Processing of Plastics

Overview: From Polymer to Finished Product

In this chapter, the focus is on how plastics are turned into usable objects once the polymer material already exists. We consider only the key ideas of processing methods and what makes them different. Details of polymer structure and properties are handled in other chapters.

For most methods, three questions are central:

• In what physical form is the plastic during processing? (granules, powder, melt, solution, elastic “rubber-like” state)
• How is the shape generated? (pressure, pulling, pressing into a mold, blowing with gas, coating)
• What happens during cooling/solidification and possible post-treatment (e.g., tempering, joining, surface finishing)?

Below, processes are grouped by the typical feed form and the kind of product made.

Thermoplastic Processing Methods

Thermoplastics soften on heating and harden on cooling, without chemical change. Their ability to remelt is the basis of most high-volume processing operations.

Extrusion

Extrusion is a continuous process used to produce long, constant cross-section products.

Basic Principle

• Feed: usually dry granules or powder of a thermoplastic.
• In a heated, rotating screw (in a cylinder), the polymer is:
• transported
• compressed
• melted and homogenized.
• The melt is forced through a shaped opening, the die.
• On exiting the die, the hot shaped strand is cooled (typically by air or a water bath) and then cut or wound.

Typical Products

• Pipes, hoses, profiles (e.g. window frames, cable insulation).
• Films and sheets (with flat dies).
• Coatings on wires and cables (wire coating by extrusion).
• Granules: melt can also be extruded as strands and cut into new pellets.

Process Features

• Continuous operation, very high throughput.
• Shape is constant along the length; cross-section defined by the die.
• Melt temperature, screw speed, and cooling rate must be controlled to avoid defects (e.g. surface roughness, internal stresses, dimensional deviations).

Injection Molding

Injection molding is used for mass production of complex, usually three-dimensional parts.

Basic Principle

• Thermoplastic granules are plasticized in a heated cylinder with a reciprocating screw.
• A measured shot of melt is injected at high pressure into a cold metal mold.
• The melt fills the mold cavity, cools, and solidifies.
• The mold opens; ejector pins remove the finished part.
• The mold closes again, and the cycle repeats.

Typical Products

• Housings and casings (electronics, household items).
• Toys, caps, closures, and technical components.
• Gears, clips, fittings, small structural parts.

Process Features

• Very high dimensional accuracy and reproducibility.
• High mold cost, but low part cost at large quantities.
• Precise control of:
• melt temperature and pressure,
• mold temperature,
• cooling time.
• Cooling and solidification can create internal stresses and shrinkage; mold design and processing parameters must compensate for this.

Blow Molding

Blow molding produces hollow thermoplastic bodies like bottles and tanks.

Extrusion Blow Molding

• A molten tube (parison) is extruded vertically.
• The parison is captured in a mold that closes around it.
• Compressed air is blown in, forcing the hot tube against the cold mold walls.
• After cooling, the mold opens and the hollow part is removed.

Typical products: bottles, containers, fuel tanks, toys, and technical hollow parts.

Injection Stretch Blow Molding

• First step: injection molding of a preform (test-tube-like object).
• Second step: reheating, then stretching and blowing the preform in a bottle mold.
• Used for transparent, mechanically strong bottles (e.g. PET beverage bottles).

Process Features

• Economical for large volumes of hollow bodies.
• Wall thickness distribution depends on parison/preform temperature, stretching, and blow pressure.
• Stretching can improve mechanical strength and barrier properties by orienting polymer chains.

Thermoforming

Thermoforming shapes pre-made sheets or films of thermoplastic.

Basic Principle

• A plastic sheet is heated until it becomes soft and rubbery, but not fully melted.
• The heated sheet is draped over or into a mold.
• Vacuum, compressed air, or a mechanical plug shapes the sheet against the mold.
• The formed part is cooled and trimmed; excess material (skeletal frame) is removed.

Typical Products

• Packaging trays, cups, blister packs.
• Refrigerator liners, thin-walled housings.
• Disposable plates and containers.

Process Features

• Suitable for relatively thin-walled parts with moderate detail.
• Lower tooling costs than injection molding; good for larger-sized parts or medium production runs.
• Mechanical properties and wall thickness depend on sheet thickness, degree of stretching, and temperature profile.

Processing of Elastomers and Thermosets

In contrast to thermoplastics, elastomers and thermosets undergo chemical curing (crosslinking) during processing and cannot be melted again afterward.

Elastomer Processing

Elastomers are processed in a rubbery state and then crosslinked (vulcanized).

Mixing and Shaping

• Raw rubber (natural or synthetic) is mixed with:
• fillers,
• plasticizers,
• pigments,
• curing agents (e.g., sulfur systems).
• Common mixing equipment: internal mixers, open mills.
• Shaping methods:
• extrusion (hoses, profiles),
• calendering (sheets),
• compression molding or transfer molding (seals, gaskets),
• injection molding variants designed for elastomers.

Curing (Vulcanization)

• After shaping, parts are heated (in presses, molds, autoclaves).
• Crosslinking (e.g. sulfur bridges) converts the soft rubber into an elastic, dimensionally stable material.
• Time–temperature profiles are chosen so that vulcanization is complete without thermal degradation.

Typical Products

• Tires, seals, O-rings, hoses, damping elements, conveyor belts.

Thermoset Processing

Thermosets are formed from low-molecular precursors or partially reacted resins that solidify via crosslinking during shaping.

Molding Methods

• Compression molding:
• Pre-measured amount of resin plus filler is placed in an open, heated mold.
• The mold closes, pressure is applied.
• Heat triggers curing; the part solidifies in the mold.
• Transfer molding:
• The resin is preheated in a chamber and then “transferred” into the mold cavities under pressure before curing.
• Injection molding for thermosets:
• Similar to thermoplastic injection molding but with careful temperature control to avoid premature curing in the machine.

Casting Processes

• Resin casting:
• Low-viscosity resin systems are poured or injected into molds (often without high pressure).
• Curing occurs at ambient or elevated temperature.
• Typical for: electrical encapsulations, decorative items, prototypes.

Typical Products

• Electrical components (switch housings, coil encapsulations).
• Heat-resistant handles, structural parts in appliances.
• Fiber-reinforced thermosets (when combined with fibers, see below).

Fiber-Reinforced Plastics and Composite Processing

Many structural applications use plastics reinforced with fibers (glass, carbon, aramid). Processing must shape both matrix and reinforcement.

Lamination and Hand Lay-Up

• Layers of fiber fabric or mat are placed in/onto a mold.
• Liquid resin (thermoset) is applied manually or by simple tools.
• Air is removed (e.g. by rollers, vacuum bagging).
• Curing occurs at ambient or elevated temperature.

Typical products: boat hulls, small series composite parts, repair patches.

Resin Infusion and Vacuum Processes

• Dry fiber preforms are placed in a mold and covered with a vacuum bag.
• Resin is drawn into the fiber structure by vacuum (resin infusion, VARTM).
• Curing yields shaped fiber-reinforced thermoset components.

Typical products: wind turbine blades, large composite structures, automotive parts.

Prepreg and Autoclave Processing

• Prepregs: fiber materials pre-impregnated with partially cured resin.
• Layers of prepreg are laid into a mold in defined orientations.
• The lay-up is consolidated and cured under heat and pressure (often in an autoclave).

Typical products: aerospace components, high-performance sporting goods, high-strength structural parts.

Pultrusion and Other Continuous Composite Processes

• Pultrusion:
• Continuous fibers are pulled through a resin bath and then through a heated die.
• Curing occurs in the die; a continuous profile emerges.
• Typical products: rods, beams, profiles for construction.

Film and Fiber Production from Plastics

Certain processing routes specifically produce very thin or very fine products.

Film Production

Blown Film Extrusion

• A molten tube is extruded vertically and inflated into a bubble with air.
• The bubble is stretched and cooled, then collapsed into flat film.
• Used for: packaging film, bags, agricultural films.

Cast Film and Sheet

• Melt is extruded through a flat die directly onto chilled rolls.
• Cooling and stretching control thickness and surface quality.
• Used for: packaging films, technical films, and thicker sheets for later thermoforming.

Fiber and Filament Spinning

Melt Spinning

• Thermoplastic is melted and pumped through a spinneret with many small holes.
• Emerging filaments are cooled and solidified.
• Subsequent drawing (stretching) orients polymer chains, increasing strength.

Typical products: synthetic fibers for textiles, industrial yarns, filaments for 3D printing.

Solution and Dry/Wet Spinning

• For polymers that cannot be melt processed, a polymer solution is extruded.
• Solvent is removed by evaporation (dry) or coagulation in a bath (wet).
• Used for specialty fibers.

Shaping by Calendering and Coating

Calendering

• A heated plastic mass is passed through a series of counter-rotating rollers.
• The gap and speed of the rollers define sheet or film thickness.
• Often used for:
• PVC films (flooring, artificial leather),
• thin coatings and membranes.

Process features:

• Good surface finish and thickness control.
• Suitable for certain thermoplastics with appropriate viscosity.

Coating Processes

• Plastics can be applied as:
• melt coatings (e.g. extrusion coating of paper or metal foil),
• solution or dispersion coatings (e.g. paints, varnishes, adhesives),
• powder coatings (especially for thermoset powders).

Typical applications: protective layers, decorative finishes, barrier layers for packaging, anticorrosive coatings.

Joining, Welding, and Assembly of Plastic Parts

After shaping, many plastic parts must be joined into assemblies or combined with other materials.

Fusion Welding

Methods for thermoplastics where the joint region is melted and fused:

• Hot plate welding: surfaces are melted on a heated plate, then pressed together.
• Ultrasonic welding: high-frequency mechanical vibrations create localized heating at the interface.
• Vibration and friction welding: relative motion under pressure generates heat.
• Laser welding: laser energy selectively melts a joint line.

Used for: tanks, containers, housings, automotive parts, small technical components.

Solvent and Adhesive Bonding

• Solvent welding:
• A solvent softens or partially dissolves the plastic surfaces.
• On contact, the material interpenetrates; solvent evaporation solidifies the joint.
• Adhesive bonding:
• Special adhesives (often based on other polymers) create joints between similar or different materials.
• Surface preparation (cleaning, roughening, primers) is important.

Used where welding is not feasible or where dissimilar materials must be joined.

Mechanical Joining

• Screws, snaps, clips, rivets, and inserts (often metal) are integrated or added.
• Heat staking and insert molding:
• Metal elements can be embedded during molding or by localized heating.

Advantages: disassembly and recyclability can be improved, depending on design.

Influence of Processing on Properties and Quality

Processing conditions strongly affect the final properties of plastic products.

Thermal and Mechanical History

• Too high processing temperatures or long residence times can cause:
• degradation (chain scission, discoloration),
• gas evolution (bubbles, voids).
• Excessive shear stresses in processing equipment can reduce molecular weight and alter mechanical properties.

Morphology and Orientation

• Cooling rate affects crystallinity in semi-crystalline polymers:
• Fast cooling → lower crystallinity, usually more transparent, lower stiffness.
• Slow cooling → higher crystallinity, higher stiffness, possible opacity.
• Flow and stretching during processing orient polymer chains:
• Can increase strength and modulus along the orientation direction.
• May introduce anisotropy (direction-dependent properties) and internal stresses.

Dimensional Stability and Warpage

• Non-uniform cooling and shrinkage can cause:
• distortion (warpage),
• internal stresses and cracking over time.
• Mold design, cooling channel layout, and processing conditions are adapted to minimize these effects.

Surface Quality

• Surface defects (flow lines, weld lines, sink marks, orange-peel, bubbles) are often linked to:
• improper melt temperature or mold temperature,
• poor venting,
• incorrect injection or extrusion parameters,
• moisture or contamination in the material.

Post-processing such as polishing, painting, or texturing can enhance appearance and function.

Environmental and Technological Considerations in Processing

Energy and Resource Use

• Many processing methods are energy-intensive (heating, cooling, compressed air).
• Efficient temperature control and machine design reduce energy consumption.
• Regrind (recycled production scrap) is often reincorporated, especially with thermoplastics; overuse or repeated processing cycles may impair properties.

Processing Aids

• Additives can simplify processing:
• lubricants and slip agents reduce friction,
• stabilizers protect against thermal or oxidative degradation,
• nucleating agents modify crystallization behavior,
• blowing agents generate foam structures.
• Some additives affect recyclability or long-term stability.

Design for Processing and Recycling

• Product design must consider:
• wall thickness and transitions,
• demolding (draft angles),
• flow paths and gating for injection molding,
• joining methods.
• For easier recycling:
• use of single-material parts where possible,
• clear labeling of polymer type,
• avoidance of hard-to-remove coatings and mixed-material composites when they are not strictly necessary.

Processing of plastics thus links the material’s inherent properties with the final shape and performance, while also influencing recyclability and environmental impact.