The compressed air pressure requirements for the PET bottle blowing process are divided into two levels: low pressure and high pressure. These must not be interchanged:

Low-pressure stage (5–10 bar)

Used for: preform conveyance, cooling of the heating oven conveyor chain, control of pneumatic valves on the blow molding machine, and pre-blowing (to initially expand the preform and conform it to the mold contour)

Typical value: 8 bar

High-pressure stage (30–40 bar)

Used for: The final stretch-blow molding stage, where high-pressure air is injected instantaneously to inflate the preform and make it conform tightly to the mold walls

Typical values: 30–35 bar (standard water bottles), 35–40 bar (carbonated beverage bottles, large-capacity bottles)

Special bottle types (5L+ water jugs, heat-resistant bottles): May require 40–45 bar

Important Note:

Do not attempt to use a single high-pressure compressor to supply both low- and high-pressure requirements, as this results in extremely low energy efficiency and shortens equipment lifespan

Correct configuration: Low-pressure screw compressor + high-pressure booster (or high-pressure piston compressor) combination

Insufficient pressure can result in under-inflated bottles, uneven wall thickness, and incomplete bottom formation; excessive pressure may cause preforms to burst or damage the mold.

Sollant Recommendation: When selecting equipment, allow for a 10%-15% pressure margin to compensate for pressure losses caused by pipeline pressure drops and filter clogging.

The PET bottle blowing industry imposes strict requirements on the oil content of compressed air, primarily for the following reasons:

1. Bottle Surface Quality

Oily air can cause oil stains, whitening, and cloudiness on the bottle walls, severely affecting transparency.

Mineral water bottles may appear “frosted,” and those failing appearance inspections are scrapped immediately.

2. Bottle Mechanical Strength

Oil molecules disrupt the arrangement of PET molecular chains during the high-temperature blowing process.

This reduces bottle strength by 15%–25%; carbonated beverage bottles may deform or rupture due to inability to withstand filling pressure.

3. Food Safety Risks

Oil contamination may migrate to the inner bottle walls, contaminating the contents (water, beverages, edible oils).

This violates food packaging safety standards (e.g., GB 4806, FDA, EU 10/2011) and may lead to product recalls or damage to brand reputation

4. Impact on Equipment

Oil residues adhere to the air valve assemblies and pilot valves of blow molding machines, causing slow or stuck valve operation

Long-term accumulation can clog precision filters, increasing pressure drop and energy consumption

Moisture in compressed air has a severe impact on the PET bottle blowing process—an impact that goes far beyond the simple issue of “rusting”:

1. Crystalline Haze on the Bottle Body 

During the high-temperature, high-pressure blowing process, moisture accelerates the premature crystallization of PET molecules.

White, hazy crystalline patches appear on the bottle walls (commonly referred to as “hazy bottles” or “whitened bottles”).

This defect cannot be eliminated through post-processing, rendering the affected bottles a total loss.

2. Reduced Bottle Strength

Uneven crystallinity leads to a 10–20% reduction in the mechanical strength of the bottle body.

Carbonated beverage bottles may fail to withstand the filling pressure of 4–6 times atmospheric pressure.

Hot-fill bottles (designed for 85–92°C) become more susceptible to deformation under high-temperature conditions.

3. Water Accumulation Inside the Bottle

Moisture enters the preform interior along with the compressed air and subsequently condenses at the bottom of the bottle after blowing.

Post-filling: Condensed water mixes with the liquid product (water or beverage) → Dilution or off-flavors → Customer complaints.

In severe cases: Microbial growth occurs at the bottle base → Food safety incidents.

4. Equipment Damage

Moisture corrodes high-pressure pipelines, valves, and bottle blowing molds.

During winter in northern regions, water accumulating in pipelines may freeze and cause blockages, leading to a complete shutdown of the production line.

Internal corrosion of pneumatic components (such as pilot valves and cylinders) → Operational failure.

5. Increased Risk in the High-Pressure Section

High-pressure air (30–40 bar) has a significantly lower “moisture-holding capacity” than low-pressure air.

For compressed air with the same absolute moisture content, the higher the pressure, the more easily liquid water will precipitate out.

To ensure the long-term, stable operation of air compressors, regular maintenance is essential. Common maintenance tasks include:

• Replacing the air filter: The air filter removes dust and impurities from the air. Replacing it regularly helps maintain air quality and extends the compressor’s service life.
• Checking the lubrication system: Lubricating oil effectively reduces friction inside the compressor. Regularly checking the oil level and replacing the oil promptly ensures smooth operation of the equipment.
• Cleaning the Cooling System: Keeping the cooling system clean ensures the equipment operates efficiently even in high-temperature environments and prevents damage caused by overheating.
• Inspecting the Exhaust System: Regularly inspect the exhaust system’s pipes, seals, and other components to ensure there are no leaks.
• Smart Diagnostics: Utilize a smart control system to monitor the equipment online, diagnose its operating status in real time, and make timely adjustments based on data feedback.