A complex manufacturing process is often broken down into smaller steps. The output of one subprocess feeds into the input of the following, and so on, until the finished product is manufactured and shipped to the clients. To ensure that the goods possess the intended quality characteristics, the outputs of every subprocess must meet the quality standards required at that phase.

Let’s use the cement manufacturing process to understand the importance of quality control in manufacturing subprocesses for overall product quality.

Cement is manufactured by combining finely crushed gypsum with clinker. Fly ash and blast furnace slag are common additives used in cement manufacturing, depending on the products' needs. Clinker is an intermediate product made from limestone. The overall cement manufacturing process can be subdivided into the following smaller processes.

§ Raw Material Extraction: This process starts with mining limestone and, subsequently, the size reduction in a crusher. The primary aim is to control limestone size so that the subsequent milling process's output is not hampered. Maintaining the chemical composition (Lime Saturation Factor, LSF) is also essential at this stage to ensure the quality of the limestone heap in the next stage.

§ Raw Material Preparation: The crushed limestone from the crusher is preblended in Stacker along with other purchased materials such as bauxite, iron ore, low-grade limestone, etc. The purpose is to get a homogenized heap of materials in terms of LSF to minimize the quality variation. These homogenized materials are then reclaimed and fed to the raw mill for further size reduction.

§ Raw Meal Preparation: The material extracted from the reclaimer is then fed to either a ball mill or a VRM for further size reduction, and thus, raw meal is produced. The raw meal is homogenized in a raw meal silo to reduce variation before being fed to the kiln.

§ Clinkering: The homogenized raw meal, produced in a raw meal silo, is then fired at a temperature of around 1400 degrees Celsius using natural gas, pet coke, or ground coal in a kiln to produce clinker. Clinker thus produced is cooled in a clinker cooler and then stored in the clinker storage silo.

§ As a subprocess to this stage, coal or pet coke is pulverized to get fine coal and fired in the kiln to generate the required temperature for clinker production. To produce the uniform quality of clinker, the quality of the raw meal and the fine coal must be controlled.

§ Grinding: The clinker thus produced is extracted from the bottom of the clinker silo and is fed to cement mills, along with gypsum, for cement preparation. Other additives, like fly ash, blast furnace slag, etc., are also used as required.  Ground cement is conveyed to the cement silo for storage and subsequent packing operation. The quality of clinker, gypsum, and other additives are the guiding factors in controlling final product quality.

§ Shipping: This is the final stage which starts from the extraction of cement from silos, its packing in bags, and final loading in trucks or wagons. For bulk loading, the packing step is avoided. The internal conditions of silos play a significant role degradation of cement quality due to moisture ingress. Also, inadequate control during product change-over while packing may cause quality-related issues. 

The above discussion clarifies that to maintain uniform and correct product quality, the manufacturing unit should develop its own Quality Plan to ensure the quality of the incoming raw materials and the quality of all the intermediate products that come out of one stage and are fed to the next stage.

Implementing Statistical Process Control techniques helps monitor quality at each stage and ensures corrective actions in real-time if deviations from the targeted value occur.

Those who want to understand further the implementation techniques of SPC and the benefits that can be reaped of, can follow my book on Amazon.com. (Refer: The Essence of Quality: Improve Operational Excellence Through SPC, Streamline Process Efficiency, Boost Customer Satisfaction, Drive Cost Savings, ... Control (Manufacturing Excellence Series): 9798322979555: Sarkar, Suman: Books)