Product review (2 stars): Um can a quality control person be present during manufacture please as this is just terrible. Not sure it is even worthy of 2 .
As others have stated, the design relies on gravity to close but the mechanics of the setup are not very precise so it does not always fully close.  If you have to add a spring or some other external device to aid in closing then you have modified the product which in turn means a design fail.

Besides the problem stated above, my unit has the black pipe coupling section with holes that do not line up with the shutter portion.  The holes are so far off that the cutting/stamping die must be waaayyyyy out of spec and I had to enlarge all eight holes to accommodate decent sized screws.  Um can a quality control person be present during manufacture please as this is just terrible.

Not sure it is even worthy of 2 stars since it needs too much custom modification.  But then again so many products do these days.