Slip joints are useful for sink drains because they make the drain traps easily accessible for cleaning. Nearly anyone who can reach the pipes is able to remove them for cleaning, repair or replacement. There are four essential parts to a slip joint: the two pieces of pipe, a nut and a rubber washer. Some less expensive kits are sold with plastic washers.
There are two common materials used for making sink drain traps: PVC, which is a plastic, and Brass, often plated to look silver in color. PVC has become more common, is cheaper and is usually reliable as long as a quality kit is used. Cheap plastic kits are also sold, but will eventually leak and are sometimes difficult to install with good seals.
Slip joints, whether made from plastic or metal pipe, fit together by inserting one pipe into the other. The ends that fit together differ in size only enough for the two pipes to fit. You slip one pipe into the other, thus the name "slip joint." The larger pipe has an end that is threaded. A nut with a rubber washer is placed onto the smaller pipe with the nut's threads facing the larger pipe's threads. The nut is threaded onto the larger pipe, joining the two pieces together.
Some kits differ a little. The smaller pipe will have a flange on it set back an inch or two from the end of the pipe that fits into the large pipe. A washer is placed against this flange on the side that fits into the larger pipe.
Another type of slip joint uses a plastic washer with a flange to fit into the end of one of the pipes. Whichever type of kit you use, it is the washer that prevents the joint from leaking.
The seal between the two pipes is made by the washer and the pressure of the nut. The pressure of the nut on the washer compresses it enough to make a tight seal at the point where the larger pipe meets the smaller pipe.
Difficulties with achieving a poor seal and having a leak often occur with cheap, thin plastic drain kits. Deformation of one of the pipes changes it from a circle to a slightly oval shape. A good pressure seal is no longer possible and the joint leaks.
Slip joints are a reliable way to join two pipes together as long as they are used for the purpose for which the pipe was intended. Sink drains are the most common use for slip joints, but other uses are possible.