Quality inspection of bottle glass - defects

There are many kinds of defects in bottle glass, which can be summarized into two categories: defects in the glass body and defects in the molding of bottle glass. Glass defects are closely related to various links of production, such as raw material processing, batch material preparation, melting, clarification, homogenization, cooling, molding and other production processes. The tolerance of glass defects depends on the purpose of the product. Generally speaking, a large number of obvious defects are not allowed in glass products, otherwise it will affect the appearance quality of the glass, reduce the uniformity and light transmittance of the glass, reduce the mechanical strength and thermal stability of the glass, and cause a large number of waste and defective products.

Due to the presence of various inclusions in the glass body, the uniformity of the glass body is destroyed, which is called a defect in the glass body. According to their different states, they can be divided into three categories: bubbles (gas inclusions), stones (solid inclusions), streaks and nodules (glass inclusions), which are internal defects.

(1) Bubbles in bubble glass are visible gas inclusions, which are composed of various gases in the glass. They not only affect the appearance quality of glass products, but more importantly, they affect the transparency and mechanical strength of the glass.
Bubbles can be divided into gray bubbles (diameter <0.2mm) and bubbles (diameter >0.2mm) according to their size; their shapes are also various, including spherical, elliptical and linear. Bubbles often contain O2, N2, CO, CO2, SO2, nitrogen oxides and water vapor.
According to the different causes of bubble generation, they can be divided into: primary bubbles (residual bubbles in batch materials), secondary bubbles, external air bubbles, refractory bubbles and bubbles caused by metal iron.
After the glass clarification stage is over, there are often some bubbles that do not escape completely and remain in the glass. These bubbles are called primary bubbles. The main reason for the generation of primary bubbles is poor clarification. In production, various methods can be adopted to increase the gas escape rate, such as increasing the melting temperature, reducing the viscosity of the glass liquid, reducing the kiln pressure and appropriately adjusting the amount of clarifier.
Due to changes in the process conditions in the kiln, bubbles (ash bubbles) appear in the clarified glass liquid during the molding stage, which are called secondary bubbles. The formation of secondary bubbles is closely related to the glass melting process, mainly due to physical and chemical factors.
The refractory material itself has a certain porosity, and the pores often contain gas. When the refractory material contacts the glass liquid, the glass liquid is sucked in due to the capillary action of the pores, and the gas in the pores is squeezed into the glass liquid. In addition, impurities such as carbon, iron, and titanium contained in the refractory material or metal enter the melt after corrosion and produce bubbles.
The gas adsorbed on the surface of the broken glass and foreign matter brought by people, such as dust, coal ash, oil and other solid and liquid inclusions, enter the glass melt and directly contact the glass liquid to produce bubbles.

(2) Stones Stones are the most dangerous defects in the glass body. They are crystalline solid inclusions that appear in the glass body and have a serious impact on the appearance and optical uniformity of glass products. In addition, due to the different expansion coefficients of the stone and the surrounding glass, local stress is generated on the interface of the glass, which greatly reduces the mechanical strength and thermal stability of the product, and even causes the product to break automatically.
Different stones have different chemical and mineral compositions. According to the causes of their formation, stones are divided into the following categories: batch stone (unmelted particles); refractory stone; crystallization stone; sulfate inclusions (alkaline inclusions); "black spots" and foreign pollutants.
Batch stone is the unmelted component particles of the batch. In most cases, it is quartz particles, which are white in color. Its edges become rounded due to gradual dissolution, and its surface often has grooves. There is a colorless ring with a high SiO2 content around the quartz particles. It has high viscosity and is not easy to diffuse, which often leads to the formation of coarse tendons. After staying in the high temperature section for a long time, the quartz particles will gradually transform into crystals of cristobalite and tridymite.
Refractory stone is caused by the long-term high temperature of the furnace chimney and breast wall of the melting furnace, and the effect of alkali gas, alkali fly and other volatiles, forming a glaze layer on the surface of the refractory material. Due to its fluidity and surface tension, droplets are gradually formed. When the generated glass droplets reach a certain weight and viscosity, they fall from the chimney into the glass liquid to form stone. In addition, the refractory material in contact with the glass liquid is corroded and peeled off at high temperature for a long time, and is mixed into the glass products to form stone.
Crystallization stone is caused by the uneven chemical composition of the glass body that causes the glass liquid to crystallize when it stays at a temperature that is conducive to the formation and growth of crystals for a long time. Crystallization stone often appears at the interface between two phases.
Sulfate inclusion stone is caused by the sulfate contained in the glass melt exceeding the amount that can be dissolved in the glass, which will be separated as slag in the form of sulfate and enter the finished product. Black inclusions and stones are directly or indirectly derived from the batch materials. They can also be formed by the introduction of chromium, iron, nickel, etc. due to careless operation, causing defects in the glass body.

(3) Streaks and nodules The heterogeneous glass inclusions in the main body of the glass are called streaks and nodules. They are different from the main body of the glass in chemical composition and physical properties. They form stress on the bottle and affect the quality and grade of the product. In terms of lightweight glass bottles and jars, the influence of streaks on molding is particularly significant and is the main defect of lightweight bottles.
From the appearance point of view, streaks and nodules protrude to varying degrees on the main body of the glass, distributed inside the glass or on the surface of the glass. They can be colorless, green or brown. Most of them are in the shape of stripes, but there are also full lines and fibers, and sometimes they are like bumps and protrude.
According to the different causes of their occurrence, streaks and nodules can be divided into four types: uneven melting, kiln glass drops, refractory erosion and stone melting.
During the melting process of the glass liquid, through the action of the "homogenization" stage, the various parts in the melt diffuse into each other and eliminate the unevenness. If the batch materials are not mixed evenly or the melting temperature is unstable, the temperature system of homogenization is destroyed, the homogenization is not perfect, and the glass liquid in the freezing zone participates in the liquid flow, resulting in the appearance of stripes and nodules.
Due to the volatilization and decomposition of the volatile substances in the melt, the content of silica on the surface of the melt increases. Or because the excess droplets with high viscosity fall into the glass body, its chemical composition is different from that of the main glass, and it diffuses very slowly in the glass melt, which will also form stripes and nodules.
The glass melt erodes the refractory material, and the eroded part may fall into the glass body in a crystallized state to form stones. It is also possible to form glassy substances dissolved in the glass body. This kind of stripes and nodules is the most common type of stone. Under the action of the glass melt in the glass body, the stone gradually dissolves at different rates. The glass body after the stone dissolves still has a different chemical composition from the main glass, forming nodules or stripes.