Stretch Forming ,Form block and Mating die method,limitations,applications,advantages and disadvantages.
STRETCH FORMING OPERATIONS:
This method is used for producing large accurately contoured sheets. It has been developed in Second World War period itself.
Stretching is the process of stressing the work blank beyond its elastic limit by moving a form block towards the blank or sheet metal. The form block has projections of exact size required on the blank which is in the form of depressions on the same blank. Stretching is mainly done for straightening a part to obtain a straight axis and uniform cross-section. During stretching the blank, the spring back occurs after completing the stretching process.
Spring back is defined as the movement of the metal to resume its original position causing a decrease in bend angle after the applied force is withdrawn. So, this spring back has to be considered to obtain exact shape and size of the blank after the stretching process. Spring back always depends on material type, thickness of the blank, hardness of the blank and bend radius. Generally large bend radius produces greater spring back on the blank. But, this spring back can be avoid by
(1) Over stretching using-V-type form blocks,and
(ii) By coining the metal slightly at the corners of the blank to remove elastic stresses called comer setting.
Methods of Stretch Forming:
The stretch forming process can be done in two methods such as
1. Fonn-block method
2. Mating-die method
1. Form-block method:
In this method, the two ends of the blank or sheet metal is tightly held by an adjustable gripper. These grippers are fixed but adjustable. Then, the form block is moved towards the blank to make the required shape. In this case, the form-block is operated by hydraulic cylinder. When the form-block moves towards the blank, the hydraulic fluid inside cylinder gets compressed and delivered through the outlet valve. The movement of the form always depends the hydraulic fluid pressure inside cylinder. The fluid is entered the cylinder when the form-block moves away from the blank after completing stretching process. In a single stretching process, we can get no need of stages in stretching. Stretching the blank can also be done by fixing the form block stationary and moving the grippers towards the form-block. It is performed by holding the blank ends in movable grippers .
2. Mating-die method:
In this method, the blank is held in movable grippers. The blank is placed between the lower and upper die. The lower die is kept stationary and the upper die is movable one which is operated by hydraulic or pneumatic cylinders.
First, the movable grippers are moved towards the lower die on which only elastic defamation takes place. Next, the upper die is moved towards the blank. When the upper die touches the blank, only elastic change takes place. Due to continuous stretching of the blank by the upper die, plastic flow of sheet metal takes place between lower and upper dies. When the upper die edges reach the top surface of the blank, the stretching process is completed.
Materials for die and form blocks:
Wood, masonite, zinc alloys and cast iron.
Advantages:
Blanks can be stretched in a single operation,
1. No need of any heat-treatments before and after the stretching process.
2. Spring back is reduced or eliminated when compared to other forming methods.
3. Direct bending is not introduced.
4. Plastic defamation is due to pure tension only.
5. Tooling costs are low
6. This method is more suitable for low volume production.
Disadvantages:
1. Blank thickness should be uniform throughout the length. (Thin sections are to be overstretehed).
2. Sudden changes in contour surfaces cannot be stretched.
3. Maintenance cost of hydraulic cylinders is high.
4, The process requires high quality form-blocks.
Limitations:
I. Uneven thickness of blank cannot be stretched.
2. Stretching of blank to the required shape of contour is limited.
Applications:
1. Production of aircraft wing and fuselage parts.
2. Production of contoured panels for truck trailer and bus bodies in automobile industry.
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