The blow molding process has been around for thousands of years and remains essentially unchanged. The main difference with the modern process is that it’s heavily automated. The general process for extrusion blow molding is as follows:

1. The plastic material is shaped into a parison, a tube with a hole only on one end.
2. The parison is then clamped to the inside of the mold.
3. Air is blown into the heated parison.
4. The plastic expands like a balloon and covers the inside surface of the mold.
5. The plastic cools, the mold is opened, and the part is ejected.

In the case of injection and injection stretch blow molding, the parison is replaced with a preform, which is produced through injection molding. The associated tools are often made from aluminum or steel and are great for thousands of parts.

Naturally, not all plastic products are suited for blow molding. Here’s where the process shines and fails.

Pros

• Pressures are much lower than with injection molding, so the tooling is generally cheaper to manufacture.
• It’s great for parts with external threads.
• It produces parts that would be difficult to make via injection molding.

Cons

• Part geometry is limited and restricted to simple forms.
• Parts are generally more expensive than with injection molding.

Blow molding is used to manufacture many different types of products, such as ducting for automotive, plastic bottles, food containers, and even things as large as garbage bins.

So how do you evaluate if your part is suitable for this process? If it’s hollow, has thin walls, or is an enclosed shape with undercuts — with which injection molding would struggle — blow molding might be the answer.

If you have something you think might be a good fit for blow molding, try one of these companies:

• Gemini Prototyping in the UK. They specialize in everything from “micro” up to large (10 liters) and offer both extrusion blow molding and co-extrusion, which allows you to combine different materials
• Maxtech in China is a blow molding specialist. The Chinese market is known for its high volume production, and this company is able to mold in a number of different materials. They also offer a number of complementary services.
• Shape Point is a US-based company with Chinese manufacturing. They manufacture blow-molded parts for a number of industries and applications.