Unit 3 The Plastic Mold Design,Unit 3 The Plastic Mold Design,3.1 Lesson 6 Plastics and Classification of Plastics,Plastic means pliable or impressionable. It is defined as the capability of being deformed continuously in any direction without breaking apart.1 Following this definition，plastics could include glass, metals, and waxThus, it is evident that name plastics is misleading and not exactly correctToday, however, the name plastics is identified with the products which are derived from synthetic resins. The synthetic resins are made by various chemical processesPlastics manufacturing is of comparatively recent date.,Today plastics are commonly found in homes, automobiles, numerous other products and machines. Some machines which utilize electricity require plastic parts with electrical insulating properties. Plastics are usually shipped to manufacturing plants as pellets or powders, and they are melted (for thermoplastics ) just before the shaping process. Plastics are also available as sheet, plate, rod, and tubing, which may be formed into a variety of products.2 Liquid plastics are used especially in the making of reinforced plastic parts. Plastics or polymers, commonly called plastics, fall into two broad classes:, Thermoplastic molding materialsThese include polymers such as ABS and polycarbonate used for toys, consumer electronic products, and more flexible kitchenware products.3 The key feature is that these plastics can be heated to a viscous fluid, molded, and cooled in a reversible, time-and time-again manner. Because the thermoplastic materials are those which soften with heating and solidify and harden with cooling, they can be remelted and cooled time after time without undergoing any appreciable chemical change., Thermosetting molding materials These include the melamine-formaldehyde used in hard plastic tableware and the epoxy resins used for glues and rein-forced cast products such as kayaks and tennis racket frames. Thermosetting products are heated until they become a viscous liquid, poured or injected into a mold, and then allowed to solidify. As thermosetting materials, chemical cross-linking occurs to create an irreversible, infusible mass. Upon the application of initial heat the thermosetting plastics soften and melt, and under pressure they will fill every crevice of a mold cavity.,Upon continued application of heat they polymerize, or undergo a chemical change, which hardens them into a permanently hard, infusible, and insoluble state. After this they cannot again be softened or melted by further heating.,3.2 Lesson 7 Thermoplastic Mold Design,Basically, there are two types of transfer molds: the conventional sprue type and the positive plunger type. In the sprue type the plastic preforms are placed in a separate loading chamber above the mold cavity. One or more sprues (the runway between the injection machines nozzle and the runners or the gates) lead down to the parting surface of the mold where they connect with gates to the mold cavity or cavities (Figure 3-1). Special press with a floating intermediate platen is especially useful for accommodating the two parting surface molds.,The plunger acts directly on the plastic material, forcing it through the sprues and gates into the mold cavities. Heat and pressure must be maintained for a definite time for curingWhen the part is cured the press is opened，breaking the sprues from the gates. The cull and sprues are raised upward, being held by a tapered, dovetailed projection machined on the end of the plunger. They can easily be removed from the dovetail by pushing horizontally. In a positive plunger-type transfer mold the sprue is eliminated so that the loading chamber extends through to the mold parting surface and connects directly with the gates (the entrance to the mold cavity).,The positive plunger type is preferred，because the mold is less complicated，and less material is wasted. Parts made by transfer molding have greater strength, more uniform densities, closer dimensional tolerances，and the parting plane (the separation plane of the two mold halves) requires less cleaning as compared with parts made by compression molding. 1 The following figure (Figure 3-2) shows a typical two-plate mold and indicates the structure of mold and the arrangement of all parts in mold.,3.3 Lesson 8 Transfer and Injection Molding,Transfer molding represents a further development of compression moldingThe uncured thermosetting material is placed in a heated transfer pot or chamber (Figure 3-3). After the material is heated，it is injected into heated closed molds. A ram, a plunger, or a rotating-screw feeder (depending on the type of machine used) forces the material to flow through the narrow channels into the mold cavity. Typical parts made by transfer molding are electrical and electronic components and rubber and silicone parts. This process is particularly suitable for intricate parts with varying wall thickness.,Inj