Mold-analysis software developing at a rapid pace.

A brand-new release of a popular family of mold-analysis software is just coming to market this month, and another major release is planned for January 1994. Together, these software upgrades are said to bring major enhancements in speed, accuracy and convenience to basic mold filling, cooling and packing analysis, plus new capabilities for modeling gas-assisted injection, processing of fiber-reinforced materials, and predicting crystallinity, residual stresses and warpage throughout a part. In addition, new interfaces to commercial CAD/CAM programs facilitate the transfer of CAD models directly into mold analysis.

The supplier, AC Technology of Ithaca, N.Y., has kept up a fairly rapid pace of development of its C-Mold family of mold-analysis programs. This month's release 3.2 follows on release 3.1 in January 1992 and release 3.0 about six months earlier. Release 4.0 is due early next year.

FASTER AND EASIER

Among the numerous enhancements in release 3.2 are increased speed in graphics and meshing through the C-View program. Citing two examples of models containing 3134 mesh elements and 5263 elements, respectively, AC Technology says meshing has been speeded up by 1.7 to five times, deleting is six times or more faster, and exiting is 1.4 to two times faster.

But the really big gain in speed is in C-Flow filling analysis, which is the first module to utilize a new "dynamic memory allocation" feature. This software "asks" the computer workstation for only as much memory as the model requires, rather than accessing the whole memory capacity of the system. This feature has two effects: First, unlimited numbers of nodes and mesh elements are available for a model. Second, speed is increased to the extent that "you can now analyze a large, complex model on a workstation as fast or faster than on a supercomputer," claims Peter Medina, president of the company's North American operations. For a smaller model with 2000 elements, AC Technology claims a 15% reduction in CPU time. The company plans to extend dynamic memory allocation to all its modules in the 4.0 release.

In release 3.2, the company has also made it much easier to "import" CAD and FEA models from a variety of CAD systems. The C-View IGES interface is said to be faster and more "robust"; it has been tested with 30 different CAD systems and is said to be "transparent" to those CAD files, retaining the identities of the original models. For example, the importation of a CAD file won't cause a circle to be broken up into a series of line segments, or a complex surface to be broken into a series of patches that would then have to be meshed separately instead of all at once. In addition, C-View 3.2 now offers a VDAFS interface, which is essentially an IGES-type translation standard developed by the German auto makers.

CAD system suppliers are also working to facilitate seamless data interchange between their software and C-Mold. For example, Intergraph Corp., Huntsville, Ala., has a new I/C-Mold interface that converts FEA models built by Intergraph's I/Flow program into proper format for direct input to C-Mold.

At press time, another new CAD interface was expected to be announced by Matra Datavision, Inc., Tewksbury, Mass.

The most dramatic news of this sort comes from Parametric Technology Corp., Waltham, Mass. It has a new direct interface to C-Mold for its Pro/Engineer and Pro/Mesh programs, which is capable of "one-button" conversion of 3-D solid models to the "2 1/2-D" shell models used for mold analysis. With one keystroke, the interface software collapses the solid model to a midplane and then applies a mesh. This answers a need frequently expressed by mold-analysis users, and Medina calls it "very impressive."

The C-Design 3.2 materials database has been expanded from 1800 to 2255 grades; obsolete materials have been purged, and remaining grades have been verified by suppliers. In the expanded database, 944 grades have the necessary data for C-Pack analysis, and 302 include pvT (pressure-volume-temperature) data for shrink/warp analysis. There also 75 sets of generic pvT data for certain resin types in filled and unfilled versions. C-Design also includes "utility tools" that will automatically interpolate data for new materials from those for related grades in the database. For example, it can interpolate the necessary thermal and flow data for a 15% filled nylon from existing data for 10% and 20% filled grades.

Another new feature of release 3.2, in response to user requests, is a report of total projected area of a part.

'THIRD-GENERATION' STRESS/WARP PREDICTION

Dr. V.W. Wang, president of AC Technology, considers one of the most significant elements of release 3.2 to be the brand-new C-Warp module. This program calculates the type and quantitative amount of deformation that will occur throughout a part (each cavity is treated separately) as a result of the accumulated residual stresses left after the pack/hold phase. Those stresses are calculated by the existing C-Pack/W module. Up to now, it was necessary to pass the C-Pack/W results to a "third-party" finite-element stress analysis (FEA) program from another vendor (interfaces are available for Ansys from Swanson Analysis Systems, Inc. and Abaqus from Hibbit, Karlsson & Sorenson, Inc.). Though users still have that option, AC Technology's own FEA program in C-Warp reportedly achieves results 10 times faster than any commercial alternative. Wang attributes this to the "dedicated" nature of C-Warp FEA algorithms.

Besides being fast, Wang believes C-Warp to be the most accurate warpage prediction available, thanks to what he describes as a "breakthrough" in shrinkage and residual-stress calculation by C-Pack/W. Although the latter has not been widely publicized before, Wang says this "third-generation" software is unique in that, unlike what he calls "second-generation" approaches, it is not based on empirical studies of simple test molds, which could be limited in their applicability to complex tools. Rather, C-Pack/W operates solely from fundamental material properties and stress/strain relationships. It takes into account the compressibility of the melt, geometrical constraints of the mold cavity, the varying pressure conditions during the pack/hold phase, and the asymmetric cooling conditions at different points on the core and cavity sides of the mold (from the integrated C-Cool analysis). The packing-phase analysis follows on the filling and cooling analyses, with the results of each step automatically feeding into the next. AC Technology calls this a "unified filling/post-filling/residual-stress analysis." C-Pack utilizes pvT data from the materials database in order to calculate local part density from pressure and temperature during packing. The upcoming release 4.0 will add a calculation of crystallinity distribution throughout the part--and through the part thickness--as a further refinement to shrinkage accuracy.

With these features, C-Warp and C-Pack/W can automatically isolate the dominant cause of shrinkage and warpage in a part. The software distinguishes the relative contribution of three separate mechanisms: uneven cooling, orientation (of fibers or polymer molecules), and nonuniform shrinkage (which can arise from such factors as nonuniform part thickness, crystallinity or density). Knowing the relative contributions of these mechanisms to a warpage problem could help in evaluating the likelihood of solving it through changes in gate size or location, wall thickness, cooling layout, or hold pressure and time.

Medina says AC Technology's goal is to "make warpage analysis as commonplace as flow analysis," by making it relatively fast and easy. With release 3.2, a user can initiate with a single keystroke an automatic sequence of filling, cooling, packing, and warpage analyses of a single model. There's also a new ability to perform a fast, "streamlined" shrinkage and warpage analysis at a preliminary stage, even before you know where the cooling lines will be placed in a mold. Bypassing the normal full 3-D cooling analysis, this "isothermal" shrink/warp prediction assumes "ideal" cooling--i.e., perfectly uniform heat transfer at all points on both mold faces.

C-Warp helps make things easy by automatically defining the statically determinate constraints needed to predict deformation. That is, it finds three nodes on the part's finite-element mesh that are farthest apart and applies the appropriate constraints on a total of six degrees of freedom of movement. No longer must the user do this manually. "No general-purpose structural-analysis program does this," says Medina. "Now, if you can do flow and cooling analysis, you can do warpage, too. You don't have to be an FEA expert."

Medina is quite pleased with the results of an independent benchmark verification study of C-Mold shrinkage and warpage predictions, performed last year by GE Plastics in Pittsfield, Mass. The part was a fairly complex photocopier base, measuring about 12 x 12 x 0.1 in. Nine experiments were performed with each of two materials (amorphous PPE alloy and crystalline PBT), varying the selection of 11 available gate locations, melt and mold temperature, max. injection pressure, and holding pressure. According to GE's report, "It was found that the C-Mold simulation provides better results than the published information |i.e., datasheet shrinkage values~ and adequately predicts the effect of process or design changes on the molded part dimensions." Figure 1 summarizes the results.

As noted above, further enhancements to shrink/warp analysis are planned for release 4.0 later this year. The current 3.2 version can handle relatively small warpage deformations, which Medina says should accommodate 75-80% of market needs. The 4.0 version will be able to predict both large and small deformations.

GAS-INJECTION SIMULATION

The C-Gasflow module, first introduced 18 months ago, incorporates several new features in release 3.2. Now, for example, it can model three variants of the process:

* Gas-pressure control, with stepped pressure profile and programmable gas-injection delay time after the start of filling.

* Automatically ramped gas-pressure profiling to ensure a consistent speed of melt-front advancement (otherwise, it tends to accelerate dramatically as the gas "explodes" into the cavity).

* Gas-volume (rather than pressure) control, with programmable delay time--i.e., the Cinpres process.

There's also an enhanced packing-phase analysis added to the gas-injection simulation. According to Medina, gas continues to penetrate into the part during post-filling, owing to shrinkage of the solidifying melt. Medina says Japanese molders want to be able to compensate for shrinkage during the holding phase by allowing additional gas, rather than melt, to enter the cavity.

Other features of C-Gasflow are time-stepped displays of the advancement of gas and melt fronts; predictions of skin-thickness distribution, required clamp force, and percentage of fill; and identification of problems such as gas-blow-through or penetration into thin wall sections.

For release 4.0, AC Technology is working on prediction of sinks (regular C-Pack software already includes a contour-map display of a "Sink-Mark Index") and plans to enhance C-Warp to predict warpage of a partially hollow part.

WHAT'S AHEAD

Release 4.0 early next year will boast as many new features as does 3.2. Among these will be automatic runner balancing, a correction for pressure losses in subgates, and enhancement of C-Cool to accommodate mold inserts of specially heat-conductive metals. AC Technology also hopes to include a new tooling materials database (to facilitate accurate heat-transfer calculations), and perhaps even a chiller and mold-temperature controller database. The latter could be used to ensure that the cooling conditions in the model (including coolant temperature and flow rate) do not exceed the capabilities of the machine--and conversely, it will help you to pick a machine with sufficient capacity. This would be analogous to the current injection molding machine database (including over 2400 models as well as generic maximum specs), which warns the user if the simulated conditions would exceed the max. injection speed or pressure, or clamp tonnage, and advises on the choice of the correct press for the job.

One of the most important new developments will be incorporation of crystallization kinetics into C-Flow and C-Pack 4.0. At last year's SPE ANTEC meeting in Detroit, a paper co-authored by AC Technology and Himont Inc., Wilmington, Del., compared mold-filling simulations of standard and nucleated Himont PP resins, with and without consideration of crystallization effects. From a graph of the results, the authors concluded, "It is obvious that the filling pressure is significantly underestimated if the crystallization effect is not incorporated into the flow simulation for PP." The 4.0 software will separately calculate crystallinity of multiple layers through the wall of the part at different locations in the mold and at different times during the cycle. Such data could prove valuable in estimating not only processability, but also finished part properties such as heat resistance, ESCR, and barrier properties.

Perhaps equally significant will be the introduction of fiber-orientation simulation to C-Mold 4.0. It will identify the different fiber-alignment patterns in the skin and core of the part, the relative proportion of skin and core thickness, average orientation distribution, orientation across the thickness, and how these patterns affect part thermomechanical properties and warpage.

Release 4.0 will also answer users' requests for more rule-based "artificial-intelligence" assistance in evaluating and troubleshooting the results of a mold analysis. C-Mold already incorporates a number of "AI"-type checks for violations of common-sense rules, such as placing a water line across a parting line or failing to connect all cooling circuits. Release 4.0 will add software that automatically searches the simulation results for any localized melt temperature, shear stress or shear rate that exceeds the manufacturer's recommended maximum for that resin (contained in the materials database). Such "intelligent interpretation" assistance will help ensure that conditions that could compromise part quality won't be overlooked.

AC Technology also plans a couple of quick-and-simple software modules for preliminary calculations of cycle time and process windows (temperature, pressure and fill time) before conducting a full-scale flow analysis.