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How does it process? There's still no good, simple answer.

How Does It Process?

How often has something like the following happened to you? 1) A resin supplier's sales or marketing person tells you of a wondrous new material; 2) you ask, "What does it process like?" 3) only to be told, "Kiind of somewhere in between polypropylene and polycarbonate." In other words: Don't ask. Just put some in your machine and see for yourself.

Last month in this space, I made a modest proposal that domestic materials suppliers get together and standardize their testing so that they can offer truly comparable data on mechanical and thermal properties of plastics. In addition, I'd like to suggest that processors need and deserve a better set of objective benchmarks of how materials mold and extrude.

Up to now, you've generally had to settle for a data-sheet value of melt index (MI) or melt flow rate (MFR). As a single datum, it tells you a little bit--about as much as the screw diameter tells you about an extruder, or the clamp tonnage does about a molding machine. Some of you may ask for, and get, graphs of melt viscosity versus shear rate, which can be useful indicators to those who know how to use them. About the only other processing-related data we have today are a handful of esoteric calorimetric data used for computerized mold-filling analysis. (I wonder why they're not often openly available on data sheets--or when they will be.)

Many of you would probably prefer a more "real-world" flow test--such as the new squeeze-flow test that one molder developed for thermoset compounds, reported on page 17. WE already have such a real-world test for injection molding--spiral flow. Unfortunately, there are so many variables that it's not possible to compare data that were not produced in the same machine and mold. Call me naive, but please permit these questions from a nonexpert:

* Couldn't some clever engineer design a standard benchtop instrument to provide repeatable spiral-flow data . . . something with a hydraulic ram and sensors for pressure, time and temperature, which could simulate injection into a standard test cavity?

* If not, might it not be interesting to try simulating spiral-flow molding with flow-analysis software? From a few basic materials data, might it thus be possible to derive an objective index of injection pressure to fill a standard cavity, or of flow length at a given pressure?

I repeat, I'm no expert, but I think the experts agree that some fresh thinking on this thorny problem is desperately needed.
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Article Details
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Title Annotation:lack of comparable data
Author:Naitove, Matthew H.
Publication:Plastics Technology
Article Type:editorial
Date:Jan 1, 1990
Words:414
Next Article:LCM and SMC share top billing at SPI composites conference.
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