Peter Ulintz Peter Ulintz
Technical Director

Pressure-Plate Strippers—Part 1

March 1, 2022

The term “stripper” applies to the stripping plate, keepers or retainers, and pressure system (springs, gas, rubber, etc.), which all aid in the stripping of stock material from around the punch steel.

1-keeper-block-systemMost die design rules of thumb recommend that stripper pressure constitute approximately 10 to 30 percent of the cutting force. In general, softer materials and tighter die clearances require higher stripping force. But, every rule of thumb involves some degree of tradeoff for the convenience of applying the rule. In the case of stripping forces, the tradeoff is die wear.

Frictional forces inside of the die influence the amount of pressure required for stripping. In high school physics I learned that “Friction is FµN,” a convenient way to remember that friction (F) is the product of the coefficient of friction (µ) times the normal force (N). 

The frictional forces associated with stripping force directly relate to the rate at which the cutting edges of the punch and die components wear. As a result, a primary design objective should be to reduce the value of the µ and N factors to produce more stamped parts between sharpening. 
Factors that influence µ include: a) lubrication, which breaks down with process heat; b) punch surface finish, which contributes to adhesive wear; c) punch hardness, which often degrades over time due to poor sharpening practices; and d) the type of material being punched.

2-spool-diametersFactors influencing N include: a) punch-to-die cutting clearance, which increases the normal force as the clearance is reduced; b) the ratio of hole size to stock thickness; c) the spacing between adjacent holes; and d) the cutting-edge conditions on the punch.

Stripper-Design Guidelines

Stripper construction plays a significant role in die reliability and durability. Consider these important design guidelines:

1. Avoid stripper bolts, which may become loose and tend to break at the undercut of the threads.

2. Use keeper blocks or spools in place of stripper bolts.

Advantages of using keepers: a) increased rigidity and reduced tipping due to more surface area, which provides improved guiding; b) a correctly designed keeper will not bend or deflect and cause jamming, whereas stripper bolts bend easily; c) keepers provide more strength under high stripping pressure, whereas large strippers would require far too many stripper bolts; and d) keepers are the best choice for long stripper-plate travel due to better guiding (Fig. 1).

3-spring-retainersAdvantages of using spools: a) reduced space required on the die shoe as compared to keepers; and b) the round shape is readily machined, which reduces machining costs.

3. If the die design requires spools of more than one length, use different diameters to prevent their improper mounting (Fig. 2).

4. Use spring cans around springs in every pad that bottoms out.

5. Use spring retainers to hold springs in the upper die should a pad be removed for service (Fig. 3).

6. For longer nitrogen-cylinder life, do not use the maximum piston travel listed in the catalog. Instead, use 85 percent of rated travel.

7. Do not preload nitrogen cylinders with the pad keepers. Provide extra travel (commonly 1⁄8 to 1⁄4 in.) on the keepers beyond full piston extension. 

8. Make the pad travel slightly more than the die work to prevent material from moving due to pad bounce.

9. In progressive dies, all upper pressure pads that push stock lifters down as the die closes must have the same travel length to maintain level strip and lift as the parts locate in each station. Otherwise, parts may be pulled out of location.

10. Provide windows in the face of large stripper pads to allow easy removal of ball-lock punches.

11. For material greater than 0.045 in. thick, provide pad balancers, which prevent damage to the die caused by pressure pads tipping during the entrance of a new material coil into the die.

12. When a pressure pad will not balance on the stock in the workstations, provide stock balancers to prevent tipping.

13. Do not locate springs over a stock balancer as they will provide no hold-down force to the part being formed. 

14. Remember that springs located halfway between a pad balancer and the material only provide one-half of the spring force on the stock material. MF

Industry-Related Terms: Case, Die, Plate, Stripper, Stripping, Surface, Thickness, Undercut
View Glossary of Metalforming Terms

Technologies: Tooling


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