A knockout plug is a removable piece of material that blocks an unused orifice. Each knockout plug is typically stamped from the same type of material as the tube or hole it blocks. The name is derived from the fact that if the orifice needs to be used, the plug can literally be knocked out. One common use for a knockout plug is the dishwasher connection found on many garbage disposals. Knockout plugs are also used in a variety of other applications, such as electrical boxes, dashboards, and control panels.
Most garbage disposals include an inlet tube that a dishwasher can be connected to. When the dishwasher drains down, it can push water into the disposal through this tube. Since not all homes have a dishwasher, disposals with this feature will typically have the opening blocked off by a knockout plug. This keeps the unit from leaking if nothing is connected to the dishwasher inlet. If there is a dishwasher, then the plug can easily be knocked out with a punch and hammer and removed prior to installation.
Another common use of knockout plugs is in electrical boxes. The box will typically have a number of possible ways to connect a conduit, each of which will be covered from the factory with a knockout plug. After the electrician has determine how many electrical conduits will connect to the box and in what configuration, he can use a hammer and punch to remove the appropriate plugs.
Control panels, dashboards, and other similar applications will often use knockout plugs. An example is an automotive dashboard, where the same unit may be used in vehicles that include different options. Rather than creating a unique dash for each application, a single dash with a variety of knockout plugs may be used. This can result in a vehicle that lacks certain options having plugs of some form on the dash where switches, gauges or lights would be located on other models.
A knockout plug typically can not be reinstalled after it has been removed, though there are often ways to simulate the functionality. The dishwasher inlet on a garbage disposal can typically be plugged with a rubber end cap. In order to prevent leakage, the cap may be fastened in place with a worm gear clamp. Electrical boxes often have replacement plugs as well, which will typically snap in place to block off any unused conduit holes and prevent electrocution.