Injection Molding Defects - Identification, Causes and Solutions 2025

Introduction - cost of defects in production

Injection molding defects are one of the largest hidden costs in the plastics processing industry. A typical production facility in Poland reports 3-8% scrap in serial production, which at an annual production value of 5 million PLN means a loss of 150,000-400,000 PLN per year.

The problem doesn't end with material costs - every defective part also represents wasted energy, machine time, quality control costs and potential claims. In the automotive industry, where quality requirements are minimum Cpk 1.67, even a small increase in scrap rate can mean losing a contract worth millions of zlotys.

Good news? Companies that have implemented a systematic approach to quality control on Tederic injection molding machines report 60-80% scrap reduction within 6-12 months. According to industry data, 6 most common defects account for 91% of all flaws - by eliminating them, you achieve dramatic quality improvement.

In this guide, we present these 6 defects, their causes and specific solutions with parameters for Tederic machines, along with a real case study of a Polish company that reduced scrap by 82%.

Flash (burrs) - 35% of all defects

Flash (burrs, overflow) is a thin layer of material (0.01-0.5mm) flowing outside the mold parting line. It is the most common defect in injection molding production.

Identification

• Thin edge of material along mold parting line or ejectors
• Can be continuous or local
• Sharp edges may pose cutting risk

Main causes

1. Insufficient clamping force

If injection pressure is too high relative to clamping force, mold plates separate during injection.

Test: Required force [T] = Projected area [cm²] × Injection pressure [bar] / 100

2. Mold wear

Mechanical wear of contact surfaces after 300k-1M cycles or dents from contaminants.

3. Excessive injection pressure/speed

Excessive pressure forces material through microscopic gaps.

Tederic Solutions

Step 1: Increase clamping force

• Increase by 10-20% (e.g., from 80% to 95% of maximum force)
• Caution: Do not exceed 100% - risk of mold damage

Step 2: Reduce injection pressure/speed

• Injection pressure: Reduce by 10-15% (e.g., from 1200 bar → 1050 bar)
• Injection speed: Reduce by 15-20% (e.g., from 120 mm/s → 95 mm/s)

Step 3: Increase material viscosity

• Lower barrel temperature by 10-15°C
• Lower nozzle temperature by 5-10°C

Step 4: Mold maintenance

• Thorough cleaning of parting line
• Inspection of surfaces for dents
• For high wear: mold regeneration (grinding)

Short shots - 18% of all defects

Short shot (short mold) is incomplete filling of the mold cavity - the part is incomplete, missing geometry fragments.

Identification

• Incomplete part - missing sections, usually farthest from injection point
• Incomplete ribs, mounting features, thin walls
• Part unusable

Main causes

1. Insufficient material shot - the injection machine does not plasticize enough.

2. Temperature too low - material solidifies before filling the cavity.

3. Speed/pressure too low - material does not reach the end of the mold.

4. Blocked nozzle - char or solidified material.

Tederic Solutions

Step 1: Increase shot size

• Increase by 5-10% (e.g., from 45mm → 48mm screw position)
• Rule: shot size should be 40-80% of screw capacity

Step 2: Increase material temperature

• Barrel zones: +10-20°C all zones
• Nozzle: +10-15°C
• Example for PP: from 200-210-220-230°C → 210-220-230-240°C

Step 3: Increase speed and pressure

• Injection speed: +15-25% (e.g., 80 mm/s → 100 mm/s)
• Injection pressure: +10-20% (e.g., 900 bar → 1050 bar)

Step 4: Nozzle cleaning

• Purge injection machine with purging compound
• Remove accumulations of solidified material from nozzle

Warpage - 12% of all defects

Warpage (deformation) is a defect where the part after removal from the mold curves, bends or twists. One of the most difficult defects to eliminate.

Identification

• Curved surfaces where they should be flat
• Flatness test: part on table - do all points touch?
• Automotive typically requires <2mm warpage for large parts

Main causes

Mechanism: Warpage results from non-uniform material shrinkage during cooling.

1. Non-uniform cooling - one side cools faster → different shrinkage → bending

2. Internal stresses - excessive packing pressure "freezes" stresses

3. Molecular orientation - molecules orient in flow direction → anisotropic shrinkage

Tederic Solutions

Strategy 1: Cooling optimization

• Mold temperature: Increase by 10-20°C (slower, more uniform cooling)
• Example for PP: from 40°C → 55°C
• Cooling time: Extend by 20-30% (allow fuller crystallization)

Strategy 2: Packing pressure reduction

• Holding pressure: Reduce by 15-25% (e.g., from 750 bar → 600 bar)
• Reduces internal stresses
• Trade-off: Watch for sink marks

Strategy 3: Material temperature control

• Reduce gradient between barrel zones
• Instead of 200-210-220-230°C → 215-215-220-220°C (flatter profile)

Note: Warpage often requires parameter trade-offs. Use DOE (Design of Experiments) to find optimal settings.

Sink marks - 25% of all defects

Sink marks (depressions, cavities) are local depressions on the part surface, usually in areas with thick cross-sections or at ribs.

Identification

• Shallow depressions (0.1-2mm) on outer surface
• Location: opposite thick sections, mounting features, ribs
• In Class A parts: unacceptable defect

Main causes

Mechanism: When a thick section of the part shrinks internally, the solidified outer layer is "sucked" inward → sink mark.

Risk factors:

• Large wall thickness (>3mm for PP, >4mm for PA)
• Non-uniform wall thickness
• Ribs thicker than 60% of nominal wall thickness
• Insufficient packing pressure

Tederic Solutions

Step 1: Increase packing pressure and time

• Holding pressure: Increase by 15-30% (e.g., 500 → 650 bar)
• Holding time: Extend by 3-8 seconds
• Continue packing until gate freeze

Step 2: Increase shot size

• More material available for packing phase
• Increase by 3-7%

Step 3: Lower mold temperature

• Faster solidification of skin layer → better support
• Lower by 5-15°C
• Trade-off: May increase warpage risk

⚠️ Note: Sink marks and warpage have opposing solutions. Find compromise settings - priority depends on application (Class A surfaces vs precise fitting parts).

Burn marks (material charring)

Burn marks (charring, black spots) are dark discolorations or charred areas resulting from local material overheating.

Identification

• Dark spots (brown, black) usually in final fill areas
• Characteristic burning odor
• Material may be brittle, weakened

Main causes

Diesel effect: Trapped air in the mold compresses during injection, temperature rises to 400-600°C, igniting the material.

Additional causes: Excessive barrel temperature, too long residence time, shear heating at high speeds.

Tederic Solutions

Step 1: Improve venting (tool modification)

• Most effective solution
• Add vents 0.02-0.05mm on parting line
• Temporary workaround: slightly reduce clamping force (flash risk!)

Step 2: Reduce injection speed

• Slower injection → less air compression
• Reduce by 20-40%
• Especially in final fill phase

Step 3: Lower temperatures

• Barrel temperature: -10-20°C
• Lower temperature = less susceptible to degradation

Step 4: Material handling

• Properly dry material (moisture → vapors → burn marks)
• PA, PET, PC: drying 80-100°C, 4-6h
• Limit regrind to 20-30% max

Weld lines - 8% of all defects

Weld lines (knit lines) are visible lines formed where two flowing material fronts meet and join.

Identification

• Thin line on part surface (0.01-0.1mm)
• Location: below holes, behind posts, at multiple gates
• In transparent materials: very visible
• Weld line strength: typically 60-90% of virgin strength

Main causes

Mechanism: Two fronts meet at low temperature → weak molecular bond → visible line, low strength.

Tederic Solutions

Strategy 1: Increase material temperature

• Barrel temperature: +15-25°C all zones
• Nozzle temperature: +10-15°C
• Mold temperature: +10-20°C (fronts remain hot longer)

Strategy 2: Increase injection speed

• Faster injection → less cooling before joining
• Increase by 20-40%

Strategy 3: Increase packing pressure

• Higher pressure forces fronts to better bond
• Increase by 15-25%

Note: Weld lines cannot always be eliminated - acceptance depends on application (Class A surfaces vs hidden surfaces vs structural parts).

Diagnostic matrix - quick troubleshooting

The table below contains quick fixes for the 6 most common defects:

Defect	First attempt	Second attempt	Root solution
Flash	↑ Clamping force +15%	↓ Injection pressure -15%	Mold maintenance
Short shot	↑ Shot size +10%	↑ Barrel temperature +15°C	Nozzle cleaning, venting
Warpage	↑ Mold temperature +15°C	↓ Holding pressure -20%	Cooling optimization
Sink marks	↑ Holding pressure +20%	↑ Holding time +5 sec	Design: reduce wall thickness
Burn marks	↓ Injection speed -30%	↓ Barrel temperature -15°C	Add vents
Weld lines	↑ Melt temperature +20°C	↑ Injection speed +30%	Gate relocation

Systematic approach: Test parameters gradually, change one variable at a time, document results. Use DOE (Design of Experiments) for complex cases.

Case study - 82% scrap reduction

PP packaging manufacturer - comprehensive optimization

Company: PP disposable cup manufacturer, Mazovia, 80 employees

Production: 200ml thin-wall cups, 8-cavity mold, 350k pcs/day

Machine: Tederic TRX-M.260

Initial problem:

• Scrap rate: 6.8% (23,800 defective cups/day)
• Defect mix: Short shots 38%, Warpage 29%, Flash 18%, Burn marks 15%
• Loss: ~420k PLN/year

6-month program - systematic approach:

Month 1-2: Data collection, Pareto analysis → Short shots = priority #1

Month 3: Short shot elimination

• Cause: Shot size 42% (too low)
• Solution: Increased to 55%, temperature +12°C
• Result: 2.6% → 0.3% (-88%) ✅

Month 4: Warpage reduction

• Cause: Non-uniform cooling
• Solution: Mold temperature 40°C → 58°C, +8 sec cooling
• Result: 2.0% → 0.6% (-70%) ✅

Month 5: Flash elimination

• Cause: Mold wear (350M cycles)
• Solution: Mold regeneration (parting line grinding)
• Result: 1.2% → 0.1% (-92%) ✅

Final results after 6 months:

• Scrap rate: 6.8% → 1.2% ✅ (-82% reduction)
• Good parts: 326k → 346k daily (+6% efficiency!)
• Savings: ~360k PLN/year recovered
• Investment: 45k PLN (regeneration + SPC software)
• ROI: 1.5 months ✅

ROI of quality investment

Quality is not a cost center - it's a profit center!

Example savings calculation

Assumptions: 5M parts/year, cost 3.60 PLN/part (material + energy + labor)

Scenario A: Defect rate 5% (current state - poor)

• Defective parts: 250,000/year
• Wasted cost: 900,000 PLN/year ❌

Scenario B: Defect rate 2% (improvement to average)

• Defective parts: 100,000/year
• Wasted cost: 360,000 PLN/year
• Savings: 540,000 PLN/year ✅

Scenario C: Defect rate 0.5% (world-class)

• Defective parts: 25,000/year
• Wasted cost: 90,000 PLN/year
• Savings: 810,000 PLN/year ✅

Typical investment costs

Process optimization: 15-25k PLN (DOE studies, test material)

• Expected improvement: 30-50% defect reduction
• ROI: <1 month

Tool regeneration + optimization: 40-85k PLN

• Expected improvement: 60-80% defect reduction
• ROI: <2 months

Full SPC system + automation: 110-215k PLN

• Expected improvement: 70-90% reduction + automated tracking
• ROI: 2-4 months
• Additional benefits: Traceability, real-time alerts, predictive maintenance

Summary and next steps

Key takeaways

1. 6 defects = 91% of problems

Flash, sink marks, short shots, warpage, weld lines, burn marks - by eliminating these defects, you achieve dramatic quality improvement.

2. Most defects have specific, identifiable causes

Systematic approach (5 Whys, Ishikawa, DOE) leads to solutions. 80% can be eliminated through machine parameter adjustment.

3. Tederic injection molding machines enable Cpk>2.0 achievement

NEO series: repeatability <0.5%, temperature control ±2°C. DREAM series: <0.3% repeatability, ±1°C. This is the foundation for world-class quality.

4. ROI of quality investment is astronomical

Typically <3 months payback for process optimization, <6 months for tooling improvements. Savings lasting for years.

5. Quality = competitive advantage

In automotive, medical, packaging - quality requirements are the entry ticket. Suppliers with Cpk>2.0 and scrap rates <1% get contracts.

What to do now - Action plan

1. Measure current state

• Start tracking scrap rate (even simple spreadsheet)
• Categorize defects by type
• Calculate cost of quality (defects × cost per part)

2. Pareto analysis - identify main problems

• Which 2-3 defects account for 70-80% of problems?
• Focus effort on main priorities

3. Root cause analysis

• 5 Whys for each major defect
• Get to root cause, not just symptoms

4. Implement solutions systematically

• Start with process optimization (machine parameters) - lowest cost
• Use DOE - change one variable at a time, measure impact
• Document successful parameter sets

5. Verify and maintain improvements

• Monitor scrap rate after changes
• Calculate Cpk (goal ≥1.67 for automotive)
• Lock process parameters, operator training

6. Continuous improvement

• Quality is a continuous journey, not a destination
• Set increasingly ambitious goals: 5% → 2% → 1% → 0.5%
• Celebrate successes with team

Need help?

The TEDESolutions team offers:

• Quality audits: On-site assessment, root cause analysis, action plan
• Process optimization: DOE studies for Tederic machines, parameter optimization
• Training: Operators and engineers in troubleshooting, SPC
• SPC implementation: Software configuration, dashboards, data integration

---