Processing, thermosets preheating

Molding compound is transferred in the cold state by pressure from the material hopper into the main injection chamber. Here it can be preheated before injection into the heated mould tool. Injection, through a special nozzle, can be either by ram or screw pressure. If screw feed is used, the screw must be of the type designed to process thermosets as opposed to thermoplastics. 

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Different foundry casting techniques are used. Included are plastic-based binders mixed with sand. Various types of molds and cores are produced that include no-bake or cold-box, hot-box, shell, and oven-cured. Usual binders are phenolic, furan, and thermoset polyester. There is the foundry shell casting, also called dry-mix casting. It is a type of process used in the foundry industry, in which a mixture of sand and plastic (phenolic, thermoset polyester, etc.) is placed on to a preheated metal pattern (producing half a mold) causing the plastic to flow and build a thin shell over the pattern. Liquid plastic pre-coated sand is also used. After a short cure time at high temperature, the mold is stripped from its pattern and combined with a similar half produced by the same technique. Finished mold is then ready to receive the molten metal. Blowing a liquid plastic/sand mix in a core-box also produces shell molds. 

Generally, the lower halve contains a cavity, while the upper part has a projection. After preheating the material, the mould is closed. As the pressure is increased, the polymer material will deform and fills up the mould cavity. In case of thermosets, the pressure and the temperature has to be maintained until the material is fully cured. For thermoplasts, it suffices to cool the mould, thereby fixating the polymer s shape. When the mould is cooled down, the object is ejected from the mould and the process can be repeated. 

Molding under pressure is called compression molding. In this process, powders or mold materials are usually preheated, and then simultaneously pressed and hardened (Figure 36-1). Generally, only thermosets containing a great deal of filler are used as mold materials, i.e., phenolic, urea, melamine, and unsaturated polyester resins. Inlays such as mats and fabrics are also frequently used. 

In extrusion, the preheated material is forced out of the extruder with a screw (Figure 12-2) or double screw and allowed to cool in a bath or in the air. Thermoplasts, elastomers, and thermosets are extruded. As a rule, thermosets are processed in torpedo-containing extruders. With thermosets, most of the curing reaction must occur in a heated pressure chamber. Pressures can approach several hundred bars. The rate of extrusion is lowest with thick-walled bodies. Tubes are extruded at rates of up to 10 m/min, films up to 150 m/min, and telephone-cable insulating material or fibers at up to 1000 m/min. In extrusion, the Barus effect (Section 11.3.1) and melt fracture (Section 7.6.1) may be observed. Tubes, films, ducts, cable insulation, and knot-free nets are produced by extrusion. 

Cascade coating n. A process for applying epoxy and other thermosets to objects such as electrical resistors and capacitors, in which finely powdered resin is poured over the preheated object. The article is usually rotated as the powder is applied. 

Transfer molding n. A molding process used mainly for thermosetting resins and vul-canizable elastomers. The molding material, usually preheated, is placed in an open pot with a hole in its bottom atop the closed mold. The cross-sectional... 

Dielectric heating can be used to preheat powder, pellets, or preforms to remove water before processing the material. Dielectric heating can also be used to generate the heat necessary for curing polar, thermosetting adhesives, and it can be used to quickly evaporate water from a water-based adhesive formulation. 

In the compression molding process, a thermoplastic or a partially polymerized thermosetting polymer is placed in a heated cavity. The material is nsnally preheated and preshaped with a form roughly similar to that of the cavity. The mold is closed and pressure is applied in order to force the material to fill the mold cavity. In the process, the polymer undergoes complete polymerization or crosslinking. At the end, the mold is opened, the part is ejected and the cycle can start again. This process wastes very little material. However, it is difficult to produce parts with close tolerances because the final size of compression molded products depends on the exact amount of the preform. Moreover, it is not possible to obtain parts with a complex shape, for example with deep undercuts (Osswald and Hemandez-Ortiz, 2006, Tadmor and Gogos, 2006). 

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