Friedrichshafen, Germany — Sensors mounted inside injection molds have proven themselves in thermoplastic molding by monitoring pressure and temperature variations and using the data to adjust process parameters.
But such sensors may have an even more important role to play in LSR and multi-component LSR/thermoplastic molding, since the extremely low viscosity of liquid silicone rubber is difficult to control until it cures in the mold.
At Fakuma 2018 in Friedrichshafen, Robert Vaculik, head of the business field plastics within the industrial process control division at Winterthur, Switzerland-based sensor producer Kistler Instrumente AG, talked about the role played by sensors in injection molding LSR.
Kistler sensors monitor both temperature and pressure within molds in real time during molding, with a piezo-electric crystal for pressure and a thermocouple element for temperature. Molders use the technology to optimize process parameters and ensure they do not deviate beyond set tolerances.
Vaculik said Kistler has had sensors for LSR in the market for some time. Within 14 sensor models designed for direct integration into molds, seven are suitable not only for thermoplastics, but also specifically for LSR processing.
Within the seven sensor series for LSR, only the 4 millimeter 6190C series monitors both pressure and temperature, while the 6 mm diameter 6152A series measures only pressure, and all of the other five monitor only temperature.
The smallest 6193B series has a 1 mm diameter and senses only temperature. Vaculik admits that "We are a little too large for some LSR parts when it comes to combined pressure and temperature monitoring," referring here to the 4mm 6190C series.
He says that sensors for LSR basically have to have a small gap in view of the low viscosity of the material. But that is a challenge, as it means there is very little play and that can potentially damage the piezo-electric crystal in a pressure or pressure/temperature sensor.
Sensors have traditionally been applied by mounting them within the mold steel. But Vaculik talked of a new approach of mounting them close to its interior surface, which measures the compression of the steel during molding. "It gives similar results, but without marking parts," Vaculik said.
Data from sensors is processed within Kistler's ComoNeo system. The latest version, with a ComoNeoPredict option, was shown at Fakuma. The technology uses model-based prediction-specified part quality, based on the cavity pressure and temperature profiles. The result should then be optimum process parameters required when working to Industry 4.0 principles.
Vaculik said: "ComoNeo is Industry 4.0 put into practice."
Industry 4.0 is an important element within a manufacturing execution system (MES), which is why Kistler acquired IOS GmbH in July 2017, after years of joint MES cooperation, and incorporated it in June 2018 into a new Kistler Aachen GmbH subsidiary in Aachen, Germany.
Stefan Holzapfel, who has 20 years of MES experience, has been managing the MES center since July and told LSR World that claims that products can control their own production, via bar codes or RFID labels, are intelligent products is false, as "the intelligence is in the MES."
Holzapfel stressed that MES is different for each producer, as they have different shop floor and machine layouts. He said customers can enter digitalization with a "MES Lite" and expand it later into "full-blown MES." Key elements are acquiring production operating data and machine data.
Holzapfel said Kistler's strength over other MES providers is its connection to machines via its sensor systems.
Another Swiss company is a significant competitor for Kistler in the sensor solutions area: Priamus System Technologies AG in Schaffhausen, a member of Barnes Group Inc. since October 2015.
Marcel Fenner, president of the Priamus System Technologies LLC subsidiary, made a presentation at the recent LSR 2018 conference in Anaheim, Calif., on quality monitor and quality balancer solutions. These use mold cavity sensors and associated electronics and software, such as Fillcontrol Control V on cold runner mold filling and Q-Button for touch-of-a-button monitoring and control.
This technology provides real-time solutions in LSR injection molding to monitor and analyze temperature, provide automatic venting, mold balancing of multi-cavity cold runner mold and weld-line control. Characteristics monitored directly in mold cavities by these systems also include short-shot detection, compression differences, shear rate, shear stress and viscosity, Fenner said.
Referring to use of the data for process stability prediction, Fenner said "cavity sensors are also the first link in the chain for Smart Factory solutions for molding operations to pick up trends before bad parts are produced." Fenner has referred to Priamus process monitoring and control systems being "basic building blocks for the fourth industrial revolution."
Priamus Managing Director Christopherus Bader has talked earlier of the low viscosity of LSR leading to larger differences in filling of multicavity molds compared to thermoplastics, as well as placing higher demands on process regulation due to the different melt flow front speeds in each cavity.
Resulting volume differences affect the degree of cure and therefore molded part quality. A typical arrangement with LSR is to use a pressure sensor near the gate to regulate viscosity and a temperature sensor at the end of the flow path to regulate over- or underfilling and cure, Bader said.
At Fakuma, Priamus showed that its Fillcontrol Control V has been used for addressing different compression airbag igniters overmolded by LSR, as well as in LSR/thermoplastic molding of oxygen masks.