Co jsou jádra v lití písku?

1. Zavedení

Cores in sand casting serve as the internal architects that shape the hidden features of metal parts—internal cavities, podříznutí, and fluid passages—that a single mold cannot achieve alone.

Historicky, craftsmen inserted simple wood or clay plugs into molds as far back as ancient Rome;

Dnes, foundries employ advanced sand‐core technologies to produce intricate geometries,

such as engine coolant jackets, hydraulic manifold channels, and turbine blade cooling circuits, are impossible to machine cost‑effectively.

In modern operations, cores account for 25–35% of total mold volume, reflecting their critical role in unlocking design complexity and reducing downstream machining.

2. What Is a Core?

V lití písku, A jádro is a precisely shaped, sand‑based insert placed inside the mold cavity to create interní mezery, such as passages, podříznutí, or hollow sections, that the mold alone cannot form.

Whereas the mold defines a casting’s externí geometrie, cores determine its vnitřní features.

Core vs. Plíseň

Zatímco mold defines a casting’s external shape, The jádro creates internal features:

• Plíseň: Hollow cavity formed by packing sand around the pattern’s exterior.
• Jádro: Sand assembly placed inside the mold before pouring to block metal flow, producing voids once removed.

Cores must integrate seamlessly with the mold, resisting molten metal pressures (až do 0.6 MPA in aluminum casting) while later fracturing cleanly for shakeout.

3. Types of Cores in Sand Casting

Cores in sand casting come in several designs, each tailored to create specific internal features—from simple holes to intricate cooling passages.

Selecting the right core type balances material usage, přesnost, pevnost, a clean‑out požadavky.

Solid Cores

Solid cores are the most basic type, ideal for forming simple hollow features in castings.

They are typically made from a homogeneous sand–binder mixture compacted into core boxes.

Due to their uncomplicated geometry, they are cost-effective and easy to produce, making them suitable for components like pipe sections, Pouzdra ventilu, or mechanical blocks with straight-through cavities.

• Výhody: Simple manufacturing, low cost for basic shapes.
• Omezení: High material usage; difficult removal from deep or narrow cavities due to lack of collapsibility.

Shell Cores

Shell cores are precision-engineered cores formed by depositing resin-coated sand against heated metal core boxes, creating a rigid, thin-walled shell with high dimensional accuracy.

This method provides excellent surface finish and strength, making shell cores ideal for high-performance applications.

• Běžné použití: Bloky automobilových motorů, Hlavy válců, and parts requiring intricate cooling or lubrication channels.
• Key Benefits: Těsné tolerance (± 0,1 mm), hladká povrchová úprava, and reduced material consumption.

Resin-Bonded Cores

Používá se v no-bake a cold-box core-making processes, resin-bonded cores provide high strength and dimensional consistency.

In the no-bake method, chemical catalysts cure the sand-resin mix at room temperature, while the cold-box method uses gas (typically amine vapors) to harden the resin within minutes.

• Výhody: Fast cycle times, Vynikající mechanická síla, suited for high-volume production.
• Průmyslová odvětví: Automobilový průmysl, Těžké stroje, pump and valve casting.

CO₂ Cores (Sodium Silicate Cores)

CO₂ cores are made by mixing sand with sodium silicate and hardening the mixture by injecting carbon dioxide gas. This process rapidly sets the core, enabling quick turnaround times.

• Silné stránky: Fast production, strong initial hardness.
• Úvahy: Difficult to reclaim; the cores may be brittle and prone to moisture absorption.
• Typická použití: Short-run or urgent jobs requiring fast core availability.

Collapsible Cores

Designed to disintegrate or weaken during or after solidification, collapsible cores simplify removal and reduce the risk of damage to the casting.

These cores in sand casting often include combustible or thermally sensitive additives that break down during the casting’s cooling phase.

• Aplikace: Large or complex castings with deep, narrow internal features—such as marine engines or structural housings.
• Výhody: Reduce stress during solidification, prevent internal cracking, and ease core knockout.

Chaplet-Assisted Cores

For heavy or unsupported core geometries, metal chaplets are used to maintain core position during mold filling.

Chaplets act as spacers between the core and mold wall and are designed to fuse with the casting without compromising metallurgical integrity.

• Případy použití: Velké průmyslové odlitky, such as turbine housings or engine frames, where core shift would otherwise cause dimensional inaccuracies.
• Výhody: Prevents movement under metal pressure; maintains internal precision.

4. Core Binders and Curing Methods

5. Core Properties and Performance Criteria

Cores in sand casting must satisfy a demanding combination of mechanický, tepelný, a dimenzionální requirements to produce defect‑free castings.

Níže, we explore the five key properties—and their typical target values—that foundries monitor to ensure core performance.

Pevnost

Cores need sufficient integrity to resist molten‑metal pressures yet break down cleanly during shakeout.

• Zelená síla (before dry cure)

• • Typický rozsah: 0.2–0.4 MPa (30–60 psi)
  • Význam: Ensures cores survive handling and mold assembly without distortion.

• Dry Strength (after binder cure)

• • Typický rozsah: 2–6 MPa (300–900 psi) for resin‑bonded cores
  • Význam: Must withstand hydrostatic loads up to 1.5 MPa in steel castings.

• Hot Strength (at 700–1,200 °C)

• • Retention: ≥ 50% of dry strength at casting temperature
  • Význam: Prevents core deformation or erosion when in direct contact with molten metal.

Propustnost

Gas generated during pouring (pára, Co₂) must escape without forming porosity.

• Permeability Number (Pn)

• • Green Cores: 150–350 PN
  • Shell & Resin Cores: 100–250 PN

• Too Low (< 100): Traps gases, leading to blow‑holes.
• Too High (> 400): Reduces core strength, risking erosion.

Slučitelnost

Controlled collapse of the core eases shakeout and accommodates metal shrinkage.

• Collapsibility Metric: 0.5–2.0 mm deformation under standard load
• Mechanisms:

• • Green Cores: Rely on moisture and clay structure to deform.
  • Resin Cores: Use fugitive additives (coal dust) or weak layers.

• Prospěch: Reduces internal stresses—preventing hot tears in deep cavities.

Rozměrová přesnost

Precision of internal features dictates post‑casting machining allowances.

Tepelná stabilita

Cores must maintain integrity under rapid heat flux from molten metal.

• Koeficient tepelné roztažnosti: 2.5–4.5 × 10⁻⁶/K (core sand vs. kov)
• Refraktorinost:

• • Silica‑Based Cores: až do 1,200 ° C.
  • Zircon or Chromite Enhanced Cores: > 1,700 ° C.

• Význam: Minimizes core shifting caused by uneven thermal expansion.

6. How Are Cores Held in Place?

Ensuring cores remain precisely positioned throughout pouring and solidification is critical: even a slight shift can distort internal passages or cause metal to invade the core cavity.

Foundries rely on a combination of mechanical registration, metal supports, a bonding aids to lock cores securely in the mold.

Mechanical Registration with Core Prints

Every pattern includes protruding “core prints” that create matching recesses in the cope and drag. These prints:

• Locate the Core in all three axes, preventing lateral or vertical movement
• Transfer Loads by bearing the core’s weight and molten‑metal pressure (až do 1.5 MPa in steel)
• Standard Dimensions typically extend 5–15 mm into the mold wall, machined to ± 0.2 mm for reliable seating

By closing the mold, the core print seats into its pocket, delivering a repeatable, interference‑fit that needs no additional hardware.

Metal Supports: Chaplets and Sleeves

When hydrostatic forces threaten to float or erode cores, foundries deploy metal supports:

• Chaplets are small metal pillars—often stamped from the same alloy as the casting—placed at regular intervals (every 50–100 mm).
  They bridge the gap between core and mold wall, carrying both core weight and metal pressures.
• Sleeves consist of thin‑walled metal tubes that slip over vulnerable core sections, shielding sand from high‑velocity metal impingement and reinforcing the core’s structure.

Po ztuhnutí, chaplets remain embedded and are either removed by machining or left as minimal inclusions; sleeves are typically extracted with the sand.

Bonding Aids: Adhesives and Clay Seals

For lightweight or precision cores, mechanical supports alone may prove insufficient. In these cases:

• Adhesive Dabs—small dots of sodium silicate or proprietary resin glue—secure core feet to the mold surface, offering initial green strength without hindering permeability.
• Clay Slip Seals—a thin coating of bentonite slurry applied around core prints—enhances friction and seals any microscopic gaps, preventing fine sand from migrating into the cavity during closing.

Both methods require minimal material yet dramatically reduce core “float” during mold handling and metal fill.

7. Core Assembly and Mold Integration

Seamless integration of cores into the mold is pivotal for achieving accurate internal geometries and avoiding defects such as misruns, core shift, or metal penetration.

Core Placement Techniques

Manual Placement

• Alignment Pins & Locators: Use precision‑machined pins on the drag and cope halves to guide cores into position.
• Tactile Confirmation: Operators should feel the core “seat” against its prints, then give a gentle tap to ensure full engagement.

Automated Handling

• Robotic Grippers: Equipped with vacuum or mechanical fingers, robots pick, orient, and place core assemblies with ± 0.1 mm accuracy.
• Programmable Sequences: Integrate vision systems to verify orientation and detect foreign objects before placement.

Mold Readiness

Before closing the cope and drag, confirm that the mold is fully prepared to accept both the core and molten metal:

• Vent Inspection: Ensure all core vents (Ø 0.5–1 mm) and mold vents are free of sand buildup to facilitate gas escape.
• Back‑Filling & Balení: Support external core surfaces by back‑filling with loose sand or using pea‑gravel backing for shell cores, preventing core deformation under metal pressure.
• Parting‑Line Clearance: Verify that no sand bridges or debris occupy the parting line, which could shift core prints or cause mismatches.

Core Binding and Sealing

• Adhesive Dab Application: For small or thin cores, spot‑apply sodium silicate or proprietary clay adhesive at core‑print interfaces to prevent core “float” during mold closing.
• Clay Slip Fillets: In green‑sand molds, brush a thin coat of bentonite slurry around core seams; this seals gaps and adds frictional resistance.

Final Assembly Checks

Prior to pouring, perform a systematic inspection to confirm core integrity and mold alignment:

• Go/No‑Go Gauges: Slip gauges over core prints to verify correct seating depth.
• Visual Inspection with Lighting: Shine angled light into mold cavity to highlight misaligned cores, loose chaplets, or gaps.
• Dynamic Vibration Test: Lightly vibrate the mold assembly; properly secured cores will remain immobile, while loose cores reveal themselves.

8. Common Core‑Related Defects & Léky

9. Reclamation and Sustainability of Core Sand

• Fyzická rekultivace (Green‑Sand): Attrition scrubbing and screening recover 70–80 % virgin quality.
• Tepelná rekultivace (Resin Cores): 600–800 °C burns off binders; yields 60–70 % reusable sand.
• Blending Strategy: Mix 20–30 % virgin with reclaimed to maintain performance while reducing landfill by 60%.

10. Aplikace a případové studie

1. Automotive Engine Blocks: Collapsible cores in water jackets achieved ± 0.5 mm over 1.5 m span, reducing machining time by 25%.
2. Hydraulic Manifolds: Cold‑Box resin cores eliminated 70 % of gas defects in intersecting channels, improving yield.
3. Turbine Cooling Channels: 3D‑printed sand cores integrated with epoxy binder produced ± 0.1 mm accuracy and cut lead time from 8 weeks to 2 týdny.

11. Závěr

Cores form the hidden infrastructure of complex sand‑cast components, enabling intricate internal features that drive performance in automotive, Aerospace, and industrial sectors.

By selecting appropriate sand types, binders, and assembly methods—and by rigorously controlling core properties and reclamation—foundries achieve high‑precision, defect‑free castings.

Těšíme se dopředu, additive core making, eco-friendly binders, and real-time property monitoring promise to advance core technology, supporting increasingly sophisticated designs.

 

Časté časté

What are cores in sand casting?

A jádro is a specially shaped insert made from sand and binders, placed inside the mold cavity to create internal voids, podříznutí, or complex internal geometries in a casting.

Cores enable the production of hollow components like pipes, bloky motoru, a tělesa ventilů.

How is a core different from a mold?

Zatímco mold forms the exterior shape of the casting, The jádro creates the interior features.

Molds are generally larger and define the outside contours, whereas cores are placed inside the mold cavity to form cavities, díry, and passageways.

What materials are used to make cores?

Most cores are made from high-purity silica sand combined with a binder system,

such as bentonite clay (for green sand), thermoset resins (for shell or cold-box cores), or sodium silicate (for CO₂ cores).

Additives may be used to enhance strength, propustnost, or collapsibility.