Watch Case Made Of Titanium Alloy Lost-wax Casting
Losed-wafer casting, also known as investment casting, is a precision casting method. Its principle involves first creating a wax model with the same shape as the watch case. Then, multiple layers of refractory material are coated onto the surface of the wax model to form a monolithic mold. Next, the mold is heated, causing the wax model to melt and flow out, leaving a cavity within the mold that matches the shape of the watch case.
Overview of Lost-Wafer Casting Technology
Losed-wafer casting, also known as investment casting, is a precision casting method. Its principle involves first creating a wax model with the same shape as the watch case. Then, multiple layers of refractory material are coated onto the surface of the wax model to form a monolithic mold. Next, the mold is heated, causing the wax model to melt and flow out, leaving a cavity within the mold that matches the shape of the watch case. Finally, molten titanium alloy is poured into this cavity. After the titanium alloy cools and solidifies, the mold is broken to obtain the desired watch case. This casting method can produce parts with complex shapes and high precision, making it ideal for products like watch cases where high accuracy in appearance and dimensional precision is required.
Advantages of Titanium Alloy in Watch Cases
Lightweight
Titanium alloy has a relatively low density, approximately 4.5 g/cm³, which is lighter than traditional stainless steel (density approximately 7.9 g/cm³). This makes wearing the watch more comfortable, without putting excessive strain on the wrist, and is especially suitable for extended wear.
High Strength
Titanium alloy has very high strength, with a tensile strength generally between 400 and 1400 MPa. This means the watch case can withstand certain external impacts and pressures during daily use, resisting deformation and damage, thus ensuring the watch's durability.
Corrosion Resistance
A dense titanium oxide protective film forms on the surface of the titanium alloy. This film has excellent chemical stability and can resist corrosion from various media, such as sweat and seawater. For watches, this effectively prevents the case from rusting and corroding, extending the watch's lifespan.
Good Biocompatibility
Titanium alloy is non-toxic to the human body and will not cause allergic reactions, making it suitable for people with all skin types. This is an important advantage for watch cases that come into direct contact with the skin.
Specific Process of Lost-Wafer Casting of Titanium Alloy Watch Cases
o. Design and Engraving: Based on the watch's design requirements, a 3D model is created using computer-aided design (CAD) software. A high-precision wax model prototype is then produced through CNC machining or hand engraving. This prototype must accurately reflect the shape, size, and detailed features of the watch case, including the crown, lugs, and markers.
o. Wax Model Replication: Using the prototype, a mold is made, and wax models are mass-produced by injecting wax. During the injection process, the temperature, pressure, and injection speed of the wax must be carefully controlled to ensure the quality and consistency of the wax model.
o Wax Model Assembly: Multiple wax models are connected together using wax rods to form a wax model assembly. The design of the wax model assembly must consider the flow of molten metal and venting to ensure a smooth casting process.
o Refractory Material Coating: The wax model assembly is immersed in a coating composed of refractory materials (such as silica sol, zircon sand, etc.) and a binder, ensuring a uniform coating on the surface. Then, a layer of refractory sand is sprinkled on top, adhering to the coating surface. This process is repeated multiple times until the shell reaches the required thickness. Generally, the shell consists of 4-8 layers.
o Drying and Hardening: After each layer of coating and sand is applied, drying and hardening treatment is required to improve the strength and stability of the shell. The drying process is usually carried out under specific environmental conditions, with controlled temperature, humidity, and ventilation. Hardening can be achieved using chemical or thermosetting methods.
The mold is placed in a steam dewaxing kettle or hot water to melt and flow out the wax pattern. The dewaxing process must be thorough to avoid residual wax affecting casting quality.
* Titanium Alloy Melting: The titanium alloy raw material is placed in a vacuum induction furnace for melting. During the melting process, the vacuum level, temperature, and melting time in the furnace must be strictly controlled to ensure the uniformity and purity of the titanium alloy composition. Simultaneously, it is necessary to prevent the titanium alloy from reacting with impurities in the furnace, affecting its performance.
* Casting: Once the titanium alloy reaches the appropriate temperature and fluidity, it is quickly poured into the preheated mold. The casting process must be fast and smooth to avoid splashing and oxidation of the molten metal. At the same time, the casting speed and volume must be controlled to ensure the quality and dimensional accuracy of the watch case.
* Case Cleaning: After the titanium alloy cools and solidifies, the mold is broken, and the watch case is removed. Then, sandblasting, grinding, and other methods are used to remove residual mold material and oxide scale from the surface, making the watch case surface smooth. * **Heat Treatment:** To improve the mechanical properties and corrosion resistance of the watch case, appropriate heat treatment is required. Common heat treatment processes include annealing, quenching, and aging. The temperature and time must be strictly controlled during heat treatment to ensure the desired effect is achieved.
* Machining and Surface Treatment: According to the watch's design requirements, the cast watch case undergoes machining processes such as drilling, milling, and grinding to achieve precise dimensions and shape. Then, surface treatments such as polishing, electroplating, and PVD coating are performed to improve the watch case's appearance quality and wear resistance.
Quality Control and Inspection
Dimensional Accuracy Inspection
Using precision measuring tools such as coordinate measuring machines and calipers, the key dimensions of the watch case are measured to ensure they meet design requirements. Dimensional deviations are generally controlled within ±0.05mm.
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Surface Quality Inspection
The watch case surface is inspected for defects such as cracks, pores, and pinholes through visual inspection and microscopic observation. Surface roughness must meet certain standards, generally requiring an Ra value between 0.4 and 0.8μm.
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Mechanical Property Testing
Tensile tests and hardness tests are used to test the mechanical properties of the watch case, such as tensile strength, yield strength, and hardness. These performance indicators must meet relevant standards and design requirements.
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Corrosion Resistance Testing
Salt spray tests and immersion tests are used to test the corrosion resistance of the watch case. After the tests, the corrosion condition of the case surface is observed to assess its corrosion resistance.
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Challenges and Solutions for Lost-Wafer Casting of Titanium Alloy Watch Cases
* Challenge: Titanium alloys are chemically reactive and easily react with oxygen and nitrogen in the air at high temperatures, forming brittle compounds that affect their properties. Simultaneously, titanium alloys have a high melting point (approximately 1668℃), requiring specialized equipment and processes for melting and casting.
* Solution: Vacuum induction melting and vacuum casting technologies are used to avoid contact between the titanium alloy and air. During melting and casting, the vacuum level and atmosphere within the furnace are strictly controlled to ensure the quality of the titanium alloy.
* Challenge: Titanium alloys react chemically with the mold material at high temperatures, leading to decreased surface quality and dimensional accuracy.
* Solution: Select suitable mold materials and coatings, such as zircon sand and yttrium oxide. These materials have good chemical stability and can reduce the reaction with titanium alloys. Simultaneously, optimize the coating formulation and application process during mold manufacturing to improve mold quality.
* Challenge: Lost-wax casting is prone to defects such as porosity, shrinkage, and cracks, affecting the quality and performance of the watch case.
* Solution: Optimize casting process parameters, such as pouring temperature, pouring speed, and cooling rate, to reduce casting defects. Simultaneously, use advanced simulation software to analyze the casting process, predict the location and causes of defects, and take preventative and control measures in advance.
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