Sheet Plastics and Thermoforming

Material properties and characteristics relevant to thermoplastic selection and fabrication in P&O.

Plastic selection is dependent on many different factors as described in the text and table below. Justify your reasoning for selecting thermoplastics by using the following properties. Use the table below this list to compare values.

• Mechanical Properties
  • Stiffness: Consider how much bending the plastic must be resist.
  • Tensile Strength : Consider how much tension the plastic will need to resist, e.g., at location of screws.
  • Izod Impact Strength : Consider how the patient may create impact forces on the device, e.g., jumping or stepping over curbs/steps.
  • Creep or cold flow (viscosity): Consider areas that need to withstand sustained loads, e.g., windows of lower limb sockets.
  • Coefficient of friction: Consider the neighboring materials and/or part body part.
• Processing/Fabrication Characteristics
  • Glass transition temperature (T_g): Consider the fabrication methods. Amorphous plastics have a large operational range (as compared to semi-crystallines), allowing the ability to bubble form. Additionally, T_g is related to the tensile strength, impact resistance, modulus of elasticity, and its operational temperature range.
  • Molecular Weight: Consider the type of printer and intended use of the component part. The lower the molecular weight the more runny the material is when in the melt state making it easier to extrude, however it stretches more when forming, and is less fatigue resistant and less impact resistant.
  • Crystalline vs. Amorphous (microscopic structure of the plastic)
  • Thermoplastic used in O&P exhibit specific crystallinity or crystal structure.  They exist in three structural forms: crystalline, semi-crystalline, or amorphous.  Shrinkage is especially important when working with semi-crystalline materials like polypropylene, polyethylene, and copolymers.  These materials have a higher coefficient of thermal expansion (CTE) when compared to PETG (amorphous), so they expand more when heated when compared to PETG, thus inversely contract or shrink more upon cooling.
    • Crystalline (Cr) polymers have highly ordered, repeating molecular arrangements that form distinct crystal structures, resulting in materials with sharp melting points (T_m), high strength, and low permeability.
    • Semi-crystalline (Semi-Cr) polymers contain both crystalline regions (ordered domains) and amorphous regions (disordered areas), giving them a balance of properties like moderate flexibility and strength with a melting temperature range rather than a sharp point.
    • Amorphous (Am) polymers have completely random, disordered molecular arrangements with no crystalline structure, making them more flexible, transparent, and exhibiting a glass transition temperature (T_g) instead of a distinct melting point.

Material Properties^[1][2][3][4]