Vrste pijeska u lijevanju u pijesak: Sveobuhvatni pregled

1. Uvod

Sand serves as the backbone of lijevanje pijeska, forming the mold cavity that directly shapes every casting.

By packing sand around a pattern, foundries create the negative impression into which molten metal flows, solidifies, and takes on its final geometry.

Sand selection plays a pivotal role: it influences surface finish, gas permeability, točnost dimenzije, and ultimately, koštati.

In the following sections, we examine the principal sand systems—green, chemically bonded, natrijev silikat, resin‑coated, and specialty sands—highlighting their compositions, svojstva, i idealne aplikacije.

2. Zeleni pijesak

Green sand serves as the workhorse mold medium in over 70% of global sand‑casting operations.

Foundries favor it for its low cost, ease of reuse, and adaptability to a broad range of part sizes and geometries.

Sastav

A typical green‑sand mixture contains:

Ključna svojstva

• Moisture Content (2–4 %)
  Ensures good sand plasticity for pattern imprinting. Too little moisture causes crumbling; too much yields poor permeability and gas defects.
• Green Strength (30–50 psi)
  Measures the unbaked mold’s ability to support molten metal without collapse.
• Permeability (200–400 PN)
  Indicates how readily gases escape mold cavities—critical to avoid porosity.
• Collapsibility (0.5–1,5 mm)
  Describes mold’s controlled deformation on solidification, reducing hot‑tear defects.

Prednosti i primjene

Green sand’s low tooling cost ($50– 200 dolara po kalupu) i reusability over 5–20 cycles make it ideal for large,

heavy castings such as engine blocks, Kućiva pumpe, and agricultural machinery components.

Foundries also utilize green sand for prototype parts, where rapid turnover and minimal upfront investment matter.

Ograničenja & Mitigations

• Tolerancija dimenzija (± 0.5–1.5 %)
  Green‑sand molds exhibit looser tolerances than resin‑bonded processes. Engineers tighten tolerances by precisely controlling clay and moisture levels.
• Washout in Thin Sections
  Extended contact with molten metal can erode fine details. Increasing clay content or applying refractory coatings to mold walls mitigates washout.

3. Kemijski vezan pijesak

Chemically bonded sand systems transform simple silica grains into high‑performance molds and cores by using synthetic resins as binders.

Foundries select from three leading resin chemistries—phenolic, furan, and epoxy—each tailored to specific strength, cure, and gas‑generation profiles.

Resin Types and Properties

• Phenolic Resins: Offer excellent thermal stability (do 300 ° C) and low gas evolution (≤ 0.2 L/kg sand).
  They achieve bench strengths of 200–300 psi (1.4–2.1 MPa) within 5–10 minutes.
• Furan Resins: Cure rapidly (1–3 minutes) with moderate gas evolution (0.3–0.5 L/kg).
  Their bench strengths reach 250–350 psi (1.7–2.4 MPa), making them ideal for medium‑run steel castings.
• Epoxy Binders: Deliver the highest strengths (300–400 psi / 2.1–2.8 MPa) and minimal gas output (< 0.1 L/kg).
  Although cure times extend to 15–30 minutes, epoxy sands produce exceptionally clean surfaces for thin‑walled aluminum parts.

Transitioning from resin chemistry to process selection, foundries choose between No‑Bake i Cold‑Box metode:

No‑Bake Process

• Mehanizam: Mix sand with liquid resin and catalyst; allow the mold to cure at ambient temperature.
• Prednosti: Simple setup, energy‑efficient (no external heating), accommodates large molds (> 2 m in length).
• Typical Metrics: Compressive strengths > 10 MPa within 2–5 minutes; bench life of 10–15 minutes for mold assembly.

Cold‑Box Process

• Mehanizam: Pack sand–resin mix in a flask, then pass a gaseous amine catalyst through the sand to trigger instant cure.
• Prednosti: Cycle times as low as 30 sekundi, ideal for high‑volume production and intricate cores.
• Typical Metrics: Compressive strengths of 10–15 MPa in under 1 minuta; low residual catalyst minimizes defects.

While chemically bonded sands deliver bench strength do 15 MPA i collapsibility adequate for complex geometries, they demand rigorous gas‑management.

Excessive gas evolution can cause pinholes and blow‑holes; thus, foundries regulate resin dosage,

optimize core-box venting, and employ vacuum or low‑pressure pours to mitigate defects.

Prijava range from large marine engine blocks—where dimensional tolerances tighten to ± 0.2 mm—to aerospace turbine housings requiring Ra ≤ 2 µm finishes.

In these scenarios, chemically bonded sands meet both the dimensional precision and surface‑quality standards that green sand cannot achieve.

4. Natrijev silikat (Staklo za vodu) Pijesak

Building on chemically bonded systems, sodium silicate sand—often called water glass sand—offers a distinctive CO₂‑curing mechanism that balances speed, jačina, i površinska kvaliteta.

Foundries employ it primarily for core making and medium‑volume castings where rapid turnaround and good finishes matter.

Binding Mechanism and CO₂ Hardening

1. Mixing: Operators blend silica sand with a liquid sodium silicate solution (8–12 wt %).
2. Mold Assembly: Technicians pack or shoot the wet sand around the pattern or core box.
3. CO₂ Curing: A stream of 100% CO₂ (flow rate 4–8 m³/h) passes through the mold.
4. Set Time: The silicate gel forms in 10–30 seconds, yielding a rigid mold ready for immediate assembly.

Thanks to this rapid hardening, sodium silicate cores can enter the flask and be poured within 1–2 minutes of CO₂ exposure, dramatically shortening cycle times compared to resin systems.

Prednosti

• Fast Cure: Complete gelation in under 30 seconds eliminates lengthy bench times, boosting throughput.
• Good Surface Finish: Cured cores exhibit surface roughness around Ra 3–5 µm, finer than green sand by 30–50%.
• Low Smoke and Odor: CO₂ curing generates negligible volatile byproducts, improving foundry working conditions.
• Reusability: When properly reclaimed, sodium silicate sand can cycle through 8–12 uses before significant strength loss.

Drawbacks

• Reclamation Challenges: High sodium carbonate content necessitates wet or thermal reclamation at 600–800 °C to strip binders—raising energy costs.
• Reduced Sand Life: Recycled sand eventually accumulates carbonate and fines, degrading strength by up to 15% after 10 ciklusi.
• Moisture Sensitivity: Ambient humidity above 70% can pre-harden mixtures or slow CO₂ penetration, requiring climate control.

Prijava

Foundries leverage sodium silicate sand when they need a balance of speed and accuracy:

• Core Making: Gas‑hardened cores for pump impellers, tijela ventila, and heat‑exchange passages.
• Medium‑Size Steel Castings: Manifolds and gearbox housings (10–200 kg range) that require moderate dimensional tolerances (± 0.3 mm).

5. Resin‑Coated Sands

Resin‑coated sands—commonly used in kalupljenje školjke—combine the precision of chemically bonded systems with the speed of high‑volume production.

By applying a thin, pre‑catalyzed resin layer to each sand grain, foundries create robust “shells” that capture fine detail and maintain exceptional dimensional accuracy.

Shell Molding Process

1. Resin Coating: Manufacturers uniformly coat high‑purity silica sand (AFS 50–70) s 1–2 wt % thermosetting resin (phenolic or epoxy).
2. Formiranje ljuske: They tumble the coated sand around a pre‑heated pattern (175–200 °C); heat cures the resin, forming a rigid shell approximately 2–5 mm thick.
3. Core Assembly: Technicians remove unbonded sand, assemble shell halves into a flask, and backfill with uncoated sand for support.
4. Lijevanje: Rapid shell production yields molds ready for pouring—often within 5 minutes of pattern removal.

Ključne prednosti

• Exceptional Surface Finish: Shell‑molded castings achieve Ra ≤ 2 µm—up to 80% smoother than green‑sand counterparts.
• Uske tolerancije: Dimensional accuracy reaches ± 0.1 mm, reducing post‑machining by 30–40%.
• Thin‑Wall Capability: Walls as thin as 1 mm with minimal hot tears or washout.
• Automation‑Friendly: Continuous shell lines produce 100–200 shells per hour, supporting high throughput.

Cost and Cycle‑Time Considerations

While shell molding requires higher upfront costs, njegov rapid cycles i reduced finishing make it economically compelling for srednji production runs (1,000–50,000 units).

Target Industries and Applications

• Automotive Turbocharger Housings: Thin‑wall, high‑thermal components benefit from shell molding’s precision.
• Aerospace Gearbox Casings: Uske tolerancije (± 0.1 mm) and fine finishes meet stringent certification standards.
• Precision Medical Devices: Complex geometries with Ra < 2 µm surfaces require almost no secondary operations.
• Electronics Enclosures: Small, intricate die‑cast alternatives use shell molds to avoid porosity and improve EMI performance.

6. Specialty Sands and Additives

Beyond standard silica mixes, foundries deploy specialty sands i additives to tackle high‑temperature service, poboljšati kvalitetu površine, and fine‑tune mold behavior.

By tailoring sand chemistry and grain characteristics, engineers optimize castings for demanding applications.

High‑Temperature Sands

When molten metal temperatures exceed 1,300 °C—or when thermal shock resistance matters—foundries substitute or blend in refractory sands:

Surface‑Quality Additives

To achieve smoother cast surfaces i minimize washout, foundries introduce fine organic or carbonaceous additives:

• Coal Dust (Sea‑Coal)

• • Dosage: 1–3 wt % of sand mix
  • Funkcija: At casting temperature, coal volatiles deposit a thin carbon layer that improves metal flow and reduces sand fusion, yielding surface finishes 20–30% better than untreated sand.

• Lustrous Carbon Additives

• • Chemistry: Blend of coal tar pitch and graphite microspheres
  • Korist: Produces a shiny carbon film in the mold cavity, further enhancing detail and preventing metal penetration into sand pores—critical for high‑precision aluminum and brass castings.

Grain Size and Fineness

A American Foundry Society (AFS) Grain Fineness Number guides sand selection:

• Coarser Sands (AFS 30–40): Ideal for heavy sections where gas escape outweighs surface requirements.
• Medium Sands (AFS 50–70): The workhorse for general engineering castings, offering a compromise between fillability and detail.
• Fine Sands (AFS 80–100): Required for thin walls, Oštri rubovi, and small features, but often blended with coarser grains to maintain gas flow.

7. Key Properties of Sand for Sand Casting

8. Selection of Sand for Specific Casting Applications

Based on Metal Type

Different metals require different sand characteristics due to their melting points and reactivity:

• željezne legure (Željezo, Čelik):
  These metals pour at high temperatures, often above 1,400 ° C, demanding sands with excellent refractoriness, metal penetration resistance, i toplinska stabilnost.
  Common choices include:

• • Chromite sand – superior thermal conductivity and resistance to fusion
  • High-purity silica sand – economical and widely available, with moderate refractoriness

• Non-Ferrous Alloys (Aluminij, Bakar, Cinkov):
  These cast at lower temperatures (600–1,100 °C) and are more sensitive to gas defects and surface roughness. Ideal sand systems include:

• • Zircon sand – low thermal expansion and excellent surface finish
  • Fine-grained silica sand – cost-effective and capable of high detail resolution

Based on Casting Complexity

• Simple Shapes: Green sand can be a cost – effective choice due to its ease of molding.
• Složeni oblici: Chemically bonded sands (especially Cold – Box) or resin – coated sands for shell molding are preferred for their precision and detail – holding capabilities.

Based on Production Volume

• Low – Volume Production: Green sand is popular due to its low cost and reusability.
• High – Volume Production: Chemically bonded sands (Cold – Box) or resin – coated sands offer consistent quality and faster cycle times, despite higher initial costs.

9. Sand Reclamation and Recycling in Sand Casting

Importance of Sand Reclamation

• Environmental: Reduces demand for virgin sand, conserving natural resources, and minimizing landfill waste.
• Economic: Cuts down on sand procurement and disposal costs, providing significant savings for foundries.

Reclamation Techniques

• Physical Reclamation: Mechanical processes like screening, attrition, and scrubbing to remove binders and contaminants. Suitable for sands with simple binders (Npr., zeleni pijesak).
• Thermal Reclamation: Uses heat to burn off binders and organic contaminants. More effective for complex binders but requires more energy and is costlier.

Reclaimed Sand vs. Virgin Sand

Reclaimed sand may have slightly different properties, such as grain size and binder content. Međutim, with proper quality control, it can meet the requirements for many casting applications.

Environmental Impacts and Cost – Benefit Analysis

While reclamation has some environmental impacts (Npr., energy use in thermal reclamation), the overall environmental benefit outweighs the impact of using only virgin sand.

Economically, the savings from reclamation usually exceed the investment in equipment and processes.

10. Future Trends in Sand for Sand Casting

Development of New Sand Materials

• Research efforts to develop new types of sand with enhanced properties, such as improved refractoriness, lower thermal expansion, and better environmental compatibility.
• The exploration of alternative materials to traditional sand types, such as synthetic sands or sands derived from waste materials.

Advancements in Binder Technology

• The development of more environmentally friendly binders with lower emissions and better performance.
• How new binder technologies can improve the strength, propusnost, and other properties of sand molds and cores, leading to higher – quality castings.

Automation in Sand Handling and Processing

• The increasing use of automation in sand casting processes, including sand mixing, kalupljenje, and reclamation.
• How automation can improve the consistency and efficiency of sand handling, reduce labor costs, and enhance the overall quality of the casting process.

11. Zaključak

Selecting the right sand type forms the foundation of successful sand casting.

From versatile green sand to precision resin‑coated shells, each system delivers unique advantages and trade‑offs.

By understanding sand composition, key properties, and reclamation strategies, foundry engineers ensure high‑quality castings, economical production, and environmental stewardship.

As sand technology advances—embracing eco‑friendly binders, digital process control, and additive manufacturing—sand casting will continue powering innovative applications across diverse industries.