I. Overview of Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE), renowned for its chemical inertness, low coefficient of friction, resistance to extreme temperatures, and electrical insulation properties, finds extensive application in chemical engineering and related fields. The processing of its sheet and rod profiles is critical to product manufacturing. Above its melting point, PTFE exhibits extremely high melt viscosity, precluding conventional melt processing. PTFE sheets and rods are typically produced by powder moulding and sintering into blanks, followed by machining to shape. The core processing steps are detailed below.
II. Raw Material Selection and Preparation
Processing commences with selecting suitable PTFE raw materials, choosing different PTFE resin powders based on product properties such as strength.
Suspension-polymerised resin: Characterised by coarse particles and excellent flow properties, it is employed for moulding large-dimension or high-strength plate and rod blanks.
Dispersion-polymerised resin: Featuring fine particles, it is often blended with additives to enhance processability. It is formed into small-diameter rods and other blanks via extrusion or similar methods, followed by sintering.
Following resin selection, preforming occurs: powder is uniformly filled into moulds and compressed under high pressure (10–50 MPa) at ambient or slightly elevated temperatures to form green compacts. This process expels air and promotes initial particle bonding, laying the foundation for subsequent sintering.
III. Mechanical Cold Working Techniques
PTFE blanks exhibit low hardness, poor thermal conductivity, and a high coefficient of thermal expansion, making cold working techniques the primary processing method.
Turning: Commonly employed for machining flat surfaces on bars and similar stock. Utilise sharp tools with strict control of cutting parameters:
Cutting speed: 50–200 metres per minute to prevent material softening and tool adhesion.
Feed rate: 0.05–0.2 millimetres per revolution for finishing passes, maintaining moderation to avoid friction-induced heating and material tearing.
Cutting depth: 0.1–0.5 millimetres for finishing operations.
Tool angles: Large rake angle (15°–30°) and clearance angle (10°–15°) to maintain cutting edge sharpness.
Milling: For machining flat surfaces, employing sharp tools with climb milling. Commonly uses high-speed steel milling cutters, relying on compressed air for cooling and chip removal.
Sawing/Cutting: For blanking or sectioning, employ band saws etc. with coarse-pitch, sharp-edged blades. Maintain moderate cutting speeds to prevent material overheating and adhesion.
Drilling and Tapping: Use sharp twist drills at low speeds with small feed rates and prompt chip evacuation. Given PTFE's low thread strength, design coarse-pitch, shallow threads and tap gently.
IV. Surface Modification Treatment
PTFE possesses low surface energy, presenting challenges such as adhesion difficulties. Following machining, surface activation treatment is often required.
Chemical Etching Method (Sodium-Naphthalene Treatment): A classic and effective technique. Workpieces are immersed in a sodium-naphthalene complex solution, where active sodium atoms attack fluorine atoms, forming a carbonised layer and introducing polar functional groups. Processing time must be strictly controlled within seconds to minutes, followed by thorough rinsing and neutralisation.
However, the solution is highly flammable, explosive, and corrosive, necessitating stringent protective measures.
Plasma treatment: Environmentally sound and safe, this method generates plasma in vacuum or specific atmospheres, where particle bombardment introduces polar groups. Effectiveness may diminish over time.
Other methods: Laser processing and similar techniques remain less widely adopted.
V. Post-Processing and Quality Inspection
PTFE components undergo post-processing upon completion of machining.
Stress Relief: High-precision or components subjected to significant machining stresses may be annealed.
Cleaning: Removal of contaminants such as swarf and oil residues.
Quality Inspection: Comprehensive testing against product standards and application requirements.
Dimensional Accuracy: Measurement of critical dimensions against tolerances using tools such as calipers.
Visual Quality: Visual inspection of surfaces and machining finish.
Performance Sampling: Measurement of density and other physical/electrical properties.
