Quality, Reliability, and Performance - Delivered
[email protected]

Enhancing Cold Extrusion Die Lifespan: Key Strategies

Last updated:
April 19, 2024
Share your like:

Table Of Contents

From analysis, it’s clear that the working conditions of cold extrusion dies are extremely harsh, leading to their short service life. This is particularly true for dies used in the cold extrusion of steel materials. Therefore, to promote the development of cold extrusion technology, in-depth research into the lifespan of cold extrusion dies is a key issue.

There are mainly two aspects to extend the lifespan of cold extrusion dies:

Reduction of unit extrusion force

a) Choosing the appropriate deformation method

When a part can be formed using different extrusion methods, the method with the lower unit extrusion force should be chosen. For example, using a process shaft to convert simple direct or indirect extrusion into compound extrusion is an effective measure to reduce unit extrusion force and extend die life.

During extrusion molding, the direction of frictional resistance is always opposite to the direction of metal flow. Therefore, the extrusion methods shown in Figure 9-15 can be used to reduce unit extrusion force.

Figure 9-15: Extrusion Using Frictional Force
Figure 9-15: Extrusion Using Frictional Force

a) direct extrusion, and b) indirect extrusion.

The components are numbered as follows:

1 – Punch
2 – Extrusion cylinder
3 – Extruded part
4 – Die
5 – Ejector rod

b) Choosing the right degree of deformation

In actual production, to increase productivity and reduce the number of deformation operations, the method of increasing the degree of deformation is often adopted, which is inappropriate. This inevitably increases the unit extrusion force, leading to early die failure. Therefore, during process design, the permitted degree of deformation should be strictly controlled.

c) Using the optimal die shape

Regardless of whether it’s a direct extrusion die or an indirect one, there must be an optimal shape that minimizes the unit extrusion force. Therefore, in actual production, numerical simulation of metal flow should be used to find the optimal die shape to reduce unit extrusion force and extend die life.

d) Choosing the right blank shape

Available data indicates that using a reasonable blank shape can reduce unit extrusion force. Therefore, the blanks used for cold extrusion are not directly taken from the cut raw material, but are processed through a preform operation.

e) Strict pre-treatment of the blank

Reasonable annealing and surface lubrication treatment of the blank before cold extrusion can significantly reduce the unit extrusion force. For example, in the cold extrusion of steel materials, the extrusion force will differ by about half depending on whether phosphatizing treatment is used before extrusion.

Enhancing the Mold’s Resistance to Damage

(1) Improving the Quality of Mold Materials and Developing New Materials

Enhancing the quality of existing mold materials and developing new ones are fundamental methods for extending the mold’s lifespan.

1) The primary way to improve the quality of mold materials is by demanding metallurgical plants to enhance their metallurgical quality. This not only requires the assurance of the chemical composition of the steel but also the purity and uniformity of the mold steel. For manufacturers, it is essential to strengthen the inspection of raw materials and timely adopt some effective process measures to improve the quality of mold materials.

2) In recent years, significant progress has been made in developing new mold materials both domestically and internationally. Numerous innovative mold materials such as new high-speed steel, steel-bonded hard alloys, base steels, and ceramic materials have been developed, providing favorable prerequisites for enhancing mold strength and extending mold lifespan.

However, it is crucial to fully understand the properties of these new materials and choose them correctly based on specific working conditions to avoid misuse, which could result in severe waste and potentially unsatisfactory usage effects.

(2) Correct Selection of Mold Materials

Correctly choosing the appropriate mold material based on the working conditions of the cold extrusion mold is a key measure for extending the mold’s lifespan and reducing costs. Therefore, the following two issues should be considered when selecting mold materials.

1) The selection of mold materials should be based on the working conditions of the mold. The actual working conditions of cold extrusion molds often vary, and the modes of failure are not completely the same. If the mold mainly fails due to wear, it is unnecessary to choose high-strength mold materials; instead, mold materials with high wear resistance should be chosen.

If the mold is subject to large eccentric forces and mainly fails by breaking (such as punch molds), mold materials with high toughness should be chosen. Experience shows that if the selected mold material cannot meet the usage requirements in some aspects, it may shorten the mold’s lifespan and cause early failures.

2) The selection of mold materials must also fully consider the economic benefits. As is well known, the prices of mold materials are generally quite high, especially for some advanced mold materials. Therefore, when selecting mold materials, it is necessary to choose more affordable mold materials as much as possible while meeting usage requirements, and absolutely avoid the arbitrary misuse of advanced mold materials.

(3) Designing a Reasonable Mold Structure

A reasonable mold structure is an important measure to enhance the mold’s load-bearing capacity and extend its lifespan. In summary, the following points should be considered.

1) Ensure the mold has sufficient strength, rigidity, reliability, and good guiding performance.

2) Avoid forming a mold structure with large stress concentrations as much as possible. For example, transition parts should be designed with a sufficiently large fillet radius; split structures should be adopted for mold cavity parts that are prone to cracking, etc.

3) Different optimization design methods should be chosen based on the type of die material used. For instance, when using hard alloys or steel-bonded hard alloys for the die, the optimization design should target zero tensile stress on the inner wall of the die.

When using tool steel for the die, both the die and the pre-stressed ring material should yield simultaneously as the target function for optimization design. This way, the potential of the mold material can be fully utilized.

4) Adopt proven new mold structures. For example, using a steel strip winding die or a wire winding die can ensure the mold has high strength while reducing the mold’s size and weight; using a stepwise combined die can bear larger radial internal pressure than a flat-mouth combined die of the same size.

(4) Adopting a Reasonable Forging Process

The distribution and form of carbides in mold steel have a decisive impact on its mechanical properties; and forging deformation is the main method to break the blocky and band-shaped carbides in steel, making them evenly distributed as tiny particles to eliminate material anisotropy and improve internal quality.

Although the mold materials supplied by steel mills have undergone a certain degree of pressure processing, they cannot meet the usage requirements and must undergo re-forging. When re-forging mold steel, the following issues should be noted.

1) Fully understand that the main purpose of re-forging mold materials is to improve mechanical properties, and only secondly to change the shape.

2) Pay special attention to the differences between forging mold materials and forging general materials, and strictly adhere to the forging process specifications of mold materials.

3) To fully break up carbides and make them disperse evenly, a high forging ratio and multidirectional extrusion method should be adopted for forging.

(5) Adopting a Reasonable Heat Treatment Process

According to domestic and international statistics on early mold failures, the largest proportion of early mold failures is caused by unreasonable heat treatment processes. Experience shows that the heat treatment process of the mold is one of the important means to improve the mechanical properties of mold materials.

To ensure the mold has high strength, hardness, wear resistance, and heat hardness, surface strengthening heat treatment processes should be added in addition to quenching and tempering heat treatment methods. This can significantly enhance the hardness, wear resistance, and fatigue resistance of the mold working surface. Currently, the following surface strengthening heat treatment processes are used for cold extrusion mold steel.

1) Carbonitriding

The simultaneous infiltration of carbon and nitrogen into the surface of steel is called carbonitriding. It belongs to a type of multi-element co-infiltration chemical heat treatment.

Carbonitriding has several advantages compared to carburizing.

① The infiltrated layer surface has higher hardness and wear resistance than carburized steel, as well as certain heat hardness and corrosion resistance.

② Nitrogen dissolved in austenite increases the stability of supercooled austenite, thereby improving the hardenability of the infiltrated layer.

③ The introduction of nitrogen can lower the temperature at which austenite forms, preventing grain coarsening.

④ The deformation of carbonitriding is smaller than that of carburizing.

Due to the series of advantages mentioned above, carbonitriding is currently trending to replace carburizing in the surface strengthening heat treatment process of molds.

Carbonitriding processes can be divided into solid, liquid, and gaseous carbonitriding according to the state of the materials used. Cyanide carbonitriding has been phased out in recent years due to its high toxicity. Gaseous carbonitriding, on the other hand, is non-toxic, provides good working conditions, is easy to operate, and easy to control. Therefore, gaseous carbonitriding is widely used currently.

2) Gas Nitrocarburizing

This refers to a type of gas carbonitriding that mainly involves nitrogen infiltration performed at temperatures below the eutectic temperature of the Fe-C-N ternary system. The surface of the mold after gas nitrocarburizing has higher strength, wear resistance, and fatigue strength. Currently, this surface strengthening heat treatment process has begun to be used in cold extrusion molds and has achieved good results.

For example, the punch mold used by a certain factory for cold extrusion of 15 steel instrument seats, when made of CG-2 base steel without gas nitrocarburizing treatment, had a mold life of 14,500 pieces, but after nitrocarburizing treatment, the average lifespan reached 27,500 pieces, with the highest reaching 45,600 pieces.

The process specification for gas nitrocarburizing is: heating temperature is 530~570℃, and the holding time is generally 1~16h.

In addition, ion nitriding, hard chrome plating, TiC vapor deposition, salt bath vanadium infiltration, boron infiltration, as well as rare earth surface engineering and nano surface engineering technologies are also being applied in the surface strengthening heat treatment of cold extrusion mold steel. They can all improve the strength of the mold surface to different extents and extend the lifespan of the mold.

(6) Ensuring Machining Quality and Adopting Novel Processing Methods

1) Strictly ensuring the quality of machining.

Higher machining quality can extend the service life of the die. Therefore, effective process measures must be taken to ensure the fulfillment of specified technical requirements. Experience shows that to achieve higher machining quality in the mechanical processing of cold extrusion dies, the following points should be emphasized:

i) After the final machining of the punch die, all transition parts should be smooth and seamless. The working part should be coaxial with the clamping part, and its shape should be strictly symmetrical. Otherwise, not only will it result in uneven thickness of the extruded piece, but the punch die itself may also bend due to unilateral stress.

ii) To ensure the high strength of the punch die and to avoid cracking due to stress concentration, the punch die should not have residual center holes at either end. Therefore, when processing the punch die, a “bump” must be reserved on the top surface for punching the center hole.

iii) The surface roughness of the punch die should generally adhere to certain requirements based on the type of raw material of the extruded piece. For steel reverse extrusion punch dies, the surface roughness Ra should be <0.2μm; for non-ferrous metals like pure aluminum, a surface roughness Ra of 0.8μm is sufficient.

However, for the working band part, regardless of whether the material being extruded is steel or non-ferrous metal, a surface roughness Ra of <0.2μm should be maintained.

iv) The lower the surface roughness of the forward or reverse extrusion die cavity, the better, generally it should be Ra<0.2μm. The die cavity that has been ground and then polished can reduce the unit extrusion force, significantly extending the service life of the die.

v) The grinding or polishing of the working part of the die should be performed after the final heat treatment. The machining allowance left before grinding should not exceed 0.1mm, and the surface roughness Ra should be <1.6μm. To remove residual stress after grinding, it is best to perform a stress-relieving annealing treatment after grinding is completed.

2) Adopting new processing methods.

In addition to general mechanical processing methods, the processing of cold extrusion dies also includes methods such as electrical discharge machining, electrolytic processing, and cold extrusion of die cavities, as well as novel processing methods such as ultra-precision machining and high-speed milling. Here, we will briefly introduce the method of cold extrusion of die cavities.

The essence of cold extrusion of die cavities is to directly process the die cavity using the cold extrusion process based on the plastic forming principle of metals. It usually involves first processing a high-hardness punch die from tool steel according to the shape of the die cavity.

Then, under the action of a cavity extrusion machine (or a large-tonnage hydraulic press), the punch die is pressed into the blank, thereby creating a die cavity that matches the shape, perimeter, and size of the punch die working part in a concave-convex inverted manner.

There are many methods of cold extrusion of die cavities, among which the most common is the cavity extrusion within the die frame, as shown in Figure 9-16. The working process is as follows: the annealed and softened blank is pre-placed in the die frame, and the punch die is pressed into the blank under the action of the hydraulic press.

Figure 9-16: Cavity Extrusion within the Die Set

1. Punch
2. Guide plate
3. Outer ring
4. Inner ring
5. Conical fit block
6. Blank

At this time, the blank metal can only flow upward, ensuring close contact between the blank and the punch die. As a result, the precision of the die cavity can reach IT6-IT7, the geometric shape is correct, and the surface roughness Ra is 0.1~0.2μm.

The punch die for cold extrusion of the die cavity is a critical component related to the success or failure of forming. Attention must be paid to the manufacturing of this punch die. Generally, the following issues should be considered:

i) To reduce stress concentration, the transition part of the punch die should not have abrupt changes and should have a minimum radius of curvature of 0.2mm.

ii) To ensure that the punch die has high strength and hardness, ultra-high-strength steel should be selected as the raw material for the punch die, and strict forging deformation and heat treatment processes should be implemented.

iii) After heat treatment, the working part of the punch die should be ground again, with a surface roughness Ra≤0.1μm.

iv) To ensure the guiding precision of the punch die, the length of the punch die guiding part should generally not be less than 1.2 times the maximum radial dimension of the die cavity.

It should be noted that the production rate of manufacturing die cavities by cold extrusion is much higher than cutting processing. More importantly, after the die cavity is formed by cold extrusion, the internal structure of the die is denser, and its strength and wear resistance are higher, thus extending the service life of the die.

However, because the strength of the die material formed by extrusion is higher and its plasticity is poorer, this processing method is only used for simple shapes and shallow die cavity processing.

(7) Rational Use and Maintenance of Dies

Understanding the characteristics of cold extrusion forming and the rational use and careful maintenance of dies are also important measures to extend the service life of dies.

During low temperatures in winter, it is best to preheat the die before use to prevent cracking. During cold extrusion production, due to the effect of heat, the temperature rise of the die is rapid, so the die should be cooled regularly.

For dies that experience large forces, after a few thousand extrusions, a stress-relieving annealing treatment should be performed at 160~180℃ for 2 hours.

For the outer and middle prestressed rings that are used repeatedly, after multiple extrusions, a stress-relieving annealing treatment should be performed at 180℃ for 2 hours; otherwise, the middle and outer prestressed rings may suddenly crack, not only damaging the die but also possibly causing personal accidents.

To extend the service life of the die, a complete set of maintenance and care system must be established. Designated personnel should be assigned to promptly adjust the press, as well as perform die repair, adjustment, and storage work. During storage and transportation of the die, rust prevention measures should be taken, and there should be limit blocks protecting between the upper and lower die bases to prevent damage.

Request FREE Quote
Contact Form

Latest Posts
Stay up to date with new and exciting content on various topics, including useful tips.
Talk To An Expert
Contact Us
Our sales engineers are readily available to answer any of your questions and provide you with a prompt quote tailored to your needs.

Request a Custom Quote

Contact Form

Request A Custom Quote
Get a personalized quote tailored to your unique machining needs.
© 2024 Artizono. All rights reserved.
Get Free Quote
You will get our expert reply within 24 hours.
Contact Form