Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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of the gates can be used to control the opening or closing of the individual gates.* See Fig. 4 for key points described by the cavity temperature curve: a) Mold steel temperature: Before the material reaches the sen- sor, the mold-steel temperature deviations from cavity to cavity can be seen. Blocked cooling lines can be quickly identified over time as the mold surface temperatures may show an increase. In addition, a mold insert is easily detected as there is a temperature drop before the material is detected in the cavity. b) Material location in the cavity: When the temperature curve spikes rapidly, the material has reached the sensor. This signal can be used for automatic switchover or transfer to hold- ing pressure.* It is possible to optimize this point using a slight signal-delay function. c) Maximum temperature: This is a "contact" temperature, not a melt temperature. The maximum point of the temperature curve will be much lower than the temperature at which the mate- rial is initially injected. d) Cooling of the cavity: The decline of the curve indicates the mold-cooling effectiveness in the specific location of the sensor. If the curve reaches the initial mold-steel temperature by the time the mold opens, the cooling is good in this location. This information can be used to optimize cooling time and reduce cycle time. USING BOTH CAVITY TEMPERATURE & PRESSURE When a cavity-pressure sensor near the gate is used together with a cavity-temperature sensor near the end of fill, material viscosity, including shear stress and shear rate, part compres- sion, and shrinkage all can be monitored and controlled. The shear rate is calculated from the time, ∆t (see Fig. 5), that the melt takes to flow the distance between the pressure sensor and the temperature sensor. The pressure measured at time T1 is used to determine shear stress. Batch-to-batch material-viscosity variations can be easily identified. This method of measuring material viscosity provides a quick, easy way to transfer molds to different machines or locations. Process conditions (shear stress and shear rate)—not just machine settings— can be duplicated. Costly tool validations can be eliminated. Shear- stress and shear-rate values obtained from mold-filling simulations can also be matched for faster startups of new molds. ABOUT THE AUTHORS Susan Montgomery has been president of Priamus System Technologies, LLC in Brunswick, Ohio, since 2002. She's been involved in plastics injection molding since 1994 and earned her Six Sigma Green Belt at GE Plastics. Vincent Gallo, senior applications engineer, joined Priamus in 2006 and has been active in plastics since he graduated with a BS in Plastics Engineering from Behrend College at Penn State in 2003. Contact: (877) 774-2687; e-mail s.montgomery@priamus.com, or visit www.priamus.com. PLASTICS TECHNOLOGY MAY 2012 41

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