Stamping material that can take a punch

Yasin ATEŞ

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10 Haz 2020
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Punching a deeper draw without cracking, tearing, work hardening, wrinkling, and fracturing​

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When a deep draw is punched, material elasticity, ductility, and springback all play roles. Applying guiding principles and parameters will help stamping manufacturers achieve a better draw.
Forming in a die can be stressful, not only to the designer and operator but especially to the material being formed. Whether you need to do a deep-draw form or create a deep form in a multicontoured part, the principal factors determining the strength of the form, repeatability, and success are the same—tension, compression, and stretching.
There are a variety of ways to produce a large form, whether it is simply a large flowing form or a deep draw. An operation is defined as a deep draw when the height of the form is greater than the diameter or width of the form.
Each material has its own limits, and you can only do so much within the confines of those limits. Understanding the limiting draw ratio (LDR) of the material and the objective for the product during the design phase is crucial and will give the engineer the best opportunity to match the final product to the design intent.
Whether your material is a cup or blank and you run a transfer press or coil-feed a strip into a progressive die, the fact of the matter is that material is material. Cracking, tearing, work hardening, wrinkling, and fracturing are going to occur at some point.

Punch and Die Design​


From a punch design standpoint, outside of completely redesigning the stamped product, you can focus on a few minor changes to the punch and die cavity to improve performance. An obvious change is adjusting radii on the forming edge of the die and edges of the punch tip. However, you must be aware that in doing so, you may alter the effects of the final stamping, as well as how the form will release from the die cavity.
A punch radius that is too big may result in additional springback that will need to be accounted for in the design. If the radius on the die is too big, that can lead to a thickening at the base of the part, increasing its size and making it more difficult to release from the die cavity.
Friction is a big concern in draw forming that can be mitigated somewhat with lubrication, but another method to reduce friction should be considered before production—improving the tooling’s surface finish. Polishing a draw punch tool is common, but it can be done wrong.
Overpolishing can change the dimensions, creating material flow issues, and underpolishing may do a disservice as well. What you may not realize, however, is that grain direction and how you polish play critical roles in material flow as well. Having a surface finish that is in line with the direction of flow, a surface barrier such as lubricant, and a way for it to flow and dissipate heat are as critical as a shiny finish. Just keep in mind that a shiny surface may not be the optimal surface.
Another key to a successful deep forming application is using the proper tool steel for the application. A lot of pressure is applied to the tools during forming, so a tool steel with high yield strength is necessary to withstand the forces being applied.
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Whether the material you are deep drawing is a cup or blank and you run a transfer press or coil-feed a strip into a progressive die, material is material; cracking, tearing, work hardening, wrinkling, and fracturing are going to occur at some point. Understanding the limiting draw ratio (LDR) of the material and the objective for the product during the design phase is crucial and will give the engineer the best opportunity to match the final product to the design intent.
Often you may feel the urge to sacrifice tensile strength for yield strength. You may not need to do that, as many of today’s tool steels can meet the yield and tensile strength requirements, allowing for longer run time before reworking or replacing the tooling.

Cold Forming, Hot Forming, Hydroforming​


If you use traditional cold forming methods, understanding the material strain and knowing how plastic deformation and springback affect the final product are key factors to successfully stamp the product. Once you have a good understanding of the material’s tensile and yield strengths, you can focus on the design and how to achieve the required results.
A common misconception about deep drawing is that the material is stretched to achieve the final form. While some degree of stretching is inevitable, it is not the process. The ideal method is drawing the material to a form using a punch and draw die that pull material into the die cavity. During the draw, that material pulled into the cavity will tend to be thicker at the tip and thinner as it gets to the base of the form.
Materials have been developed for lightweighting and strengthening to meet a specific need or regulation. Servo presses accommodate a variety of changes to stroke and speed to allow for better control of material flow. Dwelling at the bottom of the stroke will help reduce springback. Restriking can reduce springback too, without a transfer or additional tooling. All of these help with forming and productivity.
There are a number of ways to circumvent the material’s properties, but not without changing its metallurgy. Hot forming and hydroforming have taken on new life in recent years to help with forming many of the new material types. Hot stamping heats the material to change the tensile strength and allow for improved further flowing. Hydroforming applies consistent pressure. These processes circumvent traditional manufacturing problems to form without cracking, tearing, work hardening, wrinkling, and fracturing.

Forming Options​


A number of common practices alter the forming results based on the application. Evaluating the material is the first step. Once you’ve determined the material properties, you dive a little deeper into deciding the number of reductions needed, the punch and die shapes, punch radii, clearances, press speed, the force applied on the blank holder, and whether annealing is necessary.
Once you have optimized all those steps, you’ll be left with production requirements that render those approaches impossible to attain without some engineered magic.
A quick way to check what struggles may lie ahead is to run a finite element analysis (FEA) of the application to determine potential areas of concern. You may see areas of thinning or cracking that might require a design change. This is an opportunity to gather additional information and determine what can be adjusted in the manufacturing process to ensure your success.
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From a punch design standpoint, you can make minor changes to the punch and die cavity to improve performance such as adjusting radii on the forming edge of the die and edges of the punch tip, and polishing a draw punch tool.

Annealing, Lubrication​


The process of forming creates friction; friction creates heat; heat causes premature wear.
Heat also work-hardens the material. Work hardening can be a benefit to the form by adding strength in just the right places. It can also be a hindrance, causing cracking or tearing in high-stress areas.
A common practice to prevent work hardening is to anneal the part during the forming process to get the material back to a workable state. Annealing is beneficial but can be costly and time-consuming. Using lubrication and applying some design changes to the punch tooling can reduce or eliminate the need for annealing.
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not only help to reduce the heat occurrences and friction, they also help the material flow better throughout the process. Different lubricants react differently, depending on the situation. Lubricants in a draw application are under immense pressure and high temperatures. Therefore, selecting the right lubricant that can remove heat and reduce friction is most important in deep forming applications.
At the end of the day, draw forming is an art that takes science to function properly. Take the time upfront through FEA and process development to explore the optimal combination of press, material flow, design, heat dispersion, and tool integrity to maximize productivity. The additional time upfront and associated costs will be made up for in production green time.
 
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