'Expert' process control for injection.

`Expert' Process Control for Injection

Joining a growing number of introductions of compact, modestly priced, flat-screen controllers, Hunkar Laboratories Inc., Cincinnati, has just come out with a closed-loop injection machine control that costs only $6900 complete with hydraulic servovalve. Called DAC-III, it's a process controller designed to work together with separate sequence-logic and temperature controls. While it does not include some relatively little-used features of more expensive CRT controllers, such as multi-step profiling of screw back-pressure, it does offer unique "expert" features designed to simplify setup, plus full SPC capabilities and production data reporting.

First among those expert features is a brand-new Automatic Injection Profile setup program. Addressing the lack of available guidelines for a molder to use when first setting up an injection speed profile, Hunkar performed a statistical correlation of profiles actually used on 15,000 machines in the field. "Approximately 90% of them follow the same basic pattern," says company president Denes Hunkar. So that pattern was programmed into the system as a starting point. The user need only enter the setpoint value for the first step, and the program will set the 39 other points of the profile accordingly. Hunkar recommends starting with a low initial injection speed, and then ramping the entire profile up by increasing that first speed setpoint gradually until a problem such as flashing or burning is experienced, in order to obtain the fastest cycle.

The profile can also be edited manually; the user may enter just 10 points, and the program will interpolate between them for a total of 40 steps, in order to provide smooth transitions.

'EXPERT' SPC FUNCTIONS

Auto-Learn is another function of DAC-III, which incorporates certain expert software elements of Hunkar's CIM-1 network controller. This feature avoids the necessity of performing time-consuming Taguchi-style "Design of Experiments" procedures in order to set meaningful statistical process control (SPC) limits on key process variables that affect product quality. Based on computer analysis of millions of production cycles performed on thousands of machines, Auto-Learn automatically weights key variables according to their quality influence. These preset weighting factors are combined with the DAC-III controller's observation of the current machine's "signature" of process variation, in order to set SPC "windows" of appropriate width for different variables. Thus, once a machine and mold are running good parts, the operator presses the LEARN button and continues to run for another 35-50 cycles, after which Auto-Learn will set upper and lower specification limits on 35 SPC parameters. Once a setup is "learned," the system will automatically alarm if the process deviates from those settings.

Other main features of DAC-III include these:

* SQC functions--Through a serial access port, DAC-III can accept direct input of electronic scale, gauge, or bar-code readings of product attributes. SPC data from each shot are held in buffer memory during the next machine

cycle, during which time quality data on parts from the last shot can be entered and will be matched with process data from the same cycle.

* Good/bad parts discrimination--Automatic SQC designates a shot as "bad," and can divert the parts to a separate receptacle if any designated "critical" SPC limits are exceeded.

* Programmable SPC functions--You can set upper and lower limits manually on any of the 35 SPC variables, and select which variables are critical for good/bad parts discrimination.

* Downtime and reject codes--The user can create his own descriptions for up to 999 downtime and reject reasons that the operator can select when needed.

* Production data--A production database contains 50 items that can create real-time order-status displays of total good and bad cycles and parts, parts made and still to be made, cycles/hr, parts/hr, hours to run, cumulative downtime, etc. Material and personnel tracking can also be accomplished by bar-code reading or manual entry of operator codes when checking in or out of a machine station.

* Other process-control functions--Standard features include Hunkar's automatic shot correction and cushion control to correct for melt-density variations stemming from raw-material or regrind variability. There's also servo-controlled, closed-loop back-pressure control, two-step pack/hold-pressure control, and boost cutoff based on ram position or hydraulic pressure. (For all-electric machines, melt pressure at the nozzle can be substituted for hydraulic pressure.) New software for automatic cavity decompression is said to prevent "ram bounce" at switchover by ramping smoothly from high to low pressure. Also, the system automatically logs any changes made to set-points.

DESIGNED FOR NETWORKING

DAC-III is designed with local-area networking in mind. It can perform as a node in a CIM-1 plantwide-monitoring system, or on its own if linked to a personal computer or laptop for data storage. Both production and process data are automatically formatted for display by commercial database programs on the PC or laptop. SPI communication protocol is available to integrate auxiliary equipment with the DAC-III as an interface for one-point setup and control. Auxiliary-equipment process conditions can be included among SPC variables monitored by DAC-III for good/bad cycle discrimination.

DAC-III supports the MODBUS/JBUS communication protocol as standard, which is compatible with many programmable logic controllers and temperature controls. (The latter include controls from Eurotherm Corp., Reston, Va., which has a joint technical program with Hunkar.) DAC-III can, for example, serve as the interface for up to 32 temperature controllers, sufficient for many hot-runner systems.

DAC-III is intended for both new machines and for retrofits. Existing Hunkar DAT machine monitoring terminals can be upgraded to DAC-III capability for $3500, including servovalve. Incidentally, Hunkar has just introduced a new model HEPICS servovalve after two years of testing. Unlike previous models, it requires no gain adjustment or tuning.

PHOTO : Hunkar Laboratories' DAC-III injection-machine controller can take direct gauge input of part-quality data and match these with process data from the same cycle for SPC/SQC reporting.