Plastic Extrusion process

Extrusion is a continuous process which is used to produce both solid and hollow products that have a constant cross section. Examples are tubing, pipes, profiles (like window frames, curtain track), films, sheets, insulated wires etc. Extrusion process is as follows,

 

Operation procedure,

•        Fill hopper with plastic material
•        Plastic is then heated in the barrel gradually to the state that its’ able to flow sufficiently
•        Melt plastic is forced by screw press through the die and  cooling bath
•       Letting the product cool and cut to length

Extrusion rates normally vary between nine and sixty meters per minute. Thicker cross sections are extruded slowly because more time is required for initial heating and later cooling. Extruder is the main object in the extrusion process. Its main equipments are given in the figure.

Equipments of the extruder

Functions of the extruder screw is convey the resin through the extruder, mixing the ingredient together, build pressure in the extruder (so that resin will be pushed through the die), impart mechanical energy as part of the melting process. The screw can divide in to three zones according to its function. They are feeding zone which pulls the polymer pellets from the hopper, pre heat them and convey it to other zones, transition zone which compress the material conveys from the feed zone and so it has decreasing channel depth,   metering zone which is a having constant screw depth and very shallow flight depth as homogenize the melt and supply to die.

Zones of the extrusion screw

Polymerization

Polymers are long chain like molecules which can say macromolecules. Normally they are solids or liquids in the room temperature because of their high molecular mass.

Polymerization is the reaction the monomers convert in to the polymer molecules. Monomer is the starting small molecule. Generally there are three steps in polymerization reaction called initiation, propagation and termination.

Three types of polymerization reactions can be seen.

• Addition polymerization (chain growth polymerization)
• Ionic polymerization
• Condensation polymerization (step growth polymerization)

Addition polymerization

In this type monomers must have unsaturations or double bonds. Free radical initiators or ionic initiators are used to initiation of the polymerization reaction. Otherwise energy can be absorbed from UV or heat.  First initiators react with monomers and make activated monomers which having an active centers. Then propagation happens by active center propagating along the chain. There are two widely available mechanisms of termination called termination by combination and termination by disproportionation. 

 

Example for addition polymerization

Ionic polymerization

Ionic polymerization can be divided in to anionic or cationic according to the charge of the active center. In this type also monomers must have unsaturations or double bonds. 

 

Example for ionic polymerization

 

• Anionic polymerization

Anionic initiators are required.  Negatively charged active centers are propagating along the chain. 

• Cationic polymerization

Cationic initiators are required.  Positively charged active centers are propagating along the chain. 

Condensation polymerization

In this type reactive functional groups of monomers will react with each other and remove small molecules. There is no initiation, termination nor active centers. The reaction will happen as long as they have reactive functional groups like carboxylic acid groups, hydroxyl groups etc. To make chain more than one reactive functional group are needed.

Example for condensation polymerization

This reaction is exothermic one. So to control the chain size we can do things like Cool the system, Use one component excess, use some amount of monomers which are having one reactive functional groups.