Doing a good job of continuous stretching mould requires comprehensive consideration of many aspects such as design, process, material, manufacturing and debugging. The following are the key steps and considerations:
1. Design and process planning
Process decomposition According to the shape of the product and material properties, reasonable distribution of the number of times of stretching and deformation. Avoid a single stretch too large to cause rupture, or too small to increase costs. Gradual size reduction (stepwise stretching) is usually adopted. Layout of work stations Arrange punching, stretching, shaping, trimming and other work stations in the continuous mould to ensure smooth connection of all processes. The spacing of the stations should take into account the material flow and the strength of the mould. Material flow simulation Use software (e.g. AutoForm, Dynaform) to simulate the flow of metal, predict the risk of wrinkles and rupture, and optimise the mould structure.
2. Material selection and control
Material to be processed - Select materials with good ductility (e.g. stainless steel, aluminium alloy, mild steel). - Control the material thickness tolerance (within ±0.02mm) to avoid uneven stretching. - Mould materials - Select high hardness and wear-resistant materials (e.g. SKD11, DC53, cemented carbide) for key parts (convex mould, concave mould). - Surface treatment: TD treatment, chrome plating or nitriding treatment to improve wear resistance.
3. Mould structure design points
- Clearance control The gap between the convex and concave moulds is usually 1.1~1.2 times the thickness of the material (slightly larger for the first stretching, and gradually reduced for the subsequent ones). If the clearance is too small, it is easy to cause abrasion; if it is too large, it will be wrinkled. - The design of the crimping force uses a nitrogen spring or hydraulic system to provide a stable crimping force to prevent the material from wrinkling. The crimping force must be adjusted with the depth of the stretch. - Lubrication system Provide oil grooves or injection holes in the stretching area, and use high-viscosity stretching oil (e.g., with chlorine and sulfur additives) to reduce friction. - Air venting design Additional air venting holes (0.5~1mm in diameter) are provided in the concave mould to avoid gas accumulation that may lead to deformation of the parts.
4. Precision Manufacturing and Assembly
- Machining accuracy - The accuracy of key parts is controlled within ±0.005mm, using slow-feeding wire-cutting (accuracy ±0.003mm) and precision grinding machine. - Parallelism of mould base ≤0.02mm/300mm, clearance of guide pillar and guide sleeve ≤0.01mm - Assembly points - Split structure is adopted, which is convenient for adjusting single station. - Use laser alignment gauge to ensure the coaxiality of each station. 5.
5. Commissioning and optimisation
- Mould test procedure 1. run empty to test the mould action; 2. low speed (10~20SPM) test punch, observe the material flow; 3. gradually increase the speed to the design value (usually 60~120SPM). - Frequently Asked Questions - Cracking: Increase the rounding angle of the concave mould, reduce the single stretching rate, improve lubrication. - Wrinkling: Increase the crimping force, reduce the gap, add stretching rib. - Rebounding: Increase the shaping station or strengthen the pressing material locally. For rotary and cylindrical stretching products, the principle of calculating the opening size is based on the principle that the volume of the material remains unchanged, even if the material will be thinned in the stretching process, but its total volume will not change.
For complex shape stretching products, the calculation method will be more cumbersome, because its shape is also attached to the material thickness changes, even in the present 3-dimensional software, simulation and analysis software low to assist in the calculation of the case, it is still difficult to achieve the expected effect of the open material. How to determine the size of complex stretching products? Can only try the knife mouth, probably determine how much material is needed, and then design a stretching structure for continuous attempts, and finally get the right size of the open material.
Stretch coefficient stretching products need to be divided into how many steps, each step of the stretching height, size is how much is through the stretch coefficient to calculate. Different stretching structure, stretching process of the stretching coefficient is not the same, so need to be based on the actual product to make a reasonable choice. Factors affecting the coefficient of tension are: material properties, material thickness, the number of times of stretching, stretching method, mould structure, lubrication and so on.
If the test mould appears to have the product pulled apart, you can try to apply some lubricant (rapeseed oil, soapy water) to the lower mould or cover the concave mould surface material with a film can also achieve a certain effect.
6. Maintenance and upkeep
- Daily Maintenance - Clean the mould surface of oil and dirt every shift, check the status of guide pillar and spring. - Check the wear of convex/concave mould every 50,000 strokes (wear ≤ 0.02mm). - Lifetime management - Replace the wearing parts (e.g. ejector rod, guide bush) regularly. - After accumulating 500,000 strokes, the mould should be completely dismantled and overhauled.
7. Balance between cost and efficiency
- Combination of work stations Reduce the number of work stations and shorten the length of the mould by combining processes (e.g. punching + stretching). - Standardised design Adoption of quick-change structure (e.g. standard mould carrier, sub-module set), mould changeover time can be controlled within 15 minutes.
Key Data Reference | Parameters | Typical Values ||----------------|----------------------|| Single Stretch | 20%~40% (soft steel) || Concave Die Fillet Radius | 5~10 times the material thickness || Crimping Force | 20%~40% of the total punching force || Die Life | 1,000,000~5,000,000 punches || Die Life | 1,000,000~5,000,000 punches || Die Life | 1,000,000~5,000,000 punches
Through the above systematic control, the continuous stretching mould can stably achieve the dimensional accuracy within 0.05mm, and the yield rate can reach over 99%. In practice, the parameters need to be flexibly adjusted to the specific product characteristics, and the key variables optimised through DOE (Design of Experiments).
