Process simulation gathers momentum in blow molding in thermoforming.

After a slow start, computer process simulation has attracted a handful of new suppliers and is building a critical mass of high-profile users.

Radically new technology takes time to sink in. Initial acceptance may be slow, but at some point, interest starts to snowball. That appears to be what's happening with software for simulating the stretching and thinning of blow molded parisons and thermoformed sheet. Pioneering work by GE Plastics in Pittsfield, Mass., dates back to 1986. Commercial PC products have been available since 1992, but for a while they seemed to make little impact on the market.

Until recently, there were two or three suppliers of this software and just a handful of users. Software cost and technical sophistication apparently were obstacles to processor acceptance. Demand was so slow that both the first two commercial pioneers in this field - Polydynamics and C-Mold - pulled back from active marketing and technical development.

But it looks like simulation is beginning to gather steam - faster in blow molding than in thermoforming. Today, there are at least five vendors offering faster, more capable, and more user-friendly software packages. And approximately 140 firms are commercially using or evaluating simulation packages to predict wall thickness, cooling times, and shrinkage of products from bottles to bumpers.

The list of high-profile users includes resin suppliers such as Dow, Eastman, Exxon, GE Plastics, Montell, and Nova Chemicals. Toy manufacturers Fisher-Price and Mattel are also in it, as are bottle makers Schmalbach-Lubeca and Graham Packaging. Consumer-products giant Procter & Gamble, aircraft supplier Boeing, and automotive companies such as Visteon Div. of Ford Motor Co. are getting involved. Even mold builders like Husky are interested.

Why use simulation?

Most blow molders and thermoformers still rely on educated guesswork and trial and error to determine the right process conditions, which wastes time and material. "Simulation gives you guidelines for designing tooling and process conditions, and helps save you the expense of recutting a tool," says one consultant.

The data can help you optimize a mold design, parison program, or sheet temperature profile in as little as 30 minutes to a few days, compared with a week or more when no simulation is employed. "Process simulation helps us reduce debug time by 40% when we are doing mold sampling," says Curt Randall, plastics and tooling engineer at Fisher-Price Inc., East Aurora, N.Y. For molding large toys like tricycles, simulation helped determine head tooling size, die shaping, parison programs, and product wall thickness. The resulting toy was about 90% accurate in hitting target wall thicknesses, Randall reports.

"Simulation is used on every blow molding product we make," says Dr. Mohammad Usman, supervisor of CAD/CAM/CAE for the Exterior Systems Div. at Visteon in Milan, Mich. Simulation helps Visteon eliminate expensive prototype molds for bumper systems and multi-layer fuel tanks, and it highlights the need for changes earlier in the development process.

According to Tony Gennari, CAE engineer at GE Plastics' Automotive Group in Southfield, Mich., "It's imperative that we use blow molding process simulation. We use it to predict wall-thickness distribution, and that helps predict overall product performance."

Still, some sources warn that simulation will not give you absolute answers. It is just a tool that helps you identify process trends, and it may not fit every product. Paul Bristow, GE program leader for thermoforming, says processors using only one or two materials that they know well will probably continue to rely on experience rather than a computer. He sees the most opportune uses for process simulation in tight-tolerance applications like truck and car body panels, or where new materials are used, not in straightforward jobs like bathtubs.

Software suppliers say sales and licensing activity is on the rise. "We are seeing a growth rate in the use of our product of about 20-25%, and I think this will continue to be a trend in the future," says Thierry Marchal, product manager at Polyflow, a Belgian-based subsidiary of Fluent Inc.

Besides the growing number of commercial users, suppliers have granted at least 80 educational or noncommercial licenses as well. A consortium of 11 thermoforming companies was recently assembled by the Industrial Materials Institute of the Canadian National Research Council (IMI-CNRC) to develop new simulation software.

The University of Massachusetts at Amherst is conducting an 18-month project to measure process variance in simulated part trials. After the tests end in December, the university will offer a suite of simulation software tools that can be downloaded from its World Wide Web site, says David Kazmer, assistant professor of mechanical and industry engineering. "The goal is to predict part tolerances to within 0.2% of normal dimensions," he says.

Expanding range of software

At least two additional firms entered the simulation market this year, while two previous entrants plan to debut new blow molding software at the K'98 show in Germany this month. An upgrade to an existing package is in the works by another firm.

Higher speed is one hallmark of the new and upgraded simulation programs. "Early finite-element analysis of wall thickness took hours to run when you wanted to change plug dimensions or another parameter," recalls consultant James Throne, president of Sherwood Technologies. "Now, by the time you hit the button you can get a result."

Blow molding and thermoforming simulation are similar in that they model stretching of a sheet or tube until it contacts a cold mold. Nearly all simulation packages require a CAD model of the part. The model can be imported into the simulation software or can be drawn from scratch in the simulation package. The next step is to generate a finite-element mesh on the model. Then the program needs a model of the material behavior of the resin you will use. A database of common resins comes with some simulation packages, but you can also go to a commercial test lab to generate the data.

Thermoforming packages generally can model the effects of plug assist. Blow molding packages may offer a choice of extrusion, injection-blow, or stretch-blow simulations. Packages may provide "freeze-frame" images of the process at any selected point in time, and/or may animate the process sequence. Newer packages may include viscoelastic models of plastic behavior, which take into account plastic flow as well as purely elastic stretching of the melt. This can be valuable for simulating longer cycles with larger parts. It also accounts for phenomena such as sheet sag, elastic yield, and strain hardening.

Once the simulation is complete, the results can be exported to a FEA structural-analysis program to produce shrinkage estimates.

Software providers say a 3-D simulation package is the way to go. However Fluent and C-Mold also offer 2-D simulation for use when the model can be simplified. The 2-D simulation is less rich in detail, but it can be generated faster.

Software prices depend on whether they are designed for a Unix workstation or a Windows-based PC. Prices range from approximately $1900 to $45,000, depending on type of system and whether a structural-analysis package is added.

These are the current suppliers of process-simulation software and a brief sketch of their capabilities:

* IMI-CNRC this year released through its sales agent JAR Associates the BloView 3-D package for extrusion or injection stretch-blow molding. Developed through a consortium of 27 companies, it incorporates a viscoelastic model. The package has an expandable database of 20 resins. It creates an animated sequence of the process, including inflation, cooling, and warpage. A special optimization feature can automatically feed simulation results back into the program to run additional iterations in order to predict the parison or preform thickness and process conditions that will yield the most uniform product.

The current workstation software starts at $15,000. A PC version is in development. Upcoming enhancements will increase speed and add die-flow simulation to the viscoelastic models to calculate die swell. The new version will also be able to simulate multi-material coextrusion. A new simulation package for thermoforming is being developed for commercialization in 1999.

* Fluent Inc. markets Polyflow 3.6 for blow molding, a 3-D fluid-dynamic package capable of simulating multi-layer extrusion blow molding, including die flow and die swell. It also models injection and stretch-blow processes. The Unix software was supplemented by a PC version last year. Shrinkage calculations are planned to be introduced in a new 3.7 version next June. The current version costs $26,000 for an annual license, which provides technical support and use of an expandable materials library with more than 20 resins. There are 60 users.

* Compuplast markets T-SIM thermoforming simulation software developed by T-SIM cz in the Czech Republic. It runs on a Windows PC and includes a viscoelastic model and a database of common materials. A new predistortion feature shows where to position decal graphics on the sheet prior to forming. The package starts at $15,000. It has about 10 users. A brand-new package for extrusion blow and injection stretch-blow molding, called B-SIM, will debut at the K'98 show this month.

* Sherwood Technologies will introduce four new Java-based programs for thermoforming that analyze only sheet heating and cooling. The TF202 program relates material data on 34 polymers to heater emissivity, spacing, and dimensions in order to calculate heating rates and temperature profile through a sheet. TF505 addresses zoned top and bottom heating profiles to calculate local sheet temperature. TF309 calculates heating cycle times for very thin, transparent sheet. TF213 calculates heat transfer to the air during oven-to-mold transfer and subsequent cooling in the mold. These programs will cost $150 each or $500 for the set.

* C-Mold commercialized thermoforming and blow molding simulation based on GE's prototype software in 1994. It offers both 3-D and faster 2-D simulation for Unix workstations. It does not use a viscoelastic model. Features include a database of eight materials, an automatic meshing function, ability to simulate stretch-blowing, and an interface to Ansys structural-analysis software. Color-coded displays of thickness, temperature, and stretch-ratio distribution throughout the part can be "mapped back" to the original parison or sheet to help optimize parison programs or oven profiles. Price is around $45,000. There are 93 users, including 80 non-commercial ones.

* Polydynamics was the first on the market with its T-Formcad thermoforming simulation in 1992 and has sold four licenses worldwide. The 3-D package (version 5.0) for Windows 95 PCs models only sheet stretching, not cooling. It has no viscoelastic model and no materials database. An annual license costs $1900. It has 20-30 users.

* BASF Corp. has a proprietary 3-D finite-element simulation package for industrial blow molding. Not offered for sale, Moldblow simulates parison stretching in order to define an optimum parison program for complex parts like fuel tanks. The program can model extrusion, injection, and stretch-blowing, as well as parison prestretching and preblowing. It can detect defects such as crease formation.