Mold cavity vacuum

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-... 

Flexible foam is made from long-chain diols with a small amount of triols for cross-linking to give strength and resilience. Foaming is produced by reaction between measured amounts of isocyanate and water to liberate carbon dioxide. Molded products are made by pouring the reactive liquid mixture into a mold cavity at 50°C. They foam and cure in 2-10 min, after which they are temporarily crushed or vacuum-shocked to open the cell walls and insure softness. Then they are allowed to condition for several hours to finish the cure. The major products are auto seating and headrests, and furniture cushions. 

Applying vacuum in a mold cavity can be very beneficial in molding plastics at low pressures. Press can include a vacuum chamber around or within the mold providing removal of air and other gases from the cavity(s). 

To insert a product it can be just immersed in a plastic liquid prior to the plastic hardening. The product could have fixed spider type supports or retractable pins or other features to support it when molten plastic is poured or injected around it. Another approach is to place the product on a layer of plastic that is partially polymerized in a mold cavity followed by applying a final layer that physically encloses it. For certain plastics and/or products a vacuum system can be used if air pockets or voids are to be eliminated. 

In vacuum forming, the thermoplastic sheet can be clamped or simply held against the RIM of a mold and then heated until it becomes soft. The soft sheet is then sealed at the RIM, and the air from the mold cavity is removed by a suction pump so that the sheet is forced to take the contours of the mold by the atmospheric pressure above the sheet (Figure 2.39a). The vacuum in the mold cavity is maintained until the part cools and becomes rigid. 

This process has been used since the 1940s to provide better control of the molding cycle to fabricate parts that are more precise. Those that used it were Bell Laboratories (USA) and D. V. Rosato. This technology is to combine IM of the compound with a complete or partial vacuum in the mold cavity, to facilitate the impregnation of the reinforcement. The vacuum injection process requires a perfect seal around the edge of the mold. Development included applying vacuum from the hopper, through the plasticator, and into the mold. It has very little interest. 

Efficient mold cooling also depends on accurate venting. Air trapped between the mold cavity and the plastic will significantly slow down cooling and also could cause problems with the part. Certain molds incorporate vacuum lines to ensure proper contact. 

Compressed air thermoforming is a technique borrowed from sheet steel processing its main use in plastics processing is in plastic cup production. Machines are also coming into use for the production of engineering components, such as computer covers. A flexible-pressure pad is used to press the hot sheet into the mold cavity, with the aid of an air cushion maintained under pressure. Compared with vacuum forming, this... 

LIM employs two or more pumps to move the components of the liquid system (such as catalyst and resin) to a mixing head before they are forced into a heated mold cavity. In some systems, screws or static mixers are used. Only a single pump is required for a one-part resin, but usually two (or more)-part systems are used. Equipment is available to process all types of resin systems, with unsophisticated or sophisticated control systems. A very critical control involves precision mixing. If voids or gaseous by-products develop, vacuum is used in the mold. 

Resin Transfer Molding (RTM) is accomplished with the use of a reinforcement system that has been preshaped to fit the mold cavity and a low-pressure resin in liquid form that is injected into a closed mold. The injection process can be assisted with the use of a vacuum system to provide for evacuation of any volatiles and to gain molded density. 

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