| 摘要 |
A retractable needle hypodermic syringe includes a retractable needle assembly comprising a needle seat for supporting a hypodermic needle, and a generally tubular which comprises the barrel and serves as a reservoir for injectable or withdrawn fluid. The barrel is open at both ends to receive a plunger at the plunger end and a needle at the opposite end. A hollow plunger is slidably mounted within the barrel and extends from the plunger end for engagement by the user. A seal is provided between the plunger and barrel to prevent air from passing into or out of the barrel around the plunger and through the plunger end. A piston is slidably mounted within in the plunger such that movement of the plunger into the fill position locks the piston in place. A seal between the piston and the interior of the plunger prevents air or fluid from passing by the piston into the plunger. As the plunger is depressed to inject the fluid the plunger moves axially in the barrel while the piston is stationary. The relative axial movement of the plunger in relation to the piston causes a vacuum to be created within the plunger. The relatively smaller cross section of the plunger as compared to the barrel produces a corresponding greater vacuum force. The plunger in the fully depressed position releases the needle carrier from the end of the barrel and the vacuum pulls the needle carrier and needle into the hollow plunger.
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