Blow Molding vs Injection Molding: What's the Difference?

When creating a product that requires molded plastic parts, you have a variety of processes to choose from, depending on the type of application and part type you want to produce. Injection molding and blow molding are the two most popular methods of producing high-quality, cost-effective plastic parts.

But before you delve into the different molders and manufacturers, you need to understand the differences between the two processes. Here's a quick guide to help you differentiate:

What is blow molding?

The blow molding process follows the basic steps in glass blowing. The blow molding process is designed to manufacture high-volume one-piece hollow objects. If you need to make a lot of bottles, this is the process for you. Blow molding produces very uniform thin-walled containers. Also, it can do it very economically.

Blow molding offers several distinct advantages, including:

Lower cost of blow molding compared to injection molding

Mechanical costs are also generally lower

One-piece construction eliminates the need to connect halves, enabling shapes that cannot be produced by injection molding

With blow molding, the plastic tube is heated and filled with air until it essentially becomes a hot plastic balloon called a "parison." The mold is then clamped around it, trapping the plastic while the air continues to fill the parison into the shape of the part. The size of the machine and associated costs to produce blow molded products depend on the weight of the plastic pellets used in the mold.

Blow molds have greater design freedom between the mold halves, as each mold half forms its own wall shape. For blow molding, creating the mold is important, but there are variables such as wall thinning, air leaks, flash and streaks that must be monitored. For example, wall thickness variation is often an important factor for product designers to consider. Quality control is an important part of the process. Also, it is important to find qualified and trained suppliers to measure, monitor and improve every aspect of the product.

Examples of parts and products manufactured using blow molding include water bottles, shampoo and other small bottles, auto parts, stadium seats and chairs, watering cans, coolers or any other type of hollow part.

What is injection molding?

Injection molding requires extensive up-front engineering to develop detailed tooling or molds. The molds are made of stainless steel or aluminum and are injected with liquid polymer at high temperature and extreme pressure. The mold is then cooled to release the intact plastic part.

Plastic injection molding has several distinct advantages of its own, including:

Detailed, highly engineered tooling with multi-cavity mold options

Precise and efficient handling of large numbers of small parts

The flexibility to change the type of material or color being used

Efficient material usage and low scrap rates

Plastic injection molding is ideal for high volume orders and mass production in projects that require thousands or even millions of identical parts. There must be a high-precision match between the mold halves of an injection mold so that the material flow is perfectly controlled. For injection molding, making the mold is the key to creating the perfect, precision part. When the production mold is complete and the machine is ready, manufacturing begins.

The initial run will thoroughly check for any part defects. If none are found, full production can begin. Depending on the part and agreement with the molder, periodic quality checks can be performed. They will check for strength, color correctness, and any common defects that may appear, such as glitter or warping. The precision of injection molds generally makes them more expensive than blow molds.

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