- A set of tooling consists of upper punch, lower punch, and die.
- The upper punch has a shorter stem.
- The lower punch stem is longer because it travels longer distances up and down in the die for filling, compression, and ejection, thereby sealing the die hole from below during the entire process of compression.
- In rotary tablet presses, only the dies are fixed in the die table, whereas the punches are vertically mobile and are not fixed to a holder.
- They slide up and down in the turret bores, driven by cam tracks guiding the punch heads.
- The dwell time, i.e., the period during which the tablet is under full compression, is determined by the diameter of the flat part of the punch head.
TOOLING FOR ROTARY TABLET PRESSES
- The punch consists of head, neck, barrel, and stem.
- The barrel diameter and the overall length are determined by the machine turret dimensions.
- The working length (i.e., overall length minus any cup depth at the punch tip) determines thickness and mass of the tablets and needs to be controlled regularly. The variation in working length should be within ±10 µm to ensure low variability of tablet properties.
- The die depth should also be accurately cut in order to guarantee a flush fitting with the die table.
INTERNATIONAL STANDARDS:- Standardization of tooling dimensions has been agreed to facilitate inter changeability between presses and to reduce the number of spare parts. In many cases, a certain press is available with different turrets in order to standardize tooling within a tablet manufacturing company irrespective of the make of tablet presses used in R&D and production, respectively. A special turret was used for R&D on which all standard tooling can be used on one. It allows to inter change punches and dies of any sizes for trial compression.
There are two main standards:-
1) B-tools with subcategories EU19 (Europe)/TSM 19 (USA)/Japan Norm;
2) D-Tooling, also called EU1, TSM 1,
Which are classified according to the dimensions of the barrel diameter, overall length, and the overall diameter of the punch head.
- D-tooling is thicker than B-tooling; European toolings (both B- and D-types) are longer compared to TSM types.
- The head angle or head radius (in the case of ‘‘domed heads’’), respectively, determines the lead under the compression roller.
- The head flat determines the dwell time of the compression event (i.e., the period during which the tablet is under maximum pressure), which should preferably be long so as to yield mechanically resistant tablets.
- The head flat must in all cases be smaller in diameter than the neck in order to guarantee pressure transmission onto the whole punch.
- Neck shape and neck diameter are determined by the dimensions of the cam track.
- The head thickness needs to be precisely cut in order to ensure a smooth gliding of the punches through the cam track and, in the case of lower punches, to guarantee a reproducible dosing of granule in the die as well.
- There is another head shape to the D-tooling called Fette EU1 441, which is characterized by the absence of a neck and a larger head diameter (38.00 mm) allowing for increased dwell times.
- A further subcategory of B-tooling, which is referred to as B2-type (Stokes) has the same barrel diameter as the standard B-type, but the overall length of the lower punch is smaller. B2 is used on some older presses, but not manufactured any more.
- There is yet another variation on standard B-type tooling, where the overall diameter of the die is smaller. The die diameter determines the maximum tablet diameter as well as the maximum number of die bores in the case of multiple tools. A larger die is more resistant although pressure is not directly applied to it but only by lateral extension.
- The height (depth) of the die ensures a flush fitting with the die table.
- The overall die diameter— nominal size and manufacturing tolerances—enables a tight fit in the turret avoiding lateral movement, which is also supported by the die groove accommodating the locking screws.
- A protection radius or protection shoulder prevents damage to the die locking screw or scoring of the die pockets in the turret caused by burrs or sharp edges on the die.
- The die bore should be made as close to its nominal size as possible because it is this, which determines the tablet size, not the punches.
- A bore chamfer provides a lead for the upper punch to enter the die.
- Any clearance between the die wall and the punch affects the tableting behaviour, as entrapped air will escape through the gap when pressure is applied.
- Tapered dies are more expensive than standard dies.
- Tapers can be applied to both ends of the bore to allow for an inversion of the dies.
- Tapered dies not only let air escape, but also assist ejection as the stress is gradually released by ejecting to an increasing diameter thereby preventing capping and generally increasing the tensile strength of the tablets. Furthermore, their use also allows higher machine speeds.
- Shaped punches—all other shapes than round ones—and multi tip punches need to have keys on their barrel in order to prevent any rotation. Keys are available in two configurations:
- The Woodroff key and
- The flat key (European key)
- Can be distinguished by the shape of the slot milled into the barrel where the key is in many cases screwed into position.
- The vertical position of the key on the punch barrel and the length of the key itself are determined by the turret dimensions in such a way as to allow for clearance in the punch-guide keyways.
- The optimum angle of the key with respect to tablet shape is determined by the rotation direction of the tableting machine and the design of the take-off fence at the discharge point with respect to the punch tip, and therefore not standardized.
- Tablets hitting the fence with a corner should be avoided; they should rather hit it with a flat face in order to ensure proper discharge without fragmentation.
- Barrel flutes, usually on round upper and lower punches, are vertical sharp-edged grooves along the length of the punch barrel. During production, they would rotate and thereby scrape off possible depositions of material in the punch guide ways.
- Several types of dust caps and sealings fitted to the barrel-to-stem chamfer are available to prevent contamination of the tablets by machine oil dripping from the upper punch.
- Alternatively, punch bellows in silicone rubber fitted into special seal grooves for Euro-standard are available.
MATERIALS, MANUFACTURE, FINISH, AND PRESSURE TOLERANCE:- Materials and Manufacture Steels are basically classified as carbon and alloy steels which are defined by national and international standards, e.g., ASTM and DIN.
- Carbon steels contain less than 1.65% manganese, 0.6% silicon, 0.6% copper, plus boron and deoxidizers as well as carbon.
- principal additive increasing hardness. Manganese also increases hardness, particularly the ability to be hardened by tempering, and is present in all steels.
- Silicon acts as a deoxidizer. Alloy steels exceed these limits or contain additional elements, e.g., chromium, vanadium, tungsten, molybdenum, nickel, and cobalt.
- A high-nickel content increases resistance to corrosion caused by HCl released from drug salt. The composition of special steel types for tooling, their provenance, and their thermal treatment are part of the trade secret of tooling manufacturers.
- A high abrasion resistance usually corresponds to high hardness of a steel type, whereas its ductility and toughness is low and vice versa: very hard tooling may fracture, ductile material may wear off quickly.
- Hardness and ductility do not only depend on the chemical composition of the steel but also on its heat-treatment (tempering). A second tempering procedure for punch tips, which is computer-controlled and carried out at high temperatures in a vacuum furnace, softens the material and prevents fracture.
- Punches usually have a longer lifetime than dies if they are made from the same material; that is why manufacturers often choose different steel qualities.
- The surface of a die bore can become smoother or rougher during its use. The ejection force does not— apart from the composition of the tableting material and the compaction force—necessarily depend on roughness of the die wall, but to a high degree depends on the metal type of the die.
- For specific tableting tasks, the tooling manufacturer will be able to provide optimum tool properties. It may be helpful in this respect to supply him with a sample of the tableting material in question.
- 440C has the highest chromium content (17%), which increases wear resistance.
- D2 and D3 have a high-chromium content (12%) and are additionally carbon-rich (over 1.5%).
- Chromium and carbon together form stable carbide, which increases hardness.
- S1 contains tungsten (2%), which makes heat-treatment easier.
- Adhesion, sticking, picking, and filming on punches and dies can in many cases be avoided by a suitable surface finish of the tooling.
- In some cases, the surface is smoothened during continuous use of punches, whereas in other cases, depending on the material compressed, e.g., in the case of crystalline lactose, small scratches may occur on the punch tips after a few thousand compression events. On those small scratches and defects in the surface, corrosion will start.
- Any roughness of the finishing is below 1 mm peak-to-valley height, and particularly punch-tip faces and die bore walls are polished to achieve lower tolerances.