Center of pressure calculation
Calculating the pressure center is a crucial step in mold design and press selection. The pressure center is the point where all forces acting on the mold during operation apply together. Its location must coincide with the center of the press slide. Otherwise, unbalanced loading can occur, leading to increased mold wear, deformation of the press guide rails, and workpiece precision errors. Accurately calculating the pressure center ensures uniform force distribution on the mold, extending mold life (typically by more than 30%), while also ensuring smooth press operation and reducing energy consumption. The pressure center calculation should be determined using either geometric or analytical methods, tailored to the mold’s stamping process, workpiece geometry, and cutting edge layout.
For simple, symmetrical stamping parts (such as circles or rectangles), the pressure center coincides with the workpiece’s geometric center, which can be determined intuitively. For example, for a circular blanking part with a diameter of 100mm, the pressure center is at the center of the circle; for a rectangular punching part with a side length of 100mm x 50mm, the pressure center is at the intersection of the two diagonals. In this case, the die cutting edge layout needs to be symmetrical around the pressure center to ensure even distribution of blanking force across all parts. For example, for two symmetrically arranged φ10mm holes (50mm center-to-center), the pressure center is at the midpoint of the line connecting the hole centers. Calculations only require confirming that the blanking forces on both holes are equal (due to their identical dimensions). The point of action of the resultant force is the midpoint.
The pressure center of complex stamping parts must be calculated analytically. The steps are: decompose the workpiece into simple geometric units (such as rectangles, circles, and triangles), determine the area (or blanking force) and center of gravity coordinates of each unit, and then calculate the total pressure center coordinates based on the principle of force composition. The calculation formula is: X0 = (F1X1 + F2X2 + … + FnXn)/(F1 + F2 + … + Fn), Y0 = (F1Y1 + F2Y2 + … + FnYn)/(F1 + F2 + … + Fn), where Fi is the blanking force of the i-th unit (proportional to the area), and Xi and Yi are the coordinates of the center of gravity of the i-th unit. For example, an irregular part is decomposed into A (rectangle, area 1000mm², center of gravity (20,30)) and B (triangle, area 500mm², center of gravity (50,40)). Then X0=(1000×20 + 500×50)/(1000+500)=(20000+25000)/1500=45000/1500=30, Y0=(1000×30 + 500×40)/1500=(30000+20000)/1500=50000/1500≈33.3, and the pressure center coordinates are (30,33.3).
The pressure center of a multi-process die requires the combined forces of each process. Different processes (such as blanking, bending, and drawing) have different force values, so the forces and points of application for each process must be calculated separately before combining the total pressure center. For example, a progressive die includes two processes: blanking (force F1 = 100 kN, centered at (50, 50)) and punching (force F2 = 30 kN, centered at (80, 60)). The total pressure center is X0 = (100 × 50 + 30 × 80) / (100 + 30) = (5000 + 2400) / 130 ≈ 7400 / 130 ≈ 56.9, and Y0 = (100 × 50 + 30 × 60) / 130 = (5000 + 1800) / 130 ≈ 6800 / 130 ≈ 52.3. The pressure center of multi-process molds is allowed to have a certain deviation (usually ≤10mm), but it must be controlled within the guide range of the press slide to avoid excessive eccentric loading.
The pressure center of a curved cutting edge is calculated using a segmented method. The curve is divided into several small segments, and the pressure center of each segment is approximated as a straight line segment. The pressure center is then calculated to form the total pressure center. For example, a quarter-circular cutting edge with a radius of 50mm can be divided into four straight lines, with the center of gravity of each segment located at the midpoint of the segment. The force value is proportional to the segment length. After calculating the coordinates of each segment, the total pressure center is substituted into the synthesis formula. The deviation from the theoretical center of the circle is typically ≤1mm. For complex curves, CAD software can be used to assist in the calculation. The cutting edge outline can be drawn in the software, and the “Query” function can be used to directly obtain the pressure center coordinates with an accuracy of ±0.1mm, significantly improving calculation efficiency.
Adjusting the pressure center is a crucial step in mold design. When the calculated pressure center deviates significantly from the press center (exceeding 10mm), correction is necessary by adjusting the cutting edge layout or installing a balancing block. When adjusting the cutting edge layout, the pressure center can be offset by repositioning some cutting edges (e.g., symmetrically shifting the orifice). The amount of each shift is calculated based on the deviation. When installing a balancing block, auxiliary cutting edges (such as a blank cutting edge) are added to the non-working area of the mold to generate a counterforce to balance the offset load. The balancing block’s force must equal the offset force (Foffset = Ftotal × e/L, where e is the offset distance and L is the balancing block’s lever arm). For example, if the total pressure is 100kN and the offset is 15mm, and a balancing block is placed 100mm from the center, the required balancing force, Fflat, is 100 × 15/100 = 15kN. The balancing block’s cutting edge length is determined by calculation.
In actual production, the pressure center needs to be verified through trial molds. When installing the mold, use a dial indicator to check the levelness of the upper mold base. If the pressure center deviation is too large, the levelness will be out of tolerance (>0.05mm/100mm). Observe the forming quality of the workpiece during mold trial. If the burr on one side is too large or the mold is worn on one side, it means that the pressure center is offset and needs to be recalculated and adjusted. For large molds, pressure sensors can be used to measure the force distribution at each point. The actual pressure center can be determined through data analysis, and then compared with the calculated value and corrected to ensure that the mold is evenly stressed. By accurately calculating and adjusting the pressure center, the service life of the mold can be extended by 30%-50%, the energy consumption of the press can be reduced by 10%-15%, and the economic efficiency of production can be significantly improved.
