A process is a method of doing something and generally involves a number of steps or operations. Process design is the developing and designing of the steps. Everything we do involves a process of some description. Banking, preparing a meal, and driving a car are all processes.
Factors Influencing Process Design
A number of factors influence process design, but the following are among the most important.
Product design and desired quality. The product's design determines the basic processes needed to convert the raw materials and components into the finished product. Generally there are a variety of different machines or processes that can do the job. Whichever the process designer selects will depend on the quantity to be produced, the available equipment, and the quality needed. The desired quality affects the process design because the process must be capable of achieving that quality and doing it repeatedly.
Flexibility. If a wide variety of products is made, the process must be flexible enough to respond to changes quickly. For example, if a full-service restaurant sells a variety of foods, the process must be flexible enough to switch from making full-course meals to making pizzas. Just as the process must be flexible in what it produces, the equipment and personnel must be flexible and capable of doing a number of different jobs.
Depending on the product design, volume, and available equipment, the process engineer must design the system to make the product. Based on material flow, there are three ways that processes can be organized: line-flow, intermittent, and project fixed position. We will discuss the first two.
Which system is used will depend upon the volume, the range of products to be produced, and the ease or difficulty of moving material. All three systems can be used to make discrete units such as refrigerators or bread, or to make nondiscrete products such as dairy or petroleum products.
Workstations needed to make the product, or family of similar products, are grouped together in one department and are laid out in the sequence needed to make the product. Work flows from one workstation to another at a nearly constant rate and with no delays. The typical line-flow pattern is shown in figure 2.
These systems are suitable for a limited range of products that are made or assembled in the same way, for example, automobiles, televisions, and microwave ovens. Because workstations are arranged in the sequence needed to make the product, the system is not suitable for making a variety of different products. Television sets cannot be made on an automobile assembly line. Therefore, the demand for the family of products must be large enough to economically justify setting up the line. Flow systems are usually cost-effective for the following reasons:
• Because workstations are designed to produce a limited range of
similar products, machinery and tooling can be specialized.
• Because material flows from one workstation to the next, there is
very little buildup of work-in-process inventory.
• Because of the flow system and the low work-in-process inventory,
process lead times are short.
• In most cases, because of the volume produced, line-flow systems
substitute capital for labor and standardize what labor there is into
Because it is so cost-effective, this system of processing should be used wherever, and to whatever extent, it can be.
An intermittent process is characterized by production in lots or batches at intermittent intervals. Thus, any one workstation must be able to process many different parts. Setup times are relatively short, and the work center can be shifted quickly from one operation to another. However, run times will be longer than for a line-flow system. Generally equipment and labor are organized into departments based on similar types of equipment or skills. Work moves only to those workstations required and will skip the rest. This results in a jumbled flow pattern, as shown in figure 3.
Intermittent operations are very flexible in being able to change either product or volume. However, this flexibility causes severe problems for controlling the flow of work. It usually creates a high level of work-in-process inventory and, because the process is more manual, quality may suffer. In addition, the product will spend much time waiting to be worked on, and the throughput time (the time from start to completion) will be long.
If the volume of work exists to justify it, line-flow processing is cheaper than intermittent processing for the following reasons:
• Because there are no, or few, changes in setups, setup costs are low.
• Because workstations can be designed for specific products, run
costs are low.
• Because products move continuously from one workstation to an
other, work-in- process inventories are low.
• Because work flows through the process, costs associated with con
trolling production are low.
But there must be enough volume of specific parts to use the capacity of the line and justify the capital cost.
To Be Continued