Eccentric presses

Those with a single set of tooling— single station (or single-punch ) or eccentric presses... 

The basic unit of any tablet press is tooling consisting of two punches and a die, called a station. The upper and lower punches come together in the die that contains the tablet formulation. Principally, two different types of machines are used, the eccentric and the rotary press. The eccentric press produces about 50-130 tablets per minute. The rotary press has a multiplicity of stations arranged on a rotating table with the dies. A few or many thousands of tablets can be produced per minute. There are numerous models of presses, manufactured by a number of companies, ranging in size, speed, and capacity. 

During the compression process on an eccentric press, there are other pressure ratios at the upper and lower punches. The pressure at the upper punch is usually higher than the pressure at the lower punch. A part of the pressure is lost in the material and in the resulting radial friction force against the die wall during the compression [1,2],... 

On a rotary press the filling of the die and the following compression process is done at the same time at different stations. The compression is carried out in the simplest case with two rolls touching the upper and lower punches and compressing the powder mixture. In contrast to the eccentric press, the upper and lower punches exert pressure on the tablet mixture from both sides at the same time. 

Tablets compressed on a rotary press generally show a more consistent hardness when the upper and lower sides of the tablets are compared, where the upper side of tablets compressed on an eccentric press is usually harder than the lower side [3],...

Figure 2 illustrates the difference in compression profiles of the upper and lower punches and the punch movement with fictitious rotary and eccentric presses. 

Another reason, to use a rotary press rather than an eccentric press for tablet compression is the dwell time is usually shorter on a rotary press (see Figure 3). 

A direct correlation between the results of an eccentric press and a rotary press cannot always be drawn. In addition there are many different tablet presses with different settings and possibilities. These problems can be overcome by using a compaction simulator early in the development stage. An advantage of such a simulator is its versatility, that is, all types of presses can be simulated with small amounts of solid. The problem, however, is that such a simulator is very expensive. 

Moldenhauer, H., Htinerbein, B., and Kala, H. (1972), Recording of pressure-path diagrams from an eccentric press using piezoelectric measurement, Pharmazie, 27, 417—418. 

Despite the availability of published designs for instrumenting rotary presses, much of the early work on compaction properties of materials was done on instrumented single punch eccentric presses primarily, due to the relative ease of sensor installation, as well as availability of punch displacement measurement. ... 

Eccentric tablet presses are single station tablet presses that use an eccentric shaft connected to a rotating wheel to control the displacement of the upper punch into the die. The Encychpedia of Pharmaceutical Technology provides a good summary of the construction and function of an eccentric tablet press <25). The details of its operation are not reported here since it is the only simulation of the compression cycle that is of interest to this chapter. In an eccentric press, the displacement profile of the upper punch is sinusoidal and the displacement rate is controlled by adjusting the rotation rate of the eccentric wheel. The lower punch remains stationary during the compression and acts only to enable uniform die filling prior to tablet formation and tablet ejection after tablet formation. 

Tablets are produced using tablet presses. While these presses vary in their output, from approximately 3,000 tablets per hour to more than 1 million per hour for the fastest machines, the principle of manufacture remains the same. Powder is filled to a specified depth in a die and compressed between two punches. The compression force is ended by removal of the upper punch, and the lower punch then moves upward in the die to eject the tablet. Presses can be divided into two types, single punch (or eccentric) presses and rotary presses.

Single punch presses are sometimes referred to as eccentric presses because the movement of the punches is controlled by an eccentric cam. The tabletting cycle of a single punch machine is represented schematically in Figure 11.16. The powder hopper is attached to a feed shoe which oscillates horizontally. 

Figure 4 F series slide machine with eccentric press between feed and forming area.

Danger zones of the machinery and equipments must have fixed protections, mobile protections, devices of mobile protections and safety devices interconnected (BRASIL, 2011). Something extremely hard in some kind of eccentric presses, for example, that have tools that are not suitable to electrical systems and nor fixed protections ... 

FIGURE 1 Native dextran tablet press in eccentric machine shows variability of color due to different pressure between upper and lower punches. 

The first machines for the production of tablets were simple hydraulic or manually operated presses. Later, eccentric and rotary tableting machines were developed. Today, eccentric tableting machines are only used for research, in early development, or for special applications. 

The equipment employed for tablet compression is generally categorized according to the number of compression stations and dislocation mode. Therefore, eccentric model presses have only one compression station (one die and one pair of punches, upper and lower) while rotary models have multiple compression stations (each station with one die and one pair of punches, upper and lower). The basic difference between the two types of compression equipment is that for eccentric models the compression force applied during compression is due to the upper punch whereas for rotary models it is mainly applied by the lower punch. 

The mash is conveyed from the fruit mill to the press, and to and from the heat exchanger, by means of valveless eccentric spiral pumps, large impeller pumps or slow-running piston pumps. 

In 1952-1954 Higuchi and his group have instrumented upper and lower compression, ejection, and punch displacement on an eccentric tablet press, and pioneered the modern study of compaction process. This work was followed by Nelson who was the member of the original group. 

Barra, J. Doelker, E. Instrumentation of an eccentric tablet press. In Data Acquisition and Measurement Techniques Munoz-Ruiz, A., Vromans, H., Eds. Interpharm Press Buffalo Grove, IL, 1998. 

The underflow of the cyclone 7 is withdrawn by an eccentric worm pump 8 and delivered to a belt filter press 9 yielding a highly dewatered cake and a filtrate consisting essentially of water but loaded with small concentrations of sulfuric acid, furfural, and by-products. This filtrate is recycled to tank 1 for preparing the feed stock slurry. Due to this scheme, most of the sulfuric acid is recovered and reutilized, the only loss being the quantity contained in the cake. This loss is replaced in tank 1. Analogously, the water leaving the system with the cyclone vapor and the cake is also replenished in tank 1 so that the overall mass balance is satisfied. 

FIGURE 13 The difference in the pressure-displacement waveforms as microcrystalline cellulose is compressed on an eccentric and rotary tablet press. Source Adapted from Ref. 26. 

Figure 192 represents eccentric or crank-type drives that convert rotary motion to linear, reciprocating movement. The mechanisms feature a small final rate of pressing speed (approaching bottom dead center) and high loading with low torque at maximum compression (at bottom dead center). The stroke can be adjusted on the eccentric cam or Pitman link. Normally, this method is used when force is applied from only one side and, typically, it drives the top punch. 

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