Troubleshooting Appearance Defects in Automotive Plastic Injection Molding

Identifying, Analyzing, and Correcting A-Surface Issues

Kurt Beyerchen

Nissan Technical Center North America

What is Quality?

Phillip Crosby’s DefinitionQuality means conformance to

requirements, not goodness. Quality is achieved by prevention, not

appraisal. Quality has a performance standard of

Zero Defects, not acceptable quality levels.

Quality is measured by the Price of Nonconformance, not indexes.

What is a Defect? If quality is a conformance to

requirements, a defect is a deviation from those requirements

Requirements are created based on customer expectations

Therefore, a defect is a deviation from customer expectations A source of dissatisfaction A limiter of use

Customer Expectations High quality materials

Planned execution

Seamless integration

Customer ExpectationsHigh quality materialsAppropriate glossConsistent surface finish, visual and

tactile Good touch characteristics Character of texture Consistent color

A sense of value; getting more than is paid for

Customer Expectations Planned Execution Parts have an intended appearance; no

parts look like they’re from the reject binThe right material for the jobNo stress whitening or signals of

impending failureNo warpage

Customer ExpectationsSeamless Integration Solid surface – no appearance of

cracks or splitsThought given to compatibility and

harmonyUniform appearance of surface, no

visible parting lines or non-intended changes

Customer Expectation AnalysisDesign Trends – Other Consumer

Goods, Competitor ProductsTGR / Wants TGW / WarrantySales trends

If the cost of quality is defined as the price of nonconformance, lost sales and/or market share can be considered to be part of that price

Communicating Customer Expectations

Design Trends

in Other Products

Sales, Market,

Dealer Feedback

Press

TGW

Warranty, JD Power,

Complaints

TGR / Wants

Customer Surveys

JD Power

OEMSPEC

Material Selection Customer Specs

Appearance Requirements Impact Resistance Chemical and UV Resistance Weight

Cost Raw Material Per Pound Finished Part After Forming

Incumbency Existing Equipment Familiarity

Material SelectionSurface Appearance

Inherent Gloss Match to Master / Surrounding Parts

Color Gloss Grain

Surface Aspect Sink / Read Through

Paintability

Material Properties Inherent gloss Chemical Resistance Dimensional Stability Impact Resistance Flame Resistance / Flammability Useful Temperature Range Hardness Abrasion Resistance Creep Resistance Tensile Strength Fatigue Endurance Electrical Conduction / Resistance Compression Set Resilience Dielectric Strength Adhesive Compatibility Colorant Technology Compatibility Weight / Specific Gravity Transparency Weldability Antistatic

UV / Sunload Resistance Chemical Resistance Gas Resistance Paintability Laser Markability Crystallinity Cost / Price Availability Recyclability Regrind Post-Processing Machinability Melt Flow Process Compatibility Shrinkage Viscosity Melt Point / Processing Temperature Fillers Lubricative Properties Hygroscopic Reinforcement Fume Release (ex. Formaldehyde)

Material Properties Related to Moldability Melt Flow Viscosity Melt Point / Processing Temperature Fillers Multiple-Shot Application Compatibility Regrind Colorant / Colorant Carrier Lubricative Hygroscopicity Crystallinity Use of fillers / reinforcements

Problem Solving Analysis Machine

Barrel / nozzle temp, injection speed / pressure, shot size, contamination, nozzle size, pack pressure, cushion

Mold Cavity / core temp, venting, runners & gates, polishing,

mold cleanliness, surface finish, part thickness / geometry Material

Moisture, contamination, regrind, processing temperature, colorant

Operator Cycle time, post-ejection handling, process consistency

Barrel Temperature Effects High barrel temps can cause degradation of

resin and colorant, causing the part to appear discolored or have black streaks

Low barrel temps may increase shearing due to poor mixing, knitlines, jetting, flow lines

Injection Speed Effects High injection speeds can cause shearing,

burning, blush Low injection speeds can cause short shots,

knitlines

Identification And Correction Of Defects

Some Common Injection Molding Problems

Gate Blush

What it is: Discolored area of the part, radiating from the gate, resulting from shearing of material at the gate. May also be indicated by streaking.

Gate Blush Causes

Machine Injection fill speed too fast Melt temperature too high

or too low Injection pressure too low Nozzle diameter too small Nozzle temp too low

Burn Mark

What it is: Discoloration of material, usually yellow/brown, brown or black depending on severity. Usually located at edge of part, away from the gate. Frequently also seen as short shot in this area.

Burn Mark Causes

Machine Injection speed or pressure too

high Screw speed too high Improper compression ratio of

screw Excessive back pressure Nozzle too hot or nozzle

diameter too small Faulty temperature controller Barrel temperature too high

Mold Insufficient venting size and

/ or location Vents plugged or shutoff Improper gating size and /

or location Material

Excessive regrind use Excessive lubricant

Operator Inconsistent cycles

Discoloration

What it is: Deviation from the intended color. May be local or throughout entire part

Discoloration Causes

Machine Excessive residence time Barrel temp too high Nozzle temp too high Excessive cycle time Excessive shot size ratio

Mold Improper mold temp Inefficient cooling Inadequate venting Grease, contaminated mold

components Material

Contaminated material Incorrect regrind ratio Incorrect color blending

Operator Inconsistent cycles

Contamination

What it is: Foreign substances mixed or embedded in the part

Contamination Causes

Machine Oil and grease leaks and drips

Mold Excessive lubrication Grease, contaminated mold

components Material

Contaminated material Incorrect regrind usage Excessive moisture

Operator Poor housekeeping

Short Shot

What it is: Missing plastic or features not fully formed. Edges have smooth, rounded appearance. May be accompanied by distinct knitlines, sink, or burns in surrounding area. Generally located away from gate at the last area of the part to fill.

Short Shot Causes

Machine Injection speed or pressure

too low Insufficient material feed Insufficient press capacity Inadequate back pressure Nozzle too small Faulty check valve Excessive feed cushion

Mold Insufficient venting Mold temp too low Gates / runners too small Cross-section too thin Sprue bushing too long Sprue diameter too small

Material Improper flow rate Excessive regrind use Moisture in resin Non-uniform particle size

Part Sink

What it is: Localized depression on the surface of the part, usually adjacent or above ribs or thicker areas of the part

Part Sink Causes

Machine Barrel temp too high Insufficient injection

pressure or time Inadequate cooling time Insufficient cushion or

hold time Faulty check valve

Mold Mold temperature too high

opposite ribs Gates or runners too small

or improperly placed Excessive rib thickness

(>50% of wall) Excessive part thickness Gate land too long Unbalanced flow pattern

Material Improper flow rate Excessive regrind use

Knitline

What it is: Location in part where melt fronts meet. Incomplete adhesion occurs between the fronts, causing poor appearance and structural weakness

Knitline Causes

Machine Barrel temp too low Back pressure, injection pressure, or speed too low

Mold Mold temperature too low Gates or runners too small or incorrectly placed Gate land too long

Material Improper flow rate

Jetting

What it is: Serpentine discoloration in direction of the gate.

Jetting Causes

Machine Excessive injection speed Barrel temp too high or too

low Nozzle opening too small Nozzle temp too low

Mold

Mold temperature too low

Gates or runners too small or incorrectly placed

Gate land too long

Material

Improper flow rate

Gloss Variation

What it is: Variation in gloss on the surface of the part

Gloss Variation Causes

Machine Inadequate injection pressure Inadequate residence time Barrel or nozzle temp too low Excessive feed cushion Nozzle too small Inadequate cycle time

Operator Inconsistent cycles

Mold

Mold temperature too low

Gates or runners too small or incorrectly placed

Inadequate venting

Inadequate polishing of mold surfaces

Contaminated mold surfaces from buildup of chemical residue from outgassing or buildup of mold release (plateout)

Material

Improper flow rate

Inadequate lubrication

Moisture in resin

Bubble / Blister

What it is: Gas trapped within part

Bubble / Blister Causes

Machine

Injection temp too high

Injection pressure too low

Insufficient material feed

Improper injection temp profile

Excessive injection speed

Operator

Inconsistent cycles

Mold

Improper runners or gates

Improper venting

Section thickness too great

Mold temp too low

Material

Excessive moisture in resin

Black Specks / Streaks

What it is: Black specks or streaks visible in the part

Black Specks / Streaks Causes

Machine Excessive residence time in barrel Hang up of material in barrel or

runner system Contamination of injection barrel Degradation of material due to

malfunctioning heater bands or thermocouples

Defective nozzle shutoff Cracked injection cylinder or pitted

screw Oil leaking into injection unit

Operator Inconsistent cycles

Mold Sprue bushing rough or not

seated Burned material caused by

improper venting Contamination from grease

or lubricants Mold too small for machine

size Material

Contamination of raw material

Wrong material for mold

Pin Push

What it is: Stress whitening on the a-surface of the part, often accompanied by a bulge in the surface. Occurs above ejector pins.

Pin Push Causes

Machine

High pack pressure

Inadequate cycle time

Operator

Inconsistent cycles

Mold

Inadequate polishing of mold surfaces, especially vertical mold surfaces or ribs

Wallstock too thin at ejector pin area

Insufficient b-side draft

Angled surface at ejector pin / ejector pin sliding on backside

Insufficient cooling

Insufficient ejection surface

Material

Excessive shrink rate

Flow Line

What it is: Light or dark streaks running parallel to the direction of flow

Flow Line Causes

Machine Inadequate injection

pressure Inadequate residence time Barrel temp too low Nozzle temp too low

Mold Improper gate location Material flow over ribs

Operator Improper cycling

Flash

What it is: Excess plastic squeezing out perpendicular to the parting line, often felt as sharp

Flash Causes

Machine Excessive injection

pressure Excessive shot size ratio Insufficient press capacity Barrel temp too high Excessive cycle time Excessive residence time

Mold Insufficient venting Inadequate cooling Inadequate mold supports Poor match of shutoff area

(poor spotting) Sprue bushing too long Improper stackup

dimension Material

Improper flow rate Excessive mold lubricant

Operator Improper cycling

Part and Tooling Parameters Affecting Grain and GlossWall with greater than nominal thickness

will pull away from the textured mold surface, resulting in poor grain and high gloss

Wall with less than nominal thickness will tend to adhere to the textured surface, resulting in a flat, chalky appearance

Wall-stock variation can result in gloss and grain definition variation

Relationship of Surface Roughness to Gloss The texturing process and subsequent media blasting introduces surface

roughness to the previously smooth mold surface Gloss is closely related to surface roughness The distribution and scale of the roughness in the plane of the surface is critical Another issue to consider is the refractive index of the material, and crucially

(and often ignored) the _distribution_ and scale of the roughness in the plane of the surface -  there is typically lots of microstructure in a polymer at a similar scale to the wavelength of light. 

Fillers are often added to the material to ‘roughen’ the surface that are huge compared to this crucial wavelength, so it is the coupling between the large and small scale roughness that is the key

Leather seems to succeed by blending large- and small-scale  roughness

Mold Temperature Effects on Gloss High cavity temps can decrease gloss by

allowing material to flow easier, therefore enhancing transfer of tool surface texture

High cavity temps can increase gloss by increasing material shrink, causing decreased dwell time on tool surface and decreasing transfer of surface texture

How does mold temp affect gloss? It depends!

Molding Parameters Affecting Color and GlossMold TemperatureBarrel Temperature Injection SpeedPack PressureScrew SpeedCycle TimeBack Pressure

The “Big Three”

Molding Parameters Affecting Color and GlossMold temperature has the greatest effect

Can affect L*, a*, b*, GlossMachine barrel temp has a large effect

Can affect L*, a*, b*, Gloss Injection speed has a lesser effect

Mainly affects L*, Gloss; slightly affects a*Pack pressure has a slight effect

Slightly affects L*, Gloss; minor affect to a*

Molding Parameters Affecting Color and Gloss

General rule*a*, b*, Gloss are positively correlated

If measured a*, b* or Gloss increase or decrease, the other two will follow suit

L* is negatively correlated with a*, b*, Gloss If L* increases or decreases, measured a*,

b*, Gloss will react oppositely If a*, b*, or Gloss increase or decrease, L*

will react oppositely

* Rules are made to be broken

Color Master Recommendations Since color is linked to process, develop masters

in materials that are the same as the materials that will be used for parts, and set process parameters within normal operating ranges

If a master must be processed within a small processing window, change to a combination of machine+mold+material+operator that will yield larger process parameters

Develop a robust process early on in the mastering process

Color Matching RecommendationsSince color is linked to process, try

some minor changes to the process first

Select materials that are less sensitive to process changes

Develop a robust process early on in the matching process

Lessons Customers dictate specs Continued failure to meet the correct specs will

result in lost customers Injection molding involves balancing multiple

factors Problem solving analysis should be done one

variable at a time A consistent process yields predictable results Plan for success early

Credits GE Plastics Uniform Color Company Westlake Chemical Parallel Design Dr. H. Assender, Oxford University The Detroit Colour Council Nissan