Disposable injection pen

A disposable injection pen, comprising:

• a pen casing;
• an injection head mechanism;
• and
• a trigger mechanism;
• wherein the injection head mechanism comprises a barrel arranged in the pen casing and a needle arranged on a top of the barrel;
• and a rubber plug is provided in the barrel;
• and
• the trigger mechanism comprises a protective sheath arranged in the pen casing, and the protective sheath is configured to movably wrap the needle;
• a bottom of the protective sheath movably abuts against a sliding part;
• an outside of the sliding part is sleeved with a secondary spring whose bottom is fixedly connected to a bottom of the pen casing;
• a push rod with an open bottom is movably clamped inside the sliding part;
• an inner top of the push rod is connected to an inner bottom of the sliding part through a primary spring;
• when the protective sheath is pressed, the sliding part moves downward such that the push rod moves upward to push the rubber plug forward;
• and after an injection is completed, the push rod is detached from an inner cavity of the sliding part, and the sliding part is driven by the primary spring to hit the bottom of the pen casing to produce a sound that indicates completion of the injection
• wherein the pen casing comprises an upper casing portion and a lower casing portion fixedly clamped with each other;
• two sides of an inner wall of the lower casing portion are each provided with a protrusion;
• the protrusion is fixed to an inner wall of the barrel;
• and two sides of the protective sheath are each provided with an avoidance hole fitting the protrusion;
• wherein the sliding part comprises an outer sliding block and a main body;
• two sides of the outer sliding block are each provided with a guiding groove, and a guiding plate is fixed longitudinally in the guiding groove;
• and the main body of the sliding part is configured to slide along the guiding plate to be sleeved into an inner cavity of the outer sliding block.