machine to make plastic molds

What is Blow Molding?

Photo of three full plastic water bottles in front of white background created by the blow molding process

Blow molding is the process used for manufacturing hollow, typically plastic parts. Some examples of blow molded products are small custom plastic bottles and containers for consumer products like water, shampoo, or milk.

Blow molding is also applied to more extensive industrial jobs, like storage tanks or big plastic drums. The process focuses on creating a lot of single-piece, thin-walled containers for cheap.

Let’s dive deeper into the history, process, and materials used by Blow Molded Products.

History of Blow Molding

If you’re familiar with glassblowing, you might notice similarities between it and plastic blow molding. That’s how the inventors of blow molding came up with the idea. In 1938, two 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 industry was the availability of materials. Variable density plastics came around relatively late in the technology’s emergence but changed the game forever. 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 took 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 Most Common Types of Blow Molding?

The three types of blow molding processes are extrusion blow molding, injection blow molding, and injection stretch blow molding. Let’s take a further in-depth look at all three.

Extrusion Blow Molding (EBM)

What we refer to as blow molding is called extrusion blow molding (EBM). Here’s the basic process:

  • Plastic is melted down and formed (extruded) into what is called a parison.
  • 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 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 molding process in the world.

One advantage of IBM includes producing an injection molded neck which improves accuracy. Disadvantages include: handles can’t be used and no increase in strength.

What’s the difference between extrusion blow molding and injection blow molding?

These two forms of blow molding are quite different. The main difference between extrusion and injection blow molding is that in one, plastic is melted down and extruded and in the other, the plastic is injected. Other key differences include:

  • Injection molding is far less popular than extrusion molding
  • Extrusion blow molding is faster than injection blow molding
  • IBM can be more accurate when compared to EBM

Read More: Injection Molding vs. Blow Molding

Injection Stretch Blow Molding (ISBM)

Injection stretch blow molding uses 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. 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.

What Are the Different Materials 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:

High-density polyethylenePVC
Low-density polyethyleneTPE

How Does the Blow Molding Process Work?

The blow molding process is similar across all three types of blow molding. The following steps apply to the injection blow molding process, the extrusion blow molding process, and the injection stretch blow molding process.

  1. Plastic is fed through a hopper or screw depending on the type of blow molding machine being used.
  2. From there, the plastic is superheated and melts. It’s fed through a parison, a tube with a hole at the end.
  3. It’s then clamped in place.
  4. Compressed air is then used to inflate the parison.
  5. Heated plastic balloons then fill the mold space.
  6. 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.

Choose Us as Your Blow Molding Partner

Looking for a blow molding partner who can help you take your product to the next level? Contact us today and see how Blow Molded Products can make all the difference for your next project.