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6. ROTATIONAL MOULDING

6. ROTATIONAL MOULDING. CORPORATE TRAINING AND PLANNING. INTRODUCTION. Rotational moulding is a process of making hollow articles. The part is formed inside a closed female mould. In this process the mould rotates biaxially during heating and cooling cycle.

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6. ROTATIONAL MOULDING

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  1. 6. ROTATIONAL MOULDING CORPORATE TRAINING AND PLANNING

  2. INTRODUCTION • Rotational moulding is a process of making hollow articles. • The part is formed inside a closed female mould. • In this process the mould rotates biaxially during heating and cooling cycle. • Rotational moulded pieces are stress free because the pieces are produced without any external pressure. CORPORATE TRAINING AND PLANNING

  3. The ability to manufacture large containers of capacity 30,000 gallons as well as small items like golf ball is responsible for the growth of this process. • The Process requires relatively in expensive equipment and exerts on only small pressure on the material being formed. CORPORATE TRAINING AND PLANNING

  4. PRINCIPLE • The principle of the process is that finely divided plastic material becomes molten when comes in contact with hot metal surface of the mould and takes up the shape of that surface. • As only female mould is used, the only pressure exerted are those induced by gravity and centrifugal force. CORPORATE TRAINING AND PLANNING

  5. The polymer is then cooled while still in contact with the metal mould to get the solid copy of the surface. • Rotational moulding permits to make a wide variety of fully and partially closed items. CORPORATE TRAINING AND PLANNING

  6. CORPORATE TRAINING AND PLANNING

  7. CORPORATE TRAINING AND PLANNING

  8. ADVANTAGES AND DISADVANTAGES Advantages • The major advantage of rotational moulding as compared to other plastic moulding processes is that it can make very large parts. • It requires comparatively low cost input. • The products are stress free with strong out side corners. There are no weldlines, sprue or gate marks. CORPORATE TRAINING AND PLANNING

  9. Here impact toughness is improved and failure due to brittleness is reduced • The external dimensional details can be easily moulded with better surface glossiness . • The colour changes in the product can be made easily.Similarly mould changes can also be done rapidly. • Multilayer moulding is also possible for providing chemical resistance and strength to the part. CORPORATE TRAINING AND PLANNING

  10. Good control over wall thickness variation is also achievable as compared to blow moulding or thermoforming. • Moulding can be done with metal inserts and minor undercuts. • No scrap or very little scrap is produced. • Low tooling cost. CORPORATE TRAINING AND PLANNING

  11. DISADVANTAGES • The moulding cycles are longer compared to blow moulding and thermoforming. • In case of big parts loading and unloading is very labour intensive. • The process is not suitable for parts with wall thickness less than 0.03”. • The conversion of plastic granules to powder form increases the equipment and process cost. CORPORATE TRAINING AND PLANNING

  12. LIMITATIONS • It is an open moulding process and so there are no cores inside the hollow parts • Surface details and dimensions can only be provided and controlled on the side of the part. • The process requires heating and cooling of not only plastic material but also the mould as well. • The long heating cycle increase the possibility of thermal degradation. CORPORATE TRAINING AND PLANNING

  13. It is not suitable for materials with less heat resistant to withstand the long heating cycle. • The material must be capable of being pulverised into fine powder that flows like liquid. • Removal of plastic sticking onto the surface of cavity requires careful application of mould release agent. CORPORATE TRAINING AND PLANNING

  14. ROTATIONAL MOULDING PROCESS • Loading • Heating & Moulding • Cooling • Unloading CORPORATE TRAINING AND PLANNING

  15. CORPORATE TRAINING AND PLANNING

  16. LOADING • This step includes weighing of the charge for a particular product then transferring it to the open cold mould. • The mould surface usually coated with a mould releasing agent. • The raw material can be in the form of powder or liquid state. CORPORATE TRAINING AND PLANNING

  17. The wall thickness can be controlled by varying the amount of raw material charged. • After the material is charged the mould is closed and clamped to the arm of the machine. • Then the mould is moved to an oven for heating CORPORATE TRAINING AND PLANNING

  18. HEATING & MOULDING • The mould fixed to the arm now moved to a closed chamber where it undergoes intense heating. • During heating the mould rotates in two planes perpendicular to each other. • The rotational speed varies in the range of 0-40 rpm on minor & 0-12 rpm on the major axis. • 4:1 ratio is the most commonly used for symmetric article. CORPORATE TRAINING AND PLANNING

  19. For moulding unsymmetrical products a wide variability of ratios is necessary. • The revolving motion distributes the plastic material uniformly over the inside surface of the mould. • The plastic material fuses into layers to form a hollow article. • In case of hot air oven the temperature should be between 200ºc to 500ºc. CORPORATE TRAINING AND PLANNING

  20. The moulding cycle time varies from 2 to 20 minutes depending upon the wall thickness of the article. • The wall thickness can vary from 2 to 12mm or more. • The heating chamber should be large enough to house the mould and rotate it freely. CORPORATE TRAINING AND PLANNING

  21. COOLING • For cooling the mould is transferred to the cooling station while still rotating. • The cooling should be made as quickly as possible to avoid the plastic part to shrink away form the mould. • Otherwise the part will get distorted. • Cooling can be done by air or water. To provide faster cooling cold water is sprayed over the mould. CORPORATE TRAINING AND PLANNING

  22. UNLOADING • After cooling the mould is transferred to the unloading station. • In this step the mould is opened and the cooled part is taken out. • It can be done manually or with mechanical assistance. • The ejection can also be done by forced air. • The mould is cleaned and the charge is loaded for the next cycle. CORPORATE TRAINING AND PLANNING

  23. HEATING SYSTEM IN ROTATIONAL MOULDING • The rotational molding process heats and cools both the mold and the plastic material. • Cavities are build up with materials having high thermal conductivity, in order to minimize the time required for heat to pass through the wall of the cavity. CORPORATE TRAINING AND PLANNING

  24. Rotational molds may be heated by either an open-flame method, a hot air re-circulating oven method, or by a hot-oil jacketed mold system. • Molten Salt-it leads to corrosion. • Infrared Heater-Very efficient but costly method. • The most used system is a re-circulating hot-air oven. CORPORATE TRAINING AND PLANNING

  25. RE-CIRCULATING HOT AIR OVEN METHOD • In this system a positive displacement circulating fan distributes air through a system of ducts into the swept volume of the oven. • The capacity of the fan (cubic meters of air per minute), will determine the number of air changes per minute. CORPORATE TRAINING AND PLANNING

  26. On contemporary machines, air should be changed in the oven approximately 25-30 times per minute in order to provide an effective heating for the mold. • Direction of the air in the oven is generally caused by the directional louvers so that no “dead spots” are created. • The static pressure capability of the fan system provides force to push the air over the mold and provide the scrubbing action of the hot air on the mold. CORPORATE TRAINING AND PLANNING

  27. The absorption of the heat by the mold transmits through to the powder to create the molded parts. Re-circulating Hot Air Oven Heating CORPORATE TRAINING AND PLANNING

  28. The medium for heating hot-air ovens may either be natural gas or oil with a modulating burner. • In some cases, electric heaters are used to generate the hot air environments. • The regulation of air temperature in the swept volume of the oven is controlled by sophisticated electronic temperature control devices. • The time that the mold remains in the oven is known as “Oven residence time”. CORPORATE TRAINING AND PLANNING

  29. The oven residence time necessary to cure a part will depend upon the wall thickness of the part, the type of plastic material being used to mold the part, and the conductivity of the metal of the mold. • Aluminum with a higher conductivity, allows heat to transfer from the air stream to the mold and the powder at a much faster rate than does steel. • Thinner gauge aluminum helps to increase the conductivity. CORPORATE TRAINING AND PLANNING

  30. OPEN FLAME HEATING • In the case of the rock and roll machines there was no heated oven; an open –flame method used whereby a manifold of gas jets was placed to evenly heat the mold. • As the mold rotated about the major (rolling) axis, the heat was imparted directly onto the mold surface, and transferred through to the plastic. • The machine was inexpensive to manufacture, but the operating cost were significantly more than the closed oven type of heating system. CORPORATE TRAINING AND PLANNING

  31. All of the thermal energy not imparted to the mold went into the atmosphere creating increased temperature in the work environment and the loss of energy. • Open flame machines are still used for very large tanks that are too large to fit in conventional re-circulating ovens or where the quantity of tanks is so small as to not justify the expense of building a large oven. CORPORATE TRAINING AND PLANNING

  32. Open Flame Rock and Roll Machine CORPORATE TRAINING AND PLANNING

  33. HOT OIL HEATING METHOD • The hot oil jacketed mold system was one of the earliest systems used for rotational molding. • In closed oven with re-circulating hot air, their will be heat losses due to the extra volume in the oven not filled by the mold. • The jacketed mold maintains the mold temperature very close the the temperature of the hot oil being used. CORPORATE TRAINING AND PLANNING

  34. Therefore, the hot oil system generally uses a lower temperature for molding since the oil is in direct contact with the mold and imparts the heat energy very quickly. • The difficulty of using jacketed hot-oil molds is that the expenses of the molds is considerably more than used in the other types of heating. CORPORATE TRAINING AND PLANNING

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  36. THE HEATING TIME DEPENDS UPON THE FOLLOWING FACTORS. • Part Size • Wall thickness • Resin CORPORATE TRAINING AND PLANNING

  37. THE SPEED DEPENDS UPON THE FOLLOWING FACTORS • Part size • Geometry • Resin • Heating rate • Thermal conductivity of mould metal CORPORATE TRAINING AND PLANNING

  38. Sometimes due to over heating the air inside the mould gets expanded and some internal pressure builds up, which may distort the mould as well as the part. • To avoid this, mould can be vented. • To provide vent a small pipe is placed which runs from inside to outside of the mould. To prevent entry of the vent pipe a small amount of glass fiber can be added. CORPORATE TRAINING AND PLANNING

  39. ROTATIONAL MOULDING MACHINES Three basic types of machines are : • Batch type • Semiautomatic type • Continuous or rotary type CORPORATE TRAINING AND PLANNING

  40. Batch type is used in prototype or low volume production. This method requires less capital but most involvement of manual labour. • Continuous or rotary type method include three basic stations arranged 120º apart from arms attached to a central hub containing the drive mechanism. • Advantage of this system is minimal labour and high production rate. CORPORATE TRAINING AND PLANNING

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  43. ROTATIONAL MOULDING BY CAROUSEL-TYPE MACHINES • The carousel type machine is a three-station rotary indexing type with a central turret and three cantilevered mould arms. • Individual arms are involved in different operations simultaneously so that no arms are idle at any time. • All operations are automated and at the end of each cycle the turret is indexed 120º, thereby moving each mould arms to its next station. CORPORATE TRAINING AND PLANNING

  44. Newer carousel machines being offered today have four arms. • The additional arm can be used in a second oven, cooler or load station, depending on, which is the most time-consuming part of the over all cycle. • The four-arm carousel machines increase the production by allowing the indexing from station to station to occur more frequently than could be managed on a three-arm machines. CORPORATE TRAINING AND PLANNING

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  46. MULTILAYER ROTATIONAL MOULDING • It is used to combine two different colours of the same material or two dissimilar material into one part. • It offers potential advantages of increased stiffness. • When solid and foam are combined, improved barrier properties and permeation resistance by using thin inner or outer layer of low permeable material. CORPORATE TRAINING AND PLANNING

  47. It is a two staged process in which the skin of one material is combined with an inner layer of another material. • The first shot of material moulds in the normal fashion and the material adheres to the mould. CORPORATE TRAINING AND PLANNING

  48. When adhering and curing of the layer is completed the mould is removed from oven and second shot material is added. • While producing very thick parts care should be taken not to thermally degrade the outer layers at the expense of optimizing properties of inner layers. CORPORATE TRAINING AND PLANNING

  49. The double process is at its best when two walls adhere to each other. • Two different colors of virgin and reprocessed combinations of the same material would be ideal. • Dissimilar materials such as nylon and PE that don’t bond to each other are being used, but there are some limitations. CORPORATE TRAINING AND PLANNING

  50. Two materials must be chemically compatible. • They should have similar processing temperature and similar co-efficient of thermal expansion. CORPORATE TRAINING AND PLANNING

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