In the production of pyramid tea bags, ultrasonic welding and tag sealing are two of the most important technical steps. When using PLA (polylactic acid) materials, many manufacturers encounter problems such as weak welding, thread detachment, or tag delamination.

One of the most common root causes is material incompatibility, especially when PLA is welded together with nylon components.

This article explains why this happens and how to avoid these costly mistakes.

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The Core Issue: PLA and Nylon Are Not Compatible 

PLA and nylon are two completely different types of polymers:

• PLA: A polyester made from lactic acid.
• Nylon: A polyamide made from diamines and diacids.

Their molecular structures, polarity, melting temperatures, and melt viscosity differ greatly. Because of this, they cannot fuse together during ultrasonic welding.

This incompatibility leads to serious production failures.

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What Problems Occur When PLA Meets Nylon? 

1. Ultrasonic Welding Failure 

Ultrasonic welding works by using high-frequency vibration to create frictional heat, melting the contact area so materials can fuse together.

Ideal condition:

• PLA mesh + PLA thread

• Similar melting points (around 150–160°C)

• Melt blends well

• Strong, clean, airtight sealing

But when nylon thread is used:

• Melting temperature mismatch: Nylon melts at around 220°C, much higher than PLA. PLA overheats or degrades before nylon begins to melt.

• They do not bond: Even if both melt, PLA and nylon behave like oil and water. They repel each other and cannot form molecular bonding.

Final results:

• Extremely weak welds (easily pulled apart)

• Poor sealing → leakage, moisture absorption

• Visible defects → burnt edges, discoloration, rough welds

2. Tag (Laminate) Problems 

If the tag coating or base film is nylon while the pyramid tea bag body is PLA, the issues continue:

Recycling contamination

• PLA is valued because it is industrially compostable.

• Nylon is not compostable.

• Mixing nylon with PLA means the entire tea bag can no longer be certified compostable.

Tag detachment

Because nylon and PLA do not adhere well, the tag may:

• curl

• lift

• fall off during use

Degradation problems

A nylon layer acts like a barrier, preventing microbes and moisture from reaching the PLA. This slows or stops the composting process.

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Why Do Manufacturers Make This Mistake? 

• Misunderstanding nylon’s performance: Nylon is strong and durable, so some assume it is a good choice for threads or laminations.

• Existing supply chain: Some factories already have nylon materials on hand and use them without considering compatibility.

• Lack of polymer knowledge: Many producers do not fully understand melt compatibility between plastics.

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The Correct Solution: Use Compatible Materials 

1. For welding thread 

Use the same material as the pyramid tea bag mesh, such as:

• PLA thread

• Or compatible biodegradable polyesters

2. For tags 

Use:

• PLA film tags

• PLA-based coating

• Other compostable and PLA-compatible polyesters (e.g., PBAT, PBS)

• Paper tags with PLA coating

Avoid nylon completely.

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Perform Proper Compatibility Testing 

To ensure consistent quality, manufacturers should conduct:

• Welding strength test: Pull tests to confirm strong bonding.

• Sealing test: Check for leakage and airtightness.

• Compostability certification: If selling compostable pyramid tea bags, all components—including mesh, thread, tags, and inks—must pass standards such as:

  • BPI (USA)

  • DIN CERTCO (Europe)

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Conclusion

Mixing nylon components into a PLA pyramid tea bag system is a fundamental design mistake. It leads to:

• Production failures (weak welding, leaks)

• Product quality issues

• Loss of compostability

• Environmental contamination

For brands producing eco-friendly PLA pyramid tea bags, ensuring material compatibility across ALL components is essential for product quality, brand reputation, and environmental responsibility.