Extrusion Blown Film Moldling Process and Control

 

 

To produce plastic films of different types and specifications, in addition to selecting an appropriate extruder, blow molding die head, and auxiliary equipment, it is also necessary to choose suitable molding methods and process conditions. Only in this way can operations run smoothly and ensure high-quality films.

 

 

Based on the direction of film traction, the production of blown films can be divided into three types: flat extrusion flat blowing, flat extrusion upward blowing, and flat extrusion downward blowing, with flat extrusion upward blowing being the most common.

 

1. Flat Extrusion Flat Blowing Method

The process flow of the flat extrusion flat blowing method is shown in Figure 1-15. This method uses a straight-through die head, and both the die head and auxiliary equipment have relatively simple structures, making equipment installation and operation convenient. However, the extruder occupies a large footprint. Due to the upward flow of hot air and downward flow of cold air, the cooling of the upper part of the film bubble is slower than the lower part. When the plastic has a high density or the bubble diameter is large, the bubble tends to sag, resulting in poor film thickness uniformity and difficulty in adjustment. Typically, this method is suitable for molding PE and PVC blown films with a width of 600 mm or less.

 

 

2. Flat Extrusion Upward Blowing Method

The process flow of the flat extrusion upward blowing method is shown in Figure 1-16. This method uses a right-angle die head, where the material discharge direction is perpendicular to the flow direction in the extruder barrel. The extruded tube is drawn vertically upward, inflated, pressed, and fed into the traction roller. The main advantage of this method is that the entire bubble is supported by the cooled, tough upper segment, ensuring stable film traction. It can produce films with a wide range of thicknesses and widths (e.g., diameters exceeding 10 m). Additionally, the extruder is installed on the ground, requiring no operating platform, making it easy to operate with a small footprint and allowing a wide range of film thicknesses with relatively uniform thickness. The main disadvantages are that the hot air around the bubble rises while cold air sinks, which is unfavorable for cooling; the 90° bend in the die head increases material flow resistance, potentially causing material decomposition at the bend; and the factory height must be greater. Additionally, the die head and auxiliary equipment have complex structures.

 

 

3. Flat Extrusion Downward Blowing Method

The flat extrusion downward blowing method also uses a right-angle die head, but the tube is drawn vertically downward, as shown in Figure 1-17. The traction direction of the bubble is opposite to the hot airflow from the die head, facilitating bubble cooling. Additionally, a water jacket can be used to directly cool the bubble, significantly improving production efficiency and film transparency. This method offers good cooling effects, and the film is drawn downward by gravity into the traction roller, making it more convenient than the upward blowing method. It also allows faster production line speeds and higher output. However, the entire bubble is supported by the non-solidified plastic segment, making it prone to breaking when producing thicker films or at high traction rates, especially for plastics with higher density. The extruder must be installed on a high operating platform, increasing installation costs and operational inconvenience. Due to rapid cooling with the water jacket, this method is suitable for resins with low melt viscosity and high crystallinity (e.g., PP resin) and can be used to produce highly transparent packaging films.

 

 

Among blown film production methods, the flat extrusion upward blowing method is the most commonly used. The following describes the production operations for this method, as shown in Figure 1-18.

 

 

1.Heating: Heat the extruder and die head to the required temperature using heaters and maintain the temperature for a period.

2.Feeding and Extrusion: Once the extruder and die head meet the insulation requirements, start the extruder and add a small amount of plastic (powder or pellets) to the hopper. Initially, the screw rotates at low speed. After the molten material passes through the die head and is inflated into a bubble, gradually increase the screw speed, fill the hopper, and observe whether the extrusion volume around the bubble is uniform. If the bubble is skewed or has uneven thickness, adjust the surrounding temperature and gap width. For areas with faster discharge, lower the temperature and tighten the screws; conversely, increase the temperature and loosen the screws.

3.Lifting: Gather the molten material exiting the die head, lift it, and introduce a small amount of air to prevent adhesion.

4.Feeding Rollers: Feed the lifted bubble into the nip rollers, which press the bubble into a folded film. The film is then guided to the winding device via guide rollers.

5.Inflation: After feeding the bubble into the rollers, inflate it with air and adjust the traction rate and blow-up ratio to ensure the film's fold width and thickness meet requirements. Since the air in the bubble is sealed by the nip rollers, it barely escapes, maintaining constant pressure in the bubble.

6.Adjustment: Film thickness tolerance can be corrected by adjusting the die gap, cooling ring position, air volume, and traction rate. The film width tolerance is mainly adjusted by controlling the inflation size.

 

 

1. Molding Temperature

Temperature control is critical in the blown film process, directly affecting product quality. For heat-sensitive plastics like PVC, precise temperature control is essential, and the proper coordination of heating temperature and time is crucial.

The processing temperature is set to achieve the optimal melt viscosity in the flow state, ensuring qualified products. Different raw materials require different temperatures; for the same material, different film thicknesses require different processing temperatures; and even for the same material and thickness, different extruders may require different temperatures. Thinner films require better melt flowability, so for the same material, a 20 μm film requires a much higher heating temperature than an 80 μm film.

 

Temperature control can be divided into two methods: one gradually increases the temperature from the feed section to the die, while the other keeps the feed section temperature low, sharply increases the temperature in the compression section (controlled at the material's optimal plasticizing temperature), and reduces it in the metering section to maintain the molten state. The die temperature should keep the material in a flowable state and may be the same as or 10–20°C lower than the barrel's end temperature, depending on the screw's length-to-diameter ratio.

 

For heat-sensitive plastics like PVC, the barrel temperature should be lower than the die head temperature to prevent overheating and decomposition in the barrel. For plastics like PE and PP, which are less prone to overheating, the die head temperature can be lower than the barrel temperature, aiding bubble cooling and stabilization, thus improving film quality.

 

Temperature control is complex and requires a thorough understanding of material properties and processing conditions to optimize heating temperatures. The extrusion temperature control ranges for commonly used blown films are shown in Table 1-5.

 

Table 1-5 Extrusion Temperature Control Ranges for Commonly Used Blown Films

 

Film Type			Barrel Temperature / °C		Connector Temperature / °C		Die Head Temperature / °C
PVC (Powder)	High-Speed Blowing	160 ～ 175 170 ～ 185		170 ～ 180 180 ～ 190		185 ～ 190 190 ～ 195
PE PP			130 ～ 160 190 ～ 250		160 ～ 170 240 ～ 250		150 ～ 160 230 ～ 240
Composite Film	PE PP	120 ～ 170 180 ～ 210		210 ～ 220 210 ～ 220		200 200

The heating temperatures of the barrel and die head significantly affect molding and film properties. Excessively high temperatures can make the film brittle, particularly reducing longitudinal tensile strength. High temperatures may also cause periodic transverse vibration waves in the bubble. If the temperature is too low, the resin may not be fully mixed and plasticized, resulting in irregular material flow, affecting uniform stretching, gloss, and transparency. Low processing temperatures can also cause "fish eyes" on the film surface, where crystal points are surrounded by annual ring-like patterns, with thinner film around the crystal points. Additionally, low temperatures reduce the film's elongation at break and impact strength.

 

2. Blow-Up Ratio

The blow-up ratio not only determines the film's fold width but also affects various film properties. Therefore, the selection of the blow-up ratio should consider both the fold width and performance. The blow-up ratio is also limited by the plastic's properties (e.g., molecular weight, crystallinity, melt tension). Table 1-6 lists the optimal blow-up ratio ranges for different types and applications of films for reference.

 

Table 1-6 Optimal Blow-Up Ratio Ranges for Various Films

Film Type	PVC	LDPE	LLDPE	HDPE(Ultra-Thin)	PP	PA	Shrink Film, Stretch Film, Cling Film
Blow-Up Ratio	2.0~3.0	2.0~3.0	1.5~2.0	3.0~5.0	0.9~1.5	1.0~1.5	2.0~5.0

 

A larger blow-up ratio improves the film's optical properties because irregular material flow in the molten resin can extend longitudinally and transversely, making the film smoother. Increasing the blow-up ratio also enhances impact strength, transverse tensile strength, and transverse tear strength, while longitudinal tensile strength and tear strength decrease relatively. Both directional tear strengths stabilize when the blow-up ratio exceeds 3. Longitudinal elongation decreases with an increasing blow-up ratio, while transverse elongation remains relatively stable, only increasing when the die's annular gap widens.

 

3. Traction Ratio

When the traction rate is increased (larger traction ratio), the irregular material flow from the die cannot fully relax before cooling and solidifying, resulting in poor optical properties. Even increasing the extrusion rate cannot prevent a decline in film transparency. At a constant extrusion rate, increasing the traction rate disrupts the balance of longitudinal and transverse strengths, increasing longitudinal strength while decreasing transverse strength.

 

The blow-up ratio and traction ratio represent the multiples of transverse expansion and longitudinal stretching, respectively. If both are increased, the film thickness decreases while the fold width increases, and vice versa. Thus, the blow-up ratio and traction ratio are critical parameters determining the film's final dimensions and properties.

 

4. Film Cooling

Cooling is critical in blown film production, and the degree of cooling significantly affects product quality. The bubble's travel time from the die to the traction rollers is generally over 1 minute (not exceeding 2.5 minutes). Within this short time, the bubble must be cooled and solidified; otherwise, the bubble may stick together under the pressure of the traction rollers, affecting film quality.

 

The cooling rate during blowing affects the bubble shape, as shown in Figure 1-19. Figure 1-19a shows the bubble shape with a slower cooling rate. In actual production, this shape forms when the cooling ring is positioned lower, the air volume is small, and the air temperature in the cooling ring is not very low. Figure 1-19b shows the bubble shape when the film is cooled immediately after leaving the die head. In actual production, this shape forms when the cooling ring is low, the air volume is large, and the air temperature is very low. Figure 1-19c shows the bubble shape when the film is rapidly cooled at a certain distance from the die head. In actual production, this shape forms when the cooling ring is positioned higher, the air volume is large, and the air temperature is very low.

 

 

 

Common abnormal phenomena, their causes, and solutions in blown film production are listed in Table 1-7.

Table 1-7 Common Abnormal Phenomena, Causes, and Solutions in Blown Film Production

 

Abnormal Phenomenon	Causes	Solutions
Difficult Film Traction	1. Die head temperature too high or too low 2. Large thickness variation on one side 3. Dirty raw material with many burn spots	1. Adjust die head temperature 2. Adjust film thickness for uniformity 3. Replace raw material, clean die head and screw
Film Breakage	1. Die head temperature too high or too low 2. Impurities or decomposed material in the melt 3. Blocked filter or die 4. Excessive traction speed 5. Uneven film thickness or excessive blow-up ratio	1. Adjust die head temperature 2. Clean die or replace resin 3. Replace filter, clean die 4. Reduce traction speed 5. Adjust film thickness, reduce blow-up ratio
Skewed Bubble	1. Machine body or die temperature too high 2. Connector temperature too high 3. Uneven film thickness	1. Lower machine body or die temperature 2. Lower connector temperature 3. Adjust film thickness to be uniform
Uneven Film Thickness	1. Uneven die gap 2. Core mold deformation 3. Uneven die head temperature 4. Excessive blow-up ratio 5. Uneven cooling 6. Unstable compressed air	1. Adjust die gap 2. Replace core mold 3. Inspect die head heating ring 4. Reduce blow-up ratio 5. Adjust cooling air volume for uniformity 6. Inspect air compressor
Film Wrinkling	1. Uneven film thickness, wrinkles at thin areas 2. Insufficient or uneven cooling 3. Misalignment of herringbone plate or traction rollers with die head 4. Excessive herringbone plate angle 5. Uneven traction roller pressure 6. Inconsistent winding tension 7. Excessive blow-up ratio, irregular bubble shape 8. Environmental air influence	1. Adjust film thickness for uniformity 2. Enhance cooling or reduce production speed 3. Correct centerline alignment 4. Reduce herringbone plate angle 5. Adjust traction rollers 6. Adjust winding rollers 7. Reduce blow-up ratio or replace die, adjust bubble shape 8. Stabilize environmental airflow
Poor Transparency	1. Low plasticizing temperature 2. Insufficient freeze line height 3. Excessive traction speed 4. Small blow-up ratio	1. Increase molding temperature 2. Raise freeze line height 3. Reduce traction speed 4. Increase blow-up ratio
Film Surface Blooming	1. Low machine body or die temperature 2. Excessive screw speed 3. Screw temperature too high or too low 4. Improper formulation	1. Increase machine body or die temperature 2. Reduce screw speed 3. Adjust screw cooling medium flow 4. Improve formulation
Impurities or Burn Spots on Film Surface	1. Impurities in raw material 2. Broken filter screen 3. Resin decomposition 4. Uneven mixing	1. Screen raw material 2. Replace filter screen 3. Clean die head 4. Strictly control kneading and plasticizing process
Bubbles on Film Surface	Wet raw material	Dry raw material
Holes or Tears in Film	1. Impurities in raw material 2. Blocked or broken filter screen 3. Excessive blow-up ratio 4. Excessive traction ratio 5. Blocked die 6. Foreign objects on cooling plate or rollers scratching film	1. Replace raw material 2. Clean or replace filter screen 3. Adjust blow-up ratio 4. Reduce traction ratio 5. Clean die 6. Clean herringbone plate or guide roller surfaces
Watermarks or Cloudiness on Film Surface	1. Low extruder barrel temperature 2. Low die temperature 3. Excessive screw speed 4. Filter screen holes too large or insufficient layers 5. Improper raw material selection	1. Increase barrel temperature 2. Increase die temperature 3. Reduce screw speed 4. Replace with finer filter screen or add layers 5. Select appropriate raw material
Fish Eyes or Hard Blocks on Film Surface	1. Low extruder temperature 2. Contamination with other resins 3. Low melt flow rate of raw material 4. Poor screw plasticization	1. Increase extrusion temperature 2. Replace raw material or remove contaminated resin 3. Use appropriate raw material 4. Replace qualified screw
Obvious Seam Line Marks	1. Excessive die or connector temperature 2. Material decomposition at core rod shunt	1. Lower die or connector temperature 2. Remove core rod and clean
Sticky Film (Poor Opening)	1. Excessive molding temperature 2. Insufficient cooling 3. Excessive traction speed 4. Excessive nip roller pressure 5. Improper formulation (insufficient opening agent)	1. Lower temperature, especially die temperature 2. Enhance cooling 3. Reduce traction speed 4. Adjust nip roller pressure 5. Increase opening agent dosage
Uneven Winding	1. Uneven film thickness 2. Insufficient cooling 3. Uneven herringbone frame gap 4. Traction roller misalignment 5. Air trapped in film tube, causing wrinkles	1. Adjust film thickness 2. Enhance cooling 3. Correct herringbone frame gap 4. Adjust traction roller clamping force for uniformity 5. Remove trapped air to eliminate wrinkles
Uneven Edges of Film Roll	1. Insufficient winding tension causing edge run-off 2. Dull film-cutting blade 3. Inconsistent winding tension	1. Increase winding tension appropriately 2. Replace with new blade 3. Inspect tension control system