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Sheet Plastics and Thermoforming

Thermoplastic Use in P&O General behavior/characteristics
High-Density Polyethylene (HDPE) Prefab devices  
Low Density Polyethylene (LDPE) Flexible inner, pediatric AFOs, upper splints
  • Very low stiffness (flexible) – used for non-weight bearing
  • Creep – cold flow with sustained pressure
  • Blemishes easily – avoid touching surface when hot
  • Not notch sensitive – still, smooth edges
EVA – Ethylene Vinyl Acetate (proflex, orfitrans, northvane) Flexible inner liners  
Polypropylene (PP) Lower limb orthoses
  • Moderate stiffness – used for weightbearing
  • Blemishes easily – avoid touching surface when hot
  • Notch sensitive – cut circumferentially to relieve stress
  • May warp – leave vacuum on 2x melting time
Copolymer (CP) a PP and PE blend  
  • Moderate stiffness
  • Creep – cold flow with sustained pressure
  • Blemishes easily – avoid touching surface when hot
  • Moderately notch sensitive – polish edges to reduce crazing
Procomp Lower limb orthoses  
Thermoplastic Elastomer (TPE) Foot orthoses  
PETG – Polyethylene Terephthalate Glycol Test sockets  
Polycarbonate Face masks  
Kydex Spinal orthoses  

Structure:

  • Crystalline (Cr)
  • Semi-crystalline (Semi-cr)
  • Amorphous (Am)

1) Know these plastics: PP, LDPE, PETG (Vivak), copoly, Kydex, EVAs: proflex, orfitrans, northvane, ProComp

Standard AFO thickness is 3/16”

2) How to justify selection of materials for P&O devices using these reasons: Stiffness, Tensile Strength, Izod Impact Strength, Viscosity: Melt Strength and Creep, Coefficient of friction, Molecular Weight

3) Fabrication issues:

Plastic Shrinkage is dependent on Extrusion Direction

Plastic warpage is due to Uneven Cooling

Quenching reduces the fatigue resistance

Overstretching can lead to premature cracking

Avoid thermoplastic failure

License

Lab Safety, Tools, and Techniques in Prosthetics and Orthotics Copyright © by Sue Spaulding; Amanda Barizo; Susan Kapp; and bryan roehr. All Rights Reserved.

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