Polymer Processing

Polymer processing operations

Polymer processing operations are first classified by whether the material flows steadily or is handled as discrete charges. Continuous operations such as extrusion, calendering and film blowing keep the polymer moving through a die or between rolls, which suits products of uniform cross-section. Discontinuous operations produce one part per cycle from a charge or a preformed sheet, and thermoforming is the standard example.

In practice, the process is chosen from the geometry of the product. The pairings to know are:

• Extrusion — steady forcing of molten polymer through a die to make constant-section products such as pipes.
• Injection blow moulding — a preform is made by injection and then inflated in a mould, which is the usual route for hollow bottles.
• Blow moulding — a heated parison is inflated against a mould to give hollow containers such as soft drink bottles.
• Thermoforming — a heated sheet is drawn onto a mould and then trimmed, giving blister packs and disposable cups.
• Pultrusion — fibres are pulled through a resin bath and a heated die, giving constant-section reinforced profiles such as fishing rods and poles.
• Rotational moulding — powder in a heated, rotating closed mould gives seamless hollow parts such as rainboots and large water tanks.
• Filament winding — resin-impregnated continuous fibres are wound on a rotating mandrel, which is the method for fibre-reinforced pressure vessels.
• Blown film extrusion — an extruded tube is inflated into thin film, which is then converted into carry bags.
• Calendering — molten polymer is squeezed between heated rolls to make continuous flexible film.
• Compression moulding — a charge is pressed in a closed, heated mould. Plastic trays are a typical thermoformed product, although compression moulding of trays is also accepted in the key for this pairing.

The screw of an extruder is itself divided into zones, each with a distinct job from the hopper to the die.

Die swell is the expansion of an extrudate as it leaves the die. The melt is viscoelastic, so its chains are oriented during flow and elastic strain is stored. On exit the stresses relax and this stored strain is recovered, which increases the cross-section of the extrudate. Viscous flow alone does not store recoverable strain, and plastic deformation is permanent, so neither explains swell.

For semi-crystalline polymers, the forming temperature for thermoforming must lie in the window T_g < T < T_m. Above T_g the amorphous regions become rubbery, so the heated sheet can be stretched into the mould. Below T_m the crystalline domains keep the sheet from flowing away. Below T_g the sheet is glassy and too stiff to form, and above T_m it loses its shape.

Single screw extruder output

The output of a single-screw extruder is governed by drag flow and pressure flow. Drag flow is produced by the relative motion between the barrel and the screw and increases with screw speed. Pressure flow runs against it and depends on the restriction at the die. Under open discharge, with no die and therefore zero pressure drop, the net volumetric output equals the drag flow.

For a single-flight screw with mean diameter D, screw speed N in revolutions per second, channel depth H, flight spacing W and helix angle \theta, the barrel surface velocity is \pi D N. Its component along the channel is \pi D N \cos\theta, so the drag flow is Q_d = \tfrac{1}{2}\pi D N H W \cos\theta. The helix angle here is measured from the plane perpendicular to the screw axis, so the cosine factor applies.