Blow Molding vs. Injection Molding

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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.

injection molding

Different Products

Perhaps the most noticeable difference between parts made by injection molding and parts made by blow molding is that injection molding creates solid parts, while blow molding creates hollow parts. If you’re producing something that needs one rigid wall, injection molding may be what you’re looking for. Some common examples of parts created with injection molding are bottle caps, hair combs, and housings for computers and televisions.

If you need a piece that can be either flexible, structural, or can hold a fluid, blow molding is more likely what you need. The most common example of a blow molded product is a bottle. Billions of bottles are created at very low prices using blow molding. But blow molding can also be used to make a variety of more industrial part shapes like coolers, fuel tanks, and stadium seats.

Different Processes

With injection molding, melted resin is injected into a hollow mold until it is completely filled. The injection mold is held together under intense pressure, sufficient enough for every part of the interior to be filled with high pressure molten plastic resin. The size of the machine and cost of producing a product is based on the amount of “tonnage”pressure required to hold the mold together.

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.

Unit costs for injection molded and blow molded parts are comparable for similar component dimensions & weights.

injection molding machine

Different Molds

Injection molds must have a high precision match between mold halves so material flow is perfectly controlled. With injection molding, creating the mold is 90% of the battle. Once you have a functional, error-free mold, the rest of the process should be fairly routine. The precision of an injection mold generally makes it more expensive than a blow mold.

Blow molds have more design freedom between mold halves since each mold half forms it’s own wall shape. With blow molding, the mold is only 50% of the battle. 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 both processes. It’s important to use a supplier that has an expert quality team that is trained to measure, monitor, and improve every aspect of the product.

Do you need injection molding machine?

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