As we have created a potential bottleneck within the model, the next step will be to insert a Buffer between the objects Source and Testing.
Delete the first SingleProc.
Connect the Source with the Buffer and the Buffer with Testing i.e. Source - Buffer - Testing.
Open the dialog window of the Buffer and go to the tab Times.
Change the Processing time to 0.0000.
Go to the tab Attributes and change the Capacity from 4 to 20.
Apply the changes and close the window.

On running the model you will notice that the buffer holds more than one part.
You will notice that rather than the buffer becoming blocked immediately, it accumulates parts to a maximum of twenty. Once the buffer's default capacity has been