Heat softening methods

In vacuum and blow forming, sheets produced by extrusion are shaped by vacuum or pressure forming. Heat-softened sheet is pressed into a mould by atmospheric pressure when a vacuum is created between the mould and the sheet. Fig. 24.4(a). Plastic bottles are made by blowing instead heated tube is clamped in a split mould and expanded with compressed air to take up its shape (Fig. 24.4b). Both methods are cheap and quick, and can be fairly accurate. 

Condensate drainage devices, 70 148 Condensate polishing ion exchange in, 74 417 in steam-generating systems, 23 227 water softening method for, 26 122-123 Condensate return, in heat pipes, 73 226 Condensate return systems, 70 147-148 Condensate systems, in industrial water treatment, 26 136-137 Condensation, 9 281-282. See also Polycondensation control of VOCs by, 26 679-680 ketone, 74 570... 

As lead slowly oxidises at a red heat, two methods may be used to prevent ik One is to cover the surface of the lead with a layer of fine charcoal or even wood ashes. Another and a better plan, when the work will admit of its use, is to fioat on the top of the lead a thin iron plate, fitting the veesel in which the lead is contained, but having a hole in the centre or on one side, as most convenient, and laige enough to readily admit the ticles to be tampered or softened. 

Free forming n. A variant of sheet thermoforming in which a bubble is blown into the clamped, heat-softened sheet, either by applying a vacuum to the side that will be convex or pressure to the underside. The method has been used most with cast-acrylic sheet for applications where the best possible optical properties are foremost, such as airplane canopies. 

Fig. 15. The Arrhenius dependence of the dynamic viscosity Tj in the mixture obtained by both spin microprobe (open symbols) and spin macroprobe (filled symbols) methods. Inset D-band ESR spectra of nitroxide radical (solid line) and single-crystal DBTTp3PtBr (dashed lines) introduced into a frozen and heat softened nujol/tert-butylbenzene mixture (1 10) at f = 0 (upper spectrum) and arbitrary f at T < Tg (lower spectrum) time period. (From Ref. 89 with permission.]...

Injection Molding. The method of forming a plastic to the desired shape by forcing the heat-softened plastic into a relatively cool cavity under pressure. 

Polymers are characteri2ed as thermosetting and thermoplastic with respect to the methods by which they are joined. Thermosetting polymers are permanently hard and do not soften upon the apphcation of heat they are joined by mechanical fasteners and adhesives. Several methods have been devised to join thermoplastic polymers, as weU as thermoplastic composite materials, which soften upon heating. 

Dimensional Stability. The wet heat resistance of PVA fiber is indicated by the wet softening temperature (WTS) at which the fiber shrinks to a specified ratio. At one time, the WTS was not more than 95°C for nonacetalized PVA fiber, but improvement of WTS has been achieved by improvement in heat-drawing and -treating techniques other methods proposed include suppression of polymerization temperature of vinyl acetate (36) and employment of alkafi spinning (37). 

Thermoforming. Thermoforming is the most common method of fabricating sheet into three-dimensional packaging. In conventional thermoforming, the sheet is heated to its softening point or just below the melting temperature. The softened plastic is forced by differential air pressure into an open-top mold to assume the shape of the female mold. The mold is chilled and the plastic sheet solidifies and is then removed from the mold. 

Refractoriness. Refractoriaess is determined by several methods. The pyrometric cone equivalent (PCE) test (ASTM C24) measures the softening temperature of refractory materials. Inclined trigonal pyramids (cones) are formed from finely ground materials, set on a base, and heated at a specific rate. The time and temperature (heat treatment) requited to cause the cone to bend over and touch the base is compared to that for standard cones. 

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

Trinidad asphalt has a relatively uniform composition of 29% water and gas, 39% bitumen soluble in carbon disulfide, 27% mineral matter on ignition, and 5% bitumen that remains adsorbed on the mineral matter. Refining is essentially a process of dehydration by heating the cmde asphalt to ca 165°C. The refined product averages 36% mineral ash with a penetration at 25°C of about 2 (0.2 mm), a softening point (ring and ball method) of 99°C, a flash point (Cleveland open cup) of 254°C, a sulfur content of 3.3%, and a saponification value of 45 mg KOH/g. The mineral matter typically contains... 

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). In the Vicat test a sample of the plastics material is heated at a specified rate of temperature increase and the temperature is noted at which a needle of specified dimensions indents into the material a specified distance under a specified load. In the most common method (method A) a load of ION is used, the needle indentor has a cross-sectional area of 1 mm, the specified penetration distance is 1 mm and the rate of temperature rise is 50°C per hour. For details see the relevant standards (ISO 306 BS 2782 method 120 ASTM D1525 and DIN 53460). (ISO 306 describes two methods, method A with a load of ION and method B with a load of SON, each with two possible rates of temperature rise, 50°C/h and 120°C/h. This results in ISO values quoted as A50, A120, B50 or B120. Many of the results quoted in this book predate the ISO standard and unless otherwise stated may be assumed to correspond to A50.)... 

The softening point is defined as the temperature at which the resin flows under a given load on heating. Several standard methods have been proposed to determine the softening point of resins. 

When the material is allowed to cool it solidifies again. This cycle of softening by heat and solidifying on cooling can be repeated more or less indefinitely and is a major advantage in that it is the basis of most processing methods for these materials. It does have its drawbacks, however, because it means that the properties of thermoplastics are heat sensitive. A useful analogy which is often used to describe these materials is that, like candle wax, they can be repeatedly softened by heat and will solidify when cooled. 

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