How to Choose and Stabilize Sand Molding Processes? — Resin Sand, Water Glass Sand and Lost Foam

In casting production, the molding process more directly determines dimensional accuracy, surface quality, production cost and shop-floor environment than melting. Faced with multiple routes—resin self-hardening sand, sodium silicate sand, clay green sand, and lost foam—how should a foundry choose, and how can it stabilize the chosen process? Drawing on industry technical literature, this article compiles several immediately actionable points.

Selection Must Pass Four Tests: Quality, Batch Size, Cost, Environment

Feng Shengshan divides sand casting into physical hardening (clay green sand, lost foam, V-process, etc.) and chemical hardening (sodium silicate sand, resin sand). Selection must first meet casting quality requirements: machine-molded clay green sand can stably reach CT8–10, while chemically bonded sand is used for one-off/small batches with higher precision. Second, it must match production volume—large batches of small parts favor mechanized clay green-sand lines or resin-sand core lines; medium batches can use resin self-hardening sand or CO2 sodium silicate sand; one-off heavy parts best use pit molding at the lowest cost. It must also fit the enterprise's own equipment, floor space and staffing, and factor in environmental impact: producing one tonne of casting consumes about one tonne of new sand and emits about one tonne of waste sand; sodium silicate sand is odorless, non-toxic and emits no irritating harmful gases, and is regarded as one of the molding processes most likely to achieve green, clean foundry operations.

Self-Hardening Resin Sand: Temperature, Humidity and Curing Time Are the Shop-Floor Key

Wu Laifa and Cui Mingsheng point out that resin self-hardening sand is "chemical reaction plus forming": temperature should be 15–30°C and humidity as low as possible; in winter, roughly every 8°C drop halves the curing speed, so working time or hardener composition may need adjustment. Sand temperature too low (about -5 to 5°C) delays and unevenly hardens the resin. From the end of mixing, molding should generally finish within 2–4 minutes, and stripping at 15–20 minutes: too early collapses the mold, too late makes stripping difficult. For raw materials, silica sand for cast iron should exceed 92% SiO2 and for cast steel exceed 95%, with mud content ≤0.2% and moisture ≤0.3%; resin nitrogen grade is chosen by alloy type, with low- or no-nitrogen resin preferred for steel castings. The gating system should be rapidly sealed, the draft angle taken at 0.8°–1.2°, and coating applied 2–4 hours after molding so that the not-fully-cured mold is not "burned" by solvent.

Sodium Silicate Sand: From CO2 Gas Hardening to Organic Ester Self-Hardening and Composite Processes

Ordinary CO2-blow hardening sodium silicate sand has simple equipment and low cost, but low mold/core strength, a high addition of 7–8%, and high moisture absorption, poor collapsibility and difficult reclamation. Switching to organic-ester self-hardening lowers sodium silicate addition below 3.5%, improves collapsibility, gives dry reclamation reuse ≥80%, low gas evolution, and reduces casting cracking. To address long-standing problems: stored sodium silicate "ages" and lowers bonding strength, which can be restored by physical modification (magnetic field, ultrasound, heating) or chemical modification (adding polyacrylamide, polyphosphate); surface powdering of molds/cores is mostly due to NaHCO3 precipitation, so control moisture, shorten blowing time and close molds promptly, and add about 1% syrup to prevent powdering; sudden strength loss in humid storage can be mitigated by using lithium sodium silicate, raising modulus, or adding starch hydrolysate. If sodium silicate addition can be lowered below 2% and residual Na2O in reclaimed sand kept within 0.25%, sodium silicate sand can become an environmentally friendly sand with essentially zero waste-sand discharge.

Lost Foam Casting: Clear Advantages, but Do Not Rush In Blindly

Li Zengmin analyzes that lost foam casting needs no molding or parting, greatly simplifying processes; dimensional accuracy approaches investment castings, shakeout is easy, sand reuse is high, and initial investment is small, suiting gray and ductile iron parts—especially boxes and housings with uniform 10–20 mm walls in medium/small batches (about 10,000 pieces or more). But it is not a panacea: pattern quality, coating performance, vacuum stability and dry-sand vibration compaction are all highly sensitive, and process theory is still largely empirical. He particularly reminds enterprises to "cut the coat according to the cloth"—first run process tests and small-batch trials, inspect suppliers' sites and multiple users' track records, rather than blindly pressing low prices and rushing in; cultivate an engineering team that understands both principles and details, and digest advanced foreign technology rather than copying traditional sand methods.

Choosing a molding process is only the start; stabilizing it is the source of competitiveness. Whichever route is used, whether the tapping iron temperature and metallurgical quality are qualified directly shows up in defects such as sand washing, porosity and cold shuts. While strengthening molding-floor management, foundries should pair melting with a molten-iron thermal analyzer and fast temperature tools, controlling tapping temperature and iron composition, to supply qualified, stable iron to molding and pouring and form a two-end quality closed loop between "mold and metal."