Answer to Question 1
The surge tank, when evacuated, provides a location for the air to flow when the tank is opened to a closed chamber. The atmospheric pressure pushes the material out of the chamber and into the surge tank. Without the surge tank, the chamber would have to be evacuated by a pump which can be a rather slow process.
Answer to Question 2
The mold could have an undercut on which the part might be hung-up. The vent holes might be so large or the material so soft when entering the mold that it penetrated into the vent holes and these are holding the part in the mold. The part may have shrunk into a male mold. The draft angles may be too small for easy release. The mold surface may be rough so that the part could have mechanically entrained into the mold surface or the surface may be so smooth that the part is electrostatically attached.
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