6 ROTATIONAL MOULDING CORPORATE TRAINING AND PLANNING 1

6. ROTATIONAL MOULDING CORPORATE TRAINING AND PLANNING 1

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 2

• 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 3

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 4

• 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 5

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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 8

• 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 9

• 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 10

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 11

LIMITATIONS 1. It is an open moulding process and so there are no cores inside the hollow parts 2. Surface details and dimensions can only be provided and controlled on the side of the part. 3. The process requires heating and cooling of not only plastic material but also the mould as well. 4. The long heating cycle increase the possibility of thermal degradation. CORPORATE TRAINING AND PLANNING 12

5. It is not suitable for materials with less heat resistant to withstand the long heating cycle. 6. The material must be capable of being pulverised into fine powder that flows like liquid. 7. Removal of plasticking onto the surface of cavity requires careful application of mould release agent. CORPORATE TRAINING AND PLANNING 13

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

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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 16

• 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 17

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 18

• 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 19

• 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 12 mm or more. • The heating chamber should be large enough to house the mould and rotate it freely. CORPORATE TRAINING AND PLANNING 20

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 21

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 assistance. manually or with mechanical • 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 22

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 23

• Rotational molds may be heated by either an open-flame method, a hot air recirculating 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 24

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 25

• 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. 26 CORPORATE TRAINING AND PLANNING

• 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 27

• 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 28

• 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 29

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. 30 CORPORATE TRAINING AND PLANNING

• All of 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 31

Open Flame Rock and Roll Machine CORPORATE TRAINING AND PLANNING 32

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 temperature of the hot oil being used. CORPORATE TRAINING AND PLANNING 33

• 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 34

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

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

• 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 38

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

• 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 40

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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 43

• 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 occur more frequently than could be managed on a three-arm machines. CORPORATE TRAINING AND PLANNING 44

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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 46

• 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 47

• 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 48

• 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 49

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

A. Double Wall B. Foam Only D. Solid + Foam + Solid C. Solid + Foam E. Solid - Foam CORPORATE TRAINING AND PLANNING 51

MATERIAL CONSIDERATIONS FOR ROTATIONAL MOULDING • All thermoplastic materials can be rotationally moulded. • HDPE, LLDPE, PVC, PC, ABS, PS, Acrylics, Nylon , TPU, SAN Polyesters are the materials which are commonly used. • The various properties considered in selecting the proper material are grindability, particle distribution, particle mesh size, pourability, bulk density and fusability. CORPORATE TRAINING AND PLANNING 52

• The material should be able to ground to a fine powder and the common size is about 300µ and maximum size is upto 400/500 µ. • To provide fine grinding the high speed impact mills are used. • The particle size distribution should also be uniform to provide uniform conductance of heat. CORPORATE TRAINING AND PLANNING 53

• The most common mesh size for rotational moulding ranges from 16 to 50. • The material should produce less volatiles during heating. CORPORATE TRAINING AND PLANNING 54

MATERIAL PREPARATION • A process used to reduce the pallets or granules to a smaller size is called grinding or milling. • In this process the granules fed into the centre of two plates, each with a series of radially arranged cutting edges. • One plate is held stationary while other is rotated at high speed. • The gap between the cutting edges of the two plates is narrower at their peripheries than the centres. CORPORATE TRAINING AND PLANNING 55

• Any individual granules subjected to cutting action, generates frictional heat and increases the temperature of metal cutting face. • Hence the temp must be controlled so that it doesn't raise beyond the melting point of granules. • This ground particles will be passed through a series of vibrating sieves through which the finer particles will fall and be collected for use. • The oversized particles held on the sieve are conveyed back to the mill for further disintegration. CORPORATE TRAINING AND PLANNING 56

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MOULD MATERIALS • Moulds are not so expensive, but entirely depends upon the quality level of the moulded parts and the method of heating to be used in the process. • Three types of mould materials in common use are ØCast aluminum ØSteel sheet metal ØElectroformed copper-nickel CORPORATE TRAINING AND PLANNING 58

• Cast aluminium moulds are widely used for small to medium sized parts requiring number of cavities. • Steel sheet is preferred where surface finish is not critical and for the larger moulds of simple design. • Electroformed copper-nickel moulds are expensive but offer a very smooth finish. most • This type of moulds are best when very intricate surface and precise detail is required on the finished part. CORPORATE TRAINING AND PLANNING 59

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PROCESS VARIABLES • There are many potential variable in the rotational moulding process, that can affect the size of the part being produced. • If there is any variation in the amount of plastic material charged into the cavity, the wall thickness will change accordingly. • The shrinkage and part dimension also vary with a change in wall thickness. CORPORATE TRAINING AND PLANNING 62

• The speed and ratio of rotation determine the number of times a specific location on the cavity passes through the puddle of plastic material and the direction in which it enters and exits the puddle. • A change in these molding machine settings can affect the uniformity of the wall thickness of part. • The molding machine speed, ratio of rotation, oven temperature and other processing parameters must accommodate all the parts being moulded. CORPORATE TRAINING AND PLANNING 63

• Variation in over time, temperature and air velocity can affect final part size. • The hotter the plastic material becomes, the more it expands, the material will then contract or shrink more as it returns to room temperature. • The speed with which plastic material is cooled will affect shrinkage. Cooling the material quickly will result in a low shrinkage factor. CORPORATE TRAINING AND PLANNING 64

• Cooling the material slowly increase shrinkage, but the shrinkage will be more uniform. • These Variations in shrinkage encourage warpage and make it difficult to maintain uniform dimensions. • Variations in the amount of mold release used can increase or decrease the tendency of a hollow part to pull away from the cavity as the part cools and shrinks. CORPORATE TRAINING AND PLANNING 65

• The best approach is moulding parts to close tolerance to establish the optimum moulding cycle and then maintain those conditions. • The speed of rotation of the mold must be slow enough to ensure the gravity holds the plastic material in a puddle in the bottom of the cavity. CORPORATE TRAINING AND PLANNING 66

FAULTS AND REMEDIES IN ROTATIONAL MOULDING PROBLEMS POSSIBLE SOLUTIONS Raise the oven temperature. Production rate is too low Use a (or the heating cycle too long) powder. higher-melt index Use a lower density powder. Increase the speed of rotation. Powder fuses across deep Use a lower-density powder. narrow draws in the mould resulting in incomplete mould Use a higher-melt index powder. fill Modify the mould, if possible. CORPORATE TRAINING AND PLANNING 67

Raise the oven temperature. Increase the heating cycle. Use a higher-melt index powder. Moulded piece contains bubbles Use a lower-density powder. Or Improve mould wall uniformity. Piece has a rough Decrease the wall thickness, if possible by inside surface decreasing the amount of powder in every charge. Make sure the interior surface of the mould is dry. CORPORATE TRAINING AND PLANNING 68

Decrease the heating cycle. Piece is yellow, brown or other discolouration action. Lower the oven temperature. Check the mould wall for contamination such as rust. CORPORATE TRAINING AND PLANNING 69

Clean the mould surface and apply a suitable mould release agent. Decrease the heating cycle or oven temperature. Piece sticks in the mould Increase the cooling cycle. Vent the mould. Modify the mould so that the part tapers to a slightly larger dimension toward the end of the mould through which it is removed CORPORATE TRAINING AND PLANNING 70

Use a low density powder. Piece is brittle Use a lower-melting index powder. Redesign the part to eliminate sharp corners. Decrease the cooling rate. Lower the oven temperature. Rotate the mould during cooling. Piece is warped Vent the mould. Improve the wall uniformity. Allow short air cooling before water cooling. CORPORATE TRAINING AND PLANNING 71

ROTATIONAL MOULDING PROCESS Vs BLOW MOULDING PROCESS • Rotational moulding process have clear advantages over other process like blow moulding and injection moulding. • The ideal shape for a part for Blow Moulding is a cylinder that is closed on one end with a small opening at the other end. The best shape for a rotationally moulded part is ball. CORPORATE TRAINING AND PLANNING 72

• Extrusion blow moulding machine cost more than the rotational moulding machine for a given size and capacity. • Blow moulding machines are powered by electricity which is 40 % more costly than the natural gas that is typically used for heating in the rotational moulding process. CORPORATE TRAINING AND PLANNING 73

• The moulds for blow moulding are normally higher in cost than rotational moulding. • Blow moulding has advantage over rotational moulding of being able to process many thermoplastic materials including ABS and PPO. • Multilayered walled parts like fuel tanks are blow moulded but much more costly moulding machines are required. CORPORATE TRAINING AND PLANNING 74

• The blow moulding process has advantage of being able to process materials as-received in pallet form. It eliminates the cost of pulverizing the pallets into fine powder. • Blow moulding process ideally suitable for capacity upto 1000 its. But container with capacity 10, 000 or 50, 000 Hrs are most common in rotational moulding process. • Blow moulding is preferred for larger volume, lighter duty barrels. Rotational moulding dominates the market for smaller volume specially barrels with improved toughness. CORPORATE TRAINING AND PLANNING 75

ROTATIONAL MOULDING & ITS APPLICATION Polyethylene • Industrial products : Tanks drums, containers, nesting pallets, floor maintenance machine components and tanks, medical carts, viedo game housings, news paper and magazine vending machines, tool chests, shipping cases. • • Transport Products. Consumer Products. Recreational Products. Agricultural products. CORPORATE TRAINING AND PLANNING 76

POLYPROPYLENE • PP is specified for many of the same applications as PE. PP’s higher stiffness and increased temperature resistance allow it to perform in applications where PE isn’t quite enough. • All these application takes advantage of PP’s chemical resistance, stiffness, and heat deflection temperature. CORPORATE TRAINING AND PLANNING 77

• Other applications include large chemical shipping drums, radio active material containers and high-temperature air ducts. • PP is a relatively new as rotational molding material. CORPORATE TRAINING AND PLANNING 78

PVC • PVC material has many applications like. • Industrial Products : Flexible and rigid airducts, machine feet, air and water filters, gaskets, tires and floor scrubber squeegees and bladders. • Medical Products : Examination chair arms, Flexible anesthesia face masks, blood pumps, respiration squeeze balloons and anatomical teaching models. CORPORATE TRAINING AND PLANNING 79

• Consumer products : Figurines and life-size statuary, soft cushioning furniture, picture frames, artificial fruit and toys. • Recreational Products : Soft, cuddly, and noise-making squeeze toys; life like doll and animal heads and body parts; toy wheels, energy-absorbing sports helmet liners. CORPORATE TRAINING AND PLANNING 80

NYLONS • Nylon becomes the choice where PE isn’t quite good enough. • Nylon is a choice when the application requires more temperature resistance, tensile strength, or chemical resistance in contact with oil and gasoline. • Typical uses for nylon include fuel storage tanks, high temperature ducts, truck radiator surge tanks, large chemical shipping bottles, air horns, pressurized water treatment tanks, air intake manifolds. CORPORATE TRAINING AND PLANNING 81

POLYCARBONATE • PC’s impact strength allowed it to take this application away from glass. This material’s excellent out door weatherability is another important consideration. • Other uses include pressurized beer containers and dispensers, air-cleaner housings, heating and intake ducting, illuminated traffic signage, transparent food and medical containers. CORPORATE TRAINING AND PLANNING 82