Introduction to Performance Evaluation
This section lays the groundwork needed for understanding and utilizing
the performance evaluation model. The purpose, scope, and assumptions
underlying the model are explained.
It should first be stated that the cost-based comparison of the two
process variants is only half of the picture, and that a process
comparison model is incomplete without some sort of relative
performance evaluation. Hence for the sake of completeness, a simple
methodology for such is proposed. Based on collaboration with several
SM and MM molders, including Space Limited, Multishot, and Black &
Decker, several important performance issues were identified. Based on
this information, corresponding performance aspect definitions and
measurement techniques were developed as detailed below.
The purpose of this model is similar to that of the cost estimation
model. The only difference is that performance measurement metrics will
be used in stead of a cost-based metric. Specifically, the model is
designed to help decision makers choose between two process variants
based on several measurable performance characteristics. The CAD model
of the products would be input to the model, and with minimal other
user inputs, the performance aspects would be evaluated and output
side-by-side so a decision could be made. Based on the numerical
results obtained through evaluation of the model, the exact decision
process is left up to the product designers. Specific decision-making
methods (such as decision-based-design or multi-objective optimization)
are not considered.
The model is designed to evaluate several performance aspects of a
product through geometric analysis of the CAD model. Although full
automation is the eventual goal of this model, much of the CAD analysis
will be manually performed for the time being (i.e. certain things must
be input, like the mold parting direction, etc.).
Unlike the cost estimation model, this model would not have a graphical
output like quantity-cost curves. This model's output is simple
numerical values that can be displayed as a table for relative
comparison purposes. Each row of the table would correspond to an
independent measurable physical characteristic, and the columns would
represent the measured values for both SMM&A and MMM variants.
Unlike the cost estimation model, this model would require very few
equation evaluations - much of the needed information should be
extracted directly from the CAD files.
The model will not use a weighting system to rank the various
performance aspects. This is because the relative performance of the
performance aspects is both subjective and application-specific. For
instance, for a certain product, weight may be a very important
consideration, while assembly clearances are not. We will leave this
weighting to the model user.
In stead of providing a concrete decision-making model, this model is
simply intended to help evaluate certain relevant physical
characteristics of the molded products. The output of this model can be
input to a suitable decision-making model, such as a multi-objective
optimization routine. It is recommended that a strategy, such as
plotting Pareto-optimal frontiers be used to help the decision makers
choose the better variant based on their specific needs.
Model Scope and Assumptions
The scope of the
model is the same as the cost estimation model. All of the assumptions
still hold, but are less important for this model, as they are not used
to evaluate most of the performance characteristics.
Model Application Example
This model could
be used in any of the application examples of the cost estimation
model. Here, instead of evaluating the manufacturing costs of each
variant, their relative performance in several separate categories will
be evaluated. This model is intended to supplement the cost estimation
model by providing several separate performance metrics that can be
used in conjunction with cost to choose the better variant.