The bus and special vehicle windscreen business is interesting area in the automotive glass industry. As other niche-market areas, such as the bullet proof glass or high performance glass for special vehicles, it differs completely from. The marketing and sales, production technologies and logistics require a different approach than the basic automotive glass business. For a technology oriented company ready to apply the latest technologies it opens up interesting opportunities for strengthening operations in this field.

Bus windscreen business still based on local operations

The windscreens produced are usually sold to local bus manufacturers, as well as replacement market. The division into OEM and aftermarket is more obscure. The production quantities are low and the number of different shapes large. Like the bus manufacturers, the glass manufacturers are smaller units often operating locally. Small orders combined with large sizes and difficult shapes that are hard to pack economically effectivey restrict long transport distances. Bus windscreen production is thus quite decentralized in smaller and medium production units. On the other hand, the average size is growing larger and the shape is becoming increasingly more difficult to bend. This tendency will require a lot of resources in the development of proper processing technologies, especially in regard to bending. A certain level of expertise and state-of-the art production equipment are thus required for achieving viable production.

The bus designer looks for different things than the car designer. In most of the countries the bus windscreen is still divided into left and right pieces, whereas in Europe the large monoblocks are more common, especially in well equipped charter and long distance buses. A large monoblock windscreen is aesthetically superior and provides better visibility outside, since the central frame can be avoided, but it is much more expensive to replace. Due to the lower speeds, aerodynamics are not a critical factor, the glass being installed in a vertical position. In some models the windscreen has been divided in two parts horizontally, with the upper screen having more inclination than the lower one. The wings may be bent upto almost 90° to provide a better view outside. Spherical shapes are gaining ground in new designs.

Due to the fact that most of the large screens are fitted manually and fixed with rubber rings, the shape and sag tolerances are generally not very tight. The problem here may arise only with larger pieces and spherically bent glasses with deep sagging.

Bending Process is the Key Factor

The basic windscreen manufacturing process is the same for bus windscreens as well; shape cutting, grinding, washing, printing, bending, prepressing, autoclaving and quality control. Although there are many subprocesses involved, the key process for optical quality and shape accuracy is the bending process. Without perfect bending, the whole process fails. A slightly curved, small half piece is fairly easy to process with traditional laminating and bending equipment, but large sized monoblocks demand more sophisticated production means.

Batch processing in single chamber furnace

Bus windscreens have been traditionally bent in slow single chamber furnaces. For additional capacities the manufacturers have simply added more bending units to the factory floor.

An individual furnace is a single or double chamber furnace with a closed wagon (or chamber). It has the following advantages: suitable for short series, ample scope for shape control, low tooling cost. The disadvantages are: poor repeatability, yield dependent on the production mix, labour intensive operation, and if several furnaces are needed, they require a lot of floor space.
Due to the strict quality requirements for optics, most of the furnaces for bus windscreen bending are electrically heated. The bending of bus windscreens is always based on gravity.

Combined bending performance through new double stage construction

Glassrobots developed a serial bending furnace for large bus windscreens at the end of 1980's. The first fully automatic version was introduced in 1993. With more than a dozen units sold in 1997, the company established its position as the leading supplier of bending furnaces for large bus windscreens.

The LTFBA furnace combines the benefits of a versatile single chamber furnace with the performance and capacity of a serial bending furnace. The glass is heated up and cooled down in consecutive sections. The higher the capacity needed, the more sections will be added. Bending is always completed in a closed, perfectly controlled section. A modular construction allows the bender to choose exactly the capacity required and extend it even after installation.

The LTFBA furnace is a double level tunnel furnace with preheating track above and a cooling track below. Once the glass is loaded, the fully automatic control conveys the glass through the preheating sections to the bending section and then back to the unloading/loading section through cooling. The control is based on continuous measurement of glass temperature by optical IR pyrometer. The standard version features one, two or three preheating sections and one bending section, depending on the capacity. The lower track has 1-3 cooling sections and unloading/loading station.

Preheating is completely electrical guaranteeing proper heating in all circumstances, even with some empty wagons in the process. The higher the glass temperature, the better heat control it requires to give the desired shape. All the prebending sections have individually controlled heaters, and the bending section have individually controlled 3-part or 5-part heaters.

Latest Improvements

In order to further improve the performance, Glassrobots has paid attention to the following areas and come up with important new features that significantly improve the efficiency of the bending process.

1. Heat Control during the bending to reach more complex shapes with better shape accuracy
2. Repeatability to comply with the most stringent OEM standards
3. Ergonomy
4. Ease of maintanance
5. Efficiency in operation cost

Heat Control

Some of the windscreens of today have such complex shapes that they can not be bent in basic furnaces with traditional heating systems. A wrap-around corner and tight radii mean something more to be bent. In gravity bending the key issue is the directing of heat to the right areas. Traditional means, such as heat covers and ratiation mirrors are still acceptable in today's lower capacity furnaces, but faster cycle times require more easier and automatic means. Glassrobots provides the LTFBA furnace with vertically adjustable crease heating elements divided into 3 or 5 individually controlled parts. This proven design dating back to company's architectural furnaces in the late 1980's is inexpensive and reliable. By lowering the elements, the radiation power can be increased up to more than 10 times as much as with fixed heating elements. Scatter radiation is markedly lower with the heating element in the lower position.

Better Repeatability

Glassrobots has developed a new control concept for mixed production, which upgrades the repeatability of the furnace in mixed production to a new level. The patented new feature is called FuzzyBend (pat.pend.). The system eliminates the effects of changes in the mixed production, heat balance of the furnace, external temperatures and variations in the supply voltage by adjusting the heating power according to the differences in the actual glass temperature and the target curve using fuzzy logic control and SSR/PLC power regulation. To keep the shape always the same, the heat profile is maintained.

Ergonomy and ease of maintenance

Glassrobots furnaces have traditionally been designed taking into account the furnace operator as well. The graphic operator interface with new Windows based control software makes the operation of the furnace as easy as the normal office computer. The furnace windows have heat reflective coatings for operator comfort.

Handy loading/unloading devices facilitate glass handling and minimize the load on the spinal column, thus reducing health problems among the operators. The loading device is available in two different versions.

The Condition Monitoring and Maintenace System eliminates the expensive random inspections and surprise repairs and thus minimizes downtime. The system provides the operator with maintenance schedules and instructions, supervises the functions of the furnace and gives an alarm whenever something threathens to go wrong. The system gives the operator valuable time to react to the malfunctioning of the furnace and thus saves on maintenance costs.

The system also includes remote diagnostics software, which enables Glassrobots' technicians to diagnose the control based problem and in most cases to remedy it through the telephone line.

Efficiency in Operating Cost

The serial furnace was developed in the first place to be affordable as an investment, but also affordable in terms of operating costs. The tunnel construction guaranteed a low energy consumption and an opportunity to run short series in mixed production (a different windscreen
in each wagon). Thanks to automation, the furnace requires less attendance, enabling the operator to perform other jobs while the furnace automatically bends the glass.

Glass breakage during preheating has been a problem for many benders. Glassrobots has avoided the problemin most cases by supporting the large windscreens during the preheating. The support has to be removed or lowered before the actual bending begins.

Future Perspectives

In the future, the windscreen processor will have to tackle technologies that improve the functional properties of the windscreen. A lot of research and development is being done on such issues as solar control glasses, microwires and coatings used in heatable windscreens, integrated antenna systems, integrated sensors, etc.

The key factors to the success continue to be the same: development matching future technologies, cost efficiency, process quality and product quality.