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Tablet presses : an overview

Single punch and die reciprocating presses

Rotary punch and die presses

What is the working principle of tablet presses ?

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Section summary
1. What is tablet press ?
2. How does the powder stick together due to compaction ?
3. Tablet press working principle
4. Tablet press manufacturers

Tablet presses, and more generally rotary punch and die presses, are used in various industries to compact powder and granulates and then produce a tablet that can be easily handle, sold, and absorbed by the customer. Those machines are of course widespread in pharma industry and also now in the growing food supplement industries. The same compaction principle is however found in heavier industries for example producing refractory bricks although the machine size is different and is rather not of the rotary type but more a single stroke machine.

1. What is a tablet press ?

A tablet press machine is an equipment which allows to agglomerate powder or granules thanks to compaction. The process, dry, consists in having a blend of powder or granules (powder which have been submitted to a prior step of granulation such as wet agglomeration), introduce the powder in a die, and apply a high pressure with a couple of punches. The pressure is compacting the powder, deforms it, and makes it adhere together.

  • Reciprocating punch and die presses : the simplest machine have a single die and operate according to a reciprocating movement with upper and lower punches alternatively closing and opening to compact and then eject the compacted powder under the form of a tablet or a briquette. These machines have a low throughput but are commonly used to manufacture metal pieces for example, or refractory bricks.
  • Rotary punch and die presses : the principle remains the same, punches are closed on product contained in a die to compact it, but many punches and dies are mounted on a rotating ring which allows to tremendously increase the punching frequency. These machines are however used mainly in pharma and produce very small compact of powders of few grams. The number of tablets produced is therefore high, but the mass throughput of the machine remains a few dozens or hundreds kg per hour.

2. How does the powder stick together due to compaction ?

Compaction is basically a dry process during which solids particles are made to stick together and keep a certain shape thanks to the action of a high pressure. When compacting the powder, different phenomena take place that allows to get a tablet or a briquette (for bigger machines), those phenomena are listed below, although only some of them may be at play for a given application [Pietsch] :

  • Creation of solid bridges in between particles by
    • Chemical reaction
    • Melting or sintering
  • Adsorption layers
  • Non valence associations
  • Molecular, electrical, magnetic forces in between particles (for example Van Der Waals forces)
  • Interlocking

In any case, the compression forces must be such that the solids particles are withstanding a plastic deformation [Perry] which is leading to a very intimate contact in between particles leading to some of the bonding phenomena mentioned previously. A typical compression cycle and its effect on the solids particles is represented in graph 1. It should be noted that the materials, after compaction, and once the pressure is released are usually slightly expanding due to the release of compressed air and / or a form of elastic recovery of the material [Pietsch].

Different stages of pressure agglomeration

Graph 1 : the mechanisms of pressure agglomeration with the different stages reached and impact on the material

In some cases, the addition of a binder is required if compression of the bulk solids is only yielding weak bonding that do not give enough mechanical strength to the compact (tablet, briquette...) produced.

3. Tablet press working principle

The compaction process in a tablet press machine is done following these steps :

  • Die filling : the powder to compact flows from a hopper to the die
  • Volume adjustment and Die scrapping : the lower punch is adjusted to measure the right volume of the compact and a scrapper is removing the excess of product
  • Compaction : the upper and lower punch close and exert a high pressure on the powder, leading to compaction
  • Punch release : the upper punch is lifted, no more compaction pressure is applied
  • Tablet ejection : the lower punch is raised to remove the tablet which is then collected for further processing (typically packaging). The punch is then lowered to be ready for a new cycle

Working principle of a rotary punch and die tablet press

Figure 1 : working principle of a rotary punch and die press used for tabletting

These different actions can be done on a single punch and die press, for example for lab machines, or for compressing a large amount of materials, but the different components can also be arranged on a turret that allows to increase a lot the punching frequency, albeit with a small amount of material, as it is the case in pharma. The figure below is showing the working principle of a rotary punch and die press to produce tablets in pharmaceuticals or food supplement industries.

4. Tablet press manufacturers

Tablet presses are found in major industries such as pharmaceuticals, food supplements or ceramics, thus many manufacturers have developped tablet presses to compete in this extensive market.

When contacting a manufacturer, whether for a new or second hand (used) tablet press, make sure to check its references and engage in a detail conversation with the sales and technical staff to understand the degree of knowledge of the vendor and how best he will be able to support you when operating and maintaining the press.

Source

[Pietsch] Agglomeration Processes (Phenomena, Technologies, Equipment), Pietsch, Wiley-VCH, 2004

[Perry] Perry's Chemical Engineer's Handbook, Section 21 Solid-Solid operations and processing, page 21-105, McGraw-Hill, 2008


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