Mold cooling/heating

Rotational Molding. Hodow articles and large, complex shapes are made by rotational mol ding, usuady from polyethylene powder of relatively low viscosity (57—59). The resin is in the form of a fine powder. A measured quantity is placed inside an aluminum mold and the mold is heated in an oven and rotated at low speed. The resin sinters and fuses, coating the inside of the mold. The mold is then cooled by water spray and the part solidifies, dupHcating the inside of the mold. 

Powdered glass and crushed carbon are placed in molds and heated to 1000°C, at which temperature the carbon is oxidized, forming gas bubbles which causes expansion of the glass mix. The cellular material is then annealed and, after cooling, cut to size. 

Hollow products can be made by placing powdered plastic in a closed mold, tumbling and heating it until the plastic has coated the walls of the mold, cooling it to solidify the product, and opening the mold to remove the product. 

Slush Molding. This is the reverse of dip molding. A female mold is used to give any desired surface finish on the product. The mold is preheated, plastisol is pour into it, and gels onto the surface of the mold. The excess liquid plastisol is poured out, and the internally- coated mold is heated to gel, then cooled to solidify the rubbery product, and the product is stripped out of the mold. Typical products are arm rests, head rests, road safety cones, anatomical models, dolls, toys, and auto parts. 

In contrast to standard cooling, which keeps the mold temperature at a certain temperature below transition temperature, the surface of the mold is heated up with the help of an inductive heating almost to the melt temperature in order to gain a lower melt viscosity during filling. Compressed air causes problems. So the air in the mold must be evacuated by a vacuum pump. This is necessary to provide complete part filling as well as to prevent the burning of plastic by the compressed, hot air at the bottom of the cavity. 

The clamshell machines have only one arm. The same location provides mold loading, heating, cooling, and unloading. It uses an enclosed oven that also serves as the cooling station. 

This process resembles the open molding process except it is closed like a two-part compression mold (Chapter 14). A measured amount of plastisol is poured or pumped into the closed mold cavity, similar to close molding except that a slight pressure of about 5 psi (34.5 kPa) is applied. The mold is heated to fuse the plastisol then cooled. Later the mold is opened and the product stripped out. This process can provide for accurate thickness control, filling very complex shaped parts, and so on. 

Once the cavities are loaded, the clamp is closed and molding begins. Heat and shear initiates cross-linking, which controls the cycle time. The parts do not need to be cooled... 

Another type of rubber that is used frequently is thermoplastic rubber. Components are fabricated in a process that is similar to that used for common hard plastics, such as polyethylene or polystyrene, but the final product is an elastic material with properties otherwise equivalent to those of thermoset rubbers. No chemical reactions are involved in the processing of a thermoplastic rubber. The fabrication process consists of heating the rubber compound until it liquefies, injecting the liquid into a mold, cooling the mold, and finally removing the closure from the mold. The process is reversible. Closures can be remelted and remolded into different shapes or sizes as desired. 

The second polymer (sample E) was an ethylene-hexene copolymer which Is slightly branched, and had an My of 211,000 and a nominal density of 0.955 g/cm. Specimens approximately one millimeter thick were prepared from both polymers by molding flat sheets from which dumbbell-shaped bars were cut with a die. The sheets were molded by heating the polymer for ten minutes at 440K In a press at which time the heat was turned off and pressure applied. The polymer was then cooled In the press over... 

To avoid repetition of our company s trade name we will, in this chapter, use a generic name to describe the equipment and its process. We will call it the "Fluid Film Process". The fluid film process permits a high speed continuous treatment of sheet, strip, panel and billet material requiring cooling, heating, pressure confinement or shape and surface control. Many products presently manufactured in molds or platen presses can now be much more rapidly produced continuously on machinery incorporating fluid film processing. 

In the first molding step, half of the fluoroplastic resin was charged into the mold. Next, the bearer and core assembly was set in the mold. Then the other half of the resin was dispensed into the mold. The hold-down plate was used to compress the contents of the cavity while the mold was heated to a temperature above the melting point of the fluoroplastic resin. After cooling, the part can be removed and machined to the required specifications. 

The principle of injection molding is very simple. The plastic material is heated above its melting point, resulting in the conversion of the solid polymer to a molten fluid with a reasonably low viscosity. It is then forced into a closed mold that defines the shape of the article to be produced. There, the material is cooled until it reverts to a solid, then the mold is opened and the finished part is extracted. Although the principle may be simple, the practice of injection molding is anything but simple. This is a consequence of the complex behavior of plastic melts and the ability of the process to encompass complicated products. The essential mechanisms of injection molding are heat transfer and pressure flow. 

Injection molding is the most used technique for manufacturing end products directly from a thermoplastic raw material. In this process, articles are formed by injecting the molten polymer into a mold. This method is also used on rubbers and thermosets, though in these cases the mold is heated rather than cooled in order to promote the vulcanization or curing reaction [14-16]. 

While the heating cycle has virtually no effect on the properties of the finished part, the cooling rate determines part shrinkage, final density, brittleness, and other physical properties. Mold cooling is accomplished by the use of forced air and/or application of water spray. 

The process is often used with polyethylene and is limited to making open-ended containers. The mold which represents the exterior shape of the product is filled with powder. The filled mold is heated in an oven, causing the powder to melt and thus creating a wall of plastics on the inner surface of the mold. After a specific time to build the required wall thickness, the excess powder is dumped from the mold and the mold is returned to the oven to smooth the inner wall. The mold is then cooled, and the product is removed. The product is strain free, unlike pressure-molded products. In polyethylene this is especially significant if the product is used to contain oxidizing acids. 

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