, which is sufficient to rotatably support the spherical door  90 , on the link holder  70 , so that the spherical door  90  is seated in the shaft  10 . Meanwhile, the second spring  65  is maintained between the link  80  and the coupling part  44  without generating any elastic force. 
     FIGS. 10   b  and  11   b  show the state where the knock part  30  and the cartridge  40  of the internal operating module  100  move forward by a predetermined stroke distance, for example, 4 mm, when the user presses the knock part  30  to lock the switch  20 . 
   In this case, the first spring  60  is operated similarly to the state shown in  FIGS. 10   a  and  11   a.  But, in comparison with the state shown in  FIGS. 10   a  and  11   a,  the first spring  60  applies a larger elastic force to the link holder  70 . Further, the second spring  65  applies a force generated during the forward movement of the cartridge  40  to the link  80 , in a form of an elastic restoring force. 
   Thus, as shown by the arrow f of  FIG. 11   b,  the link  80  moves forward by the predetermined stroke distance. The parallel pin  82  of the link  80  slides along the pin slits  92  of the spherical door  90 . Thus, the spherical door  90  rotates by the rotating angle of +90° in a direction W. 
   Therefore, the passage  91  of the spherical door  90  is opened in the axial direction of the cartridge  40 . Further, the link  80  and the nib  41  which have moved forward are placed in the passage  91 . 
   At this time, the nib  41  of the cartridge  40  is positioned in the passage  91  so that a writing tip of the nib  41  is not in contact with any wall of the passage  91  of the spherical door  90 . Thus, no portion of the spherical door  90  is covered with ink of the nib  41 . At this time, a user can see an end of the nib  41  positioned inside the nib hole  11 , with the naked eye. 
     FIGS. 10   c  and  11   c  show the state where the internal operating module  100  is further moved forward by a predetermined stroke distance, for example, 12 mm, to lock the switch  20  to the stepped slot  13 . 
   As described above, the user rotates the switch  20  by a predetermined rotating angle so that the switch  20  is stopped by the stepped slot  13  of the shaft guide slit  12 . Thus, the internal operating module  100  is secured in the shaft  10 . 
   Further, the nib  41  and a part of the nib extension part  45  are projected out of the nib hole  11 . 
   Since the first and second springs  60  and  65  move forward by the total stroke distance, for example, 16 mm, the first and second springs  60  and  65  are maximally compressed within elastic limits thereof. Thus, the first and second springs  60  and  65  retain maximum elastic restoring forces. 
   In such a state, the user can write with the writing instrument of this invention. 
   Conversely, when the user manipulates the switch  20  to unlock the switch  20 , the nib  41  is retracted into the shaft  10 . 
   That is, the user rotates the switch  20  in a direction which releases the coupling shaft  22  of the switch  20  from the stepped slot  13  of the shaft guide slit  12 . 
   In this case, the nib retracting operation is executed in reverse order to the nib projecting operation shown in  FIGS. 11   a  through  11   c.  Through such a nib retracting operation, the nib hole  11  is closed by the spherical door  90 , and the nib  41  is safely sealed in the shaft  10 . 
   That is, as soon as the first and second springs  60  and  65  extend in an axial direction by the elastic restoring forces, the switch  20 , the knock part  30 , and the cartridge  40  of the internal operating module  100  move backward. 
   At this time, the nib extension part  45  moves backward together with the cartridge  40 , but the link projections  86  are maintained in their stopped state. When the nib  41  is retracted into a position shown by  FIG. 10   b  or  11   b,  the cartridge guide grooves  46  of the nib extension part  45  allow the link projections  86  to slide relative to the cartridge guide grooves  46 . 
   Thereafter, when the inner surfaces of the front portions of the cartridge guide grooves  46  come into contact with the outer surfaces of the link projections  86 , the nib  41  is placed in the passage  91  of the spherical door  90 . 
   Later, as the cartridge  40  moves further backward, the inner surfaces of the cartridge guide grooves  46  come into contact with the outer surfaces of the link projections  86  of the link  80 , and then the cartridge guide groove  46  pulls the link projections  86  of the link  80  backward. 
   In this case, the parallel pin  82  of the link  80  rotates the spherical door  90  by the rotating angle of −90°. At this time, the spherical door  90  seals the nib hole  11 . 
   &lt;Second Embodiment&gt; 
   A slide-type writing instrument with a dry prevention unit, according to a second embodiment of this invention, will be described in detail in the following with reference to  FIGS. 12 through 18   m.    
   As shown in  FIG. 12 , the writing instrument of this invention opens or closes a nib hole  11  of a shaft  10 . A switch  20  is provided in the shaft  10  to be locked and unlocked, and is coupled to a knock part  30  which transmits a pressing force of a user. A cartridge  40  is axially fitted into the knock part  30 . A spring  66  is coupled to the cartridge  40  to return the cartridge  40  to an original position thereof, using an elastic force of the spring  66 . An O-ring  50  is slidably installed in the shaft  10  to seal a gap between the shaft  10  and the cartridge  40 . Further, a link holder  70  is installed in the shaft  10  while an end of the link holder  70  being axially inserted into the O-ring  50 . A link  80  is slidably installed in the link holder  70 . A spherical door  90  is also installed in the shaft  10 . A parallel pin  82  of the link  80  engages with pin slits  92  of the spherical door  90 , thus rotating the spherical door  90  by a predetermined rotating angle corresponding to a sliding motion of the link  80 . 
   Further, the second embodiment of this invention can be applied to a cartridge control unit of a conventional knock-type ballpoint pen as well as a cartridge control unit using the switch  20  and the shaft guide slit  12  according to the first embodiment. 
   Of course, the knock part  30  of this invention may be designed in a similar manner as a clip having a gear seating part in which a gear unit is rotatably seated. The structure having such a clip is disclosed in detail in Korean Patent No. 10-2003-55414 and 10-2003-56940, which relates to a safe knock-type writing instrument and is filed with KIPO by the same applicant as this invention. 
   As shown in  FIG. 13 , the cartridge  40  has a nib extension part  45  which is designed as follows. 
   That is, a clutch part  47  having a circular cross-section is provided on an outer circumference of an end of the nib extension part  45  so that a diameter of the clutch part  47  is larger than a diameter of the nib extension part  45 . 
   A stepped portion  47   a,  having a smaller diameter than the clutch part  47 , is provided at an end of the clutch part  47 . 
   In order to pull the link  80  of  FIG. 12  backward, the clutch part  47  includes a tapered surface  47   b  at a position around the stepped portion  47   a.  A clutch recess  47   c  having a semi-circular cross-section is provided at a position around the tapered surface  47   b,  and a non-skid projection  47   d  having a semi-circular cross-section is provided at a position around the clutch recess  47   c.    
   The spring  66  is fitted over the nib extension part  45  to be supported by a stepped junction between a tank part  43  and a coupling part  44 . 
   Such a spring  66  is seated on a spring seat  56  of the O-ring  50 , which will be described in detail hereinafter. 
   Thereafter, the spring  66  axially biases either the O-ring  50  or the cartridge  40 , within a stroke distance of the cartridge  40 , during the operation of the writing instrument. 
   For example, when the cartridge  40  and the knock part  30  move forward by a predetermined stroke distance, the spring  66  is compressed, and thereby retains an elastic restoring force which makes the spring  66  return to an original state thereof. In this case, the predetermined stroke distance of the cartridge  40  is equal to a stroke distance of the knock part  30  or a distance between an original position and a projecting position of the nib  41 . Further, when the switch  20  is released to move the cartridge  40  and the knock part  30  backward, the elements coupled to the cartridge  40  as well as the knock part  30  return to original positions thereof by the elastic force of the spring  66 . 
   Further, when a pressing force of a user to move the cartridge  40  and the knock part  30  forward is applied to one of the O-ring  50 , the link holder  70 , and the spherical door  90 , the spring  66  functions to attenuate impact on the O-ring  50 , the link holder  70 , and the spherical door  90 . 
   As shown in  FIG. 14 , a smaller diameter hole  53  and a larger diameter hole  54  of the O-ring  50  form an alignment with a central axis of the O-ring  50 , thus allowing the O-ring  50  to be fitted over the coupling part  44  of the cartridge  40 . 
   The spring seat  56  is provided at a position around the smaller diameter hole  53  to be axially projected. Further, a first sealing projection  55  having a semi-circular cross-section is provided at a junction between the smaller diameter hole  53  and the larger diameter hole  54 . The first sealing projection  55  is in contact with an outer circumferential surface of the nib extension part  45 , thus sealing a gap between the nib extension part  45  and the O-ring  50  during the sliding motion of the cartridge  40 . 
   Further, a second sealing projection  57  having a semi-circular cross-section is provided on an outer circumferential surface of the larger diameter hole  54 . The second sealing projection  57  is in contact with an inner circumferential surface of the shaft  10 , thus sealing a gap between the shaft  10  and the O-ring  50  during the sliding motion of the cartridge  40 . Preferably, the second sealing projection  57  may further have a grooved outer surface, thus reducing friction between the shaft  10  and the O-ring  50  during the sliding motion of the cartridge  40 , and maximizing sealing performance. 
   As shown in  FIG. 15 , the link holder  70  is supported by the O-ring  50  which is fitted over the nib extension part  45  and is biased by the spring  66 , so that the link holder  70  makes the spherical door  90  of  FIG. 12  come into contact with a rounded surface inside the nib hole  11  of the shaft  10 . 
   An axial length of a rear part  72  of the link holder  70  according to the second embodiment is slightly longer than that of the first embodiment. 
   The rear part  72  of the link holder  70  has a diameter corresponding to a size of the larger diameter hole  54  of the O-ring  50 . 
   A link mount hole  79  is formed at a center of the link holder  70 . In this case, the link  80  freely slides forward in the link mount hole  79 , but limitedly slides backward due to the constructions of the link  80  and the link holder  70  which will be described in the following. 
   That is, support steps  72   a  are provided at an end of an inner circumference of the rear part  72  of the link holder  70  to stop fitting steps  80   c  of the link  80  shown in  FIG. 16  in detail, thus limiting the sliding motion of the link  80  (see,  FIG. 18   c ). 
   Further, an inclined step  76  extends from a middle portion of the link mount hole  79  in a circumferential direction to correspond to the shape of the link  80 , thus preventing the link  80  from being adhered to the link holder  70  by impurities after a lengthy use, and aligning the link  80  with the center of the link mount hole  79 . 
   As shown in  FIG. 16 , an inner circumference of a link hole  89  of the link  80  is formed to correspond to the link mount hole  79 , the support step  72   a,  and the inclined step  76  of the link holder  70 , so that the link  80  is inserted into the link holder  70  to be sealed in the link holder  70 . 
   In a detailed description, a larger diameter part  88  is provided on the inner circumference of the link hole  89  in front of an inclined surface  83 , while a smaller diameter part  87  is provided on the inner circumference of the link hole  89  in back of the inclined surface  83 . In this case, the smaller diameter part  87  corresponds to inner circumferences of elastic pieces  80   a,  which will be described later. 
   A plurality of elastic slits  85  are formed on the link  80  at regular angular intervals, for example, 90° in a circumferential direction of the link  80 . In this case, only an end of each elastic slit  85  is opened. 
   The elastic slits  85  are provided on a rear portion of the link  80 , and the plurality of elastic pieces  80   a  are respectively provided between the elastic slits  85 . 
   The fitting step  80   c  is provided on an outer circumference of a rear end of each elastic piece  80   a  of the link  80 , while an elastic projection  80   b  is provided on an inner circumference of the rear end of each elastic piece  80   a.    
   In this case, each elastic projection  80   b  is shaped to be seated in the clutch recess  47   c  of the clutch part  47  shown in  FIG. 13 . 
   There is a close relationship between the operation of each elastic projection  80   b  and the operation of the clutch part  47 . 
   That is, when the cartridge  40  starts sliding in an axial direction, the clutch part  47  also starts sliding. 
   At first, the elastic projection  80   b  of each elastic piece  80   a  is seated in the clutch recess  47   c  of the clutch part  47  shown in  FIG. 13  while being supported by the non-skid projection  47   d.  Further, each elastic projection  80   b  temporarily engages with the clutch part  47  of the cartridge  40  by a frictional force corresponding to an elastic force of the elastic projection  80   b.    
   Thus, the link  80  having the elastic projections  80   b  slides along with the cartridge  40  having the clutch recess  47   c,  while engaging with the cartridge  40 . 
   Further, when the link  80  is stopped by the step  93  of the passage  91  of the spherical door  90  (see,  FIG. 17 ), the link  80  does not slide any more. 
   In this case, when the sliding motion is continuously executed within a predetermined stroke range, the elastic pieces  80   a  are flared relative to the elastic slits  85 , and then return to an original state thereof. In such a process, the elastic projection  80   b  of each elastic piece  80   a  is removed from the clutch recess  47   c  of the clutch part  47 . As a result, even when the link  80  is stopped, the cartridge  40  can move forward. 
   Further, a parallel pin  82  is provided at a lower end of a front portion of the link  80  to be perpendicular to a central axis of the link  80 . In this case, the parallel pin  82  is slightly outwardly projected at opposite ends thereof. The opposite ends of the parallel pin  82  are chamfered to correspond to the rounded part of the spherical door  90 . 
   The parallel pin  82  engages with pin slits  92  of the spherical door  90 , so that the link  80  functions to rotate the spherical door  90  at a rotating angle of ±90° in a direct transmission manner. 
   Referring to  FIGS. 12 and 17 , the spherical door  90  serves as a driven unit of the link  80 . As a result, the spherical door  90  serves as a door to open or close the nib hole  11 . 
   For a smooth operation, the door  90  has a roughly spherical shape. Further, a passage  91  is formed to pass through a center of the spherical door  90  while being opened at both sides of an outer circumference of the spherical door  90 . 
   The plurality of pin slits  92  are provided at eccentric positions of the spherical door  90  to receive the parallel pin  82  of the link  80 . 
   In this case, the pin slits  92  are formed at opposite sides of the passage  91  to be inclined at a predetermined angle. 
   The pin slits  92 , formed to be inclined at the eccentric positions of the spherical door  90 , receive the parallel pin  82 , thus functioning as a cam slit. That is, the pin slits  92  receiving the parallel pin  82  convert a link sliding force into a rotating force that rotates the spherical door  90  within a predetermined angular range. 
   Further, the step  93  is provided on an inner circumference of a front portion of the passage  91 , thus limiting a stroke of the link  80 . 
   The assembly of the above-mentioned elements will be described in the following. 
     FIG. 18   a  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   b  is a sectional view taken along the line M—M of  FIG. 18   a,  and  FIG. 18   c  is an enlarged view of a portion encircled in  FIG. 18   b.  As shown in the drawings, the above-mentioned elements are assembled in the shaft  10 . 
   That is, the cartridge  40  is seated in the shaft  10 , and the knock part  30  is fastened to an end of the cartridge  40  through a force-fit method. 
   The ring  21  of the switch  20  is rotatably provided between the knock part  30  and the cartridge  40 . 
   When a user desires to lock the switch  20 , the coupling shaft  22  of the switch  20  moves forward along the shaft guide slit  12 , together with the knock part  30  and the cartridge  40 . Subsequently, the coupling shaft  22  rotates at a junction between the shaft guide slit  12  and the stepped slot  13  at a rotating angle corresponding to a depth of the stepped slot  13 , and then is locked to the stepped slot  13 . 
   Meanwhile, when the user desires to unlock the switch  20 , the coupling shaft  22  of the switch  20  is rotated at the same rotating angle as the locking angle but in a direction opposite to the locking direction. Thus, the coupling shaft  22  of the switch  20  is released, and then moves backward along the shaft guide slit  12  by the spring  66 . 
   In a detailed description, at a first stroke start position; the spherical door  90  closes the nib hole  11 . 
   That is, the passage  91  of the spherical door  90  is perpendicular to the central axis of the cartridge  40 . 
   The O-ring  50  is arranged between the link holder  70  and the spring  66 . In this case, the larger diameter hole  54  of the O-ring  50  surrounds the rear part  72  of the link holder  70 . Further, the first sealing projection  55  of the O-ring  50  is in surface contact with the outer circumferential surface of the nib extension part  45  of the cartridge  40 , while being in close contact with the non-skid projection  47   d.  Furthermore, the second sealing projection  57  is in surface contact with the inner circumferential surface of the shaft  10 . 
   If any slit is formed on the nib extension part  45 , the sealing performance may be deteriorated. However, according to the present invention, no slit is formed on the outer circumferential surface of the nib extension part  45 . 
   As described above, because a seal is accomplished by the O-ring  50  and the spherical door  90  is in close contact with the nib hole  11 , an interior of the shaft  10  between the nib hole  11  and the O-ring  50  is sealed, thus preventing ink of the nib  41  from being dried up. 
   Further, the O-ring  50  surrounds the rear part  72  of the link holder  70 , so that a volume of the interior is smaller than a volume of the interior according to the first embodiment, thus reducing inflow of air, and thereby more efficiently preventing the drying of ink. 
   Meanwhile, the nib  41  is surrounded by the link  80 , and the link  80  is inserted into the link mount hole  79  of the link holder  70 . At this time, the elastic projection  80   b  of each elastic piece  80   a  of the link  80  is seated in the clutch recess  47   c  of the clutch part  47 , thus allowing the link  80  to move along with the clutch part  47 . 
   Further, the spring  66  acts an elastic force, which is sufficient to rotatably support the spherical door  90 , on the link holder  70 , so that the spherical door  90  is seated in the shaft  10 . 
     FIG. 18   d  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   e  is a sectional view taken along the line N—N of  FIG. 18   d.    FIGS. 18   d  and  18   e  show the state where the knock part  30  and the cartridge  40  move forward by a very short stroke distance, for example, 2 mm, when a user manipulates the switch  20  or the knock part  30  so that the knock part  30  moves by the stroke distance. 
   At this time, the spring  66  applies a larger elastic force to the O-ring  50  and the link holder  70 , according to the moving distance. 
   Further, the clutch part  47  of the cartridge  40  and the link  80  coupled to the clutch part  47  move by the above-mentioned stroke distance. At this time, the non-skid projection  47   d  of the clutch part  47  is separated from the first sealing projection  55  of the O-ring  50 . 
   In this case, the parallel pin  82  of the link  80  slides along the pin slits  92  of the spherical door  90 , thus rotating the spherical door  90  at a rotating angle of +45°. 
     FIG. 18   f  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   g  is a sectional view taken along the line O—O of  FIG. 18   f.  As shown in the drawings, when the user further presses the knock part  30  and the knock part  30  further moves by a predetermined stroke distance, for example, 2 mm, the knock part  30  and the cartridge  40  further move forward by the stroke distance. 
   In this case, the parallel pin  82  of the link  80  further slides along the pin slits  92  of the spherical door  90 , thus further rotating the spherical door  90  at a rotating angle of +45°. 
   As a result, the passage  91  of the spherical door  90  is aligned with the central axis of the cartridge  40 , thus opening the nib hole  11 . At this time, the link  80  and the nib  41  are placed in the passage  91 . 
   At this time, the nib  41  of the cartridge  40  is positioned in the passage  91  so that a writing tip of the nib  41  is not in contact with any wall of the passage  91  of the spherical door  90 . Thus, no portion of the spherical door  90  is covered with ink of the nib  41 . At this time, a user can see an end of the nib  41  positioned inside the nib hole  11 , with the naked eye. 
     FIG. 18   h  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   i  is a sectional view taken along the line P—P of  FIG. 18   h.    FIGS. 18   h  and  18   i  show the state where the knock part  30  further moves forward by a predetermined stroke distance, for example, 12 mm, when the user stops pressing the knock part  30 . 
   Further, the user rotates the switch  20  by a predetermined rotating angle so that the switch  20  is stopped by the stepped slot  13  of the shaft guide slit  12 . Thus, the knock part  30  and the cartridge  40  are secured in the shaft  10 . 
   In this case, the nib  41  and a part of the clutch part  47  are projected out of the nib hole  11 . 
   Since the spring  66  moves forward by the total stroke distance, for example, 16 mm, the spring  66  is maximally compressed within elastic limits thereof. Thus, the spring  66  retains a maximum elastic restoring force. 
   In such a state, the user can write with the writing instrument of the present invention. 
   Conversely, when the user manipulates the switch  20  to unlock the switch  20 , the nib  41  is retracted into the shaft  10 . 
     FIG. 18   j  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   k  is a sectional view taken along the line Q—Q of  FIG. 18   j.  As shown in the drawings, the user rotates the switch  20  in a direction which releases the coupling shaft  22  of the switch  20  from the stepped slot  13  of the shaft guide slit  12 . 
   In this case, the compressed spring  66  extends to return to an original state thereof while the knock part  30  and the cartridge  40  move backward. The nib retracting operation is executed in reverse order to the nib projecting operation. 
   That is, the clutch part  47  of the cartridge  40  moving backward is inserted into the link hole  89  of the link  80 , and each elastic piece  80   a  of the link  80  engages with the clutch part  47 , as described above. 
   In this case, the link  80  also moves backward along with the cartridge  40 , so that the parallel pin  82  of the link  80  rotates the spherical door  90  at a rotating angle of −90°. 
   At this time, the spherical door  90  closes the nib hole  11 , and the nib  41  is safely sealed in the passage  91  of the spherical door  90 . 
     FIG. 18   l  is a side view of the writing instrument according to the second embodiment, and  FIG. 18   m  is a sectional view taken along the line R—R of  FIG. 18   l . As shown in the drawings, when the spring  66  returns to the original state thereof, the knock part  30 , the cartridge  40 , the nib  41 , and the link  80  return to the first stroke start position. 
   &lt;Third Embodiment&gt; 
   The general construction of a slide-type writing instrument with a dry prevention unit according to the third embodiment remains the same as the slide-type writing instrument according to the second embodiment, except that the number of a spring included in the writing instrument of the third embodiment is one more than the number of the spring included in the writing instrument of the second embodiment, and a spherical door is rotatably coupled to rotary support brackets of the link holder, so that the shapes and assembly of the spherical door, a cartridge, an O-ring, a link holder, and a link are slightly different from those of the second embodiment. Thus, the same reference numerals are used throughout  FIGS. 1   a  through  20   f  to designate the same or similar components, and the common elements will not be described hereinafter. 
     FIG. 19  is an exploded perspective view of the slide-type writing instrument with the dry prevention unit, according to a third embodiment of the present invention, and  FIGS. 20   a  through  20   f  are views to illustrate assembly and operations of the writing instrument of  FIG. 19 , in which  FIGS. 20   a,    20   c,  and  20   e  are side views of the writing instrument,  FIG. 20   b  is a sectional view taken along the line S—S of  FIG. 20   a,    FIG. 20   d  is a sectional view taken along the line T—T of  FIG. 20   c,  and  FIG. 20   f  is a sectional view taken along the line U—U of  FIG. 20   e.    
   As shown in  FIG. 19 , in the writing instrument of the third embodiment, a switch  20  or a conventional cartridge control unit is installed between a knock part  30  and a cartridge  40 . The writing instrument includes a first spring  66  which restores the cartridge  40  to an original position thereof using an elastic force of the first spring  66 . An O-ring  50  is slidably installed in the shaft  10  to seal a gap between the cartridge  40  and the shaft  10 . A link holder  70  is provided in the shaft  10  while an end of the link holder  70  is axially inserted into the O-ring  50 . The writing instrument also includes a link  80  which is elastically supported in the link holder  70  by a second spring  67 . The link  80  includes a plurality of elastic pieces  80   a ′ engaging with a clutch part  47 , so as to smoothly and rapidly slide. Further, a spherical door  90  is provided in the shaft  10 , and rotates at a predetermined rotating angle, as a parallel pin  82  of the link  80  is coupled to pin slits  92  of the spherical door  90  and the link  80  slidably reciprocates. 
   The cartridge  40  has characteristics as follows. That is, an axial length of a coupling part  44 ′ provided between a tank part  43  and a nib extension part  45  is relatively shorter, because the second spring  67  is installed in the shaft  10 . 
   Further, a sliding circumferential part  45   s  is further provided on the nib extension part  45  of the cartridge  40 . In this case, the sliding circumferential part  45   s  has a diameter which is smaller than a maximum diameter of the clutch part  47  but is larger than a diameter of the nib extension part  45 . 
   The sliding circumferential part  45   s  is in contact with both a smaller-diameter hole of the O-ring  50  and a support step provided at an end of an inner circumference of a rear part  72 ′ of the link holder  70 , thus more efficiently sealing a gap between the cartridge  40  and the shaft  10 , and minimizing a gap to the second spring  67 , and minimizing an internal area for the second spring  67 , and thereby enhancing operational performance and minimizing the amount of air which may enter the interior of the shaft  10 , therefore minimizing the drying of ink. 
   The second spring  67  has a diameter which is sufficient to be inserted into a link mount hole  79  of the link holder  70 . The second spring  67  is coupled to the link holder  70  to be supported by the support step, thus axially biasing the link  80 . 
   In this case, a pair of rotary support brackets  78  are provided at opposite sides of a front portion of the link holder  70 . Each of the rotary support brackets  78  has a seating depression  77  so that each hinge shaft  99  of the spherical door  90  is rotatably seated in the corresponding seating depression  77 . 
   Such a construction minimizes frictional resistance between the spherical door  90  and the link holder  70 , thus minimizing vibration or noise which may be generated due to the spherical door  90  and the link holder  70 . 
   Further, an axial length of the rear part  72 ′ of the link holder  70  is longer than the axial length of the rear part  72  of the first embodiment so that the link holder  70  receives the second spring  67  therein. 
   Similarly, the O-ring  50  fitted over the rear part  72 ′ of the link holder  70 , has a longer spring seat  65 ′. 
   Particularly, an axial length of a larger diameter hole of the O-ring  50  is similar or equal to an axial length of the rear part  72 ′ of the link holder  70 . 
   Further, the link  80  is provided with a plurality of elastic pieces  80   a ′ having a simple circumferential surface, without the fitting steps  80   c  of the second embodiment. 
   The projection and retraction of the nib according to the third embodiment will be described in the following. 
     FIG. 20   a  is a side view of the writing instrument according to the third embodiment, and  FIG. 20   b  is a sectional view taken along the line S—S of  FIG. 20   a.  As shown in the drawings, when a user presses the knock part  30  with the user&#39;s finger, the cartridge  40  axially moves forward. At this time, the O-ring  50  and the cartridge  40  coupled to the knock part  30  keep closed. 
     FIG. 20   c  is a side view of the writing instrument according to the third embodiment, and  FIG. 20   d  is a sectional view taken along the line T—T of  FIG. 20   c.    FIGS. 20   c  and  20   d  show the state when the knock part  30  and the cartridge  40  move forward by a predetermined stroke distance, for example, 4 mm. At this time, the spherical door  90  rotates at a rotating angle of +90° in an opening direction, thus opening the nib hole  11  of the shaft  10 . 
   At this time, the cartridge  40  and the link  80  are placed inside the passage of the spherical door  90 , because the cartridge  40  is coupled to the link  80 , in a similar manner to the engagement between the clutch part  47  and the elastic pieces  80   a  according to the first embodiment. 
     FIG. 20   e  is a side view of the writing instrument according to the third embodiment, and  FIG. 20   f  is a sectional view taken along the line U—U of  FIG. 20   e.  When the cartridge  40  and the knock part  30  further move forward by a predetermined stroke distance, for example, 12 mm and thereby move forward by a predetermined total stroke distance, for example, 16 mm, the nib  41  and a part of the clutch part  47  are projected out of the nib hole  11 . When the switch  20  or the conventional cartridge control unit is locked in such a state, the user can write with the writing instrument. Meanwhile, the nib retracting operation is executed in reverse order to the nib projecting operation described above. 
   As described above, the present invention provides a slide-type writing instrument with a dry prevention unit, which is constructed so that a user needs not open or close a cap, thus being convenient to use, and which is constructed so that a nib hole of a shaft is opened or closed by a projection or retraction of a nib, thus preventing the drying of ink. 
   The slide-type writing instrument with the dry prevention unit is constructed so that an interior of the shaft storing the nib is sealed by a spherical door and an O-ring, thus prolonging a life span of ink, therefore maximizing product performance. 
   Further, the slide-type writing instrument with the dry prevention unit is constructed so that the spherical door is opened or closed in a direct transmission manner. Thus, a first spring and a link holder make the spherical door be in close contact with a rounded surface inside the nib hole, while controlling the operation of the spherical door, thus having excellent operational performance. 
   The slide-type writing instrument with the dry prevention unit is constructed to control an operation of the nib by holding the shaft corresponding to a body by one hand and manipulating a switch with the user&#39;s finger, thus being very convenient to use. 
   Further, the slide-type writing instrument with the dry prevention unit is constructed to remarkably reduce a space of the shaft for receiving the nib due to the coupling method between a clutch part of a nib extension part and elastic pieces of a link, thus reducing the amount of air entering the shaft during the operation of the spherical door, therefore more efficiently preventing ink of the nib from being dried up. 
   The slide-type writing instrument with the dry prevention unit is constructed to use a single spring or two springs. Regardless of using the single spring or two springs, the spherical door is rapidly opened or closed. Further, the nib is not in contact with the spherical door, so that it is possible to receive the nib in the shaft to be safely retracted into or projected out of the shaft. 
   Further, the slide-type writing instrument with the dry prevention unit is constructed so that no slit is formed on the nib extension part, thus enhancing a sealing effect using the O-ring. 
   Furthermore, the slide-type writing instrument with the dry prevention unit is constructed so that the O-ring is installed in the shaft while surrounding a part of the link holder, that is, the O-ring does not slide and the nib extension part of the cartridge slides while passing the O-ring, thus having more excellent sealing effect. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.