referring now to the drawings and in particular to fig1 and 2 , a medical syringe is indicated generally by the numeral 20 . the syringe 20 has a hollow cylindrical barrel 22 ( also shown in fig3 ) which is open at the rear end and has a main body portion 24 and a hollow front end extension 26 of reduced diameter from the body portion 24 . the barrel also has transverse flanges 28 at the rear end thereof which are gripped by the fingers of a user of the syringe 20 . a hollow plunger 30 ( also shown in fig5 and 6 ) is inserted into the open rear end of the barrel 22 and is axially slideable therein . the plunger 30 has a resilient seal 32 encircling the plunger near the front end and sealing against an inner surface 34 of the barrel main body portion 24 . the seal 32 fits in an annular groove 36 ( shown in fig6 ) near the front end of the plunger 30 . both the barrel 22 and the plunger 30 as well as the other later described parts of the syringe 20 are made preferably of radiation resistant thermoplastic material such as polypropylene or the like by injection and coining molding or injection and compression molding or extrusion and compression molding of multiple cavities rotary stations . the plunger 30 has a rear end cap 38 which is snapped in place by means of an annular groove 40 which receives an annular rib 42 ( shown in fig6 ). the plunger 30 has an annular rib or protrusion 31 which holds the plunger in centered axial alignment within the barrel 22 . this rib 31 also serves to prevent the plunger 30 from being pulled out of the barrel 30 if the rib comes in contact with a reduced diameter portion 25 near the rear end of the barrel 22 as shown in fig4 . the front end of the hollow plunger 30 is sealingly closed by a disc shaped end wall 44 which is preferably molded as an integral part of the plunger 30 . the end wall 44 has an annular notch 46 extending around the periphery thereof which provides a thin membrane portion and a notch effect which makes the wall 44 easier to sever from the plunger as will be explained later . referring now to fig7 and 8 is a hollow stem 48 having a tubular body portion 50 , a radially outwardly extending annular flange or disc 52 molded integrally on the rear end of the stem 48 , and a threaded portion 54 on the front end thereof . the flange 52 has an annular notch 55 which is similar to the notch 46 in the end wall 44 and produces an annular membrane and a notch effect . this notch 55 and the notch effect makes it easier for the stem 48 to be severed from the barrel 22 and release the stem 48 from the barrel 22 . as an alternative , the annular notches 46 and 55 and their notch effect can be placed on the opposite side of the respective part on which they are shown in the drawings . the main objective of the notches is to provide a thin membrane portion and notch effect which is easy to sever when the plunger 30 is depressed . the notches 46 and 55 are circumferentially aligned with the annular cutting edges 78 and 80 so that the cutting edges pass through the thin membranes when cutting is performed . referring now to fig9 is a needle assembly 56 comprising a needle hub 58 to which is attached a needle 60 by suitable bonding adhesive . the hub 58 has a threaded portion 62 which engages the threaded portion 54 on the stem 48 . the threads in the present example are inclined at a steep angle and are commonly know as a &# 34 ; luer - lok &# 34 ; configuration . the needle and hub assembly 56 is screwed onto the stem 48 which in turn is attached to the barrel 22 preferably by ultrasonic bonding . annular ridge 64 on the flange 52 makes contact with ledge 66 to permit ultrasonic bonding of the stem 48 to the barrel 22 . it should be noted at this point that a sheath or cap 68 as shown in fig1 through 12 is shown in chain dotted lines in fig1 and 13 snapped onto an annular ridge 59 on the needle hub 58 . the sheath 68 has ribs 70 which makes it easy to grasp and rotate when screwing the needle assembly 56 onto the stem 48 . as shown in fig1 , the sheath has a plurality of longitudinal grooves 72 inside the ribs 70 . the grooves 72 are positioned to match with a plurality of fins 74 extending radially outwardly from the hub 58 as shown in fig9 . when the sheath 68 is snapped onto the hub 58 an annular inwardly facing groove 76 near the rear end of the sheath 68 snaps over the ridge 59 on the needle hub 58 and holds it in place . the engagement of the fins 74 and the grooves 72 permits the sheath 68 and the needle assembly 56 to be rotated together as a unit to screw the needle assembly 56 onto the threaded portion 54 of the stem 48 . the interior of the needle hub 58 has a threaded portion 62 which engages the threaded portion 54 of the stem 48 . it may be seen that various sizes of needles can be interchangeable by selecting and attaching the desired size of needle assembly 56 to the stem 48 . fig1 shows an enlarged version of the syringe 20 shown in fig2 with the plunger 30 approaching the flange 52 of the stem 48 . at this point it should be noted that the plunger 30 on the front end thereof has a sharp annular cutting edge 78 on the plunger 30 which severs the flange 52 of the stem 48 when the plunger 30 is depressed to a certain position . likewise the flange 52 of the stem 48 has a cutting edge 80 which severs the wall 44 of the plunger 30 . the cutting edges 78 and 80 are both inclined at an angle of approximately 5 degrees to a plane which is perpendicular to the axis of the syringe 20 . this angle of inclination is necessary to provide a pointed force and shearing action on both the plunger end wall 44 and the flange 52 when the plunger 30 is depressed forwardly beyond a certain distance . in operation fig1 shows the plunger 30 as the forward end approaches the front end of the barrel main body 24 . assuming that a liquid medication has been drawn into a liquid chamber 82 between the barrel 22 and the plunger 30 , as the plunger 30 approaches the front end of the barrel main body 24 , most of the liquid medication has been injected into the patient . as the plunger 30 is depressed further forward , the cutting edge 78 on the front end of the plunger 30 contacts and severs the flange 52 thereby releasing the stem 48 and permitting it to be propelled rearwardly by a compression spring 84 as shown in fig1 . as may be seen in fig1 , before release of the stem 48 , the spring 84 is held under compression between the flange 52 on the stem 48 and an annular retaining band 86 which is formed as a thickened portion on the inside of the front end extension 26 . immediately after the plunger severs the flange 52 , the cutting edge 80 on the rear end of the flange 52 severs the front end wall 44 from the plunger 30 and permits the stem 48 and the needle assembly 56 to be propelled rearwardly by the compression spring 84 into the hollow interior 88 of the plunger 30 as shown in fig1 where it comes to rest inside the hollow interior 88 of the plunger 30 as shown in fig1 where it is held in a retracted position by the spring 84 . as the plunger 30 is depressed to sever the stem flange 52 the cutting edge 78 can enter an annular channel 90 which provides clearance for easier forward movement of the plunger 30 and easier deflection of the flange 52 and also receives any remaining liquid medication from the liquid chamber 82 . it should be recognized that the important components which cause the release of the stem 48 and needle assembly 56 and the plunger end wall 44 are the cutting edges 78 and 80 and the notch effect at the annular notches 46 and 55 . these edges cooperate to permit the retraction of the needle 60 by being propelled rearwardly by the spring 84 . the inclined plane in which the edges 78 and 80 lie is the primary reason that these edges create a pointed force and shearing action on the disk 52 and the wall 44 and in which the notch effect is provided on the notches 46 and 55 , thereby providing a more positive and reliable release and retraction of the needle assembly 56 and stem 48 . it can be seen that the annular cutting edge 80 telescopes inside the annular cutting edge 78 , and in the embodiment showing in fig2 through 16 , this results in the severing of the stem disk 52 slightly before severing of the plunger end wall 44 . referring now to fig1 , another embodiment of the invention shows multi - angle cutting edges 78a and 78b on the plunger 30a and multi - angle cutting edges 80a and 80b on the stem disk 52a . these cutting edges perform the same function as the cutting edges 78 and 80 but with a different angle configuration . another embodiment shown in fig1 shows cutting edges 78c and 80c in which the axial length of the stem mounted cutting edge 80c is longer than cutting edge 78c on the plunger 30c . this causes the end wall 44c to be severed and released before the disk 52c . it can be seen that other needle release and retraction means can be used without departing from the scope of the inventions as long as the syringe uses a hollow plunger with a means of hydraulically sealing off the interior of the plunger from the fluid chamber of the syringe . it should also be recognized that the detailed contours and proportions of the various components such as the needle holding components and cutting edges can vary from some of the illustrations shown in the drawings and the components can be made from various alternative materials from those disclosed herein without departing from the scope of the invention . these and various other modifications can be made in the embodiments shown and described herein without departing from the scope of the invention .
a syringe for giving medical injections which has an internal mechanism for releasing and retracting a needle into the syringe after the injection has been given and retaining the needle and a hollow plunger inside the barrel of the syringe to reduce the risk of accidental needle sticks . the interior of the hollow plunger is initially hydraulically sealed off by a plunger front end wall from the fluid chamber of the syringe to prevent fluid from entering the interior of the hollow plunger . after the plunger has been depressed to expel fluid from the barrel , further depression of the plunger causes cutting punch type members to sever a disc shaped flange portion of a stem type needle retaining member and the plunger front end wall thereby permitting the spring to propel the needle and needle retaining member into the hollow plunger thereby retracting the needle and retaining member inside the barrel .