Blow & Rotational Molding

Blow and rotational molding are plastic molding processes in which manufacturers use thermoplastic polymers to create hollow, seamless objects. Custom mold and design manufacturers also use rotational molding for thermosets. The parts created using blow or rotational molding range in size from small, 5-milliliter plastic bottles (0.15-ounce capacity) to large, 38,000-liter storage drums (10,000-gallon capacity). While the two processes sometimes compete, they have their own niche applications. Blow molding, for example, is better suited for the mass production of small disposable containers. Rotational molding is better for creating large hollow shapes.

Interested in metal injection molding? Read more about it here.

Blow Molding

Blow molding is a process in which manufacturers use air pressure to inflate soft plastic into a mold cavity. It’s a critical industrial process for making one-piece hollow plastic parts with thin walls, such as bottles and containers. Because many blow molded items are used for mass marketed consumer beverages, blow molding manufacturers typically organize production with high quantities in mind. Manufacturers borrowed the molding technique from the glass industry, making them a competitor in the disposable and recyclable bottle market.

Blow molding consists of two steps:

Step 1: Fabricating a starting tube made of molten plastic called a parison (the same term used in glass blowing)
Step 2: Inflating the parison into the desired final shape

Manufacturers form a parison using extrusion or injection molding techniques.

Extrusion Blow Molding

Custom injection molding companies use extrusion blow molding in high-production operations for making plastic bottles. The sequence is automated and is usually integrated with downstream operations, such as bottle filling and labeling. In general, blown containers must be rigid. A container’s rigidity depends on several factors, such as wall thickness.

The extrusion blow molding cycle illustration below outlines the steps blow molding manufacturers take during the molding process:

  • Extrude the parison
  • Pinch the parison at the top and seal it at the bottom around a metal blow pin as the two halves of the mold come together
  • Inflate the plastic tube so it takes the mold cavity’s shape
  • Open the mold and remove the solidified part

Injection Blow Molding

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In the injection blow molding process, custom mold and design manufacturers inject-mold the starting parison instead of extrude it. Compared to its extrusion-based counterpart, the injection blow molding process has a lower production rate. Therefore, it isn’t as widely used.

The injection blow molding process involves:

  • Injection-mold the parison around a blowing rod
  • Open the injection mold and transfer the parison to a blow mold
  • Inflate a soft polymer to conform to a blow mold
  • Open the blow mold and remove the blown product

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Injection blow molding: (1) parison is injection molded around a blowing rod; (2) injection mold is opened and parison is transferred to a blow mold; (3) soft polymer is inflated to conform to a blow mold; and (4) blow mold is opened and blown product is removed.

Stretch Blow Molding

Custom injection molding companies use a variation of injection blow molding called stretch blow molding. The technique involves extending a blowing rod downward into the injection molded parison (see step 2). This stretches the soft plastic and creates a more favorable stressing of the polymer than conventional injection blow molding or extrusion blow molding. As a result, the structure is more rigid, has higher transparency, and is more impact-resistant.

For this type of custom plastic injection molding, polyethylene terephthalate (PET) is the most popular material used. PET is polyester with low permeability. The stretch-blow-molding process strengthens it. The combination of PET’s properties makes it ideal for making containers for carbonated beverages.

Stretch blow molding steps include:

  • Injection molding the parison
  • Stretching
  • Blowing

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Stretch blow molding: (1) injection molding of the parison; (2) stretching; and (3) blowing.

Blow Molding Materials and Products

Blow molding manufacturers are limited to thermoplastics. Polyethylene (PE) is the polymer of choice for blow molding because of its high density (HDPE) and high molecular weight polyethylene (HMWPE).

When comparing the properties of HDPE and HMWPE with a more affordable low-density PE in regard to the stiffness-related requirements of the final product, it’s more economical to use the more expensive materials because it’s possible to make a container’s walls thinner.

Other materials used for blow molding include:

  • Polypropylene (PP)
  • Polyvinylchloride (PVC)
  • Polyethylene terephthalate

Disposable containers for packaging liquid consumer goods make up the bulk of products made using blow molding. Custom injection molding companies use the technique to make other products, too, such as:

  • 55-gallon shipping drums for liquids and powders
  • 2,000-gallon storage tanks
  • Automotive gasoline tanks
  • Toys
  • Sailboard and small boat hulls
    • Small boat hulls are made using a single blow molding and cutting the finished product into two open hulls

Rotational Molding

Rotational molding, or rotomolding, uses gravity inside a rotating mold to create a hollow form. The technique is an alternative to blow molding for making large, hollow shapes. Custom mold and design manufacturers use it primarily for thermoplastic polymers, but thermoset and elastomer applications are becoming more common. Compared to blow molding, rotomolding tends to be better for more complex external geometries, large parts, and lower production quantities.

The rotomolding process includes the following steps:

  • Loading a predetermined amount of polymer powder into a split mold cavity
  • Heating and rotating the mold on two perpendicular axes so the powder coats the mold’s internal surfaces, gradually forming a fused layer that’s uniform in thickness
  • Opening the mold and unloading the part

The rotational speed during the rotomolding process is relatively slow. This is because gravity, not centrifugal force, causes the uniform coating of mold surfaces.

Rotomolding Molds

Compared to injection molding and blow molding, rotational molding molds are simple and less expensive. The production cycle, however, takes longer—ten minute or more. To balance the advantages and drawbacks of production, a multi-cavity indexing machine (e.g., the three-station machine in the diagram below) performs the rotational molding. This machine’s design allows the indexing of three molds in sequence throughout three workstations so all three molds work simultaneously:

  • First station: An unload-load station where the finished part is unloaded from the mold and the powder for the next step is loaded into the cavity
  • Second station: A heating station that contains a heating chamber where hot-air convection heats the mold as it rotates; temperatures inside the chamber reach 375°C (700°F), depending on the polymer and item molded
  • Third station: The cooling station that cools the mold using forced cold air or a spray of water to solidify the plastic molding inside

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 Rotational molding cycle performed on a three-station indexing machine: (1) unload-load station, (2) heat and rotate the mold, and (3) cool the mold.

investment-casting-truck-industryCustom mold and design manufacturers create a variety of items using rotational molding, such as:

  • Toys (e.g., hobby horses and playing balls)
  • Boat and canoe hulls
  • Sandboxes
  • Small swimming pools
  • Buoys and other floatation devices
  • Truck body parts
  • Automotive dashboards
  • Fuel tanks
  • Luggage pieces
  • Furniture
  • Garbage cans
  • Fashion mannequins
  • Large industrial barrels
  • Containers
  • Storage tanks
  • Portable outhouses
  • Septic tanks

The most popular materials used for rotomolding include:

  • Polyethylene, particularly HDPE
  • Polypropylene
  • Acrylonitrile butadiene styrene (ABS)
  • High-impact polystyrene

Sinotech is a U.S.-based custom mold and design manufacturer that offers quality blow molded and rotational molded parts at competitive prices. For more than 12 years, we have audited, qualified, and worked with QS-9000- and ISO-certified rubber molding factories in The Pacific Rim. We’re dedicated to managing your project on-site for a lower cost while delivering the same quality, service and terms you expect from a domestic supplier.

What molded parts do you need? Contact Sinotech today and tell us the details to get the process started.

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