Why can't the pressure head rebound after pressing the lotion pump?

Update:14 Sep 2020
Summary:

Why can't the lotion pump rebound after being pressed? […]

Why can't the lotion pump rebound after being pressed?
In order to avoid this situation, the following points should be checked during the production process:
First, check whether the spring length is the standard length.
The difference is very small, and the influence of the length on the piston cannot be ignored. If it is too short, it will cause the piston to miss its return stroke, directly causing the piston to not rebound or slowly rebound after pressing the head.
Second, check whether the thickness of the piston fits the valve body.
In the movement of the piston, any tiny gap will result in weak suction and unable to depress the discharge body. Because plastic has the characteristics of thermal expansion and cold contraction, ordinary pistons are very important to ensure the quality of the emulsion pump.
Finally, let us analyze the ontology.
Due to the stretchability of plastic, if the injection molding process is improperly operated, insufficient injection will cut the thin layer of the main body, resulting in uneven inner wall, and inevitably there will be gaps in the piston stamping process, resulting in no discharge .