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What is a suction plunger?

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Suction plunger A suction plunger is used for clearing a blockage in a toilet. It is sometimes known as a coopers plunger.
Rubber plate on a suction plunger The suction plunger has two plates:

A rubber plate…

Plastic plate section on a suction plunger ….and a plastic one
Flexible rubber head The plastic plate section provides the flexible rubber with reinforcement. The plate prevents the rubber from over bending, which can result in splitting and tearing.

When the plunger is in use, the plastic plate helps to generate a greater plunging force, by stopping the rubber flexing too far and allowing water past the seal.

Because of the shape of the suction plunger it can only be used in toilet gullies, and in open drain ways. As it has no cup, it cannot create a seal over a drain.
Suction plunger in gulley The rubber part forces the water down the drain when plunging. This part of the plunger makes contact with the gulley, creating a barrier against the water and forcing it down towards the blockage.
Suction plunger in toilet gulley, showing direction of force The plunger is pushed into the gulley by pressing down on the T section of the handle, forcing the water down the drain under pressure. Because the plunger fits right down in to the gulley