Process constraints are the way in which the system can tell if it has made a 'good' or a 'bad' part. Any one or all the 11 constraints listed in the figure below can be used in each optimization run.
In this example, the Minimum Material Density box was checked, and a Constraint Value of 0.999 was entered. In other words, all parts of the casting must have a material density value equal to or greater than 0.999 in order to be considered 'good'.
Therefore, it is important to create the gating system out of Riser Material. If the risers were made of casting material, there would always be porosity present and the optimization would fail.
One interesting technique is to create the riser contact out of casting material. This will, in effect, create a safety factor in the model, since the riser contact will have to be sound to satisfy the constraint. If the contact is created as a separate object from the rest of the riser, this can be done easily.
If you choose more than one constraint, each constraint must be satisfied for the part to be considered 'good'.