Properties, Plastics, and Processing

Molecular weight distribution (MWD), also discussed earlier, is an indication of the relative proportions of molecules of different weights and lengths. It shows the breadth of distribution or the ratio of large, medium, and small molecular chains in the resin. If most of the molecules have about the same MW, the MWD is classified as narrow. A wide or broad MWD implies a large variation in MW. Fig. 2-30 compares wide and narrow MWDs. The MWD is independent of both density and the melt index (MI) and must be taken into account in considerations of both processing and product performance. A narrow MWD enables much better, and narrower, process control. Two plastics with the same MI and density will process very differently if their MWDs are dissimilar. 

Most materials producers provide designers with understandable classification terminology that is directly useful for designing the best part at the lowest price. Comparisons 

In order to understand potential problems and their solutions, it is helpful to consider the relationships of machine capabilities, plastics processing variables, and part performance (see Chapter 7). 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 (Ib./hr.), and the like. Processing variables are more specific as parameters than are machine conditions, as the melt in the die or the mold temperature, the flow rate, and the pressure used. 

The distinction between machine conditions and fabricating variables is a necessary one to avoid mistakes in using cause-and-effect relationships to advantage. If the processing variables are properly defined and measured, not necessarily the machine settings, they can be correlated with the parts properties. For example, if one increases cylinder temperature, melt temperatures do not necessarily also increase. Melt temperature is also influenced by screw design, screw rotation rate, back pressure, and dwell times (see Chapter 7). It is much more accurate to measure melt temperature and correlate it with properties than to correlate cylinder settings with properties. 

The problem-solving approach that ties the processing variables to parts properties includes considering melt orientation, polymer degradation, free volume/molecular packing and relaxation, cooling stresses, and other such factors. The most influential of these four conditions is melt orientation, which can be related to molded-in stress or strain [10-12]. 

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