SPC

The Use of Statistical-Process-Control (SPC)



The Use of Statistical-Process-Control (SPC)

Plastics Compounding

Few resins are useful in their natural form and are therefore mixed with other materials to improve and enhance their properties and thereby make them more useful for a variety of applications. The process by which ingredients are intimately mixed together into as nearly a homogeneous mass as possible is known as compounding. And because of the nature of both the resin and the other ingredients, compounding requires a wide range of mixes such as dry powders, slurries, pastes, doughy consistencies, and a corresponding range of mixing operations. Controlling these processes is the concern of the compounders.

Several years ago, SPC was thrust upon American manufacturers as a requisite for doing business with the major auto makers. Among plastics processors this SPC mandate impacted parts producers first. In short order, the resin suppliers followed suit, and it became obvious that compounders were going to have to implement SPC too. Therefore, quality has become a major concern of compounders these days, as unavoidable as death and taxes, and equally unsavory. Over the past 14 years, the quest for high-quality incoming materials, which began in the auto industry with General Motors, has been embraced by even the smallest customers.

Review of SPC/SOC in Manufacturing Side of Plastics Compounding

Our manufacturing sector needs process control because all systems consisting of machines, processes, and individual workers vary in their performance. This tendency toward variation makes it impossible for any system to produce the exact same product, hour after hour, day after day.

SPC is a scientific approach to manufacturing that employs statistical methods to analyze and control a production process. It is basically a monitoring technique that uses statistical signals to indicate whether the process should be changed or left alone.

The approach involves inspection of the process and uses statistical signals before production takes place. They indicate a situation that could result in the production of nonconforming products. The feedback loop is timely enough to allow corrective action, and no bad products are made. For example, when a process experiences seven consecutive points beneath the target or average, corrective action would be taken.

SPC/SOC in the Real World of Plastics Compounding

Many compounders are “doing” SQC charts these days by generating “wallpaper” just to impress an auditor. These graphical representations of data, collected from the material, run on an after-the-fact basis. Therefore, they cannot be used to gain control over the current process effectively. Consequently, the implementation of such a limited statistical control program may satisfy some of the customers' needs but does nothing to improve the compounder's competitive position.

Controlling the Plastics Compounding Process

Not surprisingly, quite a few compounders think the most difficult aspect of the process to control is the blending of the material, achieving a homogeneous mix of fillers and reinforcements in the base resin. That is probably why compounders are in business. The difficulty of blending prevents automotive manufacturers from buying fiberglass and dumping it into polypropylene or ...