Bending technology is widely used in the boiler and pressure vessel industry, air conditioning manufacturing industry, automotive industry, aerospace industry, shipbuilding and other industries. The quality of the bending pipe will directly affect the structural rationality of these industries, safety, reliability, etc. Therefore, in order to bend a high-quality pipe, it is necessary to master the processing skills of the pipe under different process conditions. For cold bends, we often divide into mandrel bending and non-mandrel bending.
(A) Mandrel bender and non-mandrel bender
Bending without a mandrel is relatively simple. Generally, a tube with a small diameter, such as a 6-8 mm tube, which has more support on its own, it is not necessary to use a mandrel, and the ellipticity is within 5% of the tube’s outer diameter. Some galvanized pipes and iron pipes with large bending radius and thick walls can also be made without mandrels. However, for thin-walled pipes with small bending radius and large diameter, a mandrel should be used to support the inner wall of the pipe to prevent its cross-section from turning oval and wrinkling at the inner side.
(B) When mandrel bending is required
Hippo summarized a formula based on long-term practical experience, which can help customers quickly determine whether a mandrel is needed to complete the bend.
Generally, pipe bender standard dies include: pipe bending die, clamping die, pressure die, the mandrel and wiper die are optional. D is the outer diameter of the tube, T is the wall thickness of the tube, and R is the bending radius. The difficulty of bending the pipe depends on the wall thickness and bending radius of the pipe. The smaller the wall thickness, the smaller the radius, the more difficult the processing.
We use relative wall thickness tx=t/D and relative bending radius Rx=R/D as the process parameters for the bend. For the pipe fittings with Rx> 3D and tx> 0.04, you can use standard molds. For the pipe fittings with Rx <3D and tx <0.04D, you can add mandrel, wiper and other technical measures to ensure the bending quality.
(C) How to choose the right mandrel
With mandrel bending is relatively technically difficult, how should we correctly use mandrel bending to enhance the quality of the bend?
1. The first is to determine the type of mandrel.
In general, the mandrel can be divided into two categories: one is the hard mandrel and the other is the soft mandrel.
For hard mandrels, the cylindrical mandrel (or cylindrical ball-head mandrel) has a simple shape and is easy to manufacture, so it is more common than claw-shaped mandrels.
For soft mandrels, due to the spherical node mandrel is very flexible and can swing in any direction and can adapt to various deformations. Therefore, the spherical node mandrel is often used when bending thin-walled or small bending radius pipes.
At the same time, we have to decide the ball joint quantity according to different relative bending radii, relative tube wall thicknesses, and bending angles. If the number of ball joints is small, the expected effect cannot be achieved; If the number is large, it is difficult to manufacture and it is not convenient for the pipe to penetrate.
2. The second is to determine the size of the mandrel ball.
The gap between the mandrel and the tube inner diameter is also an important factor affecting the quality of the bend. If mandrel ball diameter is too small, the tube inside may occur wavy wrinkles, can’t play a role in preventing the tube outside from flattening; If the diameter is large or the mandrel ball outside is not smooth enough, the tube wall would be strained, or even break.
In a word, the selection of a reasonable mandrel diameter and its adequate lubrication are indispensable elements for the quality of the bend.
You could refer to the following empirical formula: d ≈ (0.94 to 0.98) D to decide the mandrel ball size.
d- mandrel diameter
D- tube inner diameter size.
3. The third is to ensure the mandrel toughness and lubrication
In order to avoid scratches on the inner wall of the tube, the mandrel shall be sufficiently tough and carefully deburred and polished; the inner lumen of the tube shall also be carefully cleaned and, if necessary, wet-blown sand. The arbor should also be rigid enough to avoid creases caused by shaking, and the proper lubricant should be used.
(D) How to use the mandrel correctly
The position where the mandrel penetrates into the pipe and begins to bend seriously affects the quality of the bent pipe.
If the punctured position is too far forward, the pipe outside material would be easily get stretched and can be thinner.
If the penetration position is too far back, the corresponding supporting effect cannot be achieved, and the tube easily collapses, and even wrinkles on the inside.
So, how exactly do we get this right? In fact, we can combine a formula to select: e ≤ (1/4 ~ 1/2) D. The letter e in the formula represents the position where the mandrel penetrates into the pipe at the beginning of bending, the letter D represents the inner diameter of the pipe. hen the specific value can be obtained by calculation.
Finally, after determining the position scale, we should first conduct a trial bend based on this value. During the trial bending process, make appropriate adjustments in accordance with the specific conditions, so that the mandrel reaches the appropriate position. In this way, the resulting bending products can be satisfying.
In addition, when bending square (rectangular) pipes, the cross-section shapes of the mold and the mandrel are different from the round steel pipe, but the selection and use of the mandrel and the bending method are basically the same.
