9 Special Processing Technologies in Mold Manufacturing

1. EDM

EDM

(1) Basic principles

Electrical Discharge Machining is a special processing method that uses electro-corrosion generated during pulse discharge between two electrodes immersed in working fluid to remove conductive materials. Also known as electrical erosion machining.

EDM is suitable for the processing of complex parts such as small precision cavities, narrow slits, grooves, corners, etc. When it is difficult for the tool to reach complex surfaces, where deep cutting is required, and where the aspect ratio is particularly high, EDM is better than milling machining. For the processing of high-tech parts, milling the electrode and re-discharging can improve the success rate.

Compared with the expensive tool cost, EDM is more suitable.

In addition, where it is specified that EDM is required, EDM can provide a fire-patterned surface.

With the rapid development of high-speed milling, the development space of EDM has been squeezed. At the same time, high-speed milling has brought greater technological progress to EDM. For example, high-speed milling is used to manufacture electrodes.

Due to the realization of narrow area processing and high-quality surface results, the number of electrode designs is greatly reduced.

In addition, using high-speed milling to manufacture the electrodes can also increase the production efficiency to a new level, and can ensure the high precision of the electrodes, so that the accuracy of EDM is also improved.

If most of the processing of the cavity is completed by high-speed milling, EDM is only used as an auxiliary means to clear the corners and trim, so that the remaining amount is more uniform and less.

(2) Basic equipment: EDM machine tools

(3) Main features

  • Able to process materials and complex shapes that are difficult to cut by ordinary cutting methods.
  • No cutting force during processing.
  • Does not produce defects such as burrs and knife marks.
  • The tool electrode material need not be harder than the workpiece material.
  • Direct use of electrical energy processing for easy automation.
  • After processing, the surface will produce a metamorphic layer, which must be further removed in some applications.
  • The purification of the working fluid and the processing of the smoke pollution generated during processing are troublesome.

EDM has the following characteristics

Can process any high strength, high hardness, high toughness, high brittleness and high purity conductive materials.

No obvious mechanical force during processing and is suitable for processing low-rigidity workpieces and fine structures.

The pulse parameters can be adjusted as required, and rough machining, semi-finishing and finishing can be performed on the same machine.

The pits on the surface after EDM are good for oil storage and noise reduction.

EDM production efficiency is lower than that of cutting.

Part of the energy is dissipated on the tool electrode during the discharge process, resulting in electrode loss and affecting the forming accuracy.

(4) Use range

Molds and parts for machining complex shaped holes and cavities.

Processing all kinds of hard and brittle materials such as cemented carbide and hardened steel.

Processing deep pores, special-shaped holes, deep grooves, narrow slits, cutting flakes, etc.

Machining various tools and measuring tools such as forming tools, templates and thread ring gauges.

Three conditions necessary for EDM

  1. Must use pulsed power
  2. An automatic feed adjustment device must be used to maintain a small discharge gap between the tool electrode and the workpieceelectrode.
  3. Spark discharge must be performed in a liquid medium with a certain insulation strength (10 ~ 107Ω · m).

Not all mold steels can be used for mirror surface EDM. Some mold steels can easily achieve mirror effect, while some mold steels cannot achieve mirror effect anyway. At the same time, the hardness of the mold steel is higher, and the effect of EDM on the mirror surface is better.

Please refer to the table below for various materials and mirror surface processing properties.

various materials and mirror surface processing properties

2. WEDM

WEDM

(1) Basic principles

WEDM (Wire Electrical Discharge Machining) using continuously moving thin metal wires (called electrode wires) as electrodes, the workpiece is subjected to pulse spark discharge to etch metal and cut into shapes, also known as wire cutting.

(2) Basic equipment: WEDM Machine Tool

(3) Main features

In addition to the basic features of EDM, WEDM has some other features:

① It is possible to process any two-dimensional curved surface with a straight line as a generatrix without manufacturing tool electrodes with complicated shapes;

② Can cut narrow slits of about 0.05 mm;

③ Not all surplus materials are processed into waste during processing, which improves the utilization rate of energy and materials.

④ In the low-speed WEDW cutting process where the electrode wire is not recycled, the electrode wire is constantly updated, which is conducive to improving the processing accuracy and reducing the surface roughness.

⑤ The cutting efficiency achieved by WEDM cutting is generally 20 to 60 mm2/mins, up to 300mm2/mins; the processing accuracy is generally ±0.01 to ±0.02mm, up to ± 0.004mm.

The cutting efficiency achieved by WEDM cutting is generally 20-60mm2/min, up to 300mm2/min. The processing accuracy is generally ±0.01 to ±0.02mm, and the maximum can reach ± 0.004mm.

The surface roughness is generally Ra2.5 to 1.25 microns, the highest can reach Ra0.63 microns.

The cutting thickness is generally 40 to 60mm, and the thickest can reach 600mm.

(4) Use range

Mainly used for processing:

Various complex shapes and precise and small workpieces, such as punches, dies, convex and concave dies, fixed plates, discharge plates, forming tools, templates, metal electrodes for EDM, various fine holes, narrow slits, arbitrary curves, and etc.

It has outstanding advantages such as small machining allowance, high machining accuracy, short production cycle, and low manufacturing cost, and has been widely used in production. At present, EDM machine tools at home and abroad have accounted for more than 60% of the total number of electric processing machine tools.

WEDM is a technology to realize the size processing of workpieces. Under certain equipment conditions, reasonable formulation of processing technology route is an important link to ensure the quality of workpiece processing.

The process of WEDM machining of molds or parts can generally be divided into the following steps.

Analysis and review of drawings

Analyzing the drawings is a decisive first step in ensuring the quality of the workpiece and the comprehensive technical indicators of the workpiece.

Taking the punching die as an example, when digesting the patterns, we must first pick out the workpiece patterns that cannot or cannot be easily processed by WEDM.

There are roughly the following:

  • The workpiece has high surface roughness and dimensional accuracy, and cannot be manually ground after cutting.
  • The workpiece with a narrow slit smaller than the diameter of the electrode wire plus the discharge gap, or the workpiece with rounded corners formed by the discharge gap of the electrode plate derrick is not allowed at the corners of the figure.
  • Non-conductive materials.
  • Parts whose thickness exceeds the wire frame span.
  • The processing length of the workpiece exceeds the effective stroke length of the x, y pallet, and requires higher accuracy.

In accordance with the conditions of the wire cutting process, careful consideration should be given to surface roughness, dimensional accuracy, workpiece thickness, workpiece material, size, fit clearance, and stamped part thickness.

Programming considerations

  1. Determination of die clearance and transition circle radius

Reasonably determine the die clearance.

The reasonable selection of the die clearance is one of the key factors related to the life of the die and the burr size of the punched parts. The die clearance of different materials is generally selected in the following range:

Soft punching materials, such as copper, soft aluminum, semi-rigid aluminum, bakelite, red cardboard, mica, etc., the gap between the convex and concave die can be selected from 10% to 15% of the thickness of the blank.

For hard blanking materials, such as iron sheet, steel sheet, silicon steel sheet, etc., the gap between the male and female die can be 15% to 20% of the blanking thickness.

This is the actual empirical data of some wire cutting punching dies, which are smaller than the large gap dies that are popular internationally. Because the surface of the wire cut workpiece has a melted layer with a brittle structure, the larger the processing electrical parameters, the worse the surface roughness of the workpiece, and the thicker the molten layer. With the increase of die punch times, the crisp and loose surface of this layer will be gradually worn away, and the die gap will gradually increase.

Reasonably determine the radius of the transition circle. In order to improve the service life of general cold stamping dies, transition circles should be added to the lines, line-circle, and far intersections, especially at small angle corners. The size of the transition circle can be considered according to the thickness of the blanking material, the mold shape, the required life, and the technical conditions of the punched part. As the punched part becomes thicker, the transition circle can also increase accordingly. Generally can be selected in the range of 0.1 to 0.5㎜.

For the transition circle with thin punching material, small die fit clearance and no increase of punching part is allowed, in order to obtain a good convex and concave die matching clearance, a transition circle is generally added at the corner of the figure. Because the electrode wire processing trajectory will naturally process a transition circle with a radius equal to the electrode wire radius plus a single-sided discharge gap at the inner corner.

  1. Calculatingand write processing programs

When programming, you should choose a reasonable clamping position according to the condition of the ingredients, and determine a reasonable starting point and cutting route.

The cut-off point should be taken at the corner of the figure, or at a place where the convex tip can be easily removed.

The cutting route is mainly based on the principle of preventing or reducing mold deformation. Generally, it should be considered to make the figure close to the side of the clamping, and the final cutting is easy.

  1. Procedure tape and proofing tape for processing

After making the paper tape according to the program sheet, be sure to check the program sheet and the prepared paper tape one by one. Then use the calibrated paper tape to input the program into the controller before trying to cut the template, and the simple and reliable workpiece can be directly processed. For molds with high dimensional accuracy requirements and small clearances between convex and concave dies, thin materials must be used for trial cutting. The precision and clearance can be checked from the trial cut parts. If it is found that it does not meet the requirements, it should be analyzed in time to find out the problem, and the procedure can not be formally processed until it is qualified. This step is an important part to avoid scrapping the workpiece.

According to the actual situation, you can also input directly from the keyboard, or directly transfer the program to the controller from the programming machine.

3. Electro Chemical Machining

Electro Chemical Machining

(1) Basic principles

Based on the principle of anode dissolution in the electrolytic process and with the help of the formed cathode, a process method for processing a workpiece into a certain shape and size is called electro chemical machining.

(2) Use range

Electro chemical machining has significant advantages for the machining of difficult-to-machine materials, complex shapes, or thin-walled parts. It has been widely used, such as machining gun barrel rifling, blades, integral impellers, molds, shaped holes and shaped parts, chamfering, deburring and etc. And in the processing of many parts, electro chemical machining technology has occupied an important or even irreplaceable position.

(3) Advantages

It has wide processing range. Eelectro chemical machining can process almost all conductive materials, and it is not limited by the mechanical and physical properties of the material such as strength, hardness, and toughness.

The metallographic structure of the material after processing basically does not change. It is often used to process hard-to-machine materials such as hard alloys, superalloys, hardened steel, stainless steel and etc.

(4) Limitations

The processing accuracy and processing stability are not high; the processing cost is higher, and the smaller the batch size, the higher the additional cost of a single piece.

4. Laser processing

Laser processing

(1) Basic principles

Laser processing is to use the energy of light to focus on the lens to achieve a very high energy density after focusing, and the material will be melted or gasified in a very short time and is etched away to achieve processing.

(2) Main features

Laser processing technology has the advantages of less material waste, obvious cost effects in large-scale production, and strong adaptability to processing objects. In Europe, laser technology is basically used for welding of special materials such as high-end car shell and base, aircraft wing and spacecraft fuselage.

(3) Use range

Laser processing is the most commonly used application for laser systems.

The main technologies include: Laser welding, laser cutting, surface modification, laser marking, laser drilling, micromachining and photochemical deposition, stereolithography, laser etching, and etc.

5. Electron beam machining

(1) Basic principles

Electron beam machining (EBM) is the processing of materials by using the thermal effect or ionization effect of high-energy convergent electron beam.

(2) Main features

This kind of machining method has the features of high energy density, strong penetration ability, wide penetration range, large weld width ratio, fast welding speed, small heat affected zone, and small work deformation.

(3) Use range

Electron beam processing has a wide range of materials, and the processing area can be extremely small. Processing accuracy can reach nanometer level to realize molecular or atomic processing.

Its productivity is high, the pollution caused by processing is small, but the cost of processing equipment is high. It can process micro holes, slits, etc., can also be used for welding and fine lithography.

Vacuum electron beam welding axle housing technology is the main application of electron beam processing in the automotive industry.

6. Ion Beam Machining

Ion Beam Machining

(1) Basic principles

Ion beam machining is to achieve the processing by accelerating and focusing the ion current generated by the ion source on the surface of the workpiece under vacuum.

(2) Main features

Because the ion current density and ion energy can be accurately controlled, the processing effect can be precisely controlled to achieve ultra-precision processing at the nanometer level, even at the molecular and atomic level.

During ion beam processing, the pollution generated is small, the processing stress and deformation are extremely small, and the adaptability to the material to be processed is strong, but the processing cost is high.

(3) Use range

Ion beam machining can be divided into two types according to its purpose: etching and coating

1) Etching

Ion etching is used to process the air bearing of the gyroscope and the grooves on the dynamic pressure motors, with high resolution, good accuracy and repeatability. Another aspect of ion beam etching applications is the etching of high-precision patterns, for example, electronic components such as integrated circuits, optoelectronic devices and optical integrated devices. Ion beam etching is also used in thinning materials to make the coupon of the transmission electron microscope.

2) Ion Beam Coating

There are two forms of ion beam coating processing: sputtering deposition and ion plating. Ion plating can be plated with a wide range of materials.

Metal or non-metallic films can be plated on metal and non-metallic surfaces. Various alloys, compounds, or certain synthetic materials, semiconductor materials, and high-melting materials can also be plated.

Ion beam coating technology can be used to plate lubricating films, heat-resistant films, wear-resistant films, decorative films, electrical films and etc.

7. Plasma arc machining

Plasma arc machining

(1) Basic principles

Plasma arc machining is a special processing method that uses the thermal energy of a plasma arc to cut, weld and spray metal or non-metals.

(2) Main features

1) Microbeam plasma arc welding can weld foils and sheets.

2) It has a small hole effect, which can better realize single-sided welding and double-sided free forming.

3) Plasma arc has high energy density, high arc column temperature, and strong penetrating ability. 10 to 12mm thickness steel can be welded through both sides without opening groove. Plasma arc machining has fast welding speed, high productivity and small stress and deformation.

4) The equipment is complicated and the gas consumption is large, so it is only suitable for indoor welding.

(3) Use range

It is widely used in industrial production, especially the welding of copper and copper alloys, titanium and titanium alloys, alloy steel, stainless steel, molybdenum and other metals, such as missile shells of titanium alloys, and some thin-walled containers on aircrafts used in military and cutting-edge industrial technology such as aerospace.

8. Ultrasonic machining

Ultrasonic machining

(1) Basic principles

Ultrasonic machining is a special machining which uses ultrasonic frequency as a tool for small amplitude vibration, and through the hammering effect of the abrasive free from the liquid between it and the workpiece on the machined surface, the surface of the workpiece material is gradually broken. The abbreviation is USM.

Ultrasonic machining is commonly used for perforation, cutting, welding, nesting and polishing.

(2) Main features

It can process any material, especially suitable for processing all kinds of hard and brittle non-conductive materials.

The machining accuracy of the workpiece is high, the surface quality is good, but the productivity is low.

(3) Use range

Ultrasonic processing is mainly used for various hard and brittle materials, such as punching (including round holes, special-shaped holes and curved holes, etc.), cutting, grooving, nesting, engraving, deburring of batches of small parts, mold surface polishing, grinding wheel dressing and other aspects in glass, quartz, ceramics, silicon, germanium, ferrite, gems and jade.

9. Chemical Etching

Chemical Etching

(1) Basic principles

Chemical Etching is a special processing that uses the acid, alkali or salt solution to corrode and dissolve the workpiece material to obtain the workpiece with the desired shape, size or surface state.

(2) Main features

1) Can process any metal material that can be cut without being restricted by properties such as hardness and strength.

2) Suitable for large-scale processing, and can process multiple pieces at the same time.

3) No stress, crack or burr is generated, and the surface roughness reaches Ra1.25 to 2.5μm.

4) Easy operation.

5) Not suitable for narrow slots and holes.

6) Not suitable for eliminating the defects such as uneven surfaces and scratches.

(3) Use range

It’s suitable for large area thickness reduction processing and machining complex holes in thin-walled parts.

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