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简体中文 The carbide pressing process is a core link connecting powder preparation and sintering forming, which directly determines the density uniformity, dimensional accuracy and subsequent sintering performance of the green compact. As the power core of the pressing process, the selection of press type, structural characteristics and regulation of key parameters (such as maximum pressing force, pressing speed, machine stiffness, etc.) constitute the basic guarantee for the quality of green compacts; while the die, as a key equipment directly endowing the powder with forming shape, its cavity structure design, material selection, core rod positioning and lubrication system optimization are the core means to solve problems such as uneven density, difficult demoulding and surface defects during the pressing process. The degree of collaborative matching between the two directly affects the qualification rate, production efficiency and comprehensive performance of cemented carbide products. This paper will systematically sort out the core technical points of presses and dies in the cemented carbide pressing process, including the classification of presses, structural principles, regulation principles of key parameters, as well as the design requirements and optimization strategies of dies, so as to provide technical reference for the precise control and quality improvement of the cemented carbide pressing process.
Press
As the core equipment of the pressing process, the performance characteristics of the press directly determine the upper limit of pressing quality. Common presses for cemented carbide pressing are mainly divided into two categories: mechanical presses and hydraulic presses. In addition, there are classifications such as automatic presses and non-automatic presses, single-direction presses and double-direction presses. In recent years, with the advancement of technology, servo presses have also begun to be applied in the field of high-precision pressing. The precision of the press is determined by the manufacturer, and users need to select different types according to product precision requirements. For example, for indexable inserts with strict peripheral dimension requirements, double-direction hydraulic presses can achieve constant density or constant height pressing, which is conducive to controlling dimensional consistency. There are significant differences in the performance parameters of different types of presses and their influence mechanisms on carbide pressing quality.
System Classification of Presses
There are various classification methods for presses, and the most common classification basis is their power source and transmission mechanism. According to this standard, presses can be divided into three categories: mechanical presses, hydraulic presses and servo presses with the characteristics of both. In addition, auxiliary classification can also be carried out according to frame structure, application and other aspects.
Brief Introduction to Mechanical Presses
Mechanical presses use electric motors as the prime power, and convert the rotational motion of the motor into the linear reciprocating motion of the slider through mechanical transmission mechanisms. Its core feature is the use of a flywheel to store and release energy to achieve stable stamping operations. According to the different transmission mechanisms, mechanical presses can be mainly divided into the following types:
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Crank/Eccentric Press: This is the most common type of mechanical press. The electric motor drives the flywheel to rotate, transmits power to the crankshaft or eccentric shaft through the clutch, and then converts the rotational motion into the vertical linear motion of the slider through the connecting rod. This structure is suitable for high-speed and high-precision processes such as punching, blanking and shallow drawing.
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Toggle Press: The toggle mechanism amplifies force and displacement through an articulated connecting rod system, usually providing extremely high pressure and a short pressure holding time at the end of the stroke. This characteristic makes it very suitable for processes requiring high precision and high pressure at the final stage, such as coining, finishing and powder metallurgy pressing.
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Screw Press: The electric motor drives a large screw to rotate, driving the meshed nut and the connected slider to make linear motion. Manually operated screw presses have a simple structure and are suitable for small laboratories or maintenance occasions. Modern friction-type or electric-type screw presses, due to their characteristics of long stroke and controllable energy, have special applications in fields such as forging.
Brief Introduction to Hydraulic Presses
Hydraulic presses use hydraulic oil as the working medium, and their basic principle is Pascal’s law. The hydraulic pump converts the mechanical energy of the electric motor into the pressure energy of the hydraulic oil, and the high-pressure oil is sent into the hydraulic cylinder to push the piston (or ram) and the slider to produce linear motion. Figure 2 shows the structure of a complete hydraulic press system, which mainly includes core components such as a hydraulic power unit, a control valve group, a hydraulic cylinder and a frame.
The main advantages of hydraulic presses are their huge pressure (up to tens of thousands of tons), stepless adjustment of stroke and pressure in the entire range, and the ability to maintain rated pressure in the entire stroke range. This makes it particularly suitable for processes such as deep drawing, lamination, molding and heavy forging. According to the frame structure, hydraulic presses can be divided into C-type open type and gantry closed type. The latter has better rigidity and is suitable for large-tonnage and eccentric load working conditions.
Servo Presses
Servo presses are the culmination of technological development in recent years. They use servo motors directly as the power source, and drive the slider to move through the combination of precision reducers with mechanisms such as screws or toggle levers. It abandons the traditional flywheel and clutch, and realizes digital closed-loop control of the slider’s position, speed and pressure. Servo presses have outstanding advantages such as energy saving, low noise, extremely strong process adaptability (customizable motion curves) and high intelligence, and are gradually becoming the development trend in the field of high-precision forming.
Basic Structure and Working Principle
Although there are many types of presses, their basic structures can be summarized into the following core subsystems: power system, transmission system, execution system, support system and control system.
Structure and Principle of Mechanical Presses
Taking the most typical crank press as an example, the electric motor of the power system drives the flywheel to rotate continuously to store kinetic energy. During operation, the clutch engages, and the energy stored in the flywheel is transmitted through the crankshaft (or eccentric shaft) and connecting rod of the transmission system. The core of the execution system is the slider, which makes precise vertical reciprocating motion along the frame guide rail under the drive of the connecting rod to complete the processing of materials in the mold. The fuselage (C-type or closed frame) of the support system provides rigid support for the entire press and bears all working loads. Its working principle is essentially to convert the rotational kinetic energy of the flywheel into the impact force and energy of the linear motion of the slider through the crank-slider mechanism.
Structure and Principle of Hydraulic Presses
Its power system is a hydraulic power unit, which is responsible for providing high-pressure oil. The transmission system is composed of various control valves (directional valves, pressure valves, flow valves) and pipelines, which are used to control the flow direction, pressure and flow rate of the oil. The execution system is the main hydraulic cylinder, and the high-pressure oil pushes the piston in the cylinder to generate a huge linear thrust. Many hydraulic presses are also equipped with an ejection cylinder for demoulding the workpiece after forming. The support system is also composed of a rigid frame consisting of an upper beam, a lower beam and columns.
Its working principle is based on hydrostatics: the high-pressure oil output by the hydraulic pump enters the main cylinder through the control valve, acts on the large area of the piston, and generates a huge thrust according to the formula “pressure = force/area”. The output force can be steplessly adjusted by adjusting the oil pressure, and the stroke position and speed can be precisely controlled by controlling the inlet and outlet volume of the oil.
Maximum Pressing Force
Maximum carbide pressing force is the most basic technical parameter of the press, which directly determines the size range and density level of the pressable products. The pressing force range of industrial-grade cemented carbide presses is widely distributed, from 100kN for laboratory-scale to 1250kN or even higher for large-scale industrial equipment. For special applications such as ultra-high pressure pressing processes, the peak force can reach 1750-2250kN. The selection of pressing force needs to consider the following factors:
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Product Geometric Dimensions: Large-size green compacts or those with a large height-diameter ratio require higher carbide pressing force to ensure sufficient densification of the bottom area; the ratio of green compact height to diameter generally does not exceed 4, otherwise uneven density or delamination is likely to occur due to pressure attenuation.
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Powder Characteristics: The cobalt content, WC particle size distribution and powder fluidity in the WC-Co mixture all affect the required pressing force; the physical properties of the powder (such as hardness and softness), purity, particle size and shape also change the carbide pressing resistance.
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Target Density: The pressing pressure usually needs to reach 100-500MPa to obtain green compacts with sufficient green strength; the optimal unit pressing pressure range for general-grade mixtures is 1.24~2.33T/cm² (about 12.4~23.3MPa), but the actual value needs to be adjusted according to the powder characteristics.
In actual production, higher pressing force is not better. When the pressure exceeds the plastic deformation limit of the material, it is easy to cause excessive crushing of powder particles, forming stress concentration points, which will lead to cracks or delamination defects during demoulding. Therefore, the pressing force needs to be scientifically set according to the specific product characteristics, usually controlled within 70-85% of the material’s yield strength, and pressure fluctuations caused by changes in single weight should be avoided.
Pressing Speed
Pressing speed has a decisive influence on powder flow, air discharge and density distribution. The carbide pressing speed range of industrial-grade cemented carbide presses is 3-15kN/s (some equipment is expressed by punch speed, which can reach 7m/s, but the unit needs to be unified). The control of pressing speed needs to consider the following factors:
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Powder Air Discharge: Excessively high pressing speed (>15kN/s) is likely to cause air between powders to be unable to be discharged in time, forming delamination defects; the pressing principle should be fast first and then slow to ensure sufficient air escape.
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Pressure Transmission: Appropriate pressing speed (8-11kN/s) is conducive to the uniform transmission of pressure in the powder body and reduces uneven density distribution.
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Equipment Response: High-precision pressing requires the press to have fast response capability and be able to precisely control the loading rate; for example, servo presses can achieve stepless speed regulation.
Studies have shown that the optimal pressing speed varies for different plasticizer systems. When using PEG-1500 plasticizer, the optimal carbide pressing speed is 8-10kN/s; when using SCDII plasticizer, it needs to be 9-11kN/s. This difference stems from the change of plasticizers on the flow characteristics of powders, and the optimal parameter combination needs to be determined through process tests. In addition, the pressure holding time (usually adjustable within 0~5 seconds) also affects the density uniformity, and it is necessary to maintain an appropriate time under the maximum pressure to promote air discharge and particle engagement.
Machine Stiffness
Machine stiffness refers to the ability of the press to resist deformation under load, usually expressed in kN/mm. High stiffness is an important prerequisite for ensuring pressing precision, especially when pressing slender products with a large height-diameter ratio. The methods for measuring the stiffness of cemented carbide presses include:
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Room Temperature Copper Swelling Test: Calculate the equipment stiffness by measuring the deformation of the copper sample during pressing.
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Direct Deformation Measurement: Use high-precision displacement sensors to measure the deformation of the frame during carbide pressing; the stiffness value B can be calculated by the formula B=ΔF/ΔHp, where ΔF is the pressure difference, ΔHp is the green compact height difference, and a larger B value indicates better rigidity.
The stiffness value of a typical mechanical press is about 10t/mm (about 100kN/mm), while the new high-stiffness testing machine can reach 310t/mm (about 3100kN/mm). In actual production equipment, the stiffness value is mostly in the range of 1000-1500kN/mm. Insufficient machine stiffness will lead to the following quality problems:
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Actual Pressure Loss: Frame deformation consumes part of the carbide pressing energy, making the actual pressure applied to the powder lower than the set value.
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Uneven Pressure Distribution: Deformation leads to the offset of the relative position between the punch and the cavity, exacerbating the uneven pressure distribution; especially in presses with side pressure, non-verticality or non-levelness will cause delamination and dimensional deviation.
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Deterioration of Dimensional Precision: The elastic deformation of the frame directly affects the height dimensional precision of the green compact; when the rigidity is good, an increase in single weight results in small changes in green compact height and large increases in density, and vice versa.
Modern high-precision presses adopt an optimized crank-rocker-slider mechanism, which significantly improves equipment stiffness through mechanical structure optimization, realizes a smooth mold clamping speed and a stable pressure holding interval, and is particularly conducive to the uniform densification of cemented carbide powders.
Other Key Performance Parameters
In addition to the above core parameters, the following performance indicators also have an important impact on pressing quality:
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Stroke Precision: Including no-load stroke precision and load stroke stability, which affects the control of green compact height; the higher the precision, the better the dimensional consistency of products.
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Repeat Positioning Precision: Determines the dimensional consistency of green compacts in mass production, and is the focus of quality control in the pressing process.
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Pressure Holding Capacity: Some pressing processes need to maintain a certain pressure for a certain time (such as adjustable within 0~5 seconds) to eliminate the influence of elastic aftereffect and improve density uniformity.
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Energy Capacity: Especially under high productivity conditions, the equipment needs to have sufficient available energy to avoid pressing defects caused by insufficient energy.
The selection of presses needs to comprehensively consider product characteristics, process requirements and production efficiency. For conventional insert products, a 250kN-class mechanical press can meet the needs; for large-scale mining tools, rolls and other products, a high-performance press with a pressing force of more than 1000kN is required. The selection principle is to choose equipment with high stiffness, good precision and low energy consumption on the premise of meeting process requirements, and pay attention to the matching between die quality (such as roughness, shrinkage coefficient) and the press.
Die
As a process equipment that directly contacts the powder and endows it with shape, the design quality and manufacturing precision of the die have a decisive influence on the pressing quality. The design of cemented carbide dies needs to comprehensively consider multiple factors such as material characteristics, product shape, demoulding method and production efficiency, which is a complex process of multi-objective optimization. During the pressing process, the total pressure includes net pressure and pressure loss, and pressure loss is the main cause of uneven density distribution of green compacts, which needs to be improved by optimizing the die design.
Cavity Structure and Material Selection
Cemented carbide dies are mainly composed of cavity, punch, core rod and other components, and their structural design needs to meet the following basic requirements:
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Sufficient Strength and Rigidity: The cavity must withstand pressing pressure up to 200-300MPa without deformation.
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Excellent Wear Resistance: Cemented carbide powder has high hardness, which causes severe abrasive wear on the cavity material.
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Good Thermal Stability: Frictional heat generated during the carbide pressing process may cause local temperature rise.
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Precise Dimensional Precision: Directly affects the geometric shape and dimensional tolerance of the green compact.
Common cavity materials include:
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Cemented Carbide: Has excellent wear resistance and is suitable for mass production.
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Tool Steel: Lower cost but limited service life.
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Ceramic Materials: Suitable for products with specific shapes but poor impact resistance.
The surface roughness of the cavity has a significant impact on demoulding resistance and green compact surface quality. It is recommended that the roughness Ra value be controlled at 0.4-0.8μm. Too low roughness will increase powder-wall friction, leading to pressure transmission loss; too high roughness will cause difficult demoulding and poor surface quality. Practice has proved that using a die with a very high cavity wall finish and applying lubricating oil on the cavity wall can reduce the external friction coefficient and improve the density distribution of the green compact. In addition, the larger the height-diameter ratio of the green compact, the greater the density difference. Therefore, the cavity design should minimize the height-diameter ratio to promote density uniformity.
Core Rod and Positioning System
For products with holes, the design quality of the core rod directly affects the hole position precision and inner hole quality. The design points include:
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Rigidity Design: Slender core rods need to have sufficient bending resistance to avoid eccentric deformation during pressing.
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Guide Structure: Set a guide section to ensure the coaxiality between the core rod and the cavity.
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Floating Mechanism: Avoid hole position offset caused by relative movement between the core rod and the powder during pressing.
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Demoulding Method: Adopt forced demoulding or stepped demoulding structure to reduce demoulding resistance.
The coordinated motion control of the core rod and the punch is particularly important for the pressing of complex-shaped products. Modern CNC presses can realize independent motion control of multiple punches, and improve the uniformity of density distribution by optimizing the pressing curve. The uneven density distribution in the green compact can be greatly improved by the double-direction carbide pressing method, which requires integrating corresponding mechanisms in the design of the core rod and the punch.
Die Lubrication System
Die lubrication plays an important role in reducing friction, preventing material sticking and extending die service life. Lubrication methods include:
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Powder Mixing Lubrication: Directly mix solid lubricants such as zinc stearate into the powder.
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Cavity Wall Spraying Lubrication: Spray liquid lubricant into the cavity before each carbide pressing.
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Self-Lubricating Cavity Wall: Prepare a lubricating coating on the cavity surface.
The selection of lubricants needs to consider compatibility with subsequent sintering processes. Sulfur-containing lubricants may cause brittle phases in the final product, so the dosage must be strictly controlled. Usually, the added amount of lubricant is controlled at 0.1-0.5wt%, and excessive amount will leave void defects after sintering. Applying lubricating oil on the cavity wall can reduce the external friction coefficient and improve the density distribution of the green compact, which is the key principle for optimizing the lubrication system.
निष्कर्ष
In summary, as the two core elements of the cemented carbide pressing process, the performance matching and parameter optimization of the press and die are the keys to ensuring the quality of the green compact. The selection of the press needs to accurately match the power type (mechanical, hydraulic, servo) and core parameters (pressing force, pressing speed, stiffness) according to product characteristics, so as to avoid defects such as uneven density and cracks caused by parameter imbalance; the die design should focus on strength, wear resistance, positioning precision and lubrication compatibility, and reduce pressure loss and demoulding resistance through cavity optimization, material adaptation and lubrication system upgrading. In actual production, it is necessary to establish a coordinated matching mechanism of “press parameters – die design – product characteristics”, scientifically set pressing process parameters according to powder characteristics, product geometric dimensions and target performance, and optimize the die structure and lubrication scheme at the same time, so as to achieve stable and controllable green compact quality. In the future, with the popularization and application of intelligent equipment such as servo presses, and the improvement of die design and manufacturing precision, the cemented carbide pressing process will be further promoted towards high efficiency, high precision and intelligence, providing a more solid technical support for the production of high-end cemented carbide products.
