Use of variable frequency drives in injection molding saves money
In the search for ways to reduce costs, decreasing overall electric usage can give a significant boost to your plant’s bottom line. When identifying opportunities to save energy, consider your injection molding operations. A boon for mass production, injection molding is a common process found in many manufacturing plants. An opportunity for cost savings of 25 to 50% or more comes in the way of process cooling systems, which are one of the largest energy users in an injection molding operation.
In these plants, plastic is melted, then injected into a mold. A shot of cooling fluid then travels through the system to cool the material in the mold and help set the plastic.
Pressure fluctuations are seen in the system when the water valves open and close. When all the valves are closed, it can create a shock in the system. That is why injection molding operations typically have a continually running bypass loop. While this approach allows the valves to remain open and prevents the system from over-pressurizing, it comes at a potentially high energy cost.
An alternate – and much more energy-efficient – approach is to optimize the system by adding a variable frequency drive (VFD) to the motor and eliminating the bypass loop. Energy savings can then be achieved by varying the speed of the pump to accommodate changes in pressure and cycling multiple pumps on and off, based on demand.
VFDs are adjustable-speed drives that control ac motor speed and torque by varying motor input frequency and voltage. Switching from a fixed- to a variable-speed method of cooling
makes it possible for pumping systems to change speed and maintain peak efficiency when output demand changes in an injection molding operation.
A VFD, in other words, is a necessary first step. But it is only the first step. To achieve maximum system efficiency and reliability, it is important to understand system demand and implement a solution to optimize the motor, drive and pump and overall process. The following covers what is involved.
Moving to closed-loop systems
Injection molding cooling systems have traditionally operated as open-loop systems, designed to maintain a constant pressure and flow rate (speed). Injection molding machine controllers access cooling fluid by opening and closing valves based on the machine’s process logic, which measures die temperatures and opens/closes values to allow cooling.
An open-loop system, in other words, is like driving a car with the gas pedal held to the floor and using the brake to control its speed. The motor will run at full speed (which is of course a waste of energy).
A closed-loop system, on the other hand, operates similarly to cruise control in a car. The desired speed is chosen, and it’s up to the car’s engine to do whatever it takes to maintain that speed up and down hills.
A similar closed loop result is accomplished in an injection molding cooling process by adding a pressure sensor to the system, making it possible to measure pressure so the system can self-adjust. The molding process still operates as it did in the earlier example. Instead of running full speed, however, the cooling control adjusts the pumps and drives to maintain pressure and flow.
The amount of energy saved by these additions depends on the operator’s ability to tweak the closed loop process. That requires a focus on process measurement. To achieve the greatest control, it pays to use a pressure sensor with a high resolution signal and a drive that provides PID control.
Use of a PID loop
A proportional integral derivative (PID) controller is an instrument that uses a control loop feedback mechanism to regulate temperature, flow, pressure, speed and other process variables in a system.
A PID functions in many ways like a home thermostat. In the case of a cooling pump, the user identifies a set point for liquid flow that must be maintained and designates tolerances and other parameters related to it, such as:
- How close to the set point the system must remain to achieve and maintain optimal performance
- Whether any movement away from the set point should be limited to just above or below the original setting
- How quickly the system should return to a set point after deviating from it.
When a PID loop runs in a drive, it watches the process feedback loop to determine how fast the process is changing. It then adjusts the drive’s output to compensate for those changes.
The speed with which it makes those changes matters. To understand why, consider a driver who wishes to maintain an average speed of 55 mph. In doing so, the driver functions similarly to a PID. After pressing firmly on the gas pedal, the car accelerates quickly, reaching, say, 45 to 48 mph in a matter of seconds, but also consuming an above-average amount of fuel.
The driver may then reduce the pressure on the gas pedal, providing feedback to the car’s engine that the driver wishes to slow the rate of acceleration. This allows the car to relatively glide to a set point of 55. While it may take longer to raise the speed by these last 10 mpg, it also use less gas to get there.
Once the desired speed is reached, a driver doesn’t automatically slam on the breaks if the speedometer edges up to 60 mph, nor pump aggressively on the gas pedal if it drops to 53 mph. Staying that close to the speed “set point” uses more gas – and creates a much bumpier ride – than making changes gradually.
The same is true with process cooling systems. Programming the drive to stay too close to the set point – or return to it as quickly as possible – may be unnecessary and foil optimization efforts. Time is better spent on understanding the dynamics of a system and running it in the best efficiency range of the pump.
[subhead]Pump selection, running time
The optimization process also requires taking a close look at ways to optimize pumping efficiency based on load. Consider, for example, an injection molding operation that has a single 100 hp pump that runs continuously at a fixed speed, pressure and flow. That same pump will run if two, three or four of the plant’s injection molding lines are running.
If that 100 hp motor is replaced with two 50 hp motors, however, consider what happens. Production on one or two lines may start with only one pump in operation. When the pump reaches a set point, the second motor may kick on. As new lines are brought on or taken off production, the pumps can operate together or cycle back and forth. By sharing running time, wear and tear can be distributed evenly between the pumps.
By spreading the use between two or more pieces of equipment – one the master, the other slaves – the pumping curve can be optimized. An analysis of flow curves against head pressure makes it possible to determine the best way to sequence pump operations to produce optimal pressure. Energy use can also be evaluated to create a load duty cycle that describes the production schedule and cooling loads associated with different levels of production.
By itself, the addition of a VFD can help to improve system efficiency. Switching from open to closed loop motor control by setting the VFD to operate in PID mode combined with proper pump sequencing can help process cooling systems such as injection molders, reducing energy costs by 25 to 50%. This type of process optimization will both increase energy savings and maximize the return on investment of system efficiency projects.