Sensor Technology on All-Electric Machines

Without question, the number of fully electric injection moulding machines is increasing. Some forecasts say that within the next 20 years, up to 50 percent, maybe more, of all machines in operation will be electric. Others say that could happen in as few as 10 years, depending on economic factors. Many machine manufacturers have centered their presentations around new or improved all-electric product lines at recent trade exhibitions.

Electric machines are certainly not new. A year ago, Cincinnati Milacron said that 4,500 of the ACT/Roboshot machines were in operation around the world, and there are obviously many more from other manufacturers. The reasons for this trend are pretty well-known: clean operation, reliability, reduced maintenance and energy costs, and the possibility for pinpoint control of highly precise machine movements, particularly where servomotors are used.

It is clear that, since electric motors cannot bring sufficient force directly onto a ball spindle, electric machines by definition use mechanical/toggle clamps. Then, since there is no hydraulic fluid, the familiar pressure sensors can't be used, and machine positioning by drives and gears, as opposed to hydraulic cylinders, eliminates the use of linear potentiometers.

So how are certain pressures and motions measured on the electric machines?

Sensors Provide Answers

Sensors are the solution being used by virtually all makers of electric machines, so IMI visited Bruno Schlaepfer, president of Schlaepfer Sensors, Hagenbuch, Switzerland, which supplies many of those sensors. Schlaepfer's immediate response was unequivocal: there would be no all-electric machines without sensors. Unlike hydraulic machines, which at the low end can still be set manually, this is impossible with the electric toggles, so the sensors become, as Schlaepfer says, ". . . the fingertips of the operator."

The measurement generally considered most critical is injection pressure. On an electric machine, it can be measured at the nozzle, at the back end of the injection unit, or on the structure of the machine. Because of backpressure, this needs to be a static measurement. Nozzle pressure is the best for closed loop control, according to Schlaepfer, which means a static measurement at 3,000 bar (45,000 psi) pressure, with temperatures up to 400°C (750°F). In those conditions, the often highly abrasive material passing the sensor tends to cause rapid wearing of the sensor diaphragm, even when it is made of stainless steel, and the sensor cable is exposed to a harsh environment. This approach is used by only one maker of electric machines at present.

The majority of manufacturers use a load cell

at the back end of the screw, similar to the pressure sensors used on hydraulic injection cylinders. It is more accurate than the hydraulic system, says Schlaepfer, because it is on the screw and thus much closer to the actual pro-cess. Schlaepfer's company makes a wireless load cell that can be mounted on the rotating part of the screw base, and at present, only one machine manufacturer uses this high-end approach. There are almost as many ways of mounting sensors on the machine--on bolts, beams, etc.--as there are machine designs (see Figure 1), but they each share the advantage of being an integral part of the machine.

Nozzle Touch Force

Though rarely done on electric machines, nozzle touch force can be measured by a manufacturer-integrated system consisting of simple proximity switches checked by an additional sensor. This system, called Nozzle Check, can be used to measure, adjust, and calibrate the touch force and can simultaneously measure nozzle pressure. It can check proper function of the entire injection unit and will reveal screw wear at an early stage. Tools have recently been developed to calibrate this system in the moulder's plant.

Clamp Force/Tiebar Strain

As with all toggle clamp machines, measuring the clamping force/tiebar strain is necessary at least periodically, for example during setup and/or preset maintenance. It can also be done online to maintain maximum control. Measuring the elongation of each tiebar tells you if the clamping force is evenly distributed. Making a total of the individual measurements will tell you the actual clamping force.

These measurements can be used to benefit both the finished product and the machines and moulds. An unbalanced system can give problems with cavity filling, particularly in multicavity moulds. Balancing helps assure lower scrap rates and the absence of flashing problems. On

the machine side, balancing supports parallelism, which helps avoid excessive wear or outright damage to moulds and to the machine itself, including broken tiebars.

Some machine manufacturers adjust the clamp force once and then make subsequent calculations for different moulding jobs using the elasticity of the machine. That assumes, says Schlaepfer, that the elasticity is linear and will be the same with different moulds at different temperatures. But during normal operation those parameters change, so the adjustments, at least for machine parallelism, need to be reset periodically.

Choice of Technologies

Strain gauge sensors (figure 2) may be placed on the surface of the tiebars between the platens or inside the tiebar using a hole specifically drilled in the end of the tiebar for this purpose. Some manufacturers supply machines with the holes predrilled, or the holes can be drilled later. The effect on tiebar tensile strength is minimal. A probe with a strain gauge sensor on the end is inserted in the hole and measurements are taken. The probes (figure 3) install quickly and can be easily moved from one machine to another.

Four sensors can be used during setup to assure parallelism, then one can be left in to maintain control. There is a precise digital readout of tiebar strain on the probe. Additionally, one to four tiebar probes can be connected to monitors or computers, and it is easy to record data for quality programs such as ISO 9000. Schlaepfer offers Windows-based software for online monitoring and recording of tiebar strain (figure 4) and clamp force, and can supply a complete laptop computer solution, including the machine interfaces, that can be easily moved from machine to machine.

Strain rings that mount around the outside of tiebars offer moulders a flexible, economical tool. They install and uninstall quickly, yet still offer very high accuracy and repeatability. Each contains two diametrically opposed sensors and can be used like the internal sensors: four for setup and one during operation.

Some manufacturers use the sensors, internal or external, for feedback in a closed loop control system. They measure in microns and are accurate enough that they are used in moulding CD-ROMs where acceptable tiebar strain deviation is ±2 percent or less, compared to ±5 percent specified in Euromap protocols.

Another type of sensor generally referred to as clamp-on can be mounted magnetically on the tiebar to measure elongation, but not left on during operation. It is highly accurate and generally used for setup and by service technicians. There are also surface mounted strain gauges for use directly on moulds and platens that offer a solution for measuring parallelism of tiebarless machines.

Strain gauges can be bonded to the surface or pressed on, as in the case of the strain rings. Bonding offers permanence, but it takes well over half an hour to install one, and time to uninstall it before mounting a new one. The rings mount by tightening one screw, and the interface cable easily threads on by hand. The rings also offer an obvious advantage in portability between machines. Even though the Euromap #7 protocols for checking tiebar elongation/strain specify the use of strain gauges, there are other ways to measure.

Dial gauges are the most traditional but they are less accurate, must be reset to zero manually, and can be influenced by tiebar bending. Ultrasonic measuring has lower resolution and measures relative strain among the four tiebars rather than absolute numbers. Piezo quartz sensors are only for mounting inside the tiebar.

Future Bus Systems

Sensor systems used on electric machines, particularly if they remain on during operation, need to be EMC (electromagnetic-compatible) safe due to the heavy electromagnetic fields generated by the machine's motors. Signal amplifiers for the sensors must also be safe for this environment. In the near future, according to Schlaepfer, we can expect to see bus signal systems such as CAN Bus used with sensors for EMC protection. Besides saving on cables, using the bus will increase measurement speed. Strain gauge sensors and amplifiers, at present, are state of the art and therefore more costly than older measuring systems. As with most new high technology, however, it is expected that the cost will decrease as production volumes increase, and volume will increase with more electric machines entering the field.

Bruno Schlaepfer also mentioned an intriguing possibility for measuring cavity pressure using these externally mounted strain gauges. Because the toggle system is so stiff, the load caused by the cavity pressure can be measured by the tiebar sensors. Though it is not true numerical cavity pressure, the signal correlation is sufficient for process optimization without the need to install costly, completely dedicated, pressure transducers within a mould. Schlaepfer cautioned, however, that this is a developing technology dependent on the shape of the moulded part, flow paths, and other considerations. He is hopeful, but says more work needs to be done.

When There Are No TiebarsCOLOR>

Though the manufacturers of tiebarless machines point out that the extreme mass of the U-frame assures parallelism, the platens and the machine itself can be bent in use by such things as nonparallel moulds, nonsymmetrical cavities, a noncentered gate, or incorrect mould setup. Obviously, you can't measure tiebar stress to address the problem, but there is sensor technology available to detect both machine and platen bending. Two surface mounted washer type strain gauges on opposite sides of the U-frame will detect machine bending. A single gauge of the same type on the fixed platen will accurately measure surface strain and the direction of maximum bending can be found by simply rotating the gauge. Because they are press-mounted, these washer gauges can be installed and moved very easily.

Sprueless manufacturing of ceramics by hot runnersCOLOR>

As a first in the area of ceramics, a sprueless, complicated ceramic product will be manufactured using hot runner channel technology. This ceramic cup will be produced on an Allrounder 320 S 500-150 injection moulding machine during the Ceramitec Show in Munich, Germany, in October. It will be demoulded and automatically positioned on a tray for insertion into the kiln by an integrated three-axis CNC robotics handling system, which treats fragile green parts with care.

Even with the increased manufacturing complexity, the basis for production is the central control of all machine, handling, and mould functions. The central control is managed by the logical structure of the Selogica controller and the sequence editor, which is directly oriented to the production process. Using this central operating philosophy, even the most complicated sequence of moulding and handling processes can be simply and easily arranged directly on the monitor. The available programming range, as well as the extended control, documentation, and help functions of the Selogica controller, contribute significantly to the simplification of reproducibility with powder injection moulded parts.