by Robert Sedlock, Director, Natoli Scientific
The tablet manufacturing process has many challenges. An understanding of the steps leading up to the final compressed tablet is crucial to the task of producing a high-quality tablet. The final tablet weight is a direct response of the volumetric filling of the die during the tableting process. Thus, the powder filling process dictating the final tablet weight is a critical factor in producing quality tablets.
The final tablet weight is an essential factor responsible for a tablet meeting acceptable quality attributes. The tablet weight is not only related to the active ingredient’s dosage, but it also directly affects the applied compression force and thus the resulting tablet strength, friability, disintegration, and dissolution times.
Key Considerations In Proper Powder Filling
Multiple steps and variables must be considered before the actual powder is filled into the die cavity. The most important factors are the hopper, the feeder, fill cam selection, and powder characteristics.
Powder is initially loaded into the hopper of a tablet press and must have adequate flow properties to successfully enter the gravity or paddle feeder (aka a force feeder). Typical powder flow issues found in the hopper are ratholing, bridging, and segregation. The hopper design plays a role in this as hoppers with sharp angles and rough surfaces can inhibit powder flow. Hoppers designed for mass flow with high quality surface finishes can address these issues. Understanding the rheological properties of the powder allows for the proper hopper design and a significant reduction of powder flow issues.
Vibration from the tablet press can cause powder particles size segregation with larger particles percolating upward. Products that require high tonnage of compression force may cause excessive vibration from the punch/roller interaction; however, quality tablet presses designed for high compression forces will absorb the impact shock and may help remediate segregation issues.
The amount of powder in the hopper determines the head pressure of the powder leading into the feeder, which can lead to tablet weight variability. It is common to have a low-level proximity probe installed on the hopper to alert the operator that the hopper is approaching being empty. However, consideration is warranted for positioning the sensor so that the charge of material is always above a certain level to maintain the required head pressure.
The feeder is the final component that delivers the powder to the die cavity. Common feeder designs can be gravity or force-fed with single or multiple paddles. Many variables are associated with the feeders, including size, clearance above die table, paddle designs, and speed. The speed of the paddles in the feeder is determined by speed of the turret, number of punches installed on the turret, and powder flow characteristics.
Paddles maintain a constant charge of powder in the feeder for proper powder dosing into the die cavity. Tablet press operators must be vigilant to avoid running the paddles at an excessive speed, which can cause issues with packing of material causing flow restriction, overbending of the formulation components, and segregation of particles by size. The correct feeder speed is the slowest speed that maintains acceptable final tablet weight consistency.
When dealing with highly compactible materials, packing may cause particle adhesion and prohibit the transfer of powder into the die.
Overblending is an issue when higher levels of lubricant (e.magnesium stearate) are used in the formulation and the excessive paddle speed causes more efficiency of the lubricant and lubricant coats a higher percentage of the particle surfaces. This overblending of lubricant can cause tablet quality issues, including lower tablet strengths, capping, and extended disintegration/dissolution times. This issue is exacerbated when powders are recycled from the fill cam/overfill. This is detailed later.
Fill Cam Selection
The fill cam is a lower cam located under the feeder that deliberately overfills the die cavity. Following the fill cam is the dosing cam, which pushes out the excess material to achieve the target weight. A scraper bar removes the excess powder and directs it to a waste area or back into the feeder along a recycling path.
This two-step filling process is important. As the overfilled material is pushed up from the dosing cam and scraped off, the powder is consolidated and will better assist in a proper dosage in the die cavity. On the other hand, an excessive overfill and scrape off will cause issues with either low material yields or excessive recycling and potential overblending as described earlier.
It is important to know that the proper fill cam size should be selected based off the dosing cam position. The recommended overfill is 2-4mm. Achieve the target weight by approximating the fill cam size by calculating the dosing cam depth and simply adding 2-4mm. The dosing cam is based on the tooling geometry, material bulk density, and tablet weight. First, calculate the volume of powder to be compressed by dividing the final tablet weight by the bulk density of the material. Then divide the volume of powder to be compressed by the punch tip’s cross-sectional area, typically provided on the tool drawing.
The above discussion focused on the equipment aspects of providing the tools to properly fill the die cavity. The powder blend and particle characteristics also play a major role in the die-filling process. If powder simply doesn’t flow, the resulting tablet is affected. Flow can be enhanced with excipients, like glidants (e.g., talc or silicon dioxide) and granulation techniques, including dry and wet processes. Candidate formulations should be evaluated for their flow characteristics and performance under typical tablet press conditions.
The best method available for predicting powder performance through the tablet-making process is a measurement of powder rheology utilizing one of the new instruments available on the market.
Understanding the powder die filling process is crucial to provide a batch of tablets that meets the expectations of patients and regulatory authorities. Powder characteristics, tablet weight, and equipment engineering are key factors in the success of high-speed tablet manufacturing.
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