The high precision press brake toolings occupy a very important position and function in the sheet metal processing system.
However, due to the differences in equipment, materials and technology of various manufacturers in the process of manufacturing bending die, the life and the precision gap of bending die are greatly caused.
Therefore, this article makes a brief analysis and expatiation on some related content in the production process of bending die, and hopes to give some reference to sheet metal users when choosing bending molds.
Sheet metal processing industry is an important support force in the machinery manufacturing industry.
The bending process is an indispensable process in sheet metal fabrication, and the daily use frequency is relatively high.
However, in order to effectively improve the quality and production efficiency of the parts, it is very important to choose a good bending die.
The main content of this article is how to realize the good quality of bending die from production.
The Status of Press Brake Toolings
At present, there are several difficulties for sheet metal users in the demand of press brake toolings:
(1) There is a certain gap compared to the bending die made in Euro, for example, the mold brand is uneven, the mold quality is not stable, the precision is not good.
(2) Using imported bending die, the price is high and the delivery time is long.
(3) If the demand is non-standard mold, the communication channel with the mold manufacturer is less, and the timeliness is poor;
(4) The industry did not implement a set of mold standards, leading to the manufacturers of the mold cannot be exchanged. In turn, the user of sheet metal have no choices when using the mold. They can only purchase the bending from the press brake manufacturers.
Core Elements of Mold Quality
The press brake bending die material should choose 42CrMo produced by regular steel enterprises.
Compared with other materials, 42CrMo materials have excellent mechanical properties.
It has the characteristics of high strength and hardenability, good toughness, small deformation during quenching, high creep strength and persistent strength at high temperature.
The bending die materials also have T7A and T8A on the market.
Although these materials can be treated by heat treatment to a certain surface hardness, due to the shallow depth of the quenching layer, the tool core is soft and easily deformed.
Due to price constraints, many of the bending die materials are relatively inexpensive materials.
In order to obtain qualified and uniform hardness, the process of heat treatment is very important.
The hardness of the bending die is 47±2HRC, which means that the hardness of any position of the mold should be between 45~49HRC. And the depth of the hardness layer is over 10mm.
The approximate process of heat treatment is:
First weld the hoisting ring on the end face of the blank mold after rough milling, then put it into the heating furnace up to 880℃. When the whole mold reach the temperature→discharge→quenching oil→cooling→check deformation→cold correction→tempering→hardness testing.
The tempering time is different according to the batch.
Complete the above steps and basically finish the heat treatment of rough milling blank mold.
The above steps have several key points:
- Suspend heating and suspend quenching oil cooling on end welding rings.
Due to the shape of the bending die is irregular geometry, when it is cooled, the cooling is uneven and vary speed, which will resulting in large deformation and uncontrollable.
Therefore, only the suspended combustion mode and suspended cooling can effectively minimize the deformation of the mold.
In the process of heating and cooling, the hoisting ring is welded together with bending die before put into the heating furnace.
If the mold is overweight, it is easy to cause the fracture between hoisting ring and mold under the temperature of 880 ℃,.
Due to the heating restriction of heat treatment, the bending die factory insists on making the press brake bending die standard length 835mm.
If the mold has defective in heat treatment, the service life of the die will be greatly reduced.
- Cold correction.
After the product comes out of the oil furnace, due to the irregular shape of the mold itself, the deformation amount is not controllable.
Therefore, after the mold is out of the oil furnace, each product needs to detect deformation.
If the deformation is too large, cold correction is required.
Many manufacturers choose to use the old fashioned method of flame gun burning to rectify the deformation.
The heavy burning position will cause local softening, and there are safety hazards in daily use.
The following service life of the bending die should be achieved after strict heat treatment process control:
When bending the stainless steel plate with the thickness of 2.0mm (including 2.0mm) and the cold plate under 3.0mm (including 3.0mm), adopting the lower die with V opening 6-8 times the thickness, The R angle of the bending edge can withstand the frequent bending of 2000000 times (convert into years, calculated based on the bending die can be used for 200,000 times within a year, and can be used for 10 years) and the abrasion of the cutting edge within 0.03mm.
Is the low price really the cost-effective?
If calculated based on the number of use, you will see that it’s not like that actually.
Only professional heat treatment technique and qualified material can ensure the bending die durable.
Precision CNC forming grinding
The mold material and heat treatment mentioned in the first two points are mainly concerned with the life of the bending die.
Then the processing method of the cutting edge and V groove are the key process to ensure the precision of the die.
There are the traditional grinding method and CNC Precision Grinding.
In the whole manufacturing process The traditional grinding method, most is human intervention; however, for CNC grinding, the size formation and precision of the products are controlled by the equipment and CNC controller throughout the whole grinding process.
Therefore, the consistency and stability of the molds are fundamentally different.
It is also the difference between domestic and foreign manufacturers in the technique causing the bending die has no industry standard. Each manufacturer produces the mold with the size of their own.
If there is a uniform size standard, sheet metal users can be more independent when choosing the bending die manufacturer, without the need to accommodate a particular mold or manufacturer.
Currently, the most popular length of press brane bending die is 835mm, 500mm and 515mm.
Under the premise that the accuracy can be guaranteed, the light weight mold can improve the production efficiency and reduce the labor intensity of the production personnel.
Fig.1 Commonly used top punch.
Fig.2 Different V slot ratios for the lower die.
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