Moisture Is the Enemy of PET and PLA

PET and PLA are hygroscopic. Pellets absorb moisture from the air, and when that moisture meets the melt in the extruder barrel it hydrolyses the polymer chains. The result is measurable: intrinsic viscosity drops, sheet loses toughness, and the finished cup cracks at the rim or turns hazy. In PLA the same mechanism produces brittleness and a narrow processing window.

This is why the LSI series from Shantou Longsheng Industry Co., Ltd. is specified with a drying and crystallising system rather than a simple hopper.

What the System Does

Crystallisation first. Amorphous PET pellets soften and stick together around 70–80°C. Crystallising them at higher temperature before drying prevents the hopper from bridging and lets the material reach drying temperature evenly.

Drying second. Hot dehumidified air circulates through the pellet bed, driving moisture down to the low levels PET and PLA require before melt processing. Dew point and residence time are the two parameters that matter.

Then extrusion. A single screw with a controlled compression profile melts and pumps the dried material to the die, and the sheet is calibrated and cooled on a three-roll stack.

What the Converter Sees Downstream

• Higher clarity — no hydrolytic haze, so transparent cups and lids stay transparent
• Better toughness — retained molecular weight means the rim survives trimming and denesting
• Stable gauge — consistent melt pressure gives a flatter thickness profile across the sheet
• Fewer rejects — bubbles and gels from residual moisture largely disappear

Gauge consistency is the point that reaches furthest. A cup machine assumes a certain sheet thickness; when the sheet varies, wall distribution varies, and the filling line notices before the QC lab does.

Where the Sheet Goes Next

LSI series sheet feeds directly into Longsheng cup making machines — FS-8556 full servo, HSS-8556 hydraulic servo and HAI-8556 hydraulic — and into multi-station thermoforming machines for trays, clamshells and lids. Because sheet and forming equipment come from one supplier, gauge, width and heat-history expectations are aligned at the design stage.

The company also builds the DSI-PP multilayer co-extrusion line for PP and the SI-PE single screw line for PE, so a converter running several materials can keep one process philosophy across the plant.

Company Background

Founded in 1998 in Shantou City, Guangdong Province, Longsheng operates a 30,000 square metre facility, holds 25 authorised patents (1 invention, 24 utility model) and is a high-tech enterprise.

For sheet extrusion projects, visit the official website: https://www.stlongsheng.com