CAE cuts the guesswork in blow molding & thermoforming.

How does a blow molder know which parison program will give the correct material distribution for desired strength in a finished part? How does a thermoformer know what pattern of oven heat zones is needed to achieve the same goal? Now processors have more at their disposal than experience-based guesswork and trial-and-error experimentation. Recently developed software can predict the degree of stretching that will occur at every point on the parison or sheet as the part is blown or formed. Such software can also predict the final thickness that will result and the rate at which every point on the part will cool to a suitable ejection temperature (see PT, June '92, p. 13; July '92, p. 9; May '94, p. 63).

The latest example of such software became commercially available just last month from AC Technology, Ithaca, N.Y. C-Mold blow molding and thermoforming software originated with the PITA/STAT program developed in 1986 for internal use by General Electric Corporate R&D in Schenectady, N.Y., and GE Plastics, Pittsfield, Mass. Over the last two years, the software has been further developed for commercial release by a consortium of AC Technology, GE Plastics, Dow Plastics (Midland, Mich.), Ford Motor Co. (Dearborn, Mich.), Johnson Controls Inc. (Manchester, Mich.), Mitsubishi Kasei of Japan, and CSI Applied Research, part of the Italian Montedison Group. Also participating in the consortium were two consultants--Dr. Lawrence R. Schmidt of LR Schmidt Associates in Schenectady, N.Y., and Dr. James L. Throne of Sherwood Technologies, Inc., Hinckley, Ohio--and researchers at Cornell University in Ithaca, N.Y., State University of N.Y. in Buffalo, and Pennsylvania State University in Erie.

PREDICTING PART STRENGTH

C-Mold offers both 3-D finite-element analysis and a simpler and much faster 2-D analysis. The latter is said to be sufficiently accurate for modeling simple axi-symmetrical parts, such as a round bottle, formed box, or even a long, relatively symmetrical object such as a blow molded bumper. Though it takes an order of magnitude longer, 3-D analysis is more accurate for more complex parts.

C-Mold automatically generates a finite-element mesh for the geometric model and permits the user to manually add more elements in particular areas of the part. Once C-Mold has calculated the final part thickness, it can automatically output its results to Ansys, a popular third-party finite-element structural-analysis program, to simulate how the part will perform under load. (In development are C-Mold interfaces to Abaqus, another popular FEA program, and to AC Technology's own structural-analysis program in its shrinkage/war-page module.)

MODELING THE PROCESS

C-Mold models the progress of parison inflation or sheet stretching in a number of steps selectable by the user. Animation permits viewing the entire process at once from any angle. At each step in the sequence, C-Mold can display a color-shaded contour "map" of thickness, stretch ratio, or cooling time of each node or mesh element in the model. Another color-coded map shows which parts of the parison or sheet are in contact with the mold surface at each step in the process simulation.

These displays help processors identify excessively thin or thick spots in the finished part and areas of the part that are last to cool. C-Mold also predicts the blowing or forming pressure needed to produce the part. Special routines permit modeling plug-assist thermoforming and stretch-blow molding.

Quick answers from 3-D analysis can be derived by selecting critical nodes of interest and obtaining a nodal plot of what happens at just those nodes, ignoring the rest of the part.

Another valuable feature allows the thickness, stretch-ratio, or other data on the final blown or formed shape to be projected back onto the original parison shape or flat sheet. This could suggest how to modify the parson program or pattern of sheet-temperature zones to improve material distribution. Consortium members are finding other benefits, too. One used C-Mold to help design an elliptical die shape for a shampoo bottle. And consultant Throne adds that the software is very useful for thermoforming plug design, which he says has been "a big unknown."

WHAT DATA ARE NEEDED?

In order to set up an analysis, C-Mold requires a geometric model of the mold and initial parison or sheet dimensions, created in the C-Mold modeler or preprocessor. The program also asks for initial melt temperature, coolant temperature, times and distances for mold movements, and definition of any preblowing, prestretching, and depressurization stages.

In addition, the program requires certain material-property data: density, specific heat capacity, thermal conductivity, and transition (ejection) temperature. These are similar or identical to data required for injection mold-filling analysis.

The only special material data required by the program are a set of "hyperelastic material constants" that provide information analogous to an elastic modulus for the melt, which is assumed to be an infinitely elastic membrane. These constants may be derived at one or more melt temperatures, depending on whether the cycle time is long enough to allow significant melt-temperature change before the final part shape is achieved. AC Technology has defined a test method for obtaining uniaxial materials constants, which was described in two student papers at this year's SPE ANTEC in San Francisco. Uniaxial stretch data are said to be relatively quick and easy to obtain and to give a reasonable approximation of reality. But for higher accuracy, biaxial stretch data can be obtained through test methods developed by GE Plastics and Penn State.

At present, the C-Mold materials database includes around a dozen generic materials, including HDPE, ABS, HIPS, PPE alloy, polycarbonate, PC/PBT alloy, and acrylic.

FUTURE ENHANCEMENTS

AC Technology officials freely admit that blow molding and thermoforming process simulation is still in its early stages--about where injection molding simulation was 10 years ago. Consortium members have tested C-Mold on complex shapes such as blow molded car bumpers and gas tanks and are satisfied with the results on at least a qualitative basis. To provide higher quantitative accuracy, consortium members have asked AC Technology to work on several refinements to the software:

* The most important will be substituting a viscoelastic material model for the current hyperelastic model. This will permit the software to take into account such real-world phenomena as elastic yielding and strain-hardening behavior in polypropylene and troublesome "aneurisms" (a form of yield behavior) in PET blow molding. Prediction of sag in thermoforming will be an especial benefit, Throne predicts.

AC Technology sources note that a viscoelastic model will be helpful mainly for long-cycle industrial blow molding and thermoforming. In most packaging and other small-parts applications, cycle times are said to be too short (shorter than the melt-relaxation time) for viscoelastic effects to be significant.

* Consortium members see a need to add extrusion process modeling to the blow molding simulation. What happens to the melt upstream in the extruder and head affects behavior such as die swell.

* Consortium members want to be able to model multilayer coextrusion with different resins or recycle layers, as well as multimaterial sequential coextrusion of hard/soft parts.

* Current cooling simulation is highly simplified. A future enhancement will interface existing C-Mold cooling software so that users can determine effects of placement and number of cooling channels and of different mold materials.

* Other future projects include scrap-volume estimation, explicit identification of vent holes (which will aid tool designers in minimizing air entrapment), and adaptive mesh generation. This last will automatically add mesh elements where they are needed--e.g., to small details of a part or to areas that undergo a high degree of stretching. That will ameliorate current problems encountered in simulating very deep draws or high stretch ratios.

COMING SOON ON PCs

C-Mold currently runs on workstations; a PC version will be released in December. Price of the workstation version is $35,000 for the analysis module only. If you don't already have C-Mold for injection molding, you'll also need the modeler (preprocessor) and graphic visualization module (post-processor), which raise the price to $43,600. The PC version will cost $14,000 less.