Rotational molding wall thickness

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. 

Wall thickness uniformity Incorrect mold rotation because of equipment problems or the wrong rotation ratio Variation in the thickness of the mold wall resulting in uneven heat transfer, thus improper melting and fusion rate of the resin Variation in heat transfer to the mold... 

The molds are mostly made of steel or aluminum. Since the forces during rotational molding are relatively small, wall thicknesses of 1.5 mm will be sufficient. 

RRM can produce uniform wall thicknesses even when the product has a deep draw from the parting line or small radii. The liquid or powdered plastic used in this process flows freely into corners or other deep draws upon the mold being rotated and is melted/fused by heat passing through the mold s wall. 

Table 14.3. Initial viscosity, minimum viscosity, n =- rate of viscosity increase, At /AT, temperature at which viscosity is equal to 20 Pa.s, T20 ( C), and deviation in part wall thickness for moldings obtained at 230°C during 8.5 min and rotation rate of...

Referring again to Fig. 8.31, the minimum wall separation (X) should be no less than 5 times the wall thickness (W) (X > 5WO, except for extreme situations when 3 times the wall thickness can be used. However, tack-offs (P, Q, R) used to stiffen the part are feasible. The combined wall thickness (S) should be 1.75 times the wall thickness (S = 1.75W0. Inserts, such as the one at A can also be used in rotational molded parts. The recommended draft angles will vary according to the material as indicated in Table 8.15. 

Holes can be molded-in rotational molded parts by using core pins to which the resin does not adhere. Bosses can be molded by molding a raised cylinder such as that at T. The tip of the cylinder is then cut off to leave an opening. The diameter of this type of hole should be at least 5 times the nominal wall thickness (D > 5W). Threads, both inside and outside, such as the one at S, are readily rotational molded. Recommended tolerances are provided in Table 8.17 (see Fig. 8.37). 

The disadvantages of rotation molding include material costs that are relatively high because most materials are produced as pellets and then must be reduced to a fine powder. The process is not suited for production of parts with wall thickness of less than 0.030 in. It is also not suited, generally, for large production runs of small parts. 

Wall distribution is determined by the rotation ratio. The fewer rpms in a certain ratio, the more uniform the wall thickness. Resin, molds, and final application must be taken into consideration to establish the most efficient cycle. 

Large hollow products having uniform wall thicknesses can be produced by rotational molding. Objects having capacities as large as 8.52 X 10 m with wall thicknesses of 6 X 10 m can be molded. Process units for rotational molding operations are not costly. 

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