Design of the Double-Action Drawing Die for Woks
The design of double-action drawing die for frying pan is a key technology in the manufacturing of metal cookware. It is suitable for deep drawing of frying pans made of stainless steel, aluminum alloy and other materials. Its double-action structure can realize the synchronous control of edge pressing and drawing, and effectively solve the wrinkling and cracking problems during the deep drawing of thin materials. As a typical rotating body part, frying pan usually has the characteristics of large diameter and shallow drawing (such as diameter 300-360mm, depth 50-80mm), so the die design needs to focus on controlling the uniformity of material flow during the deep drawing process. In the initial design, the drawing coefficient needs to be calculated based on the thickness of the frying pan material (usually 0.8-1.2mm for stainless steel) and yield strength (such as 304 stainless steel σs≈205MPa). The first drawing coefficient is generally 0.55-0.65 to ensure that the plastic deformation of the material is within a safe range. At the same time, the parameters of the double-action press need to be determined. The outer slider controls the edge pressing force, and the inner slider provides the drawing force. The strokes of the two need to match, and the stroke of the outer slider is 10-15mm larger than that of the inner slider.
The mold structure consists of a punch, a die, a blank holder, and an outer slide drive mechanism. The punch is made of ductile iron (QT600-3) or alloy cast iron, and the surface is polished to Ra0.4μm after quenching (hardness HRC45-50) to reduce friction resistance during deep drawing. The die is made of Cr12MoV steel, and the hardness reaches HRC60-62 after deep cold treatment. The radius of the edge fillet is 5-8 times the material thickness (such as 0.8mm stainless steel, the fillet radius is 5mm) to promote the flow of material into the die. The gap between the blank holder and the die is 1.1-1.2 times the material thickness to ensure uniform edge holding. An annular lubrication groove is set on the surface of the blank holder. The groove is 3mm wide, 1mm deep, and 10mm apart. It can store lubricating oil and reduce the friction coefficient. The outer slider driving mechanism is connected to the blank holder through a connecting rod. The blank holder force can be changed by adjusting the length of the connecting rod. The blank holder force needs to be calculated based on the material and is usually 30% – 50% of the drawing force.
The design of lubrication and cooling systems has a significant impact on the drawing quality. Special lubricants are required during the deep drawing process. Chlorinated paraffin-based grease is commonly used for stainless steel deep drawing, while refined mineral oil can be used for aluminum alloys. Apply it on the contact surface between the blank and the die with a thickness of 0.05 – 0.1 mm to form an effective lubricating film. The cooling system is set inside the die, using an annular water channel with a diameter of 8mm and a distance of 15-20mm from the die working surface. After water is passed through, the die temperature can be controlled below 50°C to avoid lubricant failure due to frictional heat. For automated production lines, an automatic oil spraying device can be designed to evenly spray lubricant on the blank before it is fed into the mold. The spraying amount is controlled by a flow meter to ensure that the amount used per square meter of blank is 50-80ml.
The mold positioning and guiding accuracy requirements are strict. The blank positioning adopts a combination of positioning pins and positioning rings. The inner diameter of the positioning ring is 2-3mm larger than the blank diameter. The positioning pins are evenly distributed on the inner side of the positioning ring (3-4) to ensure that the center of the blank coincides with the mold axis and the eccentricity is controlled within 0.5mm. The guiding mechanism adopts a double guide column structure with a guide column diameter of 30-40mm. The guide sleeve is made of bronze with a built-in graphite lubrication sleeve and a matching clearance of 0.02-0.04mm to ensure that the coaxiality error of the punch and the die does not exceed 0.03mm. For large frying pan molds, an auxiliary guide must be set between the outer slider and the blank holder, and a sliding bearing guide is used to ensure that the blank holder moves smoothly and the blank holder force is evenly distributed.
The optimization of drawing process parameters and mold debugging need to be combined with simulation analysis. The drawing process is simulated by finite element software (such as Dynaform) to analyze the material thickness change rate. When the local thinning rate exceeds 20%, the blank holder force distribution needs to be adjusted or the die radius needs to be increased. When testing the mold, a small batch trial is first carried out to measure the diameter, depth and surface quality of the wok mouth. The mouth diameter tolerance is controlled at ±0.5mm, and the depth error does not exceed ±1mm. The blank holder force is debugged using a step-by-step adjustment method, gradually increasing from low to high until wrinkles are eliminated and there is no cracking. The blank holder force of 304 stainless steel woks is usually 15-20kN. In terms of mold maintenance, the oil and metal debris on the surface of the die need to be cleaned every 500 pieces produced, and the wear of the edge radius needs to be checked. When the radius of the fillet increases by more than 0.3mm, it needs to be re-grinded. The surface of the punch is polished every 1,000 pieces of work to maintain a surface roughness of Ra0.2μm to reduce scratches during drawing.
