Blow Molding

Thermoforming

Stamping

Miscellaneous

POLYFLOW can simulate all phases of various blow molding processes: Extrusion Blow Molding (EBM), Injection Blow Molding (IBM), Stretch Blow Molding (SBM) and 3D extrusion blow molding. For EBM, this includes the stage of the extrusion of the parison with, if desired, moving mandrels. Similar methods are used to simulate the thermoforming process, including pre-stretching and pressure or vacuum-forming.

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POLYFLOW simulations give you a window on the blow molding and thermoforming processes, allowing you to see how they evolve with time and how potential troubles might develop. The software is accessible in a user-friendly environment and provides graphical feedback and detailed information to determine the range of optimal process conditions and/or material properties and to correct potential problems.

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In particular, POLYFLOW's robust numerical algorithms account for large displacements of the parison or sheet and predict the contact between the fluid and the mold and other mechanical devices during the closing of the mold and parison pinch-off and the parison inflation. Moving molds or plugs can be considered, including the effect of slip, as well as complex geometries (gas tanks, containers, bottles with handles) and multi-layer blow molding.

For the entire process, POLYFLOW provides information on:

• the effects of gravity on the sagging of the extruded parison
• the pinch-off and inflation process and possible blowability limitations and the location of flashes
• the process sensitivity to operating parameters and materials properties
• the fluid temperature distribution resulting from cooling during the shaping
• the thickness and extensions distribution, the results of which can be transferred to structural analysis software for further analysis
• the cooling phase resulting from contact with the die including the crystallization

In POLYFLOW, the parison is modeled as a fluid. Standard rheological data are only required for a simulation. POLYFLOW offers the largest library of fluid models (including viscous, generalized Newtonian and KBKZ viscoelastic) for a realistic representation of the material properties of the melt. Both a membrane model approach and a full three-dimensional elements approach are available and all current capabilities are available both for single and multilayer configurations.

The initial (extruded or injected) parison shape and mold geometry are set-up by standard CAD tools or extracted from parison extrusion simulations. The melt model and the operating conditions are selected interactively and also stored for further analyses. It is then easy to modify the geometry, the fluid properties or the process conditions to select the best process scenario. With these data, POLYFLOW calculates the flow within a few hours on standard PC or Unix platforms and produces a sequence of "snapshots" of the process with detailed information about various features of interest. These can smoothly be animated to offer additional insight in the transient effects of the blow molding and thermoforming processes.

Blow Molding

Process Description

The Challenges

Benefits Gained

Advanced Numerical Aspects

Process Description

The extrusion blow molding or injection (stretch) blow molding process is the method of choice to form any kind of hollow product such as bottles, gas tanks, or car bumpers. The process has four stages: extrusion, pinch-off, blowing and cooling. Each phase presents numerous challenges and the success of the whole process depends upon the success of each phase.

The final goal of the extrusion blow molding process is a blown bottle with a uniform thickness or at least a formed container where the blown products minimum thickness is above the minimum thickness everywhere else. In order to meet this final objective the parison extruded in the first phase must have the proper thickness profile.

Extrusion

A thin parison is extruded through a die whose die lip is usually, but not always, circular. In order to adjust the thickness profile of the parison, the inner die mandrel can move up and down according to a predefined transient motion. This will open or close the aperture at the die lip creating a more or less thick parison. The influence of the gravity is quite significant in this process since as the material is extruded and the parison grows, its weight will pull the material downward, stretching the extruded parison and modifying its thickness profile.

For gas tanks and bottles, multi layered parisons (up to 7 layers) are usually considered in order to gain the mechanical, chemical and optical features. In addition to the global thickness profile, the designer has to make sure that each layer also keeps the proper thickness.

Pinch-off

After the parison has been extruded, it is cut and placed between the two halves of the mold. Next, the molds are moved toward each other to close the container. Part of the parison is crushed between the two halves of the mold, creating the scraps. This is lost material, since it will be cut before the bottle is shipped. An objective of the designer is to reduce the quantity of material wasted in this section.

The quality of the welding line is very important. This is usually a weakness of the blown part.

Blowing

When the cavity is closed, air (or another gas) is injected into the cavity in order to blow the melt parison. Typical blowing pressure of a few bar, is applied along the inner parison. Potentially, this blowing pressure can change as a function of time

As the parison is blown, some sections of it hit the mold. Step-by-step, the parison takes the exact shape of the mold leading to the final blown product.

Cooling

When the parison has been blown, heat is exchanged by both the mold and possibly by the injection of cold air. The thin parison is quickly cooled down to room temperature. This often induces some phase change and crystallization.

The Challenges

The goal of the blow molding process is to obtain a blown product whose parison is neither too thin (otherwise, the container would break at the thinnest section as soon as it is under constraint) nor too thick. A too thick section is a waste of material and money. Also, a well defined thickness profile must be obtained at the end of the process.

Numerous difficulties prevent the mold designer from reaching this objective in the first trial.

Phase change and crystallization will induce additional deformation and residual stresses. This weakens the final bottle and modifies its thickness profile.

During the blowing phase, the parison is blown like a balloon, generating a lot of stretching. Viscoelastic effects such as strain hardening sometimes prevent a too-large decrease of the parison thickness during the blowing phase. However, the more the parison is blown, the thinner the final thickness. Also, the sections first in contact with the mold will not deform much whereas the sections of the parison where the fly will be the longest could see their thickness reduced a lot. These problems make it very difficult to guess a priori what the final thickness will be considering the initial thickness of the parison.

The pinch-off is definitely a critical phase of the process. The quality of the welding line must be maintained. The difficulty is even greater for multilayer blow molding processes. It is then important to make sure that each layer is still present along the welding line despite some different rheological behavior and sometimes very different viscosities.

Finally, the extrusion step is perhaps the most critical, since it determines the initial thickness distribution of the parison. The difficulties include the swelling and sagging of the parison, together with heat exchange with the environment air and the moving inner mandrel. The motion of the mandrel provides a degree of freedom for the designer in adjusting the initial thickness of the parison. Trial-and-error is usually necessary in order to find the relevant transient motion.

Benefits Gained

POLYFLOW is able to simulate the different phases of both the extrusion blow molding and the injection (stretch) blow molding processes. It allows you to understand the influence of each parameter independently. All the challenges described above can be analyzed and solved using POLYFLOW.

2D axisymmetric simulation of the (co)extrusion phase can be carried out taking into account the swelling, the sagging, the heat exchange and the moving mandrel. As a result of the die geometry, the flowing material and their operating conditions, it is possible to determine what the thickness profile of the initial parison would be.

Next, you start from this initial thickness distribution to blow the product and calculate the final thickness profile. Most of the time this final thickness is far from meeting the target requested by the product designer. Also, the thickness profile of the initial parison, hence the motion of the inner mandrel of the mold, has to be modified. Step-by-step you are able to improve the quality of the final product, making sure that the thickness is above the minimum value everywhere.

Certainly, most of the key parameters of the simulation such as the inflation pressure and the speed of the mold are time-dependent, possibly with very complex functions of time. These dependencies can be simulated either by using the typical functions already predefined in the code, or by specifying arbitrary time functions.

Temperature profiles across the parison are another key point that POLYFLOW simulates. As soon as the parison is blown or during the blowing process itself, the heat exchange between the polymer and the mold is calculated. The parison is cooled down while the mold is slightly heated in the vicinity of its inner cavity. Simulating the cooling process is especially important since this is usually a long part of the the whole process (a few seconds). Also, the cooling stage must be long enough to allow for the completion of the cooling while remaining as short as possible in order to keep the cycle time low. In addition, change of phase, i.e. the crystallization of the material during the cooling may be calculated during the simulation. It is then possible to get another evaluation of the quality of the final product.

The blow molding process is a typical example where biaxial and possibly uniaxial (stretch blow molding) extensional behaviors are important. Also, the strain hardening phenomenon, i.e. the increasing resistance of the resin to the extension as the deformations increase, can not be neglected. Otherwise, inaccurate results would be calculated. Polyflow has implemented differential viscoelastic models in 2D and KBKZ viscoelastic models in 3D in order to take this behavior into account.

Most of the time a membrane approximation for the parison is considered. This strongly reduces the computational time required to solve a simulation while barely affecting the quality of the results. Indeed, the thickness dimension (~1 mm) is very very small compared to typical dimensions of the blown product (from a few cm up to 1 m). Here, the parison is modeled as a surface deforming in a 3D environment. However, in some specific situations where the temperature, velocity or thermal gradient across the thickness can not be neglected, a full 3D simulation including volume elements across the parison can also be simulated.

Advanced Numerical Aspects

One of the greatest challenges of blow molding simulation lies in the fact that there is a parison flying in open room until it comes into contact with a mold whose geometry can be quite complex. We have developed an intelligent contact algorithm that detects whether the parison is still flying or whether it has already reached the mold, whatever its shape.

Furthermore, the boundary conditions switch from a free surface without heat exchange (or limited heat exchange with the surrounding air) toward an adherence or partial slip condition and an intense heat exchange with the mold.

Due to the very large deformation undergone by the parison, very specific and powerful remeshing rules have been implemented. One of the them, the Lagrangian remeshing technique, tracks each material point so that it is possible to understand where each point is coming from in the initial parison.

Thermoforming

Process Description

The Challenges

Benefits Gained

Advanced Numerical Aspects

Process Description

Thermoforming is the method of choice to form any kind of large flat product such as a car door panel, a truck wind deflector, or a yogurt container. The process has three stages : extrusion, blowing and cooling. Each phase presents numerous challenges and the success of the whole process depends on the success of each phase.

The final goal of the thermoforming process is to have a blown product with either a uniform thickness or at least a formed container where the minimum blown product thickness is above the minimum thickness everywhere else. In order to meet this final objective, the parison extruded in the first phase must have the proper thickness profile.

Extrusion

A thin sheet is extruded through a coat hanger die. The thickness of the extruded product should be as uniform as possible. Only the edge of the sheet where border effects are observed will be cut.

Often, multi layered sheets (up to 7 layers) are considered in order to gain from the mechanical, chemical and optical features of each resin. In addition to the global thickness profile, the designer has to make sure that each layer keeps the proper thickness.

Blowing

The initial sheet is pre heated. Usually a non-uniform map of temperature is considered in order to locally modify the resistance of the material to the deformations. Next, air (or another gas) is blown onto the sheet in order to deform the melt sheet. Typical blowing pressure of a few bars is applied along the inner parison. Potentially, this blowing pressure can change as a function of time. As the parison is being blown, some sections of it hit the mold. Step-by-step, the parison takes the exact shape of the mold, leading the the final blown product.

Another technique consists of creating a vacuum between the sheet and the male mold, so the melt sheet is attracted toward the mold, quickly acquiring the shape of it.

Cooling

When all the parison has been blown, heat is exchanged by both the mold and possibly by the injection of cold air. The thin parison is quickly cooled down to room temperature. This often induces some phase change and crystallization.

The Challenges

The final goal of the thermoforming process is to obtain a product neither too thin (otherwise, the container would break at the thinnest section as soon as it is under constraint) nor too thick. A too thick section is a waste of material and money. Also, a well defined thickness profile must be obtained at the end of the process.

Numerous difficulties prevent the mold designer from reaching this objective in the first trial.

During the cooling of the parison, due to the density variation on the temperature, residual flow of the polymer or glass is observed. Furthermore, phase change and crystallization will induce additional deformation and residual stress. This weakens the final bottle and modifies its thickness profile.

During the blowing phase, the sheet is blown like a balloon generating a lot of stretching across the parison. Viscoelastic effects such as strain hardening can sometimes prevent a too large decrease of the parison thickness during the blowing phase. However, the more the sheet is blown, the thinner the final thickness. Also, the sections first in contact with the mold will not deform much whereas the sections of the sheet where the fly will be the longest could see their thickness reduced a lot. These problems make it very difficult to guess a priori what the final thickness will be, considering an initial thickness of the parison.

Benefits Gained with POLYFLOW

POLYFLOW is able to simulate the different phases of the thermoforming process, including the sheet (co)extrusion stage. It allows the user to understand the influence of each parameter independently. All the challenges described above can be analyzed and solved using POLYFLOW.

Full 3D simulation of the extrusion or coextrusion of the sheet process is simulated. This provides valuable information, such as the thickness profile of the extruded sheet as well as the position of the interface(s). It also tells you the width of the sheet with a constant thickness. This is usually what is used for further thermoforming applications.

If you are buying your plastic sheets from an external company, these sheets usually come with a constant thickness. You start from this initial thickness distribution, but usually specify a non uniform temperature map in order to adjust the final thickness distribution of the formed product. Due to the temperature dependence of the viscosity, the resistance of the material to the deformations will change a lot if the material is cold or warm. By adjusting the initial temperature map, you fine tune the local behavior of the sheet during the forming process.

Most of the time this final thickness is far from your final target. Also, the temperature map of the initial sheet, hence the preheating process, has to be modified. Step-by-step, you are able to improve the quality of the final product making sure that the thickness is above the minimum value everywhere.

Certainly, most of the key parameters of the simulation, such as the inflation pressure and the speed of the mold, are time dependent, possibly with very complex functions of time. These dependences can be simulated either by using the typical functions already predefined in the code or by specifying arbitrary time functions.

The temperature profile across the sheet evolves during the process. As soon as the sheet is blown, or during the blowing process itself, the heat exchange between the polymer and the mold is calculated. The sheet is cooled down while the mold is slightly heated in the vicinity of its inner cavity. Simulating the cooling process is especially important since this is usually a long part of the whole process (a few seconds). Also, the cooling stage must be long enough to allow for the completion of the part cooling while remaining as short as possible in order to keep the cycle time low. In addition, change of phase, i.e. the crystallization of the material during the cooling may be calculated during the simulation. It is then possible to get another evaluation of the quality of the final product.

The thermoforming process is a typical example where biaxial extensional behaviors are important. Also, the strain hardening phenomenon, i.e. the increasing resistance of the resin to the extension as the deformations increase, can not be neglected. Otherwise, inaccurate results would be calculated. POLYFLOW has implemented differential viscoelastic models in 2D and KBKZ viscoelastic models in 3D in order to take this behavior into account.

Most of the time, a membrane approximation for the sheet is considered. This strongly reduces the computational time required to solve a simulation while the quality of the results is barely affected by this approximation. Indeed, the thickness dimension (~1 mm) is very very small compared to typical dimensions of the blown product (from a few cm up to 1 m). In this case, the sheet is modeled as a surface, deforming in a 3D environment. However, in some specific situations where the temperature, velocity or thermal gradient across the thickness can not be neglected, a full 3D simulation, including volume elements across the sheet, can be simulated.

Advanced Numerical Aspects

One of the greatest challenges of the thermoforming simulation lies in the fact that there is a sheet flying in the open room until it comes into contact with a mold, which may be of complex shape. We have developed an intelligent contact algorithm that detects whether the sheet is still flying or whether it has already reached the mold, whatever the shape of it.

Furthermore, the boundary conditions switch from a free surface without heat exchange (or limited heat exchange with the surrounding air) toward an adherence or partial slip condition and an intense heat exchange with the mold).

Due to the very large deformation undergone by the sheet, very specific and powerful remeshing rules have been implemented. One of the them, the Lagrangian remeshing technique, tracks each material point so that it is possible to understand where each point is coming from in the initial sheet.

Stamping

Process Description

The Challenges

Benefits Gained

Advanced Numerical Aspects

Process Description

The stamping process is very similar to the thermorforming process used for large flat plastic products and practically identical to the stamping process used in the metal industry. The process has three stages : extrusion, blowing and cooling. Each phase presents numerous challenges, yet the success of the whole process depends on the success of each phase. The major difference between it and the thermoforming process is the presence of additional moving parts that will either speed up the process by pushing the sheet of plastic or avoid some undesired crease.

The final goal of the stamping process is a blown product of either uniform thickness or at least a formed container where the minimum blown product thickness is above the minimum thickness everywhere else. In order to meet this final objective the parison extruded in the first phase must have the proper thickness profile.

Extrusion

A thin sheet of plastic is extruded through a coat hanger die. The thickness of the extruded product should be as uniform as possible. Only the edge of the sheet where border effects are often observed will be cut.

Often, multi layered sheets (up to 7 layers) are considered in order to gain from the mechanical, chemical and optical features of each resin. In addition to the global thickness profile, the designer has to make sure that each layer keeps the proper thickness as well.

Blowing

The initial sheet is pre heated. Usually a non-uniform map of temperature is considered in order to locally modify the resistance of the material to the deformations. Next, a male mold starts moving, pushing and deforming the sheet towards the female mold. This stamp motion allows the process to go faster than simply blowing the sheet. However, this process can lead to a local tearing of the sheet, completely ruining the product. Also, a modification of the process could be to first blow the sheet of plastic while the stamp starts moving a round, blown shape of the blown sheet usually prevents any residual tearing while keeping the benefit of a faster process.

During the blowing phase, air (or another gas) is blown onto the sheet in order to deform the melt sheet. Typical blowing pressures of a few bars are applied along the inner parison. Potentially, this blowing pressure can change as a function of time. As the sheet is deforming, some sections of it hit the mold. Step-by-step, the sheet takes the exact shape of the mold, leading to the final blown product.

Another technique consists of creating a vacuum between the sheet and the male mold, so that the melted sheet is attracted towards the mold, quickly acquiring the shape of it.

Cooling

As soon as the sheet is in contact with the mold, heat is exchanged by both the mold and possibly by the injection of cold air. The thin parison is quickly cooled down to room temperature. This often induces some phase change and crystallization.

The Challenges

The final goal of the stamping process is to obtain a product whose thickness is neither too thin (otherwise, the container would break at the thinnest sections as soon as it's under pressure) nor too thick. A too thick section is a waste of material and money. So, a well defined thickness profile must be obtained at the end of the process.

Numerous difficulties prevent the mold designer from reaching this objective in the first trial.

During the cooling of the product, due to the density variation of the temperature, residual flow of the polymer or glass is observed. Furthermore, phase change and crystallization will induce additional deformation and residual stress. This weakens the final container and modifies its thickness profile.

During the blowing phase, the sheet is blown like a balloon, generating a lot of stretching across the thin sheet. Viscoelastic effects, such as strain hardening, could sometimes prevent a too large decrease of the plastic sheet thickness during the blowing phase. However, the more the sheet is blown, the thinner the final thickness. Also, the sections first in contact with the mold will not deform much whereas the sections of the sheet where the fly will be the longest could see their thickness reduced a lot. These problems make it very difficult to guess a priori what the final thickness will be considering the initial thickness of the parison.

Benefits Gained with POLYFLOW

POLYFLOW is able to simulate the different phases of the stamping process including the sheet (co)extrusion stage. It allows you to understand the influence of each parameter independently. All the challenges described above can be analyzed and solved using POLYFLOW.

Full 3D simulation of the extrusion or coextrusion of the sheet process can be done. This provides valuable information such as the thickness profile of the extruded sheet as well as the position of the interface(s). It also tells you the width of the sheet with a constant thickness. This is usually the part that is used for further stamping applications.

If you buy your plastic sheets from an external company, these sheets usually come with a constant thickness. You starts from this initial thickness distribution but usually specify a non uniform temperature map in order to adjust the final thickness distribution of the formed product. Due to the temperature dependence of the viscosity, the resistance of the material to the deformations will change a lot if the material is cold or warm. By adjusting the initial temperature map, you fine tune the local behavior of the sheet during the forming process.

Most of the time this final thickness is far from your product design target. Also, the temperature map of the initial sheet, hence the preheating process, has to be modified. Step-by-step, you improve the quality of the final product, making sure that the thickness is above the minimum required value everywhere.

Certainly, most of the key parameters of the simulation such as the inflation pressure and the speed of the mold are time-dependent, possibly with very complex functions of time. These dependencies can be simulated either by using the typical functions already predefined in the code or by specifying arbitrary time functions.

The temperature profile across the sheet evolves during the process. As soon as the sheet is blown, or during the blowing process itself, the heat exchange between the polymer and the mold is calculated. The sheet is cooled down while the mold is slightly heated in the vicinity of its inner cavity. Simulating the cooling process is especially important since this is usually a long part of the the overall process (a few seconds at least compared to the longer inflation time). Also, the cooling stage must be long enough to allow for the completion of the cooling part while remaining as short as possible in order to keep the cycle time low. In addition, change of phase, i.e. the crystallization of the material during the cooling may be calculated during the simulation. It is then possible to get another evaluation of the quality of the final product.

The stamping process is a typical example where biaxial extensional behaviors are important. Also, the strain hardening phenomenon, i.e. the increasing resistance of the resin to the extension as the deformations increase, can not be neglected. Otherwise, unaccurate results would be calculated. Polyflow has implemented differential viscoelastic models in 2D and KBKZ viscoelastic models in 3D in order to take these behaviors into account.

Most of the time a membrane approximation for the sheet is considered. This strongly reduces the computational time required to solve a simulation while the quality of the results is barely affected by this approximation. Indeed, the thickness dimension (~1 mm) is very, very small compared to typical dimensions of the blown product (from a few cm up to 1 m). Here, the sheet is modeled as a surface deforming in a 3D environment. However, in some specific situations where the temperature, velocity or thermal gradients across the thickness can not be neglected, a full 3D simulation including volume elements across the sheet can also be simulated by POLYFLOW.

Advanced Numerical Aspects

The stamping process is typically a transient process. However, the use of an implicit transient scheme in order to solve the equation allows for the use of a large time step. An algorithm monitors the size of each time step and adjusts them according to the convergence criteria and the difference between the predictor and corrector models used in POLYFLOW.

One of the greatest challenges of the stamping simulation lies in the fact that there is a sheet flying in open room until it comes in contact with one or several mold(s) whose geometry can be quite complex. Also, we have developed an intelligent contact algorithm that detects whether the sheet is still flying or whether it has already reached the mold, whatever the shape may be.

Furthermore, the boundary conditions switch from a free surface without heat exchange (or limited heat exchange with the surrounding air) towards an adherence or partial slip condition and an intense heat exchange with the mold).

Due to the very large deformations undergone by the sheet, very specific and powerful remeshing rules have been implemented. One of the them, the Lagrangian remeshing technique tracks each material point so that it is possible to understand where in the initial sheet each point is coming from.

Miscellaneous

Dish Shaping

Tube Coating

Glass Pressing

Compression Molding

Roll Coating

There are many other applications where the contact capabilities of POLYFLOW are very useful. Every day, our users come up with new ideas that make use of these specific contact features. Here are a few of these ideas.

Dish Shaping

The dish shaping process is an interesting process where a sample of material (glass, clay, polymer) is dropped on a rotating mold. Due to the high rotational speed, the material tends to move toward the rim of the rotating dish. This action causes the gob to deform strongly. The initial free surface moves away. During this deformation, the free surface falls onto the rotating mold. Using the contact detection features of POLYFLOW, we are able not only to predict the deformation of the free surface but also where and when it will enter into contact with the mold.

Thanks to powerful remeshing techniques available in our code, very large deformations have been calculated, completing the whole process. Information such as the local velocity, the temperature map, the local shear rate, stresses, etc can also be calculated.

Tube Coating

For specific materials that can not undergo high shear rates (fluoropolymers such as FEP), the pressure coating process, where the material is quickly stretched as it touches the moving cable, can not be applied. In order to avoid the polymer deterioration, a smoother contact between the FEP and the wire is created after the polymer has left the die. The contact between the moving cable and the flowing polymer occurs after the die lip. There is a vacuum created in order to push the resin towards the cable. The material comes into contact with the cable at a position that can vary depending on the operating conditions.

Glass Pressing

Glass pressing is a widely used process to form glasses. In this process, a gob of glass at high temperature falls into a cylindrical cavity. Next, the stamp comes down and presses the melt glass. The material climbs up along the narrow channel left between the moving down stamp and the mold. It can easily be understood that very large deformation of the initial mesh is encountered. Despite the powerful remeshing techniques available in POLYFLOW, the mesh deformation can be so large that a new mesh needs to be defined. Then, an interpolation between the two meshes allows us to proceed with the simulation while restarting with the previous results of temperature, velocity, stresses. Heat exchange with the mold is particularly important in this process.

Compression Molding

The compression molding process is similar to glass pressing, but the material is not the same (polymers are more widely used for compression molding) and the shape of the male mold is usually more complex. For the compression molding process, POLYFLOW simulates the stamp coming down into the layer of polymer. The contact between the moving mold and the polymer is detected by the code so that the free surface deforms accordingly. The material is possibly pushed away from its initial position.

Roll Coating

A thin layer of polymer leaves the die. After some distance in the open air, the polymer is squeezed between two rotating rolls. Sometimes, the thin layer is simply pulled by a single rotating roll. Many parameters can influence this process and move the position of the contact point between the rotating roll(s) and the polymer. Also, the contact detection technique available in POLYFLOW allows us to easily determine the position of the contact as well as its evolution as a function of time or due to any perturbation introduced into the flow.