Why forehearth quality dictates forming quality
Every forming defect — settle wave, washboard, baffle mark, choked neck, leaner — has a process root cause that traces back through the IS machine to the gob, and through the gob to the forehearth. If the forehearth delivers thermally uniform glass at the right setpoint, the IS machine has a fighting chance. If the forehearth delivers ±15°C variance, no amount of forming-side tuning can compensate.
Gob temperature variance — the headline KPI
- Within section: variance gob-to-gob within a single section over an hour. Target ±2°C.
- Across sections: variance between sections at the same moment. Target ±3°C.
- Hour-to-hour: variance over 8h shift. Target ±5°C.
- Across crews: variance across 24h. Target ±5°C.
Riser, gathering, cooling air
Riser temperature setpoints, cooling-air distribution along the forehearth roof, and gathering temperature stability are the three main forehearth control levers. Riser-tuned setpoints often haven't been re-validated in years; cooling air imbalances are common and silently corrected for at the IS machine.
What's measurable and what's not
- Measurable: gathering temperature, riser temperatures, gob weight variance, gob shape via cameras
- Indirect: gob temperature uniformity (via gob shape and forming defect signatures)
- Often un-measured: cross-forehearth temperature profile (worth installing if not present)
How to install daily forehearth discipline
Three habits stick the most: (1) gob temperature uniformity reported daily on the shift huddle board; (2) weekly forehearth review attended by forming, conditioning and quality leads; (3) any forming defect mode investigation begins with 'what was the gob doing 30 minutes ago?'