Cast zinc mold

The zinc is normally melted in a gas, oU, or coal-fired reverberatory furnace with a capacity up to 100 tons or in a low frequency induction furnace with a capacity of a few tons. The more highly aUoyed compositions are more effectively melted and mixed in low frequency induction furnaces. The furnace must be refractory-lined to eliminate iron pickup by the molten metal. The metal temperature is maintained below 500°C to minimize loss by oxidation. A ladle is used to transfer the metal for casting into molds the pouring temperature is usuaUy ca 440°C. Zinc scrap is not generaUy suitable for remelting because it may contain undesirable impurities. 

Although more and more zinc sheet and strip are produced in continuous mills, some is stiU produced by rolling slabs cast in open or closed book-type molds made of cast iron (124—127). The casting temperatures are between 440 and 510°C, mold temperatures between 80 and 120°C. The contact surfaces of the mold must be smooth and clean to allow unrestricted shrinkage of the cast slab. Mold lubricant is not necessary, but if used should be held to a minimum. Slabs cast in open molds must be skimmed immediately to remove surface oxide. Rolling slabs are cast 1.87—10 cm thick. 

Zinc alloy (kirksite) high-quality molds can be made from cast zinc alloys, offering good non-porous surfaces but are relatively heavy, with lower heat conductivity than aluminum, requiring closely spaced cooling channels. 

As blow molds do not have to withstand high pressure, a wide selection of construction materials is available. The ultimate selection will depend on a balance of the following factors cost, thermal conductivity, and required service life. The more commonly employed materials for small parts are aluminum and aluminum alloys, steel, beryllium copper (Be/Cu), and cast zinc alloys (Kirksite, etc.). Aluminum molds are excellent heat conductors, are easy to machine, can be cast, and are reasonably durable, particularly when fitted with harder pinch blades and neck inserts (Table 4-9). 

Before the most suitable materials can be selected, the cost, product, and process requirements must be checked out. Table 1.6 lists some of these materials used in blow molds. Molds for large volumes (> 60 L) are manufactured using either cast zinc or aluminum. For molds with a volume > 1,000 1, aluminum should be used for weight reasons. The pinch-off edge areas are mostly made from steel (for casting molds or molds from forged aluminum) to make deflashing easier or to automate it and to increase the die life. In the last 5 years, the CNC-milled production blow mold (made from aluminum) has established itself. The aluminum industry has developed new materials for this application (Table 1.6). 

Large quantities of zinc are used to produce die castings, which are used extensively by the automotive, electrical, and hardware industries. An alloy called Prestal(R), consisting of 78 percent zinc and 22 percent aluminum, is reported to be almost as strong as steel and as easy to mold as plastic. The alloy said to be so moldable that it can be molded into form using inexpensive ceramics or cement die casts. 

Forming-Die Alloys. The tonnage of slab zinc used in this appHcation is small. The use of zinc alloy dies started in the aircraft industry during World War II (119). Zinc-based alloys cast in sand and plaster molds continue to be used for short-mn dies for steel and aluminum stampings in the automotive and aircraft industries (120). Considerable cost savings are realized with these low melting zinc-based alloys which are easy to poHsh, machine, weld, and remelt. 

Zinc rolling slabs have been cast successfully by semicontinuous direct-chill casting methods. This is the preferred method for superplastic zinc alloys which, because of their large freezing range, display unacceptable surface shrinkage when cast in open molds. 

Production molds are usually made from steel for pressure molding that requires heating or cooling channels, strength to resist the forming forces, and/or wear resistance to withstand the wear due to plastic melts, particularly that which has glass and other abrasive fillers. However most blow molds are cast or machined from aluminum, beryllium copper, zinc, or Kirksite due to their fast heat transfer characteristics. But where they require extra performances steel is used. 

Many toxic pollutants were detected in the process wastewaters from metal molding and casting processes. The toxic pollutants detected most frequently in concentrations at or above 0.1 mg/L were phenolic compounds and heavy metals. The pollutants include 2,4,6-trichlorophenol, 2,4-dimethyl-phenol, phenol, 2-ethylhexyl, cadmium, chromium, copper, lead, nickel, and zinc. Each type of operation in the foundry industry can produce different types of pollutants in the wastewater stream. Also, because each subcategory operation often involves different processes, pollutant concentrations per casting metals may vary. 

Zinc is malleable and can be machined, rolled, die-cast, molded into various forms similar to plastic molding, and formed into rods, tubing, wires, and sheets. It is not magnetic, but it does resist corrosion by forming a hard oxide coating that prevents it from reacting any further with air. When used to coat iron, it protects iron by a process called galvanic protec-... 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

PET s broad range of properties make them ideal replacements in certain applications for such metals as die-cast aluminum or zinc also in thermoset switches, relays, sensors, etc. Applications include packaging (particularly stretched injection blow molded beverage bottles and film), appliances, electrical/electronics, etc. PET finds applications in such diverse end uses as fibers for clothing, films, botdes, food containers, and engineering plastics for precision molded parts.118-120 A wide range of applications is possible because of the excellent balance of properties PET possesses and because the degree of crystallinity and the level of orientation in the finished product can be controlled. 

Cast materials are used for RIM molds with reasonable success. One such material is Kirksite, a zinc alloy casting material. Kirksite molds are easy castable, are free from porosity, will polish and plate well, and have been used with favorable results. 

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