Binders (also known as forming agents) are critical additives in the powder metallurgy process of cemented carbides. They serve three primary functions during the pressing (forming) stage: enhancing powder flowability, improving binding properties, and increasing green strength. These functions ensure the compact maintains its structural integrity during demolding, handling, and prior to sintering.

The primary functions of binders in cemented carbide manufacturing

In the powder metallurgy of cemented carbides, binders (also called forming agents) play critical roles, including:

Improving Powder Flowability

Reduces interparticle friction, enabling homogeneous mold filling and uniform compaction.

Prevents powder segregation (e.g., separation of WC and Co).

Enhancing Green Strength

Provides sufficient “green strength” to prevent cracking or edge chipping during handling or demolding.

Minimizes elastic aftereffects (post-compaction expansion).

Lubricating the Mold

Reduces friction between powder and die walls, lowering compaction pressure and extending mold life.

Improves surface finish and minimizes defects (e.g., delamination, cracks).

Facilitating Debinding

Must be fully removable (via thermal decomposition or dissolution) before sintering to avoid carbon residue or impurities that degrade alloy properties.

Performance Requirements for Binders
The binder must possess the following characteristics:

Excellent Compatibility

Uniformly mixes with WC-Co powders without agglomeration or sedimentation.

Chemically inert to powders (e.g., no oxidation of cobalt).

Suitable Melting Point and Viscosity

Melting point must align with compaction temperatures (typically room temperature to 100°C) to ensure:

Liquid-phase homogeneity during mixing.

Solid-phase strength during pressing.

Too high moderate viscosity leads to impedes powder flow.

Too low moderate viscosity leads to insufficient binding force.

High Binding Capacity and Lubricity

Binding capacity: Ensures green strength (flexural strength typically ≥5 MPa).

Lubricity: Reduces compaction pressure (e.g., from 600 MPa to 400 MPa).

Controlled Debinding Behavior

Broad debinding temperature range (e.g., 150–500°C) to prevent cracking from rapid volatilization.

Low carbon residue after debinding (<0.1%) to avoid disrupting alloy carbon balance.

Environmental and Safety Compliance

Non-toxic, low volatility (e.g., water-soluble PEG outperforms solvent-based rubber binders).

Meets industrial emission standards (e.g., sulfur- and chlorine-free).

Cost-Effectiveness

Low-cost and readily available (e.g., paraffin wax is more economical than rubber).

Recyclable or easy to dispose of (e.g., PEG can be water-washed and recovered).

Types of Binder

When manufacturing cemented carbide products, selecting the right binder is crucial for quality and efficiency. Here’s a detailed comparison of the three most common binder types to help you make the best choice for your application.

Paraffin Wax

Characteristics:Composition: Hydrocarbon-based, solid at room temperature with low melting point (50-70°C)

Best for: Small, simple-shaped carbide products

Advantages:

Excellent lubricity reduces die friction

Low debinding temperature (200-400°C) simplifies processing

Cost-effective and readily available

Limitations:

Lower green strength (prone to cracking)

Potential carbon residue during high-temperature debinding

Temperature-sensitive – requires dry storage

Pro Tip: Ideal for mass production of standard inserts where cost is key.

PEG (Polyethylene Glycol)

Characteristics:Composition: Water-soluble polymer with adjustable molecular weight (PEG-2000/4000)

Best for: Complex-shaped tools and precision molds

Advantages:

Higher green strength for intricate shapes

Water-soluble – enables aqueous pre-debinding

Minimal carbon residue

Limitations:

Hygroscopic – requires humidity control

Narrow debinding window (200-300°C)

More expensive than paraffin

Pro Tip: The go-to choice for premium cutting tools requiring precision.

Rubber (SBR, etc.)

Characteristics:Composition: Polymer elastomer requiring organic solvents (e.g., acetone)

Best for: Large, high-density components like rolls and mining tools

Advantages:

Highest green strength

Excellent elasticity prevents cracking

Limitations:

Challenging debinding (500°C+)

Potential sulfur contamination

Environmental concerns with solvents

Highest cost

Pro Tip: Reserved for specialized applications where extreme strength is critical.

Compatibility Principles Between Binders and Wet Milling Media

Paraffin Wax

• Requires organic solvents (e.g., ethanol, acetone)
• Limited solubility in ethanol alone – heating often needed

Recommended Medium: Ethanol + 10-20% acetone (enhances solubility)

 

PEG (Polyethylene Glycol)

• Excellent water solubility
• Requires oxidation protection for cobalt

Recommended Medium: Deionized water + 0.5% antioxidant (e.g., oxalic acid)

 

Rubber Binders

• Only soluble in strong organic solvents

Recommended Medium: Pure acetone (requires sealed system to prevent evaporation)

 

Performance Comparison of Three Major Binder Systems

Binding Strength

Rubber binders provide the highest strength due to their polymer chain structure, making them suitable for large compacts. PEG offers moderate strength ideal for complex geometries, while paraffin wax has the lowest binding strength as it relies solely on physical bonding.

Debinding Process

Paraffin wax can be removed at relatively low temperatures between 200 to 400°C, though carbon balance must be carefully controlled. PEG requires aqueous pre-debinding followed by thermal cycling, but is sensitive to moisture. Rubber binders demand high-temperature pyrolysis above 500°C and carry risks of sulfur contamination.

Residue Effects

Paraffin may leave carbon residues that affect the WC/Co ratio, requiring adjustment of carbon potential during sintering. PEG leaves virtually no residue, making it excellent for high-purity alloys. Rubber can leave sulfur residues that reduce the alloy’s corrosion resistance.

Economic Considerations

Paraffin wax has the lowest initial cost but may incur additional expenses for carbon management. PEG provides the best value for precision components and mass production. Rubber is the most expensive option and is only justified for specialized heavy-duty applications.

Selection Summary

For cost-sensitive production where simple processes are preferred, paraffin wax is suitable but requires careful control of dimensional stability during debinding. When high precision and environmental considerations are priorities, PEG is the optimal choice though it needs humidity-controlled storage. Rubber binders are reserved for applications requiring maximum strength and large components, provided that high-temperature debinding equipment is available.

Modern developments are creating hybrid binder systems that combine the advantages of these materials, such as PEG’s performance with paraffin’s cost benefits through advanced formulation techniques.