Flash / Burr
Causes, Solutions & Prevention in Injection Molding
Flash is excess material that escapes the mold cavity through gaps at parting lines, vents, or slides.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Holding pressure too high
- Fill weight above 98%
- Barrel temperature above TDS limit
- Clamp force not verified
🔲 Mold
- Support blocks misaligned
- Vent depth too deep for MFI
- Cavity imbalance >5%
- Parting line deformation
- Slide construction undersized
🧪 Material
- MFI too high (low viscosity)
- Moisture causing hydrolysis
- High regrind ratio
🏭 Machine
- Injection pressure exceeds clamp force
- Corrosion on mold or platen surface, or mold marks on platen from undersized mold mounting.
- Moving platen sag on machine.
- Platen deflection due to mounting undersized mold on machine.
- Moving platen sag on machine.
- Tie-bar deflection
🗂 Mold Design
- Mold support block placement incorrect — cavity pressure deflects platen
- Platen deflects despite support blocks.
- Cavity balance is not adequate.
- Vent depth not suitable per material standards.
- Slide construction and cylinder insufficient.
🔧 Mold Condition
- Trapped plastic material in cores, parting lines, or screw areas prevents proper closing.
- Mold polishing not performed per procedure
- Incorrect or improperly executed mold modifications
📐 Part Design
- Wall thickness variations exist — pressure requirement increases.
- Flexible core designed for hole area
- Parting line meets at 90° causing flash feel
- Gate positioned at finger-touch area of part
📋 General
- Machine clamp force insufficient
- Moving platen sag
- Tie-bar deflection
✅Solutions & Corrective Actions
⚙️ Process
- Reduce holding pressure in steps
- Keep fill weight <98%
- Verify clamp force numerically
- Reduce barrel temperature
🔲 Mold
- Reposition support blocks
- Re-cut vents to correct depth per MFI
- Balance runner system (target <5% imbalance)
- Repair parting line
🧪 Material
- Try material with lower fluidity (lower MFI).
- Material MFI value should be controlled at incoming inspection together with lot number.
- Slider can be added.
- Moisture causes hydrolysis in material, reducing molecular weight and disrupting chain lengths — material becomes more fluid. Material must be dried with dehumidifying dryer, drying temperature and time per TDS.
- Regrind increases fluidity — reduce regrind ratio, reduce regrind particle size. Proper screen selection can reduce regrind size.
🏭 Machine
- Mold should be moved to a higher tonnage machine. If hydraulic, check for leaks. Part projected area should be calculated per material.
- Platen and mold surfaces must be cleaned. Proper mold must be mounted.
- Moving platen sag should be checked every 6 months. Platen bushing wear should be checked. Platen parallelism should be checked. Moving platen deflection should be checked.
- Do not mount mold smaller than machine minimum mold size
- Moving platen sag should be checked every 6 months. Platen bushing wear should be checked. Platen parallelism should be checked. Moving platen deflection should be checked.
- Tie-bar deflection should be checked periodically. If one tie-bar breaks, all four must be replaced.
🗂 Mold Design
- Support blocks must be centered in mold — add blocks under hot runner cavities too.
- Center support block should be 0.03 mm higher
- In multi-cavity molds without balance, each cavity can receive different pressure — this causes flash in some cavities depending on injection speed. Equal distance from material entry to each cavity eliminates this. Acceptable imbalance is 1-5%. Imbalance% = (Heaviest cavity - Lightest cavity) / Heaviest cavity × 100
- Vent depths should be welded closed if needed and re-cut to correct depth.
- Slide construction must be robust with support bushings at required locations. For hydraulic slides, piston adequacy must be calculated. Locking system must be added.
🔧 Mold Condition
- Mold steel selected too soft — higher hardness should be selected. Process settings should be optimized to reduce startup scrap. Crushed areas in mold should be repaired with laser welding.
- If mold design uses spring-loaded fixed platen, mold protection is bypassed and material jams go uncontrolled. Spring-loaded mold designs should be avoided.
- Molds should undergo periodic polishing. If polishing procedures are not followed in order, parting line mismatch occurs. Polishing must be done per procedures by qualified personnel and results verified.
- Hasty modification decisions cause problems in mold structure and in-mold rheology, invalidating initial moldflow analyses. Modifications should be avoided if possible. When mandatory, design, production and tooling departments should decide together.
📐 Part Design
- Reduce wall thickness variation in design — add radii to transitions
- Hole should be repositioned
- Design should transfer to core side
- Gate location or type should be changed
📋 General
- DOE 4-run test: High/Low melt x High/Low mold temp
- Switch to lower MFI grade
- Check tie-bar deflection
🛠Step-by-Step Troubleshooting
Check Process Parameters
Reduce holding pressure in steps
Inspect the Mold
Reposition support blocks
Verify Material Condition
Try material with lower fluidity (lower MFI).
Check Machine Settings
Mold should be moved to a higher tonnage machine. If hydraulic, check for leaks. Part projected area should be calculated per material.
❓Frequently Asked Questions
What causes flash / burr in injection molding?
Flash / Burr is caused by several factors including: Holding pressure too high; Fill weight above 98%; Support blocks misaligned; Vent depth too deep for MFI; MFI too high (low viscosity); Moisture causing hydrolysis.
How do you fix flash / burr in injection molding?
Start by checking key parameters: Clamp force (kN), Holding pressure, Injection speed, Melt temperature. Reduce holding pressure in steps
Which plastics are most affected by flash / burr?
Common materials include: PP, PE, PA6, POM, TPE. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent flash / burr in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Clamp force (kN), Holding pressure; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between flash / burr and similar defects?
Thin plastic film on parting line, vents, or around ejector pins. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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