Daily glass products are rich in variety and widely used, and glass molds are important basic process equipment in the industrial production of daily glass products. They are known as the “mother of daily glass industry” and are an important symbol of a country’s daily glass manufacturing industry. China’s glass mold industry started in the 1970s, rose rapidly in the 1980s, achieved quantitative improvement in the 1990s, and achieved rapid qualitative improvement in the new century. Glass mold companies are relatively concentrated in Shajiabang, Yantai, Langfang, Chongqing and other regions in Jiangsu.

1 History of glass molds
1.1 Early glass molds
Around 100 BC, glass bottles were handmade, and there were no molds as molding tools. In ancient times, people used iron pipes to pick out the right amount of glass liquid to blow into glass bottles. In order to make the shape of glass bottles consistent, clay was kneaded into the shape of glass bottles and fired to shape them, or carved into the shape of glass bottles on stones as molds. At the end of the eighteenth century, wood was used to make molds.

1.2 Invention and use of metal molds

After the first industrial revolution, metal steel was widely used in various fields. In 1811, metal molds were first invented and used in Britain. In 1821, Britain invented a metal two-flap mold that could be opened to produce glass bottles. In 1847, American Magun Joseph invented and applied for the use of cast iron molds. It was not until 1866 that cast iron molds for glass bottles used continuous cooling air that good results were achieved.
The widespread use of metal molds has made the appearance of glass bottles uniform, and the finish of glass bottles and the life of molds have been improved to a certain extent. From then on, the manufacturing level of glass bottles ushered in a new era.

1.3 Development of metal molds
(1) Two-flap model. In 1860, metal molds were two-flap molds from the mold to the bottom. They were copied from the wooden mold style and had a complete seam line after molding. During operation, after filling the liquid into this mold, use the handles on both sides to close it. After the bottle is blown into shape, open the mold and clamp the bottle out. This type of glass bottle has a seam line on the bottom of the mold. The glass bottle is greatly affected by this line and cannot stand stably when placed.
(2) Three-flap mold with small bottom. The two-flap mold mode is changed to a three-flap mold mode, and the mold is separated from the mold bottom, and the mold bottom is added. The mold consists of three parts. The mold bottom was originally designed to be very small, commonly known as the small bottom type. The glass bottle made by this mold has a deep seam line on the bottom of the bottle, so this type of glass bottle is also unstable when placed.
(3) Three-flap mold with high bottom. After improvement, another three-flap mold with high bottom was designed, which is also composed of three parts. The mold bottom is like a high cylinder. Its mold bottom occupies half of the entire bottle body, and the seam line is in the middle of the bottle body. Although this type of glass bottle has no seam line at the bottom of the bottle, there is a circle of seam line in the middle of the bottle body, which affects the appearance and labeling of the glass bottle. Because the bottom of the mold is relatively high, it is easy to cause the glass bottle to be pinched after molding.
(4) Mechanized two-step molding form. From 1880 to 1890, the “press-blow” method and “blow-blow” method of bottle making were invented. During this period, glass containers were still semi-mechanized. After the drip feeder was successfully invented, glass molding gradually developed into full mechanization.

2 Performance requirements and materials of glass molds
2.1 Performance requirements of glass molds The mold is in direct contact with high-temperature glass liquid and conducts the heat of the glass. The quality of its material directly affects the quality of glass products and the life of the mold. The mold should be easy to machine, have a small expansion coefficient, be resistant to shattering, have good thermal shock, strong thermal conductivity, high adhesion temperature, and be corrosion-resistant. The mold material should meet the following requirements:
(1) The material is dense and has good processing performance, especially the part in contact with the hot glass liquid should have a quenching layer;
(2) Good thermal conductivity, low thermal expansion coefficient and good thermal stability;
(3) Good chemical stability and strong antioxidant ability;
(4) Good wear resistance and a certain hardness;
(5) Has a high adhesion temperature.

2.2 Commonly used glass mold materials
Currently, the most widely used mold material is alloy cast iron, which contains alloy components such as nickel, chromium, copper, and vanadium. In recent years, with the increase in the speed of bottle making machines, copper-based alloy materials with good thermal conductivity have begun to be used. They have the advantages of high thermal conductivity, strong corrosion resistance, and long service life. A mold may be scrapped due to local damage. Strengthening the protection of vulnerable parts will extend the overall life of the mold. The commonly used method is to spray weld on vulnerable parts. The spray welding material is a nickel-based alloy with a hardness of up to RC30~RC40.
Commonly used glass mold materials are:
(1) Ordinary low-alloy cast iron. Cast iron is currently widely used as a glass mold material both at home and abroad;
(2) Ductile iron, which has poor thermal conductivity and cannot meet the requirements of mechanized production. It is suitable for manufacturing small bottle molds;
(3) Vermicular graphite cast iron, which has similar mechanical properties to ductile iron and better thermal conductivity than ductile iron, and has good comprehensive performance;
(4) D-type graphite cast iron. Most imported molds are made of D-type graphite cast iron;
(5) Alloy steel. In recent years, alloy steel has been used to make molds. Stainless steel has poor thermal conductivity and is prone to overheating and adhesion during use. It has high requirements for mold cooling, structure and lubrication;
(6) Nickel-based alloys can improve the finish of glass products, but this type of material is relatively expensive;
(7) Copper-based alloys, which can increase machine speed compared to cast iron, are easy to repair molds, and have good glass bottle molding quality;
(8) Mold coating. Glass molds are partially thermally sprayed. layer, which can increase the life of the mold, but the mold cost is high.

3 Mold use management
(1) Mold preheating. The mold is preheated before use, which can quickly reach the mold temperature balance point, reduce the number of times the mold clamps the initial blank, reduce mold damage, and improve the product molding rate.
(2) Mold installation. The various parts of the mold should be handled with care and installed correctly. When installing the mold, special handles and clamps should be used to ensure that the molds fit well. If the installation is improper, the mold will not open and close smoothly, the product will have quality defects, and the mold will be damaged prematurely.
(3) Mold handling. Improper mold handling will cause damage to the inner cavity surface, edges and corners, and joint surfaces. The mold should be transported in a closed state. It is best to use a locking clamp and manufacture a special mold transport vehicle according to the shape of various molds.
(4) Mold lubrication. The purpose of mold lubrication is to prevent high-temperature glass material from contacting the mold and causing adhesion on the surface. This can reduce mold wear, enhance the stability of mold operation, reduce product defects, and improve the finish of bottles and cans.
(5) Cooling of the mold. The molding process of the bottle is the cooling process of the glass material. The cooling of the glass material is achieved by cooling the mold. The cooling condition of the mold has an important impact on improving product quality and production speed. The main principle of mold cooling is: strengthen cooling of high-temperature parts, reduce cooling of low-temperature parts, or even keep them warm. The cooling system of the mold has been continuously reformed, including water cooling, side air cooling, vertical cooling, etc. At present, vertical cooling has the best effect, which can make the mold cool evenly.
(6) Exhaust and vacuum of the mold. After the glass material enters the mold cavity, it must be in close contact with the mold wall, so the air in the mold must be exhausted. Poor exhaust can easily cause various defects, such as insufficient bottle mouth, poor bottle body molding, reduced bottle capacity, etc. The exhaust of the mold on the initial mold side includes: exhaust of the core, exhaust of the mouth mold, exhaust of the initial mold, and exhaust of the blind head; the exhaust of the mold on the finished mold side includes: exhaust holes at the mold wall, exhaust at the bottom of the mold, exhaust of the blow head, and exhaust of the end face of the blow head. When the initial blank is formed in the forming mold, it is close to the inner cavity of the finished mold from bottom to top. The exhaust holes are generally distributed in the middle and upper part of the inner cavity of the finished mold. The diameter is generally 0.4-1.2 mm. The diameter in the middle is smaller and the diameter in the upper part is larger. The aperture is small in the part with high temperature and large in the part with low temperature. The principle is to meet the exhaust requirements without affecting the appearance quality of the product.
(7) The service life of the mold. The mold life is a relative value. Its service life is difficult to define uniformly. It is related to the user’s quality requirements for the bottle and the bottle manufacturer’s quality requirements for the product and the mold. The factors that affect the life of the mold are: ① mold material and processing accuracy; ② product quality standards; ③ the way to transport, install and replace the mold; ④ the number of times the product is replaced; ⑤ cleaning method; ⑥ maintenance process; ⑦ usage method, etc. At present, the service life of molds in various domestic bottle factories varies greatly, generally 100,000 to 600,000 times, with an average of 300,000 times for the initial mold and 350,000 times for the finished mold. The life of more advanced copper alloy molds abroad can reach more than 2 million times.

4 Mold maintenance management
Mold maintenance is to restore the mold to the greatest extent and extend the mold service life. Mold maintenance will also cause wear to the mold cavity while repairing the mold. If the maintenance is not done properly, the mold service life will be greatly shortened. Therefore, reasonable maintenance management is very important. At present, the best maintenance method is to use laser cladding technology for repair.

4.1 Mold service life The mold service process can be divided into three periods: new mold period, service period and end of service period.
(1) New mold period. It can also be called the running-in period. When the new mold is used for the first time, the coordination between the various parts is not in the best state. The product appearance is good, but the yield rate is not very high.
(2) Service period. After a period of running-in, the mold enters the service period. During this period, the various parts of the mold open and close smoothly, the operation is stable, the product quality reaches the best, and the yield rate is also the highest.
(3) End of service period. After multiple uses and repairs, the mold wears more and more seriously, the product quality decreases, the capacity increases, and the yield rate decreases.

4.2 Staffing
Mold maintenance is a highly technical job, and maintenance personnel should be trained as planned. In addition to having the skills of a fitter and gas welder, they must also master professional knowledge in bottle production, such as the various processes of bottle making, the role of the mold, and understand product defects caused by the mold.

4.3 Establishing mold files
After a new mold is taken out from the mold library, the mold manager must fill out a mold maintenance card and track and record each mold. The content includes: important part size value, water capacity value, machine time, number of times on the machine, the name of the repairer, the repaired part, and the service life when scrapped.

4.4 Mold maintenance management

4.4.1 Mold maintenance process
(1) Recycling. The mold recycling personnel will go to the machine to recycle the replaced molds to be repaired according to the time.
(2) Cleaning. After the mold is used, it will produce oil stains, carbon deposits, and oxide layers on the mold surface. The cleaning process is to remove these pollutants. Common cleaning methods include dry sandblasting and wet sandblasting.
(3) Pre-inspection. After the mold is cleaned, the mold inspector will inspect it according to the mold scrapping standard. For molds that can be repaired and used, a repair form should be filled out, indicating the repaired parts, and handed over to the maintenance personnel for repair. The scrapped molds are marked as scrapped and stored in the scrap area. For large molds such as finished molds, bottom molds, and primary molds, try not to scrap them individually, and repair them as much as possible.
(4) Repair. The maintenance personnel repair the mold according to the requirements of the mold repair form. The specific steps include: ① Spray welding repairs the damaged parts. Before spray welding, the parts to be repaired should be surface treated with grinding wheels, emery cloth, files, etc. to prevent the spray welding from being loose. When the spray welding part is large, the entire mold should be preheated; ② After the spray welding is completed and the temperature is lowered, use a grinding wheel or manual processing to repair. When repairing, first flatten the spray welding part, grind the gap between the joint surfaces, and then repair the matching parts. When repairing the joint edges, the two halves of the mold need to be aligned to avoid misalignment; ③ Clear the exhaust holes or vacuum holes of the molding mold; ④ After the repaired mold is self-checked by the repairman, it is sent to the polishing machine for polishing.
(5) Polishing. The polishing quality mainly depends on the mold material. The finish of the old mold after polishing is slightly lower than that of the new mold. The polishing amount should be controlled within an appropriate range. If it is too large, the mold loss will be large and the mold service life will be reduced; if it is too small, the mold finish will not be enough, affecting the appearance quality of the bottle.
(6) Inspection. The mold inspector inspects the repaired mold piece by piece according to the mold inspection standard. Those that fail to meet the repair requirements but can be repaired will be returned to the repairman for further repair. Those that fail to meet the repair requirements and cannot be repaired will be scrapped. The inspection items are: ① Inspection of each matching surface, including the matching gap between the joint surface and each mold; ② Inspection of each matching size; ③ Inspection of water capacity. After a period of use, the mold and the initial mold should be inspected for water capacity. For molds with larger capacity, the number of uses can be reduced or the use can be suspended. If the capacity exceeds the scrap standard, it should be scrapped.
After the inspection, the inspector should fill in the mold maintenance card, mold scrapping form, etc.

4.4.2 Issues to be paid attention to in mold maintenance
(1) Ensure the expansion gap of the joint surface of the mold and the initial mold. When the mold and the initial mold are heated and clamped, thermal expansion deformation will occur on the mating surface of the two halves of the mold, which will make the upper and lower joints not fit well, resulting in too large a mold seam line. When the mold is designed, sufficient expansion gap is left on the joint surface. When the maintenance worker repairs the joint surface, it is easy to reduce the expansion gap or even make the expansion gap disappear. Therefore, it is necessary to pay attention to the expansion gap of the joint surface during maintenance. If the expansion gap is reduced and cannot be repaired manually, it can be sent to the mold processing factory for repair with a CNC machine tool.
(2) At the end of the mold, the size of the bottom blade changes greatly, resulting in a large gap at the junction with the bottom mold blade. You can use a method of one mold with one bottom, that is, the bottom mold is not interchangeable. At the end of the use, many edge sizes and matching sizes will also exceed the tolerance range, seriously affecting the product quality. It cannot be repaired by maintenance workers. At this time, you can consider spray welding and then putting it on the machine tool or sending it to the mold factory for repair. Such as the edge at the ends of the finished and primary molds, the contact between the upper and lower clamps of the finished and primary molds, the edge of the bottom mold and the blind head, and the part where the primary mold holds the mouth mold.
(3) When repairing the concave or convex parts of the mold cavity with product logos, machine recognition mold codes, trademarks, patterns, words, etc., you must be careful to avoid damage to the parts that cause the mold to be scrapped.

5 Conclusion
With the improvement of living standards and the enhancement of people’s awareness of health, green and environmental protection, the awareness of glass containers and the promotion of glassware will become more extensive, and the application of matching glass molds will increase greatly. At present, glass mold companies in Germany, France, the United States, Belgium, the Czech Republic and other countries have competitive advantages in medium and high-end glass molds with their technological advantages and strengths. In the past 10 years, Chinese glass mold companies have continuously increased their investment in technological transformation, greatly improved process innovation and quality management, and have been recognized by the international market in terms of equipment, research and development, quality and service. Independent research and development of new materials, new processes and new technologies has enhanced competitiveness and laid a solid foundation for the high-quality development of daily glass in my country.