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Plastics testing.

Plastics testing is only as good as its relation to reality. While the many existing standardized procedures point helpful directional arrows, creative customizing of tests is often necessary to provide more meaningful simulation of actual product requirements.

Volumes of tests and specifications for plastic materials from both the American Society for Testing and Materials (ASTM) and the International Standards Organization (ISO) help designers establish useful baselines. The careful designer knows, however, that the road from the test world to actual conditions is often strewn with unknowns and oversights that the straightforward "traditional" test may not reveal. As a result, companies frequently must develop their own modified test procedures, which provide more meaningful guidelines for material processing and/or design of end products.

As Barbara Furches, group leader, Testing and Color Technology, Dow Plastics, points out, "Some of these procedures become standard methods within the industry, after review and round-robin testing through ASTM or a trade organization." In any event, plastics testing, far from a cut and dried technology, is a constant challenge, requiring savvy analysis to determine what the acquired data really mean.

Furches says that the best known standard tests--those quoted in "typical property data" sheets provided by material suppliers--such as tensile, notched Izod, flexural, melt-flow rate, thermal properties, and so on, are designed for comparison of materials prepared and tested under controlled conditions. "Since these are single-point tests, they do not necessarily predict the behavior of a material in an application in which temperature, stress, and other environmental factors are not controlled. An increasing number of tests being developed through ASTM and ISO do address long-term properties such as creep, fatigue, and the environmental effects of light, heat, and chemicals. As these data become part of typical property or design manuals, part designers will be better able to compare plastic materials for real world use." Meanwhile, in the continuing search for valid information, modifications to standard test procedures or development of special tests are par for the course.

OFF THE BEATEN TRACK

The components plant of GM's Inland Fisher Guide Division, for example, stepped off the beaten test track when it had to validate a material selection for one of its customers. The problem was to verify that a selected plastic would provide the necessary long-term heat stability.

The ground rule was that the material should be reliable in the high-glass-surface-area "greenhouse" environment of the new car design, even in extreme desert conditions and with use of special tinted protective glass.

The Syracuse, N.Y., facility is the third largest plastics processing plant in North America. Its throughput, which is 100% dedicated to plastics manufacturing, was 41 million lbs in 1991, down from a peak of 70 million in 1988.

Initially, when heat-aging testing was done on standard test plaques, ABS was selected for plastic parts such as interior sidewall panels, including doors and quarter panels. Inland Fisher Guide's process engineering department, picking up on the project, considered that standard test plaques may not be reflecting the built-in failure-inducing stresses of actual molded part configurations. A decision was made to modify the simplified plaque mold, which radically diverts from typical production tooling, to include real-life elements such as ribs, bosses, holes, contours, provisions for attachments, and a smaller gate (rather than the typical easy-flow, stress-free, fan-edge gate used for standard plaques), which would duplicate the shear in actual mold flow conditions.

Test plaques then were molded of neat ABS, polypropylene, and talc and mica-filled polypropylene, using the standard plaque tooling and the modified "real life" tooling, and heat-aging tests to 220|degrees~F were performed on the molded specimens.

The standard ABS plaque passed the test with flying colors, but the plaque molded in the modified real-life tool failed grossly. The other polypropylene samples molded in the modified tooling also failed under the maximum specified temperature conditions, except for the mica- and talc-filled samples.

The results with the modified talc-filled polypropylene test plaques then were confirmed by molding full-size door and quarter panels and subjecting them under standard procedures to thermal cycling on actual checking fixtures in a large environmental chamber. Selection of a vendor to develop an ultraviolet-stabilized compound with pigments that are compatible with the talc filler has now proceeded to the running of pilot parts, with plans for going into production this winter.

SIDE IMPACT REGULATIONS

Under the new Federal Motor Vehicle Safety Standard 214, relative to side-impact collision, 10% of a company's overall domestic car sales will have to pass the government requirements by 1994, 25% by 1995, 40% by 1996, and 100% by 1997. The legislation applies to the design of door and instrument panels, knee bolsters, and other interior parts that could be affected by side impacts.

Testing normally involves simulating side impacts by hitting a stationary car, containing accelerometer-instrumented dummies, with a moving deformable barrier traveling at 33-1/2 mph. The test limits for thoracic trauma index cannot exceed 90 Gs for a 2-door vehicle and 85 Gs for a 4-door vehicle, as measured using side impact dummies equipped with accelerometers in the chest and back areas. The pelvic acceleration for either vehicle must not exceed 130 Gs.

To meet the test requirements, typical design methods include reinforcing the car's side structure, providing energy-absorbing foam cushioning inside the door, or combining the two approaches. Since individual crash tests involving actual cars could cost upwards of $100,000, the preference, if not necessity, is to dynamically test the foams in a simulated impact environment at potentially up to 35 mph. One procedure is for the car company to run an actual crash test to verify the full-scale design and then to proceed with further independent foam tests to gain additional data or to assist in continued development.

Dennis McCullough, senior chemist, Automotive Molded Foam Group, Miles Inc. Polymers Division, Polyurethanes, says that dynamic testing of polyurethane foam that simulates up to 35 mph impacts is normally not a major theoretical problem. In a straightforward vertical drop test of a weight, about a mile of acceleration is gained with each foot of drop. For a 35 mph impact test, then, the drop tower would have to be about 35 feet high.

But as McCullough points out, a 35-foot tower in an enclosed laboratory facility is not the most feasible structure. Miles's existing drop-tower impact tester provides an acceleration equivalent to only about 6 mph.

Miles now is solving the problem of the inordinate drop-tower height requirement by combining an 8-foot drop height with a horizontal sled. A free-falling weight, attached to a steel cable, provides gravitational acceleration for about 8 feet, whereupon a system of pulleys takes over and provides the mechanical advantage to simulate a 35 mph impact. Load cells set up behind the foam sample and accelerometers in the impacting head allow measurement of force, and a linear voltage displacement transducer mounted in the head measures the foam deflection.

Another advantage of the horizontal setup is that the weight is released when the foam is impacted, thus avoiding the repeated impacts incident to using the free-falling weight in the conventional drop-tower test. The resulting force deflection curve provides the data needed for evaluation of the amount of energy absorbed by the polyurethane foam as a measure of its impact resistance.

McCullough also says that the new test setup will include either a high-speed video camera or high-speed photography to facilitate study of the actual dynamic impact conditions during the test procedure.

APPEARANCE VALIDATION

Miles Inc. also says that molders of glass-filled housing parts have become more demanding in their surface requirements. Generally, they want a resin-rich surface with little or no fiberglass apparent, even in grades containing high percentages of the reinforcement. At the same time, because of cycle-time concerns, there has been a reluctance to use oil-heated molds, (where steel temperatures reach 250|degrees~F), which has been the traditional means of achieving resin-rich surfaces with glass-reinforced polycarbonate grades. Instead, the preference is to use water-cooled molds at more normal (175|degrees~F) steel temperatures for polycarbonate processing.

Though adequate for color appraisal, the use of standard molded color chips did not accurately predict the actual surface appearance of production parts with complex geometries, according to Nelson R. Lazear, manager, Technical Marketing, Makrolon. Lots that appeared similar, based on color-chip molding, performed very differently, with respect to surface appearance, in customers' shops when actual parts were molded.

Miles developed a new molding test, utilizing a complex tool geometry that more closely simulated the actual part and that more accurately predicted the surface appearance of the lot material when it was molded in an actual production tool. The test was applied in a series of designed experiments where key extrusion processing variables, which significantly affected how well the material molded relative to surface appearance, were identified. As a result, new process control procedures have been implemented during the production of glass-reinforced polycarbonates, and the surface appearance of these products has been improved.

ONE DUST PARTICLE

Standard quality control procedures are usually not adequate to ensure the extremely high-purity levels of resins used in the manufacture of compact discs and CD-ROM data storage discs. For example, in CD-ROM products, only one dust particle molded into the disc surface can lead to the loss of 2k (one page) of information.

Ramesh M. Pisipati, materials specialist, Optical Memory Project, Makrolon Technical Marketing, says that to evaluate the resin purity at the needed quality level, Miles has installed a light-scattering device that provides the ability to monitor the material down to sub-micron-sized impurities. In this way, the effectiveness of changes made in production to eliminate contaminants can be measured directly.

The requirements of plastic materials are constantly evolving. Historically, with transparent polycarbonates, for example, the resin contained high-molecular weight particles, or "gels," which could be seen in the finished products. For Miles, the gels presented two challenges: how to measure them quickly in a production environment, and how to control gel formation.

The initial test methods required visual inspection of the polycarbonate resin that had been extruded into a thin film. The quick, routine method easily determined the number and size distribution of larger particles measuring 0.6 mm and up. However, the smaller particles were more difficult to monitor. Use of a gel magnification technique facilitated closer inspection of the smaller gels. With the new method, the effects of the manufacturing process are monitored, and adjustments are made quickly to meet customer requirements.

SIMPLIFYING THE PROCEDURE

The development by Rohm and Haas of a new acrylic copolymer modifier that improves the thermoformability of polypropylene presented another significant challenge. The need was to establish an efficient single test that would verify the benefits of the modified material's capabilities over a broad range of applications.

Extensive thermoforming trials, requiring multiple tests of different part designs, previously demonstrated the improvements in thermoformability, relating the modifier to positive effects on sag control, melt strength, and low-shear viscosity. The individual tests demonstrated that the modified material enhances processing speed, permits the polypropylene to thermoform over a wider temperature range, and is more forgiving of temperature variations across the sheet.

However, showing the full range of potential performance for a given sheet material in a single test, rather than by several tests, permits a clearer perception of the material performance through better control of experimental conditions, including differences in sheet temperature; time and pressure; and sheet-to-sheet variations.

Rohm and Haas's new integrated thermoforming test is applicable for extruded sheets down to 15 mils thick and up to 1/4-inch thick. Operating in a state-of-the-art thermoforming machine, a specially designed four-cavity mold is used; each cavity differs in draw ratio so as to provide, in the one test, a more comprehensive overview of the material's formability and stretchability.

The four cavities are arranged in a square configuration, with ridges and troughs, so that each 8- by 4-inch mold cavity "sees" a symmetrical, equivalent portion of the sheet. The symmetrical placement of the four cavities facilitates direct comparison of part quality by ensuring that the thermoforming of each quadrant of the sheet will be a function only of the material itself and will not be influenced by the placement of the molds. By varying the time and temperature settings, the operator thus can determine the forming window, or the upper and lower forming temperature limits, to produce acceptable parts at each of the different draw ratios. As applications engineer Tom Frantz explains it, "The single test evaluates material performance in a way that relates to the wide range of equipment, forming conditions, and part requirements that are found in the thermoforming industry today."

INTEGRATING TWO TESTS

Dynamic mechanical rheological testing (DMRT) provides data such as a material's elastic modulus, viscous modulus, damping factor, viscosity, and glass-transition temperature, all of which relate directly to the molecular architecture of the sample under study. The data can be linked to the polymer's basic structure and its molecular weight, molecular weight distribution, and degree of branching. If changes must be made to correct a problem, the direction of change can be guided by this fundamental information.

A basic innovation in the conventional test procedure, however, now increases the efficiency of generating the needed information. DMRT is used to measure the rheological properties of thermoplastic materials in both the melt and solid forms, the data being obtained, for example, with a rheometer such as the Rheometrics Dynamic Analyzer (RDA II).

Until recently, says Thomas A. Luckenbach, technical marketing, Rheometrics, Inc., the melt typically was tested between parallel metal plates and the solid was tested using a rectangular-shaped bar that applied a torsional force to the specimen.

Basic limitations have been that, first, two separate sample and holding systems are required, and second, data on the region of transition between the liquid and solid state are at best sparse and usually nonexistent.

A new test system now enables the melt and the solid test to be done on only one sample and one holding system on the existing RDA II. The Combined Melts and Solids (CMS) bilevel fixture system employs a parallel plate (8 mm diameter by 3 mm high) attached to the center of a 25-mm-diameter plate with a serrated surface. The two concentric plates function in combination with a rimmed, 42-mm-diameter by 8-mm-deep bottom cup.

Programming and automatic control of the linear positioning of the respective plates along their vertical axes, combined with programmable temperature control of the sample as it transitions from the solid to the melt phase, are achieved through a combination of hardware and software. The automatic adjustment of the plate positions at the appropriate times and sample temperatures then provides, without operator attention, a continuous set of data on the solid and the melt through the melting zone. The data discontinuity between the solid and melt phase is thus eliminated in the greatly simplified single-sample setup.

The CMS system is broadly applicable to the testing of other meltable solid materials, including waxes, hot-melt adhesives, and foods (such as chocolate).

BULK FLOWABILITY

A major molder needed reliable data on the flowability of polyethylene powders through its bulk materials handling system. Quantum Chemical Corp.'s USI Division runs a standard test to measure the flow of its Microthene polyolefin powders. Basically, the test consists of filling a funnel with the powder and then measuring the time it takes for all the material to flow out.

Recently, during work with a major molder, it was found that the current flow test does not provide a reliable indication of powder flow for the company's Microthene F microfine polyethylene resin. When the powder is shipped in hopper cars and stored in silos, its heavy weight causes the material to agglomerate, which then impedes the powder flow.

Quantum developed a new flow measurement test, in which a small sample of the powder is put into a cylinder and a weight is placed on top of the sample. After a short time, the cylinder is raised slightly and the test apparatus is pulled horizontally to shear the compressed powder cake. The force needed to initiate the shear flow is measured, and it is called the "yield strength."

Quantum has found that the yield strength data provide excellent correlation to the flowability of the powder. The company has used the new test method to develop improved powders that have excellent flow when shipped and stored in large quantities

THE EXTRA MILE

Sometimes, rewarding testing of plastics depends on expanding the use of existing test equipment by continuing beyond a commonly accepted point to generate multiple rather than single curves. "Often, in evaluation of thermal viscosity breakdown, as a case in point, the interaction between residence time and melt temperature is overlooked," says James Reilly, polymer applications manager, Kayeness Inc. "This is especially true for the newer engineering thermoplastics because they tend to be processed at very high temperatures and consequently push the limits of the polymer's thermal stability."

Typically, Reilly continues, when a capillary rheometer, such as the Kayeness Galaxy, is used to determine the viscosity behavior of, say, a nylon 6/6 material, a single run at a given temperature provides a curve that depicts viscosity degradation as a function of residence time. However, the problem, Reilly contends, is that generating only the single curve at a nominal process temperature does not take into account both the reversible and irreversible effects of temperature on viscosity. The damage to the material caused by the irreversible effects is related directly to the decay of viscosity with time at each temperature.

In addition, the melt temperature, because of shear heating of the material by the extrusion screw, often can exceed the equipment's barrel setpoint temperature by as much as 20|degrees~C or more. Conducting the test at only the nominal setpoint temperature for the extruder runs the risk of missing an explosive increase in the degradation of the material at the true melt temperature.

Thus, generating additional viscosity vs. time curves above and below the nominal process temperature provides a more complete view of the processing window. The comparative slopes of the individual curves indicate the rate of irreversible thermal degradation and can be used to establish boundaries for the material's residence time and process temperature.

"With the rheometers and software packages available today," Reilly says, "the guidance from such curves, combined with accurate melt-temperature measurement of the process, can save literally tons of material from trial and error methods and can minimize the need for extensive applications testing. It is more readily possible to quantify the damage penalty for increased processing temperatures, and end-users can be better protected by the awareness of the temperature and residence time boundaries that secure the performance of their 'top dollar' material."

Reilly says that running the series of thermal viscosity breakdown curves, and then using the same data to generate a second graph showing the percent decrease from the initial viscosity--as a function of temperature at a series of fixed residence times--provides a quick, experimentally established assessment of the thermal viscosity breakdown of the specific material.

He adds that "increasing extrusion temperature is often advantageous because it lowers the viscosity of the material, allowing for more throughput at the same horsepower. But if the viscosity drop is due mainly to thermal degradation, the production gains will be negatively offset by performance failures of the final application."

BUILDING KNOWLEDGE

Thus, creativity in the testing of plastics can come in many forms. In all cases, the effort is to expand knowledge, even possibly to dispel illusion. The reward is to come closer to reality by carefully building on the often limited capabilities of existing standard test procedures.
COPYRIGHT 1992 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:following testing regulations and customizing testing procedures to meet actual product requirements
Author:Wigotsky, Victor
Publication:Plastics Engineering
Article Type:Cover Story
Date:Aug 1, 1992
Words:3271
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