Extrusion Moulding Machine Heads for Blown Film

Extrusion Moulding Machine Heads for Blown Film

A comprehensive guide to the design, functionality, and applications of various die head structures in the extrusion moulding process for plastic film production

In the field of plastic processing, extrusion moulding plays a pivotal role in the production of blown films. The machine head, or die, is a critical component in this process, directly influencing the quality, consistency, and characteristics of the final product. The design and configuration of the extrusion moulding machine head determine factors such as film thickness uniformity, strength properties, and production efficiency.

This detailed overview explores the various structural forms of machine heads used in blown film extrusion moulding, their respective advantages and disadvantages, and key process parameters that affect performance. Understanding these components is essential for optimizing extrusion moulding operations and achieving desired film properties.

1. Machine Head Structures in Extrusion Moulding

Blown film extrusion moulding utilizes several different machine head structures, each with unique characteristics suited to specific applications. The selection of an appropriate machine head depends on factors such as the type of plastic material, desired film properties, production volume, and specific processing requirements. The most commonly used designs include side-fed mandrel dies, center-fed crosshead dies, spiral dies, rotating dies, multi-manifold dies (lotus type), and co-extrusion composite dies.

(1) Mandrel Type Die (Side Feed)

The mandrel type die structure, as shown in Figure 3-4, is a fundamental design in extrusion moulding. In this configuration, the plastic melt is compressed through the machine neck and flows to the mandrel, where it splits into two streams. These streams flow 180° around the mandrel before recombining.

After recombination, the melt surrounds the mandrel and flows through the annular channel of the die to the die opening, where it is extruded as a thin tube. This tube is then inflated into a film using compressed air, a critical stage in the extrusion moulding process.

Key Components (Figure 3-4):

  1. Machine neck
  2. Die
  3. Adjusting screws
  4. Clamping ring
  5. Die core
  6. Die base
  7. Mandrel
  8. Die body
  9. A - Flow channel
Diagram of a mandrel type die showing the flow path of molten plastic through the extrusion moulding process

Figure 3-4: Mandrel type die used in extrusion moulding

Advantages

  • Low material residence volume within the die
  • Only one flow weld line, reducing risk of plastic degradation
  • Simple structure that is easy to disassemble and maintain
  • Particularly suitable for extrusion moulding of PVC films
  • Efficient material flow characteristics in extrusion moulding processes

Disadvantages

  • Uneven flow rates within the die can cause film thickness variations
  • Potential thickness irregularities at the flow recombination point
  • Risk of mandrel "centering deviation" (misalignment between mandrel and die)
  • Difficulties in controlling die gap in extrusion moulding operations
  • Too large a gap requires increased draw ratio and blow-up ratio, creating operational challenges
  • Too small a gap increases back pressure, reducing extrusion moulding output

The typical gap for this die type in extrusion moulding ranges from 0.1 to 1.2mm. The design of the mandrel splitter is crucial in determining final film quality. If the splitter recombination point is too sharp, it can cause a thick stripe in the film with excessively thin areas on either side.

Conversely, if the splitter has too much curvature, the weld line may be thin and a stagnation point may form, potentially causing material degradation in the extrusion moulding process. Splitter design often requires empirical adjustment with sufficient tolerance, and final optimization typically occurs through practical testing and modification in actual extrusion moulding conditions.

Compressed air plays a vital role in the extrusion moulding process after the material exits the die, expanding the tube to the desired dimensions while maintaining uniform thickness throughout the film's circumference.

(2) Crosshead Type Die (Center Feed)

Crosshead type dies come in two configurations: horizontal and right-angle designs, as shown in Figures 3-5 and 3-6. This design is another important innovation in extrusion moulding technology.

Horizontal dies are used in horizontal extrusion and horizontal blowing processes, while right-angle dies are used in horizontal extrusion with upward or downward blowing. Both designs share similar forming components but differ in their material feeding mechanisms, a key distinction in extrusion moulding systems.

Components of Horizontal Center Feed Die (Figure 3-5):

  1. Flange
  2. Machine neck
  3. Flow divider
  4. Die body
  5. Adjusting screws
  6. Core die
  7. Die
  8. Die clamping plate
Illustration of crosshead type die showing center feeding mechanism in extrusion moulding

Figure 3-5: Horizontal center feed die for extrusion moulding

Components of Right-Angle Center Feed Die (Figure 3-6):

  1. Flange
  2. Die connector
  3. Adjusting screws
  4. Die sleeve
  5. Die clamping plate
  6. Core die
  7. Connecting rod
  8. Nut
  9. Flow spreader
  10. Die
  11. Die body
Comparison of horizontal and right-angle crosshead dies used in extrusion moulding processes

Figure 3-6: Right-angle center feed die configuration for extrusion moulding

Advantages

  • More uniform melt pressure around the core die, improving film thickness consistency
  • Eliminates "centering deviation" issues common in other extrusion moulding designs
  • Provides stable flow characteristics beneficial for consistent extrusion moulding
  • Well-suited for producing films with consistent mechanical properties

Disadvantages

  • Larger internal volume results in longer material residence time in the die
  • Not suitable for processing heat-sensitive plastics in extrusion moulding
  • Core die supports create multiple weld lines in the melt
  • Weld lines can potentially affect the quality and strength of blown films
  • More complex design compared to mandrel dies, increasing maintenance requirements

The crosshead design represents a significant advancement in extrusion moulding technology, offering improved thickness control compared to simpler designs. However, the trade-off comes in the form of multiple weld lines and longer residence times, making this design less suitable for materials sensitive to heat or shear in the extrusion moulding process.

(3) Spiral Type Die

The spiral type die, illustrated in Figure 3-7, represents a sophisticated design in extrusion moulding technology. Its structural characteristic is a mandrel shaft with 3 to 8螺纹形流道s (threaded channels) that guide the material flow.

In a typical spiral core die, the melt flow pattern (shown in Figure 3-8) begins with material entering through the bottom center of the die, then splitting into two streams flowing toward the edges. These streams enter the螺纹形流道s and move upward in a spiral pattern, eventually merging before reaching the die's sizing section.

During the spiral ascent, the melt spreads through the螺纹形流道间隙s (thread gaps), gradually forming a thin film layer. As the molten plastic flows through the die, it rotates around the core die, with the depth increasing in the spiral section and between the walls in the sizing section. This unique flow pattern in extrusion moulding ensures uniform distribution of the tube parison thickness around the core die circumference.

Cutaway view of spiral type die showing internal flow channels used in extrusion moulding

Figure 3-7: Spiral type die structure for advanced extrusion moulding

Melt flow visualization in spiral die showing the spiral flow pattern during extrusion moulding

Figure 3-8: Melt flow characteristics in spiral die extrusion moulding

Key Advantages in Extrusion Moulding:

  1. No weld lines in the final film due to the continuous spiral flow pattern, significantly improving film strength and appearance compared to other extrusion moulding designs
  2. Higher die pressure results in superior film properties with better mechanical characteristics
  3. Exceptional thickness uniformity around the film circumference, a critical factor in high-quality extrusion moulding
  4. Easier installation and operation compared to complex crosshead designs
  5. Robust and durable construction that withstands the rigors of continuous extrusion moulding operations
  6. Excellent material distribution, reducing waste in extrusion moulding processes

Despite its many advantages, the spiral die design shares a common limitation with crosshead dies: the extended residence time of material within the die. This makes it unsuitable for processing heat-sensitive plastics in extrusion moulding applications, as the prolonged exposure to heat can cause material degradation and quality issues.

The spiral die represents a significant technological advancement in extrusion moulding, offering superior film quality for applications where material residence time is not a critical factor. Its ability to produce weld-line-free films makes it particularly valuable for high-performance applications in the extrusion moulding industry.

(4) Rotating Die

The rotating die, exemplified by the cross-shaped rotating head shown in Figure 3-11, is a specialized design in extrusion moulding that addresses thickness uniformity issues common in fixed dies. This innovative approach in extrusion moulding introduces controlled rotation of either the die, the mandrel, or both components during the extrusion process.

The rotation helps to average out any thickness variations that might occur due to slight imperfections in die geometry or material flow characteristics. This self-compensating feature makes rotating dies particularly valuable in high-precision extrusion moulding applications where consistent film thickness is critical.

Components of Cross-shaped Rotating Die (Figure 3-11):

  1. Commutation contact ring
  2. Thermometer
  3. Adjusting screws
  4. Die
  5. Core die
  6. Core die support
  7. Die housing
  8. Connector
  9. Bearing components
  10. Drive mechanism
  11. Gear
Cross-shaped rotating die assembly showing drive mechanism and rotating components in extrusion moulding

Figure 3-11: Cross-shaped rotating die for precision extrusion moulding

Advantages

  • Significantly improved thickness uniformity in extrusion moulding
  • Compensation for minor die imperfections and flow irregularities
  • Reduced sensitivity to material viscosity variations
  • Enhanced optical properties due to more uniform molecular orientation
  • Ideal for high-precision extrusion moulding applications

Considerations

  • More complex design requiring additional mechanical components
  • Higher initial investment compared to fixed dies
  • Increased maintenance requirements for rotating components
  • Potential for rotational speed-related quality issues
  • Requires precise control systems in extrusion moulding lines

Rotating dies represent a specialized solution in extrusion moulding for applications where the highest level of thickness uniformity is required. While they add complexity to the extrusion moulding process, their ability to produce consistently uniform films makes them invaluable for premium applications in the packaging and industrial film sectors.

2. Machine Head Process Parameters in Extrusion Moulding

In blown film production, regardless of the extrusion moulding machine head design, several critical structural parameters must be carefully considered and controlled. These parameters directly influence the quality, characteristics, and production efficiency of the extrusion moulding process, ultimately determining the performance of the final film product.

(1) Blow-up Ratio

The blow-up ratio (α) in extrusion moulding is defined as the ratio between the diameter of the inflated bubble (D₁) and the diameter of the die opening (D). This fundamental parameter in extrusion moulding establishes the relationship between the film规格s (specifications) and the die dimensions.

The blow-up ratio typically ranges from 1.5 to 3.0 in most extrusion moulding applications. For ultra-thin films, this ratio can be increased to a maximum of 5 to 6, though this requires precise control over all aspects of the extrusion moulding process.

During production, compressed air pressure must remain stable to ensure a constant blow-up ratio, a critical factor in maintaining consistent film quality in extrusion moulding operations. As the blow-up ratio increases, film thickness uniformity tends to decrease, presenting a significant challenge in extrusion moulding process control.

Excessively high blow-up ratios can cause bubble instability and film wrinkling, both serious quality issues in extrusion moulding. The selection of an appropriate blow-up ratio must consider its direct impact on film layflat width and various mechanical properties, while also taking into account the specific characteristics of the plastic material being processed.

α = D₁/D

Where:

α = Blow-up ratio

D₁ = Bubble diameter

D = Die opening diameter

(2) Draw Ratio

The draw ratio is another critical parameter in extrusion moulding, representing the ratio of the film take-up speed to the polymer melt extrusion speed at the die lip. This parameter significantly influences the molecular orientation and mechanical properties of the film produced in extrusion moulding.

In combination with the blow-up ratio, the draw ratio determines the overall orientation balance in the extrusion moulding process. A higher draw ratio typically results in increased tensile strength in the machine direction, while a higher blow-up ratio enhances properties in the transverse direction.

Balancing these ratios is essential in extrusion moulding to achieve the desired combination of mechanical properties for specific film applications. The optimal draw ratio varies depending on the polymer type, film thickness, and performance requirements, requiring careful calibration in each extrusion moulding setup.

(3) Die Gap Width

The die gap width refers to the annular opening between the die and mandrel at the exit of the extrusion moulding machine head. This dimension directly affects the initial thickness of the extruded tube before inflation, making it a critical parameter in extrusion moulding process control.

Proper die gap selection in extrusion moulding depends on the desired final film thickness, the blow-up ratio, and the draw ratio. The relationship between these parameters can be expressed as: Final thickness ≈ (Die gap × Die diameter) / (Blow-up ratio × Take-up diameter).

In extrusion moulding operations, die gap adjustments are typically made using precision screws that control the position of the die relative to the mandrel. This allows for fine-tuning of the gap to compensate for any thickness variations detected in the final film, ensuring consistent quality in extrusion moulding production.

Die gap measurement showing the precise annular opening between die components in extrusion moulding

Precise die gap control in extrusion moulding

Summary of Extrusion Moulding Machine Heads

The selection of an appropriate machine head design is crucial in extrusion moulding for blown film production, as it directly impacts film quality, production efficiency, and material suitability. Each design—from the simple mandrel type to the sophisticated spiral and rotating dies—offers unique advantages and limitations that must be carefully considered based on specific application requirements.

Understanding and optimizing key parameters such as blow-up ratio, draw ratio, and die gap width is essential for achieving consistent, high-quality results in extrusion moulding operations. As extrusion moulding technology continues to advance, machine head designs are becoming increasingly specialized, offering improved performance and greater process control for modern film production challenges.

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