Ultrasonic Welding Leak Test Failure: Causes & Solutions

In ultrasonic plastic welding, a joint that looks good isn’t always a joint that seals well. 

Many production teams experience the same frustrating scenario: the weld line appears uniform, 

pull strength meets spec, and the cycle time is stable—but the part fails the leak test (pressure decay,

 bubble test, or helium leak detection).

Leak failures rarely come from a single factor. In most cases, they are the result of combined effects

 from part design, material behavior, process settings, tooling/fixturing, and even the test method itself. 

This article provides a practical, engineering-focused breakdown of the most common causes and the 

most effective solutions—plus a step-by-step troubleshooting roadmap you can use on the factory floor.

1) First: Identify the Leak Failure Pattern (This Saves Hours)

Before changing parameters, classify the failure. Different failure patterns usually point to different root causes:

A. Consistent leak failure (almost every part fails)

Typical cause: insufficient melt or incomplete fusion, often driven by joint design, horn contact, 

or low energy transfer.

B. Random leak failure (some pass, some fail)

Typical cause: variation—part fit-up, fixture repeatability, dimensional tolerance stack-up, or

 inconsistent clamping.

C. Time-dependent leak failure (passes now, fails later)

Typical cause: residual stress, micro-cracks, creep relaxation, or seal deformation after cooling/aging.

Why this matters:

If you don’t know which pattern you’re dealing with, you’ll chase symptoms with parameter tweaks 

and never stabilize the process.

2) Root Cause Category #1 — Joint Design Issues (Most Common)

Even with a perfect machine, the joint must be designed to create controlled melt flow

and form a continuous sealing ring.

Common design-related leak causes include:

Cause 1: Energy director too small / discontinuous

If the energy director is undersized or interrupted, the joint may “stick” in spots but never

 build a continuous hermetic seal.

✅ Solutions

• Increase energy director height and angle (within material limits)

• Ensure a continuous sealing path (avoid breaks in the perimeter)

• Use a shear joint for demanding leak requirements (better melt containment)

Cause 2: No controlled melt containment (flash becomes a leak path)

If molten plastic is forced outward or inward unpredictably, it can form voids, channels, or

 inconsistent collapse—leading to leaks.

✅ Solutions

• Add a flash trap / overflow groove

• Use tongue-and-groove features to control flow

• Balance wall thickness to prevent differential melt

Cause 3: Weld line too close to ribs, bosses, or snap features

Nearby structures can pull heat away, deform during welding, or cause local stress concentration.

✅ Solutions

• Increase distance between seal line and structural features

• Add local stiffening under the joint to reduce flex

• Redesign ribs to avoid “hinge” movement

3) Root Cause Category #2 — Process Settings (Energy Delivery Problems)

When leak failures occur, many teams raise weld time and pressure first. That often makes things worse—

by creating stress cracks, marking, or flash. Instead, focus on how energy is delivered.

Cause 4: Under-weld (insufficient melt / incomplete fusion)

Parts may look fused, but micro gaps remain.

✅ Solutions

• Increase amplitude (more effective than time alone)

• Switch to energy mode or collapse distance mode for repeatability

• Add hold time to stabilize the seal while cooling

• Check stack tuning and horn-face contact

Cause 5: Over-weld (excessive melt → voids and seal collapse)

Too much energy can squeeze out molten material, trap bubbles, or create weld-line thinning.

✅ Solutions

• Reduce amplitude, especially on thin walls

• Use collapse distance limits to prevent over-collapse

• Reduce pressure if it is forcing melt out too aggressively

• Optimize trigger force to ensure consistent start

Cause 6: Inconsistent collapse (random leak failures)

If your collapse varies part-to-part, your seal continuity will vary too.

✅ Solutions

• Use collapse distance control with strict limits

• Improve fixture rigidity and part seating

• Monitor peak power and collapse signatures for drift

4) Root Cause Category #3 — Fixturing & Alignment (The Hidden Leak Generator)

Fixturing issues cause more random leak failures than most teams realize.

Cause 7: Part rocking or incomplete seating

If the part tilts slightly, the horn may weld one side more than the other, leaving micro gaps.

✅ Solutions

• Add locating features that constrain all axes

• Increase fixture support directly under the seal line

• Use vacuum or nest inserts to prevent tilt

Cause 8: Horn misalignment or uneven contact pressure

Even slight angular misalignment can create uneven energy distribution.

✅ Solutions

• Verify horn parallelism and face flatness

• Use carbon paper or pressure-sensitive film to check contact uniformity

• Ensure horn/fixture rigidity; eliminate flex under load

5) Root Cause Category #4 — Material & Molding Variation

Sometimes the welding process is fine—the parts are not.

Cause 9: Moisture, contamination, mold release residue

Moisture can cause bubbling; mold release can prevent wetting and fusion.

✅ Solutions

• Dry hygroscopic materials (PA, PC, PET, etc.) to spec

• Ban mold release on sealing surfaces (or strictly control type and application)

• Improve handling to avoid oil/dust contamination

Cause 10: Warpage / shrink variation → poor fit-up

Leak tests fail when the joint line has gaps before welding.

✅ Solutions

• Improve molding process control (pack/hold, cooling balance)

• Add design features for self-alignment

• Tighten tolerances around seal line

• Measure pre-weld gap using gauges or 3D scans

6) Root Cause Category #5 — Leak Test Method Errors (Yes, the Test Can Lie)

Leak test failures sometimes come from the test, not the weld.

Cause 11: Wrong test pressure / stabilization time

Pressure decay tests can fail if the part expands, temperature changes, or the stabilization time is too short.

✅ Solutions

• Add stabilization time before measurement

• Control test temperature (plastic expands with heat)

• Set pressure based on part stiffness and design

• Calibrate decay thresholds against known-good samples

Cause 12: Fixture sealing leaks during test

If the test fixture O-ring leaks or seats inconsistently, good parts may fail.

✅ Solutions

• Validate the fixture with a solid reference plug

• Replace O-rings on schedule

• Use consistent clamp force and alignment for testing

7) A Practical Troubleshooting Roadmap (Use This on the Shop Floor)

Here’s a fast, systematic approach that avoids guesswork:

Step 1: Confirm the failure pattern

Consistent / Random / Time-dependent?

Step 2: Locate the leak path

• Bubble test to identify location

• Helium sniffing for precision

• Dye penetration (where applicable)

Step 3: Compare weld signatures

Look at:

• peak power

• energy

• collapse distance

• cycle time

• hold phase stability

A good process leaves a consistent “fingerprint.”

Step 4: Validate fixture rigidity and alignment

If random failures exist, suspect the nest.

Step 5: Adjust the right knob

• Under-weld → increase amplitude or energy

• Over-weld → cap collapse distance, reduce amplitude

• Random leaks → fix seating + alignment, then tune

Final Takeaway: Hermetic Success is a System, Not a Setting

Ultrasonic leak failures are almost never solved by “just increasing weld time.” The most reliable

approach is to treat the seal as a system:

• Design for controlled melt flow

• Deliver energy consistently

• Clamp and locate parts rigidly

• Control material and molding variation

• Validate the leak test fixture and method

When these five pillars are aligned, leak performance becomes repeatable—

high yield, stable process, and fewer surprises.