Plastic Extrusion Process

PLASTIC EXTRUSION PROCESS

It is a process of forming a continuous piece of plastic by forcing it through a shaping orifice with or without the presence of heat. The opening through which the resins are forced gives the product its form, resulting in consistent thickness and gauge control. 

Plastic Extrusion Terms 

Cooling Tank: A tank typically containing water through which extrusion is constantly passed for cooling. 

Cure: The technique of cross-linking a plastics material. Adiabatic Extrusion: A type of extrusion whose only source of heat is the conversion of the drive energy through the viscous resistance of the plastics mass in the extruder. 

Back Pressure: The resistivity of molten plastic material to forward flow. 

Extrudate: The product or result of an extrusion process. An extrudate is a product or material forced through a shaping orifice as a continuous body. 

Extruder Size: The minimal inner diameter of the extruder barrel 

Extrusion Coating: A coating technique in which molten plastic feeds directly from an extruder die into a nip-roll assembly combined with the substrate. 

Haul-off:  Also called a “caterpillar,” it is an apparatus used for the continual removal of extrudate from the die. 

Barrel:  The part of the extruder encasing the screw or plunger. 

Barrel Liner: The sleeve forming the inner surface of the barrel. 

Calendering: The process of pressing or smoothing material between rollers. 

Cladding: Sometimes referred to as “sidings,” it is extruded PVC-U boards that are used as outdoor weather-resistant façade panels. 

Heat Aging: The unique process of aging a thermoplastic or thermoset product and examining the percentage of retained physical and chemical properties after exposure to heat for a prolonged period of time. 

Melt Strength: A term that refers to the strength of molten plastic. 

Outer Die Ring: The element of tubing tie that shapes the outer surface of a tube. 

Compression Section: The transition section of a screw channel in which a reduction in the screw channel volume occurs. 

Decompression Section: The section of a two-stage extruder in which an increase in screw channels volume occurs. 

Die: The component on a plastics extruder affixed to the extruder head through which the melt is pushed to form the desired profile. 

Die Plate: In moulds, the main support for the punch or mould cavity. 

Dry Blend: A free flowing blend of compound or resin and other ingredients as prepared for an additional manufacturing operation specifically for extrusion or moulding. 

Pellets: Resins or mixtures of resins with compounding additives in the shape of similar-sized tablets and granules that have been extruded or chopped into short segments to prepare them for moulding operations. 

Ram Extruder: A barrel with a temperature control, wherein a plunger pushes material in a melted state to the die. 

Screw: A helically grooved rotating element inside the barrel of a screw extruder. The main purpose of a screw is to melt and feed raw material from the feeder to the die, but it also homogenizes, compresses and pressurizes the material. 

Screw Extruder: A machine comprised of a barrel with a temperature control. It houses one or more rotating screws, which pass plastic materials from the feed aperture and move them in the form of melt under pressure through a die. 

Take-up: An apparatus for reeling extruded material. 

Trunking: An extruded PVC-U channel used to contain and protect pipes or cables. 

Vacuum Sizing: A procedure utilizing a sizing die with a vacuum applied to the outer surface of the extrudate. 

CLASSIFICATION OF EXTRUDERS: 

There are several methods of classifications of extruders, considering the operation (viz., continuous and batch), orientation of the barrel (vertical and horizontal), mechanism of the flow generation (screw, disk, drum, barrel, two-stage disk extruders, etc.).

Batch – Type

1. Ram Extruders

2. Reciprocating screw extruders

Continuous –Type

1. Screwless Extruders

          Disk Extruders

          Drum Extruders

          Other Extruders

2. Screw Extruders

          Single-Screw Extruders (SSE)

          Twin-Srew Extruders (TSE)

          Multi-Screw Extruders 

RAM EXTRUDERS

1. Ram Extruder is discontinuous

2. It uses a ram or piston operating in a cylinder

3. The material is softened by thermal energy from the outside heaters

4. After softening the material is forced out by the ram movement.

5. After that rams retracted, the cycle is repeated.

Advantages

1. There is no violent agitation of the material

Simple process control.

2. Material that cannot undergo shearing can be processed.

3. Material having high viscosity (e.g. PTFE, UHMHDPE) can be processed.

4. Power consumption is low.

Disadvantages

1. Large temperature gradient between the ram cylinder wall and the interior of the plastic bed. 2. The gradient may lead to degradation of the surface layer of the resin.

3. Low throughput

4. Discontinuous process.

Types of Plastic Extrusion Process:

1 SINGLE-SCREW EXTRUDERS (SSE)

2 TWIN SCREW EXTRUDERS (TSE)

3 SHEET EXTRUSION

4 PIPE EXTRUSION

5 CO-EXTRUSION

6 BLOWN FILM EXTRUSION (FILM BLOWING)

7 CAST FILM EXTRUSION

8 FOAM EXTRUSION