Differences in Precision Casting Wax Mold Materials

The terms "low-temperature wax" and "medium-temperature wax" originated from early classifications of mold materials in investment casting. Although these terms are still widely used, strictly speaking, they should be referred to as "low-temperature mold material" and "medium-temperature mold material."

With the rapid advancement of science, technology, and industrial production, the applications of investment casting have significantly expanded. However, the performance and availability of traditional wax-based materials have struggled to keep pace with the industry's rapid development. As a result, various materials, including organic compounds, plastics, and inorganic salts, have been introduced. Some are used as fillers to enhance wax-based materials, while others directly replace wax for mold-making. Consequently, the terms "wax mold" and "wax material" have gradually been replaced by "mold material" and "investment mold." Nevertheless, in practice, people still commonly refer to these as "wax molds," though the modern "wax mold" is quite different from its traditional counterpart.

Due to the wide variety of mold materials and their diverse compositions, classification is complex. There are two widely accepted classification methods:

1. Classification by Thermal Stability
   Thermal stability is a critical property of mold materials, closely related to their melting points. Generally, the higher the melting point, the greater the thermal stability, and vice versa. Based on melting points, mold materials can be divided into three categories: high-melting-point, medium-melting-point, and low-melting-point mold materials. From the perspective of thermal stability, these correspond to high-temperature, medium-temperature, and low-temperature mold materials.

   Therefore, the commonly mentioned "low-temperature wax" and "medium-temperature wax" are clearly distinguished based on the thermal stability of the mold material (or wax material). This is the first key distinction.

2. Classification by Base Material Composition
   Based on the development of modern mold materials, they can be classified into wax-based, rosin-based, plastic-based, water-soluble, and filled mold materials. This classification aligns with the framework outlined in Wang Leyi’s Investment Casting Technology. The first classification method (by thermal stability) is generally consistent across various sources. However, the second method (by composition) varies slightly. Some online sources categorize mold materials into four types: wax-based, resin-based, filled, and water-soluble mold materials. Similarly, Practical Investment Casting Technology edited by Jiang Buju divides them into non-filled mold materials (wax-based and resin-based), filled mold materials, and water-soluble mold materials, which is largely consistent with the above. Another classification, from Yamaya Hiroshi’s Practical Precision Casting Technology, is presented in Table 1 (Classification of Mold Materials).

Regardless of the classification method, the core principles remain the same. Modern advancements have introduced a new category: mold materials for rapid prototyping.

Below, we elaborate on the mold materials categorized by thermal stability:

1. Low-Temperature Mold Materials
   Low-temperature mold materials typically have melting points below 70°C. They exhibit low strength, poor thermal stability (around 30°C), significant shrinkage, and dimensional instability with changes in ambient temperature. Their surface roughness is relatively high. A common example in China is a mold material composed of 50% paraffin and 50% stearic acid. These materials are simple to produce, have high recyclability, can be reused, and are cost-effective. They are typically used for paste pressing and manual operations, with dewaxing performed using hot water. These materials are often paired with water-glass binders.

2. Medium-Temperature Mold Materials
   These primarily include rosin-wax-based and filled mold materials, with melting points between 70°C and 100°C. They offer higher strength, better thermal stability (around 35°C), lower shrinkage, and good dimensional stability. Molds made with liquid pressing have finer surface roughness and excellent replication performance. These materials can be recycled, though their production process is more complex and costly. Dewaxing can be done using water, steam, or microwaves. They are typically used with silica sol binders and produced using wax injection machines.

3. High-Temperature Mold Materials
   These include rosin-plastic-based, plastic-based, and salt-based mold materials, with melting points above 100°C. They offer high strength, excellent thermal stability, and minimal shrinkage, but their production process is complex, and they are not recyclable. Dewaxing is typically performed using flash firing, water, or organic solvents, and the cost is high. Water-soluble mold materials and those used for 3D printing fall into this category. According to Investment Casting Technology, high-temperature mold materials have significant thermal expansion, which can cause shell cracking during dewaxing due to insufficient shell strength.

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