Dalian hollow forming (such as extrusion blow molding, injection blow molding) is one of the core processes in plastic processing, and air pressure regulation is a key link in determining product quality. Reasonable air pressure control can achieve products with uniform wall thickness, precise shape, and smooth surface. Conversely, it is prone to defects such as uneven wall thickness, deformation, and bubbles. The following summarizes the core techniques of air pressure regulation from the dimensions of air pressure magnitude, stage control, stability assurance, and material adaptation:

1、 Accurate matching of air pressure: adapted to product and material characteristics

The pressure directly affects the blowing effect of the billet, and it needs to be dynamically adjusted according to the product size, material viscosity, and wall thickness requirements:

Product size: Large sized products (such as 200L barrels) require a higher main blowing pressure (0.6-1.2MPa) to ensure that the billet fully fits the corners of the mold; The air pressure of small products (such as 500mL bottles) can be reduced (0.3-0.8MPa) to avoid excessive inflation and local thinning.

Material viscosity: High viscosity materials (such as PP and PVC) have strong intermolecular forces and require higher air pressure (main blowing 0.7-1.2MPa) to overcome resistance; Low viscosity materials (such as LDPE) are prone to swelling, and the air pressure should be low (main blowing 0.4-0.7MPa) to prevent cracking.

Wall thickness requirements: For thick walled products (such as industrial containers), the main blowing pressure should be slightly higher, and the holding time should be extended to ensure cooling and shaping; Thin walled products (such as food packaging bottles) should have a stable air pressure to avoid local thinning caused by instantaneous impact.

2、 Staged adjustment of air pressure: optimizing the entire inflation process

The air pressure for hollow forming needs to be precisely controlled in three stages: pre blowing, main blowing, and holding pressure, with different goals for each stage:

Pre blowing stage: low pressure (0.1-0.3MPa), short time (0.1-0.5s), the core is to suppress the sagging of the billet and preliminarily expand and shape it. Excessive air pressure can easily lead to premature mold sticking of the billet (local thinning); If it is too low, the billet will sag severely (thick at the bottom and thin at the top). The timing of pre blowing should be matched with the temperature of the billet: pre blowing should be advanced when the temperature is high to prevent excessive softening and sagging of the billet.

Main blowing stage: High pressure (0.4-1.2MPa), fast and stable, to ensure that the billet fully adheres to the mold. The air pressure needs to be gradually increased (rather than instantaneous impact) to avoid the formation of bubbles or the rupture of the mold due to the entrainment of air by the airflow. The main blowing time is based on the complete adhesion of the billet to the mold (which can be determined through the transparent observation window of the mold).

Pressure holding stage: Medium and low pressure (0.2-0.5MPa), stable and continuous, ensuring product cooling and shaping. The holding time should cover the cooling of the product to below the glass transition temperature (such as PE about 80 ℃) to prevent shrinkage and deformation after demolding. Low holding pressure can easily cause product dents, while high pressure can increase mold load.

3、 Ensure air pressure stability: avoid fluctuations affecting quality

Pressure fluctuations are the main cause of unstable product quality, which needs to be strengthened from three aspects: gas source, equipment, and pipelines:

Stable gas source: equipped with high-precision stabilizing valve (accuracy ± 0.01MPa) and air filter (regularly replace filter element) to prevent impurities from blocking the pipeline; The air compressor needs regular maintenance to ensure stable output pressure (such as 0.8-1.0MPa).

Smooth pipeline: The diameter of the blowing tube needs to be adapted to the product (such as using a large barrel with a diameter of 10-15mm and a small bottle with a diameter of 5-8mm) to avoid insufficient airflow caused by a too small diameter; Pipeline joints need to be sealed (regularly checked for leaks) to prevent pressure loss.

Equipment calibration: Regularly calibrate the pressure regulating valve (such as once a month) to ensure that the parameters are consistent with the actual output; Check the sensitivity of the solenoid valve action to avoid delay and deviation in the timing of inflation.

4、 Dynamically adjust based on material characteristics

The melting characteristics of different plastics vary greatly, and targeted optimization is needed for pressure regulation:

PE: Low viscosity, easy to sag, slightly lower pre blowing pressure (0.1-0.2MPa), moderate main blowing pressure (0.5-0.8MPa), short holding time (fast cooling).

PP: High viscosity, fast crystallization, slightly higher pre blowing pressure (0.2-0.3MPa) and earlier timing (to prevent sagging), high main blowing pressure (0.6-1.0MPa) is required to ensure mold sticking.

PVC: Poor thermal stability, sufficient main blowing pressure (0.7-1.2MPa) and fast inflation speed (reducing high temperature residence time), short holding time (avoiding decomposition).

PET: When injecting and blowing bottles, the pre blowing pressure is crucial (0.15-0.3MPa), which needs to be matched with the heating temperature (high temperature leads to low pre blowing pressure), and the main blowing pressure is high (0.8-1.5MPa) to ensure uniform bottle wall.

5、 Optimize air pressure by combining product structure

Complex structural products require targeted adjustment of air pressure distribution:

Pattern/concave convex structure: The main blowing pressure needs to be increased (1.0-1.5MPa) to ensure clear details; Pre blowing needs to be more precise (such as locally strengthening airflow) to avoid uneven wall thickness at the pattern.

Bottle mouth/bottom: The bottle mouth needs to be accurately positioned during pre blowing (with the blowing needle centered), and the air pressure should be fine tuned to prevent deformation; The problem of bottom sagging can be solved by increasing the pre blowing pressure (0.2-0.3MPa) or advancing the pre blowing time.

Alien products: using segmented inflation (such as multi zone air pressure control) to reduce air pressure in thin-walled areas and increase air pressure in thick walled areas, achieving overall uniformity.

6、 Collaborative other parameters: achieving overall optimization

The air pressure needs to be coordinated with temperature, mold, and inflation time:

Match with temperature: high billet temperature → reduced air pressure (prone to inflation); Low temperature → increased air pressure (overcoming hardness).

Coordination with mold: low mold temperature → short holding time, slightly lower air pressure; High mold temperature → long holding time, stable air pressure.

Match with inflation time: high air pressure → short inflation time; Low air pressure → Long inflation time, balance to avoid defects.

7、 Air pressure adjustment strategies for common defects

Uneven wall thickness: Improper pre blowing pressure → Adjust pre blowing pressure/timing; Pressure fluctuation → Check for air source leakage or replace the pressure regulator valve.

Bubble: The main blowing impact is too large → reduce the initial air pressure or use a gradual pressure gradient; High billet temperature → reduce temperature+fine tune air pressure.

Deformation: Insufficient holding pressure → Increase holding pressure/extend holding time; Release air too quickly → release air slowly (such as in sections).

Rupture: high air pressure → reduced main blowing air pressure; Pre blowing too late → Pre blowing the timing in advance.

The air pressure regulation of hollow forming is a combination of "theory+experience": better parameters need to be determined through trial blowing (recording the pressure and time of pre blowing/main blowing/holding pressure), and the wall thickness and shape should be regularly sampled and inspected during production, and dynamically fine tuned. At the same time, strengthen equipment maintenance (such as pipeline sealing and air pressure valve calibration) to ensure stable air pressure and ultimately achieve efficient and high-quality production.