Processing, thermoplastics terms Links

In principle MID manufacture is not restricted to any given base material. In fact, very different polymers can be used. The choice of material and production process is primarily linked to the mechanical, thermal, and electrical requirements of the end purpose of the finished MID. The materials group of thermoplastics is normally subdivided on the basis of heat distortion resistance or long-term service temperature on the one hand, and the price of the raw materials on the other [93]. 

The dynamic cross-linking process is used to produce thermoplastic elastomers from mixtures of crystallizable polyolefins and various rubbers. Variations of basically the same method are employed to produce novel, stable polymer alloys by performing chemical reactions during extrusion of such mixtures. In that case, the cunent industrial term is reactive extrusion. Such processes are used, for example, to improve processability of LLDPE s into tubular film (by introducing long chain branches during extrusion with low levels of peroxides) or to... 

Unlike thermoplastics, which are simply melted, thermoset resins chemically react from low-viscosity liquids to solid materials during processing, a process termed curing. Structurally, thermosets differ from thermoplastics because of the presence of cross-links in the former, which means that thermosets cannot be reshaped or recycled once the chemical reaction occurs. One advantage of thermosets vs. thermoplastics is that wetting the filler becomes much easier with a low-viscosity material. By far the most common thermoset composite is automobile tires, which consist of a polymer made from styrene and butadiene monomers and carbon-black filler. The actual recipe used is much more complicated, and can include other monomers or polymers, as well as other fillers. In the absence of filler, the cured resin is rubbery at room temperature, which makes tires a... 

Another challenge in terms of molding and process technique is presented by hard-soft composites pairing thermoplastics with elastomers. Elastomers are superior to the thermoplastics when it comes to dynamic sealing qualities, temperature resistance, and useful life. The injection molds used in this process are cooled in the area of the thermoplastic cavity, whereas the cavities into which the elastomer is injected have to be kept within the cross-linking temperature range of 180-200° C. 

If we return to our structural classifications and isolate the two ends of the spectrum— linear and highly cross-linked polymers—we find that we can establish still another method of classification based on processibility. Linear polymers can be melted or thermally softened and as such can be termed thermoplastic. On the other hand, the highly cross-linked material cannot be melted or appreciably softened. In fact, such a polymer will thermally decompose or bum before it melts. The highly cross-linked polymers are classified as thermosetting polymers, or thermosets. One further distinction between thermoplastics and thermosets is that the former are generally somewhat soluble whereas the latter are insoluble. Table 1-1 summarizes the two classes. 

The techniques of forming thermoplastics are obviously influenced by the fact that the molecules of thermoplastics do not cross-link on heating and therefore can be maintained in a softened state while being made to flow under pressure into a new shape. The properties of each thermoplastic, however, sometimes limit the nature of the possible processes in which it will be involved. Some, for example, will be flexible enough to be ejected from a mould having undercuts in it others will be too brittle to withstand such a treatment. It is therefore preferable to know about each plastics in terms of properties and applications in order to appreciate which forming techniques may be most suited to it. 

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