ANSYS POLYFLOW

POLYFLOW offers a wealth of advanced features, such as complex rheological models (including the widest suite of viscoelastic models in commercial CFD software today), very large free surface deformations (using the arbitrary Lagrange-Eulerian (ALE) technique), contact detection, native fluid-structure interaction (FSI), and a unique mesh superposition technique (MST) to simulate the flow surrounding twin screw extruders or overlapping impellers.

An easy-to-use interface and expert system guides the user during the problem setup, solution and postprocessing phases.

Direct export to the ANSYS Mechanical, ANSYS Structural and ANSYS Professional formats allow designers to model their virtual manufacturing process inside ANSYS POLYFLOW, before performing mechanical testing or drop tests on the virtual part rather than on an idealized one. By using ANSYS POLYFLOW various optimization solutions for either the initial temperature field in thermoforming, or for die balancing tasks in relation to the extrusion and co-extrusion process, customers can cut both time and cost-to-market.

ANSYS POLYFLOW includes a range of efficient and robust solvers, such as the fully coupled solver, the multi-frontal solver and the iterative solver, to allow customers to run simulations with larger meshes than ever before. Faster simulation enables optimization and automatic die balancing for rubber profiles or plastic extrusion and co-extrusion dies. The die designer can ask ANSYS POLYFLOW to find out the geometry that leads to the best velocity profile across the die lip while considering the constraints that have been defined. Then the design task can be finalized using the unique inverse die design capability of the code. The extrudate profile can then be exported to ANSYS Mechanical to run various mechanical testing and to investigate whether manufacturing or design adjustments are necessary.

Optimization is also available for the ANSYS POLYFLOW thermoforming application which allows the mold designer to require the software to adjust the initial map of temperature to get a desired thickness profile at the end of the process, this leads to better packaging products. The results of the thermoforming process can be seamlessly exported to ANSYS Mechanical to test the virtual thickness distribution for typical mechanical testing such as compression, traction, impact analysis and drop test. This capability is also available for blow molded products such as bottles and gas tanks.

Glass forming applications, whether related to gob forming, bottle blowing, pressing or making drinking glass, increasingly use numerical simulation to provide better insight into the complex deformations and thermal patterns that occur during the forming process. These simulations are extremely challenging since they involve very large deformations coupled with steep thermal gradients. Cooling is a delicate phase of the process where residual stress and defect can appear. The thermal stress relaxation model in ANSYS POLYFLOW allows customers to evaluate the relaxation of the stress during the cooling phase in order to detect the emergence of defects in the early stage of the cooling.

Mold and parison for an automotive water tank. Courtesy of MANN+HUMMEL GMBH

When designing the different components of a car, the gas and water (windshield washer fluid) tanks are often the last ones to be considered, because their shapes can be adjusted easily to fit the available space. The constraint is usually rather simple: maximize the inner volume of the tank by using as much of the remaining space as possible. This is why gas and water tanks often have complex, sometimes unusual shapes. For gas tanks, safety regulations require that the thickness of the walls be larger overall than a minimum value in order to avoid any failure in the event of an accident. If the gas tank walls are too thick, however, it needlessly increases the weight of the part and uses an excessive quantity of polymer, which impacts the cost. To balance these requirements, several companies are now using POLYFLOW software to simulate – and optimize – the blow molding process for gas and water tank manufacturing.

The blow molding process consists of two stages: the extrusion of a cylindrical tube of polymer, also called the parison, and the blowing process, where the material is forced to conform to a mold. During the extrusion of the parison, gravity and die head motion allow the designer to adjust the parison thickness profile, which is normally non-uniform. Once extruded, the parison is positioned in between the two halves of the mold. These molds have the complex imprint of the desired tank. The top and bottom of the parison are squeezed by knives to seal the tube so that air can be blown into it. The air inflates the parison, just like a balloon, while the mold gradually closes. When the two halves of the mold are completely closed, an increase in pressure ensures that the blowing parison takes the exact shape of the mold, which is the desired gas and water tank shape.

Thickness distribution of the EVOH layer for an automotive water tank; this low permeability material is one of several layers used in the parison. Courtesy of MANN+HUMMEL GMBH

Using POLYFLOW, the free surface of the deforming parison is tracked as it is inflated. The material is allowed to stretch and thin until contact between the deforming free surface and moving mold is detected. The result of the numerical simulation illustrates not only the final shape of the part, which is similar to the mold, but more importantly, the thickness map of the blown parison. Often, the final thickness of a blow-molded product shows regions where the blown parison is too thin (dark blue) to be acceptable for safety reasons, or too thick (yellow and red) to be economically attractive. To compensate for a non-optimized blown parison, POLYFLOW has tools to suggest a different profile thickness for the extruded parison that will lead to a more uniform final thickness map for the blown product, independent of the complexity of the shape. Other results of the simulation include an extensions map, which indicates how much the material has stretched to reach the final shape, the permeability of the finished product, the weight of the flashes (waste material at the edges), as well as the inner volume.

Courtesy of Forsheda

The shaping of rubber compounds by extrusion is a widely used process for producing finished products such as profiles for automotive applications and construction. The transition from the flow in the die to the freely streaming polymer gives rise to changes in the local velocities, which induce deformations in the extruded profile. When a designer’s knowledge is combined with the insight provided by numerical simulation, several designs can be rapidly evaluated so as to yield the required product. This results in a reduction of the number of trial dies, with subsequent reductions in costs, time-to-market, and scrap material.

Numerical result of the die design task: die (gray), deformed extrudate (red)

In an attempt to reduce die design costs, engineers at Forsheda in Sweden collaborated with the POLYFLOW product team in Belgium for analyses of dies for extruded rubber products. A particularly helpful feature of the POLYFLOW software is the availability of an “inverse extrusion” function, an automatic die lip design functionality that uses an advanced deforming mesh technique. The result of a calculation that uses this function is a die design that takes material deformations into account so as to produce the desired finished product.

As a part of this project, two synthetic rubber materials were simulated, each of which was characterized by a power law viscosity. Constraints on the pressure drop across the die forced the length of the die to not exceed a pre-defined value for a given extrudate speed. A larger pressure drop would require an investment in equipment which was not in line with the scope of the project.

The complexity of the die geometry resided in the juxtaposition of thin and thick parts, leading to a strongly unbalanced velocity profile. A first simulation revealed that the velocity through the thin slit region was 10,000 times slower than that across the wider body opening. The next analysis, therefore, involved two stages: a die balancing task to get a satisfying velocity profile across the die lip and an “inverse” simulation to automatically calculate the die lip shape to compensate for the residual deformation.

The first attempt to improve the product led to encouraging results, even though experimental work revealed some discrepancies with the targeted profile. An analysis of these results suggested that advanced physics such as partial slip and thermal effects (such as viscous heating) were playing a significant role. By comparing the flow pattern revealed by the simulation and the actual deformation of the material, a better understanding of the most efficient techniques to modify a given die was reached.

The steel die used for the extrusion process

With limited information regarding the flow pattern and material behavior, the numerical task for the engineers at Forsheda was a real challenge. Sliding behavior, geometric details, and thermal effects proved to play an important role in this process. As a result of the simulations, the rheological and sliding behavior of the material became better understood, leading to fundamental understanding that could be applied to other similar applications.

This text is a summary of the work published at the ESAFORM conference, Liege, Belgium, April 2001.

Case Study Courtesy: Courtesy of Vygon S.A.

Rapid technological developments in health care have been accompanied by the emergence of many new products. Vygon S.A. pioneered the development of sterile, single-use, disposable products, and today is an industry leader pursuing specialized production methods that meet the rigorous quality and cleanliness requirements of medical devices. CFD analysis, using POLYFLOW, has helped Vygon optimize performance and reduce the design time for manufacture of their flexible tubing products.

“Using POLYFLOW’s design capability and our design experience, dies can now be designed and tested in two days instead of two weeks, a considerable time-to-market reduction.” Pierre Roy,Director of Research, Vygon S.A.

Design and manufacture of flexible tubing products is made complex by several factors. Often the tubes must transfer multiple fluids, with each fluid flowing in a separate lumen whose cross-sectional area controls the maximum possible flow rate. At the same time, flexibility and small size are typical design goals. Hence the design and manufacture of the tubing must be optimized to yield a very specific cross-sectional shape (see figure). The tubes are manufactured using an extrusion process, requiring a die design that can reliably deliver the desired extrudate shape.

Pediatric tri-lume tubing cross-section, illustrating the complex extrudate shape required from the extrusion die.

Using POLYFLOW, Vygon has performed several die designs for multi-lumen tubes, including precise control of the internal lumens. On the basis of an imposed profile shape and operating conditions, POLYFLOW computed the required die shape. The dies were then manufactured and tested by Vygon and a profile very close to the required one was obtained on the first trial. In addition, the resulting dies have been more robust and stable to handle, yielding more reproducible products with negligible tuning. The result has been increased production rates and a reduction in the number of extruders operating. Photo courtesy of Vygon S.A.

Courtesy: Anthony D. Cato and Dan D. Edie, Clemson University

Carbon fibers are manufactured through heating and stretching processes. Poly-Acrylo-Nitrile (PAN) and pitch are the two most common raw products used to produce carbon fibers. Unlike the synthetic PAN based fibers, pitch is a coal-tar petroleum product that is melted, spun, and stretched into fibers. CFD can very effectively model the melting in the extruders and spinning through the spinneret .

POLYFLOW is capable of predicting the temperature rise due to viscous heating during high shear flow of mesophase pitch through the spinneret capillary. Typically, these spinnerets have an exit diameter of 150 mm and the wall shear stresses are in the range 2500 1/s to 7500 1/s. This study focuses on identifying flaws in spinneret design in terms of recirculation in the tapered spinneret capillary, while spinning the high temperature viscoelastic melt. Such recirculating flow can cause degradation of the mesophase pitch and disrupt the structure of the spun fiber. Using Fluent’s POLYFLOW software, the spinneret design can be modified to have smoother outflow through the capillary.

Stream function distribution for shear rate of 7500 /s showing recirculation

Matthew R. Hyre, Virginia Military Institute, Lexington, VA

Gob formation at the feeder

Industrial glass container forming is a complex sequence of unit processes that leads up to the actual forming process in an individual section machine. The forming process can be roughly divided into several steps that begin with the formation of a glass gob at the feeder, followed by the transfer and loading of the gob into a blank mold. The shape of the glass gob and its orientation before it falls are important components of the manufacturing process of many glass products. Large deviations from the ideal gob shape and trajectory can have severe consequences on the penetration of the glass into the transfer equipment and molds, and asymmetric loading of the gob into the blank molds can cause uneven temperature and wear patterns on the mold interiors. Traditionally, gob shape control has been conducted by trial and error based on past experience and operator knowledge, but recent advances in numerical techniques and computer capabilities have made the numerical modeling of the gob forming processes feasible.

A numerical study was performed recently using POLYFLOW to investigate the importance of the initial gob formation and transfer on the formation of glass bottles. The simulation modeled the formation of the gob at the feeder, and the transfer of the gob to the blank mold. Techniques such as thermo-mechanical coupling, mesh-to-mesh interpolation, and mesh superposition of the plungers on the glass were employed. Remeshing techniques were used that allowed a continuation of the calculations despite very severe mesh deformations. By evaluating the extent to which feeder plunger motion and gob transfer equipment affect gob shape and weight, a systematic methodology to control these parameters can be developed.

Gob transfer to a blank mold

The foam industry is rapidly consolidating and becoming increasingly driven by cost factors. Time-to-market is key for the design of foam products, such as flexible insulation products, which Armstrong manufactures for the heating, cooling, and plumbing markets. Die-making is a barrier in decreasing time-to-market, particularly with the demand for cheaper, lighter, and custom profiles. More an art than a science, die-making still depends almost entirely on trial and error. ANSYS Fluent helped to reduce the number of die iterations leading to a cut in cost and time-to-market. Working together, Armstrong and Fluent have shown how CFD can augment the traditional trial-and-error design process.

Estimated return on investment on an annual basis from using modeling for die design

In this project, partially funded by the European Commission, Armstrong Insulation Products provided geometrical and rheological data. Fluent provided POLYFLOW software and expertise in simulation of foam production and extrusion. The research team’s goal was to show that CFD simulation can predict the die geometry necessary for achieving the desired shape. As such, CFD provides a valuable “first guess” from which to start the die-making process, ultimately reducing overall time and increasing performance.

Two foam profiles were studied – one simple and one complex. The single bubble growth model in POLYFLOW was used. This model divides the foam into spherical microscopic unit cells of equal and constant mass, each made up of a liquid envelope and concentric spherical gas bubble. This model describes important qualitative features of a real system of numerous bubbles growing in close proximity. Rheological data provided by Armstrong allowed for fitting a power law for the viscosity, while the foaming model parameters were chosen in order to have the right densities at the inlet and outlet, and to obtain reasonable bubble sizes.

Fluent used POLYFLOW’s unique automated design tool to calculate the optimal die geometry. The design was built and tested. Some differences existed between the numerical results and the experimental shape, but it took minimal trial-and-error to adjust the die and obtain the desired result. The benefits of this approach were clearly shown as the cost of die design and time-to-market were significantly reduced. The return on investment for using POLYFLOW for die design assistance quickly becomes significant when the number of dies produced per year exceeds 3 of complex shape or 10 of a more simple geometry.

Roy Christopherson, REXAM Flexibles Ltd., Bristol, England

The use of thin flexible films for the packaging of disposable sterile medical devices is a large and growing part of the medical packaging market. In most cases, the packaging format used for medical devices is a formed pack produced using a thin polymeric film sealed to a top web of paper, which permits the ingress of the sterilization gas but is resistant to bacterial penetration post sterilization. In addition, to keep the cost of the packaging to a minimum and to reduce environmental impact, it is desirable to use as thin a polymeric web as possible. In the case of a syringe pack, the film thickness may typically be 65–150m, reducing to as low as 15–35m in the corners after the thermoforming process. This is adequate for providing a sterile environment, but may not be sufficiently rugged for the life and demands of the packaging. For instance, during transit from the manufacturing site to the end user, it is important that the package remains intact with no holes or pits forming in the film. A small hole of 10 microns will allow airborne bacterial spores to ingress into the pack, leading to a sterilization failure.

At REXAM, one of the top consumer packaging companies in the world, transit tests have been devised to simulate and quantify levels of packaging failure for syringe packs. The rates of failure typically average less than 0.2%, with the two primary causes being abrasion and puncture by the syringe. Failure due to puncture was of primary interest to REXAM engineers. They wanted to develop a technique to predict failure accurately and use this knowledge to “reverse engineer” their packaging, so that it would be less prone to puncture. The approach they chose involved two computational software packages: POLYFLOW, to model the thickness distribution of the thermoformed pack; and MSC.Marc™, a stress analysis code, to model the strain rate of the thermoformed packaging and predict probabilities for puncturing the pack. When combined, these two simulation techniques could be powerful predictors of mechanical strengths for a given type of syringe packaging.

POLYFLOW CFD simulation of the “coffin” thermoformed product packaging, showing the thickness distribution. Predictions for thickness at five locations on the coffin surface were found to be in very good agreement with experimental measurements for bo

POLYFLOW CFD simulation of the “coffin” thermoformed product packaging, showing the thickness distribution. Predictions for thickness at five locations on the coffin surface were found to be in very good agreement with experimental measurements for both materials tested.
REXAM engineers validated their modeling approach for a typical 10ml syringe package using two different film packaging materials. Both films were thermoformed into a “coffin” style die for the 10ml syringe. In the experimental tests, randomly chosen packs were punctured using a Lloyd Tensile Tester. CFD models for the two cases were set up in POLYFLOW, using the physical properties, including the special rheological behavior, of each material used. A membrane approximating approach was used to simulate the thermoforming process in order to reduce the computational time required. The CFD predictions were in excellent agreement with measurements for one of the films, and in good agreement for the other. The puncture resistance simulations using MSC.Marc were also in very good agreement with measurements, thus confirming the suitability of this dual simulation approach for analyzing this type of film packaging. REXAM believes that the ability to assess material changes in the packaging design will lead to significant time and cost savings in their manufacturing processes in the future.