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The process of blow molding involves the formation of plastic products using thermoplastics. These products have a hollow design and are formed through the heating and inflation of plastic tubes known as “preforms” or “parison.”

The manufacturer places preforms between dies that create the final desired product shape. Air expands the tubing, thinning the walls to conform to the mold design. After the blow molding process is completed, the manufacturer cools, ejects, trims, and prepares the product for secondary manufacturing processes.

Blow Molding – A Brief History

Blow molding has close ties with the glass-blowing manufacturing process. The molten states of plastic and glass allow for easy product formation through the introduction of air to the material. The “free-blowing” of glass has been around since 1 B.C. and refined into an alternative mold-blowing manufacturing process around 1 A.D.

Natural rubber was the first alternative material to glass used in blow molding manufacturing applications in the 1850s, with Samuel Armstrong pioneering and patenting the process. In the 1930s, the Plax Corporation developed the first blow molding machine prototype. This device featured a process involving the blowing of cellulose acetate.

The English company Imperial Chemical Industries was the first to introduce low-density polyethylene (LDPE) into blow molding in 1939. LDPE’s highly effective and efficient properties saw the explosion of blow molding processes in product manufacturing, and by the 1950s, it was a commercial manufacturing method.

Blow Molding Applications

Packaging and bottling are the primary applications for blow molding in the modern manufacturing environment. These products create around 49% of the market share demand for blow molding. Other sectors where blow molding features as a valued manufacturing process include consumer products, construction, and transportation.

In the 2020s, the CAGR of the $78 billion blow molding industry will grow 2.8% annually through 2027. Some of the common raw materials used in blow molding include polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE).

The Process of Blow Molding

The blow molding process involves melting, extruding, and blowing (molding) the material before cooling it and ejecting it from the mold. The process can feature different heating and cooling cycles and the inclusion of colorants or additives in the compounding process. These additional processes depend largely on the design and application of the final product by the market.

Here is an overview of the blow molding process from start to finish.

Plastic Resin Charging or Feeding

The charging of plastic materials is the first stage of the blow molding process. The manufacturer feeds the material into a silo or hopper via vacuum pumps. The rotary feeder at the base of the silo controls the feed rate of material into the extruder or plasticizer before compressed air feeds it into the hopper for extrusion. Sometimes, the manufacturer uses vacuum processes to send the material into the hopper without requiring compressed air.

Blow molding process

Blow molding process

Melting or Plasticizing

The plastic material undergoes heating and kneading as the resin moves through the extruder. Heating bands or elements wrapping around the extruder barrel melt the polymer. The screw on the extruder features several sections that feed, compress, and meter the material as it moves through the barrel. The screw provides sufficient compression and shearing to homogenize the material before extruding it.

Preform Injection (Parison Extrusion)

This part of the process involves the preparation of the preform for inflation. It extrudes the plastic through the injection or free extrusion, creating a preform (parison) mold.

Sealing (Clamping)

This part of the process captures the preform in a split die. In extrusion blow molding processes, the die seals the ends of the preform, leaving a single hole for the injection of compressed air.

Blow Molding

The process stage involves injecting compressed air into the preform. The air inflates the material and molds it into the shape of the die to create the final product according to the design profile.

Cooling and Ejection

After forming the product, the manufacturer cools the material to stabilize its dimensions before ejecting it from the mold and starting the process again.


Blow molding usually results in flash occurring at the die seams. The machine features a deflashing tool that trims the flash as the dies come together and clamp the parison material. It’s common for the flash to appear at the bottom and top parts of the finished product at the opening, which accommodates the injection of compressed air. A rotating blade trims the flash, and the machine grinds the flash materials before sending them back to the extruder.

Leak Testing

The final stage involves quality control processes that test the finished product for leaks. This quality check is common in packaging and bottle manufacturing. The manufacturer pressurizes the container with a machine checking for leaks using a pressure monitoring system. If there is a leak, the machine rejects the product and recovers the material for future use.

Wrapping Up

Blow molding is a highly efficient manufacturing process and ideal for high-volume production processes. It’s a cost-effective manufacturing method and is common in countries around the globe.