Design Of Round Tube Shrinkage And Expansion Molds

Design of round tube shrinking and expanding die
The design of round tube shrinking and expanding die is a special technology for shrinking (shrinking) or expanding (expanding) the ends of metal round tubes (such as air-conditioning copper tubes and automobile oil pipes). It can achieve a tube diameter change rate of 10%-30%, a roundness error of <0.1mm after shrinking, and a taper accuracy of ±0.5° for expanding. It is suitable for copper, aluminum, and low-carbon steel tubes with a diameter of 5-50mm and a thickness of 0.5-3mm. The core is to guide the axial flow of the material through the conical working surface of the die. The material shrinks inward when shrinking and expands outward when expanding. The stress distribution in the deformation zone needs to be controlled (not exceeding 90% of the yield strength of the material). The die taper angle is determined according to the tube diameter change rate during design: the shrinking taper angle is 15°-30° (small angle for large diameter change), and the expanding taper angle is 30°-60° (60° standard cone for quick connection).

The mold structure consists of a conical core (shrinking) or a conical die (expansion), a positioning sleeve, a clamping mechanism and a thrust device. The shrinking core is made of Cr12MoV steel (HRC58-62), the working surface is chrome-plated (thickness 0.01mm), the cone surface roughness Ra0.05μm, and the core length is 2-3 times the tube diameter (40-60mm for φ20mm tube) to ensure uniform deformation of the material. The expanding die is made of cemented carbide (WC-Co 90%), the inner cone surface is honed (roundness <0.005mm), and the contact length with the tube end is 5-8 times the wall thickness (5-8mm for 1mm thickness) to prevent cracking of the expanding edge. The inner diameter of the positioning sleeve is 0.1-0.2mm larger than the outer diameter of the tube, and the length is 1.5 times the tube diameter to ensure the coaxiality of the tube axis and the mold cone surface (error <0.05mm).

The clamping and thrust parameters must be precisely matched. The clamping mechanism uses a three-jaw chuck or hydraulic clamp. The clamping force is 1.2 times the material yield strength × tube circumference × wall thickness (φ15mm×1mm copper tube is about 3-5kN) to prevent the tube from slipping (axial displacement <0.1mm). The thrust device (hydraulic cylinder or servo motor) provides axial thrust: shrinking thrust F=πtσs (Dd) cotα (t is wall thickness, σs is yield strength, D/d is the diameter before and after shrinking, α is the semi-cone angle), the thrust of the aluminum tube shrinking from φ20mm to φ16mm is about 8-10kN; the expansion thrust is 1.2-1.5 times the shrinking thrust to ensure sufficient plastic deformation of the material.

The process parameters and lubrication are strictly controlled, the forming speed is 50-100mm/min, the copper and aluminum tubes are at the high speed end, and the steel tubes are at the low speed end to avoid work hardening. Special metal forming oil (viscosity 20-40cSt) is used for lubrication to form an oil film (thickness 2-5μm) on the working surface of the mold. Graphite grease (solid content 5%-8%) can be used for copper tube expansion to improve the lubrication effect. For pipes with a deformation rate of >20%, multiple forming passes are required: the first shrinkage/expansion to the intermediate size (such as the first completion of 15% when the target deformation rate is 30%), annealing treatment (copper tube 300-350℃/1h) and then secondary forming to reduce the risk of cracking.

Quality inspection and mold maintenance standards require measuring tube diameter (deviation must be <0.05mm) and roundness every 10 pieces during mold trials. If ovality exceeds tolerance after necking, adjust the clamping force uniformity (three-jaw pressure difference <5%). If cracking occurs at the flared edge, increase the die radius (by 0.1mm at a time) or reduce the deformation rate. After every 1,000 pieces, inspect the core and die taper for wear (reground if profile error >0.03mm). Replace the locating sleeve if wear exceeds 0.1mm. During maintenance, clean the mold surface of oxide scale daily, and relubricate the clamping mechanism with lithium-based grease weekly to ensure smooth movement.