Minimizing Defects During Glass Bending Process

Knots, wrinkles or even fabric thickness variations can create optical defects. Contaminants or oils can change the way STS fabrics behave over time, which can lead to defects. Longer lasting fabric reduces furnace and production downtime, saving costs.

STS fabrics are typically used in the bending area and / or the tempering area of automotive glass production lines. They often are stretched over a 3D mould for an automobile windshield, backlites or sidelites (windows) and serve as an insulating, soft pad between the mould and the glass. Without such fabrics, the glass would form more optical defects; there would be too much heat transfer between the glass and the mould, and there would be excessive breakage of the glass.

Detail of stainless steel fabric on a windshield mould.

THE PRESS BENDING PROCESS

In the glass bending process, flat glass is fed into the front end of a furnace that is typically 20–30 yards long. The glass travels through the furnace along a roller conveying system and is slowly heated to the point of malleability.

At that point, the glass is transferred to a 3D mould that has been covered with a STS fabric. The soft glass is pressed against the mould to achieve the desired curves and 3D shape. This pressing can occur before or just after the exit end of the furnace. In either case, the glass is at an extremely high temperature of 500–600°C (932–1112°F). Stainless steel is ideal for this application because the number of fabrics that can withstand such temperatures, for an extended time, are few.

This pressing into shape is usually assisted by a suction system in the mould itself. The fabric diffuses the air flow so it doesn’t create vacuumrelated optical defects. A substantial amount of force is applied to the fabric during the vacuum / pressing process. This, coupled with hot glass’ cutting the metal fibres, eventually wears out the fabric.

Fabric durability varies depending on the demands for the forming process— one shift for a very complex application to as long as a week for simpler, flatter pieces of glass.

In some processes, the glass is pressed into the mould just after it exits the furnace when the temperature is lower. In this case, fibreglass fabric can be used. About 20 years ago, most companies converted to using STS fabrics rather than fibreglass due to their durability and ability to handle higher temperatures.

PYROTEK STS FABRIC

Pyrotek’s “zero-defect” 100 percent, knitted STS fabrics are obtained through a textile company with a time-intensive, complex forming process with stringent cleaning and quality control processes. They are abrasion resistant, flexible and stretchable. The fabric is available in various widths, weights, thickness and air permeability. Different fabric compositions are designed for windshields, as well as sidelite, quarterlite and backlite applications.

Pyrotek fabrics have 10–15 percent more airflow than other such fabrics on the market. Depending on how porous the fabric is, a manufacturer should adjust the pressing pressure on the fabric accordingly. The forces related to gap settings between the mould and press ring and suction forces should be minimized without sacrificing glass quality. Using less force saves energy and fabric life.

TIPS FOR OPTIMIZING FABRIC AND TECHNIQUES

Achieving maximum quality and cost benefits of STS fabrics requires a keen knowledge of failure mode analysis and desired glass quality specifications.

• Holes–In some cases, a company will use a fabric until a hole is worn through or it folds over, creating optical defects in the glass. A heavier or thicker material could increase the life of the fabric in the customer’s process.

• Patterns in the glass–If the glass develops a pattern from the fabric, an alternate knit structure may be in order. STS fabrics typically have different characteristics on either side, so turning the fabric over is an option.

• Optical variation–If digital quality control devices record optical variation, then the customer might need a thinner fabric.

• Sagging–In some cases, fabric relaxes over time in heat and sags, which can cause the fabric to fold over and create optical defects. This issue might be solved with a lighter fabric or one with less inherent mechanical stretch in the structure.

• Lack of air flow–If the vacuum system is struggling to pick up the glass and suck it against the mould, a more porous fabric may be needed.

• Imperfect stretching procedure–Sometimes optical issues in the glass stem from the process by which a manufacturer stretches the fabric before it is placed on the mould, so a change in stretching procedures can sometimes create an improvement.

Pyrotek’s global network of application engineers work with customers to determine the optimum fabric for their processes.

www.pyrotek.info/glass