Blow Molding vs. Rotational Molding
Learn how the blow molding and rotational molding processes compare to each other.
Though blow molding and rotational molding have some similarities, they vary in terms of production cost and volume, wall thickness control, and application. The main difference between these two molding techniques is the working principle. Blow molding is a high-pressure, hollow-part plastic forming process that is used to mold pieces of low complexity. By contrast, rotational molding, or rotomolding, can be defined as a low-pressure thermoforming process used to create hollow plastic parts.
Rotational molding is the more versatile of the two, boasting great design flexibility and low startup costs. Blow molding, on the other hand, produces products on a shorter cycle time but is less flexible in terms of design. Initial setup costs are also very high. Blow molding comes into its own in high-output situations. It can be up to 40% cheaper than rotomolding if the output volume is high enough. If production demands are lower than 3,000 units a year, then rotomolding will be more cost-effective.
Blow Molding Definition and Comparison to Rotational Molding
Blow molding is an ancient technique that can be dated to at least the first century BCE in the form of Syrian glassblowing. Later refinements allowed glassblowers to create bottles. The more modern blow molding process matured in the 1930s when the first automatic blow molding machine was introduced by the Plax Corporation.
Most modern blow molding deals with thermoplastics. A hollow plastic parison (also known as a preform) is placed at the mouth of a mold cavity and heated until properly malleable. Pressurized gas is then forced into the parison’s open end until it expands to match the mold cavity’s shape. The product then gets cooled and crystalized so it doesn’t deform while being removed from the mold. The end result is a hollow plastic vessel with thin walls.
Blow molding services are ideal for large-scale production. It’s best known for soda and water bottles but also finds uses in engineered gas tanks and electrical enclosures. Because it is so cost- and time-efficient, it is favored over rotomolding in high-output industrial settings. Figure 1 shows the process of blow molding:
Advantages of Blow Molding Compared to Rotational Molding
Compared to rotomolding, blow molding has several advantages:
- Blow molding cycles are shorter.
- The mold itself lasts far longer.
- Blow molding produces thin, lightweight parts with parting line integrity.
- It is more cost-effective for production rates higher than 3,000 units per year.
- Blow molding offers better wall-thickness control and is fully automated.
Disadvantages of Blow Molding Compared to Rotational Molding
Some of the disadvantages of blow molding compared with rotational molding include:
- Blow molding has less design flexibility.
- The tooling cost is very high compared to rotomolding.
- With blow molding, low-volume production quantities are more costly.
- The surface finishes produced by blow molding are not always aesthetically pleasing.
Rotational Molding Definition and Comparison to Blow Molding
A form of rotational molding was employed by ancient Egyptians who used this technique to create ceramics. Between 1940 and 1950, an updated rotational molding was developed for the plastic industry in the US but didn’t become popular because it was so slow. However, in the past few decades, improvements in the plastic powders used in the process have made it more viable.
Rotational molding involves a lot of heat but low pressures. A hollow mold is filled with powdered plastic resin and then rotated or spun. It is transferred into an oven where the resin melts and ultimately coats the walls of the mold. This is then followed by a cooling step. The mold keeps rotating until the resin fully hardens into its final shape. The mold can then be opened and the part removed.
Rotomolding is best suited for large, single-piece hollow parts or double-walled open containers like kayaks, oil tanks, and coolers. Unlike blow molding, rotational molding produces no weld lines or pinch-off seams. The product thus doesn’t require secondary finishing processes. Rotomolding is more efficient at lower production volumes – fewer than 3,000 per machine annually. This makes it ideal for start-ups, inventors, and small businesses.
To learn more, see our guide on What is Rotational Molding.
Advantages of Rotational Molding Compared to Blow Molding
Rotomolding has the following advantages over blow molding:
- Rotational molding offers more versatility and design flexibility.
- Rotomolding is more cost-effective for production rates of fewer than 3,000 units per year.
- Rotational molding has cheaper tooling due to the low pressures involved.
- The final products are stronger and more durable than blow molded parts.
Disadvantages of Rotational Molding Compared to Blow Molding
The following are considered disadvantages of rotational molding compared to blow molding:
- Rotomolding tools need regular replacement or refurbishment.
- Voids in the end product are possible.
- Rotomolded parts have thicker walls, so they’re heavier and use more material.
- Rotomolding has poor wall-thickness control.
- Rotational molding can be more labor intensive than blow molding.
| Attribute | Blow Molding | Rotational Molding |
|---|---|---|
Attribute Tooling price | Blow Molding $200,000 / lifetime | Rotational Molding $150,000 / yr |
Attribute Piece price | Blow Molding ~$22 | Rotational Molding ~$35 |
Attribute Part weight | Blow Molding Light | Rotational Molding Heavy |
Attribute Part porosity | Blow Molding Low | Rotational Molding High |
Attribute Durability of mold | Blow Molding High | Rotational Molding Low |
Attribute Design flexibility | Blow Molding Limited | Rotational Molding High |
Attribute Cycle time | Blow Molding Often less than 1 min | Rotational Molding Often more than 30 min |
Attribute Production rate | Blow Molding 70 parts/hr | Rotational Molding 2 parts/hr |
Blow Molding vs. Rotational Molding: Comparison of Key Characteristics
Cost comparison: The initial tooling costs of blow molding are greater than those for rotational molding. However, in the long run, blow molding can save you up to 40%. This is especially true for high-output production lines that turn out more than 3,000 units per year.
Speed comparison: Blow molding is a much faster process than rotational molding. Cycle times for the former are often less than 1 minute, whereas rotomolding cycles can stretch past 30 minutes. Blow molding’s speed makes it far more energy and time efficient.
Volume comparison: Because of the difference in molding cycle times, blow molding is capable of higher production volumes compared to rotomolding. More specifically, blow molding machines can output 70 parts/hr, whereas rotomolding may be limited to only 2 parts/hr.
Materials comparison: Common rotational molding materials include Low-density Polyethylene (LDPE), Linear Low-Density Polyethylene (LLDPE), High-Density Polyethylene (HDPE), and Crosslinked Polyethylene (PEX). Meanwhile, common materials for blow molding include LDPE, Polypropylene (PP), Polyethylene Terephthalate (PET), Polyvinyl Chloride (PVC), and HDPE.
Alternatives to Blow Molding and Rotational Molding
Some alternatives to both blow molding and rotational molding are:
- Thermoforming: With thermoforming, a plastic sheet is heated until it is fully pliable, whereafter it is shaped around a single-sided mold and left to cool to the new shape. Thermoforming is similar to both blow molding and rotomolding in that all three processes involve heating the plastic.
- Vacuum Forming: With vacuum forming, a plastic sheet is heated and then pulled against the mold via suction. Similar to blow molding and rotational molding, vacuum molding is also a thermoforming process that uses heat to mold plastic.
How Xometry Can Help
Xometry provides a wide range of manufacturing capabilities including CNC machining, 3D printing, injection molding, laser cutting, and sheet metal fabrication. Get your instant quote today.
Disclaimer
The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.