• by MEOT
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• 2026/5/13

Preform Blushing, Crystallization, Sticking? Meto Engineers Summarize 5 Causes and Solutions

You open the mold. The preforms should drop cleanly. Instead, some are stuck. Others come out white and cloudy instead of clear. And on some, the neck finish looks crystallized — rough, opaque, and likely to leak.

Blushing (whitening). Crystallization (white, brittle areas). Sticking (preforms that won‘t eject). These three problems often appear together. They frustrate operators, reduce output, and increase reject rates.

The good news: these problems are solvable. Meto engineers have helped hundreds of customers diagnose and fix these issues. This article explains the five most common causes — and provides practical solutions for each.

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Part 1: What Do These Defects Look Like?

Before diagnosing causes, it is important to correctly identify the defect.

Blushing and crystallization are sometimes confused. Blushing is stress-induced whitening without structural change. Crystallization is actual crystal formation that changes the material properties.

Quick test: Heat the area with a hot air gun. Blushing may disappear. Crystallization will not.

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Part 2: Cause #1 — Low Melt Temperature

The Problem

PET must be processed within a specific temperature range (typically 270–290°C for amorphous PET, 280–300°C for bottle-grade). Too cold, and the material does not flow or pack properly.

Symptoms:

• Blushing near gate area

• Poor filling of thin sections

• High injection pressure required

• Sticking (cold material grips cavity)

The Solution

Meto tip: Do not rely solely on barrel setpoints. Use a melt probe to measure actual melt temperature. Barrel setpoints can be 20–30°C higher than actual melt.

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Part 3: Cause #2 — Excessive Moisture in PET

The Problem

PET is hygroscopic. It absorbs moisture from the air. When wet PET is processed, hydrolysis occurs — polymer chains break, releasing water vapor and causing defects.

Symptoms:

• Blushing throughout preform (not just gate)

• Splay marks (silver streaks)

• Reduced IV (intrinsic viscosity)

• Brittle preforms

• Sticking (degraded material adheres to cavity)

The Solution

Meto tip: Many drying problems are invisible. Test moisture content with a proper analyzer. Do not guess.

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Part 4: Cause #3 — Poor Cooling Design or Operation

The Problem

Uneven or insufficient cooling creates hot spots. Hot spots cause:

• Crystallization (slow cooling in PET‘s crystallization temperature range)

• Sticking (material remains soft and adheres to cavity)

• Blushing (stress from uneven contraction)

Symptoms:

• Crystallization in specific areas (not uniform)

• Sticking on some cavities but not others

• Longer cycle times than expected

• Hot mold surfaces (measured with thermal imager)

The Solution

Meto design solution: If your mold has straight-drilled cooling channels, consider upgrading to conformal cooling. Conformal channels follow the preform contour, eliminating hot spots and reducing cycle time.

Meto tip: A thermal imaging camera is invaluable for diagnosing cooling problems. Hot cavities show as bright spots. Cold cavities show as dark spots.

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Part 5: Cause #4 — Injection Speed and Packing Issues

The Problem

How the mold fills affects stress distribution. Too fast or too slow can cause problems.

Too fast injection:

• High shear stress causes blushing

• Material may degrade (friction heating)

• Potential for jetting or turbulent flow

Too slow injection:

• Premature cooling before fill complete

• High orientation stress (blushing)

• Poor packing (sink marks, dimensional issues)

Symptoms:

• Blushing at gate or flow paths

• Inconsistent preform weight

• Gate seal issues

• Sticking (uneven packing creates mechanical lock)

The Solution

Meto tip: Perform a “short shot” study. Run the mold partially filled to see how material flows. This reveals flow patterns and stress points.

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Part 6: Cause #5 — Mold Surface and Venting Problems

The Problem

Even with perfect process settings, preforms can stick or blush due to mold condition.

Poor surface finish:

• Rough surfaces grip the preform (sticking)

• May cause stress whitening during ejection

Inadequate venting:

• Trapped air compresses and heats (burns material)

• Prevents proper cavity fill

• Creates sticking from vacuum effect

Symptoms:

• Sticking in specific cavity locations

• Burn marks near vent areas

• Inconsistent fill (last cavity to fill shows problems)

• Audible popping during mold opening

The Solution

Meto design solution: Proper vent placement is part of mold design, not an afterthought. Meto molds include strategic venting based on flow simulation.

Meto tip: Do not use steel tools to clean vents. They damage the edges. Use brass or purpose-made vent cleaning tools.

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Part 7: Quick Diagnosis Guide

Use this table to narrow down the cause based on what you observe.

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Part 8: Real Customer Examples

Example 1: Blushing at Gate

Customer: New water bottle line, Southeast Asia
Problem: 8-cavity preform mold produced blushing at gate area on every preform
Diagnosis: Melt temperature too low (measured 255°C at nozzle)
Solution: Increased barrel and hot runner temperatures to achieve 275°C melt
Result: Blushing eliminated within 30 minutes

Example 2: Neck Finish Crystallization

Customer: CSD bottler, South America
Problem: Preforms had white, crystallized neck finishes — caps would not seal
Diagnosis: Neck cooling channel was scaled from hard water, reducing flow by 60%
Solution: Chemical cleaning of cooling channels + water treatment installation
Result: Crystallization disappeared; neck finish returned to clear

Example 3: Sticking on 4 of 32 Cavities

Customer: Large water brand, Thailand
Problem: Specific cavities consistently stuck; others ejected fine
Diagnosis: Cavity surface roughness (Ra 0.8μm) on affected cavities
Solution: Repolished affected cavities to Ra 0.2μm
Result: Sticking eliminated; mold ran 2 more years without recurrence

Example 4: All Three Problems — Blushing, Crystallization, Sticking

Customer: Juice bottle producer, Europe
Problem: New mold produced all three defects intermittently
Diagnosis: Multiple causes — wet PET (moisture 0.012%), low melt temperature (260°C), and poor venting
Solution: Dried PET to 0.004%, increased melt temperature to 280°C, added vents at problem cavities
Result: Clean, clear preforms; reject rate dropped from 5.2% to 0.7%

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Part 9: When the Problem Is the Mold — Not the Process

Sometimes, no amount of process adjustment fixes the problem. The issue is the mold itself.

Mold-related causes of blushing, crystallization, and sticking:

If you have tried all process solutions without success, the mold may be the problem. Meto offers mold audits to diagnose design or manufacturing defects.

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Part 10: Preventing Problems on New Molds

The best time to prevent blushing, crystallization, and sticking is before the mold is built.

What Meto does to prevent these issues:

When you order a preform mold from Meto, these issues are addressed in the design phase — not discovered during production.

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Conclusion: Diagnose, Then Solve

Blushing, crystallization, and sticking are frustrating problems. But they are solvable. Most cases trace back to one of five causes:

1. Low melt temperature

2. Excessive moisture

3. Poor cooling

4. Injection speed or packing issues

5. Mold surface or venting problems

Use the diagnosis guide in this article to identify the cause. Then apply the recommended solution. If the problem persists — or if the mold itself is the issue — Meto engineers are ready to help.

Contact Meto today for technical support on preform quality issues. Send us photos of your defective preforms. We will help you diagnose the cause and recommend a solution.