Thermoplastics rapid cooling

Thermoplastics have two properties which make them particularly suited to ultrasonic welding (a) low thermal conductivity and (b) melting or softening temperatures of between 100 and 200 °C. As soon as the ultrasonic power is switched off the substrate or bulk material becomes a heat sink, giving rapid cooling of the welded joint. When more traditional conductive heating is used for welding however the thermal... 

Some of the more interesting and innovative work has occurred in areas combining aspects of more than one chemistry type. For instance, moisture-curable thermoplastic adhesives have received much attention. Hot melt adhesives have been developed that contain active, moisture-curable isocyanate groups. The compositions provide rapid processing on assembly lines because a reasonable bond is formed as soon as the thermoplastic adhesive cools from the melt. However, bond strength and performance improve with time as the composition is slowly crosslinked to a thermoset by reaction of the isocyanates with atmospheric moisture.96,97... 

Microphase separated triblock thermoplastic materials have complex melt morphologies when flowing, which may be trapped by rapid cooling in a variety of states. 

A method of rapidly cooling thermoplastic parts when they are removed from the mold, usually by submerging the parts in water. 

Just like the injection molding machine, only thermoplastic processing equipment is recommended for TPE extrusion. In addition, TPE extrusion does not require a final vulcanization or crosslinking (curing) step. It only requires rapid cooling of extrudate to room temperature. There are some differences between thermoset rubber and... 

Carbon fibers from isotropic pitch Isotropic pitch or a pitch-like material, such as molten polyvinyl chloride, is melt spun at high strain rates to align the molecules parallel to the fiber axis. The thermoplastic fiber is then rapidly cooled and carefully oxidized at a low temperature (<100 °C). The oxidation process is rather slow, so as to ensure stabilization of the fiber by cross-linking to make it infusible. However, upon carbonization, relaxation of the molecules takes place, producing fibers with no significant preferred orientation. This process is not industrially attractive due to the lengthy oxidation step, and because only low-quality carbon fibers with no graphitization are produced. These fibers are used as fillers in various plastics to form thermal insulation materials. 

This memory is often unwelcome. Sometimes we prefer for thermoplastic parts to forget their original shape and stay put, especially when the parts must be coined, formed, machined, or rapidly cooled. Occasionally, however, this memory or instability can be used advantageously. 

All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and erystallises slowly producing sink marks. 

Thermoplastic polymers are also prone to recrystallisation processes, particularly after rapid cooling employed in many manufacturing processes. Sometimes these are very evident as with PET (Figure 2.5), though sometimes the overall annealing effect does not show an overall exothermic event. The kinetics of recrystallisation processes are usually described by the Avrami equation discussed below. 

This is also important in determining processing behaviour. The thermal conductivity of most mineral fillers is about one order of magnitude higher than that of thermoplastics and their incorporation considerably increases the conductivity of a composite. This effect is beneficial in processing as mouldings can be expected to heat up and cool down more rapidly, leading to shorter cycle times [68]. 

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