The Art and Science of Extrusion: A UK Manufacturer's Perspective
Understanding Extrusion
Extrusion is a fascinating process where hot material is pushed through a form or die to create a continuous profile, such as a tube. To achieve the hollow section of a tube, a pin or mandrel is placed inside the die, often with positive pressure applied to the internal cavities through the pin. Once the new product emerges, it requires cooling, which is typically done using a water bath. This bath is maintained in a vacuum-controlled environment to prevent the tube from collapsing. Common shapes for extruded tubes include round, square, and triangular, but virtually any continuous shape is possible. At our facility, we use various materials for extrusion, with clear tubes often made from PETG, PET, or propionate.
Challenges in Extrusion
One of the main challenges we face with PET material is managing the temperature differences. PET needs to be extruded at approximately 500°F (260°C), a significant increase from the 400 to 425°F (205 to 218°C) range for other materials. While this might not sound drastic, balancing the six or seven heat exchangers and the head temperatures, each with its own adjustable settings, can be quite tricky. The speed of the injection screw must also be carefully controlled to avoid overheating. Adding to the complexity is the need to manage the water flow and vacuum chamber, making the process a challenging yet rewarding endeavour.
What is PET?
Polyethylene Terephthalate, or PET, is a durable and strong material widely used in packaging applications, especially bottles. Known for its transparency, chemical resistance, and high strength, PET is the most common plastic material in the world. Its uses include:
- Plastic bottles for mineral water and carbonated soft drinks
- Tape applications
- Thermoforming sheets
- Food and cosmetic packaging
- Electronics
What is PETG?
The addition of “G” in PETG stands for Glycol, transforming it into a thermoplastic polyester that offers high chemical resistance and improved formability during manufacturing. Glycol helps prevent common PET issues such as cloudiness, fragility, and overheating, making PETG ideal for 3D printing and various other applications. PETG combines the beneficial properties of glycol and PET, reducing the overheating challenges faced by manufacturers. Its uses include:
- Medical packaging
- Beverage containers for cooking oils, drinking water, and soda
- Display stands and machine protection covers due to its ease of colouring
Recycling PET vs PETG
Despite their chemical similarities, PET and PETG differ significantly in terms of recyclability. PET, identified by resin code 01, is widely recycled in the UK through household recycling streams. In contrast, PETG, with resin code 07 (mixed/other recycling), is often excluded from recycling programs due to the higher temperatures required for processing. This difference in recyclability means that PETG is more likely to end up in incineration or landfill, despite being almost unbreakable, affordable, and durable.
Conclusion
At Sinclair & Rush UK, we strive to provide an honest, scientifically-based dialogue to help you make informed decisions about plastics and their alternatives. Understanding the nuances between materials like PET and PETG, and the challenges involved in their processing, is crucial. Currently we are able to re-use all waste generated from the production process and pass on cost efficiencies. Our goal is to equip you with the knowledge to make educated choices and intelligently defend your position. By working together, we can contribute to making our planet a better place.
