4.1.3 FEATURES AND PROPERTIES

As discussed earlier, features are entirely obtained from the original CAD model. Conventional CAD systems do not contain feature abstractions, so a given CAD representation is submitted to a series of transformations. These transformations result in two graph-based representations, the Noodles data model and the feature graph.

The preparation of information for processing by the expert starts with the model. The expert may work with a blue-print, a CAD rendering on the computer screen, or an initial mock-up of the part. The model itself probably does not have the information the expert needs to make an immediate assessment of its moldability. Indeed it could be a simple solid model, such as that of Figure 15 with only the topological layout explicit.The molding expert looks at the structure and must congnitively extract protrusions, depressions and other salient features of interest for any evaluation he or she must do.

Figure 15 Unclassified Solid Model

As discussed in the previous section, one feature-obtaining method is feature extraction whereby features are recognized and created from the low level CAD structures. In the [Gadh89] review of these methods, the alternative design with features approach, in addition to the creation of unrecognized secondary features, could adversely limit the amount of design variation possible.

For purposes of integrating the expert system with existing commercial CAD systems and methodologies that do not treat features, the feature extraction technique is preferred. Within the DFM system, features are abstracted from the Noodles geometric solid modeling system and extracted feature parameters are placed in data tables.

Recognition systems tend to converge on abstracting basic forms of protrusion, through hole, blind hole, and slot [Gadh et al 89][Ferreira & Hinduja 90][Gavankar and Henderson 90]. Additionally, handles or bridges are forms that are often identified for domains other than plastic injection molding. Within this system, the DDPF also applies heuristics to classify protrusions into elongated ribs and non-elongated bosses.

In plastic injection molding, a distinction must be made between elongated protrusions and non-elongated protrusions. For purposes of easier conceptualizing, an elongated protrusion is called a rib and a non-elongated protrusion is called a boss. Classification of these form is made by comparing lengths and widths. For efficiency, the classification is done before the primitive feature graph level is reached. although it could be done at a higher level.

Table 6 Simple Features in Plastic Injection Molding

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Name Description

Rib Elongated protrusion.

Boss Non-elongated protrusion.

Through Hole Pronounced depression that goes through part.

Blind Hole Pronounced depression with a bottom.

Slot Any depression like a blind hole that has one or more open sides.

Base Whatever has not been identified as a feature.

Properties must be predefined beforehand and evaluated with the feature extraction process. Features without property values are computationally useless. As such, there is a fine interaction between the development of rules and the development of symbolic features to be used by those rules.

The molding expert and the design sees parts from two different directions. The part of the rib furthest from the base, for example, is called the top of the rib by a designer. An expert, however, sees only the mold impression of the rib, and since it would be the deepest depression in the mold, call that portion of the rib the "bottom". As such, direction neutral property names must be considered.

Table 6 - Simple Features in Plastic Injection Molding