Rotational moldings

Rotational molding is used to make hollow objects. In rotational molding, a carefully measured amount of powdered polymer, typically polyethylene, is placed in a mold. The mold is then closed and placed in an oven where the mold turns about two axes as the polymer melts, as depicted in Fig. 3.73. 

Modem Plastics Encyclopedia, volume 53. McGraw-Hill, New York, 1976. 

Crawford. Rotational Molding of Plastics. Research Studies Press, Somerset, 1992. 

Denton. The mechanical properties of an smc-r50 composite. Technical report, Owens-Coming Fiberglass Corporation, 1979. 

Elemans. in Mixing and Compounding of Polymers, I. Manas and Z. Tadmor (Eds.) Hanser Publishers, Munich, 1994. 

Rotational molding is an economical process used to produce medium-to large-sized hollow parts from polymeric materials. It is a shear- and pressure-free process with processability of limited polymeric materials. Viscosity, surface tension, and thermal properties of polymer influence the sintering behaviors. Changes in economy and competition with other process methods shift from these techniques to rotational molding. The type of materials could be used in the process is mainly limited to poly-ethylenes [1]. 

Typical applications of rotational molding include chemical tanks, automotive and commercial aircraft parts, backyard play equipment, toys, etc. Rotational molding requires long cycle time, which puzzles the overall success of the technology. The cycle times are restricted by the time required to heat up and cool down the mold and the product by the slow heat convection process. 

Polymer melt viscosity is the most important rheological property, which is dominant in sintering. Increased viscosity lowers the mobility of the melt, resulting in the reduction of sintering rate, which increases the cycle time of the process. 

Melting, percentage crystallinity, and crystallization point of the polymer strongly affect the onset of sintering. Crystallinity affects the mechanical properties of the end product while processing with rotational molding. 

Mold charged with Powdered Polymehc matehal 

Fluoropolymers can be fabricated into hollow objects by rotomolding. They are, however, hard to mold due to their relatively high melting point andmelt viscosity. The most frequently rotomolded fluoropolymers are PFA, FEP, PVDF, ETFE, and ECTFE. Fluoropolymers constitute about 1% of all rotomolded parts. 

Increased stock temperature Increased mold temperature Increased part thickness Increased injection pressure Filler addition Increased polymer orientation Straight melt flow during molding 

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. 

Resin and fillers of different density are separated by rotational forces. Even when a mixture of different particle sized resin is used, the finer particles manage to sift through the larger particles to the mold surface. If a material of higher density such as filler is added, this will, by itself, enhance the movement of heavier particles to the mold surface to cause cross-sectional inhomogeneities in the composition. This has been confirmed by studies involving incorporation of 

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). 

Mold This process is particularly suited economically to producing small production runs and large-sized products, because molds are not subjected to relatively any pressure during molding and inexpensive thin sheet metal molds can thus be used in many applications. Lightweight cast aluminum and 

Cost When it is necessary to equal the production rates of other processes, the mold cost with RM may exceed that of other processes such as flow molding. The plastics used in RM are generally more expensive than the pelleted plastics used in many other processes, because they must be more finely and evenly powdered, such as to a 35 mesh. However, this process generates low levels of regrind or scrap, even when it is operating poorly. Products can have no flash at all if properly designed molds are used. 

Some combinations of materials are not feasible with this method. For instance, after molding the first layer against the mold wall, the second material cannot have a higher melt temperature, which, of course, would melt the first layer, probably causing them to mix. 

With RM one inherent overall disadvantage exists. It is that the complete cycle for a single mold is significantly longer than it is for many other processes. However, in many cases it is possible to run multiple molds on each arm or arms, to offset the effect of hav- 

Products in this process can have deep sections and relatively sharp comers. However RM flat, particularly large relatively uniform wall thickness surfaces are difficult if not impossible to produce. This process can be used to mold complex products that may require three or four split molds. Also, different finished surfaces are obtained. For example, the products surface finish is dictated by the inside surface of the mold. This makes it easy to obtain smooth as well as textured surfaces on the product. Raised or depressed letters, fluting, and other decorative inscriptions may also be molded. 

A measured amount of powder or liquid thermoplastic (TP) is placed in the cavity that is mounted on a turret arm capable of rotating the mold. The mold in the oven spins biaxially with rotational speeds being infinitely variable, usually ranging up to 50 rpm on the minor axes and 12 rpm on the major axes. A 4 1 rotation ratio generally is used for symmetrically shaped parts. A wide variety of ratios are necessary for molding unusual and complex shapes. 

This mold action permits uniform distribution of the plastics that is forced against the inside surface of the cavity. Following a prescribed cycle, the heat of the oven fuses or sinters the plastic and goes into the cooling chamber. The solidified product is removed from the mold and the cycle is repeated. This process permits molding very small to very large products. To improve product properties, hasten product densifi-cation, reduce air voids, reduce cure time, etc. 

Elements Rotational Molding Blow Molding Thermo Forming 

Orientation in part none high very high 

Part detailing ok very good good, with pressure 

RM is a simple, basic, four-step process that uses a thin-walled mold with good heat transfer characteristics. This closed mold requires an entrance for insertion of plastic and, most important, the capability to be opened so that cured parts can be removed. These requirements are no problem. Liquid or dry-powder plastic, equal to the weight of the final part, is put 

The next step involves heating the mold while it is rotating. Molds can be heated by a heated oven a direct flame a heat transfer liquid, which is either in a jacket around the mold or sprayed over the mold or electric-resistance heaters, placed around the mold. With uniform heat transfer through the mold, the resin melts to build up a layer of molten plastic on the mold s inside surface. 

After the required heat-time cycle is completed, the mold is ready for cooling, which is accomplished with the mold rotating continually. Cooling is usually done by air from a high-velocity fan or by a fine water spray over the mold. 

After cooling, the final step is to remove the solid part and reload the mold with plastic. 

This process is capable of molding small to large hollow items with very uniform wall thicknesses, using certain plastics. Production rates, compared to those of other processes, can be low. However, the total cost of equipment and the production time for moderate-sized and, especially, large parts are also low. Large parts range up to 22,000 gal (85,200 L) in size, with a wall thickness of 1.5 in. (0.6 cm). One tank used 5,300 lb of XHDPE the first charge was about 3,300 lb, followed by 1,000 lb and finally another 1,000 lb. 

Chemical Engineering Department, Alicante University, Spain 

Rolallonai moulding Injection moulding Bloivy moulding Film exirusion Sheet exirusion Tube exirusion 

In respect to markets and applications, they vaiy from countiy to country. In the USA over the 44% of all goods produced by the rotational molding sector are in the Toys and 

Complexity limited to large, hollow parts of uniform wall thickness. 

Large flat surfaces should be avoided due to distortion and difficulty to form. Use stiffening ribs. Internal walls need to be well spaced. 

Molded-in holes, bosses, finishes and lettering all possible at added cost and limited accuracy. With rotation speed variation, can build up thicker layers at key points in the mold. 

Sharp corners difficult to fill in the mold. Radii shouid be as generous as possibie (greater than five times the wall thickness) and tend to become thicker than the waii thickness on moiding. 

Metal or higher-melting point plastic inserts can be molded-in. 

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Rotational Molding. Large containers and some toys are manufactured with a specialized technique caHed rotational mol ding. A rotational mol ding machine contains several large metal molds that can be rotated in two perpendicular planes. A load of fine LLDPE particles is introduced into each mold, and the mold assembly is transferred into an oven. Inside the oven, heated molds rotate at speeds ranging from 10 to 40 rpm. The polymer powder melts and is uniformly distributed on the internal surface of each mold. After the container is formed, the mold assembly is removed from the oven and cooled at which point the plastic containers are removed. 

Rotational Molding. Hodow articles and large, complex shapes are made by rotational mol ding, usuady from polyethylene powder of relatively low viscosity (57—59). The resin is in the form of a fine powder. A measured quantity is placed inside an aluminum mold and the mold is heated in an oven and rotated at low speed. The resin sinters and fuses, coating the inside of the mold. The mold is then cooled by water spray and the part solidifies, dupHcating the inside of the mold. 

Rotational molding is used to form large shells of thermoplastic resin and chopped strands for such appHcations as agricultural tanks and fertilizer hoppers. The resin and chopped glass are placed in the metal mold that is then rotated in an oven where the thermoplastic resin melts and deposits the fiber on the metal surface. When cooled, the mold is opened and the part is removed. 

PVC Plastisols 1940 M-H VG G G G Slush and rotationally molded, foamed, extruded Used in coating machines to cover paper, cloth and metal... 

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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. 

EngrDesHndbk, Rotational Molding of Plastic Powders , AMCP 706-312(1975) 48) Anon,... 

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