An injection of polymer melts into the air, instead of into the mold. Acts as a “test run” to make sure that the machine is operating properly.
The cylinder is pulled back from contact with the sprue bushing and the injection piston is moved forward, forcing melted plastic from the nozzle. If you catch the melt in a metal cup as it exits from the nozzle, you can use a pyrometer probe to check the temperature. This method is one of the more accurate measurements of the melt temperature.