A391 Mat Proc L02

• Thermoplastics

  Structure: Linear or branched

  Can be melted or remolded by using heat

  No chemical changes

  Good for mass production
• Thermosets

  Structure: crosslinked or network

  Undergoes curing, although it takes longer hours, it gives off better properties

  Once cured, cannot be remolded through heating
• Thermoplastics are usually formed using pellets to form polymer melts

  • Forms amorphous polymer and semicrystalline polymer

  Thermosets are usually formed using powders or liquids
• Materials selection properties (Thermoset vs Thermoplastic)

  Thermosets:

  • Good mechanical & chemical properties
  • Good thermal stability
  • Good flow behaviour
  • Low processing temperature (curing rate around 100-150)
  • Low shrinkage
  • Low impact strength
  • Long molding cycles
  • Poor remolding/reshaping/recycling capabilities
  • Short raw material shelf life
• Material Properties Selection (Amorphous thermoplastic vs Semi-crystalline thermoplastic)

  Amorphous:

  • Low shrinkage
  • Low warpage
  • Good tight tolerance
  • Good transparency
  • High impact strength
  • Poor flow behaviour (chains get entangled)
  • Poor chemical resistance
  • Poor wear resistance
• Additives


  Blended with resins to improve properties such as:

  • improving impact strength
  • Protect polymer against UV degradation
  • Increase resistance to ignition
• Compounding


  The process of blending additives with resins
• Common Additives


  • Fillers: Lower cost, warp control
  • Reinforcements: Improve mechanical properties
  • Pigment: Color
  • Impact modifiers: Improve impact strength
  • UV stabilizers: Protect polymer against UV degradation
  • Heat stabilizers: Protect polymer against heat degradation
  • Fire retardants: Increase resistance to ignition
  • Polymer blend: Mix different polymers for different properties
• Extruder


  Commonly used for thermoplastic processing
• Extrusion


  1. Pellets fed into hopper

  2. Pellets conveyed by screw rotation

  3. Pellets melted by heater

  4. Shear forces homogenize polymer melt

  5. Extrudate shape determined by die orifice
• Sections of an extruder

  • Feed Section: Pulls pellets from hopper and preheats them
  • Compression Section: Compresses and softens the polymer by heating and mixing (plasticates) it
  • Metering Section: Homogenizes melt and builds pressure
• On its own, an extruder can be used to shape the cross sections of pipes and tubes
• To create hollow profiles, air channels are sometimes built into the die
• Compounding using extruder


  1. Blender: Blend resin and additives (no melting)

  2. Extruder: Melts and homogenizes

  3. Cooling: Cools and solidifies melt

  4. Pelletizing: Cuts extrudate into pellet form
• Instead of single screw extruders, twin screw extruders are commonly used for compounding
• Twin screw extruder


  The large shear forces generated allows for efficient mixing
• Blow molding


  Inflating a hot, hollow, thermoplastic preform or arison inside a closed mold so its shape conforms to that of the mold cavity
• Blown film


  Extruded tube is expanded by inflating it with air before it solidifies, stretching it into a film
• Thermoforming


  Plastic sheet is heated to soften it and drawn by vacuum into the mold cavity
• Co-extrusion


  Layers of different polymer with different functions (e.g. barrier against oxygen, food compatibility) are extruded using separate extruders
• Injection molding


  Mold is closed and clamped, pellets are fed into hopper and melted by heater bands, screw moves forward to inject plastic into mold cavity under pressure, part cools and solidifies, mold opens and part is ejected
• Hygroscopic


  Polymers that absorb water
• If the moisture is not removed before molding, at high molding temperatures, the absorbed moisture will turn into steam, leading to defects in the part
• Moisture can also chemically react with the polymer (hydrolysis), leading to reduced strength and viscosity
• Drying


  The more commonly used drying technology used today is the dessicant dryer
• Viscosity


  The measure of "internal resistance" of the fluid to flow
• High viscosity fluid has poor flow properties
• In order to fill the mold quickly, the viscosity of the resin should be low
• The layer of fluid nearest the channel wall moves slowest due to friction with the channel. Similarly, the layer of fluid furthest from the wall moves the fastest
• Shearing motion


  Adjacent layers of fluid move parallel to each other with different speeds, producing a shearing motion
• The viscosity of polymer melt is NOT just a constant!
• Shear thinning


  When viscosity decreases with increased shear rate
• Above the glass transition temperature, the viscosity of amorphous polymer decreases gradually with increasing temperature
• Above the melting temperature, the viscosity of semicrystalline polymer decreases sharply and to a much lower viscosity than amorphous polymers
• During processing the molecules, fillers and fibres can be oriented in the flow and greatly affect the properties of the final part
• Anisotropic


  Due to the velocity profile, any molecule that is flowing in a non-parallel manner will have ends travelling at different velocities. The faster end of the molecule will move ahead while the slower end drops back, this orientates the molecule along the direction of flow.
• If the part is frozen fast enough, the molecular and fiber orientation will be "frozen in". As a result, the part will have anisotropic properties
• Shrinkage


  Polymer shrinks when it solidifies from melt. Shrinkage refers to a geometric reduction in the size of the part.
• Shrinkage is a cause of sink marks and voids in the part
• Shrinkage


  Polymers will shrink primarily due to changes in temperature and/or pressure