How Does Temperature Control Affect PP Blown Film Properties?

Polypropylene is a semi-crystalline thermoplastic with low density, excellent mechanical properties, good chemical resistance, heat resistance and good insulation. It has been widely used in packaging industry. In various applications, blown films stands out as an important form whose performance is directly affected by temperature control. The influence of temperature control on the properties of polypropylene blowing film is systematically discussed in this paper from four aspects: extrusion temperature, mold temperature, cooling temperature and the synergistic effect of blow molding ratio and temperature.

Extrusion temperature is the main parameter in polypropylene blowing film production, which directly influences melt the flowability and plasticization quality. PP has a melting point range of between 155 and 165 degrees Celsius and a decomposition temperature of more than 300 degrees Celsius, so the extrusion temperature are typically between 180 and 240 degrees Celsius. This range must be precisely tailored to the characteristics of the raw material (e.g. melt flow and molecular weight distribution) and process requirements.
1.1 Low Extrusion Temperature (180–200°C)
the extrusion temperature is too low, the flowability of polypropylene melt is reduced, which leads to surface defects such as ``fish eyes' 'and weld lines. These defects arise from incomplete melting of resin, leading to localized high crystallinity and the formation of opaque particle structures. In addition, low temperature can increase the shearing effect of screws, which may lead to molecular chain breakage and decrease the tensile strength and fracture elongation of the films.
1.2 High Extrusion Temperature (220–240°C)
High temperatures make the melt flow better. But too much heat speeds up the breakdown of polypropylene. This causes problems like gel particles and ripples. Temperature changes make the melt viscosity uneven. This also makes the film's lengthwise thickness change. For example, if the temperature change in the drum is more than ±5°C, the film thickness error may go over 10%. This hurts how well the film works in later steps.
1.3 Temperature Uniformity
The temperature of all sections of the extruder must be uniform to avoid local overheating or overcooling. Temperature fluctuations will cause the melt viscosity to be inconsistent and the longitudinal thickness of the film to change. For example, if the temperature fluctuation in the drum is greater than ±5°C, the film thickness deviation may exceed 10%, seriously affecting subsequent processing performance.

Mold temperature is the key to controlling the initial shape of polypropylene melt as it exits the ring mold and forms stable bubbles. Usually set between 220 and 230 degrees Celsius, the mold temperature must be coordinated with extrusion temperature to ensure uniform melt flow.
2.1 Low temperature (<220°C)
Low temperatures reduce melt flowability, leading to defects such as water ripples on the surface of bubbles. When the melt does not relax completely at the outlet of the mold, a water wave is generated, forming a periodic wave. In addition, low temperatures increase the the brittleness of bubbles, making them prone to rupture during traction.
2.2 High Die Temperature (>230°C)
While high temperatures enhance melt flowability, they can also disrupt bubble stability. Mould temperature is too high, will reduce melt viscosity, causing bubbles in the expansion process, resulting in uneven thickness of the film. High temperatures may also accelerate the oxidative degradation of polypropylene, producing odors and discoloration.
2.3 Die Gap and Temperature Uniformity
The ring die clearance shall be controlled within 0.8 mm to 1.2 mm and the edge of the die shall be evenly spaced. The uneven gap will cause the melt flow difference in the extrusion process, which will lead to film thickness deviations. At the same time, automatic temperature measurement and control system based on thermocouple must be used to control mold temperature accurately in order to avoid local overheating or overcooling.

Cooling temperature is the core parameter that determines the crystallinity and transparency of polypropylene blowing film. Polypropylene is a semi-crystalline polymer PP's crystallinity directly influences its physical properties (e.g. tensile strength and fracture elongation) and optical properties (e.g. transparency and glossiness).
3.1 Water Cooling (15-25°C)
Water cooling is the circulation of cooling water around bubbles, which can significantly reduce the crystallinity of the film and improve transparency. When the cooling water temperature is controlled between 15 and 22°C, the film can be more than 90% transparent, with a smooth, blemish-free surface. If the water temperature is too high (>30°C) and the film is not cooled enough, crystallization will increase and transparency will be rapidly reduced. Conversely, if the water temperature is too low (<15°C), the film becomes viscous and exhibits poor opening properties, although transparency is further enhanced.
3.2 Restrictions on Air Cooling
Air cooling is the use of air rings to cool bubbles. The equipment is simple and requires less space, but it is difficult to precisely control cooling speed. Polypropylene, with its high crystallinity and low melt strength, is prone to bubble wobble and uneven thickness when using air for cooling, and the film transparency is generally lower than water cooling. Therefore, the application of air cooling in the production of polypropylene blower is less, and the application in the production of polypropylene blower is more prevalent.
3.3 Cooling Water Flow Rate and Uniformity
Cooling water flow must be coordinated with water temperature. Increasing the flow rate at 15–25°C can enhance cooling, but too much can affect bubbles and cause wrinkles. Insufficient flow rate results lead to inadequate cooling and reduced film transparency. In addition, cooling water rings must be kept horizontal to avoid uneven water flow and subsequent film thickness deviations.

Blowout ratio, that is, the ratio of bubble diameter to mold diameter, directly affects the transverse tensile strength and thickness uniformity of the film. Temperature control must be optimized in concert with the explosion ratio to achieve the optimal balance of the mechanical properties of the films.
4.1 Blow-Up Ratio Limitations at Low Temperatures
When extrusion temperature or mold temperature is low, the PP melt is less fluid and necessitates a smaller burst rate (usually less than2.0) to prevent bubbles from bursting. Under these conditions, lower transverse tensile strength of the film is lower, but higher longitudinal tensile strength due to the influence of traction.
4.2 Optimization of blasting ratio at high temperature
High temperature enhances the fluidity of the melt, expanding the explosion ratio to 2.0-2.5. This greatly improves the film's transverse tensile strength and thickness uniformity film. However, a high bloating ratio (>2.5) can cause bubbles to wobble and even burst.
4.3 Dynamic Adjustment of Temperature and Explosion Ratio
In practice, temperature and blow-up ratios must be dynamically adjusted according to the characteristics of raw material (e.g. melt flow rate) and product requirements (e.g. film thickness and tensile strength). For example, the production of high-transparency films may require lower extrusion temperatures (200–220°C) and a smaller blow-up ratio (1.8-2.0), while high-strength films require higher extrusion temperatures (220–240°C) and a higher burst ratio (2.0-2.5 ° C).

In addition to transparency and mechanical properties, temperature control plays an important role in the heat shrinkage, chemical resistance and processing stability of polypropylene blowing films.
5.1 Heat Shrinkage
The heat shrinkage of polypropylene films is closely related to crystallinity. Lower cooling temperature, the lower the crystallinity of the film and higher heat shrinkage. For example, films cooled at 15°C showed a longitudinal heat shrinkage of 1.5% and a a transverse heat shrinkage of 1.8%, while films cooled at 25°C show a heat shrinkage less than 1.0%.
5.2 Chemical Resistance
High extrusion and cooling temperature can improve chemical resistance of the film. Higher temperatures encourage the molecular chains in polypropylene to align more closely and higher crystallinity, increasing their resistance to acids, alkalis and salts. For example, films produced at 220-240°C may lose less than 5% of their tensile strength after 24 hours of soaking with a 5% hydrochloric acid solution, while films produced at 180-200°C may lose more than 10% of their tensile strength.
5.3 Process Stability
Temperature control also affects the stability of film processing. Large fluctuations in extrusion temperature or die temperature can lead to defects such as uneven thickness and wrinkles, complicating subsequent processes such as printing and laminating. Therefore, modern polypropylene blower often adopts computerized, automatic control system and thermocouple-based temperature measurement control to ensure the stability of production.
6. Conclusion:
Temperature control is a key step in the production of PP blown film production, which directly affects the transparency, mechanical properties, thermal shrinkage and processing stability of the film. By optimizing extrusion temperature, mold temperature, cooling temperature and blow molding ratio, the properties of the films can be precisely regulated. For example, high-performance PP blow molding films with transparency ≥ transparency ≥ 90% strength ≥ 45 MPa, thermal shrinkage ≤ 1.5 ≤ 1.5% be produced using an extrusion temperature temperatures of 200-220°C, mold temperatures of 220-230°C, cooling water temperatures of 15-22°C, and blow molding ratios of 2.0-2.5 ° C. With the development of materials science and processing technology, temperature control will become more and more intelligent and precise, providing strong support for polypropylene blowing film to be widely used in food packaging, pharmaceutical packaging, agricultural film and other fields.