TABLET COMPRESSION: MACHINE DESIGN
Press Design and Layout:- Modern rotary tablet presses are typically designed in separate machine sections (press zones). 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
With the proper separation of these areas, only the compression zone is exposed to material, thus reducing 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 double-sided machine has
- two feeding stations,
- two sets of precompression 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.
UPPER CAM SECTION:- The upper cam section is typically shrouded and sealed to prevent exposure of 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:-
- 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 dwell 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.
- Upper punch lowering cam:- The upper punches are lowered into the die cavity by the upperpunch 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.
- Upper precompression and main compression rollers insertion depth adjustments:- Insertion depth for both precompression 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. 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. Insertion depth can be varied between 2 and 6mm on most machines and is typically maintained between 3 and 4 mm. For precompression and main compression, the insertion depth should be maintained at approximately the same position. On most modern rotary tablet presses, the adjustments for precompression and main compression insertion depth are independent. However, on many older designs, the precompression roller is attached to the main compression roller assembly and its position is measured relative to the main compression roller position. In this way, the ratio of precompression to main compression remains constant as machine adjustments are made.
- 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 pullup 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 possibility of machine damage.
- 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. Many modern rotary tablet presses use polymer composite cams for non-impact points. These cams have excellent qualities in that they provide superior abrasion resistance and have self-lubricating properties minimizing cam and tool wear, heat generation, and noise, and ultimately resulting in increased machine speeds. However, cams that undergo impact (e.g., ejection cam) and stress (e.g., weightregulation cam) require metal construction with good impact resistance. For this purpose, an aluminum–bronze alloy provides superior abrasion resistance and excellent impact strength.
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 material. Proper maintenance and setup of the compression section is critical for optimal press performance. The primary components of the compression section are explained in the following section:-
- 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 (rat holing) in the granulation. The discharge outlet of the hopper should be as large as possible reaching into the feeder to prevent material bridging. On many machines the base of the hopper is equipped with a valve to shut off material flow to the feeder if necessary. Depending on the nature of the granulation, the hopper valve can contribute to material bridging. For materials with very poor flow characteristics, a slide valve may be preferable to a butterfly valve.
- Gravity Feed Frame:- Older machines typically employ gravity feed frames which rely on gravitational and turret rotational forces to achieve die fill. 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.
- 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 chamber/paddle system typically performs better than the two chamber/ 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.
- Alternate systems offer level sensors that are designed to provide a constant quantity of material to the feeding chambers, thus also eliminating the effect of head pressure on material flow.
- The force feeder chambers contain material baffles that function to prevent the material from randomly packing in the chambers, which results in non-uniform fill.
- Optimal systems provide minimal energy input and minimal particle mixing while providing uniform fill.
- The speed of the paddles can be sychronized with the die table speed minimizing tablet weight variation. The appropriate paddle speed can be determined by using a force-control system that displays the standard deviation of the compression force. The optimal feeder speed is determined by adjusting the feeder speed to achieve the lowest standard deviation in the compression force, which corresponds to the least weight variation.
- A rectangular paddle design is typically used to minimize powder mixing in the feeder. However, for materials with poor flow characteristics (bridging in the hopper) due to interparticle friction, a round (or wedge) paddle design can improve flow by forcing interparticle slippage. Under these circumstances, round paddles provide a mixing effect with possible impact on uniformity, compressibility, and dissolution.
- The feeder height above the die table surface is very important to minimize product loss and prevent scaling of low melting materials. The feeder height is usually maintained between 0.05 and 0.10mm(0.002–0.004 in.). Very fine particles may require a feeder height of 0.025mm (0.001 in.).
- 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 re-circulation 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 flinging of material out of the die cavity at high rotational speeds.
- Precompression and Main Compression Rollers:- After die fill and scrape-off, the punches rotate to the precompression station where an initial force is applied to the compact. The tablet is frequently partially formed during the precompression 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 precompression roller.
- The compression rollers are made of premium tool steels and are surface hardened. Because the compression characteristics of powders are time-dependent (the exact extent of this dependency depends on the primary modes of deformation), the final tablet properties depend not only on maximum compression forces but also on the rate at which these forces (rate of deformation) are applied and removed.
- On a rotary tablet press, the rate of deformation is determined by the tangential velocity of the punch and the compression roller diameters.
- The tangential velocity of the punch is a product of the press speed and the die table circumference (i.e., die table rpm × 3.14 × pitch circle diameter). As the tangential velocity increases, the rates of compression and decompression increase while the overall compression time decreases.
- The roller diameter affects both the rate of compression and decompression. As the diameter increases, the rates of compression and decompression decrease. The roller diameters should be as large as possible to provide the lowest possible rates of compression and decompression. If compression problems exist, the longest time for compression should be allowed by running at low press speeds and running on machines with a small pitch-circle diameter.
- The compression rollers are typically mounted to a block assembly that is adjusted by an eccentric or a vertical slide adjustment.
- 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 take off to prevent tablet backup; modifications are necessary for shaped tablets. On high-speed machines it is critical to move the tablets off the die table as quickly as possible.
- Under some circumstances, repositioning of the Plexiglas cover on the tablet stripper to provide minimal clearance between the tablet and the cover may prevent shingling of tablets.
- The height of the lower punch at the point of scrape-off should always be checked to verify that it is not below the die table surface.
- Typically the lower punch should protrude approximately 1–2mm from the die table surface at the point of scrape-off.
- For deep concave tablets, a protrusion height above 2mm may be necessary.
- Material Recirculation:- Material is recirculated 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 recirculate 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).
- The point of re-entry of the granulation into the feeder corresponds to the location where the lower punch enters into the fill cam when it is flush with the die table surface. Therefore, the material from the recirculation channel is typically the material in contact with the lower punch face and is the first material to be filled for each die cavity. The effect of the material in the recirculation channel should always be evaluated when compression problems occur, which can be associated with the lower punch.
- Dust Extraction:- Adequate dust extraction is necessary to maintain high-speed operation for extended periods of time. 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.
- However, the proper balance of dust extraction without high levels of material loss must be determined. If the dust extraction level is too high material could be extracted from the die cavities and the recirculation channel.
- Furthermore, the dust extraction systems preferentially removes the fine particles. Therefore, if the granulation is a direct-compression blend where the active constituent is of fine particle size, minimum dust extraction levels combined with minimal recirculation may be necessary to prevent a loss of active constituents (resulting in possible low assay).
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 precompression and main compression roller positions.
Additionally, any motors necessary for automatic machine adjustment are contained in this section.
- 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.
- Typical fill cams have a range of approximately 10mm with an increment range of 4mm (e.g., 0–10, 4–14, 8–18, and 12–22 mm). Special-order or very shallow fill cams are also available (e.g., 0–6 mm).
- A deep fill cam can cause over-packing of the feeder, which could result in jamming, temperature increases, and over-mixing of granulation or lubricant.
- 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 is 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.
- The lower punch rides on the dosing rail that maintains the lower punch at a constant height at the final fill depth.
- In order to minimize vertical movement (and subsequently punch chatter) the head of the lower punch is held tight against the lower dosing rail by the holding-down cam which is spring loaded.
- To ensure that the holding-down cam is tight against the head of the lower punch, a 1–5mm (0.04–0.20 in.) gap should be maintained between this cam and the lower dosing cam when the lower punch rests on the dosing cam.
- During press setup, the fit can be easily checked by placing a lower punch in the weight regulation unit and verifying the absence of vertical movement of the punch. This function is critical to minimize tablet weight variation at high speeds.
- The lower dosing rail and holding-down cam should always be made of a tough, abrasion-resistant material; aluminum–bronze alloy is highly recommended.
- The condition of the dosing rail and the holding-down cam should be checked every 6 months to ensure optimal performance.
- If significant wear is observed, the cams should be replaced or, in the case of the holdingdown cam, reworked to provide a tight fit.
- As the lower punch leaves the dosing unit, it is pulled down slightly (approximately 2mm) by the lowering cam. Periodically the condition of the lowering cam should be checked to ensure proper lowering of the punch in order to minimize weight variation.
- Lower-punch tight sensors are typically mounted in either of the two locations. The transition cam from the fill cam to the weight regulation unit is an ideal location to measure lower punch tight forces because it is designed to raise the level of the lower punch to the final dosing height. In addition, the lowering cam can measure the counter-force to pulling down the lower punch.
- 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. 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.
- Precompression and Main Compression Rails:- The precompression rail provides the transition support for the lower punch from the weight regulation unit to the precompresison roller, while the main compression rail provides the transition support from the precompression roller to the main compression roller. The optimal press designs provide positive support with these cams by ensuring that the lower punch head flat is always in contact with the rail surface. In this way, there is no abrupt vertical movement of the lower punch as it passes to the compression rollers. Vertical movement of the lower punch before precompression and between the compression stations can cause the introduction of air into the bottom of the compact, resulting in capping at the lower-punch face.
- Adjustment of Lower Precompression and Main Compression Roller Thickness:- The position of the lower compression rollers is adjusted from the lower cam section. Typically, the machine-control panel shows edge thickness on the indicator. However, adjustment of the edge thickness actually results in adjustment of the lower roller position only. For machines that have no mechanical link between the upper and lower compression rollers, the tablet edge thickness indication on the control panel is only valid at the specific insertion depth that was set during edge thickness calibration. However, for some of the electronic, fully automated machines, the machine automatically moves the lower compression roller during the insertion depth adjustment to maintain the same tablet edge thickness.
- Ejection Rail:- After compression the lower punch impacts the ejection rail (or on some machines an ejection roller). Upon impact the tablet is broken free from the die side wall and begins to move up the die as the machine rotates. The ejection rail should be made of a tough, abrasion resistant material such as aluminum bronze alloy.
- Scrape-Off Rail:- After riding up the ejection rail, the lower punch rides on the scrape-off rail to provide a constant height for tablet scrape-off. The height of the lower punch scrape-off can be adjusted to optimize the single tablet rejection height or the tablet scrape-off height.
- Force Overload System:- Most tablet presses are equipped with force overload systems designed to prevent machine and punch damage. On most tablet presses, a maximum allowable compression force can be set. This force setting is actually the counter force to the measured compression force. If the compression force exceeds this counter force, the compression assembly will back-off thus reducing the force. Most tablet presses use a hydraulic, air, or spring-loaded system on the lower compression assemblies (both precompression and main compression). In these systems the hydraulic or spring systems are calibrated to the measured force in the die and move instantaneously during an overload condition.
LOWER MECHANICAL SECTION:- The lower mechanical section houses the main drive motor, the gearbox, the hydraulic pump, the lubrication pump, and the signal wire distribution. Proper venting and cooling of the lower mechanical section is essential to prevent machine damage and minimize heat generation. This section should be equipped with a cooling system for products that are sensitive to heat generation (e.g., contain low melting point components that are prone to picking and sticking).
ELECTRICAL SECTION:- The electrical section contains all electrical controls and components (e.g., programmable controllers, relays, contacts, and fuses), the signal wire distribution, and the integrated or remote force control systems. On many machines the electrical section is connected to the front of the press, thus minimizing space requirements.
LUBRICATION SYSTEM:- Most high speed rotary tablet presses employ automatic lubrication systems during operation.
- Effective punch lubrication is essential for the movement of the punches in the turret, in the cam tracks, and under the compression rollers.
- Punch-lubrication systems allow high speed operation over extended periods of time while minimizing cam and tool wear and reducing heat generation.
- The lubrication pump is typically maintained in the lower mechanical section and allows variation of both the lubrication interval and time duration. The quantity of oil delivered is normally determined by the oil distribution nozzles connected to the distribution manifolds.
- The upper punches are usually lubricated on the punch head flat via a felt pad located in the upper punch dwell cam or the upper punch lowering cam.
- On many modern designs the upper punch barrel is also lubricated. The punch barrel is lubricated via radially drilled holes in the turret that transports oil to the punches by rotational forces, or by overlubricating the punch neck and allowing gravity to transfer the oil down the barrel.
- Because the upper punch shaft has the greatest area of contact with the upper turret, lubrication of the punch shaft is critical to allow high-speed operation. Poor lubrication of this area can result in heat generation and metal expansion, ultimately causing machine seizure and severe damage.
- The lower punches are lubricated at the punch neck. Oil distribution lines are frequently provided to lubricate both sides of the punch neck. Subsequently, gravitational forces distribute the oil over the head of the punch. The lower punch barrel is typically lubricated by radially drilled holes in the turret.
- All presses equipped with punch-lubrication systems require oil and dust seals to prevent oil contamination in the product and dust contamination in the turret punch sockets. These seals are normally double lipped, designed to strip oil on one side and powder on the other.
- Inadequate punch lubrication can lead to excessive heat generation, which could affect tablet properties.
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