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Die Science: Choosing between pressure pads and stripper plates

Many problems result from a poorly designed or completely absent pressure pad. In the May/June 2013 issue of STAMPING Journal, I discussed the characteristics of the three kinds of pressure pads. Now it’s time to review the pressure pad styles and see how they compare to stripper plates.

Stripper Plates

A stripper plate, commonly known as a bridge stripper, is simply a plate mounted on the lower half of the tool that strips or pulls the metal from the cutting punches (see Figure 1). Unlike a pressure pad or plate, it does not apply pressure to or hold down the metal being cut.

Stripper plates usually are designed so the strip or metal can slide underneath the stripper. In most cases, the gap is about two times the material thickness. This additional gap often is necessary to allow stock or steel that is slightly bent or deformed to pass smoothly under the stripper plate. If the space is insufficient, poor feeding or jam ups can result.

Pressure Pads

A pressure pad is a pressure-loaded plate, either contoured or flat, that holds, strips, and controls the metal during cutting or forming (see Figure 2). Pads typically get the required pressure from coil, gas, or urethane springs.

A stripper pad holds the metal down tight to the die during cutting or piercing, preventing the metal from sliding or moving on the die. It also helps prevent the part from distorting and allows the cutting punches to withdraw or “strip” from the metal part during the return press stroke.

Pads that hold the metal down during forming operations such as bending and flanging are called holding or gripper pads. Like stripper pads, gripper pads can be contoured or flat and typically get the necessary holding pressure from coil or gas springs.

The drawing pad (a blank holder, binder, or pressure ring) controls metal flow during drawing and stretching operations. Its shape and surface finish are critical, as is the amount of pressure it exerts on the metal. Except when used for very small parts or parts made from soft metals such as aluminum, drawing pads rarely get their pressure from coil springs. Coil springs often don’t have sufficient travel or force to prevent the metal from wrinkling during the drawing process.

Which Is the Right Choice?

When choosing between a stripper plate and pressure pad, you should compare their advantages and characteristics and understand their typical applications.

  • Because they don’t require springs or pad guidance, stripper plates are more cost-effective to manufacture and thus cost less than pressure pads.
  • Unlike pressure pads, stripper plates require very little maintenance because there are no springs being used or mechanical motion taking place.
  • Stripper plates typically do not strip the punches the way pressure pads can. The only way they fail is if they break loose from the bottom die.
  • Unlike stripper plates, pressure pads hold the part or the carrier web flat while cutting and forming transpires.
  • Because pressure pads go up with the upper die, the lower die can be examined without pad removal. Stripper plates have to be removed.
  • When cutting shear is used, the pressure pad holds the part or strip tight against the lower die sections. This prevents the shifting of the parts or strip that can occur with stripper plates.
  • Pressure pads can accommodate slightly distorted or bent incoming material with no negative effect on feeding.
  • When forming or drawing is taking place, pressure pads hold each part tightly in its respective station, making pitch control easy to obtain and retain. With stripper plates, nothing keeps the strip from moving during drawing, so it’s difficult to maintain constant progression.

In general, use a stripper plate:

  • When flatness is not an issue.
  • For progressive dies using only a solid carrier web design with little or no form required.
  • When the incoming material can be straightened and flattened.
  • When using little or no cutting shear.

Use pressure pads:

  • When flatness is an issue.
  • For progressive dies using a solid carrier and for all progressive dies using a stretch carrier design.
  • When deep forming or drawing is taking place.
  • When a significant amount of strip lift is required.
  • When using cutting shear.

Personally, I am not a big fan of stripper plates. Based on my experience, they often result in problems that cost more than the initial cost savings at purchase. Keep in mind that pressure pads can be used in all applications, whereas stripper plates have limits. In short, my advice is to stick with a pressure pad—it gives you more bang for your buck.

About the Author
Dieology LLC

Art Hedrick

Contributing Writer

10855 Simpson Drive West Private

Greenville, MI 48838

616-894-6855

Author of the "Die Science" column in STAMPING Journal®, Art also has written technical articles on stamping die design and build for a number of trade publications. A recipient of many training awards, he is active in metal stamping training and consulting worldwide.