ROLE OF POLYMER TRANSPARENCY AND TEMPERATURE-GRADIENTS IN THE QUANTITATIVE MEASUREMENT OF PROCESS STREAM TEMPERATURES DURING INJECTION-MOLDING VIA IR PYROMETRY

The accurate and precise measurement of process stream temperatures du ring injection molding can be difficult, since the cyclic operation re sults in spatial and temporal variations of the stream temperature. Th is paper examines the application of a spectral infrared (IR) pyromete r to monitor the cooling of a polymer melt within the mold cavity duri ng a typical injection molding cycle. An outline for interpreting the radiation signal collected with the IR pyrometer is presented. The dis cussion includes theoretical aspects as well as experimental results. The theoretical approach accounts for the polymer transparency (attenu ation behavior) at the spectral wavelength of the pyrometer and also f or the temperature gradient within the polymer, thereby establishing t he concept of a critical depth for a given pyrometer/polymer combinati on. The final analysis reveals good agreement between the predicted an d measured results for the transient cooling conditions of the polymer within the mold cavity. Depending on the degrees of polymer transpare ncy used in the theoretical prediction, the deviation between the meas ured and predicted transient bulk temperatures after mold filling (dur ing the mold cooling stage) varies from +/-2 degrees K to +/-9 degrees K.