Stretch Forming
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Stretch Forming

Stretch forming process is a stamping process that uses a die to form a flat blank into an open hollow part. Stretching is widely used as one of the main stamping processes. Thin-walled parts of cylindrical, rectangular, stepped, spherical, conical, parabolic and other irregular shapes can be made by the stretching process, and more complex shaped parts can be made when combined with other stamping and forming processes. Stamping equipment is used for the stretching of products, including: stretching, redrawing, reverse stretching and thinning stretching. Stretching process: Using a platen device, the punching force of a convex die is used to pull part or all of a flat sheet into a concave cavity, forming it into a container with a bottom. The processing of the side wall of the container parallel to the direction of stretching is pure stretching processing, while the stretching processing of conical (or angular cone) shaped containers, hemispherical containers and parabolic surface containers, etc., which also includes expansion processing.

How to build cold stamping die?

1 . Collecting necessary information
When designing a cold stamping die, the information to be collected includes product drawings, samples, design task letter and reference drawings, etc. and accordingly understand the following issues.
1 ) Understand whether the product view provided is complete, whether the technical requirements are clear and whether there are any special requirements.
2 ) Understand the production nature of the parts is trial or batch or mass production, in order to determine the nature of the structure of the mold.
3 ) To understand the material nature (soft, hard or semi-hard), size and supply method (such as strip, coil or scrap, etc.) of the parts, so as to determine the reasonable clearance for punching and the feeding method for stamping.
4 ) To understand the applicable presses and related technical specifications, and to determine the moulds and related parameters, such as mould frame size, mould shank size, mould closing height and feeding mechanism, etc., according to the equipment selected.
5 ) Understand the technical strength of mold manufacturing, equipment conditions and processing skills, to provide a basis for determining the mold structure.
6 ) Understand the possibility of using standard parts to the maximum extent to shorten the mold manufacturing cycle.
2 . Stamping process analysis
Stamping processability refers to the degree of difficulty of parts stamping processing. In the technical aspect, the main analysis is whether the shape characteristics, size of the part (minimum hole edge distance, hole diameter, material thickness, maximum shape), precision requirements and material properties are in line with the requirements of the stamping process. If it is found that the stamping process is poor, it is necessary to propose modifications to the stamped part product, which can be modified only after the product designer agrees.
3 . Determine a reasonable stamping process scheme
Determine the method as follows.
l ) Carry out process analysis according to the shape, dimensional accuracy and surface quality requirements of the workpiece and determine the nature of the basic processes, i.e. the basic processes of drop, punching and bending. Generally, it can be determined directly from the drawing requirements.
2 ) According to the process calculation, determine the number of processes, such as the number of deep drawing, etc.
3 ) According to the deformation characteristics of each process, size requirements to determine the order of the process arrangement, for example, is the first punching after bending or first bending after punching, etc.
4 ) Determine the combination of processes according to the production lot and conditions, such as composite stamping process, continuous stamping process, etc.
5 ) Finally, make a comprehensive analysis and comparison in terms of product quality, production efficiency, equipment occupancy, ease of die manufacturing, die life, process cost, ease of operation and safety, etc., and determine the most economical and reasonable stamping process scheme suitable for specific production conditions on the premise of satisfying the quality requirements of the punched parts, and fill in the stamping process card (including the process name, number of processes, and the number of stamping processes). (including process name, number of processes, sketch of process (shape and size of semi-finished products), tooling used, equipment selected, process inspection requirements, sheet material specifications and properties, shape and size of blank, etc.): ;
4 Determine the structure of the mold
After determining the nature of the process, the sequence and the combination of the processes, the stamping process plan is determined and the structure of the die for each process is decided. There are many types of stamping dies, which must be selected according to various factors such as production lot, size, precision, complexity of shape and production conditions of the stamped parts.
(l) According to the production batch of parts to determine the use of simple die or composite die structure. Generally speaking, the life of simple die is low and the cost is low; while the life of compound die is long and the cost is high.
2 ) Determine the die type according to the size requirements of the parts.
If the dimensional accuracy and section quality of the parts are high, the precision die structure should be used; for the parts with general accuracy requirements, the common die can be used. The precision of the parts made by the compound die is higher than that of the progressive die, and the progressive die is higher than that of the single process die.
3 ) Determine the die structure according to the equipment type.
If there is a double-action press in deep drawing processing, it is much better to use double-action die structure than single-action die structure.
4 ) Select the die structure according to the shape and complexity of the parts. In general, for large parts, in order to facilitate the manufacture of molds and simplify the die structure, single process die is used; for small parts and complex shapes, compound die or progressive die is commonly used to facilitate production. Like semiconductor transistor shells, such as a large output and small size of the barrel-shaped parts, should use the continuous deep drawing of the progressive die.
5 ) According to the mold manufacturing power and economy to choose the type of mold. In the absence of the ability to manufacture high-level mold, should try to design a practical and relatively simple mold structure; and in the conditions of considerable equipment and technical strength, in order to improve mold life and adapt to the needs of mass production, should choose a more complex precision die structure.
In short, when choosing the type of die structure, we should consider from many aspects, after comprehensive analysis and comparison, and make the chosen die structure as reasonable as possible. See Table 1-3 for the comparison of the characteristics of various types of dies.
5 . Perform the necessary process calculations
The main process calculation includes the following aspects.
(1) Blank unfolding calculation: mainly for bending parts and deep-drawing parts to determine the shape and unfolding size of their blanks, so as to arrange the samples under the most economical principle and reasonably determine the applicable materials.
2 ) Calculation of punching force and preliminary selection of stamping equipment: Calculate punching force, bending force, drawing force and related auxiliary force, unloading force, pushing force, crimping force, etc., and if necessary, also calculate punching work and power in order to select press. The total punching pressure can be easily calculated according to the layout drawing and the structure form of the selected die. According to the calculated total punching pressure, the model and specification of the stamping equipment are selected.
3 ) Calculate the pressure center: Calculate the pressure center and ensure the center of the mold pressure coincides with the center line of the mold shank when designing the mold, in order to avoid the mold being affected by eccentric load and affecting the quality of the mold.
4)Calculation of the sample layout and material utilization rate. In order to provide a basis for the material consumption quota.
The design method and steps of the layout diagram: Generally, the first is to consider and calculate the material utilization rate from the perspective of the layout, for complex parts are usually cut into 3 or 5 samples with thick paper. Arrangement of various possible solutions, choose the best solution. Now commonly used after the computer layout sample and then consider the size of the mold, the structure of the degree of difficulty, mold life, material utilization rate and other aspects of the problem. Select a reasonable layout plan. Determine the lap edge, calculate the step distance and material width. According to the standard plate (strip) material specifications to determine the material width and material width tolerance. Then draw the selected pattern into a pattern drawing, put the appropriate profile lines according to the type of die and punching order, and mark the dimensions and tolerances.
5 ) Calculate the die clearance and working part size for convex and concave dies.
6 ) For the drawing process, determine whether the drawing die adopts the crimping ring or not, and perform the number of drawing times, the size allocation of each intermediate process die, and the calculation of the size of semi-finished products, etc.
7 ) Special calculation for other aspects.
6 . Overall design of mould
On the basis of the above analysis and calculation, the overall design of the mold structure can be carried out, and sketching, preliminary calculation of the mold closing height, roughly determine the mold shape size, the structure form of concave mold and fixing method. Also consider the following contents.
1 ) The structure form and fixing method of convex and concave die.
2) The positioning method of the part or blank.
3) Unloading and discharging device.
4) The guiding method of the mold and the necessary auxiliary devices.
5)Mode of feeding material.
6 ) Determination of the form of the die holder and the installation of the punching die.
7)The application of standard parts of the die.
8)The selection of stamping equipment.
9)Safe operation of the die, etc.



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