PTFE is a fluoropolymer that does not melt in the traditional sense. Unlike conventional thermoplastics that are melted and forced through a die, PTFE cannot be melt-processed due to its extremely high melting point (~327°C) and its tendency to degrade before it flows. As a result, PTFE extrusion uses a technique called paste extrusion for tubing and other long continuous forms.
Paste extrusion is a cold extrusion process that relies on a lubricated PTFE powder to form the desired shape. After extrusion, the product undergoes a sintering process, essentially a high-temperature bake, that bonds the particles and provides mechanical integrity.
Key Steps in the PTFE Tubing Extrusion Process
1. Raw Material Selection and Blending
The extrusion process begins with high-purity PTFE fine powder resin. This powder is typically sourced from well-known chemical manufacturers and meets tight specifications for particle size and distribution. Before extrusion, the PTFE powder is blended with a lubricant, often an organic solvent such as Isopar® or naphtha. This mixture forms a soft, dough-like paste.
Key factors during this stage include:
- Powder uniformity: Ensures consistent flow during extrusion.
- Lubricant ratio: Influences extrusion pressure and surface finish.
- Mixing time and conditions: Over- or under-mixing affects paste quality.
The paste is then aged for several hours (usually 8–24) to allow uniform lubricant distribution and promote better extrusion consistency.
2. Preforming
After the aging process, the PTFE paste is preformed into a cylindrical billet using a hydraulic press. This step compacts the lubricated powder into a dense shape suitable for extrusion. Preforming reduces voids, aligns particles, and builds structural strength before the extrusion step.
Important parameters:
- Preforming pressure: Typically ranges between 200–500 bar.
- Billet dimensions: Must match the extrusion die requirements.
3. Paste Extrusion
The preformed billet is placed into an extrusion press equipped with a die designed for tubing production. The die consists of an outer sleeve and a mandrel that determines the inner and outer diameters of the extruded PTFE tubing.
During extrusion:
- The billet is pushed through the die using mechanical or hydraulic force.
- The lubricant reduces friction and allows the paste to flow through the die.
- The extruded tube exits as a green (unsintered) tube, still soft and pliable.
Extrusion speed, pressure, and die temperature (typically kept ambient to avoid premature sintering) are closely controlled to maintain dimensional accuracy and prevent defects such as cracking or tearing.
4. Drying
After extrusion, the green tubing contains a significant amount of lubricant, which must be removed before sintering. The tubing is passed through a drying oven at moderate temperatures (~100–200°C) to evaporate the lubricant slowly and uniformly.
Key considerations:
- Drying rate: Too fast can cause blistering or collapse; too slow extends cycle time.
Ventilation and solvent recovery: Solvent fumes must be captured and safely managed to meet environmental regulations.
5. Sintering
Once dried, the PTFE tubing is sintered in a high-temperature oven at temperatures above 360°C. This step is crucial, as it transforms the soft, powder-based structure into a solid, chemically resistant, and mechanically stable product.
The sintering process involves:
- Controlled temperature ramp-up: Prevents thermal shock and warping.
- Soaking at peak temperature: Ensures full bonding of particles.
- Slow cooling: Avoids internal stresses and cracking.
Sintering conditions vary depending on tubing size and wall thickness. Thicker tubing may require slower heating and cooling cycles to ensure uniform properties throughout the cross-section.
6. Final Processing and Quality Control
After sintering, the tubing is inspected and cut to the desired lengths. Secondary processes, such as flaring, tapering, or etching (for surface treatment), may also be applied.
Typical quality control checks include:
- Dimensional analysis: Outer diameter (OD), inner diameter (ID), wall thickness.
- Visual inspection: Looks for cracks, voids, or surface imperfections.
- Physical testing: Tensile strength, elongation, and burst pressure.
- Chemical cleanliness: Ensures no residual solvents or contaminants remain.
Compliance with standards such as ASTM D3295 or ISO 13000 may be required depending on the industry.