Mold action

Bacteria and mold action on nitric esters and on smokeless proplnts 2 B3— B4... 

Bacteria and Mold Action on Nitric Esters and on Smokeless Propellants Influence of microorganisms upon stability of NC was studied as... 

Figure 4.11 Example of mold action during injection-compression (courtesy of Plastic FALLO)...

The mold can be modified to meet certain different shapes. The molding can be made either by withdrawing cores or by special press motions that partially open the mold halves (such as the compression molds used in coining to provide 2-D action 3-D mold actions are also used). The degree of foam density, wall thickness, and surface finish depends on the foam mixture (constituents and amounts). The machine controls the time cycle and the mold action required. 

This mold action permits uniform distribution of the plastics that is forced against the inside surface of the cavity. Following a prescribed cycle, the heat of the oven fuses or sinters the plastic and goes into the cooling chamber. The solidified product is removed from the mold and the cycle is repeated. This process permits molding very small to very large products. To improve product properties, hasten product densifi-cation, reduce air voids, reduce cure time, etc. 

Draft on the cavity wall such as 1° is recommended to facilitate product removal. Lower-shrinkage plastics like PC and PMMA requires l1 to 2°. Undercuts are possible, but they should be kept to a minimum. Making provisions for undercuts or no drafts usually requires higher mold costs because the mold will require action such as core pulls or splitting a mold to allow separation parallel to the undercut groove. This type of mold action usually requires additional cycle time thus increasing cost. 

Example of mold action during injection-compression... 

Where flexible material formulations are used, it is often possible to avoid the costs associated with mold action and to negate the effect of undercuts by stripping the parts from the mold. In nearly all cases, raw materials used in the process lend themselves well to coating operations once parts have been removed from the 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. 

Although the continuous casting of steel appears deceptively simple in principle, many difficulties are inherent to the process. When molten steel comes into contact with a water-cooled mold, a thin soHd skin forms on the wall (Eig. 10). However, because of the physical characteristics of steel, and because thermal contraction causes the skin to separate from the mold wall shortly after solidification, the rate of heat abstraction from the casting is low enough that molten steel persists within the interior of the section for some distance below the bottom of the mold. The thickness of the skin increases because the action of the water sprays as the casting moves downward and, eventually, the whole section solidifies. 

Hydroxy group containing tertiary amines are also used because they become incorporated into the polymer stmcture, which eliminates odor formation ia the foam (3). Delayed-action or heat-activated catalysts are of particular interest ia molded foam appHcations. These catalysts show low activity at room temperature but become active when the exotherm builds up. In addition to the phenol salt of DBU (4), benzoic acid salts of Dabco are also used (5). 

Some plastics tend to react with zinc to give an undeskable bubbling effect when molded. There is also a tendency to darken due to tarnishing by the action of sulfides. Tarnish-resistant bronzes are available in which the particles have been coated with a transparent resin (9). 

The wet pan is used for developing plasticity or molding qualities in ceramic reed materials. The abrasive and kneading actions of the mullers blend finer particles with the coarser particles as they are crushed (Greaves-Walker, Am. Refract. Jnst. Tech. Bull. 64, 1937), and this is necessary so that a high packing density can be achieved to result in strength. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

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