In plastic welding (such as ultrasonic welding, heat staking, vibration welding, and rotary welding), the welding process involves using heat and pressure to melt and rejoin the two plastic contact surfaces. In theory, if the following conditions are met, welding can be done directly without the need for pre-treatment:

• Correct material selection

• Clean welding surfaces

• Stable process conditions

This is the most common scenario in mass production lines. However, in certain cases, light, correct pre-treatment can significantly improve welding stability and yield.

Suggested Pre-Treatment Tasks Before Welding (Detailed Explanation)

1. Clean Molds and Plastic Part Surfaces (Most Important)

Purpose:

• Prevent contaminants from obstructing the melting process.

• Improve energy transmission and fusion at the welding surface.

Common Contaminants:

• Oil (lubricants, hand sweat)

• Mold release agents (anti-rust oils)

• Dust from the air

• Foreign materials from the process

Suggested Practices:

• Regular cleaning of molds to avoid oil contamination on plastic parts.

• Avoid hand contact with the welding surface of plastic parts.

• Use IPA (Isopropyl Alcohol) to lightly wipe the welding surface if necessary.

• Important: Avoid cleaners that leave solvent residue on the surfaces.

2. Remove Mold Release Agent Residue

Why Mold Release Agents Affect Welding:

• Mold release agents are often low-surface-energy materials.

• They form a separation layer that causes:

  • Insufficient welding strength.

  • Unstable welding appearance.

  • Higher likelihood of false or weak welds.

Situations That Need Extra Attention:

• External spray-type mold release agents.

• Worn-out molds with poor release properties.

• Aesthetic or sealing parts.

Improvement Suggestions:

• Prefer using internal mold release formulations for raw materials.

• Minimize the use of mold release agents.

• If necessary, perform surface cleaning or let parts sit briefly to allow the release agent to evaporate.

3. Keep the Welding Surface Dry

Impact of Moisture on Welding:

• During heating, moisture can rapidly vaporize.

• This may cause:

  • Bubbles in the weld.

  • Whitening or silver streaks.

  • Reduced welding strength.

Special Cases That Are Particularly Affected:

• High-humidity materials like PA (Nylon), PC, and PET/PBT.

• High-humidity production environments.

• Parts stored for long periods.

Suggested Practices:

• Dry raw materials and finished products based on material needs.

• Prevent condensation from forming before welding.

• Control humidity around the welding area.

4. Surface Microtexturing (Applicable to Specific Materials)

Why Certain Materials Require Surface Texturing?

Materials like PP and PE:

• Have low surface energy.

• Have very smooth surfaces.

• Can "slide" during melting, leading to unstable welding.

Purpose of Microtexturing:

• Increase friction on the contact surface.

• Improve stability in the molten zone.

• Help concentrate energy at the welding interface.

Possible Methods:

• Light sanding (only for trial runs or maintenance).

• Pre-design features in molds:

  • Energy Directors.

  • Fine patterns.

Important: Do not over-texturize, as this could lead to uneven melting or weld defects.

Dependence on Pre-Treatment for Different Welding Methods

Practical Judgement Principles (Quick Conclusions)

Situations Where Pre-Treatment Is Not Typically Needed:

• Stable materials and clean molds.

• Internal mold release formulations used.

• Controlled production environments.

• Stable welding yield.

Situations Where Pre-Treatment Is Recommended:

• Unstable welding strength.

• Poor weld appearance.

• New material or mold changes.

• Low surface energy materials (e.g., PP, PE).

• High humidity environments or moisture-sensitive materials.

By following these guidelines, you can enhance welding quality and consistency, especially in situations where pre-treatment can provide significant benefits.