Single punch presses

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

TOOLING FOR SINGLE-PUNCH PRESSES AND COMPACTION SIMULATORS... 

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. 

Single punch presses are rarely seen in production environments due to their relatively slow production rates, although there are still a number of old products that can only be successfully produced on this type of machine. They are still used in development laboratories because they require only relatively small amounts of material to produce tablets compared to most rotary machines. 

Single-punch machines produce approximately 100—150 tablets per minute. Depending on numbers of die per punch units, standard rotary presses can produce 5000 tablets/min, and even more with a double-sided rotary press. The newest high speed presses can achieve 12,000 tablets/min. 

Pharmaceutical compressed tablets are prepared by placing an appropriate powder mix, or granulation, in a metal die on a tablet press. At the base of the die is a lower punch, and above the die is an upper punch. When the upper punch is forced down on the powder mix (single station press) or when the upper and lower... 

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

Figures 15 and 16 provide a summary of the compression cycles for rotary and single-punch tablet presses. The formation of the tablet compact in these two types of presses mainly differs in the compaction mechanism itself, as well as the much greater speeds achieved with rotary type presses. The single punch basically uses a hammering type of motion (i.e., the upper punch moves down while the lower punch remains stationary), while rotary presses make use of an accordion-type compression (i.e., both punches move toward each other). The former find their primary use as an R D tool, whereas the latter, having higher outputs, are used in most production operations.

All commercial types of single station presses have essentially the same basic operating cycle (see Fig. 16), where filling, compression, and ejection of tablets from the die is accomplished by punch movement utilizing cam actions. Material is fed to the die from the hopper... 

In this type of machine the operating cycle and methods of filling, compression, and ejection are different from those of single-station presses and are summarized in Fig. 15. More specifically, the dies and punches are mounted on a rotating turret. 

Although pellets for exptl use are loaded by single operation methods in which weighed charges are pressed either by stop loading or by controlled pressure techniques, quantity production of pellets is accomplished in automatic pelleting machinery, in which the explosive is metered volumetrically by the controlled movements of punches. Single stroke presses of the types used for expls produce about 90 pellets per minute while rotary presses have rates of about 700 pellets per minute... 

Precompression using a Frogerais 0A single-punch instrumented press (Ets. Ed. Frogerais), equipped with flat punches 11,28mm in diameter (surface area, 1 cm2). The speed ofthe machine was adjusted to one tablet s... 

Some of these measurements can also be performed on compaction simulators, which are single-stroke presses specifically designed to evaluate individual materials and/or full formulations [ 12]. The simulation of short dwell times and of many different profiles for punch movement in real time are the advantages of this type of measurement. Recent work with a compaction simulator has even included a thermodynamic analysis of compaction [13,14]. 

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. ... 

For other historical information on the press instrumentation, the reader is encouraged to peruse Ridgeway Watt s volume. There have been a number of papers published on various disparate instrumentation topics, and in a recent volume by Munos-Ruiz and Vromans, there are two good articles on the subject—but, unfortunately, they deal with marginal issues of single station press and instrumented punch only. 

Like any properly made transducer, it produces an output signal proportional to the applied load. Unlike custom-made transducers that require application of strain gages directly to press parts, load cells are self-contained and can be placed on a press in specially machined cavities to be easily replaced or serviced. As a drawback, load cells generally are less sensitive or less suited to measure the absolute force than custom-made strain gage transducers. Load cells can be used on punch holders in a single station press. Another example of load cell use is a die assembly for calibration of existing traducers on a press. 

Punch displacement measurements are easily done on a single station press by attaching LVDT to the punch. On a rotary press, such measurements can be done by means of slip ring, telemetry, or instrumented punch. Punch displacement profiles may be used in conjunction with compression force to estimate work of compression and work of expansion (measure of elasticity). Because capping tendency increases with the punch penetration depth, it may be desirable to monitor actual punch movement into the die. The shape of a force-displacement curve is an indication of the relative elasticity or plasticity of the material whereas plastic deformation is desirable for stronger tablets, excess plasticity usually results in tablets that tend to cap and laminate. ... 

Fig. 14 shows a typical set of upper and lower compression profiles. One can see that the lower trace is smaller than the upper. On a single station press, only the upper punch is usually moving, and the difference is caused, mainly, by the friction of the compressed powder inside the die. 

In single-punch tablet presses (reciprocating tablet presses), the punches are fixed to the punch holders which travel up and down, and in many cases fixation screws are used. Each make has its own holder design for punches and dies, and consequently for the tooling sets as well. Therefore little can be said in general terms, but it would appear that on the whole the remarks on the rotary press tooling apply correspondingly. 

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