热加工论坛—Inlet_e

1The inlet – or: How to define the mold filling?During the simulation, the inlet represents the melt flow into the mold. Therefore, an inlet has to be defined for each filling simulation. Due to the simulation process there are certain requirements the inlet has to meet, and the mold filling process must be defined in a physically sensible way.1.1The suitable inlet for simulationTo efficiently carry out the simulation, the inlet has to fulfil the following requirements:•The inlet diameter should be a little smaller than the downsprue diameter.•After the enmeshment, the inlet should consist of at least 2 layers. The program will automatically assign one of these layers to the material group of the ingate system.•The inlet must not project into the downsprue, Array because in that case two different pouring directionswould result (see sketch). Using the overlay principle(first the inlet, then the ingate system) you can avoidthis problem.•The cross section of the ingate system‘s next 2 to 3 layers in the flow direction must be at least as big as the inlet cross section.The best way to check if the inlet fulfils these requirements is to calculate its geometry in the postprocessor using the function ‘3D-mesh‘.1.2Physically sensible fillingThe two basic equations for the calculation of mold filling are::hg p gh v ⋅⋅==ρ2High pressure die casting (HPDC)In low pressure die casting (LPDC) as well as in high pressure die casting (HPDC)processes, the inlet cross section is defined by the geometry. The inlet cross section corresponds to the cross sections of the shot chamber and the stalk respectively.The mold filling is defined considering the pouring rate. This way the first and second phases can be displayed exactly.The piston’s velocity and cross section are known values. From these, the pouring rate is calculated,e. g.:A piston = 25 cm²v piston = 100 cm/s⇒ =V& A piston ⋅ v piston = 2500 cm³/s p o u r i n g r a t e1.2.2Gravity castingAs with this casting method a time dependent pouring rate only appears in very few cases,either a pouring time or a pressure boundary condition must be defined.Defining the pouring timeFrom the total volume and the pouring time, MAGMASOFT ® calculates a constant volume flow at the inlet, e. g.:scm s cm Vdefined s t cm V full total 333500105000)(105000====&Possibly occuring problems Filling time is too shortIn this case, the pressure at the inlet is going to increase during the mold filling. This will cause a “forced filling“, similar to that known from high pressure die casting processes.In reality, the melt would flow out of the pouring basin on these conditions, butMAGMASOFT ® does not consider the basin as “open“. Therefore, the amount of liquid metal being lead into the mold through the inlet is higher than the amount that can flow off through the downsprue.Because of this, the pressure and velocity values inside the melt increase to such an extent that they do not correspond to physically sensible values anymore.Inlet cross section is too largeThe melt velocity calculated by MAGMASOFT ® can decrease to a very low level, at which it is outside the physically sensible range. In extreme cases MAGMASOFT ® opens a dialogue box with a warning.Checking the filling conditionsBy observing the melt velocity underneath the inlet, you can check if the filling conditionshave been set in a physically sensible way.AdvantagesDisadvantages• In most cases, the pouring time is aknown value.• If a pouring time is defined, the inlet cross section will be less important than if you define a pressure boundary condition.A constant pouring rate does not correspond exactly to the real conditions:• Quick casting, to fill the pouring basin withliquid metal as early as possible.• During mold filling, the pouring ratedecreases because of the increasing counterpressure inside the mold.Table 1: Estimating the definition of a pouring timeDefining a pressure boundary conditionThe definition of a pressure gh p ρ=at the inlet leads to the physically most exact display ofthe mold filling. The initial pouring phase can be simulated, and the increasing counterpressure inside the cavity is considered as the pouring rate is calculated. The pressure at the inlet does not increase.Instead the melt velocity at the inlet is calculated from the defined pressure. Here, the inlet cross section is of decisive significance. The larger the inlet cross section is, the higher the volume flow in the melt and the shorter the filling time will be as well.Pouring basinThe inlet cross section A Inlet corresponds to the downsprue cross section.The following pressure results at the inlet:hg p ⋅⋅=ρh。