Robert Sedlock, Natoli Engineering Company, discusses compaction dwell time and the impact of punch head designs
Transitioning product from one tablet press to another during scale-up has been an ongoing challenge in the industry for many decades. There are many different approaches and aspects that must be considered including the tablet press design, punch design and powder characteristics.
When developing a new formulation scientists are equipped with a small-scale tablet press to study and understand their product’s tabletability. These systems are useful at the research level but do not always transfer successfully to larger scale manufacturing machines. Ideally, developing the product on a manufacturing press would eliminate the transfer challenges but the amount of powder required to operate such a machine is very high and not cost-effective.
There are many stages that occur during tablet production on the rotary tablet press and a full understanding of their functions is crucial in the success of producing a quality tablet. Stages include:
-Powder filling from the hopper to the feed system
- Powder filling from the feed system to the die cavity
- Proper fill cam and dosing cam settings
-Centrifugal acceleration and the need for a pull down cam
-Pre compression – is it needed and how much force?
-Main compression consolidation time and rate
-Main compression dwell time
-Main compression decompression event
-Ejection force and rate
-Tablet take off – sticking and picking adherence removal
Every step in the tablet press process merits a full discussion but this article will focus on the compression dwell time and how the tablet press and punch head designs impact this process.
Discussion
Dwell time expressed in milliseconds is the time in which the punches achieve maximum penetration in the die under the main compression rollers and the punches are no longer moving vertically. In other words dwell time occurs when the compression rollers are in contact with the punch head flat. This time is a contributing factor to the tablet strength and a means for product transfer from one tablet press to another. Although turret RPM or tablets per minute are a common language for the turret speed, it doesn’t allow a true comparison for tablet presses with different turret sizes but the turret velocity is a normalisation of turret size and the dwell time is a normalisation of the turret size and punch head flat. It is important to understand the turret velocity before we can discuss dwell time.
Turret velocity:
When comparing press speed the tangential velocity or TV expressed in mm/sec allows for a true comparison. The TV is a function of the turret RPM and pitch circle diameter or PCD which is the measurement of the turret from center of the die to the opposite die center. (See Figure 1) The PCD is critical as this is where the compression event occurs.
Figure 1- Turret Pitch Circle Diameter
Examples of Different Pitch Circle Diameters
At a particular turret RPM the TV is dependent on the PCD which varies depending on the number of stations on a turret. Figure 2 depicts examples of different size machines and the relationship between turret RPM and TV.
Figure 2 - Tangential Velocity for Different Size Presses
As the turret PCD increases the required turret RPM is reduced to match a particular TV. A representable manufacturing velocity is above 1,000mm/sec. At 1,000mm/sec the larger scale NP500 press will operate at a turret speed of 34RPM where the medium scale NP400 operates at 65RPM and the smaller scale BLP16 at 84RPM. Running at 84RPM turret speed can pose some challenges with powder flow into the die cavity inhibiting the ability to evaluate tablet attributes. In fact some industry small-scale tablet presses are not able to generate manufacturing velocities due to their smaller PCD or limited turret RPM. This is where the punch head flat can be designed to provide this gap for scale up matching.
Head flat and dwell time
Compression tooling heads are designed with a flat providing a constant strain to the powder bed while under the compression rollers. The longer the head flat the longer the dwell time and depending on your powder deformation characteristics and strain rate sensitivity a longer dwell time may be needed to produce a robust tablet. Slowing the turret speed can also increase the dwell time but slowing the turret speed will decrease production rates.
There a many different types of tools used in the industry that have different head configurations. Some are designed specifically to increase dwell time while others are designed to reduce premature wear. The two most recognized tool configurations are the European ISO standard and the American TSM. These standards are provided to allow tablet press tool interchangeability, consistent quality between tooling vendors, simplified inspection processes and inventory. Within both configurations the ‘B’ and ‘D’ type tools are the most common. ‘B’ tooling is designed with a smaller body allowing a higher population of punches in a turret which is favourable for high tablet output where the ‘D’ tooling is designed with a larger body allowing for larger tablets approaching up to 1 inch. As a ‘D’ tool is larger the head profile and flat for a standard configuration is also larger providing a longer dwell time. Figures 3 and 4 provide a chart of different head designs and their respective dwell times for a given TV. Notice that the TSM Domed Std. and EU19 Std. are almost identical as their head profiles are very similar. Also notice that the dwell time is less sensitive at the higher turret rates.
Figure 3 - B Type Tooling Dwell Time Comparison
Figure 4 - D Type Tooling Dwell Time Comparison
Dwell time can be calculated by taking the punch head flat dimension divided by the turret tangential velocity. This simple calculation is helpful when transferring product to a different size press. And in the case of the large gap between TV as discussed previously with smaller R&D machines and larger scale machines, the tooling head flat can be designed by your tool manufacturer to match or closely match dwell times. The flexibility you have on your flat dimension is dependent on the punch neck diameter. To ensure the robustness and integrity of the punch, the neck diameter should be larger than the head flat and with some safety margin.
Figure 5 is an example of comparing dwell time of a production press (NP500) and an R&D tablet press (BLP16-D). The NP500 is a high speed 45 station double sided production tablet press with extended head flat tooling and the BLP16-D is a 16 station D tooled development press. The NP-500 running at 80% of its maximum speed reaches 50RPM and 17m of dwell time. With standard TSM domed heads the BLP16-D must reach 80RPM turret speed to match 17ms. It can be challenging to maintain consistent tablet weights at such a high speed. When using the reduced head flat of 7.95mm on the BLP16-D the required turret speed is now only 40RPM which is a more manageable speed for tablet weight consistency and helps reduce powder wastes.
Figure 5 - Production Press vs. R&D "D" Tablet Presses
Another challenge that can be resolved with head flat designs is transferring product from a B tooled machine to a D tooled machine or vice versa.
Figure 6 is an example of matching dwell time from an NP-500 to a BLP16 utilizing B tooling. The standard B tooling matches dwell time of the NP-500 at similar turret speeds. When using the 50% of standard B head, the turret speed is reduced to 25RPM which again, at slower speeds helps reduce powder wastes.
Figure 6 - Production Press vs. R&D "B" Tablet Presses
Figure 7 - NP500, Standard D and B Head Flats
As previously discussed the European ISO standard and American TSM are the references for the tooling specifications. They provide the acceptable tolerances to ensure proper installation, alignment and tablet consistency. The punch head flat also specifies an acceptable tolerance of +/- 0.2mm for the Euro standard and +0.00 / -0.76mm for the TSM standard. At common production rates the extreme difference is less than 1ms and as low as 0.2ms at higher production rates. Figure 8 depicts the small dwell time differences for the European tolerances. To accurately measure your punch head dimension a Horizontal Optical Comparator can be used. (Figure 9)
Figure 8 - European Punch Head Tolerances & Impact to Dwell Time
Figure 9 - Horizontal Comparator
Conclusion
Tooling designs and dwell time are one of many important parameters that play a role in tablet quality. Tooling head flats can be designed to achieve your desired dwell to assist in product transfer scalability processes or tablet robustness issues. Normalizing for turret size, speeds and tooling allows for a clearer understanding and comparison of different size tablet presses.