Numerical  simulations  have  been  found  to  be  very useful in  many areas  which lead  many  researchers  attempting  to  implement  them  into  die casting  process. Considerable research work has been carried out on the problem of solidification

including fluid flow which is known also as Stefan problems. Minaie et al in one of the pioneered work use this knowledge and simulated the filling and the solidification of the cavity using finite difference method.Hu et al used the finite element method to improve the grid problem and to account for atomization of the liquid  metal.   The atomization  model in  the  last model  was based  on the  mass transfer coefficient. This model atomization is not appropriate.  Clearly, this model is in waiting to be replaced by a realistic model to describe the mass transfer1. The Enthalpy method was further exploded by Swamina than and Voller and others to study the filling and solidification problem.

While numerical simulation looks very promising, all the methods (finite difference, finite  elements,  or  boundary  elements  etc)2suffer  from  several major drawbacks which prevents them from yielding reasonable results.

•There is no theory (model) that explains the heat transfer between the mold walls and the liquid metal.  The lubricant sprayed on the mold change the characteristic of the heat transfer. The difference in the density between the liquid phase and solid phase creates a gap during the solidification process between the mold and the ingate which depends on the geometry.  For example, Osborne et al showed that a commercial software (MAGMA) required fiddling with the heat transfer coefficient to get the numerical simulation match

the experimental results.

• As it was mentioned earlier, it is not clear when the liquid metal flows as a spray and when it flows as continuous liquid. Experimental work has demonstrated that the flow, for a large part of the filling time, is atomized.

•The pressure in the mold cavity in all the commercial codes are calculated without taking into account the resistance to the air flow out.  Thus, built–up pressure in the cavity is poorly estimated, or even not realistic, and therefore the characteristic flow of the liquid metal in the mold cavities poorly estimated as well.

•The flow in all the simulations is assumed to be turbulent flow. However, time and space are required to achieved a fully turbulent  flow. For example, if the flow at the entrance to a pipe with the typical conditions in die casting is laminar (actually it is a plug flow) it will take a runner with a length of about 10[m] to achieved fully developed flow. With this in mind, clearly some part of the flow is laminar. Additionally, the solidification process is faster compared to the dissipation process in the initial stage, so it is also a factor in changing the flow from a turbulent (in case the flow is turbulent) to a laminar flow.

•The liquid  metal velocity at the entrance to the  runner is assumed for the numerical  simulation  and  not  calculated.   In  reality  this  velocity  has  to  be calculated utilizing the pQ2diagram.

• If turbulence exists in the flow field, what is the model that describes it adequately?  Clearly, model such k−ǫ are based on isentropic homogeneous with mild change in the properties cannot describe situations where the flow changes into two-phase flow (solid-liquid flow) etc.

•The heat extracted from the die is done by cooling liquid (oilor water). In most models (all the commercial models) the mechanism is assumed to be by “regular cooling”. In actuality, some part of the heat is removed by boiling heat transfer.

•The  governing  equations  in  all  the  numerical  models,  that  I  am  aware  of neglect the dissipation term in during the solidification.  The dissipation term is the most important term in that case.

One wonders how, with unknown flow pattern (or correct flow pattern), unrealistic pressure in the mold, wrong heat removal mechanism (cooling method),erroneous governing equation in the solidification phase, and inappropriate heat transfer coefficient, a simulation could produce any realistic results. Clearly, much work is need to be done in these areas before any realistic results should be expected  from  any  numerical  simulation.   Furthermore,  to  demonstrate  this  point, there are numerical studies that assume that the flow is turbulent, continuous, no air exist (or no air leaving the cavity) and proves with their experiments that their model simulate “reality” [22]. On the other hand, other numerical studies assumed that the flow does not have any effect on the solidification and of course have their experiments to support this claim [11].  Clearly, this contradiction suggest several options:

•Both of the them are right and the model itself does not matter

•One is right and the other one is wrong.

•Both of them are wron