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

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

But before you dive into researching different molders and manufacturers, you want to arm yourself with the differences between these two processes. Here's a quick guide to help you differentiate: 

What is Blow Molding?

The process of blow molding follows the basic steps found in glass blowing. The blow molding process is designed to manufacture high volume, one-piece hollow objects. If you need to make lots of bottles, this is the process for you. Blow molding creates very uniformly, thin-walled containers. And, it can do so very economically. 

Blow molding offers several distinct advantages including:

• The costs in blow molding are lower as compared to injection molding
• Machinery costs are typically lower as well

• One-piece construction so that there’s no need to connect part halves, which can achieve shapes that injection molding cannot produce 

With blow molding, a plastic tube is heated and filled with air until it essentially becomes a balloon of hot plastic called a “parison.” A mold is then clamped around this, trapping the plastic while air continues to fill the parison into the shape of your part. The size of the machine and associated costs to produce a blow molded product is based on the weight of the plastic shot used in the mold.

Blow molds have more design freedom between mold halves since each mold half forms its own wall shape. With blow molding, creating the mold is important but there are variables as well such as wall thinning, air leaks, flash, and streaks that must be monitored. For example, wall thickness variation is often an essential factor for product designers to consider. Quality control is an important part of the process. And, it’s important to look for a supplier that is qualified and trained to measure, monitor, and improves every aspect of the product.

Examples of parts and products that are created using blow molding include water bottles, shampoo and other small bottles, automotive parts, stadium seating and chairs, watering cans, coolers, or any other type of hollow parts.

10 Must-Ask Questions eBook

What you need to know before selecting a molder for your project

Download our eBook, “10 Must-Ask Questions When Choosing a Plastic Injection Molder.” Inside, we cover key questions like:

• Does the molder guarantee the molds for the life of a project?
• How many molding machines do they have and what are their capabilities?
• Is their pricing competitive, even when comparing with offshore molders?

What is Injection Molding?

Injection molding requires a great deal of upfront engineering to develop detailed tooling or molds. Crafted from stainless steel or aluminum, molds are injected with liquid polymers at high temperatures under extreme pressure. The molds are then cooled to release complete plastic parts. 

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

• Detailed, highly engineered tooling with multi-cavity mold options
• Precise, efficient processing for large volumes of small parts
• Flexibility in being able to

   change the 

  type of material

   or color 

  that is being used

• Efficient material use and low scrap rates

Plastic injection molding is ideal for high-volume orders and mass production in projects requiring thousands or even millions of the same part. Injection molds must have a high precision match between mold halves, so material flow is perfectly controlled. With injection molding, creating the mold is the key to building a perfect, precision part. When the production mold is finalized, and the machine is prepped, manufacturing begins. 

An initial run will be thoroughly examined for any part defects. If none are found, full production can begin. Depending on the part and the agreement with the molder, quality checks can take place on a regular basis. They will check for strength, color correctness, and any common defects that can occur like flash or warping. The precision of an injection mold generally makes it more expensive than a blow mold.

At Rodon, we specialize in close-tolerance injection molding of small plastic parts, and provide a total concept solution from design and tooling to production, fulfillment, and packaging. The combined effort of our designers and toolmakers results in molds that ensure quality, precision tolerances, and optimized cycle times. We have the capacity to mold parts with complex geometries and variations in wall thickness. Our maximum part size is 10” square, although our major concentration revolves around parts with smaller dimensions.

When sourcing for a molder, we often recommend ThomasNet as an excellent  platform for supplier and product searching.  With over 500,000 suppliers, you'll easily be able to find one that matches your needs and requirements.