Application Example: Twin Screw Feeders in Pharmaceutical Tablet Press Lubrication Applications

Continuous Lubricant Feeding (Powdering of Tablet Press Tools)

The Coperion K-Tron Tablet Press Lubricant Feeder is an adaptation of K-Tron's sanitary pharmaceutical design twin screw feeder and features the addition of a specialized gas venturi for aspiration of the lubricant direct to the tablet press. Control of the precise amount of lubricant into the process is critical to the overall product quality. If too much lubricant is added the tablet can often become softer than required. This becomes critical in the making of effervescet tablets, where excessive magnesium stearate can actually cause the tablet to repel water, thus affecting the dissolution into water.

When mixing the lubricant agent directly into the mixer, a higher percentage of lubricant is often utilized than is required for the end product quality. As an alternative, when spraying this lubricant directly into the tablet press tooling, the overall consumption is reduced drastically, sometimes by as much as 97%!

The photo above shows a typical set-up for a feeder and an injector. The standard system typically comes packaged with controls. Due to their material handling characteristics, only twin screw feeders can be used for feeding lubricants.

Typical Applications and Industries

Tablet press applications can be found in the following industries:

  • Pharmaceutical:  Solid dosage form for bulk pharmaceuticals, minerals and vitamins
  • Biotechnology:  Biocatalysts
  • Food:  Dextrose tablets, peppermint tablets, sweeteners and soup cubes
  • Chemical:  Film developers, batteries, carbon tablets, explosives, pyrotechnic displays and detonators of airbags
  • Detergent:  To form solid tablets used in washing machines and dish washers
  • Ceramic:  Catalysts and packing materials of reactors and columns
  • Powder metallurgy:  Bearings and balls made of compressed and sintered metal powders
  • Nuclear: Nuclear fuels

Traditional Technology

In a common process, a predetermined amount of lubricant is added to a predetermined amount of granules or free-flowing bulk solids before it will be compressed in the tablet press. Photo on right shows traditional process

In the pharmaceutical industry the lubricant is often fed into a container  which contains the granules. Afterwards the container is mixed batchwise with a container mixer for
a defined time. The mixture is then fed into the tablet press.Figure 1: Traditional Process

In other industries the granules or free-flowing bulk solids, as well as the lubricants, are fed into a batch mixer. After a defined mixing time the mixture is transferred with a pneumatic or mechanical conveying device and fed into the tablet press (see photo above).

Because the granules and lubricants vary greatly in particle size and in bulk density, the risk of demixing is very high. That means that it is nearly impossible to reach a uniform mixture. Therefore many manufactures tend to overfeed lubricants so that the risk of a too low lubricant concentration in a tablet can be reduced a little bit.

Figure 2: Modern Process

Photo above shows modern process

Modern Technology 

In a modern process, the granules or free-flowing bulk solids are transferred with a pneumatic or with a mechanical conveying device to the tablet press. (See Figure 2)
The lubricants are fed continuously through a funnel into an injector, which is supplied with compressed air. The air in the pneumatic conveying line conveys the lubricant to a nozzle, which is fixed in the tablet press. The nozzle is mounted in a way that all surfaces of the tablet press tools, which are in contact with the granules or free-flowing bulk solids (the dies as well as the punches), are uniformly powdered with lubricant. The dust created inside the tablet press is eliminated by an exhaust air system.
Figure 3: Twin Screw Feeder and injector system in a typical pharmaceutical application

Pharmaceutical Applications

Depending on the application requirements, Coperion K-Tron supplies twin screw feeders in standard or pharmaceutical models. Some added features of pharmaceutical feeders (Figure 4)versus standard twin screw feeders include:

  • Mechanical polishing of product and non product contacting parts
  • Continuous welding of product and non product contacting parts
  • Mechanical polishing of product and non product contacting welding
  • Use of machined instead of cast stainless steel parts
  • Clear separation of the production and the technical area
  • All production relevant operations can be done from the front side (production area)
  • Design according to the current GMP guidelines
  • Easy to clean (washing-in-place possible)
  • Easy to disassemble without tools
  • Stainless steel cover for motor and gear box as an option (see figure 4)

Validation documentation as an option.

Figure 4: Twin Screw Pharmaceutical feeder

Figure 4: Twin screw feeder in pharmaceutical feeder

Volumetric or Gravimetric Control?

Depending on the application requirements, twin screw feeders can be supplied either in volumetric or gravimetric models.

The volumetric control consists of a variable speed drive (DC drive or frequency converter) and a simple operator interface. In volumetric mode, the twin screw feeder runs with a constant screw speed. Volumetric feeding may lead to relatively high variations in actual feed rate because of inconsistent screw fill and variable bulk density of the lubricant. Further, volumetric control provides no way to monitor that lubricants are discharged during normal operation or alert you in case of an empty hopper or bridge building in the hopper.

But there are some benefits with volumetric feeding:

  • Easier installation, refill, operation and control
  • Lower investment costs.

Gravimetric control means that the weight of the lubricant is constantly measured and the control keeps weight reduction per time constant by adjusting the screw rotation speed of the twin screw feeder (loss-in-weight control). Gravimetric twin screw feeders have relatively low variations in the mass flow rate because the effects of the variations in screw fill and bulk density are compensated by adjusting the rotation speed of the screws.

Because of the short time in which the tools can be powdered (a fraction of a second) high short term feeding accuracy is required to guarantee a stable process. A high short term feeding accuracy can only be achieved with a gravimetric feeding system.

The gravimetric control consists of a scale or three load cells, a variable speed drive (DC drive or frequency converter), a gravimetric control and an operator interface.

But there are some drawbacks to gravimetric feeding:

  • More complex installation, refill, operation and control
  • Higher investment costs.

The following compares long term benefits of gravimetric feeding over the lower cost volumetric alternative:



Impossible to measure and record mass flow rate

Easy to measure and record mass flow rate

No detection of too low or no flow in case of bridge building

Lubricants discharged by continuous weight measurement

Separate low level detector required

Continuous level detection built in

Less feeding accuracy (less constant feed rate)

High short term feeding accuracy (consistent feed rate)

Less uniform powdering of tablet press tools

More uniform powdering of tablet press tools

Calibration of mass flow rate of each lubricant required

Easy calibration with static weights

Harder to validate

Easier to validate


Benefits of the New Technology

Modern technology offers a simple, continuous and single stage process. That means it is simple to automate and to control this process. Everything can be fully automated, which may reduce labor costs, and no special batch mixing device is required, which reduces:

  • Investment costs
  • Operating costs
  • Cleaning effort and cost  

The lubricant consumption is only a few hundred grams to a few kilograms per hour if the new technology is used. Thus the lubricant content of the tablets is dramatically reduced. This increases the strength of the tablets which is very important for effervescent tablets and some vitamin tablets. After dissolving the effervescent tablets in water there is almost no lubricant film visible on the water surface because of the low lubricant content of the tablets.

By using a twin screw feeder with special modifications in gravimetric execution and a special pneumatic conveying device, the mass flow rate of the lubricant can be kept nearly constant. This guarantees a uniform powdering of the tablet press tools and ensures that the tablets have no adhesion to the tools, and no build-up occurs on the tablet press tools.

Future Potential 

Coperion K-Tron has supplied over 50 systems in the last three years to different customers in different countries. This technology can be used for new installations as well as for retrofits of existing installations in all industries.

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