Upper Punch Penetration: Optimizing Your Tablet Manufacturing Process

As we continue this series on tablet press setup optimization, let us delve into upper punch penetration.? Upper punch penetration refers to how deep the upper punch enters the die bore during pre-compression and main compression as measured from the top face of the die.? While there are machines with fixed punch penetration, the majority offer an adjustment with either handwheels on the top of the machine or a selection on the HMI.

When it comes to punch penetration there is a trade-off between product yield and how fast you can run the press before tablet defects such as capping, sticking or delamination occur. ?Some companies are unaware of this setting, others set it as a fixed value for all their products while best practice suggests adjusting penetration depth independently for each formulation.?

Factors to consider include:

Yield loss and penetration depth:? As the upper punch penetrates the die, a small amount of powder ejects from the die bore in a puff.? Deeper punch penetration reduces the formulation yield loss as increasing compaction pressure occurs farther from the top of the die.?

Formulation characteristics:? Powders with low bulk density (below 0.5 mg/mm^3), low moisture content or poor cohesiveness are more prone to puffing and yield loss and therefore benefit from a deeper punch penetration setting.? Powders with a high bulk density (above 0.7 mg/mm3), higher moisture content or good cohesiveness may successfully run with a shallow punch penetration at high speed with less yield loss from puffing.

Air entrapment and penetration depth:? Air entrapment within the formulation during compaction reduces tablet tensile strength, leading to capping problems, delamination, or even sticking.? Reducing punch penetration often helps as it provides less distance for air to travel before escaping.? As a result, a shallow punch penetration can solve capping issues or allow the press to run faster as it needs less time to evacuate air.

Die bore wear:? For abrasive products, the working life of dies can be an important consideration as wear rings may form quickly in the compression zone of the die bore.? Most commonly, flipping the die bore is the Go-To solution to extend die life as the compression zone within the die bore changes.? Another method to extend die working life for abrasive products is to vary the penetration depth, between batches.? This extends the working area of the die bore and extends die life but is only helpful in cases where the formulation is both abrasive and insensitive to penetration depth (i.e. not a fluffy product and not a product prone to capping).

Die taper depth vs penetration depth:? The standard taper depth of a die bore per TSM (Tableting Specification Manual) is 3/16” (4.76mm).? When a typical 2-3mm punch penetration setting is applied in conjunction with a tapered die, the top portion of the tablet is compressed within a tapered zone of the bore.? This results in increased yield loss, friability loss, and flashing.? To address this issue, the upcoming TSM 8th Edition will decrease the standard taper depth from 4.76mm to 3mm.? We recommend matching taper depth with penetration depth unless there are specific reasons to do otherwise.?

Compaction time as a function of penetration depth:? Dwell time (duration of pressure roll contact with the punch head flat) is not the only consideration when it comes to how fast you can run a press.? Compaction time (duration of pressure roll contact with any portion of the punch head) affects air evacuation.? Longer compaction times can benefit products that are prone to capping.? Factors that influence compaction time include press speed, PCD (pitch circle diameter), pressure roll diameter, punch head design, and punch penetration.? We can calculate compaction time for SUPAC (Scale-up and Post Approval Changes) purposes; contact Natoli Engineering for details.

Pre-compression versus main compression penetration depth.? It is important to ensure punch penetration at pre-compression is not deeper than what is used at main compression.? General practice is to either keep the values the same or set upper punch penetration 0.5mm to 1mm shallower at pre-compression than main compression.? The advantage of keeping them both the same is that you minimize tablet ejection forces and can run main compression punch penetration with the shallowest possible setting.? The advantage of running punch penetration deeper at main than pre- is that you gain additional de-aeration benefits at the beginning of main compression.?

To Summarize,

Best practice is to optimize punch penetration for press speed, tablet quality, and yield.? In cases where yield is paramount, running deeper is helpful.? In cases where production output is paramount, running more shallow is helpful.? For products prone to capping, it is better to run shallow (1.5mm to 2.5mm) punch penetration.? For fluffy, dry, non-cohesive formulations it is better to run deeper to maximize yield.

TDM 9/3/2024