Machine Design

Reinforced Plastics Make Metals Look Weak

More structural applications use continuous-fiber-reinforced thermoplastics thanks to easily processed materials and a better understanding of molding technology.


By Patrick Johnson
Tim Greene
Product Director
Applied Fiber Systems Ltd.
Clearwater, Fla.


Waterstick Inc., Port Perry, Ontario, uses an Applied Fiber Systems CFRTP fabric called RF6 to make an eight-dihedral-faced kayak paddle. The paddle surface is said to not only grab the water more effectively than conventional blades but also release surface pressure at eight precise locations along the outside edge of the blade. This produces a blade that has zero flutter while providing more bite per square inch of surface area. The thin blade design was made possible due to the high stiffness and impact resistance of the CFRTP material.

Powder coated with melt-fusible thermoplastic particles, the continuous-fiber filaments are woven into fabrics or braid, formed into semirigid unidirectional tapes or ribbons, or laminated into panels.

This CFRTP process weaves together strands of powder-resin-coated fibers to produce Applied Fiber Systems' TowFlex fabrics. This is in contrast to other fabrics where raw reinforcement fibers are first woven then coated. Weaving individual coated strands makes the fabric highly drapable. This is because each strand within the fabric moves freely relative to adjacent strands. In addition, the strands remain flexible because they are not fully wet out prior to molding. Complete wet out of the fabric comes during the compression-molding process.

Two types of continuous-fiber-reinforced materials have recently replaced metal in many aerospace, sporting good, and industrial applications. The first and most well known are made from thermosets. Parts made from these composites are lighter and more corrosion resistant than metals. They also form more easily into complex shapes. The other, made from continuous-fiber-reinforced thermoplastic (CFRTP) materials, offers several additional benefits compared to thermoset composites. They are tougher and better withstand impacts. They mold readily and can be recycled. They also have unlimited shelf life and emit no hazardous solvents during processing.

Typical thermoset composites are brittle and have poor impact resistance. An impact may cause little visible surface damage but makes the part dramatically weaker. Thermoplastics, on the other hand, are much tougher and withstand more severe impacts with little or no damage. But, impacts great enough to weaken the part typically cause surface damage that is clearly visible.

Thermoset resins need a chemical reaction, usually brought on by heat, to harden, or cure. The chemical reaction is irreversible; once cured, a thermoset material can't be reprocessed or reformed. And molding cycle time for thermoset materials is largely determined by the curing time. In contrast, thermoplastic matrix materials don't require a chemical reaction. They melt repeatably when heated and harden when cooled. CFRTPs work well with rapid part-fabrication methods: Their molding cycle time depends on how fast tooling and equipment heats or cools, rather than on the time needed for a chemical reaction to occur.

There have been few commercial applications for CFRTP, however. One reason has been that early product forms of these materials were tougher to process and mold. The development of highly drapable, conformable CFRTP fabrics addresses such shortcomings. They are made from thermoplastics such as nylon, polypropylene, polyphenylene sulfide (PPS), polyetherimide (PEI), and polyetheretherketone (PEEK) with carbon, glass, or aramid reinforcement fibers. They can easily be molded into complex structural shapes and are ideally suited for production quantities of between 1,000 to 50,000 parts annually.

Fiber-reinforced thermo-plastics are widely used in injection molding. The vast majority of these products use short or chopped fibers. These fibers generally measure less than 0.25 in. and will randomly orient themselves during molding. Typical injection molded parts contain only 20 to 30% reinforcement fiber. Short, randomly oriented fibers in low percentages don't provide much reinforcement. And it's often difficult to mold such material into complex, large, or thick-walled parts without voids or knit lines.

In comparison, parts molded from new CFRTPs often contain more than 60% reinforcement. Reinforcement fibers run continuously throughout the entire part in specified directions to help optimize strength and stiffness. CFRTPs contain continuous-reinforcement fiber filaments that are powder coated with melt-fusible thermoplastic particles. The uniformly coated filaments are woven into fabrics or braid, formed into semi-rigid unidirectional tapes or ribbons, or laminated into panels. The resin particles wet out and consolidate quickly when compression molded.

The first step in compression molding CFRTP fabric is to cut and assemble fabric plies. The plies create a preform that approximates the flat pattern shape of the molded part. Automated cutting equipment such as reciprocating knives or ultrasonic gear may be an option for complicated patterns manufactured in high volume. Steel-rule dies, electric rotary shears, or hand shears/scissors might work best for lower volume applications. It is often useful to machine a simple preform fixture with a cavity or recess in the shape of the flat pattern. The assembly fixture helps keep the precut fabric plies in proper order, location, and orientation. This includes any partial plies needed to build up additional thickness in specific areas.

Stacked plies get tack-welded ultrasonically or via a soldering iron before removal from the fixture. This helps keep them in the right orientation during handling and storage. The preforms have unlimited shelf life and can be produced in large quantities independent of the molding process or molds. They also need no additional lay-up labor before being compression molded into finished parts. The fabric drapes easily and makes for a straightforward molding process that forms and shapes the flat preform. In contrast, thermoset preimpregnated materials generally require cutting and splicing to mold complex shapes.

Matched steel metal molds give the best surface finish and mold life when molding CFRTP parts. Nickel-plated aluminum molds are good for moderate volumes of material processing below 600°F. Steel molds are mandatory for higher temperature molding of matrix resins such as PPS or PEEK. Registration of the upper to lower mold halves takes place through either guide pins or the mold configuration itself.

Generally, molds are designed to fully "bottom out" on the CFRTP material rather than on thickness stops. This helps maintain pressure on the material throughout the molding process. Thickness stops, however, are used to maintain flatness and help ensure that the mold doesn't "rock" during the process. They also establish a minimum part thickness. Stops should typically be 0.010 to 0.020 in. lower than the desired nominal part thickness. For relatively thin parts or plates of less than 0.15 in. the quantity of fabric plies loaded into the mold primarily controls thickness. Thicker parts or plates use a specified preform weight along with the number of plies to control part thickness.

Molds need low mass for fastest processing — quick heat up and cool down. To handle certain deep-draw parts in hot/cold shuttle-press processing, cooled platens/bolsters which approximate the part shape are added to the molding presses. This tactic helps keep heating and cooling sources close to the CFRTP and eliminates unnecessary mold mass.

Molded parts need side-wall draft angles of at least 3° to keep enough pressure on the material and to facilitate part removal. For best results, corner radii should be designed no smaller than 0.06 in.

An important consideration in thick-plate processing is the thermal-expansion mis-match between the CFRTP materials and the metal molds. This effect becomes an issue when plate thicknesses exceed 0.15 in. and width or length exceed 10 in. The mold contracts during cooling. But carbon-reinforced CFRTP materials, with a near-zero co-efficient of thermal expansion, stop shrinking once they solidify. Tooling will "trap" the part in the contracting mold perhaps damaging either the mold or part if not accounted for. Ways to combat the problem include removing the molded CFRTP plates at as high a temperature as possible and designing the mold with two adjustable sides that can move during cooling. Another approach is to make molds from materials such as Invar that expand negligibly when heated.

CFRTP fabrics generally use three variations of the compression-molding process. All employ conventional equipment and don't need rapid closing speeds or excessive press tonnage.

Single-press/heated-cooled platens — The platens in a single press are heated and cooled to reach the right processing conditions. They, in turn, heat and cool molds loaded into the press. The molds stay in the press for the entire process cycle. But the fact that both press platens and molds must heat up and cool off slows cycle speed. Cycle times range from 30 to 90 min for typical equipment and tooling depending on part thickness, mold mass, and other factors. It is useful to cool molds used in repeated cycles only enough for part removal. This helps reduce the overall cycle time.

This approach applies best in situations requiring a variety of different parts in relatively low volumes that do not need a quick molding cycle.

The molding process begins with loading of the preform into the lower mold half. Operators next install the upper mold half and load the complete mold into the press. It's then heated with pressures on the order of 10 to 50 psi. Low pressure during heating helps ensure good heat transfer and initiates the forming of the preform. Full pressure, 100 to 800 psi, comes during final compaction as the mold reaches its required processing temperature. Next the press platen is cooled to bring the mold and CFRTP part to the removal temperature.

Hot/cold shuttle press — Hot/cold shuttle presses separate the basic segments of the compression-molding process — heating, cooling, and loading/unloading — for better efficiency. Separate heated and cooled platen presses apply pressure to the mold and CFRTP material. Cycle times are short because molds shuttle between preheated hot presses and precooled cooling presses.

Cycle times are particularly fast for relatively thin parts and low-mass molds. Molds don't need individual heating and cooling systems which helps reduce cost. Molds for deep-draw parts may need to use heated or cooled press platens or bolsters. These approximate the part shape and mount to the hot and cold presses. This approach helps keep heating and cooling sources close to the material to increase processing speed.

The hot/cold shuttle-press approach is useful for combinations of different parts in moderate to high volumes. Here economics don't justify the expense of heating and cooling provisions in each mold.

The hot press station is first preheated to 50 or 100°F higher than the desired mold temperature. A preform goes in the lower mold half, the upper mold half lowers into place, and the mold shuttles into the preheated press. Low pressure, 10 to 50 psi, is applied as the mold heats for good heat transfer and to start preform shaping. Full pressure of 100 to 500 psi or more forces final compaction as the mold reaches the processing temperature range. Next, the hot press opens, pressure releases, and the mold shuttles into the cold press station. Full pressure again applies as the mold and material cool enough for part removal.

For best process economics, three molds should simultaneously cycle through each step in the process. Depending on part shape, typical cycle time with continuous processing is often 5 min or less for 0.030 to 0.125-in. parts with low-mass molds. Plates thicker than 1 in. that need heavy steel molds may have cycles on the order of 30 min.

The shuttle-press process is typically set up in either an "in-line" or "rotary" configuration. Molds for in-line processing shuttle on guide rails or roller conveyors from the loading station through a four-post hot press, into the cold press, and out. The presses are aligned back-to-back or side-by-side. Rotary configurations place the load-ing/unloading station and the C-framed heating and cooling presses around a rotary table. Molds attached to the rotary table index to each processing station.

Single-press/heated-cooled molds — Here integrally heated and cooled molds mount directly onto press platens. Processing cycles for complex-shaped parts can be fast because mold heating and cooling systems can be close to the CFRTP. Individual molds are more expensive, because each mold must contain integral heating and cooling. Press platens must also have sufficient travel to open wide enough for easy preform loading and part removal. Otherwise molds would need to be removed from the press for part removal.

The single-press/heated-cooled mold approach is especially appropriate for large production runs of a specific part or plate. Manufacturers can amortize molds over a large part count and speedy processing helps drive cost down. It is also appropriate for large or deep molds where the shuttle process is impractical.

The integrally heated and cooled mold halves are usually attached to the press platens to minimize handling. The mold temperature cycles between the processing and part-removal temperatures. The CFRTP preform loads into the mold, the press closes with low pressure, 10 to 50 psi, for good heat transfer and to initiate preform shaping. The heated mold goes under full pressure, 100 to 500 psi or more, for final compaction and forming. The mold and material then cool and the part is demolded. Cycle times can be under 5 min depending upon the mold mass, part thickness, and part geometry.

CFRTP versus standard unreinforced and discontinuous-reinforced products
THERMAL EXPANSION (CTE in./in./°F) 10 X –6
PPS resin
40% glass/PPS
40% carbon/PPS
Continuous carbon fiber/PPS
PEI resin
30% glass/PEI
40% carbon/PEI
Continuous carbon fiber/PEI
PEEK resin
30% glass/PEEK
40% carbon/PEEK
Continuous carbon fiber/PEEK
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