Mold-Filling Problems

There are numerous problems that can occur during mold filling in RTM. Some of them are trivial, but some of them very difficult to eliminate. A few important problems will be discussed in this section. 

Poor impregnation will result if the fiber volume fraction is so low that a region without fibers is formed on top of the reinforcement. This can happen, for example, if high performance fabrics with a high uncompacted fiber volume fraction are used at a too-low-fiber volume fraction. This situation is similar to the race tracking problem described earlier. Flow on top of the reinforcement can occur, even if the whole cavity thickness is filled with fibers, if 

Flow on top of the reinforcement is also mainly a preforming problem, and the best solution is to increase the fiber volume fraction or to use a more springy reinforcement where the problem is most pronounced. A decrease in injection pressure can also be a way to reduce the problem. 

When the part is of sandwich type there is a potential for a flow instability that will push the core toward one of the sides. The resulting part will have a thicker skin on one of the sides. 

The flow instability can best be understood by looking at a case with unidirectional flow (see Fig. 12.7). There will always be some nonuniformity between the sides. This will result in the flow front moving faster on one side of the core than it does on the other. The net force on the core from the resin will be higher on the side where the flow front has moved the farthest and as a result the core will be pushed away from this side. The displacement of the core will increase the permeability more and the flow front will move even farther ahead on the fast side, and so on. The process will reach an equilibrium when the reaction force from the reinforcement becomes large enough to balance the fluid pressure on the other side of the core. 

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The process interaction in cast plastic products is mainly involved with the curing processes and with mold filling problems. Voids and porous sections are a frequent problem with castings because the mold filling is done at atmospheric pressure, or low pressure, and if the product has thin sections to fill, the flow may be a problem. 

Porous areas in LCM are often caused by a mold filling problem (see Sec. 12.3.3) however, other causes also exist, (e.g., high temperatures during cure may evaporate resin monomers or dissolved gas that form voids). The actions to be taken to optimize cure for minimum void content are ... 

The integrated continuity equation is a weaker form of the full continuity equation. This is noticed in numerical solutions of mold filling problems, where continuity is never fully satisfied. However, this violation of continuity is insignificant and will not hinder the solution of practical mold filling problems. The integrated continuity equation reduces to... 

There are various special forms and simplifications of the above equation and they are given below. In subsequent chapters of this book we will illustrate how the various forms of the Hele-Shaw model are implemented to solve realistic mold filling problems. 

The second integral on the right hand side of eqn. (9.67) can be evaluated for problems with a prescribed Neumann boundary condition, such as heat flow when solving conduction problems. For the Hele-Shaw approximation used to model some die flow and mold filling problems, where 8p/8n = 0, this term is dropped from the equation. 

Once the boundary conditions are applied, the pressure field can be solved using the appropriate matrix solving routines. Note that for mold filling problems, there is a natural boundary condition that satisfies no flow across mold boundaries or shear edges, dp/dn = 0. Once the pressure field has been solved, it is used to perform a mass balance using eqn. (9.144) or (9.145). Once the flowrates across nodal control volume boundaries are known, a simulation program updates the nodal control volume fill factors using... 

During some mol ding and extmsion operations, knit line failures, incomplete mold fill, die drag, and excessive heat buildup, ie, scorch, are problems. Many of these problems are reduced or eliminated by the addition of internal lubricants such as low mol wt polyethylene or Vanfre AP-2 Special, a product of R. T. Vanderbilt. 

The heat transfer problem which must be solved in order to calculate the temperature profiles has been posed by Lee and Macosko(lO) as a coupled unsteady state heat conduction problem in the adjoining domains of the reaction mixture and of the nonadiabatic, nonisothermal mold wall. Figure 5 shows the geometry of interest. The following assumptions were made 1) no flow in the reaction mixture (typical molds fill in <2 sec.) ... 

Thicknesses from 1 mm up to at least 50 mm are possible with RTM however, several difficulties can arise for thick parts, such as the problem of race tracking at edges, air enclosures due to uneven mold filling, and problems connected to cure. 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

In this section, we will proceed to develop a finite element formulation for the two-dimensional Poisson s equation using a linear displacement, constant strain triangle. Poisson s equation has many applications in polymer processing, such as injection and compression mold filling, die flow, potential problems, heat transfer, etc. The general form of Poisson s equation in two-dimensions is... 

The most characteristic feature of injection molds is geometrical complexity. In such molds there is a need to predict overall flow pattern, which provides information on the sequence in which different portions of the mold fill, as well as on short shots, weld line location, and orientation distribution. The more complex a mold, the greater this need is. The irregular complexity of the geometry, which forms the boundary conditions of the flow problem, leads naturally to FEMs, which are inherently appropriate for handling complex boundary conditions. 

Problem 4-8. Pressure-Driven Radial Flow Between Parallel Disks. The flow of a viscous fluid radially outward between two circular disks is a useful model problem for certain types of polymer mold-filling operations, as well as lubrication systems. We consider such a system, as sketched here ... 

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