Heat bending

The forming of thermoplastics can be convenientiy carried out by applying heat, usually between 120 C and 170 C, and bending to shape. Care must be taken not to overheat, as permanent damage to the material can result. Provided no permanent damage has been done, a shaped thermoplastic sheet will return to a flat sheet on the application of further heat. 

Simple bending is carried out by locally heating along the bend line, from both sides, until the material is pliable, using a strip heater. A strip heater can easily be constructed using a heating element inside a box stmc-ture, with the top made from a heat-resisting material. 

The top has a 5 mm wide slot along its centre, through which the heat passes. Fig. 14.5. When the material is pliable, it can be located in a former and bent to the required angle, e.g. in making a splash guard for a lathe, Fig. 14.6. The material can be removed from the former when the temperature drops to about 60 °C. Formers can be simply made from any convenient material such as wood. For volume production large-scale industrial machines are available which can have multiple bending areas and advanced heat control. 

Shapes other than simple bends can be carried out by heating the complete piece of material in an oven. To avoid marking the surface, the material can be placed on a piece of brown paper. The time in the oven depends on the type of material and its thickness, and time must be allowed for the material to reach an even temperature throughout. 

Acrylic sheet material is easily worked at 170°C, 3 mm thickness requiring about 20 minutes and 6 mm thickness about 30 minutes in the oven. Again, a simple former can be used to obtain the required shape, e.g. in making a guard for a drilling machine. Fig. 14.7. 

Drape forming (also called oven forming) is carried out commercially on small and large parts where the plastics material is pre-heated in an oven to the forming temperature, placed in a mould and held in place while it cools. Applications include 

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Remove the bulb from the acid, carefully wipe the capillary dry and seal the end by means of the flame. By further heating, -bend a hook on the end of the stem for hanging on die balance... 

Bimetallic Thermometers. These use two metals of different coefficients of thermal expansion that are bonded together their heating bends the device, and an electrical contact can then be made or broken. Thermometer coils that expand/contract with temperature and make or break contact with a mercury electrode are used in household thermostats and industrial control apparatus. 

Lately nanotubes have been used in the cooling plants as capillary heat bends. In particular, researchers at the Purdue University have developed a water cooling system for heated electronics based on carbon nanotubes. Thanks to the use of nanotubes, represented as microscopic capillaries carrying water, this new cooling system does not need a pump that makes water circulate in the traditional systems of water cooling. 

Despite its iuhramt impact resistance, PC provides only fair chemical resistance. When exposed to the common cleansCT addihve ammo-ihum hydroxide, PC exhibits catastrophic change including whitening and loss of visibihty. likewise, heat bending of PC can result in aberrahons that reduce optical clarity of the protechve device. 

Many continuous extractions involving solid samples are carried out with a Soxhiet extractor (Figure 7.18). The extracting solvent is placed in the lower reservoir and heated to its boiling point. Solvent in the vapor phase moves upward through the tube on the left side of the apparatus to the condenser where it condenses back to the liquid state. The solvent then passes through the sample, which is held in a porous cellulose filter thimble, collecting in the upper reservoir. When the volume of solvent in the upper reservoir reaches the upper bend of the return tube, the solvent and any extracted components are siphoned back to the lower reservoir. Over time, the concentration of the extracted component in the lower reservoir increases. 

Flaking paint is treated by infusion of an adhesive in the areas where needed, followed by resetting the flakes on the substrate the softening of the paint needed to bend it back is effected through solvent action or heat. Losses can only be filled and inpainted. Inpainting may also be necessary when cracks become so wide as to seriously affect the visual appearance of the painting. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

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. 

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