Polymers: Classification and Fabrication

Polymer Classification

• Polymers can be classified as either natural or synthetic.
• Natural polymers include cellulose, starch, and rubber.
• Synthetic polymers include plastics, fibers, and elastomers.
• Plastics include thermoplastics like polyethylene, and thermosets like bakelite.
• Fibers include polyamides and polyester.
• Elastomers include neoprene.

Polymer Fabrication

• Polymer fabrication processing starts with monomers that go through a polymerization process to create high molecular weight polymers or macromolecules.
• These polymers can become rubbers, plastics, fibers, coatings, or adhesives.
• Plasticizers, stabilizers, and fillers are added during the processing.

Thermoplastics Classifications

• Thermoplastics are classified into amorphous and semicrystalline types.
• Examples of amorphous thermoplastics include PS and PVC.
• Examples of semicrystalline thermoplastics include LDPE, HDPE, and PP.
• Thermoplastics are also classified by price and benefit, into commodities, engineering, and "specials".

Transformation Processes

• Transformation processes have common stages, including pre-molding, molding, and shape stabilization.
• Pre-molding involves transporting the polymer and heating it to a state of high fluidity.
• Molding involves material flow through shaping tools.
• Shape stabilization involves treatment to achieve permanent dimensions through solidification and cooling.

Manufacturing Methods

• Manufacturing methods include continuous and cyclic processes.
• Continuous methods, like extrusion, are used for simple geometries.
• Cyclic methods, like injection molding and thermoforming, are used for complex geometries.

Thermoplastic Processing

• Thermoplastic processing involves different methods to form the final piece
• Injection, extrusion, and thermoforming (stretched + suction).
• Extrusion includes blowing and stretching.

Extrusion Process

• The material (pellet) is heated by friction with the spindle and cylinder, and by electric resistors.
• It is a continuous process for products like tubes, bars, sheets, and fibers.

Spindle Characteristics

• Spindles have three zones as part of the extrusion process.
• Spindles have a length/diameter ratio (L/D) of 20:1.
• Fusion is not adequate when spindles are shorter
• There is material degradation when spindles are longer.
• Compression ratio is the ratio of fillet height in the feeding area and in the dosing area, measured at 2/4.
• A value of 2 is designated for very amorphous plastics and very sensitive to variations of t.
• A value of 4 is designated for plastics with high crystallinity.
• Extruder is based on the nominal Φ of the cylinder: 25 / 300mm.

Extrusion of Plates and Films

• For films the thickness is <0.5 mm, while for iron its ≥ 0.5 mm.
• Types of nozzles:

Nozzle types

• T: Simple, cheap but do not allow easy flow.
• Fishtail: Good distribution but does not allow large plates, as much material can be degraded.
• On hanger: The best and most expensive distributions.