Injection-Molding Simulations

Injection-molding simulation has a high return on investment. 

In short, compared to other production processes, injection molding demands more of part and mold designers—experimentation after the mold is built is expensive in terms of time and money. Injection-molding simulation is relatively inexpensive in terms of project cost and offers great benefits to those using it early in the manufacturing process. 

We will not discuss the details of simulation for these processes, as many of the fundamental ideas are common to injection-molding simulation. 

Earlier we mentioned that a model exists for injection-molding simulation that allows simulation to be carried out in reasonable time on relatively inexpensive computers. This model is based on the fact that in a thin-walled part we may neglect any pressure variation in the normal direction. This assumption underlies the basis of most commercial plastic CAE analysis. 

There are three main types of analysis in injection-molding simulation ... 

TABLE 11.3 Data Needed for Injection-Molding Simulation—Simple Mold Filling Analysis of Thermoplastics and Thermoplastic Elastomers... 

TABLE 11.6 Data Needed for Injection-Molding Simulation—Additional Data for Shrinkage and Warpage Analysis ... 

The individual costs of generating material data needed for performing injection-molding simulation, including advanced shrink-age/warpage analysis (Table 11.26), comes to almost 8400 compared to a discounted price of 3000 to 4000 for group tests offered by some of the independent laboratories. 

TABLE 11.26 Individual Costs of Generating Data Needed for Injection-Molding Simulation... 

TMcmicept, Injection molding simulation software system. Plastics Computer Inc. 

A common hypothesis in injection molding simulation is to assume an infinitely rigid mold. However, in reality, the high cavity pressure can lead to mold deflection due to mold compliance. Experimental investigations (Leo and CuveUiez 1996 Delaunay et al. 2000b) have shown that the mold elastic deformation can play a significant role in the pressure-time history. 

Thermal conductivity is an important parameter used for material thermal calculations in the injection molding simulation. A variation in the thermal conductivity will alter the cooling rate and hence cause a variation in the temperature this in turn will cause the viscosity, pressure, and frozen layer to vary. Classical Fourier theory, which assumes thermal conductivity to be a constant scalar value depending only on temperature alone, is inadequate to describe the heat conduction in deformed molten polymer. Van den Brule (1989, 1990) and van den Brule and O Brien (1990) suggested that heat transport mechanisms along the backbone of a polymer chain are more efficient than those between neighboring chains. Hence, the orientation of polymer chain segments induced by flow leads to an anisotropic thermal conductivity. To describe anisotropic thermal conduction, one... 

Lipscomb et al. (1988) have shown that the flow kinematics in complex flows can be strongly affected due to fibers. However, in the injection molding simulation for flows in narrow channels, the effects can be neglected, and an orientation-flow decoupled simulation approach is used. To understand in which situations the decoupled simulation is allowed, we need to classify the flow regimes. Tucker (1991) has identified four regimes of suspension flows. His classification was based on two dimensionless numbers. The first is the particle number defined by... 

Consider a set of typical values in the injection molded composites (p — 10%, Ur = 20, Cj — O.OOland s = 0.01. FromEqs. 5.48 and 5.50, assmning the fibers are aligned, we obtain Np = 6.4 and (5 = 0.1. Then we have Np3 = 0.064. The flow belongs to regime one, and hence decoupled flow and fiber orientation calculations will be allowed. This will be the case for many, but not all, injection molding simulations for fiber-filled composites. 

For infinitesimal defOTmations considered in injection molding simulations, the total strain is regarded as the smn of elastic strain Ey and the thermal strain... 

It is convenient to divide the whole procedure of injection molding simulation into three modules ... 

On each tetrahedral element Qg, the velocity and pressure are approximated by simple piecewise varying functions. We choose the MINI-element, which has been applied to 3D injection molding simulations by PicheUn and Coupez (1998), and Yu and Kennedy (2004), following the pioneering work of Arnold et al. (1984). The velocity and the pressure filed are discretized, respectively, by... 

As alternatives to the MINI element, Rajupalem et al. (1997) and Talwar et al. (1998) used an equal-order velocity-pressure finite element formulation, as proposed by Rice et al. (1986), in their 3D injection molding simulation, and Chang and Yang (2001) used a 3D finite-volume approach based on the SIMPLE finite volume algorithm of Patankar (1980). Since the finite volume method does not... 

Compared with the grid-based methods, the SPH method handles the fountain flow more easily and naturally, without mass loss. However, for injection molding simulations, the SPH method is more time consuming, and it is difficult to enforce essential boundary conditions. 

Brincat P, Friedl C, Talwar K (1998) Extensional viscosity modeling for injection molding simulation. Society of Plastics Engineers Annual Technical Conference—AN lEC 1998 44 552-556... 

Kennedy PK (1995) Elow analysis of injection molds. Hanser Publishers, New York Kennedy PK (2008) Practical and Scientific Aspects of Injection Molding Simulation. Ph.D... 

Zhou H, Li DQ (2004) Modeling and prediction of weld line location and properties based on injection molding simulation. Int J Mater Prod Technol 21 526-538 Zhou H, Zhang Y, Wen J, Li DQ (2009) An acceleration method for the BEM-based cooling simulation of injection molding. Engineering Analysis with Boundary Elements 33 1022-1030... 

NatureWorks s injection moldable grade of Ingeo in the capillary rheometer test has shown a good fit into the Cross-WLF viscosity model (see Table 6.2). There are seven coefficients in the model and it is readily embedded into Moldflow software for injection molding simulation (Moldflow Plastic Labs 2007). Moldflow is computer software that is widely used across the plastic injection molding industry to predict and optimize the... 

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