Cooling molding

Eig. 3. Coasumable-electrode melting ia the vacuum-arc furnace (13). A, arc B, buttoa E, electrode I, iagot L, Hd M, water-cooled mold and P, pool of... 

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. 

Commercial calcium carbide, containing about 80% CaC2, is formed in the Hquid state. Impurities are mainly CaO and impurities present in raw materials. CO is usually collected for use as a fuel in lime production or drying of the coke used in the process. The Hquid calcium carbide is tapped from the furnace into cooling molds. 

Water-cooled mold mounted on oscillating table... 

In a continuous casting machine, molten steel is poured into a reservoir at the top of the machine. It passes at a controlled rate into a water cooled mold where the outer shell of the steel becomes solidified. The steel is drawn down into a series of rolls and water sprays, which ensure that it is both rolled into shape and fully solidified at the same time. At the end of the machine, it is straightened and cut to the required length. Fully formed slabs, blooms, and billets emerge from the end of this continuous process. 

Even though most of the literature on processing specifically identifies or refers to thermoplastics (TPs) as in this book, some thermosets (TSs) are used (TS polyesters, phenolics, epoxy, etc.). The TPs reach maximum heat prior to entering cool mold cavities, whereas the TSs reach their maximum temperature in hot molds (Fig. 6-3). 

Direct chill casting is the pouring of molten aluminum into a water-cooled mold. Contact cooling water is sprayed onto the aluminum as it is dropped into the mold, and the aluminum ingot falls into a water bath at the end of the casting process. 

In injection molding (Fig. 16.8) the polymer is softened in a heated volume and then forced under high pressure into a cooled mold where it is allowed to harden. Pressure is released, the mold is opened, the molding is expelled, and the cycle is repeated. 

Figure 7.3 Schematic illustration of semicontinuous casting process in which (a) molten metal solidifies in a water-cooled mold with a movable base and (b) the base is moved downward so more metal can be poured into the reservoir. Reprinted, by permission, from the McGraw-Hill Encyclopedia of Science and Technology, Vol. 11, 8th ed., p. 31. Copyright 1997 by McGraw-Hill.

During high temperature compression molding, the polymeric powder is poured into a positive pressure mold that is heated and then cooled under pressure. The cooled mold is then opened to yield a fully sintered UHMWPE article. 

This is only true for a hot runner system. However, in many injection molding processes the runner system is directly inside the cooled mold, and the flow is not isothermal. 

Hot Runners. When the molten plastic is pumped into the water-cooled mold, the cooling system solidifies both the plastic product in the mold cavities and also the plastic material in the runners. Later the solid runners must be separated, reground, and reused. This is an extra burden on the process. An alternative is to avoid cooling the runners, and actually keep them hot, so that the molten polymer in them remains ready for the next shot into the mold. 

Hot-Melt Molding. Hot molten plastisol is injected into a cooled mold to make products like fishing baits and novelties. 

Temperature melting/solidifying profiles are different for TPs and TSs. TP just melts in the plasticator and solidifies in the cooled mold. The... 

A TP is melted into a tube that is generally referred to as a parison. While still in a heated ductile and firm plastic melt state, the parison is clamped between the cavity halves of a cooled mold, so that the open top and bottom ends of the parison are trapped, compressed and sealed by the mold faces (Chapter 17). Air pressure enters the parison. Air pressure causes the parison to expand like a balloon, so that it takes up... 

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

The feed system is an unwanted by-product of the molding process, so a further requirement is to keep the mass of the feed system at a minimum to reduce the amount of plastic used. This last consideration is a major point of difference between cold and hot runner systems. The cold runner feed system is maintained at the same temperature as the rest of the mold. In other words, it is cold with respect to the melt temperature. The cold runner solidifies along with the molding and is ejected with it as a waste product in every cycle. The hot runner system is maintained at melt temperature as a separate thermal system within the cool mold. Plastic material within the hot runner system remains as a melt throughout the cycle, and is eventually used on the next cycle. Consequently, there is little or no feed system waste with a hot runner system. Effectively, a hot runner system moves the melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3 32> 326-332,490... 

An interesting effect occurs while the cavity is filling with polymer (Figure 14-45). One might expect that as material enters the gate, it would be continually pushed ahead by the material that enters afterward. However, the material that first enters the cavity immediately spreads out to contact the cool mold wall and solidifies. The material that... 

The polymer expands to take the shape of the cooled mold and solidifies while air pressure remains in the part (third diagram from the top of Figure 14-58). After cooling, the mold halves open, the part is ejected (bottom diagram) and the next preform enters the mold. 

Injection molding is one of the processing techniques for converting thermoplastics, and recently, thermosetting materials, from the pellet or powder form into a variety of useful products. Forks, spoons, computer, television, and radio cabinets, to mention just a few, are some of these products. Simply, injection molding consists of heating the pellet or powder until it melts. The melt is then injected into and held in a cooled mold under pressure rmtil the material solidifies. The mold opens and the product is ejected. The injection molding machine must, therefore, perform essentially three functions ... 

Figure 11.10 Schematic of the blowing stage, (a) The molten, hollow tube — the parison or preform — is placed between the halves of the mold (b) the mold closes around the parison (c) the parison, still molten, is pinched off and inflated by an air blast that forces its wall against the inside contours of the cooled mold (d) when the piece has cooled enough to have become solid, the mold is opened and the finished piece is ejected. (From Kaufman, H.S. and Falcetta, J.J., Eds., Introdur ion to Polymer Science and Technologies, John Wiley Sons, New York, 1977. With permission.)...

Injection molds are usually made from high quality tool-grade steel. They may include beryllium copper inserts to improve heat transfer rates for more rapid cooling. Molds are quite expensive, and often contain wear plates that can easily be replaced, so that the molds can be more easily refurbished and have a longer life. 

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