Rotational molding cycle

Figure 3.74 presents the air temperature inside the mold in a typical rotational molding cycle for polyethylene powders [7],... 

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 molding 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. 

Most practitioners deflne the flow behavior of polymers based on the melt flow index however, this property is not entirely relevant to the rotational molding process because it is essentially a shear-free and pressure-free process. The use of zero-shear viscosity has been proposed as a better way to assess the coalescence behavior of resins. Resins with lower zero-shear viscosity coalesce at a faster rate and can thus be processed using a shorter molding cycle.The coalescence of individual powder particles is initiated as the particles stick and melt onto the mold surface or melt front. As the melt deposition process continues, pockets of air remain trapped between partially fused particles and lead to the formation of bubbles. In the rotational molding process, the coalescence of particles occurs at a temperature range close to the melting point of the material thus, from a processing standpoint, low values of zero-shear viscosity at low temperatures (i.e., close to the temperature at which the particles adhere to the mold surface) are preferable. 

Fig. 4 (A) Stress relaxation curve of rotational molding grade ethylene copolymers. (B) Part density, which is indicative of the bubble content, measured after interrupting the rotational molding heating cycle at different heating times. (From Ref.. )...

Heat transfer in the rotational molding process was first modeled by Rao and Throne. Since this initial attempt at modeling the heating cycle, several studies... 

Gogos, G. Liu, X. Olson, L.G. Cycle time predictions for the rotational molding process with and without mold/part separation. Polym. Eng. Sci. 1999, 39 (4), 617-629. 

With rotational molding, a closed mold is rotated in two or more planes during fusion or solidification of the fluid polymer system. The action of the rotation deposits an even coating of polymer upon the inside of a "clam shell" type mold. After a predetermined time under heat, the molds are removed from the oven and cooled, and the finished product is removed whereupon the cycle is started again. Conventional gas fired ovens or hot salt bath sprays are used for heat transfer, and cooling is usually effected by spraying or immersion in cold water. This technique is well suited to molding completely hollow objects. 

Example 11.8 Explain the variation in the physical properties of rotational molded parts from polypropylene and polystyrene shown in Table El 1.8 with changes in the cooling cycle... 

Blow molding is one of three popular methods used to obtain hollow products from plastics. The other two methods include thermoforming and rotational molding. Blow molding is expected to produce parts in a very short cycle with little labor. The method also uses the highest pressmes which require materials having high stability. One of the... 

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