Low-pressure molding offers stiff competition

Feb. 5, 2004
Structural-foam molding can produce hollow, multilayered parts that are often bigger, stronger, lighter, and cost less than injection-molded equivalents.

Six structural foam HDPE sidewalls for material-handling containers are molded in the same cycle on one Uniloy Milacron SF1000 machine. A total shot weight of 75 lb produces 120 parts/hr. Part quantity/sizes per cycle: (2) 45 X 21 in., (2) 44 X 25 in., and (2) 34 X 16 in.
Uniloy's first 2,000-lb structural web machine produces large chamber sections for underground runoff stormwater systems. Stormtech, Old Saybrook, Conn., maker of stormwater handling systems uses the low-pressure structural web technique to make 50 in. wide X 30 in. tall X 7 ft long, 75-lb polypropylene overlapping storm water chamber sections. To conventionally injection mold the mammoth parts would take a 12,000-ton, high-pressure press.

Complete structural foam HDPE agri bin (420-gallon capacity) is molded in one cycle on a Uniloy SF1000 wide-platen machine. Total shot weight is 135 lb using 20 injection nozzles. Production rate is 15 parts/hr.
Eight structural foam HDPE air conditioner pads are molded in the same cycle on one Uniloy Milacron SF800 machine. Total shot weight is 100 lb and 200 parts/hr are produced using 50 injection nozzles.

Multinozzle structural-foam technology is growing in a big way -- bigger presses, bigger parts, and greater productivity. So says Ed Hunerberg, spokesman for the structural foam machinery business of Uniloy Milacron, Manchester, Mich. The process, one of the oldest forms of gas-assisted molding, produces more parts per platen with faster cycle times than conventional injection molding. Constructed of a foam core and a solid skin, structural-foam-molded parts have higher strength-to-weight ratios and up to 70% more load-bearing capacity than injection-molded parts of the same weight.

Structural-foam molding replaces wood, metal, concrete, and fiberglass on large-part applications demanding strength and durability. Applications include office, business, medical equipment, automotive, and recreation (boats, camping gear, toys, and playground equipment). The process also builds shipping containers, tote boxes, drums, and pallets.

Under the skin

Structural-foam molding takes its name from the injection of an inert foaming agent (such as nitrogen N2) into the thermoplastic resin melt. The gas and plastic dynamically mix under high backpressure inside the extruder barrel of the injection-molding machine (IMM). The gas dissolves forming a homogenous mixture of single-phase solution and microscopic gas bubbles. The resin/gas mixture is held under pressure to prevent foaming (cell growth) until it injects into the mold.

Molds typically fill sequentially via multiple injection nozzles located in each mold cavity. This optimizes the process and better controls part weight. The injected resin/gas mixture partially fills the cavity 80 to 90% in what's called a "short shot."

After the short shot, nozzles for the cavity close and the foam expands filling out the rest of the cavity. As the resin makes contact with the cool surface of the mold it forms a solid skin while the foaming gas in the inner core expands forming a cellular structure inside the part. During cooling the internal cell pressure of the foam eliminates sink marks on part surfaces.

Key advantages of the process include:

  • The ability to convert large structural parts to plastic.
  • Parts have low molded-in stress because short shots don't need high-pressure packing of the mold.
  • Tooling costs may be 20 to 30% lower.
  • Precise gas-injection and dynamic mixing produces foams with engineered cell structures that boost part strength and reduce weight.
  • Manufacturing and assembly operations are eliminated -- components mold as one monolithic part.

For large shot weights and big molds conventional IMMs need high clamp forces. This can limit the size or number of parts that a given piece of equipment can produce. With structural foam, clamp force is no longer a limiting factor. Low-pressure lets structural-foam presses provide 10 to 20 times the mold area per ton of clamp force over conventional high-pressure IMMs, says Hunerberg. "For example, a 1,000-ton structural-foam machine can mold a total projected area of 3,300 to 6,600 in.2 versus 300 to 500 in.2 for a conventional 900-ton IMM." The process also gives designers significant weight and material savings coupled with unmatched flexibility and control over cavity filling -- a critical advantage on large, complex parts with many walls, webs, rails, and reinforcements, says Hunerberg.

Other benefits of the process include 10 to 30% weight reduction versus solid parts, thick walls (0.125 to 0.5 in.), complex part features molded without sink marks, and low warpage. The process works with most thermoplastics and low cavity pressure lets designers specify lower-cost aluminum molds. This is a benefit for small production runs that may otherwise be too costly to produce via conventional injection molding with expensive tool steel molds.

Molders can produce "shoot-and-ship sets" per cycle (molding multiple parts in a single cycle, in one mold or multiple molds on a single platen) and increase productivity by consolidating processes through stack molding and multicolor/component molding.

Catch the Wave

Wave filling is a similar technique. It is the sequential filling of a single large part. This innovation from Uniloy provides independent hydraulic and electrical control over individual nozzles (up to 80 per machine). This control permits nozzles to open and close sequentially as a function of actual shot position to distribute material not only between different mold cavities, but also inside each mold cavity.

Nozzles can be programmed to sequentially open in a cascading pattern. This action "wave flows" material across the mold. The technique eliminates weld lines and reduces cavity pressures. All nozzles can close in series to prevent overpacking the mold in specific areas. This helps reduce part weight, flash, and molded-in stress.

Multinozzle capability and large press daylight area make possible the use of stack molds to boost productivity without increasing platen size, says Hunerberg. Nozzles of different lengths inject material into parts stacked at two or three positions in the mold. With this technique, a three-cavity stack mold produces three 52-in.-diameter water tank lids every 2.5 min on a Uniloy Milacron 1,000-ton wide platen press, says Hunerberg.

Trend: Larger presses, more parts/cycle

The trend in structural-foam molding is toward larger, more productive presses that produce multiple parts per cycle. Machines such as Uniloy's multinozzle, sequential injection version boasts the ability to process multiple molds mounted on the same platen (either side-by-side or stacked), each with different molding parameters, all in one machine cycle.

"Multinozzle sequential injection lets each mold cavity fill individually with different shot sizes and at three different injection speeds per cavity," says Uniloy Milacron spokesperson Ed Hunerberg. Shot size is repeatable to within 0.5%, while closed-loop control varies injection speed to compensate for different polymer viscosities caused by material or temperature changes. Machines with as many as 12 shot sequences let 12 different molds reside on one multinozzle machine platen.

Mold cavities can have single or multiple nozzles. The operator keys in the shot size for each cavity along with individual injection speed profiles. The first cavity fills to a specified shot size and its nozzles close. The nozzles in cavity #2 open and shot #2 injects. The process repeats until all cavities fill.

Sequential filling reduces clamp pressure requirements. This can increase machine-molding capacity by 50% or more and is particularly useful when molding parts with large projected areas or multiple large parts in the same cycle. The technique involves filling one part at a time and then delaying the next shot until cavity pressure in the first shot drops. During this dwell time the peak cavity pressure will drop more than 50% within the first 5 sec, says Hunerberg.

Machines equipped with multiple extruders not only mold two different colored parts on the same press, but also run two different materials or structural foam and structural web gas-assist parts simultaneously.

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