Process control limits are the natural limits allowed in a process. There are many different parts to a process and each part has allowable tolerances and specifications. With a quality control process, a whole batch of products may be discarded if a sample number of the products do not meet certain tolerances during a stage of a manufacturing process.
Process control limits are not the same as quality control limits. It is the limit at which the system controlling the process is in control. Anything beyond the limit is considered as an indicator that the process is out of control.
Take for example a system that mixes different paints to produce paint of a very specific color. The process is 3 different colors of paint combined together to make a unique color. Let’s say that there is 10 liters of red, 10 liters of green and 1 liter of blue and the 3 colors are mixed with equal quantities. Obviously there is not enough blue paint to make more than 3 liters of the final mix and if the minimum limit of blue paint is 1 liter, then the system is considered to be out of control and will request maintenance.