GWELL EXPERIENCE-What is the solution for the PET sheet to get yellow when extruded production

The yellowing of PET sheets during extrusion production is a common issue influenced by multiple factors in the manufacturing process. Based on the search results, here are the key causes and solutions:

 1. **Control Drying Conditions**
PET is highly sensitive to moisture and heat. Improper drying can lead to hydrolysis or thermal degradation, both of which cause yellowing:
- **Excessive drying temperature/time**: Prolonged exposure to high temperatures (above 160°C) or extended drying durations can degrade PET. Reduce drying temperature to **140–160°C** and limit drying time to **4–6 hours** .
- **Inadequate drying**: Insufficient drying leaves moisture in PET, leading to hydrolysis. Ensure moisture content is **<0.005%** using a dehumidifying dryer with **-40°C dew point** air .
- **Monitor molecular sieves**: Replace aged sieves if the dryer fails to maintain low humidity .

 2. **Optimize Melt Temperature**
High melt temperatures during extrusion accelerate PET decomposition, releasing volatile compounds (e.g., acetaldehyde) that cause yellowing:
- Keep melt temperature **<280°C** to prevent thermal degradation. Adjust extruder zones (e.g., rear zone: 240–260°C, die zone: 255–265°C) .
- Use a **barrier screw design** to reduce shear heat and ensure uniform melting .

3. **Use Stabilizers or Anti-Yellowing Masterbatches**
Additives can mitigate thermal and oxidative degradation:
- Incorporate **UV stabilizers** or **anti-hydrolysis agents** to improve thermal stability .
- Apply **fluorescent whitening agents** (e.g., Zhongshan Beihua’s XH-202 masterbatch) to neutralize yellow tones and enhance clarity .

4. **Prevent Contamination and Decomposed Material**
Residual degraded PET or impurities can trigger chain reactions:
- Replace **clogged or broken filter screens** regularly to remove carbonized particles.
- Clean extruder barrels thoroughly to eliminate residual decomposed PET .

### 5. **Adjust Cooling and Calendering**
Rapid cooling prevents crystallization-induced discoloration:
- Maintain a **short distance (8–10 cm)** between the die and calender rolls to minimize heat exposure .
- Set calender roll temperatures to **40–60°C** (upper/lower rolls) and ensure synchronized speed control to reduce stress-induced defects .

### Summary of Critical Parameters:
| Factor | Optimal Range/Recommendation | Source |
|----------------------|-----------------------------------------|--------------|
| Drying Temperature | 140–160°C | |
| Moisture Content | <0.005% | |
| Melt Temperature | <280°C | |
| Calender Roll Gap | 8–10 cm from die | |
| Filter Screen | Regular replacement (e.g., 100–200 mesh)| |

By systematically addressing these factors, manufacturers can effectively reduce yellowing and maintain PET sheet quality. For further details, refer to the technical guidelines from Webpages 6, 7, 9, and 11.