Float Glass Manufacturing Process Step by Step

Float glass is a fundamental component in modern architecture and automotive industries, known for its uniform thickness and smooth surfaces.We have shared what float glass is in the previous content. If you are interested, you can click to read. What is float glass? The float glass manufacturing process involves several critical stages, each contributing to the production of high-quality glass sheets. This guide provides an in-depth look at each phase of the process, highlighting the importance of materials like graphite in ensuring optimal results.

The process flow of a float glass production line can be roughly divided into five parts: raw materials, melting, forming, annealing, and cutting (packaging). The essential difference between the float process and other processes lies in the glass forming part.

The departments of the float glass production line can be divided into two parts: the hot end and the cold end. The hot end includes raw material, melting furnace, tin bath, and annealing lehr; the cold end includes cutting, packaging, and other links. In addition, the engineering department also performs daily operation, maintenance, and repairs on utility. The department settings and division of labor may be different in each factory.

The melting furnace is the place where raw materials are melted. The melting of glass raw materials requires a "long" process. First, the raw materials themselves need to be slowly melted under high temperature conditions. Secondly, the molten glass forms invisible convection at the bottom of the melting furnace, which makes it take a long time for the raw materials from entering the melting furnace to enter the tin bath to start forming. In this process, in addition to ensuring that various raw materials are fully melted, defects that may affect product quality, such as bubbles and stones, must be avoided.

The melting furnace can be divided into a melting end and a refining end. Both parts are built with refractory materials. The melting end mostly uses natural gas or heavy oil to heat the raw materials. The heated raw materials form molten glass liquid, which enters the refining end through the neck. The melted glass liquid in the melting end can continue to be homogenized and cooled to the temperature and viscosity required for forming in the refining end. After that, the glass liquid enters the tin bath after the canal and two tweels to start forming.

The tin bath is the glass forming room in the float process and is the most significant part that is different from other production processes. The glass liquid flows into the tin bath from the canal. Due to the density difference between the glass liquid and the tin liquid, the glass liquid itself is affected by the surface tension and spreads on the tin liquid surface, while the lower surface is naturally polished, so that a thickness of about 6.9 mm is obtained, which is called the equilibrium thickness.

The outside of the tin bath is a metal casing, the bottom is refractory bricks fixed on the bottom casing, the top is a hanging structure, the refractory bricks are installed on the top hook, and silicon carbide heating elements are installed to provide heat for glass forming. Silicon carbide heating elements are spread over all the top areas of the tin bath and can provide heat for each stage of forming.

In order to obtain products of different thicknesses, it is necessary to apply lateral tension or thrust to the molten glass to obtain products with a thickness less than or greater than the equilibrium thickness. At the same time, the molten glass is led to the transition roller (also called the lift out roller). The transition roller pulls the molten glass forward (i.e. forms a longitudinal tension) to form a glass ribbon. The molten glass is subjected to the action of lateral tension (or thrust) and longitudinal tension, as well as the combined influence of heat, cooling, and the use of auxiliary equipment to obtain products of different thicknesses and widths.

In the float glass manufacturing process, the tin bath plays a critical role in shaping the glass ribbon. One major concern at this stage is preventing direct contact between the molten glass and refractory materials. If contact occurs, the glass may stick and accumulate, disrupting production and causing serious operational issues-especially during thick glass forming, where support contact is often unavoidable.

To address this, manufacturers rely on graphite, which offers high-temperature strength, low thermal expansion, self-lubrication, and corrosion resistance. These properties make it ideal for use inside tin baths, where stable, non-reactive support is essential. Graphite components are typically categorized into daily-use parts for routine operation and cold-repair parts used during major overhauls. Among them, isostatic graphite is especially valued for its fine structure and ease of machining into custom shapes.

For high-performance graphite solutions in float glass forming, SHJ-CARBON provides a full range of products tailored to tin bath requirements-ensuring process stability and minimizing downtime.

The forming of the glass ribbon inside the tin bath requires the tin bath itself to have good air tightness and adjustability. Air tightness requires the use of sealing materials at the inlet, outlet, ADS machine, water cooler, side seal and other parts of the tin bath to maintain a good seal. The adjustability of the tin bath refers to the regulation and control of the temperature, the amount of glass liquid entering, the width and thickness of the glass ribbon, the convection of the tin liquid, the flow rate of the protective gas, etc.

Inside the tin bath, various reasons may affect the quality of the glass to varying degrees, resulting in a variety of quality defects and yield loss. Common defects include uneven thickness, dripping, tin staining, water ripples, bottom open bubbles, top tin, tin stones, exit lip scratches, curtain scratches, abrasions, plucks, crushing, printing, ramming materials affecting surface quality, etc. Different quality defects must first be inferred from the location and cause, and then effective measures must be taken. This requires technicians to accumulate rich experience and summarize and analyze in order to effectively respond.

The ultimate purpose of the annealing lehr is to release internal stresses and provide a glass ribbon that is easy to cut at the cold end. Annealing of glass is a process that uses a combination of heating and cooling to achieve the purpose.

In order to achieve the ultimate goal of stress release, the annealing lehr is usually designed into several areas, the upstream area is a closed area, and the downstream area is an open area. Different areas are divided according to temperature control. The glass ribbon is transported from the tin bath exit to the cold end by many rollers.

The float glass annealing zone is usually in the range of 566 to 496°C. This critical temperature range determines the formation of stress in the glass at room temperature.

After the glass ribbon passes through the forced cooling zone, it is transferred to the cold end, where the glass ribbon is finally cooled to room temperature, all temporary stresses disappear and only permanent stress remains. For properly annealed glass, the edges are compressive stress and the middle is tensile stress.

The most common factor affecting the yield of glass during the annealing stage is ribbon breakage, including horizontal ribbon breakage and vertical ribbon breakage. This needs to be avoided by adjusting the parameters of the annealing lehr. Common defects include scratches, warping, etc.

After leaving the annealing lehr, the glass ribbon enters the cold end and must go through online inspection, cutting, breaking, surface protection, stacking, and packaging.

The glass ribbon is first inspected online at the cold end to mark any quality defects. Some production lines will also set up manual online inspections, and the results of online inspections directly determine the subsequent cutting procedures. Cutting is divided into longitudinal cutting and transverse cutting to obtain products of different sizes. Generally, longitudinal cutting is performed first, followed by transverse cutting, and then breaking, folding, powdering, slicing, retrieval, stacking or boxing. The entire flat glass production is then completed.

Based on the characteristics of the entire process of float glass production, the production line must run uninterruptedly 365 days a year. Any unexpected shutdown of the production line is a serious production accident, which will cause economic losses to varying degrees. With the development of automatic control technology and the upgrading of intelligent production equipment, the number of accidents in float glass production today has been significantly reduced compared to the situation of relying on manual operation. However, traditional operating principles and methods still need to be mastered in case of emergency. This requires good and systematic training for operators of the production line. At the same time, there are many types of defects generated in the production process of float glass, especially surface quality defects generated in the tin bath forming stage. How to reduce and avoid the occurrence of quality defects has always been a common topic of research for float process technicians and managers.

This article only gives an overview of the float glass production process. The examples listed in the article do not represent the actual situation of any production line. They are for reference and learning by industry insiders. At the same time, we will further explore the relevant knowledge of float glass forming process. Among them, the use of graphite materials in the forming process will be described in detail. We hope that our knowledge points can provide some help to the management and technical personnel engaged in float glass. If there are any inappropriate descriptions in the article, please understand! We will continue to summarize and improve our theories and products. To this end, we will establish a product database for float glass, which will provide strong support and basis for product optimization!

Call to Action

In the production of float glass, graphite and its related products are mainly used in the glass forming stage. Graphite is particularly suitable for use in the tin bath of the float glass production line because of its high temperature strength, small thermal expansion coefficient, self-lubrication, high temperature resistance, corrosion resistance, and easy processing.

SHJ provides a full range of graphite product packages and complete graphite product solutions for float glass forming! We focus on the research and development of leading graphite materials and the provision of solutions!