Tiger Stripes, Mold Deposits, and Flow Marks in Injection Molding: Causes, Diagnosis, and Solutions

A Practical Troubleshooting Guide for Automotive PP Compounds and Large Injection-Molded Plastic Parts

This problem is particularly common in automotive applications such as bumper fascias, instrument panels, door panels, and interior trim components.

Three defects frequently cause production difficulties:

• Tiger stripes: alternating glossy and dull bands on the molded surface.
• Mold deposits: unwanted residues accumulating on mold surfaces during production.
• Flow marks: visible lines, waves, or gloss variations associated with melt flow and solidification.

These defects may appear similar, but their underlying mechanisms are different.

Some originate from polymer rheology and morphology. Others are caused by thermal degradation, mold geometry, poor venting, or unstable processing conditions.

Understanding these differences is essential because adjusting injection speed or melt temperature alone may not solve the problem.

1. Tiger Stripes: Why Large PP Parts Develop Alternating Gloss Bands

What Are Tiger Stripes in Injection Molding?

Tiger stripes are periodic bands of different surface gloss or appearance that develop along the melt-flow direction.

They are particularly noticeable on large, unpainted automotive components manufactured from impact-modified polypropylene compounds.

Typical applications include:

• Automotive bumper fascias
• Instrument panels
• Door panels
• Large exterior trim components
• Textured industrial housings

The defect often appears as alternating light and dark bands, even when the molded part has a uniform color.

On textured surfaces, these bands can become especially visible under directional lighting.

Why Are Impact-Modified PP Compounds Susceptible?

Automotive polypropylene compounds commonly contain elastomeric phases to improve toughness, particularly at low temperatures.

Typical formulations may include:

• PP homopolymer or impact copolymer
• Ethylene-propylene rubber (EPR) or EPDM
• Polyolefin elastomer (POE)
• Talc or other mineral fillers
• Stabilizers and color additives

These materials provide a useful balance of stiffness, toughness, cost, and processability.

However, multiphase polymer systems have more complex flow behavior than a single-phase polymer melt.

During injection molding, the PP matrix and dispersed elastomeric phase can respond differently to shear, stretching, and cooling.

Under certain conditions, this can contribute to unstable flow-front behavior and periodic surface appearance variations.

The Role of Melt Viscoelasticity

Polymer melts behave as both viscous fluids and elastic materials.

During flow through a runner or gate, polymer chains undergo deformation and orientation.

When the flow conditions change, some of this stored elastic deformation can recover.

A related rheological phenomenon is die swell, also called the Barus effect, in which an extruded polymer melt expands after leaving a die.

However, die swell should not be treated as the sole explanation for tiger stripes in injection molding.

Tiger stripes are more appropriately associated with the interaction of:

• Melt viscoelasticity
• Flow-front instability
• Fountain-flow behavior
• Local shear and extension
• Elastomer morphology
• Cooling and surface replication

In injection molding, the advancing melt front continuously transports hot material toward the colder mold walls.

If the flow front becomes unstable, the resulting surface structure and gloss may vary periodically.

These variations can remain visible after solidification.

Why Does Changing Injection Speed Sometimes Fail?

Injection speed strongly influences flow-front stability, but the relationship is not always linear.

Reducing injection speed may help in some cases by changing the flow-front behavior.

In other cases, lower speed increases cooling at the advancing melt front and makes surface defects worse.

Similarly, increasing speed may improve surface replication but introduce additional shear heating or instability.

Therefore, there is no universal injection-speed setting that eliminates tiger stripes.

If the compound has an unfavorable rheological response or elastomer morphology, process adjustments alone may have limited effectiveness.

2. How to Reduce Tiger Stripes in Automotive PP Parts

Material-Level Solutions

For repeated tiger-stripe problems, material selection is often one of the most effective starting points.

Compounders can investigate:

Elastomer type and content

The rubber phase influences melt elasticity, toughness, and surface appearance.

Reducing elastomer content may improve appearance in some formulations, but it can also reduce low-temperature impact performance.

Rubber-phase morphology

Particle size, dispersion, and interfacial characteristics influence both mechanical properties and flow behavior.

Improved dispersion may help stabilize surface appearance, although smaller rubber particles do not automatically eliminate tiger stripes.

Melt rheology

Two PP compounds with similar melt flow rates can behave differently during injection molding.

MFR is only a single-point flow measurement and cannot fully describe shear-dependent viscosity, elasticity, or extensional behavior.

Specialized low-tiger-stripe grades

For high-appearance automotive applications, selecting a compound specifically developed for low tiger-striping can be more effective than repeatedly adjusting the same material.

Mold-Level Solutions

Mold design can influence tiger-stripe formation through flow distribution and local shear conditions.

Potential improvements include:

• Optimizing gate dimensions and geometry
• Reducing abrupt flow-direction changes
• Improving flow balance
• Avoiding unnecessary restrictions
• Maintaining uniform mold-surface temperature

Increasing gate cross-sectional area may reduce local shear in some designs, but it must be evaluated together with gate location, packing requirements, and weld-line behavior.

Process-Level Solutions

Recommended process investigations include:

• Injection-speed profiling
• Melt-temperature optimization
• Mold-temperature optimization
• Transfer-point adjustment
• Packing-pressure evaluation
• Checking machine repeatability

The objective is not simply to make the melt flow faster or slower.

It is to identify a stable processing window that provides consistent flow-front behavior and surface quality.

3. Mold Deposits: Why Residues Accumulate on Injection Molds

Mold deposits, sometimes called mold fouling or mold buildup, are residues that gradually accumulate on mold surfaces during repeated molding cycles.

They may appear as:

• White or gray films
• Yellow or brown deposits
• Dark residue near vents
• Oily or wax-like buildup
• Localized contamination around gates

As the deposits become thicker, they can transfer to molded parts or interfere with surface replication.

This may lead to:

• Black spots
• Surface stains
• Gloss variations
• Appearance defects
• Increased cleaning frequency
• Production downtime

Mold Deposits vs. Additive Blooming

These two problems should not be confused.

Mold deposits accumulate on the mold during processing.

Blooming or plate-out on finished parts refers to substances migrating or appearing on the molded surface, sometimes after storage or aging.

Although both may involve additives or low-molecular-weight substances, their diagnostic procedures are different.

4. Four Major Causes of Mold Deposits

Cause 1: Additive Migration and Volatile Components

Polymer compounds contain functional additives such as:

• Lubricants
• Processing aids
• Flame retardants
• Stabilizers
• Pigments and dispersants
• Impact modifiers

Some additives or low-molecular-weight components may migrate, volatilize, or separate under unfavorable processing conditions.

They can then condense or accumulate on cooler mold surfaces.

The risk depends on additive compatibility, thermal stability, concentration, and processing temperature.

Not every pigment melts during molding, and not every flame retardant decomposes under normal processing conditions. The actual mechanism must be established for the specific formulation.

Cause 2: Thermal Degradation

Excessive melt temperature or residence time can cause polymer degradation.

Potential causes include:

• Excessive barrel temperature
• Hot-runner dead spots
• Long residence time
• Poor shutdown procedures
• Material stagnation
• Inadequate purging

Degradation can generate volatile compounds, discolored material, and carbonized residues.

These may accumulate near gates, vents, or other mold surfaces.

A particularly important warning sign is the combination of increasing mold deposits and black specks in molded parts.

Cause 3: Poor Mold Venting

During cavity filling, air and gases must escape from the mold.

If venting is insufficient, trapped gases can become highly compressed and heated.

This may contribute to:

• Burn marks
• Localized degradation
• Deposits near the end of fill
• Incomplete filling
• Surface contamination

Poor venting may result from inadequate vent design, blocked vents, or changes in mold condition.

Excessive clamping force can aggravate certain venting problems, but it should not automatically be assumed to be the cause.

Cause 4: Excessive Shear Heating

High shear rates can increase local melt temperature.

Potential high-shear locations include:

• Small gates
• Narrow runners
• Restricted hot-runner passages
• Sharp flow transitions
• Certain screw and nozzle geometries

For shear-sensitive compounds, excessive local heating can accelerate degradation or additive instability.

This is particularly important when processing filled, flame-retardant, or highly modified materials.

5. How to Diagnose Mold Deposits Systematically

When mold deposits appear, engineers should first determine whether the problem is gradual or sudden.

If Deposits Develop Gradually

Investigate:

1. The number of molding cycles before deposits become visible.
2. Whether deposits appear consistently in the same cavity locations.
3. Changes in raw-material batches or additive packages.
4. Mold-temperature distribution.
5. Vent cleanliness and hot-runner condition.

Gradual buildup often suggests repeated deposition of low-level volatile or migrating components, although other causes remain possible.

If Deposits Appear Suddenly

Check:

1. Actual melt temperature rather than barrel settings alone.
2. Recent process changes.
3. Material contamination.
4. Hot-runner temperature control.
5. Abnormal residence time.
6. Vent blockage or localized burn marks.

When necessary, laboratory analysis of the residue can help identify its origin.

Depending on the deposit, useful methods may include FTIR, thermal analysis, or elemental analysis.

The most reliable troubleshooting approach is to connect the deposit composition with the material formulation and process history.

6. How to Prevent Mold Deposits

Optimize Material Formulation

Select compounds with:

• Appropriate thermal stability
• Good additive compatibility
• Controlled volatile content
• Suitable processing aids
• Consistent raw-material quality

Improve Mold and Runner Design

Consider:

• Appropriate gate and runner dimensions
• Effective venting
• Elimination of stagnation zones
• Balanced cavity filling
• Accessible cleaning locations

Control Processing Conditions

Maintain:

• Stable melt temperature
• Reasonable residence time
• Proper purging procedures
• Controlled shear exposure
• Consistent machine operation

Establish Preventive Maintenance

Mold cleaning should be based on observed deposit growth and production requirements.

Surface treatments or coatings may help in selected applications, but they must be evaluated for wear resistance, adhesion, maintenance, and compatibility with the molded material.

Preventive cleaning is generally easier than removing thick, strongly adhered deposits after extended production.

7. Flow Marks: Why Melt Flow Leaves Visible Patterns

Flow marks are surface appearance variations associated with how molten plastic advances, contacts the mold, and solidifies.

Common visual patterns include:

• Wavy lines
• Concentric ripples
• Alternating gloss bands
• Streaks near gates
• Irregular flow-front patterns

The term “flow marks” is broad and may describe several distinct mechanisms.

Correct identification is essential before changing process settings.

8. Common Mechanisms Behind Flow Marks

Mechanism 1: Flow-Front Cooling and Hesitation

As the polymer melt enters the mold, its surface begins cooling.

If flow slows significantly or hesitates, a thicker frozen layer may develop.

When flow accelerates again, the changing flow-front and surface-freezing conditions can create visible bands or waves.

This is more likely near:

• Sudden wall-thickness changes
• Flow restrictions
• Complex geometry
• Regions far from the gate
• Areas with unstable filling

Mechanism 2: Jetting Near the Gate

Jetting occurs when the melt enters the cavity as a relatively free stream instead of spreading smoothly along a mold wall.

The stream can fold or coil before being incorporated into the remaining melt.

After solidification, the part may show snake-like or irregular lines.

Jetting is often associated with:

• Poor gate orientation
• Excessive initial injection velocity
• Gate geometry
• Unfavorable cavity entry conditions

Possible solutions include redirecting the melt against a wall, modifying the gate, or reducing initial injection speed.

Mechanism 3: Surface Replication and Gloss Variation

Some apparent flow marks result from differences in how the polymer replicates the mold surface.

Local variations in:

• Melt temperature
• Mold temperature
• Contact pressure
• Cooling rate
• Polymer morphology

can produce different surface textures or gloss levels.

This is particularly relevant for textured automotive interior parts.

9. How to Eliminate Flow Marks

Improve Mold Temperature Control

A higher or more uniform mold-surface temperature can improve surface replication and delay premature freezing.

However, excessive mold temperature may increase cycle time or create dimensional problems.

Optimize Gate and Runner Geometry

Gate size and location influence:

• Flow-front behavior
• Shear rate
• Filling balance
• Pressure loss
• Weld-line formation

A larger gate is not always the correct solution. The geometry must be evaluated for the specific part.

Use Multi-Stage Injection Speed

A staged injection profile may help control different filling regions.

For example:

• Lower speed during initial gate entry to reduce jetting
• Higher speed during long-flow filling to limit premature freezing
• Controlled speed near the end of fill to manage venting and pressure

The correct profile depends on the mold and material.

Evaluate Material Flow Behavior

If process and mold changes are insufficient, consider whether the selected resin provides an appropriate combination of:

• Melt viscosity
• Flowability
• Thermal stability
• Surface appearance
• Mechanical performance

A higher-MFR grade may improve filling in some cases, but it can also affect impact strength, melt elasticity, and other properties.

10. Tiger Stripes vs. Mold Deposits vs. Flow Marks

11. A Practical Troubleshooting Workflow for Injection Molding Engineers

The most effective approach is to investigate the complete molding system rather than changing parameters randomly.

Step 1: Identify the Defect

Record:

• Defect appearance
• Location
• Flow direction
• Affected cavities
• Time of occurrence
• Material grade and batch

Step 2: Check Material-Related Factors

Review:

• Material formulation
• Melt flow rate
• Drying requirements, where applicable
• Thermal stability
• Additive compatibility
• Lot-to-lot consistency

Step 3: Evaluate Mold Design

Inspect:

• Gate size and location
• Runner balance
• Venting
• Cooling channels
• Wall-thickness transitions
• Surface texture

Step 4: Review Process Conditions

Measure or verify:

• Actual melt temperature
• Mold temperature
• Injection-speed profile
• Filling pressure
• Transfer position
• Packing conditions
• Cycle time

Step 5: Confirm the Root Cause

Change one major variable at a time where practical, or use a structured design of experiments.

Compare results across repeated cycles rather than judging a single acceptable part.

For recurring automotive surface defects, a stable process window is more valuable than a temporary adjustment that produces a few good samples.

Conclusion: Surface Quality Begins with Material and Process Compatibility

Tiger stripes, mold deposits, and flow marks are not simply cosmetic problems.

They can lead to:

• Higher scrap rates
• Additional mold cleaning
• Production interruptions
• Customer complaints
• Increased manufacturing costs

More importantly, they often reveal deeper problems in the interaction between polymer formulation, mold design, and processing conditions.

Tiger stripes may require better control of compound rheology and flow-front stability.

Mold deposits require identifying the source of degradation, volatile substances, or contamination.

Flow marks require understanding the actual filling and solidification behavior.

For automotive and industrial injection molding, the most reliable solution is a coordinated approach involving material suppliers, mold designers, and processing engineers.

A stable molding process is not achieved by finding one perfect parameter. It is achieved by designing a material–mold–process system that consistently produces acceptable parts.

Related engineering guides

Explore these related material and processing guides.

• Low-temperature impact modified PP for automotive bumpers
• Automotive PP bumper injection molding
• Shear thinning and injection molding flowability
• Understanding MFI in a PP technical data sheet
• Annealing and residual stress in injection-molded parts