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Plastic Injection Molding Process Simulation

Remember Saturday Night Fever, Three Times a Lady and the Rockford Files? Then you remember the year when computer simulation of injection molding process went commercial.

So this has been a very interesting topic but I find it hard to believe we are still having it, a third of a century on. And please, there is no need to keep on saying that injection molding simulation does not replace knowledge and experience of the process because simulation never claimed to do that.Plastic Injection Molding Process I know that for a fact because – and declaring an interest here – I was an old-school ex mold-designing ex mold-commissioning molders' technical manager when I introduced Moldflow to Europe in 1981. I did not think the software devalued my hard-won expertise. I thought it enhanced it and we marketed Moldflow on that basis. I have been long gone from the company now but I don't think that principle has ever changed.

By the way, I take it that the questioner was using the name Moldflow in a generic sense in much the same way as people use Hoover to mean a vacuum cleaner. Apart from Moldflow the main injection molding process simulation players are Moldex3D, Simpoe, Sigma, Transvalor and Cadmold. Vero International's CAD/CAM product includes VISI Flow simulation. This has been developed from Giorgio Bertacchi's TM Concept software which, together with Moldflow, pioneered the technology in the late 70s and early 80s. Many other CAD/CAM products feature integrated injection molding simulation modules and these are generally provided by some of the players already mentioned.

The propositions behind plastic injection molding process simulation are very basic. First, that although our plastic materials and processes are very complex they are not special. They obey the same laws of physics as everything else and that means that prediction is possible.

Second, that expert calculation is better than expert estimate. If plastic injection molding had been simple, we would all have been calculating the process since plastic injection molding began. But it was too complex for pencil and paper, slide rule and log table so it had to wait for affordable computing to come along. That is the sole reason why injection molding process simulation by computer seems controversial while rule-of-thumb calculations for locking force seem natural.

Third, that because the materials, processes and geometries are so complex, the computer model must be imperfect but this does not mean it is useless. A law of diminishing returns ensures that beyond an economic point, further gains in model accuracy bring only slight improvements in results. The threshold moves as calculation techniques develop and the cost of computing power drops but the principle remains. Current injection molding simulation models are probably close to being as good as they need to be. With them, a plastic injection molding expert can get closer to right-first-time than without them and that's where the benefit lies. Incidentally, when we use a rule of thumb, we are using a very simplified process model. It is a lot further from reality than any computer simulation out there.

I stress again the value of experience but it can only take you so far. There's not much future now in being a me-too business. The value lies in difficult differentiating products, in things you have not done before, in things you are not sure whether you can do. Things you probably have to achieve without the time or the budget for experiments. Things where the product economics don't allow over-design as an insurance policy. You need the tools for the job and the more experienced you are, the better you will use them.

The market being not just injection molders but also OEMs, product designers, plastic materials suppliers, mold designers and mold makers. I have never been able to come up with a reliable figure but I fear it is disappointingly low; perhaps 10 or 12 percent. I think the market should see simulation as core business but then I would say that wouldn't I?
Most large companies I work with will not release tooling without mold flow simulation as part of a detailed DFM (Design For Manufacturing) process. The CEO of one of them started driving stronger cooperation between the simulation team, tooling engineers and the injection molding processing group. It's interesting how you can build experience through numerous mold flow simulations, not dissimilar to other types of numerical simulations or life experience on the floor, but flow and thermal problems can get very complex. Simulation is never 100% waterproof without experience loop back, for many companies who have closed this loop simulation has become a very powerful design tool.
- - - -> by: Paul
I am a plastics product design and troubleshooting consultant who started in the 80's with filling simulation back when we did shell analysis with meshes rarely over 5000 elements. I authored a monthly "filling analyst" series article in a magazine, have conducted training classes, and have ran simulations for well over 800 projects. Many companies today have some type of filling simulation program but few use these programs correctly. I know this because I get a good deal of my troubleshooting work from projects which have had analysis work done incorrectly. You need the right software (and good material data) but more important is a deep knowledge of tooling, materials, product design and processing. A good analyst develops the skills to make "surgical" decisions which have the greatest impact to the quality of the part with the least cost impact to the plastic injection mold tooling. Very few people can do this correctly.
- - - -> by: Nannis

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