As explained in the first part of this article, published in Glass-Technology International issue 5/2011, modern windshield design features cross curvatures over 30 millimetres in car windshields and well over 70 millimetres in bus and special vehicle windshields. Although the trend of increasing height of the windshields naturally compensates challenges related to sagging depth to a degree, centre line sagging in modern extent still creates a real challenge. Of course any modern bending furnace, serial or single chamber, features enough IR-heating power to bend even deeper sagging than required, and, in fact, it is not really a question of total IR-power either, since simply increasing the bending time over glass transition temperature will result in more sagging.
One can continue to bend the glass pair up to the physical limitations created by the glass and mould size – there is no difficulty in executing this. However, bending deep cross curvature, which potentially includes a number of points that have to remain within a narrow frame of repetition still maintaining smooth progressive sagging profile and correct cycle time, is an objective that not all manufacturers can reach.
What really creates the challenge in bending modern centre sagging is the equation of the following points:
1.    controlling deep sagging shape->bending smooth and progressive shape;
2.    repeating the sag within narrow tolerance from specified points (value adding devices in modern windshields);
3....

As explained in the first part of this article, published in Glass-Technology International issue 5/2011, modern windshield design features cross curvatures over 30 millimetres in car windshields and well over 70 millimetres in bus and special vehicle windshields. Although the trend of increasing height of the windshields naturally compensates challenges related to sagging depth to a degree, centre line sagging in modern extent still creates a real challenge. Of course any modern bending furnace, serial or single chamber, features enough IR-heating power to bend even deeper sagging than required, and, in fact, it is not really a question of total IR-power either, since simply increasing the bending time over glass transition temperature will result in more sagging.
One can continue to bend the glass pair up to the physical limitations created by the glass and mould size – there is no difficulty in executing this. However, bending deep cross curvature, which potentially includes a number of points that have to remain within a narrow frame of repetition still maintaining smooth progressive sagging profile and correct cycle time, is an objective that not all manufacturers can reach.
What really creates the challenge in bending modern centre sagging is the equation of the following points:
1.    controlling deep sagging shape->bending smooth and progressive shape;
2.    repeating the sag within narrow tolerance from specified points (value adding devices in modern windshields);
3.    bending the required cross curvature in efficient cycle time.

Causes of imperfections in windshield sagging profile
Let’s take a look of the causes of a uniform sagging profile. A perfect sagging profile is an important quality feature and is a fundamental requirementof a windshield. Without a smooth and progressive sagging profile, optical quality will suffer. Furthermore, it can affect wiper performance and potentially increase wind noise. The effect on optics is complete as the deformation in the sagging shape will result in both poor reflection distortion (waviness) and poor transmission optics. The magnitude of the effect is directly proportional to the extent of the deformation. Whereas minor flat spots might remain at acceptable levels in terms of transmission optics, the same flat spot can be unacceptable due to waviness in reflection distortion.
Legislative safety standards such as ECE R 43 stipulate only the very basic conditions for optical quality in terms of transmission optics. Passing the optical test defined in ECE R 43 is in no correlation to OEM vehicle manufacturer quality regulations. Therefore, in reality, the final product quality is determined by the vehicle manufacturer’s standards, those that today reflect well the real requirements of the vehicle final users - therefore the gap between existing legislative norms and vehicle OEM quality regulations still keeps increasing. OEM product specifications define a strict set of requirements and tests for characteristics that are directly related to the windshield sagging profile.

Controlling sagging and bending smooth and progressive shapes
The gravity bending process has a natural tendency to create non-progressive sagging shapes, where extensive sagging occurs at the printed edges, while flat spots or even inverse areas occur in central areas of the pane. Simple simulation explained in the first part of this article describing how paper sheets behave when bent to windshield-like shapes applies here too and is further enhanced by the black enamel printing applied to the windshield edges.
To overcome the natural tendency to create un-uniform cross curvature shapes, the Heating Element Controls (HEC) bending process and mould tooling have to be fine-tuned to the point of excellence. Successful end results require that all process parameters be in perfect balance. All this starts from perfect loading conditions and is followed by uniform, well balanced heating distribution from the beginning of the bending process to the point of transition temperature. Thereafter, the HEC should start to focus again on the areas where heat is required to bend the desired sagging. How the HEC will be adjusted depends on the adjustment features available in the bending furnace.
No matter what IR-heating features are available, the principle is that accurate, direct radiation is required to distinct areas for the centre line of the glass, which can be adjusted in accordance with sagging shape results.
This applies for both bottom and top directions. There are many designs enabling this, including: adjustable heating element positions, heating elements placed in reflection grooves, special midsection heating element fields, as well as extra heating elements that can be located freely in the wagon.
In product introduction, the first bending trials will provide feedback on the centreline sagging profiles’ individual nature in terms of sagging behaviour, and the HEC should now be adjusted carefully according to the results: more heating to areas with flat spots and less for partially over-heating areas. After a few test rounds, the nature of the sagging profile will become more evident and in the case of fine-tuning, the HEC alone is not sufficient enough to bend perfect profiles, and required mould adjustments should be started.
There are multiple mould tooling designs that can provide assistance in controlling IR-heat distribution. No matter how well designed the IR-heating automation is, there will always be scattered radiation that will also affect the undesired areas of the glass. The black printing enamel on the edges, in particular, absorbs all heating radiation and is affected to a great degree by the scattered radiation. Therefore, these areas will be either covered with so-called heat umbrellas or heat absorption plates installed under the glass pair. Both of these applications have their own strengths and weaknesses and which one is more suitable is case dependent.
Double-Stage mould design is also sometimes key to improved performance in sagging profile. After the engineering and installation of heat controls tools to the mould, test trials can continue. At this stage, it can be assumed that due to the absorption of extensive heating some modifications to the HEC are required again to achieve centre sagging as per product specifications and also to fine tune the sagging to be smooth and progressive. Once again, after each test round, corrective actions should be assigned according to the last bending cycle results. In the introduction trials, understanding and availability of Visual Reference Points in combination with response variable such as pyrometer temperature, will ensure faster and more consistent progress in the trials as less “blind work” or “questimations” is involved.

Repeating sagging
Repeatability of the process is one of the key factors in modern production lines as it affects the complete performance in many ways. With regards to repeating centreline sagging according to tight tolerances, key performance factors differ between products. With car and truck windshields (<2.2 square metres) the results rely more on correctly set HEC, furnace parameters, production supervision, and production planning.
Larger buses and special vehicles (>2.2 square metres) rely more on operator performance, as the bending furnaces (both singe chamber and serial) are mostly operated manually when complex modern shapes are produced. Here again know-how and the correct use of Visual Reference Points (VRP) will have an essential role. Visibility into bending furnace chambers is always somewhat limited, but mould solutions can be engineered according to individual requirements to provide reliable VRP, and ensure better process repeatability in terms of sagging.

Efficient cycle time and sagging requirements
As stated in the opening paragraph, with enough bending time any sagging can be created. This, however, does not apply well in serial bending furnace production. Certain cycle target time (CTT) and allowed cycle window are defined, and production capacity is planned accordingly. Of course, a serial furnace can be set to run according to longer CTT parameters but doing this might not be feasible or new CTT might not meet production capacity requirements.
In single chamber production, the case is simple since all production cycles are independent (to some extent) from each other, and if allowed by the production plan, individual pro-ducts can use extended production time. In serial bending furnace production of car windshields with complex deep sagging, there is a requirement for a certain level of IR-heating power, adjustment and automation features that must be available to enable mixed production along with 12-20-millimetre normal sagging range products. The first priority is sufficient bottom heating, followed by IR-heating power and finally IR-heating element controls for the middle section. In any case, general furnace parameters such as CTT must be defined according to the product featuring the deepest sagging and others adjusted to meet these parameters.

Conclusions
Mastering heat treatment processes remains the most important function of windshield manufacturers. Perfecting modern shapes is a constant challenge. To master the processes manufacturers need dedication to improve bending processes by fine tuning the HEC, furnace parameters, re-engineering and introducing new mould tooling designs. Furnace operators involved in introduction trials and production must have knowledge of VRP to enable efficient progress in introduction trials and ensure repeatability during production. To cover this topic completely in these 4,000 words is impossible, but the vital principles should be well covered here.
My upcoming book The Secrets of Windshield Manufacturing will cover this topic in further detail and provides readers with all the essentials required to produce quality windshields. Furthermore, our training programme “The Art of Gravity Bending Windshields” will provide further insight to this topic and more.

The Author
Mika Eronen is an internationally experienced Finnish safety glass professional. Mika started his career in the glass industry at Pilkington Finland in 1999. Working with the largest and most complex bus windshields in the markets, he has developed considerable knowledge on windshield manufacturing. In 2004, taken up by international challenges, Mika started international expert work carrying out new bending furnace start-ups, commissioning, customer training, and consultative projects, still developing his skills further in the art of bending process/technology and production of windshields. Today Mika is a partner in Safety Glass Experts International Ltd. His tasks include conducting on-site training and consulting projects, as well as management, planning and coordination of ongoing customer projects.