Processing melt flow rate

One method of comparing the processability with product performances of plastics is to use their MWD. A narrow MWD enhances the performance of plastic products. Wide MWD permits easier processing. Melt flow rates are dependent on the MWD. With MWD differences of incoming material the fabricated performances can be altered requiring resetting process controls. The more the difference, the more dramatic changes that can occur in the products. 

The resin must be of highest purity for optimum processing characteristics and properties. Degradation results in discoloration, bubbling, and change in melt flow rate. 

A distinction should be made between machine conditions and processing variables. Machine conditions are basically temperature, pressure, and processing time (such as screw rotation/rpm, and so on) in the case of a screw plasticator, die and mold temperature and pressure, machine output rate (lb./hr), and the like. Processing variables are more specific such as the melt temperature in the die or mold, melt flow rate, and pressure used. 

Thus, both the mean droplet size and the size distribution may be predicted using these correlations [Eqs. (26), (27), (28), or (29) and Eqs. (30), (31)] for given process parameters and material properties. For a given atomizer design, the standard deviation of droplet size distribution has been found to increase with the melt flow rate, but appears to be less sensitive to the gas flow rated5 Moreover, the variation of the standard deviation is very atomizer- and melt-specific. An empirical correlation which fits with a wide range of atomization data has the following form ... 

The properties are close to those of polyethylenes, with some differences due to the tertiary carbon linked to the backbone good mechanical properties at ambient temperature, low price, attractive price/property ratios, easy processing, chemical inertness, we absorption of water, low density, good electrical insulation even in wet media, feasibility of welding, versatility of processing methods, broad range of available melt flow rates (MFR). 

Melt flow rate or melt flow index may be obtained by applying a specified weight to force a sample of the plastic, heated to a specified temperature, through a hole of stated diameter the amount extruded in this way in ten minutes is the parameter required. The actual conditions of test may be varied according to the material and the requirements of processing but in general the rate of shear is far lower than is experienced in reality in most melt processes (in other words, this is a low shear rate test). 

The results are shown in Figure 1. PPP was comparable with the tris(nonylphenyl) phosphite in preventing process degradation, although it wasn t quite as effective. The unstabilized base resin, however, became soupy, discolored, and had a melt flow rate of more than 10 after four passes. Aryl phosphite and the PPP gave considerably less discoloration, and melt flow readings of 3.9 and 4.7, respectively, after four passes through the extruder. 

Once the process has been optimized, plastic conditions should be recorded such as fill time, peak pressure at fill, cavity pressure,184 melt temperature, mold temperature, melt flow rates, and gate seal time. Record all basic machines setpoints on the setup sheet such as the transfer time (fill time) and weight, overall cycle time, and total shot weight, part weight, % runner, etc. 

Knowledge of flow of the melt at different temperatures and shear rates is required in the fabrication processing of the resins. Melt viscosity of these polymers is a function of shear rate. For example, pol5mier viscosity changes as extrusion rate is varied. In addition to melt viscosity, melt flow rate (MFR), also known as melt flow index (MFI), is a customary flow characteristic of resins. MFR is defined as the mass of molten polymer (in grams) that flows through the die/orifice of a rheometer in ten minutes and is inversely proportional to the melt... 

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