Modular controls improve performance, lower costs for injection molding machine
Taylor's Industrial Services, HPM-Sandretto Division, of Mt. Gilead, OH has historically been one of the largest injection molding machine suppliers in the US. Its older machinery lines needed improved capabilities to compete more aggressively in today's marketplace. The company decided it needed to inject new control technology into its product line.
Taylor's Industrial Services, HPM-Sandretto Division, of Mt. Gilead, OH has historically been one of the largest injection molding machine suppliers in the US. Its older machinery lines needed improved capabilities to compete more aggressively in today's marketplace. The company decided it needed to inject new control technology into its product line. Thus, the new Freedom Series of injection molding machines was introduced.
The company's goal for the new machines was to offer higher throughput, improved shot control and wanted to facilitate easy machine repairs and upgrades. HPM also wanted to use the same system architecture that had proven successful on its die casting machine line.
The new control system (Fig. 1) is based on an open system design. Modular system components from different vendors are plugged together via a standard communications interconnect, in this case PROFIbus. Incorporating an open system architecture enables system designers to choose best-in-class components without being locked%%MDASSML%%in to the products of any one vendor.
The new control system distributes critical control functions among different system elements rather than handling all control functions through a master central controller. Open systems with distributed control elements offer higher performance because each control element can be specialized for the task it performs. In the HPM%%MDASSML%%Sandretto system, hydraulic motion control is provided by a programmable electro-hydraulic motion controller that takes high-level commands from an industrial PC and independently runs programmed motion profiles while the PC performs other tasks.
The industrial PC runs a multitasking operating system with soft PLC software to perform system control functions, and additional software to implement the HMI. Many new industrial systems are using general purpose computers running special industrial application software such as this because the systems are more flexible and expandable than dedicated-function PLCs.
Some older hydraulic motion control systems control via cylinder position sensing only, and use valving with only “open” and “closed” positions. This can result in imprecise controls, lower quality output and the need for frequent system maintenance. On/off controls should be avoided if precise operation is required.
Select the right controller
When choosing a controller for precise hydraulic motion in molding press applications, select a device that can seamlessly transition from hydraulic motion control based on actuator position information to control based on pressure, without stopping the motion. The controller should be able to drive proportional hydraulic servo valves, adding or subtracting pressure in minute amounts.
Because such a controller can produce smooth operation, hydraulic pressure transients and pressure overshoot are reduced, improving part quality by eliminating flash. The smooth injection also results in even density within the molded parts, which contributes to their strength. Also, less shock and vibration reduces the potential for hydraulic leaks and extends machine life.
Faster loop time improves shot control of the new injection molding machines. Distributing hydraulic control using a dedicated motion control module is the key to closing control loops more quickly. If an industrial PC or a conventional PLC was used, loop times could be 10 to 20 milliseconds. The best new electro-hydraulic motion controllers can have a 1-millisecond loop time. The result is more responsive hydraulic control and what HPM believes to be the tightest shot control in the industry.
With the installation of the new control system, HPM-Sandretto feels that the electronics is no longer the limiting factor to machine performance.
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