Injection Moulding vs. Blow Moulding

CustomPAK • March 7, 2018

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Are you building a product that requires moulded plastic parts? Injection moulding and blow moulding are the two most popular ways to produce high-quality, cost-effective plastic parts and products, but both are used for different applications and have different requirements. Do you know the difference?

Save time researching suppliers with this quick guide on injection moulding and blow moulding.

Different Products

Perhaps the most noticeable difference between parts made by injection moulding and parts made by blow moulding is that injection moulding creates solid parts, while blow moulding creates hollow parts. If you’re producing something that needs one rigid wall, injection moulding may be what you’re looking for. Some common examples of parts created with injection moulding 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 moulding is more likely what you need. The most common example of a blow moulded product is a bottle. Billions of bottles are created at very low prices using blow moulding. But blow moulding can also be used to make a variety of more industrial part shapes like coolers, fuel tanks, and stadium seats. See an extensive list of blow moulded products here.

Different Processes

With injection moulding, melted resin is injected into a hollow mould until it is completely filled. The injection mould 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 mould together.

With blow moulding, a plastic tube is heated and filled with air until it essentially becomes a balloon of hot plastic called a “parison”. A mould 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 moulded product is based on the weight of the plastic “shot” used in the mould.

Unit costs for injection moulded and blow moulded parts are comparable for similar component dimensions & weights.See this blow moulding design guide for more information.

Different Moulds

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

Blow moulds have more design freedom between mould halves since each mould half forms it’s own wall shape. With blow moulding, the mould 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.