Rotational-molding venting

Venting System. Most rotational molds require a venting system to remove the gas that develops in the heating cycle. Depending on the mold size, vents range from 3 to 50 mm in diameter. A 12.5-mm-i.d. tube is used for an article of 0.76-m volume. 

Through heating in the oven area, the air expands inside the mold. Conversely when cooling, the air contracts inside the mold. For this reason, it is essential to provide the rotational molds with a venting unit because the over- and underpressure negatively affect the service life of the mold and the quality of the plastic... 

Centrifugal casting is used to produce parts with fine detail relatively free from included air. The mold is attached to a horizontal spinning disk which rotates at a speed of 5 to 30 m/s. The mold must be capable of having the polyurethane added from the center of the disk. The centrifugal forces move the denser polyurethane to the circumference of the mold and the air moves to the center. Some air vents may be required, as air can be trapped on horizontal surfaces. 

Heat transfer, from the hot air in the cavity and from the wall of the mold to the powder, melts the polymer which sticks to the hot cavity (Fig. 6.42). It results in the formation of an even coating on the interior wall of the cavity. The mold is cooled while it is rotated to retain the uniform wall thickness. Finally, after the polymer has solidified, the rigid part is removed from the mold. Volatile material formed during the heating period is removed from a vent port. The rotation rate, temperature, heating rate, and cooling rate are controlled throughout the process. 

During the process, a number of key factors must be considered to ensure that the process is effective and economic venting is used to ensure that pressure inside the mold is in equilibrium with the external environment release agents must be used for most materials to ensure that they do not stick to the mold surface rotation settings must be carefully selected as the relationship between the primary and secondary axes affects the way in which the material is distributed in the final part and only the external surface of the part is in contact with the mold, particularly in the case of semicrystalline materials, such as polyethylene, which means that cooling rates must be controlled to minimize distortion while accelerating the process. 

Bridging of the resin in the mold Poor resin powder flow. Slow rotation rate. Radii of the undercuts, ribs, and other recesses are too small. Air trapped in the recesses, consider venting in tiiese locations. 

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