Rotational molding pressure

Rotolining ETEE Rotationally molded Pressure ok Max size Visual By hot patching... 

Rotational molding creates a wide variety of plastic products that cannot be made effectively, efficiently, or economically by other means. What sets this method apart from others is that it can create thin-walled, hollow parts that exhibit no weld lines or scarring from ejector pins and from the process itself. It also has the advantage of having little scrap and minimal molded-in stresses, due to the low pressure and low shear rate characteristics of the process. Finally, it can be used to make parts that are very large which would be impossible to manufacture by other methods. 

Molding employs a mobile prepolymer that may be thermoset or using a thermoplastic polymer. The polymer can be injection-molded (often for solid objects), blow-molded (for hollow objects such as bottles), rotation-molded, compression-molded, transfer-molded, or thermoformed. Casting is closely related to molding, except that the pressure is typically not used. 

Fig. 3. Core torque related to channel length versus angular velocity of core rotation (O) at different specific molding pressures (MPa/m) 1-5 2-30 3-45 4-60...

ABS structural foam can be processed by injection molding, through conventional or low pressure injection machines (Chapter 4) by expansion casting in rotational molding machines (Chapter 13) or conveyorized oven systems or it can be extruded into profiles through conventional extruders (Chapter 5). 

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. 

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. 

Crawford, R. Cramez, M.C. Oliveira, J.J. Spence, A. The importance of monitoring mold pressure during rotational molding. Proceedings of the Society of Plastics Engineers Annual Technical Conference, San Francisco, CA, May 5-9, 2002 The Society of Plastics Engineers Brookfield, 2002. 

Rotolining ETFE PVDF ECTFE All up to 250 mils (normal thickness 186 mils) Rotationally molded No seams No primer Shop only Pressure OK Limited Vacuum ability Max size 8x22 Visual Spark By hot patching Testing recommended... 

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