What is Blow Molding? Let`s Start with Its History

If you`re at all familiar with glassblowing, you might already notice some similarities between it and the plastic blow molding we`ve discussed so far. That`s how the inventors of blow molding came up with the idea. In 1938, two enterprising inventors developed a mass manufacturing technique for blow molding, patented it, and sold it to Hartford Empire Company.

One bottleneck for the growth of the blow molding industry was the availability of appropriate materials. Variable density plastics came around relatively late in the technology`s emergence but changed the game forever. Today, three of the most common polymers (plastics) used in blow molding are high and low-density polyethylene(HDPE&LDPE), polypropylene(PP), and co-polyester.

When the American soft drink industry began taking off in the mid-1970s, it marked a turning point resulting in billions of blown products over the next 20 years. Since then, many billions more have been blown, and the industry shows no signs of slowing.

What Are the Main Types of Blow Molding?

Extrusion Blow Molding (EBM)

What we refer to as blow molding is called extrusion blow molding (EBM). First, plastic is melted down and formed (extruded) into what is called a caparison. This small tube has an opening for air, and once secured into a mold, and the air is forced into it. This expands the plastic into the shape of the mold, and once the melted plastic cools, the mold is opened, and the product is removed. There are variations to this technique, but this is the basic process.

Some of the advantages of EBM include faster production rates, handles can be incorporated, and complex parts can be molded. Disadvantages include low strength, spin trimming may be necessary, and can only be used with hollow parts.

Injection Stretch Blow Molding (ISBM)

Injection stretch blow molding users two methods: single-stage and double-stage. The single-stage process includes seeing the preform manufacture and bottle blowing done in the same machine. From there, the single-stage process is then run a second time through a 3-station or 4-station machine. The latter is more expensive than the former process. Essentially, molecules are stretched during this type of blow molding vertically, then horizontally, to cross over one another to fit together. This is one of the most robust methods of blow molding.

With injection stretch blow molding using double-stage, plastic is molded into a preform using the standard injection mold process. Eventually, the preform is later fed into a reheat stretch blowing machine. Preforms are heated above their temperature threshold, then blown using pressure air via metal molds.

Some advantages of ISBM include better for short runs and even wall thickness when blowing non-rectangular shapes. The one disadvantage is severe restrictions on design.

Injection Blow Molding (IBM)

Injection blow molding is used in creating plastic objects and hollow glass in large amounts. During the process, a polymer is first injection molded using a core pin. The core pin rotates at a blow molding station through the inflation and cooling processes. There are three steps to the IBM process: injection, blowing, and ejection. This is the least used blow bolding process in the world.

One advantage of IBM include produces an injection molded neck which improves accuracy. Disadvantages include handles can`t be used and no increase in strength.

What Are the Different Materials That Can Be Used for Blow Molding?

Blow molds are often made of beryllium-copper alloys due to their excellent heat conductivity and resistance to wear and tear.

Some of the most used plastics in blow molding include:

ABS

EVA

COC

COP

Co-polyester

High-density polyethylene

Low-density polyethylene

Nylon

PET

Polypropylene

Polystyrene

PVC

TPE

What Are the Different Steps in the Blow Molding Process?

For the most part, no matter the blow molding process, these are the most common steps:

Plastic is fed through a hopper or screw depending on the type of blow molding machine being used.

From there, the plastic is superheated and melts. It`s fed through a parison, a tube with a hole at the end.

It`s then clamped in place.

Compressed air is then used to inflate the parison.

Heated plastic balloons then fill the mold space.

Once the plastic cools, the mold is opened, removing the part and sending it on to the next step in the overall manufacturing process based on what the final product is.

These steps may vary from machine to machine or process to process. For the most part, blow molding is automated, and the workflow is the same across the board. For specialized molds, it may vary with additional steps along the way to create the final product.