Ask the Stamping Expert: Which method of slug control is best?

A: Slug pulls can cause tooling breakage and be a major cause of press downtime. Without knowing the specifics, it’s hard to address a particular slug pulling issue. However, there are many ways to address this problem, each with its own advantages, and you can combine them in an extra effort to fix the issue. The thing to remember is that all methods to address slug pulls should be addressed in the initial tool design.

Following are what I believe to be the most common methods. Note that the term pierce punch is used to cover all cutting punches, not just round pierce punches. Likewise, the term cutting dies covers all cutting dies and round bushings. And a full slug, as opposed to a partial slug, has a perimeter that matches the die in its entirety.

Ejector Pins

One way to solve slug pulling is to put a hole in your pierce punch and insert a spring-loaded (headed so it does not fall out) ejector pin through the punch. The ejector will typically protrude beyond the face of the punch by some amount. As the die closes on the raw stock, the pin is pushed flush to the face of the punch. The punch cuts at press bottom dead center, the slug is freed from the stock, and the ejector pin pushes the slug down through the die.

If the ejector is not protruding enough in the free state, the slug still may come up because of the reverse suction created when the punch backs up out of the die cavity. If it’s too long, it could cause the slug to flip or rotate. A portion of the rotated slug then is above die level, which inhibits the feeding of the raw stock.

Punch Slug Retention

A second method is to grind slots or grooves across the face of the punch in various cross-hatch or single, straight-line patterns. This deforms the slug, because the cutting edge of the punch has steps on it. And when the slug is full of these deformities, it tends to stay tight in the die and not pull up. This method also helps reduce the surface area between the slug and punch face, which is probably coated with oil and less likely to stick to the punch face and pull up with the tool.

On the downside, if the grooves are too big or too deep, the steps on the face of the punch will generate slug dust the size of grains of sand, which can build up and cause scrap marks on your parts. If the grooves are too small, they will do nothing. If the grooves are just the right size, they will solve your slug pulling problem, but you will see subtle, immeasurable groove shadows on your parts. We use this method extensively on light-gauge stock 0.003 to 0.007 inch thick.

Die Slug Retention

A third method is to machine grooves in the side wall of the dies to some width and depth, depending on whether you wire-EDM your die section or split it in two halves and grind it. The grooves are angled from top to bottom on the internal wall of the die (not in the depth axis). This puts steps into the die opening or cutting edge. As you blank the slug and drive it to some depth in the die, the portion of the slug edge that is in the grooves gets deformed and tight in the die, because the grooves are angled and the slug is driven straight down.

If the grooves are too deep and too wide, you will see it on your part. A burr will develop because you are increasing the clearance, and “sand” scrap marks will appear. If the grooves are too shallow, they won’t do what they are supposed to do. If the angle of the grooves is not steep enough, the slug won’t wedge in as it is pushed down.

This method works great, and if you wire-EDM your tooling, it adds almost no cost. Die slug retention is a function of the raw material thickness and mechanical properties. Through trial and error, you will develop a matrix of groove width, depth, and angle based on the application.

Good luck, and happy stamping!