How to prevent tooling tip wear

Tip wear can lead to a host of issues for tablet design. Here, Bill Turner, technical service manager, tooling and tablets, and Kevin Queensen, technical service support tooling and tablets, Natoli Engineering, discuss the steps needed to avoid wear in tooling.

Wear to the edges of punch tips is overlooked in many tooling inspection procedures. Many factors can cause tip wear, including poor tablet and tool design, granulation characteristics, and improper steel selection for the application. While inspection technicians typically pay attention to the punch cup face, tablet defects are in fact more often related to punch tip land and outer edge wear.

While proper inspection can help prevent common tableting defects, punch tip inspection often only checks the size of the tip by measuring it with a micrometer or calipers. Wear to the very edge of the punch tip is relatively small, is undetectable using traditional measuring equipment and techniques, and may not be easily observed by unassisted visual inspection.

Punch tip edge wear can lead to problems like raised edges on the final compressed tablet, commonly referred to as “flashing” (Figure 1). In turn, tablet flashing can cause many other issues after the compression event. When tablets are subjected to de-dusting, the flashed edges may break off, leaving rough, poorly defined edges. Tablets with flashing can be difficult to film coat and can lead to loose particles adhering to the coated tablet surface. Understanding the most frequent causes of punch tip edge wear is crucial to minimising defects and downtime.

Common causes of punch tip wear 

Formulation/Granulation Characteristics

Abrasion due to granulation can cause tip wear, most commonly in the nutraceutical industry. Nutraceuticals are usually composed of different vitamins and minerals, most of which are quite abrasive. Pharma products with a higher percentage of active pharmaceutical ingredient (API) typically require higher compression forces, which may accelerate tip wear.

Abrasive ingredients also erode the land and the outer edge of the punch tip. Once wear occurs to the edge of the punch tip, particles will become trapped in the increased gap between the tip edge and die wall. As the punch moves vertically, the abrasive particles are ground between the tip and die wall, accelerating the wear to both tool components and the overall wear process.

Particle size also can affect tip wear. Small particles, known as fines, can sift into the gap between the punch tip and die bore (Figure 2). Fines, combined with the cyclic motion of the punches, can cause wear on the outer edges of punch tips. While often there is little or no choice as to the components of a formulation, controlling particle size and tip-to-die-bore clearance can reduce unnecessary tip wear and extend tool life.

Tablet Design

The design of the tablet plays an integral role in the functionality and longevity of tooling. If the cup is excessively deep or if the concavity of the cup is steep near the edge with nearly vertical surfaces, it will be more susceptible to abrasion. This type of cup design is not as robust as a standard cup and will require more land than its counterpart.

Steps to strengthen the edge of the punch cup include reducing the slope of the cup and introducing an appropriate amount of land into the tablet design. Land gets a bad reputation as many believe additional land may cause edge chipping and difficulty with film coating. Whilst tool vendors know the value of land and may incorporate it into design, many are forced to use a minimal land width of around 5% cup depth. Knowledgeable tablet designers believe that land width should be closer to 10% – and occasionally up to 20-25% – especially for difficult-to-compress tablets that require high force. Having sufficient land in your tablet design adds strength and wear resistance, both on the inside of the cup and on the outside of the tip, while creating a more robust tablet and enhancing tablet stability.

Reducing and preventing tip wear 

Tool Design (Configuration)

Refinements are made to the basic tool design to improve performance and extend life. One of the most commonly used tool options is tapered dies, which can resolve capping and laminating issues and make it easier to eject the tablet from the die. Tapered dies can also help offset mild turret misalignment by guiding the upper tip into the bore, bypassing the potential contact with the lead-in chamfer.

Selecting the proper steel type can help reduce tip wear for abrasive granulates. For very abrasive products, punches can be produced from premium wear-resistant PM (particle metallurgy)-grade steels. As the hardness increases on premium tool steels so does the abrasive wear resistance, but it must be noted that the impact toughness decreases somewhat. Some steels are required to be cryogenically tempered (-184°C, -300°F) to increase wear resistance. If your vendor does not offer this tempering process, either the steel type or your tooling vendor should be reconsidered. The combination of each steel’s unique chemistry and the related heat treatment process allows different types to attain their unique mechanical properties. This makes them ideal for the wear resistance needed for abrasive formulations yet still allows them to support the compression forces needed for the high-stress, high-cycle loading conditions of tablet compression.

Inspecting for tip edge wear

Punch tip edge wear can come from a variety of circumstances and may cause significant quality-related issues. It is important to understand and know how to identify the most common causes of this type of wear. Punch tip edge wear can be difficult to detect because traditional methods of inspection are ineffective. Fortunately, the use of a horizontal optical comparator (Figure 3) makes inspection easy – and fast. Avoiding punch tip edge wear when possible, and quickly identifying when it does occur, will provide great benefits to the quality of your tablets and to your production operation.