The die cutting edges have become worn to a bell-mouth.
The material being cut is very thin and light in weight.
The die is running with excessive lubrication and has just been sharpened.
The cutting clearance is too big or has worn off.
The slugs are not held on all sides.

Many methods can be applied to keep slugs down.
Adding ejector into the punch, if space is permitted. ( Fig. 1 ) This method is most effective and most common method in the market.

If the cutting punch is unable to put in any ejector due to the space constrain. A small shear angle ( slug killer ) can be ground on the punch face as shown in Fig. 2. Slug can be held in the die profile when slug spring back to it original shape while punch retract.

Another effective method is to machine a small groove ( 0.005 - 0.01 mm ) to the die profile to creates a precise amount of interference. The grooves were machined at an angle ( 3 - 5 deg. ) to the die surface. ( See picture below ) The slug expands into a series of this special grooves, pressure between the slug and the die land occurs, preventing it from pulling back through the top of the die and onto the work zone. The slug is then safely cleared on the next stroke from the punching zone

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Slug pulling is a problem that can be prevented by proper design and correct techniques,