Why Plastic Parts Crack or Turn White During Ultrasonic Welding

Introduction

Plastic cracking and stress whitening during ultrasonic welding are usually not caused by one machine setting alone. 

They are symptoms of excessive localized stress, uncontrolled energy input, inadequate fixture support, poor joint 

geometry, molded-in stress, or variations in the plastic material.

The first step is not to immediately increase or decrease welding time. Engineers should first identify where 

the defect appears, when it occurs, and whether the part already contains stress before welding.

This guide explains how to distinguish stress whitening from cracking, identify the most likely root causes, 

and troubleshoot the welding process systematically

Quick Answer: Why Does Plastic Turn White or Crack After Ultrasonic Welding?

Plastic parts may turn white or crack when ultrasonic vibration, pressure and heat create more localized stress

than the part can safely absorb.The most common causes include:

• Excessive welding amplitude, energy or weld time

• Incorrect or unstable welding pressure

• Poor energy director or joint design

• Insufficient fixture support

• Horn misalignment or uneven horn contact

• Sharp corners, ribs or sudden wall-thickness changes

• Residual stress from injection molding

• Moisture, fillers, regrind or material variation

• Incorrect welding frequency or control mode for the application

The correct solution depends on the defect location and failure pattern.Reducing one parameter without testing 

ultrasonic weld strength may hide the cosmetic defect while creating a weak weld.

What Is Stress Whitening in Ultrasonic Welding?

Stress whitening is a white, cloudy or frosted appearance that develops when a plastic part experiences localized 

deformation.Microscopic deformation, voids or crazing inside the material can scatter light and make the affected

area appear white.The defect is often more visible on dark, transparent or high-gloss plastic parts.

Stress whitening may appear:

• Under the welding horn

• Beside the weld interface

• Around ribs, bosses or sharp corners

• Near an energy director

• On thin unsupported walls

• Around snap-fit or molded features

Stress whitening does not always mean that the weld has lost its mechanical strength. However, it indicates 

that the part has experienced significant localized stress and that the welding process or part design should 

be reviewed.

What Is the Difference Between Whitening and Cracking?

Stress whitening changes the appearance of the plastic but may not create a complete fracture.

Cracking is more serious. It means the material has separated and may no longer meet strength, sealing, safety or 

appearance requirements.

Cracks may occur:

• Along the weld line

• Through the plastic wall

• At the base of a rib or boss

• Around a sharp internal corner

• At the edge of the horn contact area

• Several hours after welding

Delayed cracking is especially important. If the part looks acceptable immediately after welding but cracks later, 

the problem may involve residual molding stress, material condition or environmental stress rather than only 

excessive weld time.

Diagnose the Defect Location Before Changing Parameters

The location of the defect provides useful information about its cause.

Whitening Directly Under the Welding Horn

White marks beneath the horn often indicate a contact or support problem.

Possible causes include:

• Excessive horn pressure

• Excessive amplitude

• A rough or damaged horn surface

• Uneven horn contact

• Insufficient support beneath the part

• Local bending of a thin wall

Check whether the horn face matches the product contour and whether the fixture supports the part directly

below the horn contact area.

Whitening Beside the Weld Interface

Whitening close to the joint may indicate that the weld is receiving too much energy or collapsing too aggressively.

Possible causes include:

• Excessive amplitude

• Excessive weld time or energy

• Oversized energy director

• Excessive joint interference

• Poor alignment between the two parts

• Insufficient control of collapse distance

The joint should melt progressively rather than collapse suddenly.

Cracks Near Ribs, Bosses or Sharp Corners

Cracks around molded features often indicate stress concentration.

Possible causes include:

• Sharp internal corners

• Thin-to-thick wall transitions

• Ribs positioned too close to the weld

• Molded-in stress

• Ejection stress

• Poor fixture support

Parameter adjustment may reduce the symptom, but the permanent solution may require changes to

the molded part or fixture.

Cracks Through the Weld Interface

Cracking through or immediately beside the weld line may be caused by:

• Excessive energy input

• Poor material compatibility

• Incorrect joint geometry

• Misalignment

• Excessive interference

• Material degradation

• Inconsistent part dimensions

Inspect the fracture surface to determine whether the crack passed through the parent material or followed 

an incomplete weld interface.

Main Causes of Plastic Cracking and Stress Whitening

1. Excessive Amplitude, Energy or Weld Time

Amplitude controls the mechanical movement delivered by the welding horn. When amplitude, energy 

or welding time is too high, the joint can heat and collapse too aggressively.

Possible results include:

• Excessive melt flow

• Flash

• Surface marking

• Internal stress

• Part deformation

• Whitening

• Cracking

Reducing amplitude or energy may improve appearance, but the weld must still be tested for strength and

sealing performance.Do not adjust several parameters at the same time. Change one factor, 

record the result and compare it with the original process.

2. Incorrect or Unstable Welding Pressure

Both excessive and insufficient pressure can create problems.

Excessive pressure may:

• Compress the part before sufficient melting occurs

• Deform thin walls

• Increase stress around ribs or corners

• Leave visible horn marks

Insufficient pressure may:

• Allow unstable movement between the parts

• Reduce consistent energy transfer

• Create localized heating

• Produce incomplete or irregular melting

Pressure, trigger force and downspeed should be optimized together rather than treated as separate settings.

3. Poor Ultrasonic Welding Joint Design

A poorly designed joint cannot distribute ultrasonic energy consistently.

Common problems include:

• Oversized energy directors

• Energy directors with inconsistent dimensions

• Sharp joint corners

• Insufficient alignment features

• Excessive shear-joint interference

• Sudden wall-thickness changes

• Weld joints positioned too close to fragile features

A properly designed energy director should begin melting in a controlled manner and allow predictable 

joint collapse. For detailed dimensions, joint types and design principles, see our ultrasonic welding joint design guide. For sealing applications, the joint must also control melt flow without placing excessive stress

on the surrounding wall.

4. Inadequate Fixture Support

The fixture is not only used to position the product. It must also support the plastic structure against 

welding force and vibration.

Insufficient fixture support can cause:

• Part flexing

• Wall vibration

• Local bending

• Stress concentration

• Energy loss

• Cracking away from the weld interface

The fixture should support the part close to the welding area without damaging cosmetic surfaces.

Soft or uneven fixture materials may also reduce process repeatability.

5. Horn Misalignment or Uneven Contact

If the horn does not contact the part evenly, one area may receive more pressure and vibration than another.

Check:

• Horn parallelism

• Horn-to-part contact pattern

• Fixture alignment

• Product seating

• Horn surface condition

• Wear or contamination on the horn face

A contact test or pressure-sensitive film can help identify high-pressure areas when appropriate for the application. 

The horn geometry, material, surface condition and frequency tuning should also be reviewed as part of the 

ultrasonic welding horn design process.

6. Residual Stress From Injection Molding

Some parts contain significant internal stress before ultrasonic welding begins.

Common molding-related causes include:

• High injection pressure

• Uneven cooling

• Incorrect mold temperature

• Poor gate location

• Fast ejection

• Sharp corners

• Non-uniform wall thickness

Ultrasonic vibration may release or concentrate this existing stress, causing whitening or cracking.

Compare welded and unwelded parts from the same molding batch. If unwelded parts already show distortion, 

stress marks or easy cracking, machine settings alone may not solve the problem.

7. Plastic Material Condition and Variation

Material name alone is not enough to predict welding behavior.

For example, two parts labeled ABS or PC may behave differently because of:

• Different resin grades

• Colorants

• Flame retardants

• Glass or mineral fillers

• Regrind percentage

• Moisture content

• Material contamination

• Storage conditions

PMMA is also commonly called acrylic and should not be treated as a separate material category.

Semi-crystalline materials such as PP, PA and POM often require careful control of energy transmission 

and joint design. However, they are not automatically more likely to crack than amorphous plastics. 

The actual risk depends on material grade, part structure, tooling and process conditions.

Ultrasonic Welding Crack and Whitening Troubleshooting Table

Step-by-Step Troubleshooting Process

Step 1: Record the Existing Process

Record the current:

• Amplitude

• Weld time or energy

• Pressure

• Trigger force

• Downspeed

• Hold time

• Collapse distance

• Peak power

• Material batch

• Part cavity number

Without a baseline, parameter changes cannot be evaluated accurately.

Step 2: Identify the Exact Defect Pattern

Determine:

• Where the whitening or crack begins

• Whether every part is affected

• Whether the defect is linked to one mold cavity

• Whether the defect appears immediately or later

• Whether the defect follows the weld line or a molded feature

• Photograph and classify each failure mode.

Step 3: Check the Horn and Fixture

Before changing the welding program, confirm:

• The horn contacts the part evenly

• The fixture supports the welding area

• The part is fully seated

• The tooling is aligned

• There is no horn or fixture wear

• The product does not rock or flex during welding

Step 4: Optimize One Parameter at a Time

Depending on the observed defect, evaluate controlled changes to:

• Amplitude

• Energy or weld time

• Welding pressure

• Trigger force

• Downspeed

• Hold time

• Collapse distance

For parts requiring tighter dimensional or collapse control, engineers should also compare different ultrasonic welding

 control modes, including time, energy, distance and collapse-distance modes.Do not simply reduce all settings until 

the whitening disappears. The final process must still meet strength, appearance, dimensional and sealing requirements.

Step 5: Review the Joint and Molded Part

If parameter optimization produces only limited improvement, review:

• Energy director dimensions

• Shear-joint interference

• Alignment features

• Rib and boss location

• Corner radii

• Wall thickness

• Distance between the horn and weld interface

A poor joint design cannot always be corrected through machine settings.

 Step 6: Check Material and Molding Consistency

Compare different:

• Resin batches

• Color batches

• Mold cavities

• Regrind percentages

• Moisture conditions

• Molding dates

This helps determine whether the defect is a welding problem, a molding problem or a combination of both.

Step 7: Validate the Final Process

A successful process should be evaluated by more than appearance.

Depending on the product, validation may include:

• Visual inspection

• Tensile or pull testing

• Peel or shear testing

• Leak or burst testing

• Dimensional inspection

• Cross-section analysis

• Functional testing

• Repeated production trials

Manufacturers developing a new application can follow a structured ultrasonic welding validation process before

purchasing equipment or starting mass production.

How to Prevent Cracking and Whitening in Mass Production

After the process has been optimized, manufacturers should establish a controlled production window.

Recommended controls include:

• Upper and lower energy limits

• Collapse-distance monitoring

• Peak-power monitoring

• Stable material and molding specifications

• Regular horn and fixture inspection

• Defined cleaning and maintenance intervals

• Periodic destructive testing

• Part traceability by mold cavity and material batch

A servo-driven ultrasonic welding machine can provide more precise control of force, speed, position and 

collapse distance for demanding applications.However, a servo machine cannot compensate for poor 

joint design, severe molded-in stress or inadequate fixture support.

Frequently Asked Questions

Q1. Does Stress Whitening Always Mean the Weld Is Weak?

A1: No. Stress whitening is mainly a visible sign of localized deformation. The weld may still have acceptable 

strength, but appearance alone cannot confirm structural performance.The part should be tested according

 to its actual mechanical or sealing requirements.

Q2. Can Lower Amplitude Eliminate Whitening?

A2: It may help when excessive vibration is the main cause. However, reducing amplitude too far may create 

an incomplete or weak weld. Amplitude should be optimized together with weld time, pressure and collapse.

Q3. Which Plastics Are Most Likely to Turn White?

A3: Whitening is often more visible on high-gloss, transparent, dark-colored or stress-sensitive plastic parts. 

ABS, PC,  PMMA acrylic, PC/ABS and other materials may show whitening depending on grade, geometry

and molding stressNo material should be evaluated by name alone.

Q4. Can Machine Parameters Fix Every Cracking Problem?

A4: No. Parameters can solve defects caused mainly by excessive or unstable energy input.

Cracks caused by sharp corners, poor joint geometry, severe molding stress, material degradation or insufficient

fixture support may require tooling, molding or product-design changes.

Q5. Is a Servo Ultrasonic Welder Better for Cosmetic Plastic Parts?

A5: A servo welder can provide more repeatable force, speed and displacement control, which is useful for delicate or 

appearance-sensitive parts.The final result still depends on material, joint design, horn contact, fixture support 

and proper process development.

Need Help Identifying the Root Cause?

Plastic cracking and stress whitening should be diagnosed using the actual product, material and weld requirements.

TIMEAST provides ultrasonic welding sample testing, tooling evaluation and process optimization for automotive, 

medical, electronics, packaging and industrial plastic components.

Send us:

• Plastic samples

• Material information

• Product drawings

• Photos of the defect

• Current welding parameters

• Strength or sealing requirements

• Estimated production volume

Our engineers can evaluate the failure pattern and recommend suitable welding parameters, tooling and equipment 

before mass production.