Rotational molding processing

Rotational molding processes rely on a few resins and many different additives to manufacture a wide variety of different products. The choice of both the resin and the additives depends on the end use requirements and the processing conditions. 

Polyethylene is, by far, the most commonly used resin in rotational molding processes. Low density, high density, and linear low density polyethylene are all used extensively. Additionally, crosslinked polyethylene is used for parts that require high chemical or heat resistance or enhanced impact resistance. 

Stabilizers counter the effect of the high temperatures and the oxygen rich atmosphere experienced by the resin during the rotational molding process. Since some rotational molded parts require up to one hour of residence time in the oven, such stabilizers are essential. Without them, the polymer would lose its inherent properties, becoming unfit for the final application. 

Why is the feedstock for rotational molding processes either a finely divided powder or plastisol ... 

Why do the resins used in rotational molding processes typically contain a higher level of antioxidants and thermal stabilizers than the resins used to manufacture parts, by injection molding ... 

Patent Number US 6083434 A1 20000704 ROTATIONAL MOLDING PROCESS FOR PRODUCING FOAMED ARTICLES... 

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. 

With the exception of two fluoropolymers, PVF and PTFE, the rest of the resins described in this entry can be processed by standard melt-processing techniques, such as injection, transfer and blow molding, extrusion, and rotational molding. Process equipment for fluoropolymers must be made from corrosion resistant alloys because of the corrosive compound that may be produced when fluoropolymers are heated above their melting points. Higher melt viscosity of these resins may require more powder and higher pressure rating equipment. 

The rotational molding process is inherently transient, and the selection of optimal molding conditions will... 

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 ... 

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... 

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. 

Fillers are not used to any extent in products made by the rotational molding process. Rotational molding is dominated by polyethylene (close to 90% volume) to which even small addition of pigments or fillers (less than 2 wt%) causes a decrease in tensile and impact properties of the products manufactured in this process. Polyethylene is vulnerable to environmental stress cracking which is made worse if fillers are present. 

The rotational molding process takes place in two stages. During the first stage the polymer melts, sinters, and densifies, while in the second stage the melt flows on the walls of the mold and fills all channels, undercuts, and other cavities in the geometry of the... 

Rotational Lining - Rotational lining is a process by which a hollow object is lined with a plastic. The surface of the part, contrary to rotational molding process, is prepared to adhere the liner to the mold wall. See also Rotational Molding. 

Rotational molding (process parameters) oven temperature °C 364-391... 

Rotational molding. The rotational molding process, sometimes referred to as rotomolding, can produce parts ranging in size from small balls to enormous containers. The principal material used in rotational molding is polyethylene, however, some nylon parts are also made. For simple... 

Some of the important variables that need to be carefully monitored during the rotational molding process are ... 

Although the rotational molding process is carried out with very little shear on the polymer melt, the cooling rate of the polyethylene after the fabrication step remains important in order to avoid excessive shrinkage and warpage. This is due to the crystallization process that takes place as the polymer melt cools from an amorphous, less dense state to a solid, semicrystalline state, higher density solid. 

The viscosity of the polyethylene material at very low shear-rate is an important parameter for the type of polyethylene used in the rotational molding process because of the very low shear rate that is present during the molding process. This property is referred to as zero-shear viscosity of the resin. 

Table 6.19 shows some typical resin specifications for polyethylene suitable for the rotational molding process. The relatively higher Melt Index... 

LyondellBaseh, Equistar Division,A Guide to Rotational Molding, (concise description of rotational molding process), www.lyondellbaseh.com. 

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