Overview: Investment casting, also known as lost-wax casting, is an advanced near-net-shape process capable of producing precision and complex parts that closely approximate their final shape with minimal or no machining allowance. Silica sol precision casting is suitable for mass production of small to medium-sized parts (weighing 0.05~30kg). Our company specializes in silica sol precision casting, with standard products weighing 0.3~5kg and a maximum production capacity of 35kg. We have successfully piloted the production of products weighing over 50kg using specialized production equipment.
Keywords: large castings, precision casting, special auxiliary tools
1. Product Introduction: This product is a plug for marine heat exchangers, made of CF8M steel, weighing 57.2kg, with an overall wall thickness of 8mm. The original process involved stamping steel plates, forming them, and then welding the plates into tubes. This method had low production efficiency, complex manufacturing processes, high costs, and required expensive flaw detection. Furthermore, surface treatment was necessary to achieve a good surface finish, and the dimensional accuracy did not meet the application requirements. The new precision casting method eliminates the expensive welding costs, meets the dimensional accuracy requirements, and achieves a surface roughness of Ra6.3 or higher, satisfying customer requirements.
2. Trial Production Process
1) Mold Making: The mold is made of one piece of forged aluminum 6061. The mold dimensions are approximately: length × width × height = 650mm × 650mm × 420mm. The upper core is removed pneumatically.
2) Module: Wax injection is performed using a 20-ton single-station Wax Injection Machine at an injection temperature of 52℃, an injection pressure of 3.5MPa, and an injection time of 20 minutes, followed by a 3-minute pressure holding period. The electric hoist attached to the wax injection machine is used to assist in mold opening, and the wax mold is water-cooled. During the wax part turnover process, as well as for shaping and assembly, the bottom is supported by sponge. During tree assembly, a tree assembly bracket is used for support, and adhesive wax is used for bonding. The upper part of the mold head is reinforced with four auxiliary lifting platforms to increase the overall lifting strength of the module. 3) Shell Fabrication: The shell consists of 8 layers: a surface layer of zircon sand, a transition layer of 50-100 mesh mullite sand, a back layer of 30-60 mesh mullite sand (one layer), and five layers of 16-30 mesh mullite sand. Each layer requires 24 hours to dry. After the fifth layer, wire mesh is used for reinforcement. Traditional shell fabrication involves manual application of slurry, either by rain or boiling sand application. However, due to the large size of the product, manual operation is difficult. To address this issue, our company has developed a movable shell-fabrication auxiliary trolley for slurry application. This trolley can be raised and lowered for height adjustment and can rotate 360 degrees in both directions, significantly reducing the labor intensity for workers. Furthermore, the blind chambers inside the shell are filled with dry sand after four layers to prevent poor drying of the internal cavity and to simplify the shell cleaning process. The drying method is directional drying chamber suspension drying to facilitate the drying of the shell's internal cavity.
Dewaxing is performed using an electric steam dewaxing kettle, one kettle per furnace, with a dewaxing time of 15 minutes.
4) Shell Firing: The shell firing was performed using a gas-fired firing furnace at 1120℃ for 1.5 hours. Due to the weight of the shell, traditional forklift loading and pouring methods were difficult to implement. During trial production, we designed a shell firing fixture (welded from 50*50mm steel plates) to be loaded into the furnace along with the shell for easier handling. Both loading and unloading were done using a manual forklift (the forklift teeth were pre-wrapped with refractory cotton to prevent deformation from high temperatures).
5) Casting: The casting process adopts sub-casting. Since the overall wall thickness of the product is only 8mm, in order to ensure that the metal completely fills the mold, the casting temperature is 1680℃ and the casting is fast.
6) Cleaning: The post-processing flow is the normal process flow—manual shell vibration → shot blasting → cutting → grinding → shot blasting → welding repair → heat treatment → shot blasting → pickling → shot blasting → passivation → machining → pressure testing → inspection → warehousing → packaging. Due to the weight of the castings, a self-made electric hoist is used to assist in the grinding process. All product turnover in the cleaning workshop is carried out using turnover carts.
3. Summary: Precision casting using the silica sol process can produce castings weighing over 50kg, achieving dimensional accuracy meeting the CT6 grade requirements of GB/T6414-1999, and a surface roughness of Ra6.3. Specialized auxiliary tools are used during shell making, auxiliary fixtures are used for mold firing, and lifting tools are used during cleaning, reducing the labor intensity of workers. The integral casting of welded products reduces production costs. Trial production of castings has been completed, and the product is initially ready for mass production.
