The Manufacturing Process — Part 3

Dry Granulating — Dry granulating, also called Slugging, Chilsonating or Roller compaction, involves the pressing of mixed powders into an object to be reground into a precise powder. This action increases particle density, improves powder flow and captures fines.

The Dry Granulating method is used over other technologies for one or more of the follow reasons:

1. Granulate materials which are sensitive to heat and/or moisture
2. Produce a uniform particle size range
3. Improve flow properties
4. Control dust
5. Control bulk density
6. Produce uniform blends
7. Control particle hardness
8. Improve wetting or dispersion rates

Powders can be compacted using a tablet press; this is called Slugging. Once slugging is completed or powders are compacted on a Chilsonator or Roller Compactor, they are milled. It is best to Mill densified powders using a low shear mill for best results. Using a high shear mill may over-mill or result in an over production of fine particles.

Milling

Milling equipment is used to Improve flow, reduce segregation, enhance drying, and limit wide particle size distribution

Milling machinery used in the preparation of tablet & capsule formulations can be categorized as to their mechanical energy; Low, Medium or High energy mills will impart a force on the powders called shear force. Therefore, milling machinery is defined by Low, Medium and High shear applications.

Some milling machines allow for changes in the type of mechanical action used to reduce the powder to the proper final particle size range. Mills can be used to de-lump powders without actual particle size reduction.

Often different mills are used within different unit operations throughout the complete manufacturing process: At weigh-up for de-lumping, before blending for proper particle size distribution, after wet granulating to enhance drying, and after dry granulating to prepare powders for final blending and tablet compression.

Mill Application

Generally we want to be as gentle with powders as possible. Some powders have high moisture content and they may be subject to compaction within the mill; others are very hard and friable and are subject to producing “fines”. Fines are powders that are very small and “dusty”, which will pass through a 200 mesh screen.

Fine dusty particles impede the flow, do not compress well and can become air born. The air born dust can be witnessed on filters, walls, cabinets and machine components. Besides affecting yields, the dust will combine with oil and grease on the tablet press causing the punches to become tight, requiring more frequent cleaning cycles.

Common milling equipment: Low Shear Mills; Oscillators and Comils. Medium Shear Mills; Quick Sieves and Hammer Mills. High Shear Mills; Pulverizes and Hammer Mills.

Many companies do not have designated milling rooms which requires moving single mills from location to location. In this event, you must always check motor rotation before operating any milling equipment.

Powder Flow and Blending

There are at least ten (10) different variables that can contribute to the success or failure of powder flow on a tablet press. In addition to the well studied particle size, shape and distribution. There are also particle surface texture, cohesivity, surface coating, particle interaction, static electricity, recovery from compaction and wear/attrition while in the holding container.

These other non-traditional measurements, studied and appreciated, shed significant light on flow issues heretofore not fully understood: Particle size, Size distribution, Shape, Surface texture, Cohesivity, Surface coating, Particle interaction, Electro-static charge, Compaction recovery, andWear/attrition characteristics.

Most powders, without the aide of granulation and flow agents, simply cannot flow at speeds required for high speed tabletting. All powders have the capacity to form bridges, create rat holes and stick to contact surfaces. To some extent, most powder mixes exhibit some degree of each problem situation above. The issue becomes critical when any or all of the situations begin to affect unwanted change in powder flow. Bottom line: Recognize that a “good” final blend is often viewed as such because it has good content uniformity and potency, not by its ability to flow.

However, good flow is imperative to attaining a good tablet. Understanding powder characteristics will contribute to accurate blending practices.

Final Blend

The final blend represents the result of the formulating, granulating and lubrication effort. The reason we test blends is to optimize blend time, demonstrate lack of segregation after blending is completed, and confirm that specified blend conditions produce acceptable uniformity during validation.

An individual powder or finished blend may flow very well under one set of circumstance and not flow well at all under another. Notice that under Powder Flow we see attributes of the powder itself while under Powder Process we see what may happen under different processing circumstances.

The message here is for management to be aware of these potential issues on the production floor.

Uniform Blending

Materials go from an unmixed state to a state of relative homogeneous consistency. Achieving a homogeneous blend is accomplished through a combination of time and mechanical energy. Given enough time, components will pass from an unblended state to a relatively homogeneous blend and back to an unblended state.

Blend studies determine the optimum endpoint. All blends have a unique pathway to their optimum state of uniformity. Because under blending and over blending fall on either side of the optimization curve, the symptoms are somewhat similar; and include Content Uniformity problems, Weight and Hardness variation.

The most common blenders used for final blending are the V blender, the double cone blender and the tote blender. All use low shear tumble blending as the most effective way to achieve good mixing with a variety of powders and granules.

-Michael Tousey, Techceuticals

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