Meaning
Pneumatic trigger points in stretch blow moulding dictate the exact moment when the initial low-pressure air is introduced. This control parameter, called pre blow timing, coordinates the start of the pre-blow air injection with the downward movement of the stretch rod. It manages the initial expansion of the preform before the high-pressure air completes the shaping process.
Stretching Interaction
The interaction between the descending rod and the pre-blow air determines how the polymer moves along the stretch path. If pre blow timing occurs too early, the preform expands before the stretch rod can guide it, causing the material to cool prematurely on the mould shoulder. If it occurs too late, the stretch rod pushes the material into the base, creating a thick bottom and thin sidewalls.
This coordination is critical for maintaining correct material distribution.
Thickness Regulation
Adjusting this timing helps operators control the thickness of the container shoulders and base. Small shifts of a few milliseconds can significantly alter the weight distribution of the blown container. This sensitivity makes it a primary tool for fine-tuning during machine startup.
Control Calibration
Modern high-speed machines utilize digital encoders to synchronize the pneumatic valves with the mechanical carousel. Consistent valve response times are necessary to prevent drift in the pre-blow start point. This stability ensures that the weight distribution remains constant throughout a long production run.
