Sheet Plastics and Thermoforming
Thermoforming is a common practice in P&O to fabricate test sockets, flexible sockets, orthoses, and other products. It may be used with various types and thicknesses of thermoplastic sheets. Plastic selection is dependent on many different factors as described in the chart and in the text below.
| Material |
Structure |
Density (g/cm3) |
Tensile |
Elong. |
Flexural Modulus |
Izod Impact |
Fatigue* |
Additional notes |
Use in P&O |
| ASTM Method | — | D 792 | D 638 | D 638 | D 790 | D 256 | — | ||
| Polyethylene Terephthalate Glycol (PETG) (e.g.,Vivak) | Am | 1.27 | 7.7 | 140 | 310 High stiffness | 1.7 | — |
|
Test sockets |
| Polypropylene (PP) | Cr | 0.9 | 5.4 | 200 | 225 | 1.2 | — |
|
Lower limb orthoses |
| Copolymer (CP) – a PP and PE blend | Cr | 0.9 | 5 | 200 | 135 | 6 | 24 |
|
Lower limb and Spinal orthoses |
| High Density Polyethylene (HDPE) | ≤95% Cr | 0.95 | 4 | 100 | 200 | No fracture | 19 |
|
Prefab devices |
| Low Density Polyethylene (LDPE) | 60% Cr | 0.92 | 1.4 | 500 | 30 Low stiffness | No fracture | — |
|
Flexible inner, pediatric AFOs, upper splints |
| Material |
Structure |
Density (g/cm3) |
Tensile |
Elong. |
Flexural Modulus |
Izod Impact |
Fatigue* |
Other |
Use in P&O |
|
Ethylene Vinyl Acetate (EVA) – a CP of ethylene and vinyl acetate (proflex, orfitrans, northvane) |
Semi-Cr | 0.9-1.13 | 3.6 | 2.5 Very low |
|
Flexible inner sockets | |||
| Surlyn Ionomer | Am | 0.94 | 2.1-5.4 | 400 | 4-8 | 6 | — |
|
Flexible inner liners |
| Acrylonitrile Butadiene Styrene (ABS) | Am | 1.05 | 4.3 | 20 | 300 | 6.6 | 16 |
|
Dummy parts |
| Thermoplastic Elastomer (TPE) | Semi-Cr and Am | 1.7 | 31 |
|
Foot orthoses | ||||
| Polycarbonate | Am | 345 | 12-16 |
|
Face masks | ||||
| Kydex – acrylic and PVC | Am | 335 very stiff |
18 |
|
Spinal orthoses | ||||
| Procomp | Lower limb orthoses | ||||||||
| Material |
Structure |
Density (g/cm3) |
Tensile |
Elong. |
Flexural Modulus |
Izod Impact |
Fatigue* |
Other |
Use in P&O |
Justify your reasoning for selecting materials using these properties
Stiffness, i.e., elastic modulus. Flexural rigidity equals the elastic modulus (E) multiplied by the area moment of inertia (I), e.g., geometry.
- Consider how much bending the plastic must be resist.
Tensile Strength – Resistance of a material to breaking under tension.
- Consider how much tension the plastic will need to resist, e.g., at location of screws.
Izod Impact Strength – Resistance to fracture when subjected to a sudden impact load
- Consider how the patient may create impact forces on the device, e.g., jumping or stepping over curbs/steps.
Creep or cold flow (viscosity) – tendency of a solid material to slowly move or deform under the influence of mechanical stress
- Consider areas that need to withstand sustained loads, e.g., windows of lower limb sockets.
Glass transition temperature (Tg) – Temperature at which an amorphous polymer changes from a hard/glassy state to a soft/leathery state, or vice versa.
- Consider the fabrication methods. Amorphous plastics have a large operational range (as compared to semi-crystallines), allowing the ability to bubble form. Additionally, Tg is related to the tensile strength, impact resistance, modulus of elasticity, and its operational temperature range.
Coefficient of friction – amount of friction between two objects
- Consider the neighboring materials and/or part body part.
Molecular Weight – the total mass of all the atoms in a molecule
- 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.