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How to Handle Liquids in Compounding Applications

While most of the ingredients for compounding processes are in solid form, liquids are also regularly used and can offer advantages in certain situations and particular applications.

Coperion K-Tron Liquid Feeder
Coperion K-Tron’s modular liquid loss-in-weight feeders offer high accuracy addition of liquid additives in compounding processes.

Liquid ingredients such as oils or silicas can mix and disperse more effectively and can deliver faster reaction times in reactive processing. However, there are also a number of issues to consider when feeding liquids in plastics compounding operations.

Learn more about benefits and challenges when feeding liquids in these Questions & Answers.

What challenges can be encountered when feeding liquids and how can these be overcome?

Liquids tend to be incompressible, which means when they are fed the physical properties do not change. Variations in bulk density can be a massive issue in feeding bulk solids, but with liquids provided you have a consistent “volumetric” throughput you should get the equivalent “gravimetric” throughput.

Unfortunately, as flow rates and pressures increase the flow becomes less predictable, because of slippage back through the pump and cavitation (irregular filling of pump head), it becomes essential to provide a secondary measurement. As flow-meters suffer in the same way as described above the best way to measure and therefore control these materials is using a precise weighing system such as a loss-in-weight liquid feeder with high accuracy weighing technology.

Viscosity is a challenge because when a liquid is highly viscous it does not want to flow. This affects the type of pump that can be used. In most cases Coperion K-Tron recommends the use of gear pumps because they result in a very linear flow. Depending on the material characteristics we sometimes use membrane pumps instead. When a liquid is too viscous, one solution is to heat it up to reduce the viscosity. Coperion K-Tron then incorporates heating systems in the feeder to keep the liquid flowing as it is fed into the process.

Coperion K-Tron Liquid Feeder
Coperion K-Tron’s modular liquid loss-in-weight feeders are available with insulation and heating systems to keep viscous liquids flowing.

What are the latest developments in liquid feeders?

When feeding liquids, the special requirements of each application vary widely. Thanks to the new modular design of the Coperion K-Tron Loss-in-Weight Liquid Feeders, a broad range of pumps and tanks can easily be combined to constitute a reliable liquid feeding system representing the ideal configuration for the specific application.

Although each liquid feeder is custom-designed to meet the desired feed rate, the process demands, the characteristics of the liquid, and the new modular concept streamlines the layout and engineering process of a feeder, thus reducing its lead time and initial cost. The new modularity also provides benefits in terms of a uniform mechanical construction as well as identical operation and maintenance for multiple differently sized units. This simplifies feeder operation, maintenance and spares inventory which in turn saves effort and cost. The same applies for retrofits..

Coperion K-Tron’s modular liquid feeders are ideal for accurate gravimetric feeding of liquids in either continuous or batch operation. The liquid is metered by a pump outside the scale area. The feed pump is selected according to the feeding capacity which is required and the characteristics of the liquid to be fed. The modular concept makes it easy to tailor each liquid feeder to the parameters of the customer process.

The foundation of this modular liquid feeder is a base frame, available in various lengths and widths, which can be outfitted with optional feet or casters. The pump and weigh bridge are mounted on the base frame. The pump technology is selected depending on the viscosity of the liquid, for example gear or membrane pump.

The exclusive, patented Smart Force Transducer (SFT) weighing technology features vibrating wire technology and an on-board microprocessor with fine-tuned control algorithms to ensure high feeder accuracy. The storage tank is mounted on the scale; standard sizes range from 7.5 up to 300 liters, but larger custom systems have also been realized. A flexible connecting line is required between tank and pump. Depending on the needs of the application, additional options such as insulation or heating systems to maintain the process temperature can be supplied as well. ATEX configurations are also available.

Coperion K-Tron Liquid Feeder
Anatomy of a mineral oil feeder for an extrusion system

What are the most important key features in liquid feeding for sheet foam manufacture?

Within many compounding processes the liquid component is introduced downstream under melt pressure to avoid early degradation of anti-static properties, for example. Working together with an international customer in the UK, Coperion K-Tron has developed several special configurations to handle low pressure addition to an atmospheric port in the case of cross-linking promoters / catalysts in sheet foam manufacture.

The addition rate is critical, hence the need for accurate gravimetric feeding and the nature of these fluids means that the pump selection is critical, requiring extensive trials to ensure long life. We discovered progressive cavity PD pumps with stainless rotors and fluoropolymer elastomer or EPDM stators worked extremely well, with a linear flow rate, but only for a short time because the elastomer absorbed the fluid and became deformed. Switching to conventional gear pumps with a loading valve worked well until the shaft seals failed due to rubber ingress. The final solution, which is now in production on many foam lines, uses our SFT weighing technology coupled with magnetically driven gear pumps, sealed from all mechanical forces.

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