'Real-world' melt-elasticity tester.

An instrument that measures the elastic properties of plastic melts at realistic processing conditions was introduced late last year by the Maxwell Instrument Co., Inc. of Cranbury, N.J. This new company was formed by Bryce Maxwell, professor emeritus at Princeton University (see PT, Oct. '94, p. 12). The Maxwell Elasticity Tester has been designed to complement familiar melt-index measurements as a standard procedure in quality control.

According to Maxwell, processors in extrusion, injection, and blow molding should monitor not only resins' melt flow, but also the melt elasticity or elastic index for quality control. For example, variations in melt elasticity cause changes in die swell in extrusion and blow molding, leading to variability in product dimensions or weight.

In injection molding, melt elasticity directly affects mold filling. The injection force must overcome the melt's resistance to both viscous flow and elastic deformation, explains Maxwell. Melt-elasticity testing is thus an important procedure for resin quality control because it gives insight into real processing behavior.

HOW IT WORKS

Use of the Maxwell Elasticity Tester is said to be simple. Total time for performing a test - inserting a cold sample, melting, shearing, elastic recovery, and recording data - can be less than two minutes. Here's how it works:

* About one tenth of a grain of resin in pellet or any other convenient form is placed on a flat plate heated to the test temperature.

* A second disk-shaped plate is placed on top of the polymer and pressed downward. As the polymer flows, it forms a disk-like specimen of controlled thickness between the two plates.

* A drive motor rotates the upper disk counterclockwise, shearing the specimen with respect to the lower plate.

* After a specified amount of deformation has been applied, the upper disk is released from the drive and it then rotates in a clockwise direction, propelled by the elastic recovery of the specimen.

* The time-dependent elastic-recovery rotation of the upper disk is recorded by a video camera focused on a pointer attached to the disk and to a stationary scale. Elastic recovery is usually recorded for one minute.

* The video data record is displayed on a monitor. The amount of elasticity is represented by the complete curve of clockwise disk rotation. If a numerical specification of a melt-elasticity index is desired, specific points may be used, such as the "total recovery" in one minute or an amount of recovery in some shorter time (i.e., 10 sec).

* Cleaning out the used specimen can be done in less than a minute by removing the disk and lower plate, scraping off the melt specimen, and wiping with a soft cloth.

HOW IT COMPETES

A complete Maxwell instrument - including temperature-control system, video camera, video monitor, and auxiliary equipment - sells for under $10,000. The unit is intended to compete with rheological instruments costing $50,000 to $250,000. Such instruments for measuring elastic properties of melts include mechanical spectrometers and controlled-stress rheometers. "These can measure the elastic modulus of a melt but not its elastic recovery," explains Maxwell. And in the case of controlled-stress rheometers, the test may take hours. "The results do not relate to plastics processing, which takes place in a very short time," comments Maxwell.

Die-swell measurements, another test of melt elasticity, involve extruding through a die vertically downward and measuring the diameter of the extrudate using a laser optical sensor. 'This method [die swell] is about the best we have had up until now," says Maxwell. However, he notes that several questions associated with this method - such as where to measure the specimen and how to overcome curving, waving, stretching and cooling of the extrudate - are eliminated with the Maxwell instrument.