Overview of the Welding Object & Process Principle PU and TPU
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1.1 Material Characteristics of Coated Fabrics
The fabric features a polyurethane (PU) coating on the front (excellent wear resistance, flexibility, and gloss) and a thermoplastic polyurethane (TPU) coating on the back (superior waterproofness, chemical resistance, and low-temperature flexibility). Both PU and TPU belong to thermoplastic polymers, which soften and melt when heated, and re-solidify to form a bond upon cooling—this provides the fundamental condition for high-frequency welding.
1.2 Core Principle of High-Frequency Welding
High-frequency welding (also known as dielectric welding) utilizes high-frequency electromagnetic fields (typically 27.12MHz or 40.68MHz) to act on the overlapping PU and TPU coating layers. The polar molecules in PU/TPU undergo rapid oscillatory friction (up to millions of times per second) under the electromagnetic field, converting electrical energy into heat energy. This localized heating melts the coating surfaces to a viscous state, and the application of pressure promotes molecular diffusion between the layers. After the high-frequency field is turned off, the melted material cools and solidifies quickly, forming a tight, seamless weld joint.
2. Key Process Flow
2.1 Pre-Welding Preparation
- Material Pretreatment: Wipe the welding area of the PU and TPU surfaces with anhydrous ethanol to remove oil, dust, or release agents—contaminants can cause poor fusion and reduce joint strength. For fabrics with moisture content >0.5%, pre-dry at 40–50℃ for 10–15 minutes to avoid "spark discharge" during welding.
- Mold & Equipment Setup: Install a copper electrode mold matching the weld seam shape (e.g., straight line, curve, or sealing ring) on the high-frequency welding machine. Adjust the mold gap to 0.1–0.3mm greater than the fabric thickness to ensure uniform pressure distribution.
2.2 Welding Execution
- Overlay Positioning: Align the PU-coated surface of one fabric with the TPU-coated surface of another, ensuring the overlapping width of the welding area is 5–15mm (adjustable based on strength requirements).
- Electromagnetic Heating & Pressurization: Lower the electrode mold to apply a pressure of 0.3–0.8MPa to the overlapping area, then activate the high-frequency power supply. The electromagnetic field penetrates the fabric base and acts directly on the PU/TPU coatings, heating them to 120–180℃ (the melting point range of PU/TPU) within 0.5–3 seconds.
- Cooling & Shaping: Maintain pressure for 1–2 seconds after stopping the high-frequency input. The built-in cooling system (water-cooled mold) accelerates the solidification of the melted coating, ensuring the weld joint retains its shape and strength.
2.3 Post-Welding Inspection
- Visual Check: Confirm the weld seam is smooth, free of burns, bubbles, or gaps.
- Performance Testing: Conduct a peel test (required peel strength ≥3N/mm for general industrial fabrics) and waterproof test (no water leakage under 0.2MPa pressure for 30 minutes) to verify joint reliability.
3. Critical Process Parameters & Optimization
The parameters directly determine the weld quality—they must be adjusted based on fabric thickness (typically 0.2–2.0mm) and coating weight (30–150g/m²):
| Parameter | Typical Range | Optimization Basis |
|---|---|---|
| High-Frequency Power | 500–2000W | Thicker coatings (≥80g/m²) require higher power; excessive power causes coating carbonization. |
| Welding Pressure | 0.3–0.8MPa | PU coatings (softer) use lower pressure; TPU coatings (higher rigidity) need slightly higher pressure. |
| Heating Time | 0.5–3s | Calculated by the formula t = L/v (L = electrode-to-press-roller distance; v = welding speed). |
| Cooling Time | 1–2s | Extend to 2–3s for low-temperature environments (≤15℃) to avoid joint brittleness. |
4. Application Scenarios & Advantages
4.1 Typical Application Fields
This process is widely used in products requiring both flexibility and sealing:
- Protective gear: Waterproof gloves, chemical-resistant aprons (relying on TPU’s chemical resistance and PU’s wear resistance).
- Outdoor products: Tent seams, inflatable mattress interfaces (weld joint waterproof rate ≥IPX7).
- Medical supplies: Disposable protective clothing, isolation gowns (seam tensile strength meets GB 19082-2009 standards).
4.2 Core Advantages vs. Traditional Processes
| Aspect | High-Frequency Welding | Adhesive Bonding (Traditional Method) |
|---|---|---|
| Bonding Speed | 0.5–3s per joint | 2–4 hours (including curing time) |
| Environmental Friendliness | No adhesives/solvents | Volatile organic compounds (VOCs) emitted |
| Durability | Resistant to water, oil, and aging | Adhesive failure prone to moisture/heat |
| Seam Performance | Seamless, air/water-tight | Prone to gaps at adhesive edges |
5. Common Issues & Solutions
| Issue | Root Cause | Solution |
|---|---|---|
| Weld joint peeling easily | Insufficient heating time/pressure | Increase power by 10–20% or extend heating time by 0.5–1s. |
| Coating burns (yellowing) | Excessive power or prolonged heating | Reduce power by 15–30% and optimize electrode contact area. |
| Bubbles in the weld seam | Moisture in coatings or fabric | Pre-dry the fabric at 50℃ for 15 minutes before welding. |
| Uneven weld strength | Uneven pressure distribution | Replace worn electrodes and adjust mold parallelism. |
This process leverages the thermoplastic nature of PU and TPU to achieve efficient, high-performance bonding. If you need more details—such as recommended equipment models for different fabric thicknesses, or weld joint design specifications for curved seams—feel free to let me know, and I can further refine the content for your specific application scenario.