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A plunger is fittable within a cylinder of a pneumatic dispenser that discharges a viscous material. The plunger has a first portion located at the front, and a second portion located at the rear. The second portion is a hollow structure and has a circumferential wall. An inner circumferential surfa

A plunger is fittable within a cylinder of a pneumatic dispenser that discharges a viscous material. The plunger has a first portion located at the front, and a second portion located at the rear. The second portion is a hollow structure and has a circumferential wall. An inner circumferential surface of this circumferential wall has a tapered surface. The circumferential wall has a thickness dimension that decreases in the axial direction moving away from the first portion. Therefore, the circumferential wall easily displaces in the radial direction, because the bending stiffness decreases in the axial direction moving away from the first portion. The first portion is a solid structure that is more rigid than the second portion. The first portion also has a partition wall surface that separates the inner chamber of the second portion from the solid section of the first portion.

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1. A plunger for use by being fitted into a circular-shaped cylinder of a pneumatic dispenser that employs pressurized air to discharge a viscous material, wherein the plunger is configured to divide an inner chamber of the cylinder into a first sub-chamber that stores the viscous material and a sec

1. A plunger for use by being fitted into a circular-shaped cylinder of a pneumatic dispenser that employs pressurized air to discharge a viscous material, wherein the plunger is configured to divide an inner chamber of the cylinder into a first sub-chamber that stores the viscous material and a second sub-chamber into which the pressurized air is charged, the first and second sub-chambers being coaxially aligned with each other,the plunger has a first portion configured to face the first sub-chamber and a second portion configured to face the second sub-chamber, the first and second portions being coaxially aligned with each other,the second portion is a hollow structure having a circumferential wall that is configured to be coaxially aligned with an inner circumferential surface of the cylinder, the circumferential wall being an elastic structure that is elastically deformable in a radial direction of the plunger,the first portion is an at least substantially solid structure and is more rigid than the second portion,an outer circumferential surface of the first portion has a first annular groove and a first land, which extend circumferentially around an axial direction of the plunger,the first land is configured to locally oppose the inner circumferential surface of the cylinder,the first land has an outer diameter that is sized so as to provide a first radial clearance with the inner circumferential surface of the cylinder, the first radial clearance has a radial dimension that enables venting of air, which is within the first sub-chamber, from the first sub-chamber to the second sub-chamber when viscous material is filled into the first sub-chamber, while blocking viscous material from flowing from the first sub-chamber to the second sub-chamber due to the viscosity of the viscous material, and the first land is not displaceable in the radial direction with respect to the axial direction of the plunger,an outer circumferential surface of the second portion has a second annular groove and a second land, which extend circumferentially around the axial direction of the plunger,the second land has an outer diameter that is sized so as to be at least substantially in local contact with the inner circumferential surface of the cylinder such that said air venting and said viscous-material blockage are achieved, and such that pressurized air, which is within the second sub-chamber, is at least substantially prevented from flowing from the second sub-chamber to the first sub-chamber by leaking between the second land and the cylinder, and the second land is displaceable in the radial direction with respect to the axial direction of the plunger, andin a free state in which external forces are not being applied to the plunger, the outer diameter of the first land is smaller than the outer diameter of the second land. 2. The plunger according to claim 1, further having: a third land extending along an annular boundary between the first land and the second land,wherein the third land has an outer diameter sized so as to provide a second radial clearance with the inner circumferential surface of the cylinder, such that said air venting and said viscous-material blockage are achieved, andthe third land, the first land and the second land of the plunger are respectively configured to locally oppose the inner circumferential surface of the cylinder. 3. The plunger according to claim 2, wherein the outer diameter of the third land is at least substantially equal to the outer diameter of the first land. 4. The plunger according to claim 1, wherein the circumferential wall of the second portion has a thickness and a bending stiffness that decrease in the axial direction moving away from the first portion, such that the circumferential wall is more easily displaceable in the radial direction at a first end that is remote from the first portion than at a second end that is adjacent to the first portion. 5. The plunger according to claim 4, wherein: an inner circumferential surface of the circumferential wall is tapered such that an inner diameter of the circumferential wall increases in the axial direction moving away from the first portion, andan outer circumferential surface of the circumferential wall is non-tapered. 6. The plunger according to claim 1, further having: a deflector disposed on an interior side of the circumferential wall and having a work surface that is inclined relative to the axial direction of the plunger,wherein the deflector is configured to, in response to a flow of pressurized air that impinges on the work surface during operation of the pneumatic dispenser, generate, from the flow of pressurized air, forces in directions that cause the circumferential wall to radially expand, and to direct the forces onto the surface of the circumferential wall. 7. The plunger according to claim 1, wherein the first portion has a partition wall surface that separates an inner chamber of the second portion from a solid section of the first portion. 8. The plunger according to claim 1, wherein the plunger has a length in the axial direction that is about 70% or greater than the outer diameter of the first land. 9. The plunger according to claim 1, wherein the plunger has a surface coated with a synthetic resin having less adhesiveness than the surface of the plunger, whereby it is possible to reuse the plunger by removing any viscous material attached thereto by washing. 10. The plunger according to claim 3, wherein: an inner circumferential surface of the circumferential wall of the second portion is tapered such that an inner diameter of the circumferential wall increases in the axial direction moving away from the first portion,an outer circumferential surface of the circumferential wall is non-tapered such that a thickness and a bending stiffness of the second portion decrease in the axial direction moving away from the first portion and the circumferential wall is more easily displaceable in the radial direction at a first end that is remote from the first portion than at a second end that is adjacent to the first portion, andthe plunger has a length in the axial direction that is about 70% or greater than the outer diameter of the first land. 11. A pneumatic dispenser comprising: a cylinder having a circular inner circumferential surface surrounding a hollow inner chamber and a viscous material discharge port located at one end thereof, anda plunger slidably fitted in the cylinder such that the plunger divides the hollow inner chamber into a first sub-chamber that holds viscous material and a second sub-chamber, into which pressurized air is chargeable, the first sub-chamber being coaxially aligned with the second sub-chamber,wherein a first portion of the plunger faces the first sub-chamber and a second portion of the plunger faces the second sub-chamber, the first portion being coaxially aligned with the second portion,the second portion is a hollow structure having an elastic circumferential wall that is coaxially aligned with the inner circumferential surface of the cylinder and is elastically deformable in a radial direction of the plunger,the first portion is an at least substantially solid structure that is more rigid than the second portion,a first annular groove and a first land are respectively defined on an outer circumferential surface of the first portion and circumferentially extend about an axial direction of the plunger,a first radial clearance is defined between the first land and the inner circumferential surface of the cylinder and has a radial dimension that enables venting of air, which is located within the first sub-chamber, from the first sub-chamber to the second sub-chamber when viscous material is filled into the first sub-chamber, while blocking viscous material from flowing from the first sub-chamber to the second sub-chamber due to the viscosity of the viscous material,the first land is not displaceable in the radial direction with respect to the axial direction of the plunger,a second annular groove and a second land are respectively defined on an outer circumferential surface of the second portion and circumferentially extend around the axial direction of the plunger,the second land at least substantially contacts the inner circumferential surface of the cylinder such that said air venting and said viscous-material blockage are achieved, and such that pressurized air, which is located within the second sub-chamber, is at least substantially prevented from flowing from the second sub-chamber to the first sub-chamber by leaking between the second land and the inner circumferential surface of the cylinder,the second land is displaceable in the radial direction with respect to the axial direction of the plunger, andin a free state in which external forces are not being applied to the plunger, the outer diameter of the first land is smaller than the outer diameter of the second land. 12. The pneumatic dispenser according to claim 11, further having: a third land annularly extending on an outer circumferential surface of the plunger and located between the first land and the second land,wherein a second radial clearance is defined between the third land and the inner circumferential surface of the cylinder that enables said air venting and said viscous-material blockage. 13. The pneumatic dispenser according to claim 12, wherein the third land has an outer diameter that is at least substantially equal to the outer diameter of the first land. 14. The pneumatic dispenser according to claim 13, wherein the circumferential wall of the second portion has a thickness and a bending stiffness that decrease in the axial direction moving away from the first portion, such that the circumferential wall is more easily displaceable in the radial direction at a first end that is remote from the first portion than at a second end that is adjacent to the first portion. 15. The pneumatic dispenser according to claim 14, wherein: an inner circumferential surface of the circumferential wall is tapered such that an inner diameter of the circumferential wall increases in the axial direction moving away from the first portion, andan outer circumferential surface of the circumferential wall is non-tapered. 16. The pneumatic dispenser according to claim 13, further having: a deflector disposed on an interior side of the circumferential wall and having a work surface that is inclined relative to the axial direction of the plunger,wherein the deflector is configured to, in response to a flow of pressurized air that impinges on the work surface during operation of the pneumatic dispenser, generate, from the flow of pressurized air, forces in directions that cause the circumferential wall to radially expand, and to direct the forces onto the surface of the circumferential wall. 17. The pneumatic dispenser according to claim 15, wherein the first portion has a partition wall surface that separates an inner chamber of the second portion from a solid section of the first portion. 18. The pneumatic dispenser according to claim 17, wherein the plunger has a length in the axial direction that is about 70% or greater than the outer diameter of the first land. 19. The pneumatic dispenser according to claim 18, wherein the plunger has a surface coated with a synthetic resin having less adhesiveness than the surface of the plunger. 20. A plunger configured to slidably fit in a hollow circular cylinder such that the plunger divides a hollow inner chamber of the cylinder into a first sub-chamber that holds viscous material and a second sub-chamber, the first sub-chamber being coaxially aligned with the second sub-chamber, the plunger comprising: a first portion configured to face the first sub-chamber, the first portion being an at least substantially solid structure, anda second portion integrally coupled to, and coaxially aligned with, the first portion, the second portion being configured to face the second sub-chamber and being a hollow structure having an elastic circumferential wall that: (i) is less rigid than the first portion, (ii) is coaxially aligned with the inner circumferential surface of the cylinder, and (iii) is elastically deformable in a radial direction of the plunger,wherein a first annular groove and a first land, which has a first outer diameter, are respectively defined on an outer circumferential surface of the first portion and circumferentially extend about the axial direction of the plunger,a second annular groove and a second land, which has a second outer diameter, are respectively defined on an outer circumferential surface of the second portion and circumferentially extend around the axial direction of the plunger, andin a free state in which external forces are not being applied to the plunger, the first outer diameter is smaller than the second outer diameter. 21. A dispenser comprising: a cylinder having a circular inner circumferential surface surrounding a hollow inner chamber and a viscous material discharge port located at one end thereof, anda plunger slidably fitted in the cylinder such that the plunger divides the hollow inner chamber into a first sub-chamber that holds viscous material and a second sub-chamber, the first sub-chamber being coaxially aligned with the second sub-chamber,wherein a first portion of the plunger faces the first sub-chamber and a second portion of the plunger faces the second sub-chamber, the first portion being coaxially aligned with the second portion,a first annular groove and a first land are respectively defined on an outer circumferential surface of the first portion and circumferentially extend about an axial direction of the plunger,a first radial clearance between the first land and the inner circumferential surface of the cylinder is defined circumferentially around the first land and has a radial dimension that enables venting of air, which is within the first sub-chamber, from the first sub-chamber to the second sub-chamber when viscous material is filled into the first sub-chamber, while blocking viscous material from flowing from the first sub-chamber to the second sub-chamber due to the viscosity of the viscous material,a second annular groove and a second land are respectively defined on an outer circumferential surface of the second portion and circumferentially extend around the axial direction of the plunger, andthe second land at least substantially contacts the inner circumferential surface of the cylinder circumferentially therearound such that said air venting and said viscous-material blockage are achieved, and such that air, which is within the second sub-chamber, is at least substantially prevented from flowing from the second sub-chamber to the first sub-chamber by leaking between the second land and the inner circumferential surface of the cylinder. 22. The dispenser according to claim 21, wherein, in a free state in which external forces are not being applied to the plunger, an outer diameter of the first land is smaller than an outer diameter of the second land. 23. The dispenser according to claim 22, wherein: the second portion is a hollow structure having an elastic circumferential wall that is coaxially aligned with the inner circumferential surface of the cylinder and is elastically deformable in a radial direction of the plunger,the first portion is an at least substantially solid structure that is more rigid than the second portion,the first land is not displaceable in the radial direction with respect to the axial direction of the plunger, andthe second land is displaceable in the radial direction with respect to the axial direction of the plunger.