Low Density Polyethylene

Manufacturing
Polyethylene is a versatile material in its manufacturing capabilities and may be saw cut, die cut, or pinched.  It can be shaped by drape, frame, injection, vacuum, and pressure molding techniques.  Predrying is not required for thermoforming.  Moisture contained in the model should be sealed with a moisture barrier to resist bubbling and rippling.

The normal forming temperature range for Low Density Polyethylene is 270-280 F.  Once molded, it can be modified using localized heat.  They are self adhering at forming temperature and may be laminated.  At forming temperature, the plastic is extremely tacky.  One of the most beneficial properties of Polyethylenes is their excellent mold replication accuracy.

Polyethylenes may not be solvent welded or glued.  Medium strength welds can be obtained by using hot welds employing welding rods and hot air welding irons.  The welding rods are available in sizes and colors to match sheet material.

Sterilization
Polyethylenes can only be sterilized using ethylene oxide used in a 12%/88%: ethylene oxide/freon gas mix).

Precautions
Polyethylenes are combustible thermoplastics and therefore should be stored in a cool dry area.  Because they are petroleum derivitive plastics, they give off a heavy black toxic smoke that is dangerous to inhale and difficult to see through.  This material has ultraviolet stabilizers and is tolerable to most outdoor applications.  Due to their excellent mold replication properties, vacuum interface is almost always required when vacuum forming to prevent marking off.  If air holes are required to draw vacuum they should be small.  Using vacuum holes of no larger than 1/32" diameter is required to prevent marking of the plastic.

Applications
Applications of this material is ever increasing.  It can be used anytime extreme flexibility is necessary and high strength and shape retainment are not required.  Polyethylene is used in upper and lower extremity orthotics to make tongue closures for shell and cuff designs.  Applications of certain thicknesses have been used for tibial fracture orthoses.  It can be used for flexible hydrostatic control spinal orthotics.

Polyethylenes have made their greatest advancement in prosthetics.  Their use was limited to upper extremity humeral cuffs.  With the invention of the flexible brim and suction suspension upper and lower extremity prosthetics, the use of Polyethylenes has grown.  Specific thicknesses are required for these applications, but the increase in patient comfort cannot be matched by other materials.

Sheet Chart
Low Density Polyethylene is one of the most frequently used sheet resins in today’s orthotic devices due to its exceptional formability.  Polyethylene leads to excellent patient compliance. It is flexible and hypoallergenic.  It is not suitable for high strength and rigidity.