In a D1 process, the mold cavity is completely filled using velocity controlled first stage. The second stage is mainly used to compensate for shrinkage and wait until the gate freezes.

In a D2 process, the mold cavity is not completely filled using velocity controlled first stage (visible short). The second stage is used to finish mold filling, compensate for shrinkage, and wait for gate freeze.

Although the the velocity controlled injection of the D1 process is better than the traditional pressure-controlled injection of older molding machines, it still places too much material into the mold during 1st stage. In reality, most injection molding processes benefit from a short shot during 1st stage using a D2 process.

-Andy