While some of today’s technologies and processes for tablet making do not lend themselves well to increased efficiency in pharma, others are well-suited for the task. It is drugmakers’ responsibility to learn more about what options are available and how they can use them to improve speed and quality in manufacturing.
In tablet manufacturing, the traditional use of a batch-based wet granulation process presents several issues. They include low equipment utilization, high costs and energy consumption, and burdensome cleaning requirements. An alternative is dry granulation by roller compaction. This continuous process eliminates the challenges of wet granulation and offers maximum productivity in raw material processing. However, to appreciate how roller compaction can benefit manufacturing capacity and efficiency, it is important to first understand the functionality and capability of this technology.
The “How” Of Roller Compaction
The roller compaction process consists of two phases:
Phase 1: Moving the raw material through the roller and compressing powder into a solid string (referred to as a flake or ribbon)
A feed screw, which can be either single-flighted or double-flighted, conveys the raw material to the rollers, where it is dragged into the roller gap by the surface roughness of the rollers. To produce a stable and homogenous string of material—known as a flake or ribbon—it is important the material is fed evenly to the rollers during this process. With a single-flighted feed screw, the material is always fed at one point on the tip of the screw. The position of the feeding point on the screw determines the quality of the produced flake, with some areas having a higher density than others. A double-flighted screw offers more even feeding, which results in a homogenous density distribution in the produced flake. The feed screw works best if the friction between it and the raw material is low and the friction between the feed screw housing and the raw material is high. Otherwise, the material can get stuck on the feed screw and turn with it, decreasing efficiency significantly.
A vacuum unit (a filter tube that surrounds the end of the feed screw) can be used to create a layer of product at the feed screw housing, which increases friction between the housing of the feed screw and the material. Because a layer of product is forming on the filter, only a small part of de-aeration is done by the vacuum. Most de-aeration is done via the hopper and, if present, a combi-vent-feeder (CVF). A CVF is a second chamber added to the hopper, releasing air from the feed process and allowing for a small flow of product (e.g., recycle of fines, undersize, dust, or additives) to be added in the chamber.
Historically, roller compactors have been built with the rollers in a horizontal arrangement. However, this has been changed to vertical, so the feeding of the raw material to the rollers is not affected by gravity. The result is a more controlled feeding and a reduction of any loose material falling through the rollers during the production run and especially during start up and shut down. The surface of the roller also plays an important role. A smooth surface can only be used for materials that get dragged in easily while knurled, squared, or knurled-and-grooved rollers should be used for materials that need higher friction to be dragged in.
Once the raw material is between the rollers, it is compressed into the flake or ribbon (the intermediate product). The main parameter that influences the degree of compaction is the specific compaction force (SCF) that applies to the material along the roller width. This determines the tensile strength, hardness, and solid fraction of the produced flake. The degree of compaction that results from a certain SCF is strongly product-dependent. However, a trend can be seen for almost all products. At low SCFs, the slope at which strength, hardness, and solid fraction rise is strong, and it starts to level at higher SCFs. Second and third parameters are the roller speed and the roller gap. Even though these parameters do not affect the ribbon quality as strongly as the SCFs, all three parameters must be kept constant to ensure a process that delivers a homogenous product.
Phase 2: Milling
This is when the intermediate product created in the first phase is milled down to granulate in a defined particle size. The vast majority of roller compactors use some sort of screen mill for this. Some have rotating screen mills, which offer continuous milling, and others have oscillating mills. However, with an oscillating mill, the granulator has to continuously be stopped and restarted in order to change direction. By doing this, the mill creates an uneven force profile during the milling. To minimize the mechanical stress used in each step, a three-stage crushing system can be applied. During this process, a flake crusher cuts the produced ribbon into smaller flakes, a pre-granulator then further crushes down the material using a screen mill with a rotating granulator, and a fine granulator creates the desired particle size. This system offers more defined crushing in each step and reduces the quantity of fines produced due to a decrease of residence time and energy input in each milling step. Because less fine material is produced, granulate is produced without the use of an additional screening machine.
The “Why” Of Roller Compaction
Roller compaction technology can offer the following benefits, which help increase efficiency and lower costs in manufacturing:
Improved flowability of raw material
In manufacturing industries such as pharmaceutical and food, stable and reproducible powder blends are very important. For drug development specifically, each tablet must have the same content and amount of API in order to be active and effective. If the raw material used to form a tablet is a fine, dusty powder, it will not flow properly and the different ingredients of the API will eventually segregate. However, if the raw material is granular, it is free-flowing, ensuring an even distribution of material and producing a uniform granule out of the pre-blended mixture. By changing the flowability of the raw material mixture with the roller compaction process, the dies of the tableting machines can be more sustainably filled and faster. This allows the tableting machine to achieve its maximum capacities, and, therefore, production efficiency is increased.
Defined particle size
Some products are sold in granular form. As an example, salt for dishwashers is sold in a size range of two to five millimeters in order to have a linear dissolution profile. Fines would result in sludge that is not good for dish washers.
Increased bulk density, reduced volume
Raw material is transported from one facility to another via trucks, trains, or ships. The transport volume to do this is quite costly. Therefore, a densification process prior to shipment makes sense. This reduces the volume of the bulk material and thus decreases the cost for transport. A high bulk density also lowers the storage volume.
Reduced dust, recycle of material that is uncompacted and/or outside of size specifications
Dust can be very dangerous in a manufacturing environment, which is why organizations like the Occupational Safety and Health Administration (OSHA) have limits on the amount of dust allowed in a facility. Not only are employees inhaling it, which can have unknown risks, but dust also creates a potential for explosions. The dust explosion risk is decreased significantly by roller compaction due to the particle enlargement, which increases the minimum ignition energy required to start an explosion. Nevertheless, with an additional screening machine after the roller compaction process, dust can be removed completely from the manufacturing environment. Depending on the customer’s specifications, the dust either has to be dumped or can be recycled into the CVF and re-compressed. This is done with products like herbal extracts, salt, sugar, and nutritional powder (i.e., baby food). Furthermore, uncompacted material can fall off the sides of the rollers. Roller compactors can also separate this material and feed it back into the CVF. In both scenarios, recycling through roller compaction can reduce the amount of leftover material to zero.
Increased storage stability
During transport and/or storage of raw material, fine raw material is at risk of compacting and, therefore, clumping. If the particle size is changed from a fine powder to a granule, the undefined size of each particle prevents the material from sticking together. If stored over a long period of time, the granule will be still a free-flowing granule, while the original powder can cake into one lump. In addition, due to the increase in bulk density from roller compaction, the storage volume of toxic, radioactive, or otherwise problematic waste can be reduced. This is done, for example, in the nuclear industry before nuclear waste is filled into the castor.
Manufacturing For The Future
Overall, the benefits of a roller compactor are far-reaching. This technology has the capability to improve efficiency and lower costs in several ways, which is becoming increasingly important, especially in the pharmaceutical industry. The rise of novel drugs focused on the unmet needs of smaller patient populations is adding pressure to manufacturers that must find quicker and more efficient ways to produce drugs. Through the use of roller compaction, the pharmaceutical industry and other manufacturing industries seeking lower prices and a quicker time to market can achieve their goals while maintaining the profits they need as well as the quality customers expect.
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