Molding closed mold

Biodeterioration. For objects made out of organic materials, mold and insect attack are a principal cause of damage. Microbiological organisms can also be responsible for serious deterioration of outdoor stone. Museums, especially storage areas, are quite conducive to providing the conditions in which infestations can occur. Objects are stored in close proximity, and left untouched for prolonged periods, often with Htde if any ak movement. 

Cheese products generally must be maintained under refrigeration using closed flexible plastic, or plastic cups or tubs for packaging. Ice cream packaging is generally minimal, ie, lacquered or polyethylene extmsion-coated paper-board cartons, molded plastic tubs, or spinal wound composite paperboard tubs or cartons. 

The pressure of the extmder forces uniform plastic distribution throughout the mold. Cooling the mold solidifies the plastic with slight shrinkage. The mold is maintained closed by mechanical or hydraulic pressure while the thermoplastic is injected and solidified. 

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. 

Steam-Chest Expansion. In steam-chest expansion the resin beads in which gas is already present are poured into molds into which steam is injected. The steam increases the temperature close to the melting point and expands within the stmcture to create beads with food cushioning and insulating properties. Expanded polystyrene is widely used in this process for thermal insulation of frozen food packaging. 

The mold is usually prepared with orifices to permit air trapped between the sheet and the mold to escape and ensure uniform, close contact of the plastic with the mold surface. By clamping the sheet beyond the perimeter of the piece, plastic may be drawn from the peripheral areas into the mold, ensuring uniformity. Both pressure and vacuum are employed to force the softened plastic sheet into the mold. 

Horizontal rotary machines employ multiple molds in a horizontal plane on a rotary turret. As each mold approaches the extmder die exit, it opens to accept the parison and then closes. The parison is then blown into the bottle shape. The extmder must extmde on an intermittent basis or be intermittently withdrawn to provide a parison for each passing mold. 

Vertical rotary molds also employ multiple molds on a turret but rotate in a vertical plane. As each mold reaches the die exit, it grasps the parison and closes. Because of the vertical spacing between molds, intermittent extmder action is not required. Vertical wheels are used commercially for high volume apphcations. 

The Hartford-Empire 28 is a press-and-blow machine used to make articles such as drinking glasses (tumblers). It uses paste molds and the ware is rotated to avoid the mold seams jars with screw threads cannot be produced. The product leaves the machine as an almost closed, hoUow object and is finished by severing and fire-polishing with a bum-off machine. 

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

A basic dmm is often supplemented by blow-molded plastic inserts that fit closely into the interior cavity. These inserts generally are provided with molded fittings for filling and sealing. The combination, which is referred to as a composite dmm, enhances the fiber dmm for additional appHcations,... 

Iron is the preferred metal for casting steel and nonferrous metals are used in smaller amounts. Most castings are made in green sand molds, ie, uncured molds of sand, clay, and water. However, the use of sheU mol dings is growing, because such mol dings permit reproducibUity of castings with close dimensional accuracy. In addition, the simplicity of equipment procedures reduces costs. 

An iajection mol ding machine is operated by hydrauHc power and equipped with an electric motor and hydrauHc pump. The maximum hydrauHc oil pressure is ca 14 MPa (2000 psi). A hydrauHc cylinder opens and closes the mold and holds the mold closed duriag iajection another cylinder forces the screw forward, thereby injecting the melt iato the mold. A separate hydrauHc motor turns the screw to plasticate, homogenize, and pressurize the melt. Control of these movements is a combined function of the hydrauHc and electrical systems (35—37). 

Fig. 13. Injection molding machine A, hydiaulic motoi foi turning the screw B, hydraulic cylinder and piston allowing the screw to reciprocate about three diameters C, hopper D, injection cylinder (a single-screw extmder) E, no22le F, fixed platen G, tie rods H, mold I, movable platen , hydraulic cylinder and piston used to move the movable platen and supply the force needed to keep the mold closed and K, machine base.

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the mol ding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

To make a decorated plate, the mold is opened shortly after the main charge of molding compound has been pressed iato shape, the decorative foil is laid ia the mold on top of the partially cured plate, ptinted side down, and the mold closed again to complete the curing process. The melamiae-treated foil is thus fused to the molded plate and, as with the decorative lamiaate, the overlay becomes transparent so that the ptinted design shows through yet is protected by the film of cured resia. 

Secondary Fiber. Increasing costs of taw fiber, legislative mandates for recycling (qv), and avadabdity of inexpensive waste papers have contfibuted to the increased use of recycled fibers. The use of secondary fiber in the United States has grown from 22% of pulp from all sources in 1978 to close to 31% (24 x 10 metric tons) in 1993 (3). Recycled fibers ate sometimes used in special writing papers, but the principal use is for the manufacture of linetboatd, newsprint, tissue, cereal boxes, towels, and molded paper products such as paper plates and egg cartons (see Packaging materials, containers AND INDUSTRIAL materials). 

Fig. 19. Typical injection molding and reaction injection molding (RIM) machines (a) injection molding machine for thermoplastics (b) reaction injection molding machine, showing A, the closed position, where reagents circulate, and B, the open position, where reagents are mixed and dispensed to mold.

A special attribute of these processes is the abiHty to pre-position reinforcement, inserts, and core materials for stiffening ribs. Gel coatings can be apphed to the mold surface to eliminate post-mold finishing. Because both surfaces of the part are formed in a mold to close tolerances, accurate assemblies are possible, which is a requirement for many automotive or tmck body appHcations. 

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