Blow Molding vs. Rotational Molding: Differences and Comparison
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 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 is 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 is an example of a plastic bottle blow molding machine:
A plastic bottle blow molding machine.
Image Credit: Shutterstock.com/FUN FUN PHOTO
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.
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.
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.
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.
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|
$200,000 / lifetime
$150,000 / yr
Durability of mold
Often less than 1 min
Often more than 30 min
Rotational molded parts are, on average, heavier and thicker than blow molded parts. However, rotomolded parts are known to have some degree of porosity. This is because the rotation of the drum distributes resin semi-randomly. In theory, the centrifugal force with a liquidized resin should be able to evenly distribute it over the entire surface, but in practice, this is not always the case.
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. Blow molding tooling typically costs about $200,000 for the lifetime of the mold whereas upkeep and parts for rotational molding average $150,000 per year.
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.
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.
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.
Mutual 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.
Blow molding and rotational molding both:
- Form hollow plastic products.
- Are examples of thermoplastic molding processes.
An alternative to blow molding is:
- Blow Molding vs. Compression Molding: Blow molding and compression molding are similar in that they both require high pressures to form the final product.
- Blow Molding vs. Injection Molding: These processes are similar in that they are both high production-volume manufacturing technologies that form plastic into many different shapes and sizes. For more information, see our guide on Blow Molding vs. Injection Molding.
An alternative to rotational molding is:
- Rotational Molding vs. Injection Molding: These processes are similar in that they both offer design flexibility and can be used to create complex parts.
This article presented the comparison of blow molding to rotational molding, explained what they are, and discussed the different attributes of each. To learn more about blow molding and rotational molding, contact a Xometry representative.
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