Abstrict A disposable syringe comprises a syringe cylinder and a syringe
piston. The syringe cylinder is open at one end and at its other
end has an end wall, which is closed with the exception of a region
adjacent to a needle holder. The syringe piston comprises a piston
skirt, which protrudes from the syringe cylinder, and a piston head,
which is disposed within the syringe cylinder and provided with
an annular seal. The annular seal is connected to the piston head
by an annular peripheral flange. In order to provide a syringe which
can easily be actuated and is nevertheless absolutely tight, the
flange is frustoconical and flares toward the piston skirt. The
radial distance from the inside peripheral surface of the hollow
frustoconical flange to the piston skirt increases continuously.
The flange is adjoined by the annular seal, which consists of a
coaxial cylinder and freely protrudes on the side of the piston
head.
Claims I claim:
1. A disposable syringe comprising
a syringe cylinder having an inside peripheral surface, an open
first end and a second end, an end wall at said second end, the
end wall being provided with a hollow needle holder and being closed
with the exception of the region adjacent to said needle holder,
a syringe piston comprising a piston skirt that protrudes out of
said open first end of said cylinder and is provided with a handle
outside said cylinder, and a piston head disposed within said cylinder
and facing said second end thereof, the piston head having an end
face facing said second cylinder end and carrying a peripheral annular
flange that is provided with an annular seal in sliding contact
with said inside peripheral surface of said cylinder,
said end wall of said cylinder having an inside surface which,
except adjacent to said needle holder, conforms to said end face
of said piston head,
said flange having a frustoconical inside surface facing said first
end of said cylinder and said piston skirt and a frustoconical outside
surface facing said inside peripheral surface of said cylinder and
said second end of said cylinder,
said flange flaring toward said first end of said cylinder,
said inside surface of said flange and said annular seal defining
a peripheral annular space which is generally triangular in radial
section,
said end wall of said cylinder comprising a peripheral annular
portion which is generally triangular in radial section and conforms
to said annular space,
said needle holder being eccentrically disposed with respect to
said end wall and extending through said annular portion and opening
into said annular space,
the radical distance from said frustoconical inside surface to
said piston skirt continuously increasing toward said second end
of said cylinder, and
said annular seal being cylindrical and being joined to, and coaxial
with, said flange and freely protruding therefrom toward said end
wall of said cylinder.
2. The syringe set forth in claim 1 wherein said annular seal
has a smaller wall thickness than said cylinder and decreases toward
said annular seal.
3. The syringe set forth in claim 1 wherein said flange has a
smaller wall thickness than said cylinder.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a disposable syringe comprising a syringe
cylinder and a syringe piston, wherein the syringe cylinder is open
at one end and at the other end has an end wall, which is closed
with the exception of a region adjacent to a needle holder, and
the syringe piston comprises a piston skirt, which has a free end
that protrudes from the syringe cylinder and at that free end carries
a handle, and said piston also comprises a piston head, which is
disposed within the syringe cylinder and is provided with an annular
seal, which contacts the inside surface of the cylinder and is connected
to the piston head by a peripheral annular flange.
2. Description of the Prior Art
Such disposable syringes are generally made of plastic and are
intended to ensure an absolutely tight contact between the annular
seal and the inside surface of the cylinder during the intake and
delivery strokes of the piston and to permit the piston to be actuated
gently and with a minimum of force during the actual injection.
These two requirements are not satisfactorily met by the existing
disposable syringes, in which the annular seals consist of sealing
beads integrally formed on the piston head or of flanges provided
with two-legged sealing lips (Published German Application 20 25
379; French Patent Specification 15 00 009) so that the seal proper
is supported by means which are rather stiff in a radial direction
and a sufficiently strong pressure force is exerted between the
seal and the inside surface of the cylinder. But that design has
a result that the cylinder wall will expand during a storage of
the syringe for considerable time so that the sealing action is
more or less lost and it is no longer possible to actuate the piston
as uniformly as is desired because the strain has resulted in a
variation of the diameter of the cylinder. That situation cannot
be improved even by a closer fit between the seal and the inside
surface of the cylinder because this would result only in a requirement
for a stronger force for actuating the piston so that a sensible
actuation of the piston would no longer be possible. It has been
attempted to eliminate that disadvantage by an incorporation of
a solid lubricant in the material of the cylinder. But in that case
a long time must be permitted to elapse after the manufacture of
the syringe before said solid lubricant emerges from the material
for a lubricating action.
In other known syringes the piston head is caplike and fitted on
the adapter end of the piston skirt (Swiss Patent Specification
366126; French Patent Specification 1228933) and constitutes
a protruding conical sealing lip because its peripheral surface
defines a forwardly open annular wedge-shaped gap and said sealing
lip is integrally formed with an oppositely directed, second sealing
lip. In order to ensure the required tightness in both directions
in which the piston is actuated, the sealing lips must contact the
inside surface of the cylinder under a sufficiently strong pressure
force so that the seal has a high stiffness, with all the disadvantages
involved therein. That stiffness is further increased by the fact
that the piston head must be sufficiently firmly fitted on the piston
skirt.
All said known disposable syringes also involve the risk that a
leak may result from a canting of the piston as the syringe is actuated.
Such a canting cannot be prevented because there is a radial clearance
between the piston skirt and the syringe cylinder and because the
syringe is made of flexible material.
SUMMARY OF THE INVENTION
For this reason it is an object of the invention to eliminate said
disadvantages and to provide a disposable syringe which is of the
kind described first hereinbefore and distinguishes in that it can
be actuated with ease and that it is absolutely tight.
That object is accomplished in accordance with the invention in
that the flange has a frustoconical outside surface, which faces
the inside surface of said syringe cylinder and said end wall of
said syringe cylinder, and a frustoconical inside surface, which
faces said piston skirt and said open end of said syringe cylinder,
said flange flares toward said open end of said syringe cylinder,
the radial clearance between said frustoconical inside surface and
the piston skirt increases continuously, and the annular seal is
cylindrical and is joined to and coaxial to said flange and freely
protrudes toward said end wall. Owing to said shapes of the flange
and the annular seal, the seal is relatively softly supported in
a radial direction and a resilient sealing system is provided so
that the biasing forces being exerted will be very weak and an expansion
of the syringe cylinder need not be feared even during a prolonged
storage and, besides, the reciprocation of the syringe piston in
the syringe cylinder will involve only low frictional forces and
for this reason can be effected by a small actuating force. The
conical shape of the flange and the provision of the unsupported
and freely protruding portion of the annular seal will ensure a
compliance and resiliency in a radial direction so that the syringe
can be actuated as easily as is desired and the syringe is absolutely
tight. Owing to the special shapes of the flange and the seal the
pressure conditions existing in the cylinder space will influence
the sealing forces and will ensure that the pressure force under
which the seal must contact the inside surface of the cylinder will
match said pressure conditions. Any expansion of the syringe cylinder
or any canting of the piston can easily be compensated owing to
the resilience of the seal of the piston without any risk of a leak.
The compliance and resiliency of the means for supporting the seal
will be improved further if the piston head consists of a conical
member, which is mounted on the piston skirt and is continued by
the flange, and the included angle of the conical member exceeds
the included angle of the flange.
The resiliency of the seal of the piston can also be influenced
in that the annular seal, in a manner known per se, and optionally
also the flange has a smaller wall thickness than the syringe cylinder
and the wall thickness of the annular seal preferably decreases
in known manner toward its free rim and, in addition, the wall thickness
of the flange also decreases toward the annular seal.
In a desirable arrangement, the annular seal is provided in known
manner on the outside with peripheral annular sealing beads at both
end portions of its cylindrical shape because the linear contact
between the sealing beads and the inside surface of the cylinder
will result in more uniform and more accurate sealing conditions.
In an embodiment in which the end wall of the syringe cylinder,
with the exception of the region adjacent to the needle holder,
has an inside surface which conforms to the end face of the piston
head, and the needle holder is eccentrically disposed in the generally
triangular space between the flange and the annular seal and interrupts
the opposing generally triangular portion of the end wall of the
cylinder, the residual volume remaining in the syringe after the
injecting operation will be minimized and the removal of inleaked
air before an injecting operation will be facilitated.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1 and 2 are, respectively, an axial sectional view and an
end elevation showing a disposable syringe in accordance with the
invention.
FIG. 3 is an axial sectional view showing on a larger scale as
a detail the forward end of that disposable syringe.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An illustrative embodiment of the invention will now be described
in more detail with reference to the schematic drawing.
A disposable syringe 1 is composed of a syringe cylinder 2 and
a syringe piston 3. The syringe cylinder 2 and the syringe piston
3 consist each of a single piece made of a synthetic thermoplastic
material. The syringe cylinder 2 comprises at one end an end wall
5 which is closed with the exception of the region adjacent to
a hollow needle holder 4. At its other end, the syringe cylinder
2 constitutes a handle 6. The syringe piston 3 comprises a piston
head 7 and a piston skirt 8 which at its free end protruding from
the syringe cylinder 2 comprises a handle 9. The piston head 7 is
provided with a flange 10 which carries an annular seal 11 that
contacts the inside surface of the cylinder. The flange 10 has a
frustoconical inside surface, which faces the piston skirt 8 and
the open end of the syringe cylinder 2 and a frustoconical outside
surface, which faces the inside surface of the syringe cylinder
2 and the end wall 5 of said cylinder. The flange 10 flares toward
the open end of the syringe cylinder 2. The annular seal consists
of a cylinder, which freely protrudes from the flange toward the
end wall 5. The piston head 7 comprises a conical member 12 which
is mounted on the piston skirt 8 and is continued by the flange
10 but has a much larger included angle than said flange. The two
end portions of the cylindrical annular seal 11 are formed on the
outside with peripheral annular sealing beads 13 14 in sealing
contact with the inside surface of the cylinder. Besides, the end
wall 5 of the syringe cylinder 2 with the exception of the region
adjacent to the needle holder, has an inside surface which conforms
to the end face of the piston head. The needle holder 4 is eccentrically
disposed in the peripheral annular space 15 that is defined by the
flange 10 and the annular seal 11 and is generally triangular in
radial section. The needle holder 4 has an opening 16 which intersects
the peripheral annular portion 17 that is formed in the end wall
of the cylinder and is generally triangular in radial section.
The special design of the syringe piston 3 provides a resilient,
soft piston seal, which ensures an absolutely tight contact between
the piston and the cylinder and nevertheless permits the syringe
to be actuated easily. The pressure conditions occurring in the
syringe cylinder 2 will influence the contact pressures exerted
between the annular seal 11 and the syringe cylinder 2 and will
cause said forces to match the prevailing pressure conditions. For
instance, the injection operation will cause an overpressure to
be built up in the generally triangular space 15 adjacent to the
piston head 7 so that a force will be exerted which urges the annular
seal 11 radially outwardly and the sealing bead 13 at the free end
portion of the seal will be forced more strongly against the inside
surface of the cylinder so as to ensure a tight seal. During the
intake operation the negative pressure being built up in the generally
triangular space 15 will act on the flange 10 so that the sealing
bead 14 at the other end portion of the seal 11 will be strongly
forced against the inside surface of the cylinder and a tight seal
will thus be ensured also during the intake stroke.
Even variations of the diameter of the syringe cylinder 2 as a
result of an expansion of said cylinder or a canting of the syringe
piston 3 during an actuation of the syringe will readily be taken
up and compensated without any risk of a leak by the resiliency
and compliance of the annular seal 11.
Because the end face of the piston head and the inside surface
of the end wall conform to each other, the residual volume left
after an injection operation will be minimized and that conformity
and the eccentric arrangement of the needle holder will facilitate
the escape of inleaked air because any air bubbles will migrate
along the generally triangular space 15 into the opening 16 of the
holder so that such bubbles can reliable be ejected. |