Watch Case Back Made Of Titanium Alloy Lost-wax Casting
Losed-wafer casting, also known as investment casting, is a precision casting method with a long history. The principle involves first creating a wax model with the same shape as the watch case back. Then, through a series of processes, multiple layers of refractory material are coated onto the surface of the wax model to form a monolithic shell.
Overview of Lost-Wafer Casting Technology
Losed-wafer casting, also known as investment casting, is a precision casting method with a long history. The principle involves first creating a wax model with the same shape as the watch case back. Then, through a series of processes, multiple layers of refractory material are coated onto the surface of the wax model to form a monolithic shell. The shell is then heated, causing the wax model to melt and flow out, leaving a cavity within the shell that matches the shape of the watch case back. Finally, molten titanium alloy is poured into this cavity. After the titanium alloy cools and solidifies, the shell is broken to obtain the desired watch case back.
Properties of Titanium Alloy Material
Titanium alloy is an alloy composed of titanium as a base and other alloying elements. It possesses many excellent properties, making it ideal for watch case backs. First, titanium alloy has a low density, only about 60% that of steel, making watches lighter and reducing wrist strain. Second, titanium alloy has excellent corrosion resistance, resisting the erosion of sweat, seawater, etc., ensuring that the watch case back will not rust or corrode even after long-term use in various environments. In addition, titanium alloys possess high strength and good biocompatibility, and will not cause skin allergies, which are crucial characteristics for watch case backs that come into prolonged contact with the skin.
Specific Process of Lost-Wafer Casting of Titanium Alloy Watch Case Backs
o. Mold Design and Manufacturing: Based on the design requirements of the watch case back, a 3D model is created using Computer-Aided Design (CAD) software to determine the dimensions, shape, texture, and other details of the case back. Then, a CNC machining technology is used to manufacture the mold for pressing the wax model. The precision and surface quality of the mold directly affect the quality of the wax model, therefore, strict control of machining precision is required.
o. Wax Model Pressing: Molten wax, heated to a suitable temperature and fluidity, is injected into the mold and held under pressure for a period of time to ensure the wax fills all corners of the mold. After the wax cools and solidifies, the mold is opened, and the wax model is removed. A preliminary inspection is performed on the wax model to remove excess flash and burrs, ensuring that the dimensions and shape of the wax model meet the design requirements.
o. Coating: The prepared wax model is fixed onto the sprue bar to form the mold assembly. The mold is then immersed in a coating composed of refractory materials (such as silica sol, zircon sand, etc.) and a binder, ensuring a uniform coating layer on the wax mold surface. The thickness and uniformity of the coating are crucial to the quality of the shell, requiring strict control of the immersion time and speed.
o. Sand Spreading: Immediately after immersion in the coating, the mold is placed in a sand spreading device, allowing refractory sand particles to adhere evenly to the coating layer. The particle size of the sand is selected according to the different layers and requirements of the shell, generally spreading in multiple layers from coarse to fine. Each layer of sand provides a certain strength and support to the shell.
o. Drying and Hardening: After sand spreading, the mold is placed in a drying chamber for drying and hardening. During the drying process, the solvent in the coating gradually evaporates, and the binder undergoes a chemical reaction, causing the shell to gradually harden. The drying and hardening time and temperature need to be adjusted according to the type of coating and environmental conditions to ensure the shell has sufficient strength and stability.
o. Multiple Repetitions: To obtain a shell with sufficient thickness and strength, the processes of dipping in coating, sprinkling sand, drying, and hardening need to be repeated, generally requiring 5-7 layers. The process parameters for each layer need to be adjusted according to the actual situation to ensure uniform shell quality.
o. Steam Dewaxing: The prepared shell is placed in a dewaxing kettle, and high-temperature steam is introduced to melt the wax model and allow it to flow out of the shell. The temperature and pressure of the steam need to be controlled according to the material of the shell and the characteristics of the wax model to ensure that the wax model melts completely and flows out smoothly without damaging the shell.
o. Hot Water Dewaxing: In addition to steam dewaxing, hot water dewaxing can also be used. The shell is placed in hot water, allowing the wax model to melt and float to the surface. The wax is then collected for recycling. Hot water dewaxing has the advantages of simple equipment and lower cost, but the dewaxing effect may not be as thorough as steam dewaxing.
o. Heating Process: The dewaxed shell is placed in a firing furnace for firing. The purpose of firing is to further remove residual wax, moisture, and other impurities from the mold shell, improving its strength and refractory properties. The firing process requires strict control of the heating rate, generally using a segmented heating method to avoid cracking due to rapid temperature changes.
o. High-Temperature Firing: After the mold shell reaches a certain temperature, it is held for a period of time for high-temperature firing. The firing temperature and time are determined based on the material of the mold shell and the casting process requirements, generally between 900-1200℃. High-temperature firing allows the binder in the mold shell to completely solidify, forming a robust ceramic structure.
o. Cooling Process: After firing, the mold shell needs to be cooled slowly to prevent cracking due to thermal stress. The cooling rate also needs to be controlled according to the material and size of the mold shell, generally using furnace cooling or cooling in a specific cooling medium.
o. Titanium Alloy Melting: The titanium alloy raw material is placed in a vacuum induction melting furnace for melting. Because titanium alloys readily react with oxygen and nitrogen in the air at high temperatures, the smelting process must be carried out in a vacuum environment to ensure the purity and quality of the titanium alloy. Precise control of the smelting temperature and time ensures that the titanium alloy raw material is completely melted and achieves a uniform composition and suitable temperature.
o. Casting: Once the molten titanium alloy reaches the appropriate temperature and fluidity, it is rapidly poured into a preheated mold. The casting process must be carried out under the protection of an inert gas (such as argon) to prevent the molten titanium alloy from reacting with air during casting. The casting speed and volume need to be precisely controlled according to the size and shape of the watch case back to ensure that the molten titanium alloy fills every corner of the mold.
o. Case Cleaning: After the titanium alloy cools and solidifies, the mold is removed using mechanical methods (such as vibration, sandblasting, etc.). Care must be taken during the case cleaning process to avoid damaging the watch case back and ensure the integrity of the case back surface.
o. Gate Cutting: The watch case back is cut off from the sprue bar, removing excess gate and riser. The cutting process requires high-precision cutting equipment to ensure a smooth and even cut surface.
o. Heat Treatment: To improve the mechanical properties of the watch case back, heat treatment is necessary. Heat treatment processes include annealing, quenching, and tempering; specific process parameters need to be determined based on the composition and performance requirements of the titanium alloy. Heat treatment improves the microstructure of the titanium alloy, increasing its strength, hardness, and toughness.
o. Surface Treatment: Surface treatments such as polishing, sandblasting, and electroplating are applied to the watch case back to improve its surface finish and decorative appeal. The surface treatment process needs to be selected based on the watch's design style and market demands to give the watch case back a better appearance and texture.
Advantages of Lost-Wafer Casting for Titanium Alloy Watch Case Backs
High Precision
Lost-wafer casting achieves very high dimensional and shape accuracy, allowing for the manufacture of watch case backs with complex shapes and rich details. For watch case backs with unique designs and fine textures, lost-wafer casting is an ideal manufacturing method.
Good Surface Quality
Watch case backs obtained through lost-wafer casting have a high surface finish, reducing the workload of subsequent processing. Meanwhile, the surface quality of the mold directly affects the surface quality of the case back. By optimizing the mold manufacturing process, a watch case back with excellent surface quality can be obtained.
High material utilization
Lost-wax casting allows for precise casting according to the actual shape and size of the watch case back, reducing material waste. Compared with traditional machining methods, lost-wax casting can improve material utilization and reduce production costs.
Strong adaptability
Lost-wax casting can adapt to different types and compositions of titanium alloys. For watch case backs with special performance requirements, the needs can be met by selecting appropriate titanium alloy materials and casting processes. Furthermore, lost-wax casting can be combined with other manufacturing processes to achieve diversified design and manufacturing of watch case backs.
Potential challenges and solutions
Difficulty in melting and casting titanium alloys
Titanium alloys have high chemical reactivity and are prone to reacting with elements such as oxygen and nitrogen in the air during melting and casting, leading to a decrease in the performance of the titanium alloy. The solution is to use vacuum induction melting and inert gas protected casting methods, strictly controlling the melting and casting environment to avoid contact between the titanium alloy and air.
Case Cracks
During case making, dewaxing, firing, and casting, cracks are prone to occur due to temperature changes and stress concentration. The solution is to optimize the case making process, control the heating and cooling rates, and avoid excessive thermal stress on the case. Simultaneously, the structure and strength of the case must be considered during design and manufacturing to avoid areas of stress concentration.
Difficulty in Dimensional Accuracy Control
Lost-wax casting involves multiple process steps, and the process parameters at each step affect the final dimensional accuracy. The solution is to establish a strict process control system to precisely control and monitor each process step. Simultaneously, advanced measurement technology and equipment should be used to monitor and adjust the dimensions of the watch case back in real time to ensure that the dimensional accuracy meets design requirements.
High Costs
Lost-wax casting involves large equipment investments, complex processes, and relatively high material and labor costs. The solution is to reduce production costs by optimizing the process flow, improving production efficiency, and reducing the scrap rate. At the same time, strengthening cooperation with suppliers to secure more favorable raw material prices will improve the company's economic benefits.
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