In simplistic terms, punches, dies, and forming tools generally involve a fabrication process of “brute force” to pierce, cut, shape, or otherwise form a piece of material. These are often referred to as “chip-less” machining techniques. Due to the rapid advancement of PVD coating technology, today’s tool/die machinists and buyers need to know more than just the properties of the bare metal that forms the tooling blank. They need to understand the synergy involved in the complete system: tooling design, tooling raw material, tooling processing & heat treatment, coating properties, material being machined, cycle time requirements, and allowable machine downtime.
Coating Benefits for Punches & Dies
PVD coatings can add value and reduce total cost to punches, dies, and forming tools. First, the combination of high film hardness and low friction coefficients impart the favorable properties of:
● decreased sliding friction
● increased abrasive wear resistance
● reduced punch & die retraction forces
● increased surface hardness & toughness
● enhanced pill tablet release from tablet punches
● decreased galling, adhesive wear, & material pick up
● eliminated reaction to material due to a level of chemical inertness
All of these improvements must be accomplished while maintaining the critical dimensions and tolerances that punches, dies, and forming tools demand when in use; thus, the usefulness in using PVD as the means of providing the functional surfaces.
Extend Lifespan Of Punches, Dies, & Forming Tools
Punches, dies, and forming tools can experience significantly improved lifetimes when coated with PVD thin film coatings.
PVD VS CVD Coating
PVD is preferred over CVD in the coatings of punches, dies, and forming tools because of the lower temperatures that PVD uses during processing to protect the heat treated hardness and austenitizing temperatures of some of the forming tool substrate materials.
Since PVD can provide coating thicknesses in the 1–5 μm range (0.00004” – 0.00020”), parts can be machined to final size prior to coating, allowing for the resulting clearances needed in close tolerance punch and die applications.
Increase Wear Resistance
“Wear” can be defined as a way to quantify the effects of abrasion and corrosion during the contact with other materials, grit, or tools. “Toughness” can be defined as a punch, die, or forming tool’s resistance to breaking, cracking, or chipping during use. Each material used to fabricate a punch will have characteristic wear and toughness values. Those material choices range as follows:
● Low wear resistance with High toughness
● Balanced wear resistance & toughness
● High wear resistance with Low toughness
The specific forming application will determine which tool material to choose. Thus, wear resistance can be impacted by the hardness of the tool itself, plus any additional coating applied.
Tool & Coating Synergy
A PVD hard coating is able to simultaneously increase surface hardness (to protect the “cutting” edge) while decreasing the frictional forces, allowing the punch to withdraw easily and extend the lifetime of the tool. Optimum tool life is application-specific, and is best achieved by selecting the correct PVD coating and tool substrate for the actual method of forming and material being formed.
Coating Can’t Enhance Poor Quality Tool Steel
To clarify with an example, a punch made from poor quality tool steel won’t experience the benefits of the PVD coating. This is because the base punch lacks both the toughness and wear resistance needed to do the punching operation. No synergy can be gained in this case. The benefits of the PVD coating can’t overcome the severe limitations of the poor quality tool steel. There is no substitute for the proper choice of tool steel.
Prevent Material Pickup, Reduce Downtime
Materials such as stainless steel and dead soft aluminum are prone to “material pickup,” which is often generated by the punching process. Material pickup refers to the tiny pieces of material that adhere to the tool and can accumulate to levels that result in galling, scoring, or scratching of the work-piece. The reduced frictional attributes of PVD coating resist these tiny material pieces from adhering to the tool, thus extending the useful life of the forming tool and reducing machine downtime.