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What Is Blow Molding And What Is It Used For?

Blow Molding Plastics

What do plastic beverage bottles, toy dolls, fuel tanks, stadium seating, milk jugs, biohazard waste containers, and skin cream bottles have in common? They are all plastic products produced by a process called blow molding.

Blow molding is a fast, cost-effective way to manufacture hollow plastic products and packaging. Essentially, blow molding fills a heated plastic cylinder with air, causing the plastic to expand like a balloon and evenly fill a shaped mold. Then the plastic is cooled and the finished bottle, jug or tank is removed from the mold press.

Blow Molding vs. Other Types Of Molding

Blow molding and injection molding are the two most commonly used plastic molding methods.

Blow-molded products are typically hollow and more often than not used to hold liquids. Injection molding, on the other hand, can be used to make solid plastic parts, pipefittings, toys, utensils, bottle caps and a wide variety of intricately designed plastic tools, parts and products.

The process of extrusion blow molding takes place in two stages: The first step involves fabricating a tube of soft, molten plastic called a parison. The parison is positioned between two halves of a mold press. The mold is then closed around the parison and a blow pin is inserted at the bottom of the mold.

During the second step, air is forced in through the blow pin, causing the parison to inflate until it fills the mold cavity. After the plastic cools, the finished product is ready for use.

Injection molding does not use an air blower of any sort. Instead, heated plastic is injected under high pressure into a mold. The pressure ensures that every space within the mold is filled with plastic. Because no air is used, products made with injection molding are typically not hollow.

Other, less common types of molding include compression molding and rotational molding. Compression molding uses two heated mold halves to press a slug of hard plastic into shape.

Rotation molding essentially turns a mold slowly, using gravity to coat the cavity walls of the mold with molten plastic.

Which Plastic Resins Work Best With Blow Molding?

Certain polymer resins are not conducive to blow molding. Polystyrene (better known as Dow Chemical’s Styrofoam™), for example, is not commonly used in blow molding. Custom-Pak Design & Blowmolding’s article, A Guide to Plastics Commonly Used in Blow Molding, lists the following polymers as well-suited to blow molding:

  • High-Density Polyethylene (HDPE) – The #1 most common resin for blow molding, HDPE is used to bottle liquids ranging from motor oil and fuel to shampoo, in consumer-sized bottles or industrial drums.
  • Low-Density Polyethylene (LDPE) – When a high level of stress-crack resistance is required, LDPE is preferred due to its flexibility. Common applications include squeeze bottles and blown film for plastic bags.
  • Polypropylene (PP) – One of the most popular plastics in the world, PP is very similar to HDPE. But PP’s lower density and high heat resistance lends itself to medical parts, food containers, and milk jugs.
  • Polyvinyl Chloride (PVC) – Another incredibly popular polymer, PVC is tough and flexible. Traffic cones, soft equipment parts, durable plastic jars, and household cleaner containers are all products of blow molded PVC.
  • Polyethylene Terephthalate (PET) – Injection blow molding, which injects plastic into the blow mold in lieu of using a parison, turns PET plastic into water and soft drink bottles.

Some other common blow molding polymers include thermoplastic elastomers (TPE), Acrylonitrile Butadiene Styrene (ABS), Polyethylene Oxide (PPO), Polycarbonate (PC) and polyurethane.

What’s In Store For The Future?

New technologies are affecting the blow molding industry in a positive way (10 Keys to the Future of Blow Molding). Advanced simulation software is being used to predict wall thickness on blow molded plastic products. These predictions can then be used to adjust both the mold and parison in order to better regulate even plastic thickness throughout the entire finished product.

Prior to these simulation advancements, blow molding often suffered from uneven thickness in the walls as the parison was inflated to fill the mold cavity.

3D printing equipment is constantly improving prototype and models used by product design teams. The better the prototype, the more money and time will be saved in recalibrating factory equipment to perform test-runs of new products.

Finally, complex machine learning algorithms are being implemented in factory equipment and computers that affect automated communication and calibration between connected devices. For example, measurements taken at the tail end of an assembly line can be instantly sent to various machines along the line, which can then automatically adjust settings as needed without needing human supervision.

Plastics Color Stands Ready To Work With Blow Molders

Plastics Color works hand-in-hand with customers to ensure that plastics facilities are getting the specific polymer resin they need to maximize their manufacturing capabilities.

Our blow molding sample service allows clients to envision their custom formulated plastic in a finished, blow-molded bottle. This service reduces the amount of down time manufacturers often experience when they shut down their machines and recalibrate them to produce a sample run. Plastics Color’s blow molder fabricates the test run right here, on premises.

Plastics Color’s laboratory services and Solutions Center stand ready to work with clients to produce the perfect compounds, concentrates and masterbatches for any and all of their plastics needs. To find out more, please complete the contact form located at the bottom of this page.

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