Pharmaceutical tablets are generally produced on rotary tablet press shown in Fig.1, where upper and lower punches reside in the upper and lower turret, respectively. The dies are inserted into the die table and secured by die lock screws. The upper and lower turret and the die table are precisely aligned. The movement of the punches is controlled by cam tracks and compression rollers. As the entire assembly rotates, the upper and lower punches move along the cam tracks to accomplish die fill, tablet compression, ejection, and scrape-off.
Tablet compression can be separated into the two distinct yet equally important phases of die fill-weight adjustment and tablet formation as shown in Fig.2. As die fill begins, the lower punch face is initially flush with the die table surface as the lower punch enters the overfill cam at the entry of the feeder. The lower punch travels under the feeder and is pulled down by the overfill cam. At this point, the lower punch has passed through approximately 50% of the feeder and the die cavity contains more material than required.
After overfilling the die cavity, the lower punch is adjusted to a constant height as it passes into the weight-regulation unit. The constant height, known as the fill depth, is measured as the distance between the lower punch face and the die table surface. Since die fill is volumetric, the constant height of the lower punch in the weight-regulation unit provides a constant volume of material. Therefore, the fill depth is affected by the density of the granulation. Variation of granulation density between batches results in different fill depths, whereas variable granulation density within a batch results in fluctuating fill depth requirements.
As the lower punch passes from the fill cam to the weight-regulation cam, the excess material is pushed back into the feeder and scraped off at the top of the die table by the excess material stripper and directed into a recirculation channel. On many modern presses, the lower punch is lowered by approximately 2-4 mm relative to the top of the die table after the excess material stripper. This lowers the material away from the top of the die table, minimizing uncontrolled loss as the upper punch enters into the die cavity after scrape-off. Under these circumstances the upper punch does not contact the top of the material until it enters into the die, minimizing material loss and weight variation. Additionally, lowering the slug of material away from the die table surface reduces material loss due to the centrifugal force of the rotating die table.
Overfilling of the die cavity is necessary to achieve uniform tablet weights and to optimize machine running conditions. However, at times excessive overfilling can lead to other problems such as excessive wear for abrasive raw materials, particle size reduction for friable granulations, material segregation, and over-mixing of lubricant. Therefore, the effect of different machine running conditions must be evaluated for each different product separately.
The material that is directed into the recirculation channel is subsequently introduced back into the feeder at the tablet stripper or the inside edge of the feeder. It is worth noting that the paddle in the feeder at the point of material entry rotates in the opposite direction as the turret to aid in die fill and induce flow back into the feeder.
Frequently the maximum machine speed may depend on die fill characteristics due to excess tablet weight variation at high machine speeds. However, because the compression characteristics of most pharmaceutical products exhibit visco-elastic properties, the press speed may also have a major effect on the compressibility of the material. For this reason, the ability to compress a tablet adequately is often the overriding factor to consider in tablet compression.
The process of tablet formation begins as the upper punch is lowered directly into the die cavity after the excess material stripper. As mentioned previously, it is advantageous if the slug of material is lower than the die table surface as the upper punch enters to minimize uncontrolled material loss and weight variation.
After the upper punch enters into the die, the upper and lower punches begin to move toward each other as the punches ride along cam tracks toward the pre-compression rollers. At the pre-compression stage, the initial (and typically the lower) compression force is applied. Traditionally, tablet presses were equipped to apply a 20 kN maximum pre-compression force using relatively small compression rollers (approximately 100 mm (4 in.) diameter rollers). However, to improve flexibility, many modern rotary tablet presses are equipped with identical pre-compression and main compression force capabilities, allowing the application of 80-100 kN forces using 250–300-mm-diameter compression rollers.
After the application of the pre-compression force, the punches move toward the main compression rollers where the final (main) force is applied. As the punches impact the rollers, the compression force increases until the punch head flat is tangent to the compression roller and maximum force is applied (Fig. 2.3). The applied compression force is a measured value and depends on the distance between the punches and the quantity of material in the die. After the main compression, the upper punch is pulled out of the die cavity while the lower punch impacts the ejection cam to begin the ejection process. As the die table continues to rotate, the lower punch raises the tablet out of the die cavity to eject the tablet to the point of scrape-off.
Press Design and Layout
Typical sections to provide separation and isolation of the compression area from the other components are as follows:
- Upper cam section
- Compression section
- Lower cam section
- Lower mechanical section
- Electrical section
- Lubrication system
With the proper separation of these areas, only the compression zone is exposed to the material, thus reducing the cleaning and change-over-time of the tablet press. In addition to the machine sections, an understanding of other machine subsystems is necessary, such as the lubrication system and the diagnostic systems (safety systems) to achieve optimal machine performance.
Modern rotary tablet presses are either single-sided or double-sided. A single-sided machine has one feeding station, one set of pre-compression and main compression rollers, and one discharge station. These machines produce one tablet per punch station per die table revolution. A double-sided machine has two feeding stations, two sets of pre-compression and main compression rollers, and two discharge stations, and produces two tablets per punch station per die table revolution. The double-sided machine operates identically to the single-sided machine with the exception that the excess material from the first feeding station passes into the second feeding station. A double-sided machine has a higher output than a single-sided machine. Its pitch circle diameter is also greater, which could result in weight uniformity and compressibility issues.
(A) Upper Cam Section:
The upper cam section is typically shrouded and sealed to prevent exposure to the material. It consists of the upper cam track, all upper compression rollers, and all adjustments to the position of the upper compression rollers. The primary components of the upper cam section are as follows:
1. Upper punch removal/dwell cams: The upper punches are loaded and removed from the machine at this location. These cams typically reside directly above the material feeder. In many press designs, the upper punch dwells cam is designed to measure the tightness of the upper punches in the turret. A spring-loaded cam designed to raise the upper punch slightly (1-4 mm) is connected to a proximity sensor. If the punches are too tight then the spring-loaded cam falls instead of raising the upper punches, thus tripping the proximity sensor and shutting down the machine. In alternative press designs, the upper punch tightness is measured in the upper-punch pull-up cam, typically by a strain gauge measurement of the lifting force.
2. Upper punch lowering cam: The upper punches are lowered into the die cavity by the upper punch lowering cam. This cam is typically CAD optimized to minimize the acceleration and velocity of the upper punch as it enters into the die cavity. In this way, the upper punch travels in a smooth and controlled manner as it enters the die cavity, thus improving weight uniformity.
3. Upper pre-compression and main compression rollers insertion depth adjustments: Insertion depth for both pre-compression and main compression is adjusted in the upper cam section. The insertion depth determines the location of tablet formation in the die cavity relative to the top of the die table as shown in Fig. ????. It is measured as the distance at which the upper punch enters into the die at the tangent between the upper punch head and the compression roller.
4. Upper punch pull-up cam: After compression, the upper punch enters into the upper-punch pull-up cam, which removes the upper punch from the die cavity. This cam provides an excellent location to measure the upper punch pull-up force that determines the tightness of the upper punches. Compared to the upper punch dwell cam, this location has the advantage of determining the punch tightness not only in the turret but also in the die cavity. Detection of tight punches at this location prevents almost all possibilities of machine damage.
5. Cam material of construction: Both the upper and lower cam sections use cams to guide the punches while the turret rotates. These cams are typically made of various materials such as steel, bronze, or alloy. Most of the cam tracks in the turret are designed to smoothly guide the punches. However, cams that undergo impact (e.g. ejection cam) and stress (e.g. weight regulation cam) require metal construction with good impact resistance. For this purpose, an aluminum–bronze alloy provides superior abrasion resistance and excellent impact strength.
(B) Compression Section:
The compression section contains all components that are exposed to the material, such as the material hopper, the feeder, the excess material stripper, the upper and lower turrets, the die table, and the tablet stripper. Additionally, the dust-collection shrouds are located in the compression section. Proper shrouding of this area ensures that none of the upper and lower punch heads, compression rollers, and cam tracks are exposed to the material. Proper maintenance and setup of the compression section are critical for optimal press performance.
The primary components of the compression section are explained in the following sections:
- Material hopper.
- Gravity feed frame.
- Force feeder.
- Excess-material stripper.
- Pre-compression and main compression rollers.
- Tablet stripper.
- Material recirculation.
- Dust extraction.
1. Material hopper: The material hopper is an integral part of the feeding system. Typically, it is capable of holding approximately 5-10 kg of material. Low-level sensors are mounted in the hopper to signal an alarm, shut off the machine or activate a feeding mechanism to deliver more material when the product falls below this level. The material hopper should be symmetrical with steep discharge angles to promote mass flow and prevent funnel flow (ratholing) in the granulation. The discharge outlet of the hopper should be as large as possible reaching into the feeder to prevent material bridging.
2. Gravity feed frame: These feed frames provide good performance for materials with good flow properties but are typically limited to slow machine speeds. On the other hand, gravity feeders do not agitate the product and impart no energy. Therefore, they offer advantages for products where material segregation and over-mixing are of concern. For example, products that are sensitive to over-blending of magnesium stearate (i.e., exhibit capping when over-blended) may exhibit improved compressibility by using a gravity feeder as opposed to a force feeder.
3. Force feeder: Force feeders are typically multi-chamber and multi-paddle feeders. These feeders are critical to allow optimal press performance at high machine speeds with minimal weight variation. For products with good flow properties, the feeder should move the material from the overhead hopper to the dies with minimal mixing. Most force feeders contain two or three chambers and paddles. The three chambers/paddle system typically performs better than the two chambers/paddle designs. The top paddle and feed chamber are connected directly to the hopper and move the material from the overhead hopper to the filling chambers located directly above the die cavities. The top chamber eliminates the effect of the head pressure on material flow, thus providing uniform die fill regardless of the quantity of material in the hopper.
4. Excess-material stripper: The excess-material stripper is located immediately after the feeding system and scrapes off the excess material on the die table after weight adjustment. It is often overlooked during setup although it is one of the most critical components of the tablet press. A brass stripper is employed, which sits flush on the die table under spring tension. The material is scraped off just before the lowering cam. The brass stripper directs the excess material into the recirculation channel. A tail-over-die covers the die cavity after scrape-off to the point of upper punch entry. This design minimizes uncontrolled material loss due to the flinging of material out of the die cavity at high rotational speeds.
5. Pre-compression and main compression collars: After die fill and scrape-off, the punches rotate to the pre-compression station where an initial force is applied to the compact. The tablet is frequently partially formed during the pre-compression stage. Subsequently, the upper and lower punches move together under the main compression rollers where the final tablet is formed. The main compression roller is usually larger than the pre-compression roller.
6. Tablet stripper: The tablet stripper scrapes off the tablets from the lower punch and directs them down the discharge chute. On high-speed machines, special attention must be paid to the tablet takeoff to prevent tablet backup; modifications are necessary for shaped tablets. On high-speed machines, it is critical to move the tablets of the die table as quickly as possible.
7. Material recirculation: Material is re-circulated from the center of the turret into the feed frame. Some press designs include recessed recirculation channels to minimize particle attrition and prevent excess material loss to the vacuum system. It is critical not to re-circulate too much material because this can result in low product yields and can have a detrimental effect on the powder’s physical properties, which could result in poor compressibility, uniformity, and final properties (e.g. reduced dissolution rate).
8. Dust extraction: Adequate dust extraction is necessary to maintain high-speed operation for extended periods. The entire compression area should be shrouded to minimize dust infiltration into other press areas. Effective dust extraction minimizes dust and oil contamination on the surface of the tablets, which could produce black specs. Insufficient dust extraction results in excessive material build-up on the lower and upper punches leading to tight punches.
(C) Lower Cam Section
The lower cam section is completely sealed from the compression section. It houses the lower compression rollers, the entire lower cam track that guides the lower punches as the turret rotates, and all adjustments for the lower pre-compression and main compression roller positions. Additionally, any motors necessary for automatic machine adjustment are contained in this section.
- Fill cam.
- Weight regulation cam.
- Lower punch brakes.
- Pre-compression and main compression rails.
- Adjustment of lower pre-compression and main compression roller thickness.
- Ejection rail.
- Scrape-off rail.
- Force overload system.
1. Fill cam: The fill cam is designed to lower the punch to overfill the die cavity. Lower punch fill cams are typically available in a variety of sizes that are changed depending on the final fill depth as determined by the weight regulation cam. Press manufacturers recommend a fill cam in which the weight regulation cam operates in the approximate center of the fill cam.
2. Weight regulation cam: The lower punch travels from the fill cam to the weight regulation cam, which determines the final volume of material that remains in the die cavity after scrape-off. Proper design and operation of this unit are essential to ensure uniform tablet weights. In general, the unit should operate in a manner to ensure smooth punch travel minimizing punch chatter as the lower punch is raised to a precise and constant height.
3. Lower Punch Brakes: Most rotary tablet presses are equipped with lower-punch brakes that are Teflon tipped and spring-loaded to apply constant pressure to the lower punches. Alternatively, some manufacturers apply pressure to a friction belt that provides resistance on the lower punches. The lower-punch brakes act as a ‘‘retention’’ system for holding the lower punches in place during press setup. More importantly, these systems help to minimize lower punch chatter at high press speeds thus minimizing tablet weight variation.
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