Raw and auxiliary materials for coated sand

Coated sand is generally composed of raw sand, binding agent, curing agent, lubricant and special additives.

1、Raw sand.

Raw sand is the main body of the overlay sand, the requirements for raw sand are: high refractoriness, less volatiles, more rounded particles, high self-strength. , Natural silica sand is generally used, mainly because of abundant reserves and low price, which can meet the requirements of most castings. Special sand (such as zircon sand, chromite sand, ceramic grain, magnetite sand, etc.) is only used for castings with special requirements as the original sand for coating sand.

2、Binder

At present, thermoplastic phenolic resin is commonly used as a binder. Thermoplastic phenolic resin, its performance requirements are: A. polymerisation speed (hot plate method): 25-75S; B. softening point (global method): 90-105 ℃; C. fluidity (inclined plate method): 60-110mm; D. free phenol content (bromination method): ≤ 4%.

3、Curing agent, lubricant, additive.

Commonly used curing agent for cladding sand is Uroto, and its additive amount is 5%-20% of the resin. In order to meet some special requirements (such as anti-shelling, rapid polymerisation, etc.), some companies have developed a special curing agent. The role of the lubricant is to prevent agglomeration and improve the fluidity and demoulding of the coated sand. There are mainly calcium stearate, ammonium stearate and so on. The lubricant should try to choose high melting point products to prevent the phenomenon of shelling during the use of coated sand. The main function of additives is to improve the performance of coated sand. Currently widely used additives are mainly high temperature resistant additives, easy to collapse additives, toughening additives, anti-sticking sand additives and wet additives. Many laminating sand factories have unique additive formulations, additive performance on the production of special laminating sand plays an important role in the current laminating sand market is one of the core competitiveness.

Foreign coated sand performance development history. The performance of coated sand is to increase the tensile strength and adjust the relative molecular weight of the resin. The amount of resin added to the sand with additives was reduced by 20% to 30% to increase the hardening rate. Improvements in resin synthesis methods; addition of hardeners reduces shell (core) time by 20% to 40%. Lower temperature of shell mould (core). Improved resin synthesis method and molecular bonding method. Mould temperature reduced by about 25°C. Shell-core sand is flexible (aluminium alloy), introduction of substances susceptible to thermal decomposition in resin synthesis; addition of disintegrating agents to cladding sand, reduction of the cost of heat treatment of aluminium alloy castings (sand burning), reduction of the thermal expansion of cladding sand, addition of plasticising molecular substances to resin synthesis, addition of plasticisers to sand, cladding use of special sand shells (cores) to reduce cracking, reduction of billet defects, improvement of the dimensional accuracy, reduce odour, and use thermosetting resins; reduce the content of free phenols in the resin and improve the operating environment; improve the regulation of pore defect reduction to improve the relative molecular weight of the shell (core) antifouling resin; add additives to control the resin fluidity of the cladding sand for the production of large tubes and to improve the regulation of the relative molecular weight of the hollow resin when shells are made by this method. Rotary; improve the production efficiency of thin-walled parts with anti-adhesion materials added to the cladding sand; improved sand resistance; shell (core) uncoated with the addition of heat-resistant agents and lubricants; grades, eliminating the construction process and the surface finish of the castings.