Selecting a manufacturing process is just as important as the design itself. And of equal consideration are the design parameters.

Process yields product. So how do you determine which will provide the results you want for your application?

Know your application and the design requirements. Understand what materials will best achieve function. And, know what process will pro-duce those properties. It is also a good idea to consider quantity you'll be producing, especially from a cost standpoint.

There are many molding processes in the industry: injection molding, blow molding, and compression molding, to name a few. But what about rotational molding?

"You don't think of rotational molding as producing a pretty part but as producing a functional part," says Jim Leitz, a member of the Assn. of Rotational Molders Board of Directors and marketing manager for Gregstrom Corp. (Woburn, MA), a rotational molding manufacturer.

Rotational molding, otherwise known as rotomolding or rotational casting, is a thermoplastic processing method for producing simple to complex, leak-proof hollow parts that can be filled with foam. The process is best applied for producing runs of 200 to 2,000 or 3.000 parts, which range in size from small car syringes of plaslisol to large 22.000-gal vessels of polyethylene.

Using the process, engineers can design a product that will feature mold-in inserts, eliminating component count: and embedded graphics, which eliminate surface presentment required with other methods.

The process generally makes use of polyethylene powders, other powders, and liquids. However, nylon, elastomers, fluoropolymers, and polypropylene can also be used. According to Leitz, the association is working with material suppliers to create more options.

The rotational molding process can be found in automotive/transportation industries; in applications involving sporting, medical, and industrial equipment: and in toy equipment.

Design considerations when using rotational molding

• Heat transfer of material
• Location of parting lines
• Where to vent part
• How mold is clamped
• How mold is mounted for rotation
• How many parts mold will make

Process Benefits

• Ability to mold different parts at same time
• Produce little scrap
• Short lead time
• Low tooling costs
• Low initial investment (15 to 20% that of an injection mold)
• Beneficial for producing large parts with a seamless, double-wall construction
• Turnaround time of six to eight weeks, depending on complexity

Integrated Design Systems Inc. redesigned the Pulsar II (left) to decrease cost, part count, assembly time, and most importantly, lo lake advantage of rotational molding benefits. The resulting product, Pulsar III (right), Is more visually appealing at a cheaper price.

Putting the process to work

When Arch Chemical, formerly Olin Chemical Pool Products, decided to improve the design of its Pulsar II pool chlorinator, the company's rotomolder Gregstrom Corp. referred it to Integrated Design Systems Inc., a design consulting office in Great Neck, NY.

The original product was produced using rotational molding. However, the design required much machining, secondary operations, and assembly work. These factors, combined with light tolerances, resulted in a product with a limited market and limited manufacturability.

Arch Chemical called on Integrated Design Systems to improve manufacturing costs and marketability. The company redesigned the product from scratch and worked with Arch Chemical's product development team to coordinate product design and function. The firm rebuilt the model using PTC's solid-modeling Pro/ENGINEER software.

"The new design fully integrates the product's function and its aesthetics," says Michael Paloian, designer at Integrated Design Systems Inc. "A close look at the design and the benefits of the rotational molding process helped us eliminate 60 to 70% of ports. We also reduced cost by 50% and assembly lime by 80%.'

Paloian says the company used its knowledge of rotational molding to improve the design. "We creatively applied the technology to our advantage by maximizing molded-in features that ultimately resulted in part consolidation and lower cost," says Paloian. "Parts easily nested together without having to be molded precisely. When everything is assembled, the overall appearance is very precise and neat, with minimum secondary operations. We eliminated much of the machining associated with the previous design."

Paloian adds that the company also improved the assembly by stacking components. "This resulted in improved manufacturability and production consistency." says Paloian. "By reducing part count and eliminating assembly steps, we were able to make the product more reliable—it looked better, functioned better, and was ultimately more successful in the field."

The original rotomolded product consisted of nested, single-walled containers that required precise molding and extensive machining of top and bottom surfaces. The upper hopper was covered with a single-walled lid that hinged about a plastic rod. "Although the lid was functional, it required additional machining and lacked customer appeal," says Paloian.

To fix this, Paloian considered the benefits of rotational molding. "We eliminated  all the machining by molding the lid as a continuous, hollow, double-walled. 2-inch-thick part. This new lid design was very robust, easier to manufacture, less expensive, and better looking than the previous version because the process was integral to the design."

Paloian also says that when using rotational molding it is important to take advantage of the contours you can put into a mold instead of relying on machining. "Whenever you can put functionality into the part, that's where you are saving money and taking advantage of the process. The less machining and cutting you do, the better the end product will be from an appearance, reliability, and cost standpoint."

As the design progressed, Gregstrom helped Integrated Design Systems with design parameters, " Gregstrom helped us early in the design cycle by defining minimum radii, wall sections, draft angles, tolerance limitations, as well as advantages inherent in the process," says Paloian.

The original vessel featured many fittings on the outside of the unit. The fixtures were vulnerable to the external environment and had potential to leak if damaged by a service employee or outside force. Integrated Design Systems put the fittings inside the interior. This simple change, says Paloian. cleaned up the design and meant that any leaks would just fill the inside of the vessel.

ROCK&ROLL

The Rotational Molding process involves the following six steps:

• Machine operators deposit a predetermined amount of plastic, powder or liquid form, in 1/2 of a charged mold.
• The machine moves the charged mold into an oven where it rotates biaxiaily at low speed for approximately 15 to 20 min.
• Heat penetrates the mold, and the plastic melts and forms a shell over the inside surface of the closed mold until it is completely fused.
• The machine rotates out of the oven into a cooling chamber where air or water spray, or a combination of both, cools the mold. The mold continues to rotate during this time and the temperature gradually lowers.
• Manufacturers open the mold and remove the finished hollow part.

Here, a Pro/E screen image depicts the Pulsar Ill's new design features: a simplified assembly with outer components now located In the interior.

Getting the most out of the process

While designers could have chosen to manufacture this product using a number of different processes, Paloian says they all would have been more expensive and not well suited to low volumes. For instance, he says, the quantities for this application are too small for blow-molding or injection molding to be cost-effective. In addition, the wall sections couldn't have been as heavy as they are so the chlorinator wouldn't have been as robust.

"With rotomolding," he says, "you have a lot of design freedom so you can get complicated shapes, big parts, and uniform wall sections." And, because the radii can be relatively tight, Paloian adds, you can achieve intricate aesthetic detail.

Rotational molding offers a low initial investment. 15 to 20% that of injection molding, says Paloian. And turn-around time is six to eight weeks maximum, depending on complexity. In this instance, the product was rotomolded in polyethylene for its chemical resistance.

"The best designs often include the creative integration of the best material and process with the right application," says Michael Paloian. "The rotomolding process, like all manufacturing technologies, has certain limitations. If you know what they are and creatively integrate them into your design, the results can be quite exciting."

—Anna Allen, Staff Editor