Internal Flow

Film Casting

Screw Design

Internal Flow

What's going on inside the die ? Why study it?

By studying the flow between the rotating screw and the barrel, you can gain valuable insights on how to obtain better mixing or a more homogeneous temperature distribution.By performing further analysis of the flow inside the die, you can avoid recirculation zones, viscous heating and degradation of the material or reduce pressure losses. It may also be important to balance the flow at the die exit to minimize the extrudate deformations in the free flow and finally to obtain the right shape of the extrudate after the deformations.

POLYFLOW provides the tools to address these issues and the simulations provide a wealth of information over the whole flow domain which no affordable experiments could reveal. Data such as velocity and pressure distributions in the fluid, temperature and residence times, particle trajectories and stresses are typically obtained in a simulation. This vastly improved insight into the flow provides a better understanding of the process itself and points out improvements. These can then be tested on the computer without interruption of the production line.

Simulation is sometimes the only way to have a quick answer to questions like the reduction of pressure losses, the existence of stagnation zones which would yield risks of degradation, unbalanced flows in flat dies and exaggerated heat generation and transfer due to viscous heating. It also becomes possible to reduce the number of necessary and costly experiments and to reduce the global time-to-market.

Film Casting

Film casting is one of the processes widely used to produce plastic films. The plastic is extruded through a narrow slit. After the die lip, the resin is in contact with the air before touching the chilly roll rotating at high speed. The rotational speed of the roll imposes the drawing velocity at the end of the free surface. Depending upon the material and the operating conditions, different widths and thickness profiles of the film are obtained. The goal of the design engineer is to get a uniform thickness on the largest width of the film, the side sections being cut and recycled.

POLYFLOW reproduces the necking, the thickness variation through the film and the deformation of the shape of the film. The use of the differential viscoelastic models (Phan Thien-Tanner, Giesekus-Leonov) available in POLYFLOW gives a much better representation of the actual process than a simple generalized Newtonian model would. Indeed, the extensional phenomena are critical due to the high stretching undergone by the fluid.

In order to reduce the computation time and allow for the use of complex rheological models such as the multi mode, non-isothermal viscoelastic model, POLYFLOW has implemented a sheet approximation of the film. Due to the large difference of dimensions (thickness << width), a uniform stress, temperature and velocity field across the film is assumed. This does not significantly affect the accuracy of the results while it strongly reduces the computational time.

Multilayer film casting simulations can also be performed. The thickness distribution for each layer is then calculated while taking into account the different rheological behavior as well as different initial thickness for each layer.

Screw Design

Single Screw Extruder

Twin Screw Extruder

The Challenges

Understanding the flow of material around rotating single screws, twin screws or moving impellers is critical. Many defects of the final product are due to a low mixing quality while the pellets are flowing around a single screw; the pressure built by a given screw rotating at a specified rpm can be too short considering the required pressured drop for a new die that just has been cut; while mixing color or foaming additives with a plastic melt through a twin screw extruder, a good quality mixing must reach an additive dispersion as uniform as possible in order to avoid regions of different color or lower foaming intensity. For batch mixers as well, the quality of the mixing created by the rotating impeller is critical to the quality of the final product.

See animation

In addition, the requested power to apply on the rotating impeller or screw will depend upon the force and torque applied by the flowing material. Getting an evaluation of the power requirement while designing the whole device allows you to acquire the appropriate material. Furthermore, both the cost and the mixing quality will depend upon the geometry of the barrel and the moving impellers or screws, the flowing material and the operating conditions. All these parameters can be modified on the computer in order to select the best operating window.

See animation

Single Screws

Flow pattern, temperature field through the flowing material, pressure increase along the screw, residence time, local shear rate, and local stress are typical information that would be very useful to know while designing or acquiring a screw. Using POLYFLOW, you analyze how a new screw would perform in your own device before acquiring it, or you can demonstrate to your customers that the new screw you have designed will perform much better than the old one. Furthermore, you can vary the operating conditions, such as the rotational speed of the screw, and analyze the subsequent effects, such as pressure built or temperature increase due to viscous heating. This allows you to identify the optimal working window depending upon the device and the flowing material without deteriorating the resin.

Twin - Multiscrew Devices

Understanding and simulating the flow around the twin screw has always been challenging due to the complex geometry in the vicinity of the interscrew region and the large deformations induced by the two (or more) rotating screws. POLYFLOW has implemented a Mesh Superposition Technique that superimposes meshes (for the flowing materials and the screws itself). Also, non-isothermal flows around the twin screw detecting the high temperatures induced by viscous heating, quality of the mixing performed by particles tracking and statistical analysis together with other many other quantities (pressure, velocity, residence time, mixing index, local shear rate, stresses, viscosity) are calculated. They provide you with a wealth of extremely useful information about the flow of material around the screws.

Single Screw Extruder

Process Description

The Challenges

Benefits Gained

Advanced Numerical Aspects

Process Description

Melting the Material

Whether the resin is extruded, blow molded or even injected, the solid pellets delivered by the resin provider have to be melted before being formed. The solid pellets are fed through the feed hopper (2). Next the solid material is conveyed by a single screw rotating inside a closed barrel. The frictions induced at the interface between the flowing resin and the wall increase the temperature through the viscous heating process, locally melting the material.

Increasing the Pressure

The rotational motion of the screw not only conveys the melted material but compresses it as well. So, the pressure of the plastic increases along the screw. Depending on the material and the rotational speed, the pressure at the end of the screw, i.e. at the inlet of the die, will be (more or less) large. This pressure needs to compensate for the pressure drop appearing inside the die. It is also very important that the screw builds up the right pressure. Otherwise, a smaller flow rate, i.e. a smaller production rate, will limit the whole process.

The Challenges

Understanding the flow of material around rotating single screws is critical. Depending on the rotational speed of the screw, the shear rate can locally reach unexpected values. This will create some peaks of temperature that the material can not bear, leading to irreversible deterioration of the polymer grade. On the other hand, too-limited frictions due to too small rotational speed, will lead to slow or even incomplete melting of the material which could induce another dramatic failure of the final product.

Reaching the desired pressure is also a key point that will determine the flow rate, i.e. the production rate. But the pressure increase is not simply a linear function of the length of the screw or the rotational speed. Small modifications of the screw geometry, the flowing material or the operating condition (either thermal or mechanical) may lead to a significant change of the final pressure and temperature map. Many defects of the final product are due to a low mixing quality while the pellets flow around a single screw; the pressure built by a given screw rotating at a specified rpm can be too short considering the required pressure drop for a new die that just has been cut.

Benefits Gained with POLYFLOW

Flow pattern, temperature field through the flowing material, pressure increase along the screw, residence time, local shear rate, and local stress are typical information that is very useful to know while designing or acquiring a screw. These are invaluable pieces of information that you obtain from numerical simulation. You can analyze how a new screw would perform in your own device before acquiring it, or demonstrate to your customers that the new screw you have designed will perform much better than the old one. Furthermore, you may vary the operating conditions, such as the rotational speed of the screw, and analyze the subsequent effects, such as pressure built or temperature increase due to viscous heating. This allows you to identify the optimal working window depending on the device and the flowing material without deteriorating the resin.

In addition, the requested power applied on the rotating impeller or screw will depend upon the force and torque applied by the flowing material. Getting an evaluation of the power requirement while designing the whole device allow you to acquire the appropriate material. Furthermore, both the cost and the mixing quality will depend on the geometry of the barrel and the moving impellers or screws, the flowing material and the operating conditions. All these parameters can be modified on the computer in order to select the best operating window.

Advanced Numerical Aspects

The combination of non-isothermal simulation and long geometries typical for a screw requires the use of appropriate interpolation techniques in order to keep the computational time low enough. Using specific interpolation for both the temperature and the velocity field allows for very accurate results in a reasonable time (a few hours in some cases for complex screws).

Twin Screw Extruder

Process Description

The Challenges

Benefits Gained

Advanced Numerical Aspects

Process Description

The goal of the twin screw extruder is both to convey the melted material and to mix the flowing resin together with additives or tyres, by far its most important task. Melted material enters the barrel containing the twin screws. Here, the resin flows around different screw elements such as conveying elements, kneading blocks, and reverse elements. At the interscrew region, a narrow, very complex flow section appears in front of the flowing particles. This will induce the shear and extensional flows that are required for the mixing quality.

The Challenges

Understanding the flow of material around rotating twin screws is critical. Many defects of the final product are due to a low mixing quality while the pellets are flowing around a twin screw : the pressure built by a given screw rotating at a specified rpm can be too short considering the required pressured drop for a new die that just has been cut; while mixing color or foaming additives with a plastic melt through a twin screw extruder, a good quality mixing must reach an additive dispersion as uniform as possible in order to avoid regions of different color or lower foaming intensity. For batch mixers as well, the quality of the mixing created by the rotating impeller is critical to the quality of the final product.

In addition, the requested power to be applied on the rotating impeller or screw will depend upon the force and torque applied by the flowing material. Getting an evaluation of the power requirement while designing the whole device allow you to acquire the appropriate material. Furthermore, both the cost and the mixing quality will depend upon the geometry of the barrel and the moving impellers or screws, the flowing material and the operating conditions. All these parameters can be modified on the computer in order to select the best operating window.

Benefits Gained with POLYFLOW

Understanding and simulating the flow around the twin screw has always been challenging due to the complex geometry in the vicinity of the interscrew region and the large deformations induced by the two (or more) rotating screws. POLYFLOW has a Mesh Superposition Technique that superimposes meshes (for the flowing materials and the screws itself). Also non-isothermal flows around the twin screw detecting the high temperatures induced by viscous heating, quality of the mixing performed by particles tracking and statistical analysis together with other many other quantities (pressure, velocity, residence time, mixing index, local shear rate, stresses, viscosity) are calculated. They provide you with a wealth of extremely useful information about the flow of material around the screws.

In order to accurately evaluate the quality of the mixing, statistical analysis is performed by POLYFLOW. By tracking the particles during their journey around the twin screws, the software collects a wealth of local information of the temperature, flow pattern, extension, stretching, and residence time. Next, all this information is treated using statistical tools to summarize the data into a few numbers or a curve. Now, you can evaluate and compare the quality of different equipment, material and/or operating conditions.

Advanced Numerical Aspects

The simulation around the twin screw is a very challenging task due to the motion of inner solid tools (the screws) and the complex intermeshing regions between the two screws. This usually leads to excessively complex mesh generation tasks and requires unrealistic remeshing techniques. POLYFLOW has worked around these difficulties using the Mesh Superposition Technique. The basics of this technique assume that you define one mesh for each screw and a mesh for the barrel. These meshes being generated separately are easier to set up. Also, you do not need to worry about the complex mesh usually requested at the complex interscrew section.

Next, POLYFLOW superimposes the different meshes either from the screws or from the barrel and through an intelligent algorithm, the code is able to detect whether the node is inside the rotating screw or represents a flowing particle. Also, either the Navier-Stokes equations or the heat conduction problem is solved respectively.

No remeshing technique is required. Fully transient simulations including the inertia terms and the viscous heating are performed, revealing all the detail of the flow of particles.