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
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 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
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
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.