ROTATIONAL MOULDING MACHINE

 

ROTATIONAL MOULDING

INTRODUCTION

·         Rotational molding is a process of making hollow articles.

·         The part is formed inside a closed female mold.

·         In this process the mold rotates biaxially during heating and cooling cycle.

·         Rotational molded pieces are stress free because the pieces are produced without any external pressure.

·         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.

PRINCIPLE

·         The principle of the process is that finely divided plastic material becomes molten when comes in contact with hot metal surface of the mold and takes up the shape of that surface.

·         As only female mold is used, the only pressure exerted is those induced by gravity and centrifugal force.

·         The polymer is then cooled while still in contact with the metal mold to get the solid copy of the surface.

·         Rotational molding permits to make a wide variety of fully and partially closed items.

ADVANTAGES AND DISADVANTAGES

Advantages

1.     The major advantage of rotational molding as compared to other plastic molding processes is that it can make very large parts.

2.     It requires comparatively low cost input.

3.     The products are stress free with strong outside corners. There are no weld lines, sprue or gate marks.

4.     Here impact toughness is improved and failure due to brittleness is reduced.

5.     The external dimensional details can be easily molded with better surface glossiness.

6.     The color changes in the product can be made easily. Similarly mold changes can also be done rapidly.

7.     Multilayer molding is also possible for providing chemical resistance and strength to the part.

8.     Good control over wall thickness variation is also achievable as compared to blow molding or thermoforming.

9.     Molding can be done with metal inserts and minor undercuts.

10.  No scrap or very little scrap is produced.

11.  Low tooling cost.

DISVANTAGES

1.     The molding cycles are longer compared to blow molding and thermoforming.

2.     In case of big parts loading and unloading is very labor intensive.

3.     The process is not suitable for parts with wall thickness less than 0.03”.

4.     The conversion of plastic granules to powder form increases the equipment and process cost.

LIMITATION

1.     It is an open molding 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 mold as well.

4.     The long heating cycle increase the possibility of thermal degradation.

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 pulverized into fine powder that flows like liquid.

7.     Removal of plastic sticking onto the surface of cavity requires careful application of mold release agent.

ROTATIONAL MOULDING PROCESS

1.     Loading

2.     Heating & Molding

3.     Cooling

4.     Unloading

LOADING

·         This step includes weighing of the charge for a particular product then transferring it to the open cold mold.

·         The mold surface usually coated with a mold releasing agent.

·         The raw material can be in the form of powder or liquid state.

·         The wall thickness can be controlled by varying the amount of raw material charged.

·         After the material is charged the mold is closed and clamped to the arm of the machine.

·         Then the mold is moved to an oven for heating.

HEATING & MOULDING

·         The mold fixed to the arm now moved to a closed chamber where it undergoes intense heating.

·         During heating the mold 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.

·         For molding unsymmetrical products a wide variability of ratios is necessary.

·         The revolving motion distributes the plastic material uniformly over the inside surface of the mold.

·         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.

·         The molding 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.

COOLING

·         For cooling the mold 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 from the mold.

·         Otherwise the part will get distorted.

·         Cooling can be done by air or water. To provide faster cooling cold water is sprayed over the mold.

UNLOADING

·         After cooling the mold is transferred to the unloading station.

·         In this step the mold 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 mold is cleaned and the charge is loaded for the next cycle.

ROTATIONAL MOULDING MACHINES

Three basic types of machines are:

1.     Batch type

2.     Semiautomatic type

3.     Continuous or rotary type

·         Batch type is used in prototype or low volume production. This method requires less capital but most involvement of manual labor.

·         Continuous or rotary type method includes three basic stations arranged 120º apart from arms attached to a central hub containing the drive mechanism.

·         Advantage of this system is minimum labor and high production rate.

MATERIAL CONSIDERATIONS FOR ROTATIONAL MOULDING

·         All thermoplastic materials can be rotationally molded.

·         HDPE, LLDPE, LDPE, 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 grind ability, particle distribution, particle mesh size, portability, bulk density and fusibility.

·         The material should be able to ground to a fine powder and the common size is about 300µ and maximum size is up to 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.

·         The most common mesh size for rotational molding ranges from 16 to 50.

·         The material should produce less volatiles during heating.

MOULD MATERIALS

·         Molds are not so expensive, but entirely depend upon the quality level of the molded parts and the method of heating to be used in the process.

·         Three types of mold materials in common use are:

1.     Cast aluminum

2.     Steel sheet metal

3.     Electroformed copper-nickel

·         Cast aluminum molds are widely used for small to medium sized parts requiring number of cavities.

·         Steel sheet molds is preferred where surface finish is not critical and for the larger molds of simple design.

·         Electroformed copper-nickel molds are most expensive but offer a very smooth finish.

·         This type of molds is best when very intricate surface and precise detail is required on the finished part.

PROCESS VARIABLES

·         There are many potential variables in the rotational molding 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.

·         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 molded.

·         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 shrinks 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.

·         Cooling the material slowly increases shrinkage, but the shrinkage will be more uniform.

·         These Variations in shrinkage encourage war page 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.

·         The best approach is molding parts to close tolerance to establish the optimum molding 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.

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