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The Development and Application of Glass Mold Materials

The materials for glass mold include heat resistant cast iron, stainless steel and heat resistant alloy. The service life of the molds can reach up to 400-500 thousand times after the materials are processed and reinforced. 
1) Ordinary low-alloy cast iron
The cast iron has been widely used as a glass mold material since the 19th century. It has excellent casting properties, good processability, low cost and non-stick performance. It is the main glass mold material now and in the near future. Cast iron is an iron alloy of carbon, silicon and manganese, composed of graphite and matrix. 
Since the gray cast iron has excellent casting performance and good processability, it is widely used to manufacture glass molds, especially for mid-size and small-size factories. But, the graphite in the common cast iron is type A, which has poor heat resistance and growth and expansion after long-term use which will makes the structure loose and brittle and reduce its strength. It may fail due to oxidation spalling, wear, deformation. Alloy elements such as Ni, Cr, Mo, Ti, V and Cu are added to improve its performance. 
D-type graphite gray cast iron has higher strength and better resistance to oxidization and wear than A-type graphite gray cast iron. It is a kind of flake graphite. It has an austenitic skeleton structure, which improves its resistance, and a tiny, curled and blunt-end shape which reduces its cutting action on the matrix and the potential of stress concentration. Therefore, it has higher strength. 
Compared to gray cast iron, the graphite in nodular cast iron is globular, which improves the density of the materials and reduces the cutting action of graphite on the matrix. Although it has high strength and toughness and good resistance to oxidization and thermal fatigue, it has low thermal conductivity and glass products cannot be rapidly cooled in the mold cavity, which limits the forming speed and its application. It is only used to make small glass molds. 
Since the compacted graphite cast iron has similar mechanical properties to the nodular cast iron and high thermal conductivity, good resistance to oxidization and growth as well good casting properties and processability, it has good overall performance to manufacture glass molds. 
The shape of graphite has great influence on the properties of cast iron. Flake graphite has poor resistance to oxidization and growth, while the spheroidal graphite has low thermal conductivity. Therefore, compacted graphite is the ideal choice for manufacturing glass molds, especially low tin compacted graphite iron. 
2) Alloy steel
Recently, because the requirements for the quality of glass products are higher and higher, alloy steel is being used to manufacturing glass molds. In most cases, alloy steel can better meet the requirements of glass molds than cast iron. Its thermal conductivity is inferior to the cast iron, so it is prone to overheat during use and requires forced cooling, reasonable design and lubricating with silicone grease. 
Glass mold is subjected to great pressure and high temperature, thus the glass mold materials should have high strength, small thermal deformation and good heat resistance. Stainless steel is a good glass mold material with good corrosion resistance, high strength, good oxidization resistance, high toughness and small thermal expansion. 
3) Nickel-based alloy
It has better heat resistance than cast iron, so glass molds made with it have less cracks, longer service life and higher production efficiency and can improve the smoothness of the surface of glass products. Although it is expensive, it is still used to improve the smoothness of glass products and prolong the service life. Its advantage is that it can be made by casting and requires no processing. 
4) Copper-based alloy
It is mainly made of aluminum bronze. Nickel, chromium, zinc and other elements are appropriately added to improve the quality and forming speed of the glass molds. It has high thermal conductivity and can maintain the quality of glass products at high-speed molding. 
4) Heat-resistant alloy
It is better to choose alloys with low nickel. It is mainly used in the press mold. 
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