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Thermosets Get Gassed

DN Staff

September 8, 2003

6 Min Read
Thermosets Get Gassed

When the parts get thick, gas-assisted molding really gets going. This thermoplastic injection-molding technology can cut material costs and cycle times by hollowing out parts that would otherwise have solid cross sections. By reducing cycle times and material usage, gas assist technology has even helped pricier thermoplastics take over in appliance applications where thermosets had dominated for years. But thermosets may regain some of their lost ground, thanks to a patented gas assist technology for use with polyester bulk molding compound (BMC).

With good mechanical properties and heat resistance, BMC has traditionally been an attractive material for appliance makers as they've tried to replace metals in parts like handles, vent trims, control panels, and door skins. Polyester BMC, for example, has a flex modulus of roughly 1.9 million psi and a heat deflection temperature greater than 500F. These materials also cost less on a per-pound basis than some that can pass muster in high-heat applications-sometimes as much as 50% less. "BMC is simply a more convincing metal replacement than thermoplastics, especially in high heat applications," argues Len Nunnery, inventor of the new thermoset gas-assist process and quality manager at BMC Inc. (www.bulkmolding.com).

At first glance, BMC's Gas Evacuation Technique (GET) works much like the gas-assist methods used for thermoplastics. A gas port into the mold cavity allows pressurized nitrogen from a gas controller to enter the part before it cures, displacing some of the molten plastic to create a hollow channel in the center of the part. But thermoset materials behave differently than their thermoplastic counterparts from a rheology standpoint, and they require a different processing strategy.

This difference is most noticeable in the amount of molten plastic injected into the tool. Thermoplastic gas-assist processes, when used to make hollow parts, don't need to fill the cavity fully since the pressurized gas displaces some melt. According to Nunnery, this short-shot method "just doesn't work with thermosets." During his first gas-assist trials, performed on an appliance customer's mold for a 30-inch range handle, Nunnery found that the gas wouldn't core out a partial shot. Instead, it just bubbled through the molten BMC and escaped through the mold vents. "We found that BMC didn't provide the flow properties that would allow the gas to push the material against the cavity walls and create a hollow cross section," he says.

With the BMC seemingly unable to contain the gas, Nunnery almost gave up on the whole idea of thermoset gas assist. But he ultimately found a way around the problem-after making a processing mistake. With his first molding trials drawing to a close, an incorrect controller setting caused the molding machine's injection unit to push back prematurely, breaking the contact between the machine nozzle tip and the sprue bushing. "After about two seconds there was a loud pop, and a small quantity of raw BMC shot out of the sprue bushing onto the press," Nunnery recalls. When that handle came out of the tool, "it had been cored out perfectly," he says.

From that lucky break, Nunnery decided to change to a full-shot approach. "It seemed more likely that pressurized gas would move molten BMC from the center of the curing mass of a full shot," he says. During processing, he now intentionally allows the gas pressure to back off the screw soon after the part's skin cures-but before its center solidifies. He also locates the pin-valve for the gas as far away from the gate as possible to create a long gas path. Together, these strategies push uncured resin from the center of the part back out through the sprue bushing, leaving a hollow part.

After fine-tuning his method a bit, Nunnery wound up with uniformly hollow range handles whose nominal wall thicknesses ranged from 0.090 to 0.165 inch. "The wall thickness can be controlled by the amount of delay time before the gas is injected," he says. "More delay time results in thicker walls."

Working with GET will force engineers to strike a careful balance between cost savings offered by thin nominal walls and the strength offered by thicker ones. For example, some of Nunnery's hollow handles could not pass his appliance customer's lift test, which measures deflection with a 100-lb load attached to the center of the handle. Failed handles used a 15-second delay, which resulted in a nominal wall of 0.095 inches and a weight of 386 grams. But with the delay bumped up to 20 seconds, the wall came in at 0.100 inches, allowing the handles to pass the lift test without any increase in total cycle time and only an 18-gram increase in weight. "A good understanding of the end-use requirements is necessary before you can establish the final savings," Nunnery says.


Timing is everything: The Gas Evacuation Technique requires careful process timing to make hollow parts whose wall thickness strikes the right balance between material savings and strength. In these trials, parts made with a 15-second delay between initial part cure and gas injection failed an OEM's 100-lb lift test. Parts made with a slightly longer delay had enought strength to pass.

But savings there will be. Nunnery says GET typically reduces materials usage by about 30 to 40% in handles and related parts. They can also decrease cycle times by up to 50%, he says. In the case of the handle, material-related costs would have fallen from $1.46 each to $0.85, based on a 39% weight reduction, Nunnery estimates. Then there's the cycle time savings. Because less material has to cure, the hollow part molded in 35% of the time required by the solid handle.

30 x 1.25 x 0.75-inch Handle Data

Molding Time (Prior to Gas)

Cycle Time (Seconds)

Part Weight (Grams)

Unacceptable:

15 sec

45

386 (0.095-inch wall)

Acceptable:

20 sec

45

404

30 sec

55

439

35 sec

60

484

45 sec

70

525 (0.165-inch wall)

130 sec

130

662 (solid)

*Weights do not include runner or sprue.

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