Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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tips and techniques in-mold sensors Achieve Process Transparency With In-Mold Cavity Sensors By Susan Montgomery & Vincent Gallo, Priamus System Technologies In plastic injection molding, the primary objec- tive is to manufacture dimensionally and structurally consistent parts, independent of the molding machine being used. To accomplish this, molders can benefit from process optimization and control methods based upon PVT (Pressure-Volume-Temperature) relationships and automatic detection of the melt front*. These methods provide real-time infor- mation as the plastic is changing from a liquid to a solid. When the PVT behavior is consistent from cavity to cavity, then consistent parts are produced, making "lights out" molding possible. The PVT information is obtained from appropriately placed in-mold cavity-pressure sensors and cavity-temperature sensors. Although every application needs thoughtful consideration, there are some general guidelines regarding sensor use and place- ment as well as reasons why using both cavity pressure and cavity temperature information is important and beneficial. USES OF CAVITY PRESSURE Cavity-pressure sensors provide an ideal source of information for process optimization. Direct or flush-mounted piezoelectric cavity- pressure sensors are recommended due to their high rigidity, natu- ral frequency, measuring range, and signal reproducibility and linearity. If space for flush-mount sensor installation is limited, behind-the-pin sensors can be utilized for process monitoring and development. However, over time, plastic and other substances can accumulate in the ejector-pin channels, which can interfere with pin movement. This leads to unreliable pressure signal readings. In general, Priamus recommends placing a pressure sensor near the gate, within the first third of the flow path. More infor- mation is obtained from the cavity-pressure curve when the sen- sor is near the gate, compared with a sensor that is towards the end of fill. A pressure sensor near the gate will show the entire filling phase and provide more options for controls. (In some special cases, using a cavity-pressure sensor at the end of the flow path is indicated.) Specialists in in-cavity sensing, like Priamus, can offer suggestions for sensor placement in specific applications. See Fig. 1 for key points described by the cavity-pressure curve: a) Material passing the sensor: When the pressure signal rises from zero, the material is crossing the pressure sensor. Automatic detection of the melt front* at this point can be used for process control. In order to detect this, sensors and corresponding elec- tronics must be able to respond within fractions of a millisecond. b) Injection: The slope of the initial rise from zero can indicate the rate of injection. The injection profile can be opti- mized so that consistent melt-front velocity is achieved. This affects part shrinkage. c) Switchover/transfer: The beginning of the rapid increase to maximum pressure indicates when the machine transfers from injection to hold. If there is a dip in the curve at this point, the process could be transferring too early, or is indicative of hold- pressure filling. This induces internal stresses in the part. A fixed level of cavity pressure can be set and used for transfer. d) Peak pressure: The peak point on the curve is the maxi- mum pressure inside the cavity. The time to reach this point after transfer depends upon the reaction time of the molding machine. In-mold cavity-temperature and cavity-pressure sensors (left and right, respectively) each have their own merits, but offer powerful benefits when used together. *Priamus U.S. Patent 7,682,535 B2. PLASTICS TECHNOLOGY MAY 2012 39

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