Melt temperature and fill rates

Melt temperature and fill rates have been demonstrated to be very important process variables for microcellular injection molding. Recent work has studied the use of fast response thermocouples along with traditional pressure transducers to determine their effectiveness in providing practical process monitoring tools for the microcellular molding process [7]. Behind the ejector pin, pressure transducers and fast... 

Using a 4-in. test tube half filled with silicone oil and a stirrer made from heavy wire, melting points can be determined on an electric heater filled with sand (Fig. 4, p. 49). Once the sand is warmed up it has a very large temperature gradient from top to bottom, so the temperature and heating rate can be adjusted according to the depth of the test tube in the sand. 

Capillary and slit-die rheometers are used to determine the dependency of viscosity on shear rate. Since most molten polymers exhibit non-Newtonian behavior, it is important to be able to characterize this behavior. Measurements are made using a piston-driven cylinder that drives the molten polymer through a die of precise dimensions. The pressure drop across the die is measured, as is the flow rate through the die. Temperature is precisely controlled throughout the measurement. This test yields precise viscosity measurements as a function of temperature and shear rate. However, measurements tend to have artifacts in them, which need to be corrected in order to obtain true viscosity using Bagley and Rabinowitsch corrections. Capillary rheometers are also used to determine the effects of slip, a phenomenon in which the velocity of the melt at the capillary wall is nonzero. Slip has important implications for highly filled materials. 

As with microcellular extrusion, the standard injection molding process parameters such as melt temperature and cooling temperatures also affect the cell sizes and the level of weight reduction achieved as well as the other typical characteristics, such as shrinkage. The fill rates and pressure levels that develop in the injection process have an additional major effect on microcellular molded parts as compared to standard molding in that they affect the cell structure. As described in the original research, the rate of pressure drop and subsequently the number of nucle-ation sites will determine how many cells are formed and how uniform they will be. 

Yield stress values can depend strongly on filler concentration, the size and shape of the particles and the nature of the polymer medium. However, in filled polymer melts yield stress is generally considered to be independent of temperature and polymer molecular mass [1]. The method of determining yield stress from flow curves, for example from dynamic characterization undertaken at low frequency, or extrapolation of shear viscosity measurements to zero shear rate, may lead to differences in the magnitude of yield stress determined [35]. 

---