Cooling distortion temperature

As mentioned, a is quite small for polymers (due to a small k) and about 10 seconds are required for cooling a thickness of 1 mm, while the time varies inversely as the square of the thickness. One may assume that Tf < Tj, where Theat distortion temperature, meaning the minimal conditions for mold opening without any damage or fracture apply. The cooling time is apparently very sensitive to the thickness of the product and therefore... 

The most common form is the nematic, a bimdle of parallel, long, rodlike molecules. Additional cooling to the primary transition temperature, T leads to solidification into a solid crystalline phase (small crystallites). In the region between Tj and T , a liquid of very low viscosity prevails, in contrast to the high melt viscosity. The aromatic polyesters have high heat distortion temperatures (HDT). Vectra, composed of para-hydroxy-benzoic acid (PHBA) and para-hydroxy-naphtoic acid (PHNA), has an HDT of 180 C-240 C. Xy-dar, composed of PHBA, tera-phthalic acid and biphenol, has an even higher HDT of 260 C-350 C. 

In a process, a polymer is injection-molded into the shape of a sphere (radius of 0.025 m). How long should the object be cooled (mold wall temperature of 28 C) if the polymer s heat distortion temperature is 55 C ... 

Thermoplastics are resins that repeatedly soften when heated and harden when cooled (conditions that refer to fusibility ). Most thermoplastics are soluble in specific solvents and can bum to some degree. Softening temperatures vary with the polymer type and grade. Care must be taken in application conditions not to exceed the heat distortion temperature of the plastic, wherein the plastic begins to soften and potentially warp. Typical thermoplastics utilized in the automotive arena include, poly(olefins)—for example, poly(propylene) (PP) and poly(ethyl-ene)—nylon, acrylic, acetal, poly(styiene), poly(vinyl chloride) (PVC), poly-(sulfone), and the like. Also within this group are highly elastic, flexible resins known as thermoplastic elastomers (TPEs). 

The remaining cooling time should be minimised so that the surface temperatme of the component is just under the heat distortion temperature of the moulding material. This can be provided by the material supplier. 

Direct water spray cooling must be carried out with care. The spray chamber must be designed to ensure complete evaporation of all Hquid droplets before the gas enters the baghouse. Spray impinging on the chamber walls can result ia a dust mud iaside the chamber and any increase ia gas dewpoint may result in baghouse problems or atmospheric plume condensation. Spray nozzle wear can result in coarse or distorted spray and wetted bags, and water pressure failure can cause high temperature bag deterioration. 

An advantage of pack carburizing is that the charcoal pack supports the workload which minimizes distortion. However, the components must be removed from the pack for quenching, or the mass cooled to room temperature and then the components reaustenitized and quenched. 

Quenching. After solution treatment, the product is generally cooled to room temperature at such a rate to retain essentially all of the solute in solution. The central portions of thicker products caimot be cooled at a sufficient rate to prevent extensive precipitation in some alloys. Moreover, some forgings and castings are dehberately cooled slowly to minimize distortion and residual stress produced by differential cooling in different portions of the products. Cold water, either by immersion or by sprays, is the most commonly used cooling medium. Hot water or a solution of a polymer in cold water is used when the highest rates are not desired. Dilute Al—Mg—Si and Al—Mg—Zn extmsions can be effectively solution heat treated by the extmsion process therefore, they may be quenched at the extmsion press by either air or water. 

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