Sapphire transducers may take process control to a new level

Based on technology developed for the aero-space and defense industries, sapphire-diaphragm nozzle transducers, operating in direct contact with the melt, reportedly can deliver melt pressure and melt temperature readings at orders-of-magnitude-greater speed and accuracy compared to conventional systems. A process control system using signals from a sapphire nozzle transducer (Figure 1) to close the loop is currently up and running at a major domestic automotive OEM's plant. It controls shot weight at up to 10 Cpk. Six Sigma is 2 Cpk. 

"The nozzle is where you need the control," explains Armen Sahagen, technical director of Sensonetics (Huntington Beach, CA), which supplies the sapphire transducers. "Having these transducers in both the nozzle and the mold cavity will give the best results, but going with a mold transducer alone is not as good as going with a nozzle transducer alone." 

Because of their interdependency, melt pressure and temperature are unique process variables. The interaction of both affects melt viscosity, which affects everything else. Variations in peak and pack pressure at the injection nozzle affect part weight. Controlling these variables helps control part weight. 

A mold cavity transducer can only control boost-to-hold pressure transfer based on after-the-fact readings, whereas, Sahagen argues, a nozzle transducer can control the complete pressure profile, including low backpressure. He says tests conducted at customers' plants and at independent labs prove sapphire nozzle transducers used alone can outperform any mold cavity transducer used alone, including his own. 

Microsecond Response 
Sahagen adds that Sensonetics' sapphire transducer is unique for its ability to control and measure melt pressure and temperature. Signals looped to the press to maintain optimum QC limits are generated by monitoring these two critical variables via sensors in direct contact with the melt in the nozzle where, he contends, monitoring matters the most. 

Regardless of where it is located, Sahagen continues, the sapphire transducer easily bests traditional types—including strain-gauge, load-cell, mercury-filled, and quartz-piezoelectric transducers—when it comes to repeatability in real-world molding scenarios (see Figure 2). In one molding test, repeatability was to within .13 percent of the spec, some seven to nine times better than other types used in various machine and mold locations during the comparative analysis. 

Sensonetic's direct-contact, inline, real-time readings are designed to provide linear pressure measurements up to 40,000 psi and temperature readings to 800F. Response time is 100 to 500 microseconds (millionths of a second), with zero low-pressure dead bands and hysteresis. The sensor can negate the effect of temperature errors on pressure readings by briefly opening a shutoff valve to stop the machine. What's more, the sapphire diaphragms are corrosion resistant and fatigue and abrasion proof. 

A typical Sensonetics nozzle transducer retrofit kit costs $1000; $1500 with a discrete pressure/temperature monitoring unit. An older, limit-switch-controlled 1000-ton press produced 10-kg PP bumper structures with shot weight variations causing a 14 percent reject rate. Since being retrofit with the sapphire transducer in 1994, it continues to run them today to within 3g with a 6-plus Cpk.