Can't afford rejects? Try real-time cavity-pressure control.

If your parts demand the utmost consistency, you may be among the growing number of domestic molders that are adopting cavity-pressure sensing for injection molding process control. And if your parts carry too high a value to allow any to be thrown away, you may want to consider one approach to cavity-pressure regulation that corrects each shot in real time while the part is being molded.

This technology was developed by Arburg in Germany in 1987 but has not been marketed here until recently, according to Jurgen Giesow, customer-training and applications specialist at Arburg, Inc., Newington, Conn. The U.S. has lagged well behind Europe in appreciating the value of cavity-pressure control, notes John Adamowicz, who heads up Arburg's technical service. But now, domestic molders are starting to catch up, he says, especially those who produce metal-powder injection molded (PIM) parts, as well as some makers of medical products and other critical precision parts. At least a dozen such molders have recently adopted Arburg's "Process Regulation" system for real-time cavity-pressure regulation, and the company is now starting to promote it actively in North America.

Adamowicz sees PIM molders leading the move to cavity-pressure regulation because they have to meet very tight tolerances and because their rejects can be especially costly. Typical PIM part tolerances today are said to be 0.3-0.5%, but the auto industry is looking to compress the range of permitted variation to 0.1%. "That's very doable with today's closed-loop technology," asserts Giesow. However, even at current tolerance levels, PIM scrap rates can be as high as 20-30% in certain applications, he reports. High scrap rates are particularly painful for PIM molders because defective parts may not become evident for a day or two alter molding, after they have passed through the debinding and sintering stages. At that point a lot of value has been put into the parts, and they can no longer be recycled. Most PIM defects, says Giesow, result from inadequate control of injection speed and holding pressure.

WHY REAL-TIME CONTROL?

Adamowicz and Giesow believe Arburg's approach to cavity-pressure-based process control is unique. Some systems use peak cavity pressure to signal switchover from injection to holding pressure. Other systems monitor part quality by using reference cavity-pressure curves, pressure integrals, and peak pressures to qualify each shot as good or bad. In the case of a bad shot, these systems may attempt to correct the next injection cycle.

Arburg's Process Regulation system, however, seeks to maintain exact repeatability of the shape of the entire cavity-pressure curve throughout the filling, packing, and holding phases. What's more, the system continuously adjusts each cycle as it is happening, rather than waiting for the next one. This is accomplished by means of special hardware and software, which are offered as options on Arburg Series V, S, and C machines.

METAL-POWDER MOLDING WITH & WITHOUT 'PROCESS REGULATION'
(55-Ton Arburg 270V)

Last 7 of 30 Shots
Standard Control(a)

Peak Hydraul.      Peak Cavity      Cav.-Press.
Pressure,          Pressure,        Integral,
bar                bar              bar-sec

776                684              1123
777                689              1122
777                682              1114
777                692              1123
786                692              1127
784                697              1141
776                684              1119

Last 7 of 30 Shots
Process Regulation(b)

Peak Hydraul.      Peak Cavity      Cav.-Press.
Pressure,          Pressure,        Integral,
bar                bar              bar-sec

777                678              1154
776                679              1148
777                679              1152
778                679              1149
776                678              1152
771                679              1153
771                680              1148

                                  30-Shot Statistical Summary
                         Standard Control(a)   Process Regulation(b)

Peak Hydraulic Pressure, bar

Min                               773                   757
Max                               786                   791
Standard Deviation                3                     9

Peak Cavity Pressure, bar

Min                               676                   674
Max                               697                   680
Standard Deviation                5                     1

Cavity-Pressure Integral, bar-sec

Min                               1090                  1128
Max                               1141                  1154
Standard Deviation                13                    8

a Closed-loop hydraulic-pressure regulation with switchover based on
cavity pressure.

b Closed-loop cavity-pressure regulation.

Hardware modifications regulate hydraulic-oil flow on an instantaneous basis to the supply end of the injection piston and the return to tank. This "dynamic braking" function allows the screw to react faster to changing cavity pressure. In an extreme case, flow to the injection cylinder can be reversed to actually reduce pressure in the cavity if it exceeds the setpoint. Fast control response requires mounting a close-coupled hydraulic valve right at the piston. There is no hose between the valve and the piston and thus no column of oil to compress before a control action can be initiated.

Process Regulation software provides five steps of closed-loop injection-speed regulation up to the switchover point, followed by 10 steps of closed-loop pressure regulation for the packing and holding phases. The user sets the time increments between the pressure setpoints. The crucial phase from switchover to peak cavity pressure may have as many as three pressure setpoints within a total of 0.1 sec. "The key is packing the parts the same way with the same pressure," says Giesow. "That's what determines later shrinkage." Soon after peak cavity pressure is reached, the system switches back to hydraulic-pressure regulation.

Process Regulation allows the machine to compensate for changes in ambient temperature, mold temperature, hydraulic-oil temperature, material viscosity, regrind amount or quality, and color-concentrate mixing. Despite such normal disturbances, Giesow says, Process Regulation can hold cavity pressure to within 1 bar (14.51 psi) at the peak and throughout the curve. "Most molders today are happy if they can get 20-25 bar [290-360 psi] peak cavity-pressure deviation," he notes.

The add-on cost of hardware and software for Process Regulation is around 810,000-12,000. It's said to pay for itself in several ways:

* Dimensional variation is greatly reduced.

* Flashing can be eliminated.

* Molded-in stresses are reduced.

* Cycle time can be shortened by monitoring peak cavity pressure in order to set hold time more accurately.

* Faster start-ups can result from repeating an established cavity-pressure curve.

PIM MOLDING DEMONSTRATIONS

Process Regulation was put through its paces at an Arburg Inc. open house earlier this year. A 33-ton Arburg 320V machine molded impact PS. Cavity pressure was monitored with a piezoelectric Sensor from Kistler Instrument Corp., Amherst, N.Y. Figure 1 shows the variation in the cavity-pressure curve over 50 cycles that resulted with standard closed-loop hydraulic-pressure regulation and switchover by stroke position. Figure 2 shows a much tighter range of cavity-pressure repeatability throughout the curve during 50 cycles with Process Regulation.

The accompanying table displays actual data from a more recent demonstration at the PIM lab of Pennsylvania State University, University Park, Pa. A 55-ton Arburg 270V machine that was installed there this year molded a PIM compound with a PP/wax binder. Thirty shots were molded using closed-loop hydraulic-pressure regulation during holding and switchover by a peak cavity-pressure setpoint. Another 30 shots were run with Process Regulation.

Both the data on the last seven of 30 shots and the overall statistical summaries show more uniform peak cavity pressure and pressure integral for the whole curve when using Process Regulation. Interestingly, the peak hydraulic pressure varies more with Process Regulation than without it, because the machine is adjusting hydraulic pressure to maintain constant cavity pressure.