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A Basic Custom Plastic Injection Molding Machine ProcessThe Machine Controller
Machine Controls This area or component of the custom injection molding process can become quite expansive. There have been so many iterations of machine controls over the last 25 years, that it’s difficult to know where to start. Due to the fact that there are so many machines still in use today from this same time span, I will only touch on the basics of the main changes in order to keep this flowing and on topic as much as I can. The process of injection molding in it’s basic sense has remained relatively unchanged since the beginning. Molten plastic is injected under high pressure into a mold, creating a non-Newtonian flow condition, which is cooled, and then removed from the mold. There are 100’s of control mechanisms to create these conditions but they all end with the same purpose in mind and that is to produce a plastic injection molded part.
In the beginning, there were relays! Actually there were other things before that too, but we will start there. Relays, limit switches, relays, and more limit switches, were the flavor 25 to 30 years ago and I’m not referring to the newer solid state types, but the very large and now impractical mechanical relays used in many molding machines for years. As with now, these machines were provided to you with a hydraulic and electrical schematic or “ladder diagram” showing line after line of relays, some open, some closed, that all together made a required action occur. In many ways, I felt troubleshooting these machines were much simpler in a lot ways than with today’s solid state control systems. If your clamp wouldn’t close, you just looked for that line in the ladder logic, started at the end of that circuit which usually was a solenoid on a valve, activated the clamp close button and checked for voltage at that point. If there was none present at the solenoid, you back tracked to the next point along the circuit path which might be a junction block. No voltage there and back you go again until you found the first point where voltage was present, but not getting through to the rest of the circuit. That point often would be a relay or relay contact that had failed. Replace or re-build the relay and off you went again.
Now that is a simplistic approach to circuits of old, but they worked. There weren’t any touch screens, or LED read-outs, or circuit boards. Everything today is now touch screens, electronic control modules, programmable controllers, and the like. Trouble shooting is now more often done to the module level and then replaced if it’s not functioning properly with another. I would say that today’s machine controllers are much more reliable than those of old, but they are also much more expensive to maintain. Instead of replacing or rebuilding a burnt relay that cost a few bucks, you will likely have to replace a $2000 module. This is one of the costs of progress.
In a nutshell, the controller is the brain of the injection molding machine. It controls all of the functions of the injection molding machine and insures that the machine does what it is programmed to do. In the case of the newest machines, they are comprised of many sensors, such linear measurement devices, thermocouples, proximity sensors, speed sensors and many more. Think of these sensors as the nerves of your own body, or your ears, eyes, nose, and tongue. You react to what your senses tell you, just as with the machine. The controller allows you to input certain set points into it, which tells the machine what to do during the injection molding process. The sensors provide feedback to the machine controller so that it knows what’s going on and when, and what to do next based on a program that tells the controller what to do with the information that it receives. These set points that you have control over will be discussed more in a later module on process setting and controls.
Written by: WM8C, July 28th, 2006. Not for use without written permission
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