Rotational-molding speed

Rotational Molding. Large containers and some toys are manufactured with a specialized technique caHed rotational mol ding. A rotational mol ding machine contains several large metal molds that can be rotated in two perpendicular planes. A load of fine LLDPE particles is introduced into each mold, and the mold assembly is transferred into an oven. Inside the oven, heated molds rotate at speeds ranging from 10 to 40 rpm. The polymer powder melts and is uniformly distributed on the internal surface of each mold. After the container is formed, the mold assembly is removed from the oven and cooled at which point the plastic containers are removed. 

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. 

Multiple arm machines rotate molds into the different stations of the process. A three-arm machine would, for example, have one mold filling while another is heating in the oven and the third cooling in preparation of demolding. This is very efficient but requires that each stage takes the same time to accomplish. When there are three different parts, the largest part or the one with the thickest walls defines the speed for the entire process. 

In rotational molding of anionically polymerized E-caprolactam one can produce vessels up to 10 m3, fuel tanks of various shapes, and other seamless large-sized items. As a rule, the mold rotates with different speeds about the two axes in such a way that the reactive mixture flows to the mold walls with minimal net centrifugal force. When molding long items, a greater difference in rotation speeds is required than when molding round items. 

This section covers melt temperature and profile, screw rotation, injection speed and pressure, mold temperature and back pressure, cycle management, clean-out procedure, and shutdown and startup procedures. 

Rotational molding is a reasonably safe industry. Most resins used are nonhaz-ardous. Some resins may cause mild irritation suppliers provide safety information on their handling and use. Most resins used for rotational molding are ground to 500 /xm (35 mesh), and appropriate procedures should be followed to prevent a dust explosion and fire. A rotomolder is a relatively safe piece of processing equipment fans are enclosed with guards spindles and arms operate at a low speed. The oven is equipped with a temperature control. 

Rotation, Umdrehung, Drehbewegung rotation speed adjustment Drehzahlregelung rotational barrier Rotationsbairiere rotational casting/ rotational molding/ rotonnolding... 

The bulk of materials ground for rotational molding are polyethylenes (95%+ of the market). Polyethylene is a relatively tough material that can be difficult to divide. The most common method of pulverizing is performed using high-speed attrition mills that grind pellets approximately 3/16-1/4 in. (5-6 mm) in diameter down to the required particle size-distribution. 

Rotational molding creates some unusual effects that are not encountered in other molding techniques. For example, outside comers of the part are thicker (Fig. 9.9) than the general thickness of the part because the resin powder accumulates in the comers of the mold. On the other hand, inside comers are thinner because the powder tends to fall away from the mold in these areas. The recommended comer radii (Fig. 9.9) help the uniformity of the part thickness. Altering the rotation speeds around the major and minor axis (discussed later in this chapter) can control wall thickness variations. 

The axes in rotational molding have different lengths. The speed of rotation of the mold and the ratio of the two axes are very important variables of the rotational molding process. Rotation ratio is the ratio of the speed of the mold around the two axes according to Eq. (9.1). Speed is a function of the flow properties of the polymer and the ratio is related to the shape of the part. Shorter cycles and thinner part walls amplify the sensitivity of these two variables. 

The quality of the casting may be improved by eentrifugal casting, where a cylindrieal mold is spim to align reinforeing fillers and force air to the interior. Rotational molding is also used, where the mold is spun in three axes. Molds may also be heated to speed up euring. 

Molten metal is poured into a high-speed rotating mold (300-3000 rpm depending on diameter) until solidification takes place. The axis of rotation is usually horizontal, but may be vertical for short work pieces (see 1.5F). 

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