Processing, thermosets part ejection

Mold is opened and the part ejected Number of layers, consisting of a mixture of reinforcement (usually glass cloth) and resin (thermosetting), are placed in mold and contoured by roller to mold s shape. Assembly is allowed to cure (usually in an oven) without application of pressure. In modification of process, called spray molding, resin systems and chopped fibers are sprayed simultaneously from spray gun against mold surface roller assist also is used. Wet lay-up parts sometimes are cured under pressure, using vacuum bag, pressure bag, or auto-clave... 

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. 

For molding urea and melamine materials, pressures of approximately one and one-half times that needed for phenolic material are necessary. In compression molding of thermosets the mold remains hot throughout the entire cycle. After the molded part is ejected, a new charge of molding powder is introduced. Unlike thermosets, thermoplastics must be cooled to harden. So before a molded part is ejected, the entire mold must be cooled, and as a result, the process of compression molding is slow with thermoplastics. 

Figure T.8 Transfer molding process (a) the mold is closed and material is placed in the pot, (b) the plunger descends into the pot, causing material to melt and flow through runners into cavities, (c) after cure, the press opens, the plunger retracts and the parts are ejected with cull and runners. (Ref Hull, J.L., Processing of Thermosets , Modern Plastics Handbook, CA. Harper, ed., McGraw-Hill, New York, 2000)...

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). 

Runnerless Injection Molding n (1) In molding thermoplastics, a process in which the mnners are insulated fi-om the cavities and kept hot, so that the molded parts are ejected with only small gates attached. See also Hot-Runner Mold. (2) In thermoset molding, the same as Cold-Runner Injection Molding. 

For thermoplastics, the mold is cooled before removal so the part will not lose its shape. Thermosets may be ejected while they are hot and after curing is complete. This process is slow, but the material moves only a short distance to the mold and does not flow through gates or runners. Only one part is made from each mold. 

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