Processing, thermosets flash

Curing of Polyimlde Resin. Thermoset processing involves a large number of simultaneous and interacting phenomena, notably transient and coupled heat and mass transfer. This makes an empirical approach to process optimization difficult. For instance, it is often difficult to ascertain the time at which pressure should be applied to consolidate the laminate. If the pressure is applied too early, the low resin viscosity will lead to excessive bleed and flash. But if the pressure is applied too late, the diluent vapor pressure will be too high or the resin molecular mobility too low to prevent void formation. This example will outline the utility of our finite element code in providing an analytical model for these cure processes. 

Uses Processing aid, modifier, antiozonantfor high m.w. elastomers and resins, e.g., EPDM, NR, IR, SBR, CR, fluoroelastomers, SBS, SEBS, TPO, TPE, and for adhesives, sealants, coatings for low temp, flexibility Features Reactive produces liq. elastomers used alone or with solid elastomers in thermosetting and thermoplastic applies. features low vise., crosslinking, oxidation resist., good elec, props., ozone and UV resist. Properties Pale yel. vise. Ilq. HC odor m.w. 7500 sol. in nonpolar soivs. sp.gr. 0.84 vise. 690,000 cps (60 C) flash pt. > 400 E iodine no. 19 dielec. str. 740 V/mll dielec. const. 2.11 (1 kHz) ethylene/propylene 46/ 54... 

Thermoplasts and thermosets form the two largest plastics subgroups (see also Chapter 33). They have so much in common with respect to their use, and also, partially, with respect to processing, that they can be considered together. However, thermoplasts and thermosets differ with regard to the processing of their wastes. For example, edge flash and flow ribs of thermoplasts can be reprocessed, but this cannot be done with thermosets. 

Sanding n. A finishing process employing abrasive belts or disks, sometimes used on thermosetting-resin parts to remove heavy flash or projections, or to produce radii or bevels that cannot be formed during molding. 

Scrap skrap [often attributive] [ME, fr. ON skrap scraps akin to ON skrapa to scrape] (14c) n. All products of a processing operation that are not present in the primary finished articles. This includes flash, runners, sprues, excess parison, and rejected articles. Scrap from thermosetting molding is generally not reusable. That from most thermoplastic operations can usually be reclaimed for reuse in the molder s own plant or can be sold to a commercial reclaimer. 

A considerable amount of machining is done with thermosets because, with the exception of a couple of methods, the mechanical removal of molding flash is necessary in thermoset processes. Drilling holes and cutting openings is also commonplace in some processes because it is difficult, bordering on impossible, to mold them in for many designs. 

Spun bonded media production processes exploit the thermosetting properties of polymers, to form fibres that can be bonded by combinations of heat, pressure and chemical activation. Melt spinning, using conventional synthetic fibre technology, was the earliest method used for producing spun bonded filter media and continues to be of major importance, but finer fibres are produced by melt blowing and flash spinning processes. 

Like thermosets the elastomers will only cross-link in the mold by introducing additional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the... 

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