Guide To Cold Extrusion Technology And Applications for Fasteners

Cold Forging and Its Characteristics

Cold forging, also known as cold extrusion, is a subset of cold extrusion. Cold extrusion encompasses a broader range of techniques. Machines commonly used in cold extrusion include hydraulic presses, punch presses, and automatic cold forging machines. The key feature of cold forging lies in its speed.

Due to the large variety of fasteners, cold forging is widely applied in their production. However, its applications have expanded to include automotive components, transmission parts, military, aerospace, marine, and other precision parts. Nevertheless, over 90% of its use is still limited to fastener production.

The book refers to "Cold Forging Technology and Applications", while "Fastener Cold Forging Technology" is a common term used in practice. Fasteners primarily focus on threads, with detailed thread calculations outlined in P51–P57 of Cold Extrusion Technology and Applications.

Basic Theories of Cold Forging Technology

1. What is Cold Forging?

· Cold Forging: The process of using molds at room temperature to plastically deform metal to achieve desired geometry, dimensions, and quality.

· Warm Forging: Deforming metals by heating to 600–800°C with the assistance of molds.

· Hot Forging: Deforming metals by heating to 1000–1200°C with the assistance of molds.

2. The Essence of Cold Forging

Cold forging represents the essence of plastic deformation in metals.

3. Metal Plastic Deformation Flow Rules in Cold Forging

· Minimum Resistance Law

· Volume Constancy Law

4. The Three Steps of Cold Forging

Material Selection (Blank Preparation)

The raw materials for cold forging should meet the following:

· Grains and chemical compositions must meet the standard requirements for cold forging steel.

· Hardness ≤ 150° HB (≤ 80° HRB) before cold forging.

· For medium-carbon and alloy steels, they should undergo spheroidization annealing before cold drawing, with post-cold drawing hardness ≤ 180° HB (≤ 90° HRB).

Material hardness standards for cold forging:

· Low carbon steel and stainless steel hollow rivets: hardness 102° HB (60° HRB).

· Sleeve-like products: 102–133° HB (60–80° HRB), preferably 102–113° HB (60–70° HRB).

· Rod-reduction products: 133–168° HB (80–88° HRB), preferably 133–158° HB (80–85° HRB).

Design the Appropriate Forging Process
Choosing suitable deformation methods for cold forging is essential.

Improve Mold Surface Roughness and Lubrication Conditions
Surface improvements reduce friction during deformation.

Core Techniques and Common Design Methods in Cold Forging

1. Core Techniques

· Open die cold forging forming standards and design methods are referenced in Cold Extrusion Technology and Applications P12.

· Closed die cold forging forming standards and design methods are referenced in Cold Extrusion Technology and Applications P13.

Cold forging mainly consists of upsetting and extending operations. Deformation methods are categorized as follows:

· Small material, large deformation

· Large material, small deformation

· Intermediate materials with deformation at both ends

When forming fasteners or other minimal cut products, wire stock (rotational metallic wire) is often used as the blank. Selecting the appropriate wire diameter becomes critical.

Formula for Deformation:

ε=(Deformation level of material)=dD×100%ε=(Deformation level of material)=Dd×100%

Where:

· DD: Largest wire diameter (upsetting or reducing diameter selection).

· dd: Smallest wire diameter (selection for upsetting or reducing diameter).

The deformation allowance depends on material properties. For example:

ε≤30%ε≤30%: Open die cold forging is suitable.

ε>30%ε>30%: Closed die cold forging is appropriate.

Stainless steel allows up to ε≤50%ε≤50%.

Most common black metals are controlled at ε≤60%ε≤60%.

Low-to-medium carbon steels can achieve ε=85%ε=85%.

For blind holes and re-extruded hollow components, choosing the wire diameter close to the forming diameter minimizes cold work hardening.

Re-extrusion Considerations:

When re-extruding, the die remains stationary while the punch moves. In re-extruded processes:

· Blind holes can reach 3–5 times the hole diameter, depending on design.

· Low-carbon steels may even reach 9 times the diameter.

For hexagonal nuts:

· General large deformation D = 0.9–0.92 × S.

· Small deformations: D = 0.7–0.72 × S.

2. Basic Forging Processes

Cold forging involves separation and forming processes:

Separation Process:

· Cutting, trimming, and punching.

Forming Process:

· Initial upsetting, precision upsetting, extrusion, re-extrusion, combined forging-extrusion, and drawing.

3. Types of Metal Deformation Based on Movement Directions

1. Forward Extrusion: Metal movement aligns with the punch's movement direction—used for creating solid or hollow extrusions.

2. Reverse Extrusion: Metal flows against the punch's motion—used for hollow extrusions.

3. Compound Extrusion: Combines forward and reverse extrusion for complex shapes.

4. Combined Forging-Extrusion: Simultaneously upsets metal radially during extrusion—used for flanges or parts with localized large cross-sections.

4. Deformation Limits

Deformation varies by material properties. The maximum deformation permissible during a single forging is referred to as permitted deformation. Common values are referenced in Cold Extrusion Technology and Applications P35 Table 2.3–2.

5. Calculation of Cold Forming Forces

The forces involved in cold forging are calculated as follows:

Shear Force Calculation: Pq=Fτ (kgf)Pq=Fτ (kgf)

Cold Forging Force (Upsetting): Pc=γZϕσs′(1+f/3×dh)F(kgf)Pc=γZϕσs′(1+f/3×hd)F(kgf)

Forward Extrusion Force: Ps=pF(kgf)Ps=pF(kgf)

Reverse Extrusion Force: Pj=pF1(kgf)Pj=pF1(kgf)

Compound Extrusion Force: Compound extrusion pressures are generally lower than single forward or reverse extrusion pressures.

Ejection Force: Often negligible unless associated with cutting, in which case: Pd=PqK(kgf)Pd=PqK(kgf)

Detailed calculations for these forces are referenced in Cold Extrusion Technology and Applications P40–P44