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Melt thermal conductivity data become more readily available.

Melt Thermal Conductivity Data Become More Readily Available

Melt thermal conductivity data, vital to computerized flow and cooling analysis programs, are becoming more widely available, as is equipment better suited to measuring this heretofore elusive property.

GE Plastics, Pittsfield, Mass., recently developed a standard table of thermal conductivity for its product line. "We measure the ability of a material to conduct heat from 50 C (122 F) to 300 C (572 F)," says Lorene Erb Baccaro, a material science specialist in GE's marketing technology department. "We enter the data into our engineering design database. It is then available to our customers, and to our application engineers and others in the company who want to use it for heat-transfer calculations. We use the data in mold filling analysis to determine how quickly a material fills a mold or tool."

Thermal-conductivity data are also included in the Polyfacts database of Du Co., Wilmington, Del.; and Dow Chemical Co., Midland, Mich., is developing an on-line database to supply this and other information to users of its resins.

Accurate measurements of thermal conductivity is vital to computerized predictions of flow and cooling because it influences the rate of melt cooling and solidification in injection and blow molding tools and downstream of the die in extrusion. "Once engineers have data on specific heat and thermal conductivity, they can easily determine cycle times or extrusion rates," Baccaro says. As she indicates, thermal conductivity is only part of what's needed to characterize a material for mold-filling analysis; engineers must also know other parameters such as specific heat, viscosity and melt density - which are also included in the GE database.

GETTING THE RIGHT DATA

Engineers often use a single number for thermal conductivity, usually reported at room temperature, and then apply this number to the entire temperature range addressed by their computer programs. However, most users of these programs fail to take into account that conductivity is not constant and can change as much as 1% for each 18[degrees]F change in temperature. Thermal conductivity of a melt is thus quite different from the conductivity of the same plastic when solid.

The ability to measure thermal conductivity in the melt state is relatively new, GE says. Before today's advanced measurement methods, thermal conductivity was occasionally derived from thermal diffusivity measurements made under conditions of transient heat flow. This indirect method of obtaining thermal conductivity is complicated, requires expensive equipment, and is less accurate than steady-state methods over the typical temperature range of polymers.

Because of this, little diffusivity data are available on molten polymers. The existing data, says Karl Coumou, vice president of Holometrix, Inc., Cambridge, Mass., is often incorrect. Coumou, who designed the thermal conductivity analyzer used by GE Plastics, says this is especially true for data pertaining to the melt state of polymers.

The Holometrix Thermal Conductivity Analyzer (TCA) used by GE Plastics measures this parameter by the guarded-heat-flow method. Heat passing through a sample under carefully controlled conditions is measured with a sensitive transducer. It is then compared to data in the computer file that have been obtained under identical conditions on a sample of Du Pont Vespel polymide, a thermoset material of known thermal conductivity. Solid samples, 50 mm in diam. and up to 20 mm thick, are placed directly into the TCA for testing. No special instrumentation of the sample is necessary, Coumou says.

A special cell with fixed critical sample dimensions - 45-mm diam. and 4-10 mm thick - is used to test polymer melts. Because polymeric materials expand as they melt, often releasing entrapped gases, any excess molten material is expelled from the sample area into a disposable part of the cell.

The testing process is simple, reliable and completely automated, Holometrix says. It requires minimal operator attendance, since the TCA is controlled by a built-in microprocessor with menu-driven software that guides the operator through the initial setup. All the operator must do is chart the lowest the highest sample temperatures between ambient and 300 C (572 F) and the number of data points in this range to be collected.

After the initial setup, the TCA scans and analyzes the test data at regular intervals and determines when temperatures and heat flows have stabilized. It computes, prints, and stores the results while advancing to the next set point. When the test is done, thermal conductivity is tabulated and plotted as a function of temperature. Test time between data points is about 1 hr.

The material used to calibrate the TCA is crucial to its operation, Coumou says. Well-known reference materials, such as Pyrex 7740 glass or Pyroceram 9606 ceramic, are not ideal because their thermal conductivities differ from those of polymers. Du Pont's Vespel provides a standard with an operating range close to that of many polymers. "The conductivity values we use are probably accurate to [+ or -] 5%," Coumou says.

PHOTO : Using Holometrix's Thermal Conductivity Analyzer, GE Plastics created a database of thermal conductivity for its resins that helps determine accurate cycle times and extrusion rates.
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Author:Monks, Richard
Publication:Plastics Technology
Date:Sep 1, 1990
Words:834
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