Automation And Validation: The State Of The Art In Tablet Press Technology—Part 1
Frederick J. Murray, Korsch America Inc.
Tablet Press technology has continued to evolve, and over the past several years there have been advancements in both the mechanical features and control system design. In addition, the validation requirements associated with tablet press automation have continued to escalate. This paper will present an overview of the state of the art in tablet press technology, and will present a general validation outline to support a fully automated tablet press.
Tablet Press Automation
Machine Innovations (Back to Top)
With regard to mechanical design, there are a number of machine innovations that address the continued need for a higher level of GMP, improved precision and calibration of machine parameters, and improved cleaning and changeover efficiency, as follows:
- Motorized machine adjustments
- Interchangeable turret
- Heavy tonnage precompression
- Through-the-Wall technology
- Clean-in-place technology
Motorized Machine Adjustments (Back to Top)
In an effort to provide greater precision, repeatability, and to facilitate an automated product recipe capability, most modern machines are configured with motorized machine adjustments. These systems generally utilize a precision stepping or servomotor, and a displacement transducer to display machine parameters. The most common motorized adjustments are as follows:
- Dosing cam
- Tablet thickness
- Insertion depth
- Precompression roll position
In addition to the precision of the adjustment, these systems also permit a clear calibration procedure and calibration history. Finally, the use of motorized adjustments permits the frequency of these adjustments to be tracked and recorded with an electronic audit trail capability.
Interchangeable Turret (Back to Top)
Driven by the need for greater flexibility and improved changeover efficiencies, the interchangeable turret has continued to gain in popularity. In essence, an interchangeable turret will permit the use of different rotors and different tool sizes on the same machine.

With this capability, one machine can run a variety of press tool configurations. A typical example is as follows:
- 44 station, IPT BB, 13 mm max
- 35 station, IPT B, 19 mm max
- 29 station, IPT D, 25 mm max
In this way, the highest efficiency and output is achieved with each tool configuration, and the machine has the flexibility to run any size tablet. In addition, the use of an interchangeable turret permits an additional turret to be set-up in advance to permit the rapid changeover from one product to the next. The interchangeable turret is most applicable in a production environment that requires smaller batch sizes and a high frequency of product changes.
Heavy Tonnage Precompression (Back to Top)
In an effort to provide greater versatility in handling difficult products, many modern machines include a heavy tonnage precompression capability. Instead of providing a smaller roller with 10–20 kN capability for tamping and air elimination from the die, many machines are offering a precompression capability of 80–100 kN, which is in many cases equivalent to the main compression capability. This is especially useful for many vitamin and nutritional products, which require higher compression forces in order to achieve desired levels of tablet hardness. Another innovation in this area is the use of a compression dwell bar, an adjustable cam that is located between the precompression and main compression station. The compression dwell bar will maintain a compression force on the tablet of up to 10 kN, to permit additional tablet consolidation between compression stations. This system has proven to be effective on products that are "dwell-time" sensitive in nature.

Through-the-Wall Technology (Back to Top)
Through-the-wall (TTW) technology is an innovative concept that permits all services and components of a tablet press to be accessed through the rear of the machine, instead of through the machine base. In this way, the machine can be placed "through-the-wall" to permit servicing and maintenance from outside of the compression zone. As such, there is no interruption of the "white zone," or the compression room, when machine maintenance is conducted from the "gray zone," or service area. This technology is specifically applicable to product containment applications, where operator exposure to the product must be minimized.
With the TTW technology, it is possible to access all major machine components from outside the room, including the main drive, hydraulic system, lubrication system, and all central services. In addition, the electrical cabinet can also be located outside the compression room to permit complete access to the machine control hardware. Only the tablet press, touch control screen, and the tablet sampling system are in the room, thereby permitting unattended operation and minimal operator exposure.
Clean-in-Place Technology (Back to Top)
Over the past several years, there have been continued efforts to implement a wet Clean-in-Place (CIP) Technology on a tablet press. Due to the diverse range of active materials, it can be difficult to develop a CIP solution that is effective in all applications. In addition, the introduction of water to a complex, electro-mechanical machine such as a tablet press, can have longer term maintenance implications. In many cases, the CIP capability is utilized to remove the majority of a potent compound from the machine, and is not intended as the final cleaning step.
In addition to a wet CIP, there is also a dry CIP capability that can be realized with the application of barrier technology to a tablet press. In this application, the machine windows are enhanced with barrier "glove-box" technology to permit manual cleaning and removal of components while maintaining a sealed compression zone. In this way, the majority of the active material may be vacuumed, and the majority of the parts can be removed (in a sealed transport unit) to permit final cleaning with a minimized exposure risk.
Tablet Press Automation (Back to Top)
A fully automated tablet press control system is generally comprised of a series of modules, as follows:
- Password login
- Product recipe storage and retrieval
- Electronic audit trail
- Press force control
- Single tablet rejection
- On-Line tablet sampling
- Interface to other process equipment
- Central system interface
Password Login (Back to Top)
The Password Login module will limit access to the machine control system to operators, supervisors, and service personnel that have password authorization. In most cases, there are several levels of password authority that permit access to specific functions of the control system environment.
For example, the supervisor password would permit the user to create, modify, or delete product recipes. The maintenance password would permit access to the machine calibration factors. In any case, this module will restrict access to the machine, and will enable the use of an electronic audit trail to track machine adjustments as they occur during batch production.
Product Recipe Storage and Retrieval (Back to Top)
The Product Recipe Storage and Retrieval Module will permit the storage and automatic retrieval of critical machine parameters, and is especially applicable in a production environment that requires frequent product changes and smaller batch production. Once a product has been set-up and optimized for the first time, the Product Recipe module will permit the storage of all process parameters to facilitate an automatic set-up for subsequent batch production. In most cases, the Product Recipe module will store the following process parameters:
Product Recipe—Machine Parameters
Press Speed
Feeder Speed
Filling Cam Installed
Dosing Height
Tablet Thickness
Upper Punch
Insertion Depth
Product Recipe—Press Force Parameters
Press Force Setpoint
Average Force Stop Limits
Single Force Rejection Limits
Standard Deviation Limits
Maximum Tool Tip Pressure
Product Recipe—On-Line Tablet Sampling Parameters
Sample Size (number of tablets)
Sample Interval
Target Tablet Weight
Control Limits—Tablet Weight
Target Tablet Thickness
Control Limits—Tablet Thickness
Target Tablet Hardness
Control Limits—Tablet Hardness
Electronic Audit Trail (Back to Top)
The Electronic Audit Trail Module will permit storage and review of in-process adjustments and machine stops that occur during tablet production. In general, the Electronic Audit Trail Capability is comprised of two key components, as follows:
- Event log
- Alarm log
The Event Log will store all adjustments that are made to machine parameters or control setpoints, and should log the date and time, the operator of record, and the initialand finaladjustment value.
For example, if the press speed is increased during the batch, the Event Log should record when the speed was adjusted, who made the adjustment, and the initial and final press speed values. The Alarm Log should record all machine stops and record the date, time, and fault description.
In general, both the Event Log and Alarm Log files are referenced and stored by batch number. This permits subsequent correlation of batch data with adjustments and press faults that can be associated with any anomaly with the batch result. Event Log and Alarm Log data should be stored by batch number, and the data should be available for export in a standard file format for use with standardized applications such as Excel or Access.
Press Force Control (Back to Top)
Most modern tablet presses are equipped with a press force control module that provides in-process tablet weight control. These systems are designed to monitor the individual compression force on each punch station, and to provide closed loop control of the dosing cam to maintain the target tablet weight. As of now, there is not existing technology that can weigh individual tablets at the same rate at which they can be produced. The result is that a secondary parameter, the press force, must be monitored in real time.
Typically, single press forces are monitored, and after each revolution, an average press force is calculated and compared to a press force setpoint. A precision step motor on the dosing cam is then adjusted to increase or decrease the position of the lower punches in the die during die fill. These adjustments are made continuously to maintain a constant press force—or a constant tablet weight. With increased processing speed, it is now possible to calculate a "moving average" of press force to permit the control system to react to trends in the press force, as opposed to waiting a complete revolution before any adjustment is made.
The result is a more responsive control capability and, for many applications, tighter weight control. For most systems, the press force control loop is based on the main compression force, and the precompression force is monitored only to insure compliance with upper and lower limits. In addition to the real time display of average main compression and average precompression forces, most systems include the display of standard deviation of single press force values, which presents an indirect reference for tablet weight variation.
Most advanced press force control systems will display the actual press force on each individual punch, to permit a real-time assessment of press force distribution, and to facilitate diagnostic efforts related to press tool tolerances, or other processing issues. Under normal production conditions, the maximum and minimum press force should occur on random press stations. The real-time display of single press forces will permit the operator to determine if a single press tool is the source of high or low press forces. This information may then be related to an issue with the press tool tolerance, or a picking or sticking issue with a particular punch station.
Single Tablet Rejection (Back to Top)
Most press force control systems offer the capability to reject tablets based on individual press force values that violate preset limits. There are two basic methods that are utilized to reject tablets: mechanical fast gate and high-speed pneumatic reject.
The mechanical fast gate will interrupt the stream of good tablets when a high or low force is detected. Due to the mechanical response time of the fast gate, some good tablets will always be rejected with the bad tablet, with more good tablets being rejected at higher press speeds.
The high-speed pneumatic rejection system will permit the rejection of an individual tablet at all operating speeds of the machine, and will not reject an adjacent tablet, regardless of press speed. In some cases, the control system will generate a Reject Log that will record the punch station, press force, date and time of each rejection, and permit the press to be stopped if excessive rejections occur from a specific punch station.
For tablet presses that are directly linked to packaging machines, an in-process tablet rejection system is critical, as there is no opportunity for intermediate accumulation and inspection of tablets subsequent to compression.
On-Line Tablet Sampling (Back to Top)
In order to provide fully automated and unattended production, most advanced tablet presses can be configured with a tablet sampling station for in-process tablet measurement and feedback control to the press force control system. In some cases, the measurement system will monitor tablet weight only, and in other cases, tablet weight, thickness, diameter, and hardness is measured. In any case, the actual tablet weight is always the primary control parameter of any tablet sampling system.
During tablet production, a tablet sample is drawn from the machine. Again, there are a variety of methods that are utilized to transport tablets from the tablet press to the measurement system. The most basic systems will utilize a sorting gate on the good stream of tablets to direct a group of tablets to an intermediate point. A vibrator system is then utilized to separate and send single tablets to the test station. In this case, the tablets are certainly from random punch stations; however, it is not possible to obtain data on the punch station and press force associated with each tablet weight. The more advanced systems permit single tablets to be rejected from the die table—and permit the user to select tablets from random or specific punch stations. For routine in-process control, random tablets are utilized. For troubleshooting purposes, it can be a significant advantage to select a tablet from a specific punch station to permit a statistical weight assessment and physical inspection. For these systems, the individual tablet weight is associated with the punch station and press force that produced the tablet.
Once the actual tablet weight is measured, there is a comparison with the target tablet weight, and based on present control and stop limits, the press force setpoint is then adjusted to maintain the desired tablet weight. As such, there is no operator intervention, or any subjective decision made as to when a tablet weight adjustment is required.
While many machines offer a tablet weight, thickness, and hardness measurement system, there is not any system available that can handle every tablet geometry, especially with regard to a consistent measurement of tablet hardness. For more basic tablet shapes, these systems can be effective; however, for more elaborate shapes, a weight-only system is generally more appropriate. In most systems, the actual tablet thickness and tablet hardness are compared to upper and lower limits, and a closed feedback loop is generally not employed. Any adjustment to thickness will also impact the compression force, which in turn, requires an adjustment to the press force setpoint. The result is too many control loops in progress at the same time. As such, these systems are designed primarily to provide closed loop control based on tablet weight, and to stop the machine if there is any violation of tablet thickness and hardness limits.
Automated tablet sampling stations are especially applicable in high-potency applications where operator exposure in the compression room must be minimized. These systems can sample tablets automatically and will also store additional samples for subsequent in-process testing, including dissolution and friability.
All tablet sampling data is generally written to a batch report that provides a complete statistical assessment of tablet weight, thickness, and hardness data. In addition, real-time trending is displayed to confirm that the process is operating within established limits. Tablet sampling data should also be stored by batch number, and should be available for export in a standard file format. For critical batch data, there is an increasing trend to utilize a write-once, read-many media that will provide additional data security once the data has been stored.
End of Part 1
For more information: Frederick J. Murray, President, Korsch America, Inc., 12 Worlds Fair Dr., Suite K, Somerville, NJ 08873. Tel: 732-563-4222. Fax: 732-563-9499.