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