2.1 PLASTIC INJECTION MOLDING
The design and manufacture of plastic injection molded parts is a well understood domain and lends itself advantageously to computer aided engineering. Much of the underlying requirements in the knowledge of plastics and their processing is already articulated. And while the basic material characteristics of thermoplastics have been understood for decades [Dym 79], it has not been until recently with the advent of advanced computing capabilities that such characteristics could be routinely considered within the design process. In addition, the expertise of process engineers with years of invaluable experience can be captured and used within the computer environment in the form of knowledge based systems.
Plastic injection molding is a net shape manufacturing process, a process also known in some literature as "forming". In net shape manufacturing, one primary manufacturing operation determines the final shape of the product; additional manufacturing operations play little further role. As such, the entire final shape of the part can be expressed within a single CAD representation. In turn, this one representation may be the basis of extensive computational analysis.
Analytical/numerical modeling or simulation have been commonly used in manufacturability analysis. These techniques, often using finite element representations, can model the flow and cooling of molten plastic. Improper or impartial filling of the mold cavity or buildup of residual stresses may be discerned [Dixon 88].
A survey of the techniques conducted by indicates that numerical simulation techniques are limited in the scenarios to which they can apply. Also, the simplifying assumptions necessary for solving equations may make the results unacceptable with respect to the actual manufacturing process [Gadh et al 89].
A knowledge engineering approach may be able to compensate for the limits of numeric models [Frederick et al 83]. Knowledge engineering proposes that standards and proven heuristics within a given domain of expertise be encoded into representations or symbols, that are computationally manipulatable. Knowledge engineering is based on symbolic computation, as opposed to numeric computation. The knowledge based systems of concern here are sometimes referred to as expert systems.
Some expert systems specifically developed for plastic injection molding tend to provide evaluation and analysis similar to that of a numerical simulation, while providing good heuristic compensation for the recognized weaknesses of numerical simulation. These systems take a few key parameters such as the length, width, height, and material type to perform analysis[Bernhardt 87][Manzione 87]. Again, the results are based on simplified dimensioning of a part, and as with numerical models, the results may provide only first order insight into potential problems.