By Emily Newton, Editor-in-Chief at Revolutionized

Understanding the likely variables during die filling helps manufacturers achieve higher levels of uniformity in their products. Here are some crucial factors to consider that can impact the outcomes of the process.

Equipment Usage and Setup

People engaged in die filling often use presses to turn powders into tablets. In the pharmaceutical industry, this directly impacts the finished product’s weight. Thus, any inconsistencies could negatively affect the amount of an active pharmaceutical ingredient (API), plus the tablet’s strength and dissolution time. Matters associated with equipment setup and usage of a tablet press can affect die-filling results.

The Hopper

The hopper connects to a feed shoe or feeder. An operator pours granules or powder mixtures into it. Tablet press vibration can cause particle segregation, with the smallest granules drifting downward. Using a tablet press built to tolerate high compression forces can mitigate that issue.
Also, the amount of a formulation held in the hopper can affect die filling by decreasing the tablet’s weight if head pressure varies during the presses’ operation. However, installing a low-level proximity probe on the hopper can alert an operator when it’s time to refill it.

Feeder Paddle Speed

Feeders operate via either gravity or suction and feature at least one rotating paddle that keeps a constant formulation charge in the hopper. A formulation’s flow characteristics partially affect the paddle’s speed. However, an operator must monitor it carefully. If it goes too fast, overblending, packing and formulation segregation can occur.

Packing is especially concerning when using highly compactable formulations. This can limit the free flow of the formulation into the dies. 

Custom Parts

Research also indicates that operators could achieve up to a 50% reduction in dimensional variability by using a customized fluidised filler shoe instead of the stock part that comes with a press. Such machine upgrades may result in higher levels of product uniformity during die filling.

Formulation Characteristics

The makeup of the powder or particles can also affect the die-filling process. Using a rheometer to measure the powder’s internal resistance to flow familiarises a person with how it will move during die filling and may affect product outcomes.

How well a powder flows depends on numerous factors, including humidity, powder size, surface porosity and size distribution. Understanding the flow rate is a crucial part of die filling. Each die may spend only milliseconds under a feed frame’s opening.

Calculating and testing the formulation’s flow rate determines whether it matches pace with the feeder. Product uniformity results because each die receives the correct amount of product, resulting in the right weight.

Powder Surface Area

People working with powders learn that those formulations have a high surface-area-to-volume ratio. It goes up as the particle size gets smaller.

One issue with smaller particles is that they tend to stick together more readily. However, an advantage they offer is a more uniform distribution during manufacturing. Thus, a higher powder surface area could positively affect the die filling process.

Fill Type Used

Die-fill results can also vary depending on whether an operator uses a feeder that works by gravity or suction. Researchers examined the two methods while working with mini-tablets, which pose extra challenges due to their small sizes.

They used a rotary press with multiple compression tip tooling to create tablets of 1.8 and 3 millimeters. They looked at data related to the weight of the mini-tablets and the compression roller displacement to detect differences in die-filling results. Both tablet types had similar weight variations, which the researchers attributed to interparticulate friction. This limits how the powder flows and packs together.

The angle of repose test is a common way to determine the likely effects of interparticulate friction. It involves pouring powder from a narrow funnel and measuring the resulting pile’s angle. Spherical shapes typically have the lowest friction. In contrast, smaller particle sizes generally show greater friction and a steeper angle of repose.

The researchers working with the mini tablets found that the smaller of the two sizes had higher intracycle weight variations due to the smaller dies used for them. However, they also confirmed that gravity filling influenced the 3 mm tablet weight variations, and suction filling affected the 1.8 mm tablets. These results suggest that the mechanism used to fill the dies affects uniformity.

Powder Texture and Filling Velocity

Research also indicates that a powder’s coarseness or fineness can affect die fill performance. However, the fill type has an impact, too.

An investigation into two powder types poured into dies concerned milled and sieved acetylsalicylic acid as the coarse powder and mannitol as the fine substance. The results showed different segregation tendencies that affected how the dies filled. The researchers calculated the segregation index to measure the uniformity of the powder poured into the die.

Tests confirmed that powder segregation most often happened during gravity filling at high and low velocities. Conversely, suction-based, low-velocity filling caused the lowest segregation levels.

The researchers also verified that the top layers in the die became most affected by horizontal segregation. They found that coarse particles migrated down the powder pile during the final die-filling steps. That effect caused a higher concentration of coarse particles on the die’s near side. They also observed rough particle concentration at the far end, occurring when particles moved down the top surface of the powder stream.

Become More Aware of Die-Filling Process Outcomes

Whether your role involves filling dies for pharmaceutical tablets or another manufactured product, these factors will make you more confident about die-filling results before starting to work with the powder. Testing the formulation to learn more about its behavior beforehand also gives valuable information that could affect product uniformity and die-filling behavior.

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