Common PET Preform Defects and Solutions: Meto Engineers Summarize 7 Key Points
Common PET Preform Defects and Solutions: Meto Engineers Summarize 7 Key Points
Every defective preform wastes material, energy, and time. At high volume, even a one percent reject rate costs tens of thousands of dollars per year.
The good news is that most preform defects have known causes and proven solutions. You do not need to guess. You need to diagnose systematically.
Meto engineers have solved preform defects across hundreds of production lines worldwide. This article covers the seven most common PET preform defects and how to fix them.
Defect 1: Blushing
What It Looks Like
White or cloudy areas on the preform, usually near the gate or in thick sections. The preform is clear elsewhere. Blushing is stress induced whitening, not crystallization.
Common Causes
Low melt temperature
High injection speed
Poor gate design
Moisture in PET
Cold mold surface
Solutions
| Solution | Action |
|---|---|
| Increase melt temperature | Raise barrel temperature by 5 to 10 degrees |
| Reduce injection speed | Lower first stage injection speed |
| Check gate design | Ensure smooth transitions |
| Dry PET properly | Moisture below 0.005 percent |
| Increase mold temperature | Raise cooling water temperature |
Defect 2: Crystallization
What It Looks Like
White, opaque, rough textured areas, usually in the neck finish or thick sections. Unlike blushing, crystallization will not disappear when reheated.
Common Causes
Slow cooling in the crystallization zone
Poor neck cooling
Excessive wall thickness
High mold temperature
Solutions
| Solution | Action |
|---|---|
| Improve neck cooling | Check cooling circuit flow and clean channels |
| Reduce cooling water temperature | Lower to 8 to 12 degrees Celsius |
| Increase cooling time | Longer cooling phase in the cycle |
| Verify cooling uniformity | Use thermal imaging to find hot spots |
| Upgrade to conformal cooling | For persistent neck crystallization |
Defect 3: Sticking
What It Looks Like
Preforms remain in the cavity or on the core after the mold opens. May stick on all cavities or only specific ones.
Common Causes
Rough cavity surface
Insufficient draft angle
Worn or broken ejector pins
Overpacking
Mold too hot
Solutions
| Solution | Action |
|---|---|
| Polish cavities | Target surface finish of 0.2 microns or better |
| Check draft angles | Should be 0.5 to 1.0 degree minimum |
| Inspect ejector pins | Replace worn pins and verify stroke |
| Reduce hold pressure | Lower second stage pressure |
| Improve cooling | Reduce mold temperature |
Defect 4: Splay Marks
What It Looks Like
Silvery, streaky marks on the preform surface. Often appear as wavy lines or small silver spots.
Common Causes
Wet PET
Degraded material
Trapped air
Contamination
Solutions
| Solution | Action |
|---|---|
| Check dryer performance | Dew point below minus 40 degrees Celsius |
| Measure moisture content | Must be below 0.005 percent |
| Verify drying time | 4 to 6 hours at 160 to 170 degrees Celsius |
| Check for degraded material | Clean screw and barrel |
| Inspect vents | Clean vent channels and verify depth |
| Check regrind ratio | Reduce if excessive |
Defect 5: Short Shots
What It Looks Like
Preform is missing material, usually at the bottom. The shape is incomplete. May affect some cavities more than others.
Common Causes
Low melt temperature
Insufficient injection pressure
Small gate or nozzle
Hot runner imbalance
Premature gate freeze
Solutions
| Solution | Action |
|---|---|
| Increase melt temperature | Raise barrel temperature by 5 to 10 degrees |
| Increase injection pressure | Raise first stage pressure limit |
| Check gate diameter | Should be adequate for preform weight |
| Balance hot runner | Adjust valve gate timing or temperatures |
| Increase hold time | Keep gate open longer |
| Check for obstructions | Clean nozzle, gate, and runners |
Defect 6: Bubbles
What It Looks Like
Visible air pockets inside the preform wall. May be small or large. Often near the gate or in thick sections. Bubbles are internal, unlike splay which is on the surface.
Common Causes
Wet PET
Trapped air
Insufficient packing
Gas from degraded material
Solutions
| Solution | Action |
|---|---|
| Dry PET thoroughly | Moisture below 0.005 percent |
| Increase back pressure | Helps vent trapped air from melt |
| Increase hold pressure | Compensates for shrinkage |
| Extend hold time | Allow proper packing |
| Check venting | Clean or carefully deepen vents |
| Reduce screw speed | Less air entrainment |
Defect 7: Dimensional Variation
What It Looks Like
Preforms from different cavities have different weights or lengths. Variation may be subtle at 0.5 percent or significant at 2 percent or more.
Common Causes
Hot runner imbalance
Cooling variation
Cavity dimension variation
Valve gate timing issues
Temperature variation across cavities
Solutions
| Solution | Action |
|---|---|
| Balance hot runner | Adjust valve timing or temperatures |
| Verify cooling uniformity | Use thermal imaging and check flow rates |
| Measure cavity dimensions | Perform CMM inspection if suspect |
| Check valve gates | Verify all pins move freely |
| Stabilize mold temperature | Check water temperature and flow |
Meto Guarantee
Meto guarantees cavity to cavity weight variation of 0.3 percent or less, verified by trial molding before shipment.
Quick Diagnosis Table
| Defect | Primary Cause | First Thing to Check |
|---|---|---|
| Blushing | Low melt temperature or high shear | Barrel temperatures |
| Crystallization | Poor neck cooling | Neck cooling circuit flow |
| Sticking | Rough surface or ejector issues | Cavity polish and ejector pins |
| Splay marks | Wet PET | Dryer dew point |
| Short shots | Low temperature or pressure | Melt temperature |
| Bubbles | Wet PET or poor packing | Moisture content |
| Weight variation | Hot runner imbalance | Cavity to cavity weight data |
When the Problem Is the Mold
Sometimes the mold itself is the problem, not the process.
| Mold Issue | Defect It Causes | Solution |
|---|---|---|
| Poor gate design | Blushing, short shots | Gate redesign |
| Inadequate cooling | Crystallization, sticking | Conformal cooling upgrade |
| Rough cavity surface | Sticking | Repolishing |
| Worn ejector pins | Sticking, damage | Pin replacement |
| Hot runner imbalance | Weight variation | Rebalancing |
| Insufficient venting | Splay, short shots | Vent addition or cleaning |
Meto offers mold inspection and refurbishment services. If your mold causes defects despite correct processing, contact us for a diagnostic evaluation.
Prevention Starts with Better Mold Design
The best way to prevent defects is to start with a well designed mold. Meto builds quality into every mold.
| Design Feature | Prevents |
|---|---|
| Flow simulation | Blushing, short shots, imbalance |
| Conformal cooling | Crystallization, sticking |
| Mirror finish | Sticking |
| Proper gate design | Blushing, bubbles |
| Strategic venting | Splay, short shots |
| Precision machining | Weight variation |
| Trial molding before shipment | All defects verified before delivery |
When you buy a Meto mold, you receive trial molding reports, setup parameters, and a proven design. You do not receive a mold that you must debug yourself.
Conclusion
PET preform defects are frustrating but solvable. Most trace back to one of seven causes: blushing, crystallization, sticking, splay, short shots, bubbles, or dimensional variation.
Use this guide to diagnose your defect. Check the most likely cause first. Adjust the process. If the problem continues, the mold itself may need attention.
Meto engineers have helped hundreds of customers eliminate preform defects through better mold design, refurbishment, and process troubleshooting. We are ready to help you too.
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