Computation Section
Process Flow Analysis
 - Models

The process model includes a number of features that are implemented by computational procedures. The results depend on assumptions concerning the manufacturing process that are best described by providing the explicit models.

Features are selected by clicking boxes and filling number fields on the Add Process dialog. The pages of this section explain the purposes of each feature, the assumptions concerning the manufacturing process and the models used for computations. This page summarizes the models and the links on the titles and on the left lead to more detailed descriptions.


Sometimes it is necessary to set up machines in order to change from one product to another. The principal effect of a setup is wasted machine time without useful production. In order to reduce the loss, several units of product, called a lot, are processed for each machine setup. This model allocates the setup time to the units in a lot.

It may be necessary to recycle units through an operation to correct manufacturing defects or as part of the manufacturing process. The model adjusts operating time and scrap to account for the recycling.

Some operations may be performed in batches. Here when a batch size other than 1 is specified, we assume that all items in the batch are processed simultaneously. A common example is a heating process where several units are placed in a furnace and treated for a fixed amount of time.

Manufacturing operations may introduce defects into units. We assume here that a defect will ultimately cause the unit to fail. Defective units pass through the process until an inspection operation is encountered. There defective items are removed. Probability models estimate the proportion of units that receive defects and the proportion of units that are removed from the flow.
There will be times when a process experiences downtime, that is, time when the resources associated with production becomes idle. Downtime may be caused by a failure of a machine that causes production to stop until the problem is located and repaired. It may also result if some raw material is unavailable. Then production must stop until the raw material is obtained. Usually, downtime is disruptive and should be eliminated if possible, but a correct model should recognize the existence of downtime.
Queues and Inventories
Queues and inventories are the result of variability in the flow through system. Although the process flow models do not represent variability explicitly, it is possible to include models that estimate the average delays caused when units must pass through these components. These features are modeled with the Time Function feature.
Production Lines
It may be that operations and inspections are arranged in production lines in which each operation is timed to correspond to a common cycle time. The cycle time may be determined by the operation in the line with the greatest operation time or some time interval determined by other stages in the process or the demand rate for the product. When this is true the time for the operation is governed by the cycle time of the line.
Resources are used to manufacture products. A resource is characterized by time available during some interval of time, say hours per week. Production during the week uses up the time available for resources, but the resources remain for the next week. Resources include machines, space and labor. In the context of an application a variety of resources might be defined. The economic analysis allows specification of resource investment costs, operating costs, number and availability. Resources play an important role in most analyses.
Raw Materials
Every product requires material inputs into one or more operations. The amounts used depend on the product demands and the amounts of flow through the operations per unit of product. Raw materials generally have costs that affect the profitability of the product. They may also be limited in amount and may cause bottlenecks to production. The main difference between resources and raw materials is that resources have a time dimension and production uses up the time-capacity of the resources. Raw materials expenditures depend on the number of items passing through operations, rather than the time spent.


Return to Top

tree roots

Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved

Next Page