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Difference Between Injection Molding and Blow Molding

Difference-Between- Injection-Molding-and-Blow-MoldingMost people in the plastics manufacturing industry are familiar with both blow injection molding and blow molding these days. However there is still some confusion by product designers and engineers new to the industry about the difference between injection molding and blow molding . The blow molding process has become a standard for specific applications and understanding the differences between these two plastic molding processes can save design, prototyping and production time. This of course results in money savings and increases the speed of completion and delivery of your plastic part.

There are several significant differences between injection molding and blow molding:

  • manufacturing process itself
  • tolerances and margins of error
  • design considerations
  • role of the process for intended result

Main difference between injection molding and blow molding:

  • Injection molding – produces a part that is solid such as a reusable coffee cup lid
  • Blow molding – produces hollow parts such as water bottles

The blow molding process can produce plastic parts with very complex shapes. However to achieve this the wall thickness may vary from place to place depending on how much the material has to stretch when it is being blown. The thickness of a plastic part created through injection molding is determined by the relationship of the mold and the core.

You will be farther ahead if you are able to determine the best molding process for your plastic product in the product development stage. 

Be prepared to answer a lot of questions from plastic mold engineers:

  • How is your plastic part going to function?
  • Is it a stand alone item?
  • Is it a part that connects to other parts or works in tandem with other parts to achieve a result?
  • Does it need to pivot, move, expand, condense?
  • Is the weight critical to the function?
  • Will the part need to withstand extreme levels of cold or heat?
  • Does the item need to adjust to specific pressure levels or PSI?
  • Will the part hold fluids?
  • Does it need to be food safe?
  • Does it need to be flexible or does it need to be solid and rigid?
  • What length does the part need to be, width, depth?

Answering these questions when designing and manufacturing any item is good a idea. With items requiring plastic molding the large variety of plastics to choose from, the importance of accuracy, and the difference between injection molding and blow molding it’s best to determine these answers as soon as possible. When one millimeter size difference can make the item usable or unusable for its intended purpose, accuracy and creative solutions are required. That’s why we have engineers, right?

Custom Plastic Molding Solutions

Because of the many variable in producing molded plastic parts, nearly every part we produce requires a custom solution. Engineers review all variables based on the end use of the product.

Variables to consider in Molded Part Production:

  • type of plastic used
  • flexibility or strength
  • psi pressure rate required
  • withstand temperatures levels – hot or cold
  • wall thickness
  • product dimensions

Once these variables are considered the whole process will go much smoother and quicker. The use of 3D printers in the prototype stage helps to reduce time too. Designing molds and fine tuning them to perfection can take a few tries but once complete production can begin.

When injection molding is determined to be the best process for your plastic part once you have an approved mold you’re practically golden. The injection molding process begins, the product is produced and there is very little concern of production issues with the injection process.

Therein lies another difference between injection molding and blow molding. In the blow molding process getting the mold perfect is only part of the battle. During the blow molding production process there are many variables manufacturers must pay attention to. The iterative nature of the blow molding process lends itself to production variables.

Periodically checking the production line is important for quality control. If the plastic is stretched too much small pin holes or cracks in seals can result. Checking for wall thickness uniformity to ensure they are within the tolerances needed for the part to behave as it is intended is also critical. As an example, a plastic bellows is made to collapse when pressure is applied and then expand back to it’s original form once pressure is released. Consistent wall thickness helps the bellows to function correctly. If the plastic bellows collapses on one end faster than the other because the wall is thinner your plastic part may wear out quicker and may not function smoothly. Having sold process control systems in place will produce consistent parts with high quality function.

Blow Molding Production Concerns:

  • pinhole leaks
  • leaking seals
  • flexibility or strength
  • velocity or pressure of blown air
  • speed of the mold closing
  • process temperature
  • wall thickness
  • poor function
  • consistent product dimensions

You can learn more about the various plastic molding processes that are applicable to your product or new invention at Plastic news.

Do you have a new plastic product invention and you aren’t sure about which plastic mold process is the best solution for your item? Give SPI a call today, our engineers love a challenge! We will help guide you in the most appropriate and cost efficient plastic molding solution for your unique requirements.

5 Responses to “Difference Between Injection Molding and Blow Molding”

  1. I want to start manufacturing of poly carbonate suitcases , i want to know which is suitable process for suitcases blow or injection

    • Hi Shekhar, Nick Kraus one of SPI’s engineers will send an email to set up a time to talk about your product dimensions, function, etc. You can also give Nick a call at 269-849-3200. Thank you.

  2. Hello Sir, I want to start production of HDPE buckets. Which type of molding is best suited to me

    • SPI Industries June 27, 2017 at 1:03 pm

      Hi Gautam, if you would like to give us a call or contact us here, we would be happy to discuss your project with you to determine the best process for your product and how best we can assist you.

  3. Manyama benjamini July 11, 2017 at 11:15 am

    Thnx…for your good explanation