AHFS and Binders for Foundry Production

Foundry Production: The Art of Balancing Strength and Flexibility

Casting is one of the oldest metalworking technologies, yet it remains a high-tech process. The central problem: how to make a mold strong enough to withstand the pressure of molten metal, but also easily disintegrate after the casting cools?

Traditional answer: sand-clay mixtures. But they generate a large amount of scrap, require precise calibration of water and clay, and often produce surfaces with a roughness of Ra 20–25 µm (which requires further mechanical processing).

Modern answer: synthetic resin-based binders and Alkyd-based Chemical Sand (AHFS). This is a revolution in precision, repeatability, and quality.

In practice, SVK's conversion of one of our clients – a foundry in Zaporizhzhia – from sand-clay mixtures to the AHFS process resulted in a reduction of scrap from 28% to 6% in the first two months. The payback period for the transition was 4 months, considering the cost of new binders.

What is AHFS and How It Works

AHFS (Alkyd-based Chemical Sand) is a composite made of quartz sand and synthetic resin that hardens at room temperature under the action of a special catalyst. Unlike sand-clay mixtures, AHFS does not require heat – this allows the process to be very fast.

Three Main Variants of the AHFS Process

1. Cold Box Technology

The molding mixture is packed into a core box with a hydrometer, a catalyst (gas, most often CO2 or a volatile aldehyde) is applied, and the mold hardens in 1–3 minutes. Then the core box can be immediately opened and the mold extracted.

Advantages:

• Extremely fast cycle.
• The entire process can be automated.
• The mold is ready for pouring a few minutes after forming.

Disadvantages:

• Requires special equipment (core box machines).
• Higher cost of equipment and reagents compared to No-Bake.

2. No-Bake Process

The catalyst is added directly to the mixture, and it hardens slowly at room temperature (4–24 hours depending on the formula). This allows the entire process to be carried out without special equipment.

Advantages:

• Flexibility: the mold can harden while lying down.
• Lower equipment cost.
• Suitable for small-batch production.

Disadvantages:

• Longer cycle (from 4 hours).
• Harder to automate.

3. Accelerated No-Bake (Warm-up process)

The mold is placed in a warm chamber (30–50°C), which accelerates hardening to 30–60 minutes.

For other surface treatment technologies, see the article “Phosphating vs. Nanoceramics”.

Advantages of AHFS over Sand-Clay Mixtures

Three Main Binder Systems for Casting

1. Furan Resins

Basis: Furfural (from sugarcane) + aldehyde (most often formic acid).

Application: Cast iron, steel, hard alloys. Anything requiring high heat resistance.

Advantages:

• High heat resistance: the mold withstands up to 1200°C.
• Resistance to molten metal corrosion.
• Low vacuum-pressure properties.

Disadvantages:

• Generates volatile organic compounds (VOCs) during hardening.
• Requires workshop ventilation.
• Slightly worse accuracy than phenolic.

Parameters:

• Catalyst: formic acid or phosphoric acid.
• Tensile strength: 1.8–2.5 MPa.
• Mixture pot life: 3–6 hours.

2. Phenolic Resins

Basis: Phenol + formaldehyde (or epichlorohydrin).

Application: Precision casting (aircraft engines, turbines), aluminum, magnesium, copper.

Advantages:

• Highest dimensional accuracy.
• Minimal gas evolution (critical for non-ferrous metals).
• Better sand adhesion and fewer surface defects.
• Resistance to thermal shock.

Disadvantages:

• Higher costs compared to furan.
• Slower hardening (No-Bake variants 8–24 hours).
• More manual labor during molding.

Parameters:

• Catalyst: latex or organic acids.
• Tensile strength: 2.0–3.0 MPa.
• Gas permeability: low (good for aluminum).

3. Polyurethane Resins

Basis: Isocyanate + polyol.

Application: Mass production, automotive parts, mechanical engineering. For MQL in CNC machining, see the guide “How to Choose MQL for CNC Machines”.

Advantages:

• Fastest hardening: seconds–minutes (Cold Box).
• Excellent for automation.
• Good strength and accuracy.
• More environmentally friendly than furan with proper ventilation.

Disadvantages:

• Less heat resistant than furan (up to 800–900°C).
• Sensitive to moisture during operation.
• Requires careful temperature and humidity control during storage.

Parameters:

• Catalyst: DETA, TMEDA, or other amines.
• Tensile strength: 1.5–2.2 MPa.
• Mixture pot life: 15–30 minutes.

Definitions of industrial terms are in the Glossary of Industrial Chemistry.

Critical Parameters for Binder Selection

1. Mixture Pot Life

This is the time during which the mixture remains workable after the catalyst is mixed. Too short a pot life = difficult to form large molds. Too long = slow production cycle.

Standards:

• Cold Box: 1–3 minutes (intentionally short).
• No-Bake: 3–8 hours.
• No-Bake with heat: 30–90 minutes.

2. Tensile Strength

The mold must withstand the weight of the molten metal. Minimum for serial production: 1.5–2.0 MPa.

If the strength is lower, the mold will collapse under load, and you will get defects with cold castings or sand scattered in the part.

3. Gas Permeability

Molten metal generates gases, and they must have an exit through the mold. If gas permeability is low, gas cavities (voids) will appear in the casting.

Requirements depend on the metal:

• Cast iron: moderate requirements.
• Steel: more critical.
• Aluminum: very critical (requires minimal gas evolution).

4. Shakeout Ease

After the casting cools, the mold needs to be broken up and the sand removed from the part. If the binder is too strong, mechanical vibration and expensive processing are required. If too weak, it will have already disintegrated during pouring.

Binder Quality Control

Before releasing each batch, SVK conducts:

1. Bismuth test: Pouring molten material at 800–1000°C into a small mold and checking dimensional accuracy and surface.

2. Strength test: Laboratory tensile testing according to ASTM C109.

3. Gas permeability: Using the SGI (Sand, Gas, Ink flow) method.

4. Microscopy: Checking the binder structure under a microscope.

5. Mixture pot life: Recording hardening time at a controlled temperature.

Environmental Trends in Casting

As Oleg Zagorulko notes: “We observe a clear trend among Ukrainian foundries – the transition to low-emission systems has accelerated after 2023, when large EU customers began to demand EN 15339 certification and documentation of VOC levels in workshops.”

New European and US standards, including recommendations from the American Foundry Society (AFS), promote the transition to:

• Low-emission systems: Formulations with minimal formaldehyde emission.
• One-component systems: Instead of two-component (fewer opportunities for error).
• Mold recycling: Some modern resins decompose at 500°C, allowing sand to be recycled.

For EU environmental restrictions, see “PFAS Ban in EU”.

FAQ

What are AHFS and how are they better than sand-clay mixtures?

AHFS (Alkyd-based Chemical Sand) is a composite of quartz sand and synthetic resin that hardens at room temperature. Compared to sand-clay mixtures: accuracy ±0.3-0.5 mm instead of ±1-2 mm, roughness Ra 6.3-12.5 µm instead of Ra 20-25, usable casting yield 90-95% instead of 70-75%.

Which binder system to choose for mass production?

Polyurethane Cold Box systems are optimal for mass production (automotive parts, mechanical engineering). Fastest hardening (seconds-minutes), easily automated, good strength and accuracy. For cast iron and steel – furan resins (heat resistance up to 1200°C). For precision casting – phenolic resins.

What is mixture pot life and why is it important?

Pot life is the time during which the mixture remains workable after the catalyst is added. Cold Box: 1-3 minutes (intentionally short for a fast cycle). No-Bake: 3-8 hours (for large molds). Too short a pot life complicates the formation of large molds, too long slows down production.

What casting defects are caused by an incorrect binder?

With low tensile strength (<1.5 MPa) – the mold collapses under the weight of the metal, cold castings, sand in the product. With low gas permeability – gas cavities (voids) in the casting. With excessive strength – difficult mold shakeout, requiring mechanical vibration and expensive processing.

SVK: Your Partner for Foundry Production

SVK has been developing binders for casting for 25+ years. We offer:

• AHFS based on furan resins: For cast iron and steel. Proven stability during 10,000+ castings.
• Phenolic systems: For precision casting. Guaranteed accuracy of ±0.3 mm and minimal gas evolution.
• Polyurethane Cold Box systems: For mass production with a cycle of 2–5 minutes.
• Catalysts and additives: For fine-tuning the process.

Our laboratory tests your specific casting and recommends the optimal system.

Request a consultation and samples for testing directly on your equipment. SVK Test Drive allows you to test the system without risk.

Contacts:

Phone: +380 (56) 785-41-41

Email: foundry@svk.ua

Address: Dnipro, Promyslova St., 23

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Read also:

• How to Choose MQL for CNC Machines: A Complete Guide
• Phosphating vs. Nanoceramics: A Comparison of Technologies
• Graphite Lubricants for Forging: How to Choose and Optimize
• Glossary of Industrial Chemistry: 30 Terms