Materials rotational molding

Materials. Rotational molding is restricted to the use of thermoplastics that are specially formulated to woric well with the process. The normal materials of choice are primarily olefins (polyethylene or polypropylene), although PVC, polycarbonate and nylons are also used. 

Important are behaviors associated and interrelated with plastic materials (thermoplastics, thermosets, elastomers, reinforced plastics, etc.) and fabricating processes (extrusion, injection molding, blow molding, forming, foaming, rotational molding, etc.). They are presented so that the technical or non-technical reader can readily understand the interrelationships. 

Beall, G. L. Rotational Molding Design, Materials, Tooling and Processing. (1998) Hanser Gardner Publications, Cincinnati. 

Brookfield, Ct., SPE, 2002, Paper 192, Session M38-Rotational Molding Materials, pp.5, CD-ROM, 012 INVESTIGATION OF THE PROCESSING CHARACTERISTICS AND MECHANICAL PROPERTIES OF METALLOCENE POLYETHYLENE FOAMS FOR ROTATIONAL MOULDING... 

In jiggering, machines press the preceramic material into a rotating mold of desired shape. Dinnerware products are often made using jiggering. 

In rotational molding, also known as rotomolding, the mold (or cavity) is filled with material, either as a solid powder or liquid. The mold is closed, placed in a heated oven, and then rotated biaxially. The mold is then cooled, opened, and the article recovered. Powders of about 35 mesh (500 pm) are typical though different sizes are also employed. The distribution of particles and additives is determined by the mixing/rotation ratio. 

Figure 7.81 Schematic diagram of rotating mold system used in continuous extrusion blow molding. Reprinted, by permission, from A. B. Strong, Plastics Materials and Processing, 2nd ed., p. 489. Copyright 2000 by Prentice Hall, Inc.

The organic cellulose ester plastics are versatile materials and can be processed by almost any hot-processing technique used for thermoplastics. The principal techniques for all three plastics are injection molding and extrusion. Blow molding is also possible. Butyrate and propionate powder are used in tluidized-bed and electrostatic coating processes, as well as in the rotational molding process. 

One of the larger uses is for wire and cable insulation. The flame resistance makes this the material of choice for residential wiring, extension cords, and so on. Inexpensive garden hose represents another large-volume use. Sports balls can be produced by rotational molding of plastisols. The low fabrication cost allows such products to dominate the lower-price-range market.76-77... 

The coalescence of polymers is driven by the work of surface tension, which counteracts the viscous dissipation associated with the molecular diffusion within the coalescing domain. This phenomenon is often referred to in the literature as polymer sintering. In the rotational molding process, coalescence occurs at temperatures above that of the material melting point when dealing with semicrystalline polymers, or above the glass transition temperature for amorphous resins. The first analytical model describing the coalescence process was proposed by Frenkel ... 

The validity of Frenkel s model is limited to Newtonian flow and can only be used to predict the early stage of the coalescence process, when the diameter of the two spherical particles remains nearly unchanged. The inadequacy of a Newtonian model in describing the coalescence of polymers was also demonstrated in other studies, as reviewed by Mazur, and has led to the development of models as well as alternative methods for the characterization of the coalescence behavior of polymers for rotational molding applications.Based on theoretical and experimental analyses of the coalescence phenomenon, the material properties of primary interest in the evaluation of resin coalescence behavior in rotational molding have been identifled as the resin viscosity, surface tension, and elasticity. 

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. 

In addition to zero-shear viscosity, material elasticity plays an important role in determining coalescence behavior in the rotational molding process.For rotational molding grade material, this effect was first recognized from the experimental results obtained with impact modified polypropylenes and is illustrated in Fig. In this work, the relative elasticity of resins... 

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