What is blow molding

plastic milk bottle

(resource from PKGfamily)

Origin from century 19th, blowing process was firstly used for glass bottle manufacturing. Nowadays, blowing mold became the most important forming method for hollow plastic parts, and new technologies appear to improve its efficiency and application.

In general, there are three main types of blow molding:

Extrusion blow molding( EBM)

Injection blow molding (IBM)

Injection stretch blow molding (ISBM)

The most distinct way to identify EBM or IBM&ISBM bottles.

ebm bottle bottom

Instead, IBM and ISBM bottles have injection gate point.

ibm bottle bottom

Usually a EBM formed bottle has pinch off line on bottom of bottles.

Applicable material for blowing mold.

The most basic material for blowing molding is HDPE( high density polyethylene), other polyolefin also can be used for blow molding manufacturing. For different purposes, polypropylene, styrene polymer, PVC(polyvinyl chloride), PET (polyester), PU(polyurethane), polycarbonate and other thermoplastic can also be used for blow molding.

The process of blowing mold

No matter it’s EBM, IBM or ISBM, the basic principle is close. The blow molding process begins with melting down the plastic and forming it into a parison or, in the case of injection and injection stretch blow molding (ISB), a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.

The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected. Water channels are carved into the mold to assist in cooling.(from wikipedia)

This article will elaborate further on EBM only.

Common types of EBM equipment.

Continuous and Intermittent EBM are two most common variation of EBM. While shuttle type and rotary wheel type machine are more common seen types of machine belong to continuous EBM. Accumulator head machinery is the common type of intermittent EBM.

1. Shuttle blow molding machine:

shuttle machines are most used by factories in European, Latin American, Asian areas. They can be either single-sided or dual-sided machines(equipped with two sets of molding). The bottle parison extruded by flowhead constantly in middle of equipment, then mold move to the position where parison is, mold close and move back where blow pin is ready to blow air into mold cavity and start forming semi-product designed. Shuttle machines usually are used to produce calibrated neck bottles(smooth inner wall of bottle neck).

typical shuttle EBM machine

2. Rotary wheel blow molding systems

Rotary wheel machines are more popular in North American factories. They are used for  high-output production of a wide variety of plastic extrusion blow molded articles. Containers may be produced from small, single serve bottles to large containers up to 20-30 liters in volume – but wheel machines are often sized for the volume and dimensional demands of a specific container, and are typically dedicated to a narrow range of bottle sizes once built.

Rotary wheels, which may contain from six to thirty molds, feature continuously extruded parisons. Revolving sets of blow molds capture the parison or parisons as they pass over the extrusion head. The revolving sets of molds are located on clamp “stations”.

Most Rotary wheel machines is unable to produce calibrated neck finish bottles.

wheel ebm machine

Difference between calibrated and uncalibrated neck finish bottles.

  1. Calibrated neck finish bottles have smooth inner wall, while uncalibrated don’t.
  2. When bottle is designed to interference fit others, a calibrated neck finish is preferred. While bottle neck need to be fit with a surface, like heat-seal aluminum foil, a uncalibrated become better choice.