Compression/Compaction
By Dr. Keith Marshall
General Introduction
Tablets comprise the largest group of delivery systems for all prescribed and over-the-counter medications, and therefore, warrant detailed consideration of their design, development and manufacture. The main objective of this chapter is to outline contemporary approaches to the development and production of these important dosage forms. The underlying basic principles of tablet technology and their pragmatic applications will be reviewed.
Compressed tablets may be defined as drug delivery systems intended to be taken by mouth and made up of a single solid body.
Some of the most common types include:
· Immediate release, coated or uncoated (swallowable)
· Modified release, coated or uncoated (swallowable)
· Effervescent (dissolved/suspended in water before drinking)
· Chewable (chewed before swallowing)
· Lozenge (dissolved slowly in the mouth)
The most popular of these is the immediate release tablet, intended to be swallowed whole, and release the medicament rapidly in the stomach.
The Pros and Cons of Tablets
Some of the reasons for the popularity of tablets include:
· Ease of accurate dosage, patient takes one or more discrete units
· Good physical/chemical stability,because of low moisture content
· Competitive unit production costs
· Elegant distinctive appearance, easily identifiable
· High level of patient acceptability and compliance
Among the few disadvantages of tablets as a dosage form is the possibility for bioavailability problems, due to the fact that dissolution must precede absorption of the active ingredient. This requires that tablets for immediate release of medicament should disintegrate rapidly after ingestion to facilitate solution of the active component.
In addition, there is the chance that GI irritation could be caused by locally high concentrations of medicament. Also, from a patient standpoint, a small proportion of people do have difficulty in swallowing tablets, and so size and shape become important considerations.
The Sophisticated Tablet
It should be realized that tablets are rightly regarded as complex drug delivery systems, and that to trivialize their design, development and manufacture is certain to invite significant problems at some stage in development or during product life.
With this philosophy in mind, it is obvious that it will be important to begin by considering the important and relevant properties of the materials used to manufacture a tablet. There is no doubt that many tableting problems arise because of failure to pay attention to the properties of the raw materials, and/or appreciate how they are likely to behave when they are subjected to the tablet machine cycle.
Some Important Properties of Powders Intended for Tablets
During the manufacture of tablets, bulk powders or granules will be subjected to significant, and in some cases, massive applied mechanical loads. Their behavior under these circumstances may be the major factor controlling the success or failure of the manufacturing operation involved.
Deformation
All solid materials change in shape/volume when subjected to mechanical forces, perhaps better expressed as the force per unit area over which it acts, i.e., a pressure. These latter forces are sometimes considered at points in the system and are referred to as the stress on that particular region of the material. At least three types of stress may be distinguished, as shown in Figure 1:
The relative change in geometry is called strain. In the present context of particular interest are the strains caused by application of a compressive stress as shown in Figure 2.
There are several ideal model behaviors that facilitate understanding of what may be occurring in the tableting materials when under load. When force is first applied to the material in the die of the tablet press, there will be some degree of repacking of the particles, leading to a higher bulk density. This is usually limited to the initial low load region and is quickly superceded by other phenomena.
One possibility is that application of an increasing compressive load results in failure of the structure and the particle breaks into two or more pieces. This behavior, known as brittle fracture, is found in such excipients as dicalcium phosphate, some sugars, and in some active ingredients. Another possibility is that the material behaves like a spring, and the application of an instantaneous stress causes a matching instantaneous strain response, with immediate total recovery to the original geometry on removal of the load, as shown in Figure 3.
At the other extreme, the strain caused by the applied stress may continue to increase with time until the load is removed (see Figure 3). At this point, there is no tendency to recover to the original geometry. Therefore, the amount of strain is time dependent and is not spontaneously reversible. This viscous response to loading is called plastic deformation.
In practice, most solids demonstrate properties which lie between these two extremes and are described by models combining them, resulting in visco-elastic behavior. The simplest combination of these models that might equate to a real powder particle is illustrated in Figure 4. Note that in this case there will be some degree of time dependency to the deformation process, and if the solid material is not given time to deform, then the solid will have to react in some other way. This could be a major problem as the process is scale up to faster presses with less time available to make each individual tablet. Note also that there will be some time dependent recovery after the load is removed. If a sufficiently strong tablet has not been formed at the point of maximum loading, then the recovery may result in failure of the tablet structure, or at least in localized regions of it.
Effect of Massive External Forces
Because of ambiguities in existing literature, it is first necessary to clarify the definitions relating to the process of tableting. It may be defined as the compaction of a powdered or granular mixture in a die, between two punches, by application of a significant mechanical force.
The compaction process itself may be simply stated as the compression and consolidation of a two-phase system (particulate solid/air) due to applied forces. Compression is considered an increase in bulk density as a result of displacement of the air phase by solid. Consolidation is an increase in mechanical strength of the mass as a result of particle-particle interactions.
Compression
On application of external force, the bulk volume may be decreased by the several mechanisms referred to above. The significance of these deformation mechanisms to the tableting process is that if brittle fracture occurs during the unloading and ejection processes, then the tablet structure may fail. Similarly, since elastic behavior is spontaneously reversible, the tablet must be strong enough to accommodate this elastic recovery without failure.
Because most tablet formulations are mixtures and many contain organic compounds, the sequence of events as the applied force increases is likely to be a limited repacking of particles, giving way to some elastic deformation.
However, in many pharmaceutical systems, the applied force will exceed the elastic limit of the material. Subsequent compression will then be due to visco-elastic or plastic deformation, and/or brittle fracture, depending on whether the material is ductile (easily deformed) or brittle.
Consolidation
In tableting, consolidation is due mainly to the close approach of particle surfaces to each other, facilitating intermolecular bonding by van der Waals forces, for example. Alternatively, since the entire applied load must be transmitted via particle-particle point contacts, considerable pressures may develop at these points. This can cause frictional heating with a possibility of localized melting, especially if a low melting point solid is present. The resultant relief of the local stress at the point contact would lead to resolidification, forming a bridge between the particles.
It follows that the consolidation process will be influenced by:
· The chemical nature of the surface
· The extent of the available surface
· The presence of surface contaminants
· The intersurface distances
It is easy to see how the last three of these factors might affect the compaction process, since if large, clean surfaces can be brought into intimate contact, then bonding should occur. Brittle fracture (and plastic deformation) should generate clean surfaces, which the applied force will ensure are kept in close proximity. Of course, as compaction proceeds, some of the bonds that are formed will be broken to facilitate further compression. Nevertheless, the overall effect is usually an increasing number of bonded areas.
It is also important to appreciate that having compacted the material, the load must be removed and the tablet has to be ejected from the die. This will introduce new stresses into it. Therefore, at the point of maximum applied load, a structure which is strong enough to accommodate the new stresses must be developed. In other words, the mechanical strength of the tablet will be a reflection of the number of surviving bonds after it leaves the press.
On the other hand, plastic deformation is not spontaneously reversible, but is time-dependent, and therefore, tablet press speed may be a major factor. However, continuing plastic deformation during unloading and ejection may relieve the induced stresses during these parts of the tableting cycle, and thus avoid structural failure of the tablet.
Die Compaction (Tableting)
All tablet presses employ the same basic principle. They compact the granular or powdered mixture of ingredients in a die between two punches; the die and its associated punches being called a station of tooling. The simplest arrangement is illustrated in Figure 5.
Since tableting involves the two distinct processes of compression (volume reduction) and consolidation (increasing mechanical strength), any attempt to identify the dominant mechanisms in a particular case must take both into account. In addition, because time-dependent phenomena may be involved, the sensitivity to press speed must also be studied.
Simple Tablet Press Cycle
It will be convenient to study the basic process by reference to this simplest tableting cycle, i.e., that of a single station (eccentric) press. In such a system, the cycle of punch movements will be as illustrated in Figure 6, and these will generate the typical force profiles shown in Figure 7.
It is important to appreciate what forces are acting during tableting because changes in the applied forces may well swamp the effect of other variables. Therefore, formulation development must be carried out under conditions where the maximum load applied to the tablet mass is accurately known. In a simple tableting event, mechanical force is applied to the upper punch, FU, which goes through the cycle, and produces the reaction forces shown in Figure 8:
· That transmitted axially to the lower punch, FL
· That transmitted axially to the die-wall, FD
· That transmitted radially to the die-wall, FR
and since there must be an axial balance of forces, then:
FU = FL + FD
System Geometry
If the simple system incorporates flat faced cylindrical tooling, we can distinguish the following properties of material fed to the die:
· True volume of its solid components, va
· Bulk volume occupied by the material, vb, equal to the volume of the die cavity 0.25šD2H (for cylindrical geometry)
· Voidage within the material, vv, where
vv = vb - va
The extent of the residual air spaces in tablets plays a major role in dissolution by wicking liquid into the tablet structure to disintegrate it. It will be convenient to consider these void spaces in terms of the dimensionless quantity called porosity, E, defined as follows:
E (in %) = (1-va/vb)100
Force-Time Profiles
It is important to appreciate that for a given set of pressing conditions, the force generated as a result of punch movements is a function of the true volume of the solid in the die cavity and NOT the weight. It follows that comparative testing should be carried out with compression weights adjusted from the value of true density, to give a constant true volume of solid in the die during each experiment.
One of the simplest plots which can be obtained from the most basic instrumentation on a tablet press is the compaction force versus time profile, as seen in Figure 9. The area under this curve is indicative of the resistance the material offers to the compaction process. A common term found in pharmaceutical literature refers to a dwell time under load. This is now generally accepted to mean the time for which 90% of the peak load is being applied. It is also approximately the same as the time it takes for the punch head flat to traverse its contact with the lowest part of the upper compression roll of a rotary press, or highest part of the lower roll.
Figure 9: Compaction force versus Time profile
More specifically, if the compaction event is carried out slowly, then the ratio of the area to peak force, over the area from peak force, gives an approximate indication of the extent of the elastic recovery. The two areas should be equal if the material is perfectly elastic, and the ratio will be larger the more brittle fracture or plastic deformation dominates the behavior of the material.
Compressibility
A finite end to the compressional process occurs when the air spaces are completely eliminated. And there is frequently an inverse relationship between the residual porosity and the strength of the compact. This changing porosity of the tablet mass during the tableting cycle is a convenient and valuable means of following the degree of compression achieved as a result of the applied force. Several workers have attempted to analyze the porosity versus force plots, and many equations have been proposed for the force region in which most tablets are produced. All of these equations include a term for the initial porosity of the mass, just before load is applied. This means that for a given applied force, the final porosity depends on the initial porosity. However, there is no universal equation that describes the behavior of a wide range of materials over the entire force profile.
Heckel Plots
Among these many equations relating porosity to applied load is that usually credited to Heckel1 which has been widely used in tableting studies, producing data similar to that shown in Figure 10. It is based upon an equation involving the assumption that the material behaves in an analogous way to a first order reaction, where the pores are the reactant, i.e.:
Ln E-1 = KyP + Ko
where Ko is related to the initial repacking stage, or function of Eo, and Ky is a material-dependent constant inversely proportional to its yield strength Py, by the following;
Py a 1 / Ky
where Py is the point at which plastic or visco-elastic deformation becomes dominant.
Note that a high slope is indicative of a low yield pressure and hence plastic behavior should be expected at low applied loads, and vice-versa. It follows that materials that are brittle in nature will tend to give low values for Heckel plot slopes.
The curved region at the low end of the pressure scale on Heckel plots has received additional attention and has been related to the initial packing stage. It should be noted that the Heckel equation predicts that as the porosity approaches zero at higher pressures, the y axis values of the plot will approaches infinity, and therefore the linear region must be a limited one.
Porosity data can be obtained from in die measurements using instrumentation which follows punch movements throughout the compaction cycle. Alternatively, measurements of tablet geometry out of die can be made when the tablet has been ejected. It is important to appreciate that the data obtained from the two techniques may be significantly different, since the latter will include any tablet expansion as a result of elastic recovery on unloading. For this reason, the method of obtaining porosity data should be clearly stated.
Density Distributions in Compacts
An intrinsic property associated with materials compacted in a die is the development of a typical pattern of density. That shown in Figure 11, for a double-ended compaction (such as in a rotary tablet machine), is a possible result of this phenomenon and might be the cause of the typical hard core found in some tablets (seen during disintegration tests). It is most likely to be present at higher applied forces and when the thickness to-diameter ratio is large.
Frictional Effects
One of the most common manifestations of the strength of the forces at surfaces is the phenomenon of friction. This effect opposes the relative motion between two solid surfaces in contact, so that in the absence of a sustaining force, motion ceases. In the above example of the tableting event (see Figure
, the simplest equation to describe friction between the tablet materials and the die wall is:
FD = µwFR
where µw is the average coefficient of die-wall friction, an important additional factor which will now be considered.
Frictional effects undoubtedly play a major role in the progress of the compressional sequence of tableting, and we may distinguish two major components:
i. Inter-particulate friction, arising at particle/particle contacts, which can be expressed in terms of a coefficient of inter-particulate friction and which will be more significant at lower applied loads. Materials that reduce this effect are referred to as glidants; colloidal silica being one widely used example.
ii. Die-wall friction, resulting from material being pressed against the die wall and moved down it, and expressed as µw, the coefficient of die-wall friction. This effect becomes dominant at higher applied forces, once particulate rearrangement has ceased, and it is a particularly important factor in tableting. In fact, most tablets contain a small amount of an additive designed to reduce die-wall friction and which are called lubricants; magnesium stearate being the most popular choice.
A measure of this die-wall friction can be obtained by collecting FL and FU data from a single station press at different H/D ratios and then applying an equation of the form: LnFL / FU = kH / D
where the constant k includes a term for the average µw.
Experimental results demonstrate that in unlubricated systems the exact relationship is widely variable, but there is a definite trend for die-wall lubrication to result in constant FL/FU ratios. Indeed the ratio has been termed the coefficient of lubricant efficiency, the so-called R value, but is dependent on the H/D ratio and is not always linearly related to FU.
Some materials also tend to adhere to punch faces, and thus a third group of additives is recognized that minimize this phenomena and are called anti-adhesives. Talc is a common example.
Load Removal and Tablet Ejection
Tableting is a dynamic cyclic operation in that a load is applied and must then be removed to facilitate ejection of the tablet from the die. The strength of the tablet so produced, is therefore, a function of those bonds made during loading which survive the unloading and ejection parts of the cycle. For this reason, it is important to study the entire compaction event and evaluate the region beyond the point of maximum loading.
Removal of Applied Load
As the top punch moves away from the tablet surface following the point of maximum penetration into the die, the tablet may expand due to elastic recovery and visco-elastic recovery. The former is a very rapid process, while the latter may even continue after the tablet is ejected from the die.
Ejection
The process of ejecting tablets (from the die) introduces a new set of stresses into them and the tablet structure must be able to withstand them. The practice of including a lubricant in tablet formulations to reduce friction at the die wall plays a major role in minimizing the potential for failure of the tablet structure during ejection.
A typical ejection force trace from an instrumented ejection cam is shown in Figure 12. Many workers have reported relationships between the applied force to produce the tablet and that needed to eject it from the die. Among the more useful seem to be those relating the ejection force per unit area of tablet/die-wall contact to the maximum applied compaction pressure P. Many materials give linear plots, and a very steep slope in these indicates an undesirable sensitivity to compaction force levels. Some workers have used the area under the ejection force versus lower punch movement plot to obtain a Work of Ejection.
Consolidation Potential
One of the major essential properties of a tablet is that it shall possess adequate mechanical strength. Therefore, the second major component of the compaction process (in addition to compression) is the increase in mechanical strength of the tablet mass as the load is increased. This phenomenon called consolidation and the effect of every conceivable variable on tablet strength has been widely studied.
Compaction Force Versus Tablet Strength
One of the commonest tests is to make tablets at different known compaction forces and determine their strength as exemplified by the force necessary to break them. Some typical examples of compaction force versus tablet strength data are shown in Figure 13. Excessive compaction forces usually result in little increase in tablet strength and may even lead to a loss of strength. Again, it should be noted that a very steep slope in such plots is indicative of a pronounced sensitivity to compaction force levels which might be a source of problems in a production environment.
The valuable information, which this type of data can provide during formualtion development, places compaction force vs. tensile strength profiles high on the list of essential tests. However, the data (slopes) generated from the profiles give little indication of underlying mechanisms to facilitate formulation improvement, if acceptable strength is not achieved.
Energy Involved in Compaction
Intuitively, one might anticipate that the energy input necessary to form a tablet should be a more important parameter to study the process than using compaction force. It has already been noted that some of the bonds formed between particles will be broken to facilitate compression. In materials which readily bond and/or form strong bonds, a greater resistance to this compression can be anticipated, than for those where bonding is poor. The ease with which the machine can compress the material may therefore, be indicative of potential tablet strength.
The amount of energy consumed in the compaction sequence is of great interest because it affects machine requirements, and that proportion stored in the tablet retains destructive capability. Work is involved in the following processes which form a part of the compaction cycle:
1. To facilitate particle re-arrangement and overcome inter-particulate friction
2. To overcome particle die-wall friction
3. For elastic deformation deformation
4. To break bonds
5. To eject the tablets
6. To move various press parts
The first item usually only involves a comparatively small amount of work in the earlier stages of the compaction event. Overcoming friction at the die wall should also be a minimum energy requirement if the system is adequately lubricated. Items 3, 4 and 5 account for most of the energy delivered to the compacting mass as appreciable forces are applied, and many studies have been carried out to try and estimate the contribution of each.
The work required for item 6 can be separated in time from the other components, and that required to move press parts can be eliminated from the detection system.
Work of Compaction
Plots of applied force versus punch displacement give rise to the typical curve seen in Figure 14. The area under the curve (force times distance) represents the total work involved in the compaction cycle. If the hypothesis given earlier is accepted, then a proportion of this work must have gone into breaking bonds and may provide a means of predicting strength from work data.
Analysis of Force-Displacement Curves
The curved downward decompression trace arises because of elastic recovery of the compact, and in data from a single station press, the differences between upper and lower punch curves are due to frictional effects at the die wall. Consequently, the enclosed area Wn represents the net work involved in the process.
Distinctive curves, related to the stress/strain properties of the material, have been demonstrated and the results are also related to the state of lubrication. Indeed, the technique has proved particularly useful for evaluating lubricants, being a more sensitive parameter than an R value (see page 10).
The Tableting Process
Having discussed the underlying mechanisms involved in the process of tableting, the basic process and contemporary tableting equipment will now be considered. Perhaps the most important point to appreciate about tablet manufacture is that although the aim is to achieve medicament content uniformity (hence weight uniformity), the material is actually metered out by volume. This necessitates a constant die-cavity volume at the point of fill and a constant bulk density of the material in the die. This, in turn, places great importance on the qualities of the material fed to the press and, in particular, certain desirable properties of the granulation, i.e.:
· Good flow properties, i.e., optimum particle size distribution, regular shape and smooth particle surface
· Homogeneous, i.e., low segregation tendency, uniform particle density, optimum bulk density, low porosity particles
· Compactible, i.e., will actually form tablets
· Ejectible, i.e., after being formed into a tablet can be removed intact from the die
The effect of changes in these characteristics should be studied as part of the development program leading to an optimized formulation under production conditions. More specifically, changes in these factors as the processing is scaled from R&D lot sizes to full production batches must be determined; their effect minimized and documented.
Introduction to Tablet Presses
All tablet presses employ the same basic principle: they compact the granular or powdered mixture of ingredients in a die between two punches, and the die and its associated punches are called a station of tooling, as shown in Figure 5. Presses can be divided into two distinct categories on this basis:
i. Those with a single station of tooling -single station or eccentric presses
ii. Those with several stations of tooling -multi-station or rotary presses
The former are used primarily in an R&D role and for small quantities of complex-shaped or large-sized pieces, while the latter, having higher outputs, are used in most production operations. All commercial types have essentially the same basic operating mechanism; the die is filled, the mass is compacted and then the tablet is ejected. The punch movements necessary to accomplish the three parts of this cycle are obtained by cam action.
Tablet presses, especially large, contemporary production machines are powerful mechanical devices capable of exerting massive forces, via the tooling, on the mass in the die cavity (and anything else that gets in the way!). For this, and other reasons, it is important that modern high speed tablet presses be operated by qualified trained personnel and be maintained by equally experienced engineers. Regulatory requirements and prolongation of the life of these expensive items of capital equipment, dictate thorough cleaning and inspection, with good record keeping, all at appropriate time intervals.
Because of these needs, many of the latest models of production presses also incorporate high levels of electronic monitoring and control of the press operation. These additional features become a necessity when it is no longer possible for an operator to react quickly enough to ensure that the press is operating in the validated mode, or to detect a problem early enough to avoid a major failure.
Tablet Machine Design
Single Station Presses
The simple cycle of this type of press only offers the operator a limited opportunity to make adjustments, as summarized in Figure 15.
It is, therefore, important that the press is not required to compensate for poor formulations. Sizes of machines in this group vary widely from small ones, capable of making tablets up to 12 mm in diameter, at rates of about 80 tablets per minute (t.p.m.). These can exert maximum forces of the order of 20 kN. At the other end of the spectrum are large machines with maximum tablet diameters around 80 mm and capable of loads up to 200 kN or greater.
In a few cases, tablets can only be made on the single station type of machine, probably because their way of operating gives the material a longer dwell time under load, and indeed, this is their main advantage. The output rate from single station presses can be increased by the use of multi-tip tooling, but the rotary machine remains the method of choice for large scale production. Therefore, although a single station press may be used in the early stages of tablet formulation development, it is imperative that as soon as sufficient material is available, the technology should then be transferred to a multi-station press with dwell times as close to those in production as possible.
Multi-Station Presses
Multi-station presses involve the same three steps in the tabletting cycle, but these take- place simultaneously in different stations of the rotating turret carrying the tooling (Figure 16 and Figure 17) and generate a punch movement pattern as seen in Figure 18. However, the operator only has a limited number of options in adjusting the running of the press (Figure 19). The number of stations was commonly around 16 in early machines, giving outputs between 500 to 1000 t.p.m. with diameters up to 15 mm, while high speed contemporary presses have closer to 80 stations and outputs in excess of 12,000 t.p.m.
The operating cycle and methods of realizing the filling, compacting and ejection operations are basically different between the two types of presses (as listed in Table I). It is vital that these differences be fully appreciated when translating information obtained on one type of press into anticipated performance on the other.
In fact, it may be necessary to distinguish between different models of the same group, especially when considering those with high output rates. In particular, there is a large difference in the time available for the machines to make a single tablet. This parameter is particularly critical where the dominant deformation mechanism is visco-elastic or plastic.
For this reason, it is important to appreciate that contemporary tablet manufacturing formulations are adequately tested on production speed presses, before transferring the technology from the development to the manufacturing department.
The smaller rotary presses still offer the following advantages over larger, newer models:
· More flexibility in formulations they can handle
· Shorter change over times
· Less stations of tooling to buy
· Less capital cost
Contemporary Tableting Processes
The development of tableting equipment has been largely one of continuing evolution, but there is now evidence that inherent limits to further development of some press variables on existing lines are being approached. At present, and in the immediate future, one may anticipate the continuing competition between vendors and the added possibility of some more revolutionary machine designs, but with perhaps one exception, recent history is not encouraging.
Apart, perhaps, from presses designed to produce coated or layered products, in many areas the incentives for improvement have come from the users (rather than the machine manufacturers) as a result of trends in tableting operations. These include the need for higher rates of production and more uniform products, the wish to directly compress powders, a desire to automate or at least continuously monitor the process and the need for improved hygiene during operation to satisfy regulatory agencies.
High Production Rates
Having briefly reviewed some of the more basic aspects of tableting equipment, the most popular innovative trends of this technology will now be discussed. One of the more important characteristics of a tablet machine is the rate at which the machine can produce a product.
The ways in which individual manufacturers of equipment have sought to achieve higher outputs fall into four groups:
i. By increasing the rate of compression,i.e., turret speed
ii. By increasing the number of stations
iii. By increasing the number of points of compression
iv. By increasing the effective number of punches, i.e., multi-tipped types
Each of these approaches has its own particular advantages and disadvantages, are in addition, all make demands on other aspects of press design and certain general inherent characteristics of tableting have had to be taken into account.
For example:
i. Formulations developed for slower presses may not run satisfactorily on higher speed machines
ii. Some granulations may not flow faster enough to fill the dies satisfactorily, so that weight variation from tablet to tablet increases
iii. Need for a higher level of competency of the set-up personnel and machine operators
iv. Noise created by the higher speeds can increase to a level that is uncomfortable to the operator and even above that permitted by regulatory authorities
v. Capital costs for equipment may increase beyond the reach of smaller manufacturers, or those with limited profit margins
Other than following press operation instructions, probably the six key factors involved in producing satisfactory tablets at high speed are:
(i)Satisfactory robust and flowable granulation (ii)Optimized press operating conditions for that specific product (iii)Variable speed feeders, optimally set-up (iv)Satisfactory tail-over dies (seals the die cavities between filling and compession steps) and lower punch flight controls (prevents vertical movement of the dies) (v) Efficient dust extraction system (vi)Electronic self-adjusting controls to monitor and control uniformity of tablet weight and other key press functions
High Turret Speed
It is important to realize that in the fastest running presses, horizontal punch velocities may exceed 4 m/sec, and vertical punch velocities may exceed 2 m/sec. These conditions require the most careful attention to press design, construction and operation, if acceptable levels of wear and trouble-free production are to be achieved.
The Press designer must always bear in mind the inherent relationship between the centrifugal force, G, the pitch circle radius, r, and the speed, n, (rpm) of the revolving turret, i.e.:
G = k r n2
where G is in multiples of the gravitational force and, in high speed presses, may be as much as 10 times that of first generation multi-station machines.
This leads to possible movement of material in the die, which can be minimized by introducing a precompression stage, unless material is actually being lost from the die cavity. In this eventuality, it may be necessary to take advantage of variable punch penetration to carry the uncompressed material lower in the die, or virtually seal off the cavities between filling and compression points with spring loaded tail over dies.
A further problem associated with the increased turret speeds is the chance of punch velocities along the camming, particularly the weight adjustment cam, exceeding some critical value leading to a phenomenon known as punch flight. The machine modification of fitting springloaded plungers, which pressed against the punch body, originally developed as anti-turning devices, is some help in this respect, but generated cams are now the design of choice.
Precompression
Most high speed presses have a two-step compaction cycle, to facilitate removal of air from the feed, minimize loss of material from the die cavity, and reduce the work of the main compression rolls. This is achieved by introducing a second set of smaller compression rolls which can apply variable but lower forces before the material reaches the main rolls.
Many of the more recent models have two identical compression stations, and have been shown to produce stronger tablets. In fact, in some cases, a higher first compression force than the second can even be beneficial. It has been suggested that one of the reasons for this observation is that higher first compressions result in higher temperatures of the material at the second point of applying load. They fill, therefore would, be more ductile with increased plastic deformation which may increase tablet strength, due to greater interparticle bonding with fewer bond ruptures.
Several press manufacturers now provide for operating their machines under these conditions. It is important to use large diameter rolls at both stages, since small rolls and high forces can lead to tableting problems due to a higher loading rate. A further refinement, which extends the time under load, is the addition of a spring-loaded section called a dwell bar between the first and second compression stages, which is said to facilitate even higher production speeds.
Cams
For high-speed machines it has been necesssary to ensure that punch heads remain in contact with cam tracks by installing so called generated cams, so as to avoid punch flutter. In these, the punches are forced to follow a specific path by being held firmly in a cam track which controls all their vertical movements precisely.
Many of the cams in newer presses are of special plastic construction to facilitate high speed operation, prolong cam and tool life, and to produce less heat and noise. It is also essential to keep the angle of the weight adjustment cam track something less than 8°. This has involved the provision of interchangeable cams which reduce the excess of material originally filled into the die. It should be noted that higher press speeds may require more die overfill because of greater density variations in the feeder.
The gradients on ejection cams also vary widely between vendors of presses, and the debate continues as to whether rapid or slow ejection is preferable. Therefore, when switching products between machines, this factor should not be ignored.
Feeders
The original method of feeding the dies was from a stationary feed frame, material falling under the influence of gravity into the die cavities, the feed frame being designed to pass material to and fro across the die surface. More sophisticated types are needed for high-speed machines which do not rely on gravity alone, but provide a mechanism which assists material flow.
Contrary to some of the literature references, in the highest speed presses there are no longer force feeders, but devices which also partially fluidize the material and facilitate its transport into the die cavities under the partial vacuum created by descent of the lower punches. For this reason, it is advantageous to control the rate of flow of material into the feeder chamber so that there is space for expansion, i.e., it is not choke fed. Also for this reason, many recent designs incorporate three paddles (Figure 20), by having an upper one which meters material from the hopper to the twin paddles of the feeder. In earlier models, the speed of these paddle feeders was linked directly to turret speed, but it is now recognized that a variable speed, independent of the turret speed, is preferable.
It is important to appreciate that modern feeders have a multi-functional role. In addition to feeding the dies, they also carry on their leading edge, the device which sweeps the ejected tablets from the press. The trailing edge of the feeder incorporates the blade which sweeps the ejected overfill material from the die cavities just before they emerge from underneath the feeder.
Adjustments of these two additional controls, as well as an optimum clearance between feeder and rotating die table, is critical, and so the attachment and rigid locking of the feeder onto the press is a vital factor in successful operation. Even so, modern feeder designs provide rapid release for removal and are easy to strip down for thorough cleaning and inspection.
Lubrication
In the modern tablet press, the contacting surfaces of some individual components are under load and are moving relative to one another at high speeds. It is obvious that to maintain successful operation, an appropriate amount of a lubricant must be present at these surfaces at all times. It is no longer possible to expect an operator to achieve this requirement, and so presses are now fitted with ancillary equipment that provides individually measured amounts of lubricant on an optimized schedule, directly to all critical locations requiring lubrication. In many cases, transducers on the press are constantly monitoring regions where failure of the automated lubrication system would lead to increasing frictional forces, and they can stop the press or activate some alarm system.
Increased Number of Stations
The best compromise between strength of turret and maximum number of stations which can be accommodated at a given diameter, involves three variables:
i. The diameter of the dies
ii. The distance between dies (known as the web; see Figure 21)
iii. The size and geometry of the die locking mechanism
Turret Web
As more stations are fitted into the turret so the metal web between the dies becomes thinner, retaining sufficient strength and providing adequate locking becomes a design problem. Die-locking nuts should only be tightened when the entire die set is fitted; torques not exceeding 15 foot-pounds should be used and a torque wrench employed. As little as 8 foot-pounds may be adequate with new tooling.
Multi-Sequence Presses
Reducing the entire sequence of the compressional cycle into half the turret periphery, as a means of virtually doubling output, was realized in the first double-rotary presses just after the turn of the century. By this means, two tablets are produced by each station during each turret revolution.
One of the more critical factors in operating these presses is to ensure that both sides of the machine are producing tablets with closely similar characteristics. This requirement is not as easy to achieve as might be expected, and some of the differences encountered include:
i. Feed to each side of the press is not the same, due to differences in level of material in the hoppers and/or segregation in them or the bulk container (ii) The feeders are behaving differently
ii. Dust extraction not the same on both sides
iii. Punch penetration settings and/or overfill cams are different
In some machines, the tablets produced on one side of the press are carried round to the opposite side, so that there is a single product outlet from the machine. However, for the above reasons, separate outlets are to be preferred.
This concept of multi-sequencing has now been taken a stage further in a four-sided press in which the entire compression cycle is restricted to one quarter of the cycle and repeated to give a total of four tablets from each station every revolution. This Magna Press (Vector Corp.) has a rated output of almost a million tablets per hour, achieved by using a large turret (90 stations), but a low turret speed of 45 rpm.
