Molds cavity

Eree-sintered articles do not have the same dimensions as the mold cavity because they shrink at right angles to the direction of the preform pressure and grow in the direction of the appHed pressure. 

Injection Molding. Any standard design plunger or reciprocating screw injection machine can be used for PEA 340, although a reciprocating screw machine is preferred (32). Slow injection into mold cavities avoids surface or internal melt fracture, and control of ram speed is important at low... 

In the high pressure process, a resia melt containing a chemical blowiag ageat is iajected iato an expandable mold under high pressure. Foaming begias as the mold cavity expands. This process produces stmctural foam products with very smooth surfaces siace the skin is formed before expansion takes place. 

Because cycle time to inject, flow, set, open, eject, and close is finite, and the face area or platen size is limited, the effective mol ding area is increased by increasing the number of mold cavities so that the number of finished pieces per cycle may be multipHed many times. 

Injection-molded articles can be decorated by in-mold labeling or by post-mold decoration. In the former method, printed film is inserted into the mold cavity before injection. The plastic forms an intimate contact with the graphic material. Post-mold decoration includes hot stamping, dry offset printing, and decal printing. 

Liquid-Injection Molding. In Hquid-injection mol ding (LIM), monomers and oligomers are injected into a mold cavity where a rapid polymerization takes place to produce a thermoset article. Advantages of these processes are low cost, low pressure requirement, and flexibiHty in mold configuration. Conventional systems, such as isocyanate with polyol, release Htfle or no volatiles. The generation of substantial volatiles in the mold is obviously undesirable and has represented a significant obstacle to the development of a phenoHc-based LIM system. A phenoHc LIM system based on an... 

Injection Molding. In iajection mol ding a molten thermoplastic is iajected under high pressure iato a steel mold. After the plastic solidifies, the mold is opeaed and a part ia the shape of the mold cavity is removed. Geaeral descriptioas of the process and related equipment are given ia Refereaces 29—34. 

CastabEity is not to be confused with fluidity, which is the abEity of a molten aEoy to fEl a mold cavity iu every detaE. CastabEity is the ease with which an aEoy responds to ordinary foundry practice without undue attention to gating, risering, melting, sand conditions, and any other factors involved in producing sound castings. 

Beat-Cured Restm. For optimum comfort and to impede further loss of chewing efficiency, close adaptation of the denture base to contiguous oral tissues is required, which necessitates custom-made appHances. Nearly all dentures are made of acryHc resins. A wax pattern is used to form a custom denture base in which the denture teeth are embedded. A plaster or dental-stone investment spHt mold of this wax denture base and teeth is prepared. The wax portion is removed and the surface of the resulting mold cavity is painted with a separating medium, usually an aqueous solution of alginate, to aid in the removal of the cured acryHc from the plaster mold. 

Main portion of the mold containing the molding cavity, mold pins, guide pins, etc. 

Process of shaping plastic articles by placing material in a confining mold cavity and applying pressure and usually heat. 

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

In this process, resin is injected into a closed mold containing the reinforcement preform. The resin can be injected either under pressure [22] or under vacuum [23]. The potential advantages of this process are (I) low mold cost, (2) inserts can be incorporated, (3) low pressure requirements, (4) accurate fiber orientation, (5) automation possibilities, and (6) versatility. The resin formulation and process variables are selected so that no significant polymerization occurs until the mold cavity has been completely filled. This is achieved by the ad-... 

The recommended radius not only reduces the brittleness effect but also provides a streamlined flow path for the plastic melt in the mold cavity. The radiused corner of the metal in the mold reduces the possibility of its breakdown and thus eliminates a potential repair need. Too large a radius is also undesirable because it wastes material, may cause sink marks, and may even contribute to stresses from having excessive variations in thickness. 

Flow pattern Ultimately, product quality can be considered a direct outcome of a plastic melt s flow behavior in its mold cavity or cavities. Excessive restrictions and obstructions to the flow of material spell trouble in injection molding. 

Blind holes in molded plastics are created by a core supported by only one side of the mold. The length of the core and depth of the hole are limited by the ability of the core to withstand the bending forces produced by the flowing plastic without excessive deflection. For this reason, the depth of a blind hole should not exceed three times its diameter or minimum cross-sectional dimension. For small blind holes with a minimum dimension below 1/4 in., the L/D ratio should be kept to two. With through holes the cores can be longer, since the opposite side of the mold cavity supports them (3). 

Freezing action Because of the heat exchange between the flowing TP melt and the mold walls, the flow may freeze (solidify) before the product is completely filled. Products that have alternate sections with thick and then thin walls can cause problems in flow and cooling that make them difficult to fill. In some cases the plastics that have been selected for the end use requirement are too viscous to flow properly in a mold cavity, and this makes the manufacture difficult. 

Weld line Weld or knit fines where two parts of a melt join while flowing into the mold cavity can result in problems. The quality of the weld depends on the temperature of the material at the weld point and the pressure present in the melt after flowing from the gate. The higher the temperature and pressure, the more complete the weld and the better the product performance and appearance. Bringing the material to the weld point at a higher temperature and pressure requires rapid filling of the mold cavity (Chapter 3, BASIC FEATURE, Weld Line). 

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