Uptime Strategies: Tuning early model electrohydraulic control systems

Editor's note: Woody Woodrell is president of Woodrell Project Management, an injection molding consulting firm based in Anadarko, OK. You can reach him on the Internet at www.woodrell.com. This column is his fifth in a series of troubleshooting articles.

On injection molding machines with proportional control flow and/or pressure control, there is a relationship between the operator controls and the hydraulic system. The most common problem is the lack of linearity between the performance of the hydraulic system and the controls at minimum or maximum settings.

For simplicity, I will discuss one of the workhorses of the industry--the Cincinnati Milacron T-Line toggle with a PC3 control. This press is no longer produced because of improvements in electrohydraulic control and the introduction of Milacron's Vista line and other new machines. But because of the high number of T-Lines in the field, and other similar presses, I will cover them here.

Hydraulic Pressure Control

In your machine's prints you should find a sheet detailing the pots that control the zero and the gain. These must be set before you make any other adjustments. The zero and gain affect the minimum and maximum of all pressure-related functions controlled by the pressure valve driver. There are, however, two distinct differences to consider: "on board," or factory-set adjustments, and settings adjustable by setup personnel--typically by digital thumb wheel (DTW).

For example, high injection pressure and high injection speed are adjustable by setup personnel. And while low injection pressure is adjustable by setup personnel, low injection speed is not. It is adjustable only on the valve driver board via a factory-set adjustment. This is true because the part is, theoretically, volumetrically filled by high injection and only packed out by low injection to obtain the correct weight or dimensional requirements.

With the injection screw in the forward position, the DTW settings at maximum, and a gauge monitoring the injection pressure, press the injection forward switch and record the current settings of the machine. On some machines you may have to move the high injection override switch out of the way to prevent the machine from going to low injection presets.

For example, say you show maximum pressure at 70 percent of the DTW setting and anything above the 70 percent setting maintains the same pressure; and below 20 percent of the DTW setting, you stay as low as the pressure will go. You only have, therefore, adjustment over a 50-point range.

This means that with 2000 psi as the nominal you will get a 40-psi differential for every click of the DTW. An ideal setting of the zero and the gain, where the 2000 psi is divided over a 99-point range (vs. 50), equates to 20.2 psi per click of the DTW. So, every click of the DTW has a greater-than-anticipated affect on the pressure. It is important to record the original differential so the setup personnel can know what effect their adjustments will have on the machine.

Making the Adjustments

To set the pressure valve driver that controls the injection end of the machine, it is best to use the high injection DTW settings. Place the machine in a manual cycle.

With the screw bottomed out you should read maximum injection pressure on the gauge. If you don't see the required pressure, start turning the gain adjustment until the pressure is obtained. If you still can't obtain the pressure, you will need to determine why. Possible causes may include a worn pump, weak reliefs, and others.

If you are already at or close to the recommended setting, reduce the gain until the pressure just starts to drop. Now go to the zero DTW setting. You should see as low a pressure as obtainable. This is often 50 to 75 psi. Zero would be nice, but it's not obtainable with many older systems. If you can get to 100 psi at zero, call it good and go on unless you absolutely have to have very low pressure on injection (below 100 psi).

Now to the pots. As you may have guessed, if your pressure is high, trim the zero pot down; if it is low, trim it higher. If it is where you think it should be, you still need to take it one way or the other to ensure it's just starting to increase pressure as you increase milliamps to the valve itself.

As an example, a Vickers CGE 02 or CGE 06 is operated at a maximum of 400 mA. It usually reaches its maximum at about 385 mA. Also it only starts to close the metering orifice off at about 65 or 70 mA. (This can be measured by taking one of the two wires going to the valve and putting a digital volt meter in series and set it to read 0 to at least 400 mA). If you're reading 2000 psi, but your valve driver has an output of 420 mA, you can reduce the setting of the DTW by about 10 or 15 clicks before you'll see any change in the actual pressure.

If instead you dial the gain to 385 mA or so at 2000 psi, you should see an immediate reduction in pressure in proportion to the amount the DTW is reduced. Hopefully, it will be linear. If it is not exact, do not be greatly concerned. Remember, the goal is to be able to adjust the machine "some" with every click of the DTW. Of all the machines I have seen in the field, very few have had much more than 80 clicks for the entire adjustment range of 2000 psi.

When you adjust one value be sure to check the other, since they are almost always proportional to one another. Once you have the gain and zero set, you can go on to the settings that are factory set, using the minimum and maximum values you just determined. And if you've done your job well it will be adjustable with every click of the DTWs, proportional to the maximum pressure obtainable divided by the number of clicks on the DTW. For example, 2000 psi divided by 99 clicks equals 20.2 psi per adjustment increment.

After finishing with the pressure, you can move on to the speeds. It should be noted that the Vickers CGE has a prefilter that should be checked for cleanliness before setting the ranges. Also, the Dynex valve used for speed control on the earlier T-Lines (PJ-5000 series) has a prefilter in it by the force motor.

One final note: be sure to record the before and after settings, date, time, and technician doing the work. This is extremely useful when troubleshooting in the future. Good record keeping is essential to long-term preventive maintenance.