Plastic Co-Extrusion

Coextrusion is the process of pressing two or more materials through the same die to produce a single piece. When multiple plastics are combined, the result can yield properties distinct from those of a single material. Coextrusion has opened up new frontiers in material engineering and addressed several previously difficult manufacturing needs.

Coextruding a stripe of radiopaque plastic into a catheter, for example, improves x-ray quality as the catheter moves through a vein without compromising the effectiveness of the catheter itself. Coextrusion can also reduce costs by using recycled and reground scrap inside virgin material for handrails, fences and other applications. The process can be seen in projects as diverse as tubing and structural components or air blown food containers.

The Coextrusion Process

In standard extrusion, solid plastic pellets are gravity fed into a forming mechanism, where jacketed compression screws melt and feed the materials into a die. By contrast, coextrusion involves multiple extruders forming layered or encapsulated parts. Sometimes five or more materials are used in a single cycle, with each extruder delivering the precise amount of molten plastic needed for the operation..

Unlike ordinary plastic mixing, each individual plastic retains its original properties, but is combined into a compound-material part. If mixed prior to extrusion, the characteristics of the individual materials may be altered, but the end result is a homogeneous product.

Not all plastics are suitable for coextrusion because some polymers will not adhere to others, although introducing a conductive middle layer can often solve this problem. Plastics with drastically different melting temperatures are also unsuitable for the process, as degradation will occur in the lower melting material. Finally, PVC and acetals should never be coextruded together because of the potentially violent reactions that can occur when they are joined.

Coextruded Tubing

The multi-colored drinking straw is a good example of coextruded tubing’s design features. Striped tubing also serves many purposes in the medical field, in which stripes and colors can denote different chemicals. Coextrusion can produce internally hardened tubes through which a cable can be run while retaining the tube’s flexibility. Other tubes benefit from a high performance liner impervious to corrosives, or an inexpensive coating to add bulk and stability. In addition, plastic fiber optic cables are composed of a coextruded cable and jacket.

Coextruded Structural Units

Plastic is sometimes used as a substitute for wood. Manufacturers can create decking, boat docks, fences and dimensional components with “plastic lumber,” which has some advantages over natural wood. Coextrusion is a cost-effective method of fabricating many of these artificial materials. It can add titanium dioxide, a weather resistant material, to exterior structural plastics or produce decking with an inner layer of recycled plastic.

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Co-extrusion

Co-extrusionCo-extrusion is defined as the process in which two or more plastic materials are extruded through a single die. In this process, two or more orifices are arranged in such a manner that the conjoint merging and welding of the extrudates takes place and before chilling, a laminar structure forms. In co-extrusion, a separate extruder is used to fed every material to the die but the orifices can be arranged in such a manner that each extruder provides two or more plies of the same material. 



Co-extrusion may be employed in the processes of Film Blowing, Extrusion Coating, and Free Film Extrusion. The general benefit of the co-extrusion process is that every laminate ply imparts a required characteristic property like heat-sealability, stiffness, & impermeability, all of which are impossible to attain by using any single material. 



It is evident that co-extrusion is a better process than a single extrusion. For instance, in the vinyl fencing industry, co-extrusion process is used for tailoring the layers on the basis of whether these are exposed to weather or not. Generally, compound’s thin layer is extruded that contains high-priced weather resistant additives. This extrusion is done on the outside, whereas inside there is an additive package which is more suitable for the structural performance and impact resistance. 



Advantages Of Co-extrusion 

According to various internationally established and popular companies that are using the co-extrusion process continuously in their production procedures, there are a number of advantages of this process. Some of these advantages are listed below:

  • High quality mono-layer extrusion coatings in larger varieties of line speeds and widths
  • Use of lower cost materials for filling purpose, assists in saving on the amount of qualitative resins
  • Capability of making multi-layer as well as multi-functional structures that too in a single pass
  • Reduction in the number of steps required in general extrusion process
  • Provides targeted performance with the use of definite polymers in particular layers
  • Reduction in setup and trim scrap
  • Potential for use of a recycle layer

Disadvantages Of Co-extrusion 

As per a number of globally reckoned companies, there are some disadvantages related with the process of co-extrusion. Some of these disadvantages are as follows: 

  • Minor differences in physical properties are responsible for making a combination desirable, but these differences are also responsible for making the combination incompatible
  • For this process, polymers must have similar melt viscosities to sustain a laminar flow. All the viscosity differences may be more or less tolerable, according to the material location inside the composite structure along with the layer’s thinness
  • Requires more sophisticated extruder and its operator. This implies extra maintenance cost of the equipment.
  • Demands considerable planning as well as forethought in the system design.