How 3 Cavity Pet Blowing Machines Improve Bottle Manufacturing

PET bottle manufacturing is a big business. A 3 Cavity PET Blowing Machine is one possible “sweet spot” of performance and capacity, offering more bottles per cycle (compared to 1- or 2-cavity machines) but lower complexity (and therefore cost) than machines with many more cavities (6, 9, etc).

In this post, we will cover how 3 cavity machines work, their benefits, what to consider when using or buying, as well as some real-world machine specifications.

What is a 3 Cavity PET Blowing Machine?

A 3 Cavity PET Blowing Machine is a stretch blow moulding or stretch blowing system that can produce three bottles at a time per blowing cycle. It works by loading PET preforms into the machine, heating them, and then placing them in moulds where they are stretched and blown by compressed air into the mould cavities.

The “cavity” terminology refers to the number of identical bottle moulds present in the blowing section of the machine. So, a 3-cavity machine has three mould forms/moulds. That means it can simultaneously blow and produce three bottles at a time.

Core Working Principle

The following is a the typical operational flow in a 3-cavity blowing machine (fully or semi-automatic version):

Preform Loading and Preheating

PET preforms (small, thick plastic tubes with a neck finish applied) are dropped or fed into a preheating chamber or oven. Some machines have a rotary preheater or infrared lamps for uniform preform heating.

The preheating stage is critical to bring the PET preform to its glass transition/appropriate temperature, such that it can stretch and blow without defects. For multi-cavity machines, uniform heating across all three cavities is especially important to ensure all preforms are the same.

Mould Clamping & Stretching

The preforms are then moved into moulds (three cavities) where they are tightly clamped in place. The stretching rod (if present in the machine) is extended into the preform to stretch it longitudinally (for biaxial orientation), which improves the mechanical strength and toughness of the bottle.

The stretching step can improve drop resistance, clarity, top load, and overall performance.

Blowing

The moulds are now closed and locked. High-pressure air is injected into the centre of each preform to inflate it such that it expands and takes the shape of the mould cavity.

This blowing stage needs to happen under controlled air pressure, with stable low and high pressure lines, to manage the blow pressure and mould safety. For example, the Eceng 3-cavity machine below lists its blow pressures in certain bars/kg/cm2 and gas consumption.

Cooling

After the blowing stage, the bottles are cooled (either via air blow or via cooling channels in the mould that use water cooling) to solidify the PET, preserve the bottle shape, and avoid any deformations.

Cooling helps ensure bottle stability, particularly in the neck and mouth areas. If cooling is too low or uneven, shrinkage, warping, or weak necks can occur.

Mould Opening and Bottle Ejection

After cooling, the PET reaches a temperature where moulds can open and bottles can be ejected. Fully automatic machines have automatic ejection and convey to downstream operations (filling, labeling, etc) included, while semi-automatic versions may require some manual intervention for ejection/removal.

Repeat Cycle

The same cycle happens for each blowing cycle. Because three cavities exist, each cycle produces three bottles, so the machine’s hourly output is directly related to cycle time (which includes cooling time) and how quickly preforms are fed in and bottles removed.

Key Advantages of Using a 3 Cavity Pet Bottle Blowing Machine

Some of the main improvements and benefits that a 3-cavity PET bottle blowing machine can bring to your bottle manufacturing processes include:

• Higher Throughput — Producing three bottles per blow cycle increases output and the number of bottles produced per unit time, compared to single or two-cavity machines (assuming cycle times are comparable).
• Better Cost per Bottle — Typically, more bottles per cycle means lower per-bottle cost of power, labour, cooling, mould maintenance, and overheads. Materials are also better utilized. The fixed costs (mould, clamping, setup, etc) are spread over a higher number of units.
• Balanced Complexity — With three cavities, machines are more complex than single-cavity machines but less complex than large multi-cavity lines. Maintenance, mould handling, set-ups, air and cooling requirements are more manageable than with 6, 9, or many cavities. For many manufacturers, this is an easy scale-up step with a good tradeoff between added complexity vs additional capacity.
• Flexibility & Bottle Design Consistency — Because all three bottles in each cycle come from matched mould cavities (assuming uniform heating and air supply), it is easier to maintain consistency in bottle dimensions, weight, wall thickness, etc. This helps with brand image, fit of caps, fillers, labels, and proper function.
• Improved Efficiency of Resources — Better energy efficiency is achieved, especially if the machine has good preheaters, efficient heating lamps, air recovery systems, and well-designed cooling circuits. Also, by blowing three bottles per cycle, better use of air compressors and electricity per bottle produced.
• Scalability without a Huge Jump in Investment — Businesses growing from using a single or dual cavity machine, moving to a 3-cavity machine, offer a good intermediate step before investing in much larger, more expensive machines.

Important Technical & Operational Considerations

To actually enjoy these benefits and maximize results, certain technical and operational factors must be managed well:

Uniform Preform Heating

Because there are three cavities, any uneven heating between cavities can result in non-uniform quality of bottles produced; over-heated or under-heated preforms lead to weak blow, warping, or other inconsistencies. Quality of heating (oven, lamps) and performance placement precision are important.

Stable Air Pressure & Air Flow

Both the low and high pressure lines need to supply stable, clean, dry air with no fluctuations or moisture. Air filters, pressure regulators, and valves need to be of good quality. Air consumption increases as the number of cavities increases—3 active blow events require significant air volume and pressure.

Cooling Efficiency

Cooling channels in moulds must be well designed and effective. Water flow and temperature need to be tightly controlled. Cooling requirements are more pronounced for larger bottles; for smaller bottles, you may be able to get away with fewer cooling requirements—but consistent cooling still matters.

Cycle Time Optimization

The overall cycle time is the sum of preheating, clamping, stretching/blowing, cooling, ejection, and mould open/close times. Minimizing non-productive time between each step is essential. Any delays/lag in pre-feeding or bottle removal will hurt the benefit of having three cavities.

Mould Design & Maintenance

Moulds must be engineered for durability, heat resistance, and good cooling channels. Close matching of mould cavity dimensions helps avoid imbalance. Also, mould maintenance (cleaning, checking for wear, lubrication, etc) is more important as the cost of mismatch/uneven wear across cavities shows up in bottle defects.

Automation & Controls

PLC / HMI interface for recipe storage, process monitoring, and alarms. Sensors for bottle blow temperature, mould temperature, air pressure, no Preform/no blow, safety interlocks, etc. Good control systems can reduce manual interventions, human errors, and increase safety.

Conclusion

A 3 cavity PET blowing machine offers a balanced upgrade for the manufacturers. It also enhances the output while simultaneously maintaining the higher quality of the product. These machines get higher throughput which are well suited for medium scale bottle manufacturing.

At Dunamis we offer 3 cavity PET blowing machine with the required features. Call our executives now to learn more.