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Sand castings with coating
Coated (shell) sand is used, consisting of quartz sand, pre-coated with thermoplastic phenolic resin and a hardener - hexamethylenetetramine. The metal model plate is heated to 180–340 °C.
Description
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Product Description
Shell casting process (hot shell casting using coated sand, specialized for precision casting of ferrous metals)
Process principle
Coated (shell) sand is used, consisting of quartz sand, pre-coated with thermoplastic phenolic resin and a hardener - hexamethylenetetramine. The metal model plate is heated to 180–340 °C. When the coated sand comes into contact with high temperatures, the resin quickly melts, cross-links and hardens, forming a hard, thin sand shell on the surface of the model slab. Once the shell is removed, the two mold halves are joined together to form the mold cavity. Dry sand is poured around the outside of the shell for support before molten iron is poured, balancing the cost of sand casting with the dimensional accuracy of precision casting.
Special raw materials
Molding sand: coated fine sand with 2.5-4% resin content; low regenerability.
Model plate: A split mold made of cast steel or cast iron, which has high heat resistance, wear resistance and long service life with repeated use.
Release agent: calcium stearate and silicone oil to prevent the sand shell from sticking to the mold.
Ferrous materials used: small and medium precision parts made of gray and ductile iron, such as camshafts, gears, valve seats and hydraulic housings.
Step-by-step technological diagram of the shell mold casting process
Iron castings in as-delivered condition, produced by shell casting
Standard six-stage process
(1) Heating the metal model plate and applying the release agent
Heat the mold to 200–280 °C and apply the release agent evenly to ensure free removal of the sand shell.
(2) Applying sand using a tipping hopper (basic operation)
Press the heated model slab against the sand storage hopper and invert the entire assembly to completely cover the surface of the model with coated sand. Hold for 8–15 seconds. The surface sand is heated to harden into a hard shell, while the unheated loose sand is poured back into the hopper.
(3) Additional hardening (burnout) of the shell mold
Transfer the model slab with the formed sand shell to a secondary curing oven at 180-220°C for 30-90 seconds to increase the thermal strength of the shell. Shell thickness is controlled within 8–15 mm.
(4) Removing the shell, making cores and assembling half-molds
Remove the finished half-shell from the mold using the ejection mechanism. For parts with internal cavities, cores are made from coated sand at the same time, then the upper and lower half-shells are aligned and fastened together.
(5) Installation in the flask for reinforcement and filling
Place the assembled shell mold in the flask and fill the space around it with dry support sand to prevent the shell from expanding and cracking during pouring. Pour molten cast iron at a fixed temperature of 1400–1460 °C.
(6) Cooling, shelling and cleaning
After cooling for 4-12 hours, destroy the sand shell and remove the castings. Remove sprues and sprues, perform shot blasting, flaw detection and final machining.
The main advantages and disadvantages of shell casting
Advantages (compared to conventional sand casting)
Significantly increased accuracy: CT9–CT11 dimensional tolerance, stable dimensions and excellent repeatability in mass production.
Smooth surface: surface roughness Ra 6–25 µm with minimal burnt layer, which significantly reduces allowances for turning, milling and grinding.
Excellent ability to form thin-walled castings: The high rigidity of the shell mold ensures the stable production of cast iron castings with thin walls of 2-3 mm thickness, complex topography and small holes.
Extremely low sand consumption: only a thin shell layer forms the cavity; sand consumption is only 1/10 of that for conventional sand casting.
Highly compatible with automation: compatible with automatic lines for the production of shell molds in continuous mass production.
Homogeneous casting structure: controlled gas permeability of the shell significantly reduces defects, including gas holes and slag inclusions.
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