Die Science: Understanding deep-drawing and stretching variables for best results
Numerous variables affect the success of a deep-drawing or stretching process. Some are major and some minor, but you’ll need to take all into consideration when designing, building, and troubleshooting a drawing or stretching operation.
Sheet and Coil Material
Material is a major variable. No two coils of material are identical, even if they are slit from the same master coil, since no coil has the same mechanical properties from beginning to end.
This is not to imply that the metal is not within a given specification or standard, but a given spec encompasses tolerance. The process of making coil material, whether ferrous or nonferrous, involves such variables as chemistry, time, and temperature. So your process must be designed to accept the normal variation that will occur.
The six key mechanical properties to consider in any coiled metal are:
- Tensile and yield strength.
- N value, or work-hardening exponent.
- R value, or plastic strain ratio of thickness to width strain.
- Elongation percentage.
- Metal type and gauge.
- Topography or surface finish.
Each one of these properties affects the metal’s ability to flow and stretch in a deep-drawing or stretching operation.
Friction between the sheet metal and die is caused by many factors, but nothing affects friction in a drawing or stretching operation more than the application of lubricant.
Hundreds of types of lubricants are used in metal stamping operations. Most have special additives that allow them to change their frictional value with respect to heat.
For example, chlorinated lubricants have a much lower coefficient of friction when heated to about 350 degrees F than when cool. These lubricants are best suited to deep-drawing and forming higher-strength steels.
Other lubricants with additives include soap-based, oil-based, wax-based, poly-based, fatty ester, and sulfurized products. Synthetic lubricants also are available that change frictional values dramatically.
The tooling material also affects friction; for example, tools made from aluminum bronze typically have a much lower coefficient of friction than tool steel dies. Tool steel coatings, die surface finish, and material topography also affect the frictional value between the sheet material and the die and influence the amount of metal flow and stretch that can be achieved.
Heat is another factor contributing to friction. As punches and dies warm up, they expand, resulting in smaller clearances between working die sections. This change in clearances can cause ironing of the material. Insufficient clearance can increase heat even more, which can break down the lubricant (depending on its additives) and cause scoring, galling, and splitting. It’s a vicious cycle.
To prevent the dramatic effects of thermal expansion, you might need to flood lubricant on the forming sections or perhaps cool the punches and dies by running a cooling agent through them.
Forming speed influences the amount of stretch and flow that occurs in a drawn or stretched part. Think of your metal as putty: Pull it too fast and it breaks, but pull it slow and it stretches. When subjected to deep drawing, metal behaves in a similar fashion.
Metal needs time to flow into the die. Once it begins to flow, the rate of flow can typically increase. Faster speeds create more friction. More friction creates more heat. More heat can be good or bad depending on the metal type and lubricant additives.
In general, for deep drawing, slower is better. This is the reason deep double sinks are not drawn in fast crank-drive presses. In any case, changes in forming velocity affect the amount of strain and stress generated in the part. This explains why using a different press to form a part often results in slightly different part geometries and varying springback in strained areas.
Die/Blank Geometry and Holding Pressure
While the topics of die and blank geometry and holding pressure are too broad to explain in great detail here, they are very influential to the amount of flow and stretch in a metal and must be considered. The main topics to keep in mind are:
- Drawing ratio (punch-to-blank relationship).
- Blank-holding pressure.
- Part features and design.
- Part and die radii, size, and shape.
- Draw bead and draw bar geometries.
- Binder shape.
These are just a few of the most common variables that affect the process of drawing and stretching sheet metal. There are many more; take as many as possible into consideration.
Until next time … best of luck!
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.