Blank Size, Thickness, Shape, and Part Geometry

Determining the blank size is a very crucial step for a successful draw. The amount of material needed for the final product must be included within the blank. When there are multiple draws for a single product, it can be tricky to determine how much material is needed. Since the draw process causes thinning and thickening of the metals, this is an important first step to a successful draw.

Draw Radii

It is important to note the size, accuracy, and finish of the die entry radius. If the die radius is too small, the material will not easily flow. This results in stretching and fracturing of the drawn product. If the die radius is too large, the material will wrinkle after leaving the pinch point between the draw ring surface and binder. If the wrinkling is extreme, the material flow may be restricted when pulled through the die entry.

Draw Ratio

 The draw ratio is one of the most important elements of maintaining successful deep draws. The draw ratio is the relationship between the size of the draw post and size of the blank. During the forming process, the blank is pressed into a circumferential compression which creates a resistance of metal flow. If there is too much resistance, the metal will fracture. If the draw post is not big enough, the metal will stretch, becoming thinner until it cannot be formed. If the draw post is the appropriate distance from the edge of the blank, the metal will be able to flow, while becoming thicker as it enters into the die cavity.

 The formula for the draw ratio is: D/d ≤ 2 for a successful draw.

D = the blank diameter

d = plug (or post) diameter

If this ratio is greater than 2, re-draws (or break downs) are required. In our industry, this is a general rule of thumb. Certain materials may have more accurate, material-specific rule of thumb ratios. For example, Aluminum is 1.8.

Lubricants and Die Surface Finish

Adding lubricants and a polish to the die surface helps with friction and reduces the chance of galling. Galling is when two metallic surfaces slide against each other, creating friction. This can harm the product and the tooling. Applying lubricant to the blank is a very important step in the deep drawing process, to create the highest quality product, while protecting the draw post tooling. Avoiding galling enables the blank to slide easier, allowing for free flowing of the metal. 

Die Temperature

The die temperature can cause the lubricant to thicken or thin, depending on how hot the die is. When lubricants heat up, their viscosity drops and they thin out. As they get cold, their viscosity increases. Understanding this relationship is key to creating the best quality drawn part, while maintaining the quality of the die. 

It is critical to select the correct lubricant for each deep drawing process. Each lubrication brand, type, and formulation performs differently at different temperatures, depending on their intended use. Certain lubrications need to increase to a certain “working temperature” before they will exhibit any friction-fighting properties at all. In contrast, other lubricants only work in a cold or room-temperature environment. When determining the correct lubrication, the tool temperature, (mid-run and at rest) blank material, and draw severity are all taken into account.

Binder Pressure

At Toledo Metal Spinning, we use pinch and pressure to control the material flow. Binder Pressure is a machine setting that controls the upwards force and/or pressure in the press that will be applied through the draw ring/binder, which sits on top of the cushion pins. The draw ring pressure rises, while the die pressure and slide force is in a downward motion, this is how the blank is “pinched.”