Focus on Design: Carbon/epoxy drum for large-format inkjet printer

To millions of consumers who purchase inkjet printers for their computers, the digital printing process may seem mundane. That is decidedly not the case for commercial printers who use wide-format inkjet printing equipment manufactured by HP Scitex (formerly Scitex Vision, now a part of Hewlett Packard, Palo Alto, Calif.). The company’s TJ8300/8500 line of “super-fast,” wide-format, high-volume printers are capable of inkjetting full-size, four-color, high-resolution images on flexible media as large as 5 ft by 12 ft (1,524 mm by 3,658 mm) at a rate of up to 400m2/4,300 ft2 per hour — or more than 70 full-size prints. Until recently, the printer’s largest single component, its 1,275mm/4.2-ft diameter, 1,700-mm/5.6-ft in wide print drum, was fabricated entirely from metal. The drum holds the printable media and rotates it at high speed past a bank of inkjet nozzles during the print process. For that reason, it must meet some daunting performance requirements: provide a means to easily and quickly load and unload media, accelerate and decelerate quickly and remain dimensionally stable at elevated temperatures, and do it all with minimal maintenance. After encountering problems related to thermal-expansion in its metallic drums, HP Scitex turned to Cyclone Aviation Products Ltd., a subsidiary of Elbit Systems Ltd. (Bar Lev Industrial Park, Israel), to design and fabricate an alternative, says Ilan Weissberg, head of Cyclone’s composite research and development department.

“We undertook a concept study and were able to show that a carbon/epoxy drum part was the best solution for replacing the previous metallic design,” he notes. Cyclone, founded in 1970, is well known for its advanced composite aerospace parts, designed and manufactured for a variety of high-profile clients. The printing drum, however, was one of the company’s first forays into the industrial parts market. While it presented several significant challenges, the new drum design was so successful that it is now in full-scale production at Cyclone.

Minimizing thermal instability and mass

Cyclone’s material trade study evaluated several potential drum designs, including 1) a glass-reinforced nylon shell over internal polyurethane structural discs supported by a central tube; 2) an extruded thermoplastic shell with aluminum internal discs and a central tube; and 3) a carbon/epoxy laminate with steel end discs.

The selection hinged on which design could best improve on several aspects of metal drum performance, says Idan Rephaeli, Cyclone’s composites manufacturing engineering manager. First, the distance between the inkjet print head and the printable media must be consistent, from the beginning to the end of the cycle, as the drum rotates. “Because the printer has heating elements to dry the inks, the metallic drum expanded during operation,” he explains. As a result, the head/media gap had to be large enough to accommodate metal drum expansion and contraction — both the larger gap and its tendency to vary negatively affected image quality. “We wanted to minimize that gap, so low coefficient of thermal expansion (CTE) was the primary requirement,” says Rephaeli. Second, deflection of the drum surface during operation causes print distortion. HP Scitex sought a stiff and stable material that would not distort during rotation. Third, lower drum weight would reduce drum inertia, permitting faster deceleration and acceleration before and after the drum is stopped to unload and load print media, promoting faster throughput. The inherent vibration-damping characteristics of composites was a definite plus considered in the trade study, says Weissberg.

Cyclone conducted a thermal expansion comparison using samples of the potential drum materials. All of the tested materials had a lower CTE than the baseline metal drum, but the carbon/epoxy sample’s CTE was 9.5-6 °C mm/mm (5.27-6 °F inch/inch), an order of magnitude less than the polyurethane concept (9.5-5 °C mm/mm) and significantly lower than the remaining samples. Further, a carbon/ epoxy drum would have the lowest weight — 440 lb/200 kg as compared to 946 lb/430 kg for the polyurethane/nylon version, which translated to a 30 percent lower weight than the baseline steel/plastic drum. The significantly lower weight meant that a carbon/ epoxy drum would have the lowest moment of inertia at the target rotational speed of 160 rpm. Additional benefits included chemical resistance and repairability in the field if damaged. To cap it off, Weissberg notes, “it was the simplest design, with the fewest parts.”

Modeling design performance

Well-versed in numerical computational analysis, Cyclone used NASTRAN finite element analysis (FEA) software from MSC Software Corp. (Santa Ana, Calif.) to calculate the loads on the structure in order to develop the fiber architecture for the carbon/epoxy layup. Seven load cases were developed using various combinations of temperature, angular acceleration and rotational speed.

Remarks Rephaeli, “We found that in the first FEA design phase, with rotational speed of 160 rpm, the drum cylinder was deflecting, with an unacceptable runout of 0.3 mm [0.012 inch]. We concluded that a carbon/epoxy bulkhead or disc was needed in the middle of the cylinder to prevent this.” With the composite disc in place, the deflection was minimized and movement reduced to 0.1 mm/0.004 inch — well within the project specs.

Several other operational issues required design tweaking. To accommodate the gripper mechanism — a set of spring-loaded metal bars that span the width of the drum and generate 400N/90 lbf of axial force to hold the paper or other print medium in place — a series of stepped, axial grooves were designed into the drum, with material buildups along its margins to support the mechanical fasteners that affix the gripper bars.

The drum design also required a unique and complex vacuum system that helps hold the print media against the drum during rotation, says Weissberg. Like previous metal drums, the composite drum’s surface would be drilled with thousands of tiny, regularly spaced holes. Plastic tubing, divided into segments by a series of shut-off valves (shown in drawing), would be attached to one end of the drum, around the circumference, and connected to a vacuum pump (located within the printing machine, but not within the drum). When activated, the vacuum pump would pull air through the tiny holes in the laminate; a rigid, closed-cell foam, machined with a network of grooves and placed below the laminate, contains the air. The vacuum system thus can be shut down or activated in sections to accommodate the relatively small number of standard media sizes. Further, integral flanges were designed at the drum ends to allow mechanical fastening of the steel end-discs that attach the drum to the axle and the printer’s drive motor.

Maximizing design manufacturability

Cyclone selected three types of carbon fiber prepreg for the drum’s laminate: 1) 2×2 twill for the outer layer, 2) a plain weave and 3) a multiaxial (±45°) stitched nonwoven, all supplied by EPO GmbH (Willich, Germany). The reinforcments are wet out with a 250°F-/120°C-cure epoxy, and Degussa (Darmstadt, Germany) supplies the Rohacell polymethacrylimide (PMI) foam that is placed below the laminate, already machined and grooved for the vacuum system.

Weissberg stresses that for this project, a departure from their typical aerospace-related work, the company wanted to streamline production and keep costs as low as possible: “Yes, we are using prepregs,” he notes, “but the part isn’t over-designed — it’s an economical, out-of-autoclave solution.”

Cyclone designed and built the two-part steel layup tool in-house. According to Rephaeli, the drum was originally made in two parts, with a joggle for postcure bonding, but is now made in one piece. The upper and lower mold halves are clamped together and the part is layed up in one operation. First down is the twill, which forms the part’s outer surface. After layup of the foam, a single ply of woven E-glass/epoxy prepreg is layed up over the foam’s inside surface to protect the vacuum system from damage during drum changeout or maintenance. Total drum wall thickness, including the foam for the vacuum system, is approximately 15mm/0.6 inch. The layup is then vacuum-bagged and oven-cured; no postcure is required.

After demolding, the central bulk-head, which is layed up separately, is bonded inside the tube. Then, approximately 5,000 vacuum system holes, 1.5 mm/0.06 inch in diameter, are drilled by hand.

Although the company has investigated robotic drilling with the drum held in a jig, Weissberg says that, for now, manual drilling isn’t unduly slowing the process. Says Rephaeli, “The finished surface is attractive and smooth and doesn’t require any sanding or painting.” The drum’s high surface quality minimizes friction between the drum and the printed medium, making media loading and unloading easier, he notes.

After drilling, the steel end-discs are bolted to the flanges at each end of the drum and the steel axle and bearings are attached, along with the plastic tubing and valves that link the drum with the vacuum pump.

Meeting the customer’s expectation

The drum part has been in production for about two years and costs about 30 to 40 percent more than the original steel/plastic drum it replaced. Despite the higher cost, its robust design has, so far, met the customer’s five-year service life requirement, says Rephaeli. “HP Scitex is very happy with the results — production throughput of the printer with the composite drum is much higher than the capacity of the older machine.”

The company, to date, has produced more than 200 drums at a rate of about 100 per year, one of which was showcased at the 2006 JEC Composites show in Paris. Weissberg comcludes, “This project shows what can be accomplished with good design and a reasonable fabrication strategy — a composite part that can replace metal in an important application.”