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Processing using an electric injection molding machine - part 3When it come to the consistency and repeatability of the electric injection molding machines, I have yet to find anything that compares in a hydraulic version. There are simply things that the electric servo motors can do for the injection molding process that are not possible in hydraulics. With hydraulics, there is always the issue of inertia and oil compression to overcome as well as other physics of fluid dynamics, that simply don’t apply with the electric injection molding machines and servo motors. The same principles that have made robotics of any type so repeatable applies to the injection molding process as well. With an electric servo motor, if you ask for five inches per second of fill speed for two inches of your shot size, that’s what you will get. If after 2 inches, you want to slow down your fill speed to one quarter of and inch per second, for an inch and then accelerate back to five inches per second for the remainder of the shot distance, that’s what you will get. Obviously, there is still a small amount of lag time, but it is so slight that you would have a difficult time to detecting it. The versatility that this opens up for the injection molder using and electric injection unit with this type of control is phenomenal. One example that quickly comes to mind is a scenario where you have a mold with valve gates, and you are fighting a blush problem in a small area where the plastic flows around a feature in the die from thick to thin. The plastic melt front will want to flow past the thin area until pressure or resistance to flow increases enough to create the back filling you will need to back fill the thinner area. With an electric injection molding machine, you literally will have the ability to pinpoint that part of the fill process, change fill rates as you pass that point in conjunction with the valve gate firing positions, and assist yourself in working right around the issue. They respond that well. The downside to that is that with electric servo’s, you will also get what you are asking for even when you might not want it. The injection pressures that the newer electric injection molding machines are capable of is easily in the 25,000 to 35,000 PSI cavity pressure range and that can do a lot of damage to a mold if you are not making use of all the proper safety settings built into your machine to protect against it. The potential exists that you can literally “blow out” a hot runner manifold if you are not careful. If you ask for maximum injection pressure and speed, that’s what the machine will try to give you and until a safety trips either for pressure or amperage draw on the motor, you won’t be able to stop it fast enough. Once again, this is where choosing the right machine for your processing needs is important to do ahead of time. While I feel that this control is currently unparalleled by anything else in the market place today, quite frankly you simply may not need it for the type of injection molding that you or your company does. However if precision and repeatability is paramount to the success of you making a quality product, then and electric injection molding machine might be just the ticket.
Written by: WM8C, February 26th, 2007. Not for use without written permission
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