ENERGY SAVINGS

AND OTHER BENEFITS

OF AC DRIVE SYSTEMS

FOR INJECTION MOLDING

The average injection molding machine is not a careful consumer of electricity. In fact, up to 70% of the electricity consumed by some machines is never used in the molding process. However, the laws of physics state that energy (electricity) can not be created or destroyed; it can only change forms. In other words, energy can not just disappear, it has to go somewhere. This leads to the question: where does the energy consumed by an injection molding machine that is not used in the molding process go? It ends up this unused energy goes three places: heating the machine's hydraulic oil, noise, and wear and tear on the machine's hydraulic system. This explains why the hydraulic oil in an injection molding machine gets hot, and would cook if not for a cooling tower. The end effect is that you pay to pump the oil (with the machine's pump motor), then you pay to cool the oil (with your cooling tower). All of this leads us to the subject of this paper: what can be done to an injection molding machine so it consumes only what it needs, resulting in lower electric bills and a cooler, quieter machine?

The answer to the above question will be presented in three steps:

• An explanation of why injection molding machines consume more energy than they need.
• The presentation of the AC inverter drive and how it controls a motor's speed.
• How an AC inverter drive can be used to control a molding machine's pump motor and the resulting benefits.
  

Why So Wasteful?

The problem with most molding machines is that, under most circumstances, a significant amount of the hydraulic oil pumped and pressurized to run the machine is not used, but instead dumped back to tank through a relief valve. A valve is the hydraulic equivalent of a mechanical brake, a device that converts energy into heat. The source of the problem lies in two areas:

• The fixed-frequency power supplied by the electrical utilities.
• The operation of the synchronous motors used to drive the hydraulic pumps in almost all injection molding machines.
  

First lets look at the operation of synchronous motors. These motors have the word synchronous in their name because their rotational speed is synchronized to the frequency of the electricity used to power them. In case the term "synchronized" is not clear, here is a further explanation: the term synchronized is similar to the word proportional; in other words, if a motor is designed to turn at 1200 RPM when powered by 60Hz electricity, it will turn at 600 RPM if powered from a 30Hz electrical source. What follows is that if a synchronous motor is powered from a fixed-frequency electrical source, it will turn at a fixed RPM. Since the power supplied by the power utilities is fixed-frequency, the pump motors in injection molding machines turn at a fixed speed. Now if we follow the logic of this situation through to the hydraulic oil, it should become clear why too much oil is often pumped. If the pump motors are turning at a fixed speed, then the pumps are turning at a fixed speed. If the pumps are turning at a fixed speed, then they pump a fixed volume of hydraulic oil. If the hydraulic pumps can only operate at a fixed volume, the manufactures of molding machines need to specify motor-pump combinations that can operate the machine at full capacity. If machines always operated at full capacity (clamp open, close, injection and screw charge at 100% with no cooling time), there would not be a problem. However, machines rarely operate at full capacity. This leaves an opportunity to save energy if there was a way to only pump the oil needed by the machine. Until recently, this was a difficult or impossible task. But in the last decade, advances in the electronics industry have made possible a whole new breed of motor control products. One of these products, the AC inverter drive, will be the focus of the next section.

Finally, a Way to Slow Down Motors

In the past, machine designers have had only one motor control device at their disposal: the motor contactor. A motor contactor works well for stopping and running a motor at full speed, but what if an in-between speed is needed, say 50%. With a motor contactor, all that can be done is run the motor at full speed, and then dissipate the excess power down line. Recognizing the wasted energy and control difficulties created by this situation, the electronics industry developed a solution: the AC inverter drive ("drive" from hereon). With a drive, the above problem of running a motor at half speed is easily solved. Simply wire a drive between the motor and it's power source, and send the drive a 50% reference signal. Now a 1200 RPM motor will turn at 600 RPM. Send the drive a 20% reference signal, and the motor will turn at 240 RPM. Send a 100% signal, and the motor will turn at 1200 RPM. How do drives do this? The answer is simple; drives control frequency. If we refer to the above section, we will remember a synchronous motor's speed is determined by the frequency of it's electrical power. Drives provide a way to control the frequency of AC electrical power, and in turn, control the speed of synchronous motors. Now that we have a way to slow down motors, we can turn our attention to making injection molding machines more efficient.


A Leaner, Cleaner Machine

Most injection molding machines pump more oil than they need. However it is possible to reduce or eliminate this waste by implementing a system that determines a machine's requirements and pumps only the needed amount. Such a system consists of two major components: a system controller and an AC inverter drive. The system controller monitors the machine's controller and generates a signal that corresponds to the needed volume of hydraulic oil. This signal is then sent to a drive that is wired between the machine's pump motor and the motor's power source.



The basic concept of the system is simple: if the machine does not need the oil, don't pump it in the first place. The advantages of doing this go beyond energy savings. Other advantages include: better repeatability (it is easier for the machine to control the lower volume of oil), quieter machine operation, lower oil temperature, less demand on the cooling tower and less wear and tear on the machine. All of this is done without modifying the machine in any way. The machine's hydraulic system is left untouched and the connections made to the machine's controller only monitor the signals, not changing them in any way. Disconnecting a system can be done in minutes by simply moving the motor leads from the output of the drive to the output of the machine's motor contactor. All of this might seem too good to be true. In fact, when a system is properly implemented, the results can be truly amazing. We have had customers see reject rates drop so dramatically that the increased yields were worth many times the electrical power savings. The enhanced control and repeatability has proven itself many times under the scrutiny of our customer's quality control departments. There are, however, some practical considerations that need to be taken into account before deciding if an AC drive system is a wise investment for a particular machine/mold/manufacturer combination. Also, choosing the right vendor can be the difference between a positive or negative experience. The final section will discuss a few of these issues.

When do Drives Make Sense

When AC drive systems are installed on the right machines, running the right jobs, the results can be tremendous. On the flip side, there have been some cases of unscrupulous or inexperienced vendors selling systems that never lived up to expectations. Unfortunately, there is no easy way for someone without the right combination of equipment and experience to determine if a system is justified in a given situation. But we can offer a few general guidelines. Below is a list of conditions that usually indicate a good application of AC drive systems:

• Manufactures in areas with high electrical costs
• Long cooling times
• Large machines
• Older machines
• Jobs injecting soft plastic
  

The above list must be used with caution, as there are many situations where at first inspection it might not appear as though an AC drive system would be of benefit, but the final outcome was very positive indeed. We have seen such situations first hand. At a major multinational toy manufacturer in Malaysia, we installed a drive system on a 100 ton JSW running 12 second cycles. We were hoping to see electrical savings in the 40% range. After conducting before and after power survey tests we were pleasantly surprised to find that we had reduced the machine's power consumption by 53%! The reason for such a reduction was because the plastic being injected was soft and the machine needed little power to inject and recover the screw. Ultimately, the best way to decide if AC drive systems are a wise investment for your plant is to work with a vendor that has the knowledge, experience, equipment and honesty to give you accurate estimates of power savings and other benefits. Demand that any potential vendor supply references and call those references. At Concise Technologies we have been involved in the installation of hundreds of drive systems in the USA, Mexico, China and Malaysia. We have developed a proprietary power survey system that tells us not only how much electricity was consumed, but how the machine consumed it. We have the widest selection of control options in the industry. Bottom line, don't buy an AC drive system for your presses without calling us first.