Advanced Capabilities to Consider when Simulating Blow Molding in Ansys Polyflow or Discovery AIM

Ansys Polyflow is a Finite Element CFD solver with unique capabilities that enable simulation of complex non-Newtonian flows seen in the polymer processing industry. In recent releases, Polyflow has included templates to streamline two of its most common use cases: blow molding and extrusion. Similarly, Ansys Discovery AIM offers a modern user interface that guides users through blow molding and extrusion workflows while still using the proven Polyflow solver under the hood. It is not uncommon for engineers to be unsure about which tool to pursue for their specific application. In this article, I will focus on the blow molding workflow. More specifically, I will point out three features in Polyflow that have not yet been incorporated into Discovery AIM:

  1. The PolyMat curve fitting tool to derive viscoelasticity model input parameters from test data
  2. Automatic parison thickness mapping onto an Ansys Mechanical shell mesh
  3. Parison Programming to optimize parison thickness subject to final part thickness constraints

Keep in mind that either tool will get the job done in most applications, so let us first quickly review some of the core features of blow molding simulations that are common to Polyflow and AIM:

  • Parison/Mold contact detection
  • 3-D Shell Lagrangian automatic remeshing
  • Generalized Newtonian viscosity models
  • Temperature dependent and multi-mode integral viscoelastic models
  • Time dependent mold pressure boundary conditions
  •  Isothermal or non-isothermal conditions

For demonstration purposes, I modeled a sweet submarine toy in SpaceClaim. Unfortunately, I think it will float, but let’s move past that for now.  

Figure 1: Final Submarine shape (Left), Top View of Mold+ Parison (Top Left), Side View of Mold+Parison (Bottom Right)

At this point, you could proceed with Discovery AIM or with Polyflow without any re-work. I’lll proceed with the Polyflow Blow Molding workflow to point out the features currently only available in Polyflow.

PolyMat Curve Fitting Tool

With the blow molding template, you can select whether to treat the parison as isothermal or non-isothermal and whether to model it as general Newtonian or viscoelastic. Suppose we would like to model viscoelasticity with the KBKZ integral viscoelastic model because we were interested in capturing strain hardening as the parison is stretched. The inputs to the KBKZ model are viscosity and relaxation times for each mode. If they are known, the user can input the values directly. This is possible in Discovery AIM as well. However, the PolyMat tool is unique to Polyflow. PolyMat is a built-in curve fitting tool that helps generate input parameters for the various viscosity model available in Polyflow using material data. This is particularly useful when you do not explicitly have the inputs for a viscoelastic model, but perhaps you have other test data such as oscillatory and capillary rheometry data. In this case I have with the loss modulus, storage modulus and shear viscosity for a generic high density polyethylene (HDPE) material. For this material, four modes are enough to anchor the KBKZ model to the data as shown below. We can then load the viscosity/relaxation time into Polyflow and continue. 

Figure 2: Curve Fitting of G’(Ω),G’’(Ω),η() [Left], KBKZ Viscoelastic Model inputs (Right)

The main output of the simulation is the final parison thickness distribution. For this sweet submarine, the initial parison thickness is set to 3mm and the final thickness distribution is shown in the contour plot below.

Figure 3a: Animation of blow molding process

Figure 3b: Final Part Thickness Distribution

Thickness Mapping to Ansys Mechanical

The second Polyflow capability I’d like to point out is the ability to easily map the thickness distribution onto an Ansys mechanical shell mesh. You can map the thickness onto an Ansys Mechanical shell mesh by connecting the polyflow solution component to a structural model in workbench as shown below. The analogous work flow in AIM, would be to create a second simulation for the structural analysis, but you would be confined to specifying a constant thickness.

Figure 4: Polyflow – Ansys Mechanical Parison Thickness Mapping

In Ansys Mechanical, the mapping comes through within the geometry tree as shown below. The imported Data Transfer Summary is a good way to ensure the mapping behaves as expected. In this case we can see that 100% of the nodes were mapped and the thickness contours qualitatively match the Polyflow results in CFD -Post.

Figure 5: Imported Thickness in Ansys Mechanical

Figure 6: Thickness Data Transfer Summary

A force is applied normal to front face of the sail and simulated in Mechanical. The peak stress and deformation are shown below. The predicted stresses are likely acceptable for a toy, especially since my toy is a sweet submarine. Nonetheless, suppose that I was interested in reducing the deformation in the sail under this load condition by thickening the extruded parison. A logical approach would be to increase the initial parison thickness from 3mm to 4mm for example. Polyflow’s parison programming feature takes the guesswork out of the process. 

Figure 7: Clockwise from Top Left: Applied Load on Sail, Stress Distribution, total Deformation, Thickness Distribution

Parison Programming

Parison programming is an iterative optimization work flow within Polyflow for determining the extruded thickness distribution required to meet the final part thickness constraints. To activate it, you create a new post processor sub-task of type parison programming.   

Figure 8: Parison Programming Setup

The inputs to the optimization are straight forward. The only inputs that you typically would need to modify are the direction of optimization, width of stripes, and list of (X,h) pairs. The direction of optimization is the direction of extrusion which is X in this case. If the extruder can vary parison thickness along “stripes” of the parison, then Polyflow can optimize each stripe thickness. The list of (X,h) pairs serves as a list of constraints for the final part thickness where X is the location on the parison along the direction of extrusion and h is the final part thickness constraint.

Figure 9: Thickness Constraints for Parison Programming

In our scenario, the X,h pairs form a piecewise linear thickness distribution to constrain the area around the sail to have a 3.5mm thickness and 2mm everywhere else. After the simulation, Polyflow will write a csv file with to the output directory containing the initial thickness for each node for the next iteration. You will need to copy over the csv file from the output directory of iteration N to the input directory of iteration N+1. The good news is the optimization converges within 3-5 iterations.

Figure 10: Defining the Initial Thickness for the Next Parison Programming Iteration

Polyflow will print the parison strip thickness distribution for the next iteration in the .lst file. The plot below shows the thickness distribution from the first 3 iterations. Note from the charts below that the distribution converged by iteration 2; thus iteration 3 was not actually simulated. The optimized parison thickness distribution is also plotted in the contour plot below.

Figure 11: Optimized Parison Thickness (Top), Final Part Thickness (Bottom)

Figure 12: % of Elements At or Above Thickness Criteria

As a final check, we can evaluate how the modification to the parison thickness reduced the deformation of the submarine. The total deformation contour plot below confirms that the peak deformation decreased from 2mm to 0.8mm.

Figure 13: Total Deformation in Ansys Mechanical After Parison Programming

Summary

Ansys Discovery AIM is a versatile platform with an intuitive and modern user interface. While Aim has incorporated most of the blow molding simulation capabilities from Polyflow, some advanced functionality has not yet been brought into AIM. This article simulated the blow molding process of a toy submarine to demonstrate three capabilities currently only available in Polyflow: the PolyMat curve fitting tool, automatic parison thickness mapping to Ansys Mechanical, and parison programming. Engineers should consider whether any of these capabilities are needed in their application next time they are faced with the decision to create a blow mold simulation using Ansys Discovery AIM or Polyflow.

Stuff I Learned about Injection Molding with 3D Printed Tooling

3DPrinting-Injection-Molding-Pic-StauberMaking injection molding tools using 3D Printing has been a long term goal for the industry.  I knew the technology had advanced recently, but was really not aware how far it had come until I attended two seminars in Utah on the subject. In this post I’ll share what I learned, and share some content that goes into greater detail.

The Seminars

The reason for my update on this subject was a visit to PADT’s Utah office.  Our two people there, Anthony Wagoner (sales) and James Barker (engineering), told me they were doing a seminar on injection molding and I should go. I figured why not, I’m in town. Maybe I’ll meet a couple of customers.  Almost 30 people showed up to the Salt Lake Community College Injection Molding lab for the event.  Gil Robinson from Stratasys presented a fantastic overview (included in the download package) on where the technology is, how to apply it, and gave some great real world examples.  There were some fantastic questions as well which allowed us to really explore the technology

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Then the best part happened when we walked into the shop and saw parts being made right there on the machine. They had recently printed a tool and were shooting polypropylene parts while we were in the classroom next door. During the hour long presentation, Richard Savage from ICU Medical was able to fine-tune the injection molding machine and good parts were popping out. As you can imagine, what followed next was they type of discussion would expect with  a room full of injection molding people. “What material? How hot? What pressure? What is the cooling time? Do you use compressed air to cool it? Not a lot of flash, how hard are you clamping it? These features here, what draft did you need?”  Good stuff.  I got caught up in everything and forgot to grab some pictures.

I learned so much at that event that I decided to head north along the Wasatch Range to Clearfield and the Davis Applied Technology College.  About the same number of people were able to make it from medical, aerospace, and consumer products companies in Northern Utah.  Gil presented the same material, but this time we got some different questions so I learned a bit more about material options and some other lessons learned.

Then we visited their lab where I did remember to take some pictures:

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Here is a shot of different shots that Jonathan George from DATC did to dial in the parameters.  It took him about an hour, not bad for the first time using a 3D Printed tool.

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The part is actually a clam shell assembly for Christmas lights, in the shape of a snow flake. Here is what they look like on the tree itself. IMG_8235

And here is a video they made showing the process. He was able to get 950 shots out of the tool.

In talking to attendees at both events I learned of several great applications that they were going to try, varying from medical devices for clinical trials to making rubber masking tools for surface treatments. The injection molding community in Utah is very sophisticated and forward thinking.

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What I Learned

I’ll spare you the details on what we had for dinner Monday night for the Utah office holiday celebration and jump right in to what I learned.

  1. For  the right applications, you can get some very nice parts from 3D Printed tools
  2. You do need to take the process in to account and oriented the tools facing upward in the machine, add a bit more draft than usual, and keep your pressures and temperature down when compared to metal tools.
  3. For some parts, you can get over 1,000 shots from a tool, but most poeple are getting a couple of hundred parts.
  4. As with any injection molding, the magic is in the tool design and setting up the right parameters on the injection molding press.
  5. Tricky parts can be made by using metal inserts
  6. Some machining may be required on your 3D printed tool to get it just right, but that is mostly reaming holes for ejector pins and metal inserts
  7. Plastic is an insulator (duh) so plastic tools have to be cooled more slowly and with air.
  8. Conformal cooling is a great idea, but some work still needs to be done to get it to work.
  9. The mold usually fails during part ejection, so using mold release, good draft, and proper design can reduce the loading during ejection and get more parts from the tool.
  10. The material of choice for this is DigitalABS on Stratasys Connex Machines.

There was a ton more, and you can find most of it in the download package.

The big take-away from both events was that this technology works and it really does allow you to create an injection molding tool in a couple of hours on a 3D Printer. In the time it normally takes to just get the order figured out for a machined tool (RFQ, Quote, Iterate, PO, etc…) you can have your parts.

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Next Steps

Interested in trying this out yourself or learning more?  We have put together an injection molding package with the following content:

  • Polyjet Injection Molding Application Brief
  • 18 Page Polyjet Injection Molding Technical Guide
  • 12 Page White Paper: Precision Prototyping – The Role of 3D Printed Molds in the Injection Molding Industry
  • 3D Printed Injection Molding Application Guide from PADT and Stratasys
  • Presentation from Seminars
  • List of Relevant Videos
  • Four Real World Case Studies
  • Link List for Other Resources  on the Web

We have spent some time putting all this information in one place and put it into one convenient ZIP file.  Please click here to download this very useful content.

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