Extrusion techniques can be used to process most thermoplastics and some thermoset plastics. The resins most commonly extruded for medical applications include polyethylene, polypropylene, polyurethane, polystyrene, fluoropolymers, polyamide, polyester, and flexible polyvinyl chloride. A characteristic that often differentiates extruded from injection-molded plastics is the viscosity of the plastic at normal processing temperatures. Extruded plastics often have a higher melt viscosity, which allows the extrudate to retain the shape imparted to it by the die while the extrudate is in the quenching stages.Combinations of various resins can be used to gain special physical, biological, or chemical properties. Many additives can be used during the extrusion process to enhance processing characteristics of the polymer or to alter product properties. Such additives include lubricants, thermal stabilizers, antioxidants, radiopacifying agents, and colorants.
Because extrusions are not 100% contained by metal tooling, tolerances must generally be looser than other molding processes. While specialized tooling can hold tighter tolerances, generally these are the “normal tolerances” you should expect:
Wall thickness: ±.005
Cut length: ± .062 or more
Width or height: ± .010 per inch of width
Straightness: .045 bow per foot
Advantages of Extrusion
- Low cost tooling and short lead times
- Low Cost parts.
- Use of multiple materials in a variety of durometers.
- Reinforcement via fiber wrapping.
- Color matching.
Compare to other processes:
Often, extruded parts may be injection molded or thermoformed, depending on their shape. The attraction of extrusion is the low tooling cost, and shorter lead times. For high volume parts, such as sawed-off short clips, injection molding may be a lower cost option. While the piece price may be lower, design changes are required to create draft angles, and tooling is much higher. A “trough” shape can be vacuum formed, but, if it can be extruded, it will be cheaper, as the vacuum formed material has been extruded into sheet already. In profile extrusion, you convert raw resin directly to a finished part.