دليل تحسين المواد ومعلمات المعالجة للأفلام المنفوخة

I. Comparison of blown film characteristics of three biodegradable materials

Material	Melt strength	Thermal stability	Difficulty
PLA	Low	Poor (easily hydrolyzed and thermally degraded)	The membrane bubble is prone to shaking, has large lateral shrinkage, and the membrane is brittle.
PBAT	medium	better	Highly adhesive, prone to sticking, and difficult to roll up.
PBS	Low/medium	medium	Rapid crystallization, whitish and opaque film, insufficient toughness

Common challenges: melt strength is generally lower than that of traditional PE, it is sensitive to temperature, humidity and shear, and the process window is narrow.

II. PLA Blown Film: Temperature and Humidity Control, Stability is Paramount

PLA is a typical brittle and biodegradable material. The most common problems during blown film extrusion are film bubble vibration and film brittleness.

1. Raw Material Pretreatment: Drying is the First Step
   PLA is highly hygroscopic. A moisture content exceeding 250 ppm will lead to hydrolytic degradation, a sharp drop in molecular weight, and the appearance of numerous crystal points, bubbles, and even film breakage during blown film extrusion.

Drying Conditions: Hot air drying at 70–80℃ for 4–6 hours, dew point ≤ -40℃, ensuring moisture content <200 ppm.

Note: Use within 1 hour after drying, or place in a dehumidifying cabinet.

2. Extrusion Temperature Setting: Lower is Better Than Higher
   PLA has a melt temperature of 170–190℃, but a low thermal degradation threshold. Recommended stepped heating:

• Feed section: 150–160℃ (to prevent bridging)
• Compression section: 170–180℃
• Metering section: 175–185℃
• Die head: 170–180℃ (excessive die head temperature will cause melt fracture)
• Key points: The temperature difference between the end of the barrel and the die head should not exceed 10℃, and the melt temperature must not exceed 200℃, otherwise PLA will rapidly yellow and degrade.

3. Blown film process: Small blow-up ratio + large draw ratio

• Blow-up ratio: 1.5–2.5 (PE is usually 2.5–4). Too high a ratio will cause increased bubble vibration.
• Draw ratio: 3–6 (slightly higher than PE). Longitudinal stretching improves film toughness, but lateral shrinkage must be balanced.
• Frost line height: Controlled at 80–150mm. Too low a line results in too rapid cooling and insufficient bubble crystallization; too high a line leads to bubble deformation.
• Air ring: Use a dual-outlet adjustable air ring. The upper air outlet stabilizes the film bubble, and the lower air outlet cools it. The air volume should be small rather than large.

4. Common Problems and Solutions

Questions	Possible reasons	Solution
The membrane bubble was shaking severely.	Insufficient melt strength	Add 0.5–1% chain extender (such as ADR); reduce the die temperature by 5°C.
The film is brittle and easily torn.	Over-orientation or degradation	Reduce the traction ratio; check if the moisture content is excessive.
Crystal points/black spots	Degradation or impurities	Clean the screw; reduce residence time; replace the filter.

III. PBAT Blown Film: Anti-blocking and Adhesion Control

PBAT has good toughness and flexibility, but surface stickiness and winding adhesion are two major problems.

1. Process Parameter Characteristics

Temperature: Feed section 130–140℃, compression section 145–155℃, metering section 150–160℃, die head 145–155℃. PBAT melting point is approximately 120–130℃, but its fluidity increases significantly with increasing temperature; excessively high temperatures can cause film bubble collapse.

Screw Speed: Should not be too fast, 30–60 rpm is recommended. High shear will cause PBAT molecular chains to break, reducing melt strength.

2. Core Methods to Solve Adhesion

• Adding Anti-blocking Agents: Adding 0.5–2% silica or talc to the formulation forms microscopic protrusions on the film surface, reducing the contact area.
• Cooling Enhancement: Increase the airflow of the air ring and reduce the temperature of the cooling roller (if applicable) to 20–30°C to quickly set the film surface.
• Twist Tension: Use closed-loop constant tension control, with a tension value 30–50% lower than PE, and cover the surface of the take-up shaft with release material (such as Teflon cloth).

3. Common Problems

• Bubble sticking to the bubble stabilizing cage: Reduce the die temperature by 5°C, or spray an anti-stick coating on the surface of the bubble stabilizing cage.
• Severe transverse shrinkage of the film: Appropriately increase the blow-up ratio to 2.0–2.5, while reducing the draw ratio.

IV. PBS Blown Film: Controlling Crystallization, Improving Transparency and Toughness

PBS crystallizes rapidly, often resulting in blown films that are whitish, opaque, and brittle.

1. Rapid Cooling to Inhibit Crystallization

PBS crystallizes at approximately 80–100℃. Rapid cooling is necessary during blown film production to inhibit spherulite growth: The air ring nozzle should be as close to the die as possible (10–30 mm from the die opening).

The airflow should be at maximum, and the lower the air temperature, the better (refrigerated air can be used).

The height of the white frosting line should be controlled at 50–100 mm, the shorter the better.

2. Blending Modification to Improve Toughness

Pure PBS films are brittle and are usually blended with PBAT (PBS/PBAT = 70/30 to 50/50): Blending increases melt strength and improves bubble stability.

The film transitions from brittle fracture to ductile fracture, and transparency improves.

3. Temperature Settings

PBS processing window: 180–200℃, slightly wider than PLA: Feed section 160–170℃, Compression section 175–185℃, Metering section 180–190℃, Die head 175–185℃.

Note: For prolonged shutdowns, lower the temperature to below 150℃ to prevent degradation.

V. General Precautions for Blown Film Production of Biodegradable Materials

Thoroughly clean the screw and die head after each experiment. Residual biodegradable material will severely carbonize upon the next heating, forming black spots. It is recommended to use PP or PE as cleaning material and extrude until no residue remains.

Pay attention to ambient humidity. Biodegradable materials (especially PLA) will rapidly absorb moisture in environments with humidity >60%. It is recommended to use a dehumidified workshop or a drying hopper.

1. Equipment Configuration Recommendations:

• Screw L/D ratio ≥ 30, and equipped with a mixing element to improve plasticization uniformity.
• The die head flow channel is mirror polished (Ra≤0.05μm) to reduce melt retention.
• Temperature control accuracy is ±1℃, and it has a rapid cooling function (such as internal oil cooling within the die head).

Blown film production using biodegradable materials such as PLA, PBAT, and PBS presents a dual challenge to equipment configuration and process optimization capabilities. Thorough drying, precise temperature control, rapid cooling, and a suitable formulation—each step determines the final film quality. Our company’s blown film pilot line has undergone several optimizations specifically for biodegradable materials: an all-hard steel screw combined with a mixing section, a spiral mandrel-type mirror die, a dual-outlet adjustable air ring, and closed-loop constant tension winding. Hartek also provides process tuning support to help you quickly find the optimal parameter window for your specific material.

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