Why Your PET Geogrid Keeps Failing at the Node

If you manufacture PET or fiberglass-reinforced geogrids, you have seen it: the geogrid passes tensile testing on the rib, but fails at the weld node. The fiber breaks precisely where the ultrasonic horn touched it.

For years, this has been dismissed as a "process tuning issue" — adjust the pressure, tweak the weld time, change the amplitude. But the truth is uncomfortable: vertical ultrasonic welding inherently damages PET and glass fibers at the node. No amount of tuning can fix a physics problem.

This article explains exactly why it happens, why the industry's current alternatives are also flawed, and what the real solution looks like.

1. The Physics of Fiber Damage in Vertical Ultrasonic Welding

In vertical ultrasonic welding, the horn oscillates perpendicular to the workpiece surface at 15-20kHz. The vertical impact drives the horn tip into the material, creating friction between the two strip surfaces. The plastic matrix melts and flows, forming the weld.

But here is the problem: glass fibers and PET fibers are brittle under shear loading. The vertical hammering action creates a shear force at the fiber-matrix interface. For PP and steel-plastic geogrids, this is manageable because the polypropylene matrix is ductile and absorbs the impact. But for PET and fiberglass, the fibers fracture under the repeated vertical impact before the matrix has a chance to melt and flow.

The measurable consequences:

• Node strength drops to 40-60% of the parent material strength, compared to 90%+ achievable with PP geogrids
• The weld node becomes a stress concentration point — the grid fails at the node, not the rib, under load
• Production speed must be reduced by 20-40% to minimize fiber damage, directly hurting throughput
• Higher reject rates and inconsistent quality from roll to roll

Some manufacturers have tried to compensate by reducing amplitude, increasing weld time, or adding protective coatings to the fibers. These measures help at the margins but do not eliminate the fundamental problem. The vertical shear is still there.

2. What Customers Are Actually Experiencing

Based on field feedback from geogrid manufacturers worldwide, the most common complaints about PET and fiberglass geogrids produced on vertical ultrasonic lines are:

These are not hypothetical problems. They are the daily reality for manufacturers trying to produce PET and FRP geogrids on conventional ultrasonic welding lines.

3. Why "Just Switch to Thermal Welding" Is Not the Answer

Some equipment suppliers recommend switching from ultrasonic to thermal (hot plate) welding for PET materials. Thermal welding applies heat through conduction rather than mechanical vibration, so there is no fiber damage from vibration.

But thermal welding introduces a different set of problems:

• Weld time per node: 2-5 seconds vs. 0.3-0.8 seconds for ultrasonic — reduces throughput by 3-5x
• Heating elements are consumables that must be replaced regularly
• Energy efficiency is significantly worse — the entire hot plate must be heated, not just the weld zone
• Warm-up time of 10-20 minutes means wasted time between product changeovers

Thermal welding solves the fiber damage problem but creates a productivity crisis. For high-volume production, it is not a viable alternative.

4. Where the Industry Is Looking Next

The industry has recognized that a fundamentally different approach is needed. The most promising direction is linear vibration friction welding — where the tool vibrates horizontally rather than vertically, creating frictional heat without the damaging vertical shear.

But as we will cover in the next article in this series, linear vibration friction welding itself has split into two implementation paths — motor-driven and electromagnetic-driven — and both have critical engineering compromises when applied to geogrid production.

The full analysis, including detailed comparison tables and the emerging solution of torsional ultrasonic vibration, is available in our technical white paper.