This technical information has been contributed by
C-PAK Industries, Inc.

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Rotational Molder Builds Quality, Cost Savings into Parts Manufacturing

Rotational Molding

SAN BERNARDINO, Calif.--Although not as well known as other plastic parts manufacturing processes, rotational molding--also known as rotomolding--produces many parts that are in everyday use. C-PAK Industries, Inc. (www.cpak.net), for example, a rotational molding company that employs about 25 people at its 20,000-square-foot manufacturing facility in San Bernardino, California, manufactures rotomolded parts for municipal playground equipment, such as slides, simulated rock climbing walls, and cover plates for bars. The company also produces parts for various medical, automotive, aerospace, military, agriculture, and environmental applications, to name a few.

"In the automotive industry, we produce under-the-hood engine compartment components used in air intakes and air filter housings," says Candice Etchepare, owner and general manager of C-PAK Industries. "In the other industries, we produce hydraulic tanks, steam tanks, parts containers, sharps containers, cabinets, oil and fuel containers--the list goes on and on."

Rotational molding is known for making large plastic parts. But determining whether or not it's actually the best process for making parts depends not just on the size of the parts, but on the end use application and the number of parts needed on a monthly or yearly basis, according to Keith Fry, sales manager at C-PAK Industries. Other manufacturing processes, he says, might not be as economical when a customer needs only 500 large parts.

"You might need to make 10,000 to 100,000 parts with the other processes to be cost-effective," he says. "So it's not just the size of the parts, but also the production run and the end use of the parts. For someone who wants a hollow, double-sided part, rotomolding is the ideal method of production."

In the rotational molding process, molds (usually aluminum) are filled with a plastic resin and spun on a multi-axis machine at a temperature of approximately 500 degrees Fahrenheit, according to Ms. Etchepare. The resin reaches a melting point and the molten material is formed onto the mold surface while rotating. The mold is then moved from the furnace to a cooling area, where the bi-axis machine continues to spin. The material slowly cools off, building in thickness until the resin solidifies. Still extremely hot but no longer molten, the mold is moved to an additional cooling chamber, where water mist is sprayed on the molds to lower the mold temperature to a manageable degree. The mold is then moved to an unloading station, where the molds are split, revealing a finished product in its rough form. The molds are cleaned of any moisture or foreign debris, reloaded with powder resin, and the cycle begins again. A complete cycle is approximately 60 minutes.

Product makers choose rotational molding for a number of reasons, including its ability to produce virtually stress-free parts with strong outside corners and thicker walls for greater rigidity. Parts that had previously been assembled from multiple pieces can be rotationally molded as one integral part to save on fabrication and assembly costs. Economical tooling costs, the ability to mold-in inserts, and reduced weight versus metal and fiberglass parts are additional advantages. Rotational molding accommodates a variety of lightweight plastic materials, including polyethylenes (LLDPE and HDPE), plastisols, and polycarbonates, as well as ethylene vinyl acetate copolymer (EVA), crosslink Nylon, Nylon 6, Nylon 11, and Nylon 12.

Another advantage of the process, according to Fry, is that the wall thickness of a particular part can be determined by the volume of the base material that goes into the mold.

"For thicker wall thicknesses, if you have a part made from injection molding or thermoforming, you can get wall thicknesses in the hundreds of thousandths of an inch," he says. "Many of our wall thicknesses are anywhere from 1/4 inch to 1/2-inch thick." One application where these wall thicknesses come in handy is a grease tank used by fast food restaurants. "The restaurants can dump their hot fryer grease into the tanks, and the tank won't melt because we use 1/2-inch thick plastic," he says.

C-PAK can also make small parts, a capability that not all rotomolders have. "We are one of only a handful of rotomolders that can make small parts out of Nylon," says Fry. "For a military application, we've rotomolded a part out of Nylon 12 for pieces that are about the size of a cell phone. This is a pretty small part for this process, and some of these parts have inserts."

Customers are drawn to C-PAK, according to Ms. Etchepare, because of their need for high-quality products. "Our biggest strength is our product quality and craftsmanship," she says. The company assists customers with part design, as well as mold design, enlisting the skills of an in-house engineer who is actively involved in nearly every project. C-PAK also maintains the molds at little or no cost to the customer for the duration of the part contract.

"We really pride ourselves on how we maintain our molds," says Fry. "Molds really take a beating, especially when we use a crosslink material, which bonds to metal. So we clean our molds on a routine basis, and can tell when they need cleaning by the color of the release agent that we apply to the mold. We do this at no extra cost to our customer, even though it increases the per part cost by about 10 cents per part. It's kind of an insurance policy for the customer's mold, so it won't get ruined."

C-PAK Industries uses a number of different materials for tooling and products (parts or complete stand-alone units). For molds, the company might use aluminum, stainless steel, or sheet metal, for example. Several different variations of polyethylene, as well as Nylon, are used for parts manufacturing. For parts with higher temperature requirements, the company might use a cross link polyethylene, Ms. Etchepare said.

For one molding project, the company modified one of its machines to accept a rather large mold for a cargo container used to transport critical components and sensitive equipment in military applications. This particular application required a mold approximately 110 inches x 60 inches x 24 inches. C-PAK's customer had used CNC machining to produce the mold, weighing slightly over 1500 pounds, from a solid, 5000-pound chunk of aluminum billet.

According to Ms. Etchepare, C-PAK had very limited involvement in the mold design, focusing instead on the design and fabrication of the mold's exoskeleton. "We devised a mounting system used to hold the mold securely on our machine's arm and used a counter weight to balance the arm," she said.

Although modifying its machine to accommodate the large mold and its additional weight was a major challenge, according to Ms. Etchepare, C-PAK was equal to the task because most of its employees have been trained in the mechanical aspects of machinery. "We modified the furnace doors, floor, air ducting, and cooling chamber, all with in-house employees," she said. "The actions we took to accommodate our customer saved the customer money in shipping cost [versus producing the product elsewhere] and gave the customer a product built with pride and quality."

Fry explained that the machine that C-PAK modified had been designed for molding specific part dimensions. "So if we needed to make larger parts, we thought we would need to buy a larger machine," he said. "We found out that we wouldn't be able to offset the cost of a new machine based on the part's cost. So rather than go into debt, we decided to modify our existing machine and make it larger.

"This particular machine is a three-arm, four-station piece of equipment," Fry continued. "The three arms are able to move independently of each other with about 33 degrees of movement for each arm. The machine has two unloading stations, a furnace, and a cooling station. When the arms pivot and rotate through each station, the mold spins on the bi-axis portion of the machine. So it's spinning in two different directions inside of the different sections."

The problem, according to Fry, was that the exoskeleton part of the mold was hitting the inside of the furnace and the cooling structure. "We had to cut out the walls of the furnace, which were eight-inch thick walls with panels, and make new panels that were larger. We were still within the weight limits of the machine when we finished; otherwise, we would have had to purchase the larger machine."

This technical information has been contributed by
C-PAK Industries, Inc.

Click here to find suppliers

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