By: Clare Goldsberry

"Cavity pressure is to the injection molder what an EKG is to a cardiologist." That’s a quote that Karl Bauer, formerly the market manager for Kistler Instrument Corp.'s Plastics Western Region (currently regional account manager for RJG), loves to provide when he’s asked about the value of pressure transducers inside a mold.

"Information on what’s going on inside the mold creates a footprint for the processing tech," explains Bauer. "When there’s a problem with the parts, the toolmaker blames the process tech and vice versa. A transducer or pressure sensor becomes a referee by allowing you to know what’s going on inside the mold. A good curve equals a good process that makes a good part."

Molders sell machine time, so having cavity transducers in the mold reduces the time it takes to get a mold up and running and to troubleshoot a mold. Historically, 99 percent of the industry never looked at cavity pressure, notes Bauer. However, over the past 10 years it’s a tool that is beginning to catch on.

From 1992 to 1997, Kistler’s cavity pressure business doubled. Now, after a decade of documentation, pressure transducers in the mold have proven themselves to be valuable technology that isn’t going away, Bauer says. "If you can convince a customer to put a transducer in a mold you can show the customer the value in it," he adds.

The best way to fill a mold is as quickly as conditions permit. However, maintaining optimum pressure to fill the mold without overpacking and causing flash, or underfilling and creating voids, requires a scientific method instead of a wild guess because the technician can’t see through the steel. "You have to know how much material you put into the cavities," Bauer says. "We have made all kinds of progress with molding machines. Now, we need to concentrate on molds." The cavity pressure transducer allows the technician to see what’s going on in the mold. It also provides the moldmaker with the opportunity to let the molder do a better job of controlling the process and producing a better part. "A cavity pressure sensor gives you a tool that allows the processor and the moldmaker to be on the same page," says Bauer. "Once the process technician has established the correct process, the sensor can help monitor the quality of the part, thereby proving—or disproving—the mold."

Scott Harris, owner of Harris Precision Mold (Tempe, AZ) notes that use of transducers is generally driven by the customer—the molder—particularly for use in higher-end applications where predictable, more scientific molding methods are required.

Harris installed two transducers in one cavity of an eight-cavity mold in the post-gate area, at about one-third full. The second level of sensing is end-of-fill. On higher-end applications, two transducers per part are commonly used, but on multicavity tooling a transducer will be installed in one cavity. Typically, the transducer will go underneath an ejector pin or blade, explains Harris.

From a design standpoint, Robert Vaughan of Vaughan Enterprises (Walnut, CA) notes that it’s best to know going into a mold design if pressure transducers are needed. "Even the smaller versions that Kistler Instruments sells still can cause havoc with a completed mold design or finished mold if the customer decides after the fact that he needs them," he says. "Like everything else in the designing and building of a mold, space is usually at a premium." Cavity pressure monitoring is a valuable tool for processing for molders. If they know what’s going on in the cavity, they can use that to control their whole process. Jerry Schmidt, processing engineer for The Tech Group (Scottsdale, AZ) is convinced that pressure transducers are invaluable for nailing down processing issues.

"I’m working on an application where I was able to uncover the cause of a weldline problem in a multicavity mold, something I wouldn’t have been able to do without the transducers," explains Schmidt. "After doing extensive experimenting, an eight-factor DOE, and spending two days making 34 different runs, and quantifying and qualifying the different welds, I was able to quantify the cause, and then determined the best process not to get that condition.

"We noticed a variation in cavity performance. By putting transducers in the mold we noticed a distinct difference in the way the cavities were filling. We can see packing as a function of time—a correlation between those cavities not performing well and the fill-pack interval," Schmidt says. "I know what the characteristics are in the mold in a poorly performing cavity—we get the data on a hard drive. It gives a great picture of what happens not outside, but inside the mold where it counts."

Are there any downsides to using pressure transducers? Yes, says Vaughan. "If a company doesn’t train its employees in understanding the information that they are receiving from the device, frustration sets in and they become lifelong opponents of the benefits that the transducer has to offer," he says.

"We need to remember also that not all molds need a transducer, but all molds can benefit from them."

Kistler Instrument Corp.
Amherst, NY
(716) 691-5100
www.kistler.com