Closing the loop

The typical Canadian injection molder has a mix of older and newer machines, many of which have differing makes, styles or generations of machine control systems. In most cases these operations could greatly improve either process control or operational uniformity and efficiency by upgrading some or all of their machine control systems.

One testimonial to such benefits comes from a U.S. molder of electronic parts which cut its scrap rate by 50 percent after upgrading older machines to closed-loop controllers. The molder, Moldmatic, Inc., located in Pendel, PA, specializes in the manufacture of printer cartridge cases, connectors and other electronic parts. It installed Trailblazer (System 10T) closed-loop controllers from Solid Controls, Inc. on several older, 75-ton Van Dorn presses. At the same time, some of the machine’s hydraulics systems were rebuilt.

After the upgrade to closed-loop control operators saw immediate gains in the machine’s ability to monitor and control basic parameters of time, distance, speed and pressure. Screw travel was maintained within hundredths of an inch, a many-fold improvement over the control level achieved with open-loop. The improved repeatability reduced the frequency of overpacking, resulting in flash, and short shots, which leads to scrap and can damage the mold. Clamping repeatability was also improved, making it possible to set low pressure closings more accurately and further reducing the risk of damage to the mold. The closed-loop system also provided more accurate control of multi-zone barrel temperatures and reduced set-up time.

The Trailblazer is designed to retrofit to used injection molding machines with up to 1000 tons of clamping force. The system replaces all existing controls, including relays, timers, counters, function switches and pressure and temperature controls. The new controls are said to be simpler than older solid state controllers, and are interfaced to a user-friendly touch-screen.

Demand driving move to smaller controllers

The Barber-Colman/Eurotherm MACO DS series of new PC-based operator stations feature user-friendly configurations of displays and distributed architecture. They are designed to supplement the MACO 4000, 5000 and 6000 systems, which in turn incorporate the Impact closed-loop process control. The system’s auto-tuning feature automatically and continuously tunes the process, guaranteeing shot-to-shot repeatability. Other highlights include a ten-step injection, pack and hold, and recovery profiles, a 2.5 millisecond closed-loop injection control and a 10.4 in. active color matrix display. Additionally, the control package comes with full SPC capabilities.

Multi-zone temperature control, screw position, clamp speed and movement and other factors can be controlled with Omron’s series of retro-fittable programmable logic controllers (PLCs). Larger machines use PC or PLC control via a touch screen with a multi-loop controller, such as Omron’s E5ZE. Omron’s award-winning CS1 Series of PLC controllers is designed to meet greater industry demand for more compact devices without loss of programming power. The CS1 offers improvements in processing speed, information management, networking and communications abilities. Omron’s SRM1 is a fully functional PLC that fits in a shirt pocket. The SRM1 is capable of 256 networked inputs/outputs that can be distributed among multiple sites on a machine or machines.

The Simatic series of PLCs from Siemens are compact and provide a high level of programming flexibility. The S7-300 accommodates program changes and machine expansion with modular hardware and software which supports worldwide standards and open platforms.

Allen-Bradley’s Pro-Set 200 Injection Molding Control incorporates the second generation of the company’s patented Expert Response Compensation (ERC2) closed-loop control technology. It is an advanced control algorithm which provides automatic tuning of the pressure and velocity loops associated with clamp, injection and ejector motion. Any profile in the inject, clamp and ejector phases can be controlled by pressure or velocity to suit the process or molding machine. The controller lets the operator manipulate hydraulic pressure and/or velocity of the injection ram. For example, movement of the screw or ram can be divided into five or more segments with different injection speeds tailored to the characteristics of the mold or material. An optional barrel temperature module controls the thermal zones characteristic of the material. As well, a patented cold-startup routine is designed to bring machine temperature zones up to setpoint faster than traditional methods with less overshoot or temperature oscillation.

The Hettinga Advanced-Level Control (HALC) is billed as a superior alternative to PLCs and PC/PLC combinations. HALC is an all-PC based control system capable of controlling all primary and auxiliary equipment. The system provides real-time control over molding parameters and facilitates easier programming by eliminating mapping and ladder logic often used in PLC-based factory automation. Instead, programming is carried out using an intuitive, script-based language in a Windows NT format. Instructions and information on the flat-panel display can be viewed in 18 languages.

Payback on any of these control systems depends of course on the machine, as well as the materials, kinds and numbers of parts run on it. Implementation of closed-loop control, in whatever configuration, is often accompanied by upgrades to the machine’s hydraulics, cylinders, bushings, toggles and other components. Nonetheless, a significant reduction in scrap, cycle time, downtime and set-up can translate to a very quick recovery of this investment.