Currently, the use of high-speed automatic stamping lines is in the stage of popularization. However, due to costs, many enterprises still rely on more traditional stamping equipment. Although they are equipped with automation facilities such as manipulators and end pickups, which reduce personnel investment, the improvement in production efficiency is not significant.
This post mainly focuses on improving the efficiency of automatic stamping lines by addressing three key elements: the press, die, and manipulator.
As the demand for automobiles in the market increases, the demand for stamping parts is also on the rise. However, production efficiency is limited by factors such as preparation before production, lengthy die commissioning times, uncoordinated operation between the manipulator and press, and structural defects in the end pickup.
Therefore, it is essential to shorten mold change times, optimize the action matching between the press and manipulator, and improve the end pickup’s structure.
The following example examines a stamping production line made up of a traditional mechanical press and a six-axis manipulator with manual die change.
Shorten die change time
The die change process of the press includes:
① Workbench opening → ② ejector rod replacement → ③ mold positioning and installation → ④ lower mold clamping → ⑤ workbench opening → ⑥ sliding block descending and fine tuning to the lower dead center → ⑦ upper mold clamping → ⑧ adjustment of production parameters → ⑨ pressure test and re-inspection clamping → ⑩ formal production.
After analyzing the process of die change, it can be concluded that the procedures taking longer times are mainly ②, ③, ⑥ and ⑧.
To address process ②, most stamping plants purchase outsourced molds for production. In order to simplify production and use, the ejector rod and stretching pad are designed with a split structure. However, the ejector rod needs to be installed at a specific position, and each replacement takes a lot of time. If the stamping dies used in the workshop are self-developed, the ejector rod and stretching pad can be integrated or the ejector rod can be inserted on the worktable, which can significantly reduce the time required to replace the ejector rod and avoid air trapped in the die.
To address process ③, two positioning structures (V-shaped + plane positioning or 2 V-shaped positioning) can be added to the same side of the die body. This facilitates the positioning process during die installation by allowing it to be done simply with a push.
Similar structures are shown in Fig. 1.
Fig. 1 Schematic diagram of mold positioning structure
When using this structure, it is essential to ensure that the positioning pin on the equipment’s working table has a corresponding pin hole.
If there is no pin hole on the table, the table can be modified to include one.
Regarding issues with processes ⑥ and ⑧, compiling an on-site process card can specify the required parameters (such as mold mounting height, closing height, balancer air pressure, air cushion pressure, etc.), along with operation specifications. All production parameters can be monitored, observed, and adjusted in a timely manner.
Optimization of press, manipulator and end pickup
The movement of the press and manipulator is driven by the signal output from the proximity switch installed on the die. However, it is comprehensively affected by various factors, such as the die structure, press structure, PLC program, end pickup structure, manipulator deflection angle, and running track.
To enhance production efficiency, we need to focus on the following aspects.
Increase the safety distance between upper and lower molds
The safe distance between the upper and lower molds is the distance between the process part lifting from the mold surface to the lowest point of the upper mold and the position where the lower mold can be translated, as shown in Figure 2.
Fig. 2 Schematic diagram of safety distance between upper and lower molds
Increasing the safety distance is beneficial for the manipulator to enter the die in advance and grasp the parts.
Typically, the stroke of the slider is restricted by the equipment structure, so optimizing the die structure is necessary for achieving this.
For instance, consider the turnover die in Fig. 2, where there is an inserting knife on the f surface of the upper die to operate the lower die pulley mechanism.
To enhance the safety distance, relocating the inserting knife position to the side and away from the movement track of the manipulator can be considered.
Height of model surface under unified full order
When the manipulator moves at maximum speed, the height of the lower surface of the model between each process remains essentially the same. This can help to reduce the time required to handle parts between adjacent processes.
Typically, during mold development, it is advisable to minimize the height of the lower mold.
For mass production molds, if the height difference between the lower mold surface in different processes is too large, a base plate can be added to the lower mold.
Reduce the change of stamping angle between processes
The variation in stamping angle has a direct impact on the manipulator trajectory.
In general, when the stamping angle between two consecutive work sequences is smaller, the coordinated action of each axis becomes less, resulting in smoother operation of the manipulator during the picking and feeding process.
In case of left-right symmetrical parts, if there is a significant change in the stamping angle between processes, it is advisable to consider the clamping development of left and right parts.
Optimize the structure of end pickup
To facilitate the movement of the end pick-up between the molds, it is important to ensure that the end pick-up structure is capable of grasping the parts. To achieve this, the main rod should be positioned as close to the parts as possible, and the included angle formed by the auxiliary rods on both sides of the main rod should be as wide as possible.
To ensure the reliable grasping of parts, it may be necessary to increase the number of suction cups and reinforcing rods appropriately.
Reduce clutch start waiting time
In order to prevent collision between the sliding block and the manipulator during descent, the feeding signal in the press’s PLC program is typically transmitted to the press in the die to promote a delay of 500 to 1000 milliseconds before the clutch is engaged.
The delay time is gradually reduced through on-site experimentation.
Adjust equipment safety parameters
The manipulator’s “enter” and “exit” controls in the die are linked to the equipment parameters known as the “blanking angle” and “die protection angle.”
When the press slider reaches the “blanking angle,” it sends a signal to the manipulator to enter the die for the blanking operation. If the slider moves from the top dead center to the “die protection angle” but the manipulator has not exited the die, the press clutch will force an emergency brake.
Based on field observations, the press and manipulator can be operated in advance, and the single operation cycle can be shortened by gradually reducing the “blanking angle” and increasing the “die protection angle.”
To ensure the safety of the manipulator automatic line, anti-collision safety protection procedures will be implemented when multiple manipulators operate within the system coordinates. These procedures will modify the manipulator’s operations to allow it to run in advance.
Estimation of production beat of the automatic production line
(1) Calculate the displacement of the slider, Y = X3 – (X1 – X2), with X1 being the safe distance between molds, x2 being the distance from the highest point of the end pickup to the workpiece, and X3 being the slider stroke. The lower dead center of the slider is taken as the reference value of 0 when the end pickup does not interfere with the lower mold and can be moved out of the mold.
(2) Based on the calculated Y value, draw a straight line on the running curve of each equipment slider to intersect the curve, and obtain the values W1 and W2 on the horizontal axis. These values can be used as the reference values of “blanking angle” and “mold protection angle,” respectively.
(3) Calculate the difference U between W2 and W1 of each equipment. Take the same multiple V to divide the U value.
Using the multiple V value, divide the horizontal axis of the sliding block operation curve of each equipment equally, as shown in Figure 3 and Figure 4.
Fig. 3 Stroke curve of slider of 2250t press
Fig. 4 Stroke curve of the sliding block of 1000t press
(4) Draw the schematic diagram of manipulator operation, as shown in Fig. 5.
Fig. 5 Operation diagram of the manipulator
(5) Set the corresponding values according to the layout of the stamping production line and equipment parameters, and use the same time axis to synchronize the actions of the press and the manipulator.
For instance, if the stamping line consists of four presses (2250t + 1000t + 1000t + 1000t) and is equipped with a six-axis manipulator that has a maximum operating speed of 7m/s. The press spacing is 7m, and the maximum number of strokes of the automatic line slider is 10spm.
Assuming the operating time of both press and manipulator in each equally divided interval is about 0.5s (i.e., 0.5s for each displacement of I~XII; 0.5s for each displacement of KL and Mn), set the states of Fig. 3, FIG. 4, and Fig. 5 for analysis.
Then, the single-piece man-hour equals the time of one press stroke plus the waiting time of the press at the top dead center, which is 12 × 0.5 + (6-2) × 0.5 = 8 seconds/piece.
The production beat is 60 ÷ 8 = 7.5 pieces/minute.
① The first equipment in the stamping production line is usually a multi-link press that runs slowly during forming. As the production line is in a linkage state, the time taken for one cycle of actual production and operation of the first equipment is considered the production man-hour of a single piece.
② The waiting time of the press at the top dead center refers to the time when the press stops at the top dead center, and the upper feeding robot loads materials into the mold before the press starts to move again.
By enhancing the ejector rod structure, positioning mode, and standardizing the installation process, the efficiency of die installation and commissioning can increase by at least 35%.
By optimizing the end pickup structure, reducing the waiting time for clutch start, and adjusting the safety parameters of the equipment, the production capacity can increase by approximately 5%.
During the mold development phase, increasing the safe distance between molds, unifying the height of the lower mold surface, and reducing the change of stamping angle between processes can increase production capacity by around 15%.
In order to pursue higher production efficiency, the industry is also setting forth higher standards for molds and equipment. This includes selecting high-end presses, increasing the slider stroke and punching times of the press, shortening the layout between presses, implementing rapid die change systems, adjusting the traditional 4-station layout to a 5-station layout, using 7-axis manipulators, and equipping with more advanced end pickups.
With the rapid development of the economy, more advanced technologies will be applied in actual production to collectively promote the progress of the stamping industry.