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Manufacturer across automotive, medical equipment, kitchen hardware and furniture industries can relate to challenges of stainless steel tube bending process. Stainless steel tubes get deformed while bending which often led to defect called inner wall wrinkling on the inside diameter of tube, and that could affect structure as well as appearances of final parts. Here at Baorui, after 20 years working with pipe processing technology, we strongly believe that the correct mandrel design can offer a effective solution. Understanding how each parameters of mandrel affect forming quality would be the key part of successful bending.
Understanding Why Wrinkling Occurs in SS Tube Bending
Wrinkling in stainless steel tubes results from the basic principle of bending a tube. The external wall experiences tensile forces and stretches outwards as the inside wall experiences compressive forces and squashes inwardly. If there’s no internal backing on the inside surface, the walls might collapse under pressure resulting into wrinkling. Because Stainless steel alloys work-harden under stress and has greater strength, a bend requires greater bending force and requires internal support. For example Stainless Steel 304 or Stainless Steel 316 could both lead to inner wall wrinkling if there's insufficient or no supporting tool inserted inside the tube. A higher wall factor indicates greater probability of wrinkling. The wall factor is obtained by dividing the OD of the tube by the wall thickness of the tube. If a wall factor of 20 or more, then one should be really concerned about wrinkling.
The Critical Role of Mandrel Diameter and Fit
The diameter of the mandrel plays a major role in preventing the inner wall from wrinkling. Studies have shown an increase in mandrel diameter resulting into greater reduction in roundness and a decrease in the flattening of tube walls on the compressive sides. The mandrel fit has to be made so tight as much possible without galling or overstressing the tube wall. Mandrels typically have a fit within 2% to 5% of the ID. The fit is tightened in Stainless steel cases as to provide adequate support and to ensure better roundness and tighter roundness during bending. However, an overfitted mandrel can cause issues like scoring, galling and extraction problems during post bend tube bending process.
Mandrel Extension and Positioning for Wrinkle Prevention
The positioning of the mandrel against the tangency of the bend has a crucial role in ensuring free wrinkling. Mandrel has to extended forward enough that it supports the tube wall at the bending point precisely. Research results indicated that the position should be 0.5 – 1.5 mm beyond the tangency point in most cases. The mandrel should not be positioned too forward that leads to a thinning problem. For tube with thin wall factor, one can use a simple geometrical formula for determining the mandrel position, even though first off inspection always is necessary.
Ball Mandrel Configuration for Complex SS Tube Bends
In cases where stainless steel tube has high wall factor and/or bend radii are too small, ball mandrels may be used to achieve the better results in comparison to plugs. Ball mandrels comprise multiple spherical segments joined end-to-end by flexible connecting shafts. These segmented tools move in harmony as the tube is bent and can conform to the bend arc providing support along the entire length of the bending process. More number of balls and proper arrangement and size for them in the ball mandrel will improve the forming quality and minimize the possibility of the wrinkle formation. Research proved that mandrel type and arrangement of segments can control wrinkling instability and collapse by limiting transverse material flow to the inner surface. Our CNC hydraulic pipe bending machines are equipped with appropriate ball mandrels for stainless steel applications.