The purpose of a plug mandrel is to

The purpose of a plug mandrel is to prevent the tube from flattening and to bend without wrinkles or kinks. Basicprimary tooling consists of a bend die, clamp die, and mandrel.8.5.6.1 Balanced Pressure FormingThe mandrel is held in a fixed position while the tube is pulled over it. The tube stretching process is localizedon the outer radius of the bend and the material is work-hardened to retain its shape and not flatten.The material stretching is done on the forward tip of the mandrel. This force, acting on the mandreltip, supports the inner radius of the bend, holding it firmly into the bend die groove. A plug mandrel can beused to produce relatively good quality bends for tubing 3/8. diameter and smaller. Exceptions to this arethin wall tubing or a centerline radius that is less than 2 x tube O.D. There also are certain limitations fortubes larger than 3/8. diameter.Figure 8-9: Plug Mandrel Forming8.5.6.2 Unbalanced PressuresThe pressure die should be adjusted for a light pressure against the tube. The purpose of the pressuredie is to keep the tube against the bend die through the duration of bending. The pressure die also keepsthe mandrel from bending and maintains a straight tube between tangent points of bends (the portion oftubing left on the mandrel after bending). The location of the mandrel affects the amount of spring-back.The mandrel in a forward position (toward tangent) will stretch the material on the outside of the bendmore than is necessary. This increases the length of material on the outside beyond that which isrequired to make a bend. When the bent tube is removed from the bend die, it will conform to the die andthere will be little or no spring-back. See Figure 8-10 is an overstated example. The outside of the bendactually is in compression with forces acting at points A and B. Counteracting forces occur at C and D.Forces A and B tend to close the bend while forces C and D act to open the bend.Figure 8-10: Bend Force Illustration