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Tablet coating process control


Film coating of solid oral dosage forms is a well-established process in the pharmaceutical industry. Most tablets dosage forms undergo the film coating process. The main purpose for coating of immediate release film dosage form is to provide colour identification, enhance stability and taste masking. Additionally, it eases processability and handling during pharmaceutical manufacturing and thus is widely used in the pharmaceutical industry. Many solid pharmaceutical dosage forms are produced with coatings, either on the external surface of the tablet, or on materials dispensed within gelatine capsules. The coating process can be specially formulated to regulate how fast the tablet dissolves and where the active drugs are to be absorbed into the body after ingestion. There are various types of tablet coating equipment (Standard Coating Pan, Perforated Coating Pan, Fluidized Bed Coater) and techniques (Sugar coating, Film coating and Enteric coating) – working principle, design, structure, air capacity, coating composition and efficiency determine the choice.

Tablet coating process control with inline viscosity and density measurements


Over the course of time, coating processes have developed from the art of earlier years to those that are more technologically advanced and controlled such that compliance with good manufacturing practices (GMPs) is facilitated. The coating formula and process optimization using scientific methods have taken predominance over the traditional film coating methods. The process of film coating involves consistently depositing and drying a uniform coating formulation onto the surface of the substrate so as to form a uniform film. Control of process parameters is extremely important for good tablet coating.

The 3 primary components involved in tablet coating are tablet properties, coating process and coating composition. The result of the coating process is strongly dependent on the rheological characteristics of the film coating dispersion applied. Issues such as logo bridging, orange peel and spray drying are rooted in the viscous and elastic properties of the coating formulation. Determining and appraising the rheological characteristics is therefore of huge importance in order to prevent problems during the film coating process.

Moreover, in the field of instant release film coating, the process time and, as a result, the manufacturing costs are directly linked to the solid matter content of the dispersion. As viscosity is a specific characteristic of the film forming polymer, rheological investigation is very useful and important in order to select the most economical formulation. Water soluble polymers are mainly used for instant release (IR) film coating. These film formers are distributed as a powder. During the preparation of the film coating formulation, the polymers are dissolved in water. In spite of the fact that these polymers can be used for moisture protective coatings, there is no certain functionality obtained by these. However, dissolution of polymers in water can cause significant change to fluid properties of the liquid carrier. The increase in viscosity and changes in surface tension influences the droplet formation during spraying as well as film formation, and therefore, the surface quality of the coated cores. Typical IR film coating dispersions consist of different excipients – at least one or combination of different film forming polymers, pigments and optional plasticizers is needed to form a coloured film on the surface of core. Considering the list of excipients in the coating formulation, the film forming polymer is the only material causing a significant change in viscosity and flow properties of the coating dispersion influence coating result to a high extent.

In general, it can be assumed that with lower viscosity:

  • Preparation of film coating dispersion is faster
  • Risks of lump formation is lower
  • The surface quality of the coated tablets is better
  • The maximum solid matter content of the dispersion is higher
  • Processing time is shorter
  • Processing costs are lower

Each film coating formulation and film forming polymer has its characteristic limiting maximum viscosity value for processability which depends on batch size, coating equipment and the process. In order to optimise efficiency and quality, it is absolutely necessary to compensate for solvent loss by adding small amount of thinner from time to time, during the operation, to keep the viscosity optimum for use. The controls enable to orchestrate the operation of all the components to achieve the desired results.

Why is viscosity management critical in pharma-printing process?

The broad and significant factors which make viscosity management critical in pharmaceutical printing are:

  1. Coating Quality: Tablets must comply with finished product specifications and any appropriate compendial requirements, and that can be ensured. Variation in viscosity causes significant change in both solvent and water-based ink properties affecting printability, fade resistance and drying.
  2. Reduce coating errors: Viscosity control can help alleviate the frequency of miscues – sticking and picking, twinning, peeling, splitting, cracking, roughness, blistering, bridging and surface erosion.
  3. Colour: Colour consistency and colour density are highly critical to the right print quality. Controlling ink viscosity is the key to colour consistency because that is the factor subjected to the highest variability. The percent solids of fluid is the characteristic of the ink that gives it colour. Ink viscosity is an indicator to the percent solids of the fluid.
  4. Cost: Printing with incorrect viscosity harms more than just quality. Poor viscosity management drives up usage of pigments and solvents, affecting the profit margins.
  5. Waste: Materials rejected due to poor quality can be reduced with proper viscosity management.
  6. Efficiency: Eliminating manual viscosity control frees operators’ time and enables them to focus on other tasks.
  7. Environment: Lowering the use of pigment and solvent is good for the environment.
  8. Compliance: On-dose identification helps in product differentiation and enhances product safety. Perhaps to a greater degree than other industries, pharmaceutical coating demands the highest quality printing. Legibility and contrast are non-negotiable when it comes to regulatory and traceability codes.

To ensure consistent high-quality printing, the change in ink viscosity through-out the process stream is monitored in real time, making measurements from a baseline rather than simply measuring absolute values, and making viscosity adjustments by adjusting solvents and temperature to keep it within specified limits.

Process Challenges

Since spraying, coating distribution, and drying take place at the same time, tablet coating is a dynamic, complex process that is affected by many variables. In order to optimise efficiency and quality, it is absolutely necessary to compensate for solvent loss by adding small amount of thinner from time to time, during the operation, to keep the viscosity optimum for use through adjustments as conditions dictate.

Existing laboratory viscometers are of little value in process environments because viscosity is directly affected by temperature, shear rate and other variables that are very different off-line from what they are in-line. Traditionally, operators have measured the viscosity of printing ink using the efflux cup. The procedure is messy and time-consuming, particularly if the ink needs to be filtered first. It is pretty inaccurate, inconsistent and non-repeatable even with an experienced operator.

Some companies use thermal management systems to keep point-of-application at a determined optimal temperature to achieve constant ink viscosity. But temperature is not the only factor affecting viscosity. Shear rate, flow conditions, pressure and other variables can affect viscosity changes as well. Temperature controlled systems also have long installation times and a large footprint.

Conventional vibrational viscometers are unbalanced, requiring large masses to avoid large influence of mounting forces.

Rheonics' Solutions

Automated in-line viscosity measurement and control is crucial to control the ink viscosity. Rheonics offers the following solutions, based on a balanced torsional resonator, for process control and optimisation in the printing process:

  1. In-line Viscosity measurements: Rheonics’ SRV is a is a wide range, in-line viscosity measurement device with inbuilt fluid temperature measurement and is capable of detecting viscosity changes within any process stream in real time.
  2. In-line Viscosity and Density measurements: Rheonics’ SRD is an in-line simultaneous density and viscosity measurement instrument with inbuilt fluid temperature measurement. If density measurement is important for your operations, SRD is the best sensor to cater to your needs, with operational capabilities similar to the SRV along with accurate density measurements.

Automated in-line viscosity measurement through SRV or an SRD eliminates the variations in sample taking and lab techniques which are used for viscosity measurement by the traditional methods. The sensor is located in-line so that it continuously measures the ink viscosity (and density in case of SRD). Printing consistency is achieved through automation of the dosing system through a controller using continuous real-time viscosity measurements. Using an SRV in a printing process line, ink transfer efficiency is improved improving productivity, profit margins and environmental goals. Both the sensors have a compact form factor for simple OEM and retrofit installation. They require no maintenance or re-configurations. Both the sensors offer accurate, repeatable results no matter how or where they are mounted, without any need for special chambers, rubber seals or mechanical protection. Using no consumables, SRV and SRD are extremely easy to operate.

Compact form factor, no moving parts and require no maintenance

Rheonics’ SRV and SRD have a very small form factor for simple OEM and retrofit installation. They enable easy integration in any process stream. They are easy to clean and require no maintenance or re-configurations. They have a small footprint enabling Inline installation in ink lines, avoiding any additional space or adapter requirement on the press and on ink carts.

High stability and insensitive to mounting conditions: Any configuration possible

Rheonics SRV and SRD use unique patented co-axial resonator, in which two ends of the sensors twist in opposite directions, cancelling out reaction torques on their mounting and hence making them completely insensitive to mounting conditions and ink flow rates. These sensors can easily cope up with regular relocation. Sensor element sits directly in the fluid, with no special housing or protective cage required.

Instant accurate readouts on printing conditions – Complete system overview and predictive control

Rheonics’ software is powerful, intuitive and convenient to use. Real-time ink viscosity can be monitored on a computer. Multiple sensors are managed from a single dashboard spread across the factory floor. No effect of pressure pulsation from pumping on sensor operation or measurement accuracy. No effect of printing press vibration.

 Easy installation and no reconfigurations/recalibrations needed

Replace sensors without replacing or reprogramming electronics, drop-in replacements for both sensor and electronics without any firmware updates or calibration coefficient changes. Easy mounting. Screws into ¾” NPT thread in ink line fitting. No chambers, O-ring seals or gaskets. Easily removed for cleaning or inspection. SRV available with flange and tri-clamp connection for easy mounting and dis-mounting.

Low power consumption

24V DC power supply with less than 0.1 A current draw during normal operation

Fast response time and temperature compensated viscosity

Ultra-fast and robust electronics, combined with comprehensive computational models, make Rheonics devices one of the fastest and most accurate in the industry. SRV and SRD give real time, accurate viscosity (and density for SRD) measurements every second and are not affected by flow rate variations!

Wide operational capabilities

Rheonics’ instruments are built to make measurements in the most challenging conditions. SRV has the widest operational range in the market for inline process viscometer:

  • Pressure range up to 5000 psi
  • Temperature range from -40 up to 200°C
  • Viscosity range: 0.5 cP up to 50,000 cP

SRD: Single instrument, triple function – Viscosity, Temperature and Density

Rheonics’ SRD is a unique product that replaces three different instruments for viscosity, density and temperature measurements.  It eliminates the difficulty of co-locating three different instruments and delivers extremely accurate and repeatable measurements in harshest of conditions.

Achieve the right print quality, cut down costs and enhance productivity

Integrate an SRV/SRD in the process line and ensure colour consistency throughout the printing process. Achieve constant colours without worrying about colour variations. SRV (and SRD) constantly monitors and controls viscosity (and density in case of SRD) and prevents overuse of expensive pigments and solvents. Reliable and automatic ink supply ensures that presses run faster and saves operators’ time. Optimise the printing process with an SRV and experience lesser reject rates, lesser wastes, fewer customer complaints, fewer press shut downs and material cost savings. And at the end of it all, it contributes to a better bottom line and a better environment!

Clean in place (CIP)

SRV (and SRD) monitors the cleanup of the ink lines by monitoring the viscosity (and density) of the solvent during the cleaning phase. Any small residue is detected by the sensor, enabling the operator to decide when the line is clean for purpose. Alternatively, SRV (and SRD) provides information to the automated cleaning system to ensure full and repeatable cleaning between runs, thus ensuring full compliance in terms of sanitary standards of medicine manufacturing facilities.

Superior sensor design and technology

Sophisticated, patented 3rd generation electronics drive these sensors and evaluate their response. SRV and SRD are available with industry standard process connections like ¾” NPT and 1” Tri-clamp allowing operators to replace an existing temperature sensor in their process line with SRV/SRD giving highly valuable and actionable process fluid information like viscosity besides an accurate measurement of temperature using an in-build Pt1000 (DIN EN 60751 Class AA, A, B available).

Electronics built to fit your needs

Available in both an explosion-proof transmitter housing and a small-form factor DIN rail mount, the sensor electronics enables easy integration into process pipelines and inside equipment cabinets of machines.


Easy to integrate

Multiple Analog and digital communication methods implemented in the sensor electronics makes connecting to industrial PLC and control systems straightforward and simple.



Directly install the sensor to your process stream to do real time viscosity and density measurements. No bypass line is required: the sensor can be immersed in-line, flow rate and vibrations do not affect the measurement stability and accuracy. Optimize decision making process by providing repeated, consecutive, and consistent tests on the fluid.

Rheonics Instrument Selection

Rheonics designs, manufactures and markets innovative fluid sensing and monitoring systems. Precision built in Switzerland, Rheonics’ in-line viscometers has the sensitivity demanded by the application and the reliability needed to survive in a harsh operating environment. Stable results – even under adverse flow conditions. No effect of pressure drop or flow rate. It is equally well suited to quality control measurements in the laboratory.

Suggested product(s) for the Application

• Wide viscosity range – monitor the complete process
• Repeatable measurements in both newtonian and non-newtonian fluids, single phase and multi-phase fluids
• All metal (316L Stainless Steel) construction
• Built in fluid temperature measurement
• Compact form-factor for simple installation in existing process lines
• Easy to clean, no maintenance or re-configurations needed

• Single instrument for process density, viscosity and  temperature measurement
• Repeatable measurements in both newtonian and non-newtonian fluids, single phase and multi-phase fluids
• All metal (316L Stainless Steel) construction
• Built in fluid temperature measurement
• Compact form-factor for simple installation in existing pipes
• Easy to clean, no maintenance or re-configurations needed