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Engineering resins; breaking the rules.

Over the years, many plastic products essentially have been hand-me-downs, involving existing concepts for which direct material substitutions were made: plastics for metal, plastics for wood, and so on. The growth of plastics, however, will also depend upon innovative ideas that foster applications for plastics where they were previously considered illogical and inappropriate.

Parting with tradition Hot water heaters traditionally have glass-lined steel tanks to safely handle the constant working pressure. The cold water input typically is about 50 psi, so multiplication of the unit pressure by the large tank area produces stresses that result in the need for a usually bulky, heavy steel tank with a high burst strength. To provide an adequate safety factor, steel tanks are designed for pressures up to 150 psi, and they must pass burst strength tests of 300 psi.

Why not, then, use a comparatively low-cost, lightweight thermoplastic tank that would be easy to transport and to maintain? Obviously, because of the tank pressures, this would not be a viable design, and ASTM Pressure Vessel Codes, justifiably, wouldn't permit it.

If a way could be found, however, for the hot water heater to function with zero pressure in the tank, then the requirement for a heavy steel tank could be eliminated, and a plastic vessel could be used. That is what American Thermal Corp. has achieved with a unique concept based on a self-contained pressure transfer module (PTM), which bypasses the possibility of any pressure buildup in the tank itself.

Visualize a sealed module, 12 inches long and 4 inches in diameter, with a direct-connected inlet tube for the 50 psi cold water from an exterior source, inside an all-plastic reservoir tank. The cold water fed into the module then is "dumped" at zero pressure into the holding tank, where it is heated by conventional internal electrical elements. Through the action of valving in the module, the resulting hot water, at 120 [degrees] F to 170 [degrees] F, is pumped from the tank into the module and, through a direct-connected pipe, to the dwelling's water system. The water pressure in the module does not go above the input 50 psi, and the tank pressure remains at zero throughout the operation of the hot water system.

Functioning as a reciprocating positive displacement piston pump, the PTM provides the interface between the unpressurized tank and a normally pressurized house plumbing system. This is achieved by transferring the energy in the incoming pressurized cold water stream to drive the module's pistons and pump the unpressurized hot water in the tank back up to normal line pressure for delivery to the tap.

William Zebuhr, American Thermal's president and the inventor of the hot water heating system, says that the company is building units for a major demonstration program with American Electric Power of Columbus, Ohio, which will be completed later this year. Test units also are being placed in private homes.

A total of 25 different plastic parts will be used in the PTM, including Du Pont's Delrin acetal resin for the two 4-inch-diameter pistons, the valves, and a linkage-retaining bracket; Du Pont's Zytel glass-filled nylon for the inlet and outlet connectors and outlet valve housings; Himont Advanced Materials' HiGlass polypropylene and Amoco Chemical's Udel glass-filled polysulfone for the main structural centerbody; GE Plastics' Ultem polyetherimide for a critical component of the exhaust assembly for high temperature and dimensional stability; and Monsanto's Santoprene thermoplastic elastomer for the piston scals.

The PTM's outer housing is fabricated of drawn stainless steel end-cups and retained to the centerbody by a stainless steel clamp and sealed by nitrile O-rings. As the main structural plastic part, the centerbody houses the PTM's valves, operating linkages, and switching mechanism, which controls the reciprocations of the acetal sliding pistons to maintain water flow. Polysulfone will be used for the centerbody for specialty units functioning at sustained temperatures to 180 [degrees] F and above for the higher-end consumer and industrial markets. HiGlass polypropylene is planned for the same part for the lower-priced mass consumer market. Critical properties are strength, including creep and fatigue resistance; hydrolysis and chemical resistance; and stability under continuous rated temperatures. The materials also must not deteriorate after long periods in stagnant or flowing water.

Providing, by design, the ability to operate in an unpressurized environment thus results in the feasibility of using an all-plastic reservoir tank that, for a 120-gal capacity, weighs only 65 lbs, compared with 300 lbs for an equal capacity glass-lined steel tank. The all-plastic vessel is a sandwich structure consisting of a 0.1-inch-thick, leach-resistant, unreinforced, unfilled polypropylene liner; a 3-inch-thick foamed urethane core; and a 0.1-inch-thick outer polyethylene shell. Inner and outer liners are being rotationally molded for the test program, but they will be blowmolded for the production units. The plastic tank also provides an unequaled heat-insulating R factor of 25, which compares with 10 to 17 for a typical steel tank. The PTM is designed for a 20 year life, and the tank is expected to last more than 20 years.

Thus, direct material substitution, in a basically similar operating context, is not the only way to be able to take advantage of the functional and processing benefits of plastics. Often, a wholly new approach is required, one that explores creative design initiatives that recognize plastics' limitations under certain conditions, while still optimizing the application of their inherent benefits.

"Hatching" a simpler design

Circular hatch covers for hopper cars that transport plastic pellets would appear to be relatively simple, trouble-free designs. For many years they were made of aluminum and stainless steel, and more recently of glass-reinforced polyester or ABS plastics. Yet according to Donald Kleykamp, president, Re-Act, Inc., an injection and reaction injection molder, the railroad industry has sought affordable design and material changes to solve a number of significant performance problems inherent in the hatch covers. Cracking, warping, contamination of the lading, and poor chemical resistance have been typical difficulties.

The polyester/fiberglass cover now in extensive use, for example, is normally produced in a labor-intensive hand lay-up process; and, when installed on the hatch, it requires a separate exterior steel bar or strap to which heavy downward pressure must be applied to achieve effective closure. In addition, a foamed rubber sealing gasket, bonded to the cover's interior rim, tends to release over time, sometimes allowing contamination of the material. "Small fragments of contamination on the top of the pellets can cause rejection of an entire carload," says Kleykamp.

A fresh look at the long-term design, based on cooperation between Re-Act, Inc., the molder; Dev-Mark, a major component supplier to the railroad industry; Grand Model, Inc., a tooling manufacturer; and Hercules Inc., the materials supplier, led to innovations in function and processing and solved the problems. The result is a simplified one-piece hatch cover that is produced with low-cost tooling and requires no post-production assembly.

Unlike competitive arrangements in which the gasket is placed inside the hatch cover as an additional part, Dev-Mark designed a neoprene gasket that attaches directly to the periphery of the hopper car's stainless steel coaming. The gasket is designed to reduce the pressure necessary to provide adequate sealing against moisture and dirt, thus avoiding permanent setting and increasing scal life.

Retooling for a new product, however, can be expensive and time-consuming, and often a seriously inhibiting factor in commercializing a new design. Hercules' low-pressure liquid molding process for its crosslinked Metton polydicyclopentadiene-based thermoset permitted prototyping of the part with a low-pressure spray metal tooling technique; nickel-plating provides a hard, nonporous surface for as many as 5000 parts. The self-releasing polymer eliminates the need to clean the mold cavity between shots.

The 20-inch-diameter hatch cover features thick-to-thin sections without stress concentrations, and ribbing that provides the correct level of stiffness and resilience for cover closure. The lowpressure liquid molding process also facilitates incorporation of stress-free molded-in stainless steel inserts as wear points at the cover's hinge and adjustable toggle lock locations.

Navigating new "seas"

Roger Young, project manager, Dow Plastics, spends almost all of his time in the field encouraging innovative applications of engineering resins. He confirms that because of greater confidence based on many proven successes, and a much wider material data and processing base, more designers now are willing, and even anxious, to break away from traditional thinking and extend the limits of plastics technology. The predictability of computer-aided design and improved prototyping capabilities, combined with much more reliable material properties, have lowered the risk level and allowed designers to try new concepts that would have either been put on hold or not even imagined a few years ago.

Motorboat construction technology, using hybrids of thermoplastics and thermo-sets for vessels up to 25 ft long and with capability of up to 50 mph, is an active area for Young. A basic incentive is the pressure to reduce styrene emissions, for which the federal government's general standard has been reduced from 100 parts per million to a maximum of 50 ppm. The marine industry, however, CUrrently is specifically exempted from the tighter standard because, as Young puts it, "the technology is not yet available to achieve the reduction without putting many boat manufacturers, notably those constructing glass-reinforced polyester hulls, out of business. " Young relates that one leading boat builder has spent a million dollars for blowers and ductwork to protect workers from styrene emissions during the typical hand lay-up manufacturing process. The emissions are, nevertheless, still being sent into the outdoor environment.

A new concept for motorboat hulls is an offshoot of the design originally described for the JY15 sailing dinghy, marketed by Johnstone Yachts, which features a thermoplastic and thermoset laminate. The sailboat design is based on a three-step process to produce a three-layer skin comprising a thermoformable acrylic, weatherable SAN copolymer, and ABS sheet; a urethane foam core; and a structural inner hull of polyester and continuous-strand mat. Four tools were required to produce the laminated assembly.

The powerboat hull, which must be responsive to much higher requirements for strength, fatigue, and toughness, consolidates material usage and is more competitive. Producing the new motorboat hulls requires fewer steps and less tooling. A SAN copolymer sheet is first thermoformed. The sheet is then placed into one portion of a matched-metal die. Along with a contact adhesive, reinforcing glass is placed selectively in the other part of the tool and urethane foam is injected. As the foam expands, it wets out the glass fibers. The bonding of the glass to the foam prevents delamination, in spite of the significant difference in expansion coefficients between the glass-reinforced thermoset and the thermoplastic outer skin. The finished product consists of a laminate of the thermoformed outer skin, a foam core, and a tough RIM polyurethane inside surface.

The previous extra step of injecting the polyester, and the problem of styrene emissions, have been eliminated by a simplified process that also reduces the cost about 25%. Total thickness of the laminate is typically 3/4 to 1 inch, comprising the 0.110-to 0.165-inch-thick outer SAN skin, the 0.125-to 0.150-inch-thick inner polyurethane and glass layer, and the central urethane core for the remainder. The foam density ranges from 6 to 8 lbs/[ft.sup.3]. Densities up to 12 lbs/[ft.sup.3], depending upon the stiffness desired, are now possible. Several leading powerboat manufacturers are in prototype stages with the new hybrid hull concept.

A seat that conforms

The typical way to make a tilting office chair is to provide the structure and the cushioning as separate elements. As part of the Du Pont Co.'s design support to the furniture industry, according to Eric Larson, design specialist, the company participated with Shaw-Walker in developing an integrated design that uses injection molding technology to combine the necessary rigidity for the structure and cushioning and flexing in the same part. Larson says the chair is designed to be continuously adaptive to the "dynamics of sitting." Differently shaped cutouts in the chair's inner shell twist, bend, and flex to conform to the individual and the body's shifting positions. Molded of a toughened, unreinforced Rynite polybutylene terephthalate, the inner shell is fastened to a T-shaped bar at the top of the chair's tilt mechanism; it provides rigidity at the structure's outer edge and resilience from the edge to the center. An outer shell, initially designed as an aesthetic cover for the inner shell and then redesigned to be structural as well, is injection molded of Zytel nylon 6/6 with 30% glass reinforcement.

Because of the increased confidence in the materials, Larson says, more new projects are being initiated with engineering plastics, where previously, the parts would first have been designed in metal or other traditional materials and plastics would possibly have been tried later. He predicts that more firms, either affiliated with or separate from material suppliers, will emerge specifically to provide consulting services to designers who have not yet developed the expertise needed to optimize a product program with plastics concepts.

He also foresees an increase in shorter production runs for new products, with requirements for faster design and tool changes. Significant time compression from the start of a development project to production also will lead to growth of solid modeling software programs and interfaces that tie into stereolithography or selective laser sintering. Both techniques can quickly produce three-dimensional prototypes of parts.

Grams can be heavy

Even a "featherweight" tone-only pager can benefit from weight reduction achieved through imaginative design with plastic. A new pager design developed by Motorola's Communications Sector has cut its weight by about 25%, from 44 to 34 grams, and in the process also simplified the fabrication and assembly steps. Mechanical engineering group leader Rick Huber comments, "The 10 gram weight reduction makes a big difference in a shirt pocket."

The credit-card-shaped pagers, with a total thickness of 0.181 inch, typically consist of two stainless steel covers that contain the necessary electronics. The stainless steel housing is conventionally used to provide stiffness and also to function as a part of the unit's antenna system. Because of the difficulty in fabricating the openings for an LED lens, two switch buttons, and speaker ports along the part's edges, the covers are made as flat plates; the openings, and other features such as bosses with inserts and a battery holder, then normally are incorporated into a circumferential plastic strip that is adhesive-bonded between the plates. Thus, the steel housing design requires three parts and, with screw fastening of the plates, two assembly steps.

In the new pager design, the front stainless steel plate is replaced by an injection molded thermoplastic, which permits integration of the cover and edge functions in one part. Hoechst Celanese's Vectra liquid crystal polymer provides the needed stiffness, corrosion resistance, and flow that produces a wall thickness of only 0.018 inch. The plastic's lower tensile modulus-2.1 million psi compared with 28 million for stainless steel-improves the product's shock isolation capability. The material's low shrinkage rate helps eliminate sink marks and other blemishes during the molding of posts and bosses with integral metal inserts on the plate's interior surface.

The liquid crystal polymer's high chemical resistance inhibits paint adhesion. But microscopic etching of the plastic surface with a cold plasma technique modifies the top few molecular layers to improve adhesion of a sprayed gray paint. Because the circumferential strip is now a part of the plate, only simple screw fastening of the two housing parts is required.

Planning for separation

Good design has traditionally included planning for optimized assembly. To assist recycling, says Robert Johnson, manager, Technical Development, GE Plastics, planning for disassembly is a key design goal for the next decade. One objective, for example, of GE's advanced automotive instrument module concept, in which a blowmolded panel and ducting, a rigid carrier, knee bolsters, and steering column cover are integrated for simplified manufacturing and assembly, is easy separation of the segments for recycling.

More complex air intake manifolds reflect the demands of highly tuned, multivalve engines, making machining of metal more difficult and costly. GE has programs involving a lost-core process under evaluation at automotive companies, including glass-filled GTX, a blend of PPO and nylon, and Supec polyphenylene sulfide for higher temperatures and new fuels, particularly methanol blends.

The rationale for computer-aided design gets stronger as demands for product capabilities and differentiation from the competition increase, all within a compressed time frame. Validation of new designs the first time around, and quickly, is becoming more critical. With an expected increase in the Corporate Average Fuel Economy (CAFE) standard, automotive designers will be challenged more than ever to reduce car weight. However, only about a mile per gallon is gained for every 100 lbs removed from the vehicle. Johnson believes that since it will be difficult to make substantial gains by more aerodynamics, increased CAFE standards may have to be met by engine and fuel innovations as well as by weight reduction.

GE Plastics sees its expanded Engineering Design Database, to which over 100 companies now have access, playing an increasingly significant role in the process of accelerating product design. From its initial limited range of a few categories of stress-strain curves, it has now expanded to contain a section on how to design with plastics; an area on the interrelationships between design, processing, and assembly, including processing troubleshooting; and an analysis section that provides tools for factors such as stiffness, snap-fits, flow lengths for proper gating, and one-dimensional cooling relative to material thickness. In addition to stress-strain, materials data includes rheology, thermal conductivity, and thermal expansion, not as single points but in curve form.

Gerry Trantina, manager, Advanced Design Engineering, sees the database-which requires a terminal, modem, and some "engineering savvy" for best results-as providing a comprehensive design system that is helping to overcome previous reluctance to use plastics innovatively in new designs. Trantina says that after a new interactive program is added, filling analysis for a center-gated disc will be able to establish parameters such as flow length as a function of thickness, melt and mold temperatures, and pressure and time to fill the mold. The simulations will provide first-cut feasibility for a range of thicknesses by reducing a complex geometry down to discs of specific diameters. Trantina says that to predict the configuration of a blowmolded bumper, for example, where design and processing significantly affect material thickness distribution, simulation analysis could substitute for the time-consuming, and possibly premature, building of a prototype tool.

Breaking the rules

One of the cardinal rules that typically qualifies a part for injection molding is the requirement of high production to justify the tooling cost. On the JY15 sailing vessel mentioned previously, two machined stainless steel gudgeons and two pintles were initially used to attach the rudder to the hull. The four parts, which allow the rudder to swing 180 degrees, cost a total of $20. The pieces were naturals for the injection molding process, but for an anticipated volume of only 500 to 1000 boats per year, it was hard to justify cored tooling that would provide molded parts with the holes required for assembly.

A decision to take advantage of injection molding, but to simplify the tooling by not molding the holes, reduced the production cost from $5 to 90 cents per piece. Drilling the holes as a secondary operation after molding, normally a "no-no," cut tool costs to only $3200 for the gudgeon and $2900 for the pintle.

The gudgeons and pintles now are molded of Dow's Isoplast 40% long-glass-filled rigid thermoplastic polyurethane, providing a flexural modulus of 1.5 million psi and notched Izod impact strength of 8 ft-lb/inch. The properties are more than adequate for the application, and the total cost, representing a more than 80% reduction, is right. Breaking the rules achieved positive, innovative results.
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Title Annotation:Engineering resins: innovation, development, realization
Author:Wigotsky, Victor
Publication:Plastics Engineering
Date:May 1, 1990
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