Machinedesign 2635 090205 P66 Better 0

Better Injection Molding Through Bubbles

Feb. 2, 2009
Adding supercritical gases to polymer melts brings high quality parts that are lightweight and cost less to make.

Think you’ve squeezed the maximum weight and cycle time out of your injection-molded design? Users of MuCell microcellular-foam technology beg to differ. They’ve cut part weight 8 to 12% without major design changes. Those who have designed with MuCell from the start have been able to get parts up to 30% lighter than their predecessors.

Authored by:

Levi Kishbaugh
Vice President, Injection Molding
Trexel Inc.
Woburn, Mass.
Edited by Jessica Shapiro
[email protected]

Key points
• MuCell molding uses supercritical gases to cut melt viscosity and ease mold filling.
• Parts made using MuCell can weigh 10 to 30% less with little property loss.
• MuCell can cut down on cycle time, press requirements, and quality problems.

“Gas-Assist Molding: Not Just for Hollow Parts,” MACHINE DESIGN, Aug. 9, 2001
“Low-Pressure Molding Offers Stiff Competition,” MACHINE DESIGN, Feb. 5, 2004

Part weight might drive the life-cycle cost of a design, but MuCell can also shorten process time and use less raw materials. The technology lets engineers focus on part function instead of being bogged down by the limitations of standard injection molding.

MuCell mechanics
MuCell is a microcellular- foam process that starts with a gas at a temperature and pressure above its thermodynamic critical point. Supercritical fluids’ (SCFs) properties fall between those of the materials’ liquid and gaseous forms and can be tweaked with relatively small changes in temperature and pressure.

Before injection molding, technicians inject the SCF directly into the barrel of an injection-molding machine. There, a specially designed screw mixes and maintains the SCF in solution within the molten polymer. A shut-off nozzle holds pressure on the mixed melt to keep the SCF in its intermediate state.

When the melt is injected through the gate and into the mold, the pressure drop permits homogeneous nucleation of millions of micron-sized gas bubbles or voids. These microcellular voids are relatively uniform in size and distribution.

The presence of the SCF in the melt cuts the viscosity of the molten polymer 10 to 30%, so molds fill completely with less pressure. Users can also mold higher length-to-thickness ratios without significantly changing injection-molding parameters.

As the voids nucleate in the mold, during a phase called cell growth, the melt expands to fill the mold cavity more quickly than does a standard melt. The cell-growth phase replaces the pack-and-hold phase of traditional molding cycles. Getting rid of the pack-and-hold phase helps condense cycle times by 15 to 30%. Process designers can also do away with pack pressure, and press clamping force can be cut back.

The MuCell process works best with filled semicrystalline engineering resins. The base resins include polyamide or nylon (PA), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), and polypropylene (PP), as well as high-temperature polysulfones and polyetheretherketone (PEEK). Molders have successfully used mica, talc, calcium carbonate, glass fiber, and carbon-fiber fillers.

The gas used to create the SCF is usually nitrogen which generates a fine foam structure. Some users have successfully employed carbon dioxide as well.

The foam fix
In solid injection molding, process engineers must manage melt pressure and temperature, clamping pressure, mold length and thickness, and process time to get plastic through the gate to the last point of filling without freezing off, or solidifying, the material.

The material must fill the mold, or pack out, in a uniform manner along the entire flow length. If it doesn’t, finished parts can exhibit sink marks, vacuum voids, and shrinkage-induced warpage. When flatness, warpage, or bow are of great concern — as with boxes, cases, housings, and panels — or when the design calls for walls less then 2.5-mm thick, MuCell can bring a nonconforming part into compliance with its specifications.

Solid molding can also force parts to be thicker than their function would dictate to ensure moldfilling and uniform stresses at reasonable pack pressures. For instance, parts molded with PP reinforced with long-glass fibers (LGF) typically must be 2.2 to 2.5-mm thick to ensure reinforced polymer uniformly fills the mold. With LGF-specific process modifications to MuCell, designers trimmed standard part thickness by at least 20% to about 1.8 mm without significantly changing other molding parameters.

Another well-known rule of injection-molded part design is that part walls must have a uniform thickness. Molding from a thin section to a thick section or vice versa is a recipe for the quality problems mentioned above. But the gas expansion in MuCell molding provides the packing mechanism that variable wall thickness diminishes.

This means designers can create thinner nominal wall designs and add thicker cross sections as needed to meet impact strength requirements. For example, solid-molding rules say ribs should be about 60% as thick as part walls for semicrystalline materials, 70% for amorphous materials. But the MuCell process makes rib-to-wall thickness ratios of 1:1 possible.

This shift in design thinking has led to weight reductions of 30% in parts designed at the onset for MuCell molding.

The packing impetus from gas expansion means MuCell molding needs about 2,000 psi clamp force where 5,000 to 6,000 psi would be normal for traditional injection molding. MuCell users have been able to scale back machine tonnage requirements from 3,000 to 1,900 tons.

Smaller pressures mean molders can use smaller, less-expensive molds for more injection cycles. Additional benefits emerge if the mold needs modification: changes to a smaller, lighter mold cost less and take less time than the same tweaks on a mold meant to withstand high molding pressures.

One part design initiated with the MuCell process in mind is a margarine tub designed by Veriplast Solutions. The company’s superlight injection-molding technology (Slim) combines MuCell with Veriplast’s Extra Slim Label, a downgauged in-mold label.

The MuCell component lets the thin-wall cavities fill with less pressure and clamp tonnage. Freed from press limitations, designers could further slim the walls so the packaging was 10% lighter without any perceptible difference in the final tub quality.

Besides using less raw material, this kind of weight reduction lessens process energy and lowers shipping costs. In Europe it also means companies can cut their Eco Tax costs. For example, in Germany, the Slim’s 10% weight reduction for standard 15-gm tubs brings a €200,000 reduction in Eco Tax for every 100 million tubs.

In addition to the raw material savings that comes with weight reduction, MuCell can also give designers more latitude with material choice. Because MuCell cuts melt viscosity and reduces warpage, designers may be able to substitute a lower-grade material for a higher-grade one.

Designers of electrical connectors have been able to replace solid-molded syndiotactic polystyrene (SPS) or liquid crystalline polymer (LCP) with less expensive MuCell-molded polymers like PBT or nylon. Similarly, filled PP can replace modified poly(p-phenylene oxide) (PPO) that was specified solely to control part warpage.

Other electrical applications that have seen success with MuCell include socket components, wire harnesses and channels, switch components, junction boxes, fans, and insert-molded products.

Another good example of material substitution is the opportunity to change over from thermoset to thermoplastic. MuCell lets designers take advantage of the recyclability that less-expensive thermoplastics can provide.

In Europe, Barre Thomas is using the MuCell process to replace thermoset rubber with a thermoplastic urethane (TPU) for two jounce bumpers on the Citroen C5 automobile. Automotive engineers like the weight savings MuCell can offer. Potential automotive applications include air intake manifolds and valve covers. In vehicle heating, ventilation, and air conditioning (HVAC) and radiator systems, MuCell has molded end tanks, fans and fan shrouds, and motor covers. The interior of the car may have MuCell moldings in door panels, door trim, glove boxes, and speaker housings, possible because MuCell accommodates insert-molded and in-mold-decorated parts.

Process improvements
Many industries continue to replace metal with plastics like LGF-reinforced polymers. Polymer company Ticona ran industrial trials on LGF-reinforced PP molded with the Mu- Cell process. Their trials used Celstran LGF in an automotive-door module application. The MuCell-molded part showed five times less warpage, a 10 to 15% shorter cycle time, a 10% material savings, 40% less clamp tonnage, and the elimination of sink marks when compared to solid injection molding. Equally as important, the 10% reduction in raw material brought minimal property losses.

Another step the automotive industry has taken to reduce weight and improve fuel economy is to pair MuCell with a secondary expansion process called core-back expansion molding. Core-back is not a new technology, but it gets an entirely new purpose when used in conjunction with MuCell.

Core-back differs from both traditional injection- molding and standard MuCell processes. In a core-back cycle, additional gas is injected into the closed mold under pressure. The resin melt in the mold foams and fills the mold cavity. Then operators open the mold a precise amount, increasing its volume. This further expands the foam and adds bulk to the part’s final thickness.

When core-back and MuCell combine, the resulting part benefits from MuCell’s low viscosity and fast mold-filling and from core-back’s lower density. The finished product is a lighter, less dense part with good rigidity that uses less raw material. All in all, design engineers can combine processes on existing applications to realize dramatic weight reductions.

Industrial trials on automotive structural parts like instrument-panel retainers and door-panel liners have shown weight savings of up to 30%.

Engineers see the process combination eventually applying to any application that features a flat surface. Mazda recently said it will use the technology to cut weight and raw material costs on 2011-model-year cars.

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