Rotational molding continued temperature

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. 

Rotational molding, rotomolding, or roto-casting is a production process to form hollow parts of limitless size wherein powdered resin and glass fibers are charged into a split mold. The mold is then continuously rotated in a biaxial mode, in a high-temperature environment to above the resin melt temperature. When the plastic material has covered the inside of the mold and densified, the mold assembly while still rotating is cooled to room temperature. The rotation is stopped and the part removed. 

As the mold rotates past the extrusion-injection die head, the melt flows onto the circumference of the mold and is forced by the melt pressure into the cavities. Enough additional melt is supplied to create the base strip at the same time. By the time the mold has completed about one-half a revolution, the plastic has been cooled and solidified. Mold temperature is controlled by an air plenum located about one-quarter of a revolution before the injection point the molded Velcro itself is cooled by another plenum ahead of the strip-ofFpoint. The airflow is adjusted to keep the nylon (usual plastic used) sufficiently warm and flexible so that the projections can pull free from the undercuts in the cavities without damage during strip-off See extruder injectionmolding, continuous mold zipper, velocity See calculus. 

Particularly in automotive applications, the design engineer selects materials that will endure periodic fluctuations of both tensile and flexural type loads for the service lifetime of the molded part. Under-the-hood automotive conditions include extremes in ambient temperature combined with possible localized vibrations from the engine and rotation of molded parts (cooling fans) under continuous load. Although deformation will occur due to creep, there is concern regarding fatigue type failure after repeated fluctuations of the applied load. 

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