Process Modeling, Simulation, and Control

  Material Characterization, Modeling & Properties of Molded Parts

  Non-Polymer Material Processing

  CAD/CAM Related Activities

Process Modeling, Simulation, and Control

• During the initial stage of the research, effort was concentrated on the study of molding dynamics. A finite-difference numerical scheme was developed for simulating the filling of thin cavities based on 1-D flow analysis. A simple yet powerful "Coupled-Flowpath" modeling program applicable to multi-gated situations was completed during this stage which rationalized the so-called "lay-flat" approximation of cavity-filling simulation.
• The cavity-filling program was extended by developing a hybrid finite-element / finite-difference numerical scheme to handle the filling of fairly arbitrary planar geometry with variable thickness and inserts. A systematic procedure for modeling the three stages - filling, packing and cooling - of a molding cycle was developed.
• By 1985, the three-dimensional, user-friendly, mold-filling program CIMP-FLOW3D, needed by industry for producing intricate plastic parts, was developed. Since then, the finite-element / finite-difference / control-volume numerical scheme has been adopted as the technical standard by most researchers and software developers in the field.
• The program was subsequently extended to account for the compressibility of the material over the entire molding cycle of filling, packing and cooling, resulting in CIMP-PACK3D in 1989.
• A separate effort was launched to aid the design of mold-cooling systems. Starting with a simple 1-D heat-conduction calculation, the simulation progressed to a 2-D heat-transfer program using the boundary-element method. The CIMP-COOL3D program finally evolved which couples a 3-D steady-state boundary-element calculation for the mold with a transient 1-D finite-difference calculation in the cavity, matching the temperature and heat flux at the mold/polymer interface on a cyclic-averaged basis.
• The effect of viscoelasticity on injection molding has been investigated starting from the early stages of the program. In particular, effort has been devoted to measuring and predicting flow-induced birefringence and residual (thermal and flow-induced) stresses. Current effort includes the calculation of juncture pressure losses using viscoelastic modelling.
• Considerable progress has been made toward the basic understanding of the behavior of short fibers in flow fields relevant to injection molding. Work on theoretical predictions and experiments regarding fiber orientation and fiber-fiber interaction have been carried out. With some simplifications, a program has been developed to predict fiber orientation and resulting mechanical properties of injection-molded parts which are reinforced with fairly high concentrations of short fibers.
• Similar work on simulation of mold filling with thermosets has been carried out. Significant progress has been made in numerical/experimental studies on encapsulation of microelectronic chips with epoxy-molding compounds. A special slit rheometer was built to determine the viscosity as a function of the degree-of-cure as well as of temperature and shear rate. Good results have been obtained in predicting wire deformation due to wire sweep in the filling process.
• For warpage analysis of thin-walled parts, a dedicated and very efficient structure-analysis program, SHELL10, has been developed. Comparison with a high-end, general-purpose FEM code shows that SHELL10 gives comparable accuracy while requiring considerably less execution time and memory.
• Work on process control was also initiated at an early stage. Characteristics of a typical process controller were investigated and modeled. An on-line adaptive-control algorithm for controlling part thickness based on an empirical model was developed. Similar results were obtained by incorporating a predictive model in the control loop based on a 1-D packing calculation.

Material Characterization, Modeling & Properties of Molded Parts

• The study of the rheological behavior of polymers has been an essential and continuous effort of the program. Various methods of measuring and modeling the viscosity of polymer melts have been critically assessed. Substantial experimental work has been done in this regard with a variety of rheometers.
• A substantial material data bank has been developed consisting of shear-viscosity data from capillary and dynamic measurements. A new and patented instrument (K-System) using a line-heat source was developed to measure thermal conductivity of polymers as a function of temperature.
• A dual-vessel PVT device was designed and built to measure the density change of polymers over a wide range of temperature and pressure. The new device is of low-cost, is easy to calibrate/operate, and is environmentally safe. An apparatus was developed and used to assess the possible effects of thermal-contact resistance in injection molding.
• Experimental/modelling studies have been conducted on semi-crystalline polymers under both quiescent and flow conditions in order to investigate the effects of non-isothermal and stress-induced crystallization kinetics in injection molding.

Non-Polymer Material Processing

• A new research effort was launched in 1989 under a grant from the National Science Foundation to develop a novel process for Net-Shape Manufacturing of semi-solid materials (SSM). The idea was to make die-casting of metals in a similar manner to the injection molding of plastics. The new process, named "Rheomolding", makes use of a vertical injection-molding machine. The screw rotation and a controlled temperature profile along the barrel of the prototype machine have successfully produced the semi-solid state of a low-temperature metal alloy.
• Considerable work on rheological experiments and viscosity modeling of a Sn-Pb alloy has been done. A new finite-element procedure, called Net-Inflow-Method (NIM), has been developed to simulate incompressible viscous flow with inertial effects and moving free surface. NIM has also been applied to improve the numerical stability and efficiency in the filling simulation of the injection molding of plastics.
• Work has been done on rheological experiments of a ceramic-powder-filled material including the injection molding of such a material in a spiral-flow mold.

CAD/CAM Related Activities

• In the early stages of the program, an integrated CAD/CAM system, CADMOLD, was developed to help the mold designer to interactively select standard mold components. The TIPS-1 CAD system was integrated with the mesh-generation, cavity-filling, and a simple cooling-line design program.
• A system for simulating NC mold machining, applicable to milling operations, was developed. The NC simulation (NCS) program was the first software with color-shaded display which could be used for NC-toolpath verification.
• An algorithm and corresponding computer code were developed to more accurately compute the intersection of free-form surfaces. This pioneering work has evolved into a powerful geometric tool kit, SHAPES, which is commercially available.
• Work on the development of a feature-based design system for injection-molded parts was carried out. A library of commonly encountered features in plastic-part design was established to facilitate the design process. An algorithm was developed to extract mid-surfaces of thin-walled parts created from a CAD system, which can be used directly for mold-filling analysis.