Rotational Molded Products

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- valled, hollo v 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. 

What types of products are produced by rotational molding Why Give three examples. 

What are the advantages of rotational molding over injection molding What are the disadvantages  

What issues arise when using multi-armed rotational molding machines when each arm produces a different sized part  

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

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Rotational molding of vinyl plastisols is similar to the rotational molding of powder or liquid plastics (Chapter 13). Plastisol is placed in the mold and heated while the mold is being rotated. Molded products can include tanks, volleyballs, basketballs, doll heads and bodies, and various automobile parts. 

However, carbon black has been used with success as a filler in rotationally molded products. At concentrations of up to 2.5 wt% it improves the weathering resistance of the product. Since dimensional stability and the shape of articles would benefit Ifom the use of fillers and fibers, it is probable that they will start to be used in the future. When this happens, fillers will be introduced in a premixed form to assure homogeneous distribution (forces normal to the surface of product cannot cause the movement of particles towards the mold surface when particles are premixed with polymer). 

The cycle time for the manufacture of a rotation molded product can be between five to forty minutes, depending on various factors. Important influencing factors are the type and size of the molds, machine performance, material choice, and wall thickness. 

Release agents are found to affect shrinkage and warpage of rotationally molded products.There are many reasons for warpage, including polymer crystallin-... 

Rotationally molded products have walls of uniform thickness that are free of orientation and weld lines. As such they are resistant to environmental stress cracking and are therefore suitable for use in large-scale chemical containment uses. Linear low density polyethylene resins excel in such applications. Storage tanks of up to several thousand gallons capacity are used in agricultural, domestic (septic systems), and industrial applications. On a smaller scale, rotationally molded fuel tanks (chemically treated to reduce diffusion) are used on tractors, cars, and small gasoline-powered appliances. 

HoUow mol ding as the name implies is a molded product with a hoUow center such as Easter eggs, bunny rabbits, and Santa Claus. The molds used in hoUow mol ding are divided in two halves and connected by a hinge. Chocolate is deposited into one half of the mold. The mold is then closed and rotated so that the Uquid chocolate completely coats the inside surface of the mold. After cooling the molds are opened and the piece removed. 

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. 

For the folloiving products manufactured by rotational molding, indicate whether they would be made with a rock-and-roll or standard rotational molding machine ... 

In South Africa, owing to the diversity and small production volume of specific motor car models, it is not economically feasible to blow-mold HDPE fuel tanks. The only cost-effective plastic tank manufacturing process used in South Africa is rotational molding, using LLDPE with a density of 0.939 g/cm3. Some selected properties of LLDPE before and after petrol exposure are compared to HDPE in Table 16.2. It is clear that owing to weaker mechanical properties, LLDPE is far less suitable than HDPE for petrol containment, mainly because of its lower crystallinity. 

Patent Number US 6103153 A1 20000815 PRODUCTION OF FOAMED LOW-DENSITY POLYPROPYLENE BY ROTATIONAL MOLDING... 

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

The Rotosil process employed by Heraeus and Heraeus-Amersil is used for the production of tubular or cylindrical shapes. It permits greater uniformity and dimensional control than the previous process. The quartz is washed in hydrofluoric acid and distilled water to remove impurities which can promote crystallization during manufacture. The sand is then placed in a rotating horizontal steel tube where it is held at the circumference by centrifugal force. A carbon arc is passed slowly down the center of the drum to fuse the sand. A modification, using open rotating molds on a vertical axis, allows formation of cmcibles, beakers, and bowls. 

Specialized training is an absolute requirement for technical service personnel. A typical example is a person involved in supporting a polymer for which the use is the manufacture of rotationally molded consumer products. The technical service person is expected to be reasonably familiar with topics such as polymer rheology evaluations, gel-permeation chromatography, rotational molding, color science, regulatory requirements for use, mechanical and photochemical behavior of the pigmented polymer, optics, and so forth. Expertise of this variety cannot be expected to be obtained without careful... 

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