Lecture 14 Polymer Processing

Polymer Processing

Introduction to Polymer Processing Techniques

  • Polymeric materials are typically manufactured using elevated temperatures and high pressures.

  • The choice of processing technique relies on two crucial parameters:

    • Whether the polymer is a thermoplastic or thermosetting material.

    • The geometry and size of the final product.

  • Compliance with ASTM standards is often mandated to ensure reliability in processing and testing methods.

Curing

  • Curing: This is the process of hardening a thermosetting polymer via crosslinking the chains or monomers within a network.

  • Curing methods include:

    • Heat

    • UV light

    • Chemical additives

  • Examples of curing processes:

    • Vulcanization of rubber

    • UV curing of thiolenes

    • 2-part epoxy hardening

Major Processes in Polymer Processing

  • Compression Molding

  • Injection Molding

  • Extrusion

  • Blown-Film Extrusion

  • Blow Molding

  • Rotational Molding

  • Thermoforming

  • Vacuum Forming

  • Resin-Transfer Molding

  • Melt/Wet Spinning

Compression Molding

  • Compression and transfer molding can be used for both thermoplastics and thermosets.

Injection Molding

  • Involves compressing polymer granules using a ram or screw through a chamber until molten, which is then injected into a mold.

  • Typical items produced include:

    • Plastic Parts: Used for gears, toys (e.g., Legos), and cups.

Thermoplastic Extrusion

  • Components of the extrusion process:

    • Feed hopper: Contains plastic pellets.

    • Heaters: Melt the plastic.

    • Shaping die: Shapes the molten material.

  • Products from extrusion include:

    • Tubing and pipes

    • Sheets and films

  • The process also includes the use of:

    • Turning screw: Helps move the pellets.

    • Barrel: Holds the molten plastic, which is referred to as extrudate.

Extrusion Process

  • An example: The formation of Bubble Gum illustrates how extrusion occurs.

Extrusion Die Swell

  • Die swell: When a polymer exits the die of the extruder, it swells due to the sudden release of pressure, which allows polymer chains to relax.

  • Factors contributing to die swell:

    • Polymers are viscoelastic and demonstrate time-dependent stress relaxation.

    • High shear rates followed by sudden pressure release.

Blown-Film Extrusion

  • Components:

    • Heaters

    • Pinch rolls

    • Air bubble: Formed during the process.

    • Guide rolls

    • Tubing die

  • Used to create bags, films, and sheets.

Blow Molding

  • Involves using pressurized air to expand a preform into the shape of a mold. Key components include:

    • Blow mold: Cavity that shapes the material.

    • Preform and parison: The starting form that is expanded.

    • Core pin (blow stem): Assists in forming the internal shape.

A Brief History of Blow Molding

  • In the 1970s, biaxial oriented polyethylene terephthalate (PET) was developed and blow molded using a two-step process.

  • In 1977, Nisser, ASB Company from Japan initiated a one-step process for blow molding PET.

  • In the U.S. soft drink industry, the usage of plastic containers surged from zero in 1977 to ten billion in 1999.

Rotational Molding

  • Common products include:

    • Trashcans

    • Plastic tanks

    • Ice chests

    • Kayaks

  • Advanced rotational molding produces components that endure daily use.

Thermoforming

  • Used for creating various products such as:

    • Clamshell packaging

    • Food trays

    • Refrigerator liners

Vacuum Forming

  • A specialized thermoforming technique that simplifies the process utilizing vacuum systems.

Calendaring

  • Involves processing a wad of warm or molten plastic through machinery. Sourced from Billmeyer’s figure reference on conditioning equipment.

Melt Spinning

  • A process for creating fibers, involving:

    • Continuous extrusion of polymer from a spinneret to form fibers.

    • Components of the system include:

    • Bobbin

    • Melting zone

    • Heating grid

    • Packaging systems for yarn.

    • Typical fibers produced include Nylon, Polyester, and Polyolefins.

Wet Spinning

  • Used to create specific fibers such as:

    • Kevlar

    • Nomex

    • Rayon

    • Spandex.

  • The process includes creating a polymer solution and utilizing evaporation chambers.

Resin Transfer Molding (RTM)

  • A method utilized for thermosets and composites.

  • Process includes:

    • Reinforcement within a core mold cavity filled with resin.

Pultrusion

  • A continuous process for producing composite materials involving:

    • Roll of reinforced fibers.

    • Tension rollers and resin impregnation.

    • Design specifically for finished hardened fiber-reinforced polymer products.

Prepreg Processing

  • Related to the initial phases of composite shaping and alignment.

Autoclaving

  • A curing method for thermosets involving sealed environments and pressure.

  • An example includes the production of components for the Boeing 787.

Vacuum Bag Lay-up

  • A method used in composite processing including various components such as:

    • Seals and vacuum pumps for maintaining environment quality in the manufacturing process.

Curing Cycle

  • Crucial parameters for the curing cycle include:

    1. Temperature: 180°

    2. Pressure: 3-7 bar

    3. Vacuum: 0.5 mbar

    4. Time management during the curing process is necessary to achieve optimal results.

Heat Treatments

  • Annealing:

    • A heat treatment process that alters the material’s microstructure which is usually intended to increase crystallinity.

    • Results in changes to mechanical properties like strength, hardness, and ductility.

  • Quenching:

    • A rapid cooling process that serves to prevent phase transformations from occurring during cooling.