A syringe plunger is formed by a two-shot molding process. A hard plastic core is formed in a first mold, and the distal portion of the hard plastic core is subsequently overmolded with soft rubber. The result is a fully automated manufacturing process increasing quality and lowering costs, and a unique plunger having a hard plastic core and soft rubber exterior which are molecularly and permanently bonded together.

FIELD OF THE INVENTION 
The present invention generally relates to plungers used in syringes to 
vary a syringe's volume, thereby introducing or extracting fluid from 
human and animal bodies. 
BACKGROUND OF THE INVENTION 
Two-part molded plungers are available to drive and draw fluid through 
syringes. These plungers are made by fitting a molded soft rubber cover 
over a separately molded hard plastic core. The rubber cover is typically 
assembled by hand over the hard plastic core to form the completed 
plunger. When the plunger is inserted into a syringe, the soft rubber 
cover creates a seal with the inner circumference of the syringe so that 
fluid may drawn into or driven out of the syringe. The hard plastic core 
provides support for the rubber cover to prevent excessive deformation and 
possible leakage. 
Typically, that portion of the hard plastic core which supports the rubber 
cover against fluid pressure, must have a smooth exterior profile 
conforming to the profile of the rubber cover. This is necessary in order 
for the hard plastic core to support the rubber cover against fluid 
pressure generated when the plunger is driven into the syringe. Otherwise, 
the rubber cover would extrude into voids or depressions in the hard 
plastic core, weakening or damaging the rubber cover. 
Several problems have plagued prior art two-part plungers. First, hand 
assembly of the two parts of the plunger is expensive and time-consuming. 
Second, because the portion of the hard plastic core which supports the 
rubber cover against fluid pressure must have a smooth exterior profile, 
in some applications there are regions of the hard plastic core including 
large quantities of solid plastic. These large regions of molten plastic 
are difficult to adequately cool and cure in a conventional injection 
molding process, leading to delay in manufacturing and/or quality 
variations as a result of some cores being removed from the mold before 
the plastic forming the core is adequately cured. 
It would be desirable to provide a plunger that can be manufactured by a 
fully automated process, in which there is reduced potential for 
manufacturing delay and quality variation resulting from regions of 
uncured plastic. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a syringe 
plunger that is not assembled from a separate soft rubber cover and a 
separate hard plastic core into a final product, but rather is injection 
molded using a two-shot process, so that the plunger emerges from the mold 
as a complete finished product. 
A further object is to manufacture a syringe plunger which can be readily 
cooled and cured in a conventional injection molding process. 
In accordance with the invention, a two-shot mold is used to facilitate the 
manufacture of this invention. Using a two-shot mold, the plastic forming 
the plunger's hard inner core is first injected into a suitably formed 
mold. Once cured, the hard plastic core is placed into a second mold and a 
second molding is performed in which rubber is injected over the hard 
plastic core, forming a rubber exterior surface for the plunger. The 
second overmolding of rubber occurs within a reasonable time subsequent to 
the first molding of the hard plastic core, so that the rubber tends to 
molecularly bond to the underlying hard plastic core and form a single 
unit. 
In the disclosed specific embodiment, the two steps of the molding process 
occur in parallel; that is, while hard plastic is injected into one mold 
to form what will become the hard plastic core, rubber is injected into a 
second mold holding a previously molded core to form a soft rubber 
exterior. 
The core of this two-shot molded plunger is shaped so that it will cure 
rapidly, reducing warping when the core is subjected to the second 
injection that forms the soft rubber exterior. This is accomplished by 
molding the core to define at least one cavity in the distal surface of 
the core (i.e., that surface of the core which will be overmolded with 
soft rubber, and is adjacent to fluid in the interior of the syringe), 
thereby increasing the core's surface area and reducing the core's volume. 
The increased surface area and diminished volume of the core facilitates 
rapid cooling of the hard plastic core so that the plunger may be finished 
more quickly, thereby reducing manufacturing costs, and also reduces the 
likelihood for quality variation caused by deformation of the hard plastic 
core due to inadequate curing. 
Thus, the two-shot molded plunger and manufacturing process in accordance 
with the present invention reduces human labor involved plunger 
manufacturing, and reduces costs, while also reducing the opportunity for 
manufacturing defects caused by human assembly and/or inadequate cooling.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
Referring to FIG. 1, a syringe 10 in accordance with principles of the 
present invention includes a cylindrical body 12, advantageously formed of 
translucent plastic material, a narrow discharge end 14 through which 
fluid may be drawn or injected, and a flange 16 for connecting the syringe 
to an injector. Mounted inside of syringe 10 is a plunger 20 which seals 
against the cylindrical internal walls of body 12 and may be moved inside 
of body 12 to extract or inject fluid through discharge end 14. A distal 
end 22 of plunger 20 has a soft rubber exterior suitable for forming a 
seal with the cylindrical internal walls of syringe body 12. Proximal end 
24 of plunger 20 has a hard plastic surface for engagement to a plunger 
drive ram for moving plunger 20 into or out of syringe body 12. A 
mushroom-shaped button 26 extends from the proximal end 24 of plunger 20 
for engagement to jaws or other attachment devices on the plunger drive 
ram. 
Referring now to FIGS. 2 and 3, the two-shot overmolded structure of 
plunger 20 can be understood. Plunger 20 comprises a hard plastic core 30 
overmolded on its distal end with an exterior 32 of rubber. Notably, the 
overmolded rubber exterior 32 does not completely envelop hard plastic 
core 30, but rather only envelops the distal end 22 of plunger 20, 
including that portion 31 of the periphery of plunger 20 which forms a 
seal with the inner cylindrical wall of syringe body 12. At the proximal 
end of plunger 20, hard plastic core 30 is exposed. Button 26, therefore, 
is formed in hard plastic core 30, as are other structures on the proximal 
end of plunger 20, so that these areas are structurally strong and easily 
grasped by a plunger drive ram. 
Also notable in FIGS. 2 and 3, is that hard plastic core 30 has annular 
cavities 36 and 38 formed therein. Cavities 36 and 38 are enveloped by 
rubber exterior 32, and substantially filled with rubber in the completed 
plunger. As will be elaborated below, the presence of cavities 36 and 38 
in hard plastic core 30 reduces the volume of hard plastic core 30 and 
increases its surface area, thus facilitating cooling of core 30 during 
the molding process. Additional cavities 40 are formed in the proximal end 
of hard plastic core 30 for similar reasons (and to reduce the total 
volume of plastic consumed in manufacturing plunger 20). 
Referring now to the schematic FIG. 4, a manufacturing process for forming 
plungers such as plunger 20 will be understood. A molding apparatus 
includes a first section 42 and a second section 44. Sections 42 and 44 
may be pressed together to form molds for forming cores such as 30 (FIGS. 
2 and 3) and overmolding rubber exteriors such as 32 (FIGS. 2 and 3). 
Sections 42 and 44 may also be separated, as shown in FIG. 4, to eject 
molded plungers and to realign mold halves, as described below. Section 42 
includes mold halves 50 and 52, shown schematically on FIG. 4; these mold 
halves mate with mold halves formed in section 44 to form complete molds 
for forming cores and rubber exteriors for plungers. (As used herein, 
"mold half" refers to any portion of a mold which can be mated with one or 
more other mold portions to form a complete mold, and does not imply that 
the "mold half" is matable with only one other mold portion or that the 
"mold half" forms half of the complete mold, as measured by volume or 
otherwise.) 
As seen in FIG. 4, section 44 is rotatable relative to section 42, e.g. on 
a shaft 46, to provide separate alignments of mold halves in section 44 
and mold halves in section 42. 
Referring now to FIGS. 5, details of the mold halves in sections 42 and 44 
can be understood. Specifically, section 42 includes two mold halves 50 
which provide the surface outline of the distal surface of a hard plastic 
core 30 of a plunger. Notably, mold halves 50 include annular projections 
56 and 58 which define the exterior surfaces of cavities 36 and 38 of the 
hard plastic core 30 (FIGS. 2 and 3). As noted above, projections 56 and 
58 increase the surface area of the core 30 while simultaneously reducing 
its volume, facilitating cooling of the core subsequent to molding, and 
thereby decreasing the manufacturing time while increasing quality. 
FIG. 5 also shows valve structures such as 54 for controlling the injection 
of molten plastic into mold halves 50. The molten plastic supplied to mold 
halves 50 cures to a hard, structural plastic material, suitable for 
forming a structural core for plunger 20. 
When sections 42 and 44 are pressed together, mold halves 50 mate to mold 
halves 48 in section 44 to form complete molds for forming hard plastic 
cores 30. Mold halves 48 in section 44 define the exterior surfaces of the 
proximal end of hard plastic cores 30. Notable features of mold halves 48 
include a region above pin 72 for forming button 26. Further, mold halves 
48 include pins 60 (see FIG. 6A) which define cavities 40 in the proximal 
surface of plunger 20. The manner in which pins 60 interact with a plunger 
to facilitate removal of a plunger from mold halves 48 will be discussed 
below in connection with FIGS. 6A and 6D. 
Section 42 further includes two mold halves 52 forming cavities for 
overmolding rubber exteriors 32 on previously-formed plunger cores 30. 
Mold halves 52 have a generally conical shape, with features such as 
annular bumps 62 for shaping the sealing surfaces 31 of the rubber 
exterior 32 of plungers. Molten rubber is injected into mold half 52 
through an inlet, not shown in the Figs., which advantageously may be 
positioned within annular bumps 62 or elsewhere in the mold. As can be 
seen in FIG. 5, the soft rubber overmold covers 32 the entire distal end 
22 of the plunger core 30, producing a smooth rubber surface for sealing 
the interior of the syringe and driving fluid in the syringe. Notably, as 
seen in FIG. 5, rubber injected into mold half 52 fills or substantially 
fills cavities 56 and 58 in the hard plastic plunger core 30, resulting in 
a void-free plunger. 
The operation of the apparatus of FIG. 5 can be described in greater detail 
with reference to FIGS. 6A-6D. Specifically, referring to FIG. 6A, in a 
first step, sections 42 and 44 are pressed together such that a mold half 
48 in section 44 is pressed against a mold half 50 in section 42, 
producing a mold shaped in the form of a plunger core 30. Molten plastic, 
which will cure to a hard molecular structure, is then injected into this 
mold to form the plunger core 30. 
In the second step, sections 44 and 42 are drawn apart, as seen in FIG. 6B. 
As sections 42 and 44 are drawn apart, sliders 64, which are actuated by a 
spring 66, slide downward and outward away from section 42, releasing the 
cooling plunger core 30 from sliders 64 so that it may be extracted from 
mold half 50. At the same time, plunger core 30 remains held in place by 
mold half 48 in section 44, and in particular by pins 60 which extend into 
cavities 40 in the proximal end of plunger core 30. 
To prepare for the third step, sections 44 and 42 are rotated 180 degrees 
relative to each other, e.g., by rotating section 44 on shaft 46 as shown 
in FIG. 4. Doing so brings mold half 48 in section 44 into registration 
with a mold half 52 in section 42. Sections 42 and 44 and then pressed 
together to form a second mold shaped to form the exterior of the plunger, 
as seen in FIG. 6C. During this rotation, and the re-engagement of 
sections 42 and 44, the plunger core 30 formed in the first step remains 
affixed in mold half 48, specifically, pins 60 extend into cavities 40 in 
the proximal end of the plunger core 30 to hold the core in place. 
Accordingly, when the second mold is formed from mold half 48 and mold 
half 52, plunger core 30 is positioned inside of the second mold in the 
appropriate position for overmolding of rubber. 
The hard plastic core 30 begins cooling and curing immediately after 
injection of molten plastic ceases. Although the overmolding of rubber may 
be done after the core is completely cool, advantageously, overmolding is 
performed while core 30 is still curing and cooling. Accordingly, core 30 
is rapidly removed from the first mold 42 and placed into the second mold 
44, and rubber 32 is overmolded on core 30. If the core 30 is moved into 
the second mold while it is still cooling, the rubber injected in the 
second overmold will molecularly bond with the as-yet uncured plastic of 
the core 30, forming a secure bond between the soft rubber exterior 32 and 
the hard plastic interior 30. Accordingly, the separation, rotation, and 
re-engagement of sections 42 and 44 begins as soon as the core 30 has 
cooled to a sufficient extent to be self-supporting when mold half 50 is 
removed. 
In the third step, rubber 32 is injected into the second mold 44 formed of 
mold half 48 and mold half 52, producing a rubber exterior 32 on the 
plunger core 30 as seen in FIG. 6C. As noted above, the rubber injected 
during this step not only covers the distal end of the plunger core 30 and 
forms sealing surfaces 31 around the perimeter of the plunger, the rubber 
overmold also substantially fills cavities 36 and 38 in the plunger core 
30 so the resulting structure is a substantially solid plunger. 
In the final step, as the rubber 32 overmold is cooling, sections 42 and 44 
are again separated, removing the completed plunger 20 from mold half 52 
in section 42. At the same time, the completed plunger is ejected by 
motions which become apparent by comparing FIGS. 6A and 6C to FIG. 6D. 
Specifically, block 68, which is pressed to the outer surface of section 
44 by, for example, a shaft 69, is withdrawn from the surface of section 
44, causing the completed plunger 20 to move downward from the outer 
surface of section 44. Simultaneously, sliders 65, which are 
spring-actuated, for example by a spring 67, slide downwardly and 
outwardly along cam surfaces 70, withdrawing pins 60 from cavities 40 in 
the completed plunger. Ultimately, when block 68 reaches its innermost 
position, ejection pin 74 comes into contact with shaft 72, and actuates 
shaft 72 upward against the force of spring 76. The button 26 of the 
completed plunger is formed by the upper surface of shaft 72 during the 
molding process; accordingly, actuation of shaft 72 caused by ejection pin 
74 ejects the completed plunger 20 from section 44 and into a suitable 
storage bin, ready for insertion into a syringe. 
It will be noted that, from the construction of the molding apparatus, 
particularly as seen in FIGS. 4 and 5, the apparatus performs the first 
and second moldings in parallel. In particular, whenever sections 42 and 
44 are pressed together, two plunger cores are formed on a first side of 
the apparatus using mold halves 50, while simultaneously two 
previously-formed plunger cores are overmolded with rubber on a second 
side of the apparatus using mold halves 52. When sections 42 and 44 are 
subsequently separated, the two completed plungers formed in the second 
side of the apparatus using mold halves 52 are ejected, sections 42 and 44 
are rotated relative to each other, and sections 42 and 44 are again 
pressed together to place the two cores just formed with mold halves 50, 
into mold halves 52, and also to join the empty mold halves 48 from which 
finished plungers were just ejected, with mold halves 52, ready to form 
new cores. 
While the present invention has been illustrated by a description of 
various embodiments and while these embodiments have been described in 
considerable detail, it is not the intention of the applicant to restrict 
or in any way limit the scope of the appended claims to such detail. 
Additional advantages and modifications will readily appear to those 
skilled in the art. For example, the apparatus for forming plungers 
described here might be altered in various ways, e.g., by forming only one 
plunger core and overmold at a time, or by forming more than two cores and 
two overmolded exteriors simultaneously. The mold halves may mate in 
different ways and their relative motions between molding phases may be 
substantially different without changing in substance the manner in which 
a plunger is molded. The invention in its broader aspects is therefore not 
limited to the specific details, representative apparatus and method, and 
illustrative example shown and described. Accordingly, departures may be 
made from such details without departing from the spirit or scope of 
applicant's general inventive concept.