Scheduling for high volume continuous production system


Continuous or repetitive manufacturing is characterized by long production runs of identical or similar discrete items through the same sequences of operation/processing steps. The production facilities are located one adjacent to another as per the sequence of operations/processing steps for an item in order to reduce the material handling distances and cost. Such arrangements are referred to as production lines or assembly lines and the manufacturing shops are called as flow shops.

The flow shop generally represents a mass production situation (high volume production) and hence, the operations become very efficient. The production control system for continuous production is called as flow control.

The production control problem in repetitive production is different from that of job shop. Since the routings are fixed, it is not necessary to plan and prepare individual route sheets. The processing steps, routing and work methods are primarily planned along with the design of the production system.

The waiting time between operations and the work-in-process inventory are both minimal. There are usually no queues of different kinds of jobs waiting at a work center and there is no need to determine priorities and dispatching materials from each work center, is not necessary.

In this type of production, individual parts are not scheduled and tracked unit by unit. It is necessary that every component of a product must be completed at a rate that is proportional to the quantity of the component used in the end product. Repetitive manufacturing is usually scheduled at daily rates of output.

Control of this type of production system would be simple when only one model of a product is produced at a uniform rate. A special purpose plant with a capacity of the desired output rate would meet the requirements. The problem is more complicated if the demand rate varies considerably throughout the year. In such cases, the system may produce at a uniform rate, which may lead to accumulation or depletion of inventory. Alternatively, the output rate can be varied by changing the number of working days per week, the working hours per day or the number of shifts that are worked. Also, it is possible to change the size of the labor force and reassign the work to achieve a different production rate.

However, scheduling and control will become more difficult, when several models are produced on the same line and when demand varies so that, different product mixes must be made from time to time.

Several approaches that can be used to meet the demand for varying volumes and product mixes in high-volumes and repetitive manufacturing operations are:

1. To have separate production facilities for each product and to vary the production rate in response to the demand pattern. But this would require a high investment in various facilities that would seldom be fully utilized.

2. To run one large facility on a product for a while and then change to another product for a while. In this case, the rescheduling and coordinating problems could be significant.

3. To stabilize the product-mix and the production rate for an extended period so that, many of the advantages of just-in-time production can be achieved.

Estimating the time required to perform jobs, which are standardized, can be simpler as compared to that in job shops. When a sizeable volume of a standard product is produced, the direct labor hours required to produce a unit may decrease considerably as more and more units are produced. This reduction in labor hours is significant enough, so that, it should be taken into account in scheduling delivery rates and in planning capacity utilization. This phenomenon is called learning curve, or an improvement curve or a progress curve or a manufacturing progress function.

Controlling Continuous Production

The major problem in flow shop production control (flow control) is to attain the desired production rate with maximum possible efficiency (i.e. max possible utilization of resources). For this, it is necessary to determine the technological process steps (sequence of operations), the required capacity of assembly lines, the number of personnel and the amount of work per person working on the assembly line. This is done by dividing the total work content of the job into elementary or basic operations and group operations at work stations without violating their pre-determined sequence. The jobs move successively from one work station to another work station till completion. The speed of the assembly line is controlled by the required output rate, the distance between successive work station and the operation time required at each work station. By controlling the speed of the conveyor in the assembly line, the cycle time (i.e. the time for which the job is available at each work station for completion of the operation) is controlled and also the output rate on the production line is controlled.

The technique is used in such a production situation where it is necessary to nearly equally divide the work to be done among the workers, so that, the total number of employees required on the assembly line is minimized, is known as line balancing technique or assembly line balancing.