New twist in foam injection molding is revolutionizing the way molders think about lightweighting parts. A microcellular foaming process, called MuCell, reverses the longstanding view that foam molding is limited to large, thick parts. Now it is possible to apply lightweighting to thin walls (0.5-mm) and small parts with critical dimensions.

The MuCell process, developed and licensed by Trexel Inc., works by heating and pressurizing a non-flammable gas (usually nitrogen or carbon dioxide) to a “supercritical” state, in which the gas acts like a fluid. The supercritical gas is injected into the machine barrel, where plastic and gas blend into a single-phase solution. Injected into the mold, the gas expands to form highly uniform, closed cells of 50 to 5 microns diam. Conventional foam molding yields non-uniform cells and voids of at least 250 microns. The tiny cell structure of the MuCell foam reportedly impart higher mechanical properties at lower densities than are seen with conventional foaming.

The MuCell process is said to permit lightweighting of up to 10% with no loss in performance properties. It may even be possible to hike certain properties while reducing part weight, say Trexel sources. In some cases where properties are not critical, as much as 60% density reduction can be achieved.

The MuCell process can foam large, flat parts with thin sections, or small, complex pieces. Advocates say it reduces warpage, sinks, and molded-in stresses.

The microcellular process works with a wide range of materials, from polyolefins and polystyrene to reinforced engineering resins like PBT, nylon, or PEEK.

 

Processing benefits

Addition of the supercritical gas can lower melt viscosity by as much as 60%, pare process temperatures by up to 140° F, decrease molding pressures by 30-50%, and cut clamp-tonnage requirements by up to 75%. Lower clamp force means lower energy costs and enables larger parts to be molded on smaller presses. It could also allow greater cavitation, as well as use of aluminum molds. In addition, MuCell molders can completely eliminate hold pressure and hold time, and reduce overall cycle times by 20%, says Trexel president and CEO David Bernstein.

Some molders are turning to MuCell for the processing benefits, even when lightweighting is not a goal, Bernstein says. Some are using MuCell mainly to lower the viscosity of a hard-to-process material, which not only aids mold filling but also reduces stresses and orientation.

MuCell foaming can also be a solution to molding problems such as shrinkage or warpage. “We’ve seen that 2-3% addition of supercritical gas to a part can solve 99% of the molding issues. You add more gas than that only if you want to reduce weight,” says John Adamowicz, engineering manager at Arburg.

One example of MuCell’s capability was demonstrated at the NPE 2000 show in Chicago this past June. Ferromatik Milacron molded a 48-in.-long carpenter’s level of polypropylene at the show. “With solid molding, the part weight was 673 g, cycle time averaged 92 sec, and the finished part was unusable because you could not eliminate the sink marks or warpage,” says Milacron product manager Jim Esser. “If you tried to pack out the part to eliminate the sink marks, you’d get warpage. If you lowered the pack and hold pressures, the sink marks returned. And if you tried using a long pack and hold time to eliminate sinks and warpage, you would increase cycle time by 10-15 sec,” he said. With the use of MuCell, Ferromatik Milacron had a lot more success. “We ran the mold on our Magna Toggle 310-ton, wide-platen model. The wide platen allowed us to hang this large mold. We ran the part at the lowest tonnage setting on the machine, 186 tons, yet it was estimated the part could have been run at just 100 tons. We eliminated shrinkage and warpage, reduced part weight to 596 g, and cut the cycle time to 70 sec.” Esser said the customer is now evaluating the properties of the foamed parts.

“The MuCell process is not suited for every job, but where conventional injection molding fails to produce a good part, it can be a solution,” says JSW v.p. Jerry Johnson. For example, clear plastics may be an inappropriate application for MuCell, since the cells diminish clarity. Cosmetic-appearance parts may be another example, since the process tends to leave a swirl effect on the part surface. Trexel’s Bernstein says a solution to the swirl effect will debut next year. Two approaches are suggested by Kai Jacobson, manager of process engineering and customer training for Engel. One is in-mold lamination of a preformed skin to cover up the swirl, while another is to use a textured finish on the mold.

Running MuCell requires changes in some machinery components and re-thinking some design considerations for tooling. MuCell modifications add a 10-15% premium to the cost of a machine, but depending on the part and production rate, return on investment can be 12 to 14 months, according to a number of press makers.

Presses modified for the MuCell process can also run solid parts, so a molder isn’t buying a dedicated foam machine. The process is being used in both vertical and horizontal presses. Some machine suppliers say MuCell may also find a use in coinjection or multi-component applications.

 

Molders tool up

MuCell was developed initially for extrusion, but injection molders account for most of the 55 licenses issued to processors and machine builders. At present, only 10 injection molders are producing MuCell parts commercially, while others are evaluating the technology or developing new projects with customers. With MuCell blow molding starting to emerge, Trexel expects to issue a total of 200 licenses by the end of next year.

Injection machine makers are also upbeat in their forecasts for selling MuCell-capable machines. “I can’t see why it wouldn’t be on 30% of all presses we sell in a few years,” says Arburg’s Adamowicz. “We have a lot of customers who want to see what MuCell can do for their product. We are booked for months on demonstrations,” adds Engel’s Jacobson.

This year alone, five injection machine makers inked licensing agreements with Trexel, raising the total to nine: Arburg, Battenfeld, Engel, Ferromatik Milacron North America, Ferromatik Milacron Europe, Husky, JSW, Krauss-Maffei, and Van Dorn Demag. (In addition, Uniloy Milacron is licensed to supply MuCell-ready structural-foam machines.) Machine builders are licensed by Trexel to sell new machines, or retrofit existing ones, to use the MuCell process. molders must also get their own license from Trexel to practice the technology.

Engel, which became the first licensed machine builder in 1998, has more than 15 units in the field, including a 1000-tonner that is the largest MuCell machine anywhere. Arburg, which took the second machinery license in late 1999, has sold about a dozen MuCell-equipped machines from 50 to 250 tons and will install a lab system this year at its headquarters in Lossburg, Germany.

Ferromatik Milacron has four units in the field, including a 400-ton Magna hydraulic press at Trexel. Ferromatik Milacron Europe plans to demonstrate coinjection with MuCell foam as the core layer during an open house in Germany this month.

Battenfeld has one customer for a 500-ton MuCell machine, and has a coinjection unit with MuCell capability at its lab in Meinerzhagen, Germany. The lab just got another MuCell press of 880 tons.

JSW has equipped two MuCell machines, one for a Japanese customer and another to be installed at Trexel’s lab this month. JSW plans to offer the technology in the U.S. come January.

Husky has built a 176-ton MuCell press and has an order for the largest MuCell machine yet, a 2200-tonner for automotive molder MIG Plastics, Inc. in Morenci, Mich.

Of the two newest machinery licensees, Krauss-Maffei plans to have a laboratory unit for demonstrations in Allach, Germany, by December. Van Dorn will start offering MuCell technology on its machines in November.

In June, Trexel announced licensing agreements with five major suppliers of automotive parts on three continents: Johnson Controls Interior GmbH in Germany; Magna Eybl of Ebergassing, Austria; INOAC Corp., Nagoya, Japan; Takagi Seiko Corp., Toyama, Japan; and MIG Plastics.

The first automotive molder to actually use the MuCell process commercially is Injectronics Inc., Clinton, Mass., a Tier One supplier of HVAC parts. It uses Engel machines to mold microcellular foam fuse boxes, blower housings, and flapper doors of nylon and talc-filled PP.

Mar-Lee Companies in Leominster, Mass., is a moldmaker and custom molder that has produced a PP cover for a child safety gate using MuCell. The part was previously molded without MuCell on a 350-ton press but is now made on a 200-ton machine using only 100 tons of force. Cycle time was cut 17% and material savings is around 12%. Mar-Lee offers to supply other molders with turnkey MuCell molding systems based on Engel machines.

 

Low-pressure advantage

MuCell molding can be a particular advantage in electronic encapsulation, where lower filling pressure reduces the possibility of deforming wires on a coil assembly, for example. Empire Precision Molding in Rochester, N.Y., recently installed a 66-ton Arburg machine with MuCell technology. Empire plans to use MuCell to make thermoplastic-encapsulated electronic products like circuit boards, sensors, and level switches. With MuCell, lower process temperatures and pressures, as well as reduced post-mold shrinkage, make for more productive molding, says president Neal Elli. “Encapsulation of circuit boards can have yields as low as 20% in some cases. We expect to have significantly improved yields because of the decreased temperatures and pressures,” he says.

However, Elli doesn’t believe MuCell will be useful in micromolding some of Empire’s parts that are smaller than the head of a pin, weigh only 2-3 g, and already have short cycles. It’s another matter, he says, if you have a larger part with a 40-sec cycle and can shave 10% of that time with MuCell.

MuCell could also play a role in in-mold decorating and laminating, where the lower melt temperatures and pressures reduce the danger of damage to the film or fabric overlay during injection.

 

Process details

What separates MuCell technology from traditional foams is the use of nitrogen or carbon dioxide in a supercritical fluid (SCF) state. In this state, the gas dissolves into the melt much more rapidly, says Trexel v.p. David Pierick. When the gas is diffused throughout the melt, constant pressure keeps it in solution until it is injected into the cavity.

An essential piece of MuCell equipment is a free-standing SCF conditioning system from Trexel, which uses an air pump to bring the gas to a supercritical fluid state in 30 to 60 sec. Gas pressures are commonly 1000 to 5000 psi. Typically, the SCF is at a lower temperature than the melt. The machine comes in standard sizes of 10 and 30 lb/hr, though 3-lb/hr and 65-lb/hr models are coming soon.

The SCF travels through high-pressure hoses to a small manifold box mounted on the machine. The newest version of this device is simplified for faster installation. It in turn delivers SCF to injectors mounted atop the injection barrel. A solenoid metering valve on the injector is controlled by the injection-machine controller.

The Trexel manifold unit controls both the SCF flow rate and duration of flow, which can range from the entire screw recovery cycle to as little as 0.1 sec. The size of the foam cells is determined by the amount and rate of SCF flow into the barrel, as well as the temperature and pressure of the melt. All these parameters, as well as the type of plastic and gas, affect the total solubility of the gas/melt mixture, which in turn determines the melt viscosity. As a rule of thumb, if the temperature in the barrel increases, the cell size will increase. If the amount of gas increases, the cell size or the number of cells increases. The amount of gas needed to bring about a certain amount of weight reduction is different for each material, says Pierick.

Bernstein says CO2 tends to be more soluble in the melt than nitrogen. However, N2 tends to provide finer cell structure, which gives a better surface finish.

Typically the gas makes up no more than 6% of the shot weight, yet it can provide as much as a 30% reduction in part weight. The small, uniform cell size is a characteristic of SCF foaming. “The rate of nucleation is an order of magnitude faster than with conventional blowing agents. That is why we get the uniform cell structure. The high nucleation rate is caused by a rapid change in the solubility of the product,” says Pierick. Engel’s Jacobson notes that additives and fillers enhance nucleation.

Trexel has designed a special screw and barrel for the MuCell process. The screw has a 28:1 L/D to accommodate extra mixing elements. “The screw is designed to achieve a homogenous melt but at a reduced temperature,” says Pierick. It generates less shear than a conventional design. The longer screw requires some machinery suppliers to lengthen their machine bases, so Trexel is developing a 24:1 MuCell screw that fits standard machines.

Two or three barrel injection ports, each with its own SCF injector device, are used to ensure that the gas and melt get an equal amount of mixing regardless of the changing screw position during plastication. SCF flow automatically shifts from a downstream port to an upstream port as the screw retracts.

Esser says Ferromatik Milacron has modified its machine hydraulics to ensure that the melt is always kept under sufficient pressure to keep the gas in solution. Willi Meyer, project manager of Husky’s Systems Group, says the plasticating backpressure must be somewhat higher than usual for that reason. In the same vein, Ken Vaughan, market-research manager of Van Dorn Demag, cautions that “the gas process does require a certain injection speed to keep the gas in solution before it expands in the mold.”

When launching a new MuCell molding job, Engel’s Jacobson recommends, “Start with a fully solid molded part, then turn on the MuCell and add the gas at a low rate. As you increase the gas-flow rate, decrease the shot size to permit some foaming. Usually I am in production after 10 to 15 shots.”

Trexel says the internal pressure of the gas takes the place of externally applied holding pressure. Packing the part is, of course, undesirable, since no foaming would take place.

 

Tooling adjustments

Tooling may require a few modifications. Trexel recommends a hydraulic shut-off nozzle to prevent pressure loss and material drool. JSW’s Johnson is concerned about what could happen if that nozzle failed. He is helping to write a safety standard that will activate a relief valve to channel the pressurized gas to an empty tank in case the nozzle shut-off valve fails.

Machinery makers also say attention should be paid to runner and gate design. They say the reduced melt viscosity means that slimmer runners can be used, especially with cold runners. In the case of hot runners, valve gating is essential to prevent drool. The lower viscosity may let you reduce the number and size of the gates.

“You will also have to look at venting of the mold,” adds Husky’s Meyer. “After you introduce gas into the melt you need to be able to get the gas back out.” He suggests using more or larger mold vents. Parts also require about a day to stabilize after molding, while gas diffuses out of molded parts until the internal pressure equalizes with atmospheric pressure.

 

Resin research needed

Shrinkage and warpage behavior of MuCell parts have not yet been characterized for specific resins and different types and amounts of SCF blowing agents. The same goes for trends in mechanical properties versus degree of density reduction. Some resin suppliers are starting to fill these gaps.

DuPont has taken a leading role in this regard. Test parts were molded of glass-filled Crastin PBT and Zytel nylon at DuPont’s Application Technologies Center in Wilmington, Del. The parts had smooth, solid skins over cores with very small, uniform cells, DuPont reports. Shrinkage and warpage were less than usual, and injection and clamp pressures were reduced up to 50%. Mechanical properties generally declined in proportion to part density, except that stiffness was reduced to a lesser degree.

DuPont is continuing MuCell tests with a number of its materials, including Delrin acetal and Rynite PET. Other firms, such as GE Plastics and Ticona, are also evaluating the technology.