Serial bending furnace process characteristic
Serial bending furnaces suitable for mixed production are commercially available in different sizes, design and automation levels. The dominating design is a furnace with two-level design where the upper level is the...
Serial bending furnace process characteristic
Serial bending furnaces suitable for mixed production are commercially available in different sizes, design and automation levels. The dominating design is a furnace with two-level design where the upper level is the heating level and lower level is the cooling track.
The process starts with the loading actions at the loading station. Thereafter the glass&mould wagon continues to travel through the upper heating track, where it is subjected to IR-heating to pre-set temperature setpoints at the preheating sections. In the last sections of pre-bending and bending, the IR-heating is controlled by individual bending recipes for each model/wagon. The final bending section is the dominant section that sets the production speed. Therefore, in the final section the glass must reach its final shape at the set ready temperature of the bending recipe with in a certain time frame = bending cycle window. The modern furnace heating ready point is set by the glass surface temperature that is measured by a pyrometer. After the heating track, the wagon will be lowered to level 1 where it is first annealed and then cooled as it travels again to unloading to reach suitable unloading temperature.
Bending cycle window
It is important to recognize that although typically the bending recipe “ready temperature” (glass surface temperature measured by a pyrometer) releases the wagon for annealing this set point is not the most important variable. Once the a wagon is at the final bending section the duration it spends there to reach its final shape is the single most important factor in serial bending process. If this time – the bending time – at the final section varies constantly out of proportions, more than a normally allowable bending cycle time window, process repeatability will be impossible to control. Due to the nature of the continuous process, variations in bending time will cause a ‘domino effect’, where all wagons in heating or annealing stages are affected by this cycle and numerous following cycles, before the process again stabilizes, assuming the upcoming cycles will repeat the allowed bending cycle time window. The magnitude of effects to heating and cooling track is dependent in on the extent of the cycle time variation. This domino affect and furnace parameters are introduced more closely in my article Process Repeatability in Serial Production of Automotive Windshields.
Due to these reasons, every bending cycle in serial bending production should be repeated within the bending cycle window. This enables similar heating and annealing processes to take place for all glasses being processed. Modern furnace parameters usually have a certain bending cycle time setting that the automation tries to complete, cycle after cycle, as precisely as possible. Automation still has its limits and the final adjustments remain on the bending recipe and heating element configuration.
What is the allowed cycle time variation?
The allowable time frames for the bending cycle window (allowed bending cycle variation) is dependent on the bending furnace size, mixed production load, furnace operation parameters, and automation. Bending cycle time variations should never exceed 20 per cent from the target time and with perfected processing conditions even >13 per cent cycle time variation can be reached.
How to match bending ready point to the bending cycle window?
Of course it is simple to just announce: “final bending is dominant and each bending cycle should repeat the same bending time”. The parameters to be controlled to do this are also simple, but for some reasons this is where the process adjustments fails. The rule of thumb can be recognized as follows: To achieve any bending cycle window the current production and bending recipes have to be routinely altered in following manner: any given ready temperature (=ready temperature of the bending recipe) where bending results (geometry and edge size) are within the middle of the tolerance range (=0) must be met at once with the decided bending cycle time window.
Based on this we can question why the furnaces are not set to work on bending time instead of the ready temperatures, enabling a “forced pace” mode to follow constant cycle time. This is possible with most furnaces, but the function is rarely enabled to “forced pace”. The “free pace” cycle allows increased flexibility, following of pre-set heating curves, and usage temperature as a controlling measure.
What other conditions should be recognized?
Production planning of mixed serial bending production is also a key element and therefore needs attention. The loading orders should be designed to enable smooth changes between production models. There are several different practices that ease the production planning and provide smoother production runs with sustainable repeatability. Ultimately perfect production planning enables more flexibility, enabling shorter series and timely deliveries.
Although the nature of the process in reality is closer to “forced pace” than “free pace” there are functions that enable delaying the process from the loading section and this can cause severe process variation if not completed at a routine manner with in the available time.
The furnace pyrometers performance also has great importance and, therefore, routine maintenance is a fundamental part in assuring perfect repeatability. Any change in the measurements will lead to variations in process and requirements to adjust the bending recipes. If the wrong calibration, dirty lenses, inlets, interference, high temperature etc., exist for a long period of time and they are then fixed – process variation is unavoidable and cause severe disturbance to the production schedules as corrective adjustments are made.
Sadly, many times, evaluations show that manual quality documentation without sufficient cross checks leaves room for misconduct to “smoothen the variation” from what actually takes place.
Success in controlling process repeatability is a complete function that involves bending process parameter fine tuning, but also active collaboration between production planning, maintenance, the manufacturing team and quality departments. Active operator actions in process controls cannot be over-estimated as they are extremely important. The process must be monitored vigorously at all time, bending results controlled with frequent measurements, which are recorded and corrective actions to furnace parameters carried out immediately after indications of over limit process variation.
Safety Glass Experts provides expert advisory service for windshield manufacturers to improve and optimize their production performance and yield with training, expert advisory, diagnostic and tooling services. The Author is available for on-site expert advice and training or can assist remotely by email or telephone. More on this topic can be read from the author’s wide selection of articles at www.sge.fi and upcoming technical guidebook The Secrets of Windshield Manufacturing.
Mika Eronen is a Finnish safety glass professional. Mika started his career in the glass industry at Pilkington Finland in 1999. Working with the most complex bus windshields in the markets, he developed considerable knowledge on windshield manufacturing. In 2004, taken by international challenges, Mika toured globally, carrying out new hot-end machinery start-ups, customer training, and consultation projects, still developing his skills further in the art of sag bending process/technology and production of windshields. Today Mika has finished his professional specialization studies and is a founder and owner of Safety Glass Experts International Ltd. Mika has developed a passion to write about safety glass processing and, on top of his routine technical article publications, he is soon to finish the first ever complete technical guide book about windshield manufacturing, called “The Secrets of Windshield Manufacturing”.