Punch Retention – The Art & Science of Anti-flight

Olympic athletes strive for perfection, serving as inspiration to us in all the ways we can do better in what we do.?Maximizing press performance is essential – not for a medal, but to make both a high-quality product and a cost-effective product for the consumer.? Getting the highest level of performance in yield, output, and tablet quality depends on the nitty-gritty details of the press setup.? Let’s go over one of these details that directly impact press performance, setting your punch retention.??

Punch retention plays an important role in controlling tablet weight variation by eliminating uncontrolled movement (flight) of lower punches during fill.? Lower punches can bounce when hitting or coming off the weight/dosing cam.? Punches can also bounce when first hitting the lower pressure roll, so retention is critical throughout the compression cycle.? Proper punch retention dampens noise and vibration and extends the life of tooling, cams, and other press components.?

Punch retaining systems vary in from press to press and may be called different names such as punch brakes, punch retainers, punch holding plugs, or punch retention bands.?? Regardless of their construction, their purpose is to apply pressure to punch barrels to guarantee even punch movement.

The art of punch retention comes into play in setting the tension as it is done by feel.? You want the punch to be barely supported under its own weight by the retaining system. ?If the punch falls under its own weight, the retention system is too loose.? If the punch cannot be pushed with a light effort down by hand, then the retention system is too tight.?

Punch binding may occur when the press is run with a punch retention system that is too tight.? This can cause damage to the lifting and lowering cams as well as punches.?Indications include scratches and galling on punch inside head angles and punch barrels.?? ???

In cases where punches are retained in their guides with punch holding plugs (punch brakes), it is important to check their condition as these are wear parts.? Punch holding plugs (brakes) include a self-retaining screw with a spring-loaded pressure piece. Examine the plug’s end which contacts the punch barrel (may be bronze or plastic) for wear.? The spring will also lose elasticity over time degrading the performance of the retainer.? Replace punch retainers as needed.?

Taking care of punch retention systems and careful press setup is another key to success.? For specific retention system information, contact your press supplier and for additional information on press setup Natoli Engineering is ready to help you.

Todd Martin

Sr. Technical Service Engineer

Natoli Engineering

7/30/2024