The Co-Extrusion Process for Plastics

In its very basic form, the extrusion process involves melting plastic granules, pulver or pellets, pushing them through a die to create a shape. This shape is then cooled and set to the correct dimensions and then cut to the right length.  

Co-Extrusion requires multiple extrusion machines forcing two or more different materials through the same die, creating a single piece of layered or encapsulated part. The resulting part can yield properties distinct from that of a part produced using a single plastic, due to the combined materials. 

Sometimes, five or more materials will be used in a single cycle, but each individual plastic will retain its original properties while being combined into a single part. 

Common Uses of Co-Extrusion 

Plastic use has been so ingrained in our daily lives, it’s hard to imagine a world without it. Coextrusion contributes to the increased usefulness of plastic by continually combining and therefore updating the functionality of plastics we already know and use. 

• Striped Tubing – drinking straws and wiring are two common places in which we see the use of coextrusion. The striped tubing is the result of the two materials being pushed together. Coextrusion can produce internally hardened tubes through which a cable can be run – think of the green and yellow earth wires in a common plug – they retain flexibility whilst still boasting a solid exterior. 
• Structural Units – coextrusion is a way of fabricating many artificial materials, such as those used for decking instead of wood. It can add a weather resistant material (often titanium dioxide) with an inner layer of recycled plastic, you have a product that is cost effective and yet has a perfect finish. 
• Sheet Materials – coextruded sheets that resist degradation from light exposure can be used to form UV and infrared barriers that reflect damaging rays. These still allow visible light to pass through but do not yellow over time, like other materials may. 

Common Plastic Bases 

Some of the more common types of co-extruded plastic bases are: 

• PET (Polyethylene terephthalate) – This is an environmentally friendly plastic used in food and drinks packaging thanks to its durability. 
• HIPS (High impact polystyrene) – This is a hard plastic that withstands a lot of force before breaking, commonly used for toys. 
• PETG (Polyethylene terephthalate glycol) – This is a modified PET product which is good for packaging and blister packs. 

• HDPE (High density polyethylene) – This is made from petroleum, used commonly for 3D printing and hard hats. 
• ABS (Acrylonitrile butadiene styrene) – This is used in a lot of medical devices. 
• PVC (Polyvinyl chloride) – This plastic has a number of different uses, but most commonly associated with houses or structures.  

Benefits of Co-Extrusion 

As mentioned, the combination of materials to suit a job specification is one of the best advantages of co-extruded plastics.  

The most common reason for using a co-extruded plastic is to keep the cost down on parts that require a high level of finish, but also require a lot of thickness for strength. 

Seals and gaskets that require flexibility to form a seal, but also a level of rigidity so that the part can be fixed to another product such as a door, are commonly coextruded. 

Important Notes on Co-Extrusion 

It is, however, worth noting that not all plastics are suitable to use in the co-extrusion process as some polymers will not adhere to others, or the meting temperatures of two plastics are drastically different.  

This can end up resulting in the lower melting material experiencing degradation when heated to the temperature of the higher melting material. 

PVC and POM can experience violent reactions when joined, so should never be coextruded. 

The future of coextrusion is simply to continue to increase the functionality of the produced materials. Coextruded materials are already useful in a number of industries (packaging, medical and energy) and the effectiveness and usability of products in the future depends on the combination of plastics used.  

For more information about coextrusion, or to find the polymer most suitable for your needs, contact us at Rayda today.