Novel control options for injection molding machines.

Two new and unusual control options for injection molding machines from Cincinnati Milacron Plastics Machinery Div., Batavia, Ohio, are said to be useful tools to increase machine repeatability and help reduce scrap during startup. Few details were available when these products--an on-line viscometer and a materials database--were introduced at NPE last June (see PT, July '91, p. 11). This article provides more details on the products, which have since gained some exposure in the field.

The on-line viscosity monitoring and control device directly measures a material's viscosity in the machine nozzle. Monitoring viscosity, the ratio of shear stress to shear rate, is a dependable indicator of changes in either process conditions or the material itself.

A key value of the new viscometer, offered as an option on Milacron's high-end CAMAX XTL control, is that it provides insight into the effects of viscosity on the process that may not be readily evident. It also supplies an early indication of molding conditions gone awry or of changes in material composition, and is tied into the machine controller to make long-term drift adjustments to compensate for changes occurring in the process.

HOW IT WORKS

The viscometer consists of an instrumented nozzle that acts as a capillary rheometer. The extended nozzle contains a channel or capillary, and shear stress is calculated from the pressure drop across the nozzle as measured by a pressure transducer at either end. The shear rate is calculated from injection speed, using the input from the injection position transducer. The control then calculates apparent viscosity at the capillary wall, which is compared to a setpoint viscosity. Output from the viscosity control is cascaded to temperature controllers in the first two zones of the barrel, which alter the barrel temperature to maintain a consistent viscosity. The viscometer is available with an optional infrared melt-temperature probe, which will display current melt temperature on the screen.

The viscometer is said to provide a more accurate measure of flowability than does common melt-index testing. Melt index is a measure of a material's viscosity at a single, very low shear rate, which does not reflect actual molding conditions. The large difference between lab testing and real molding conditions can be misleading if used to make moldability comparisons between materials.

In contrast, the viscometer provides a realistic value of a material's viscosity under actual process conditions. The viscometer demonstrates that changes in material characteristics such as moisture content and bulk density, or in process conditions like mold temperature, have a noticeable effect on flow. Interestingly, changes in viscosity correspond to changes in fill time and transfer pressure, key indicators of the stability of a process. But the viscometer is held to be an even better gauge because it measures the viscosity directly. Viscosity can be selected as an SPC parameter and used to anticipate problems before they generate scrap.

Milacron marketing v.p. Bruce F. Kozak notes the value to processors of a means of performing incoming resin q-c that is not only more accurate than melt index, but can also be performed "on-line" at the molding press, without sending a sample to the lab first. Kozak adds that the nozzle viscometer gives molders concrete evidence of resin variability that they can show their resin supplier even when melt-index data are on spec.

EDUCATIONAL TOOL

One practical value of the viscometer, according to process development supervisor Mark Elsass, is as a support tool. "It's a monitor to show what is going on in the machine, to show how consistent things are, and an educational tool, to show what viscosity is all about." So far, Milacron has supplied about a half dozen of the devices to captive molders of electrical components. Using the viscometer in its own lab, Milacron has been able to demonstrate to customers why two batches of the same material may behave differently during molding. The viscometer option adds around $10,000 to the cost of the machine and XTL control.

MATERIALS DATABASE 

In a separate development, a materials database currently comprising 36 resins commonly used in injection molding is being offered as a new standard feature of the CAMAX XTL control. The database presents material data such as heat capacity, thermal conductivity, and shrinkage rate, as well as useful processing information, like barrel-zone temperatures, drying time and temperature, and the preferred screw tip to be used. When the molder enters a part weight, a shot stroke is automatically calculated from the resin density and machine's screw diameter. The operator then has the option of accepting the calculated shot stroke and recommended barrel temperatures or altering them. On the operator's command, the control will automatically implement the stroke and temperature values.

The resin database may be viewed as a first step toward an artificially intelligent auto-setup machine. But for now it's being marketed as a "help" tool for the machine operator, making setup information available on the shop floor if certain data are missing from the setup sheet or a question arises as to their accuracy. The user can add new materials to the database and include processing notes.