Gas-assist & recycling were focus of structural plastics meeting.

Emphasis on recycling, gas-assist molding, structural foam, and new materials was evident at April's annual SPI Structural Plastics Conference in Washington, D.C. New developments in gas injection are likely to ease the task of designing and molding gas-assist parts. In recycling, a cost-effective method for recycling PPE/nylon car bumpers back into fresh bumpers or other exterior automobile parts without paint removal was presented. New FR-ABS and ABS/PC grades, as well as a new blowing agent, were also introduced at the conference.

FOCUS ON GAS-ASSIST

Several new developments in gas-assist molding promise to help de-mystify design and processing aspects of the process. What may be the first guide to using engineering thermoplastics in gas-assist applications has been developed as a service to customers by GE Plastics, Pittsfield, Mass. The guide gives basic rules on gas-assist part design, tooling, and process control.

GE Plastics has also developed software to help predict the structural performance of gas-assisted injection molded parts. The software first calculates the amount of material left behind in a gas-assist channel (which is also done by commercial gas-assist simulation software), and then goes on to calculate stiffness or other structural properties of the hollow section and the overall part.

In another paper, GE Plastics presented an approach to optimizing processing conditions for gas-assist injection molding. The technique, which reportedly is applicable to any type of machine or processing license, uses a piezo-electric pressure transducer in the gas feed line, plus one or more mold cavity-pressure transducers, to reliably determine the actual gas-injection pressure and gas-flow rates. GE found that some control settings on a gas-injection machine produce "choked" gas flow in which the pressures in the machine exceed the actual pressures in the mold. The new technique allows the three main phases of gas-assist molding--plastic/gas filling, packing, and holding--to be clearly distinguished and analyzed separately, offering molders a better understanding of which phase is being controlled at a given time. Ability to determine the effective upper limits on processing conditions and to identify the start and finish of each the three main cycle phases reportedly will increase molders' proficiency with the process.

GAS-ASSIST AIDS COMPLEX JOB

An account of how gas-assist succeeded in an unusually complex molding job was reported at the conference by Tohoku Munekata, a Japanese molder. Munekata produced a 29-in. tv cabinet with molded-in speakers--a part that reportedly is not feasible with conventional molding. The cabinet was molded without flash in the speaker grilles, which contained 40,000 0.13-in. holes, and without damage to the core pins.

Several steps contributed to this successful result: At the design stage, flow analysis using software from Mold-flow Australia Ltd. (U.S. office is in Shelton, Conn.) was conducted to achieve proper cavity-pressure distribution and appropriate gate location. In addition, the molder and Japanese resin supplier Showa Denko  together developed a "super-high-flow" HIPS material (30 MFR) that's said to have the same level of practical strength as currently available HIPS.

When it came to molding, a cavity-pressure sensing device with feedback control to the injection molding machine maintained consistent pressure during filling. Finally, gas-assist molding, under license from Gain Technologies, Sterling Heights, Mich., was used to reduce pressure during the holding phase. Munekata says it reduced the required cavity pressure to one-fifth that of conventional molding; the required press tonnage was reduced by haft (to 650 tons) and the tool size by 30%.

RECYCLING DURABLE GOODS 

Several papers at the conference revealed successful efforts to recycle both post-consumer packaging and durable goods (auto bumpers and business-machine parts) into new structural parts.

Painted auto bumpers were recycled back into fresh bumpers without the need to remove paint during the recycling process, according to a paper by Nissan Shatai Co. Ltd. of Japan and GE Plastics Japan Ltd. The material involved was GE's Noryl GTX 6011 PPE/nylon alloy, which has been used on the Nissan Avenir and AD wagon. Painted bumpers were ground up whole, mixed with impact modifiers and varying ratios of virgin material, pelletized, and then remolded. The recycled material was found to have a melt index similar to that of the virgin material with a negligible difference in surface quality. Mechanical strength depended on the ratio of recycle to virgin, but was within the acceptable range.

GE and Nissan concluded that the PPE/PA alloy could be recycled back into painted bumpers using up to 20% recycle. Recycled material could also be successfully used at a 100% level in small automobile exterior parts such as radiator grilles.

In addition, three projects of Xerox Corp., Rochester, N.Y., focused on closed-loop recycling of PS business-machine housings and parts. In one study, Xerox and BASF Corp., Parsippany, NJ., found that glass-reinforced, flame-retardant impact PS could be recycled up to five times and in concentrations up to 100% and still maintain sufficient physical properties (tensile strength and modulus, flexural strength and modulus, Izod impact strength, and Vicat softening temperature) to be used in computer and business equipment applications over the long term.

A second Xerox study of flame-retardant HIPS copier cartridges found that the material suffered minimal property losses after being recycled up to five times, and that up to 1.5% by weight of residual toner would not affect the material's recyclability.

A third study--in which Xerox teamed with Dow Plastics, Midland, Mich., and Camoplast Inc., Richmond, Que.--found that flame-retardant HIPS structural-foam copier door panels can be recycled twice and still maintain acceptable physical properties.

Another example of recycling transformed post-consumer packaging waste into industrial parts. Collapsible, returnable material-handing containers molded by Perstorp Xytec, Inc. of Tacoma, Wash., contain 40% post-consumer HDPE milk jugs. Molded by the low-pressure structural foam process, the containers are said to have improved performance over their all-virgin-material predecessor.

AUDITING INJECTION PRESS PERFORMANCE

A practical approach to quantifying an injection molding machine's performance has been developed by RJG Associates, Traverse City, Mich., in collaboration with the Ford Motor Co. According to RJG's John Bozzelli, common molding problems such as short shots, warpage, and flash often have causes that cannot be clearly attributed to problems with the machine, mold, resin, or method of processing.

Bozzelli says the key to statistical process control is in controlling four key variables:

1) Plastic flow rate, which is critical because the melt changes viscosity as the fill rate changes.

2) Actual melt temperature, which is important to understanding the material's initial viscosity.

3) Injection-pressure profile, which determines the part weight, sinks, voids, warpage, and other characteristics.

4) Cooling rate and time, which establish a part's dimensions, molded-in orientation and stresses, and post-molding dimensional stability.

Bozzelli developed a series of 11 tests by taking each of the variables and translating them into machine requirements and limits of acceptable function.

* The key tests of a machine's ability to control flow rate or fill time reproducibly are load sensitivity, fill-time consistency, position-cutoff consistency, and shot-size consistency.

* The tests used to check a machine's ability to control temperature are melt temperature, nozzle temperature, plasticating-time consistency, and backpressure consistency.

* Three tests of pressure control are transfer time and consistency, second-stage pressure consistency, and cushion size.

* Finally, the principal test to audit for control of cooling is cycle time.

This battery of 11 tests is said to provide a tool for molders to evaluate the machine's process capability--the reliability of its mechanical and hydraulic components--independent of mold and auxiliaries, and to establish comparable values for different machines, old or new.

NEW RESINS & FOAMING AGENTS

A new flame-retardant PC/ABS blend for blow molding and extrusion has a balance of properties that until now has been available only in resins designed for injection molding, according to its supplier, Miles Inc., Pittsburgh. New Bayblend DP2-1500 is said to have uv resistance, high impact strength at ambient and low temperatures, color stability, and a generous processing window. Key properties include a flexural modulus of 341,000 psi, notched Izod impact of 14 ft-lb/in., HDT at 264 psi of 225 F, UL 94 ratings of V-0 at 0.062 in. and 5V at 0.125 in., and uv resistance of 300 hr (ASTM 4674) with a delta-E less than 1.5. In addition to excellent weld-line strength, the resin offers the melt strength necessary for blow molding large parts with uniform wall thickness and for extruding complex profiles, says Miles. Price is 82.21/lb.

A new addition to the Magnum 4400 series of ignition-resistant ABS resins was introduced by Dow Plastics. Grades in this series do not contain polybrominated diphenylether flame retardants (see PT, Oct. '92, p. 33). New Magnum 4425 can be injection molded, extruded, and thermoformed into computer and business-equipment housings. It's said to provide good processability, excellent uv stability, and improved practical toughness and heat stability when compared with other Magnum ABS resins.

Magnum 4425 features a flexural modulus of 310,000 psi, notched Izod impact of 3.5 ft-lb/in., and UL 94 ratings of V-0 at 0.062 in. and 5VA at 0.0975 in. The new grade reportedly offers 5-10% shorter cycles than competing PC/ABS blends; its melt-flow rate of 7.3 g/10 min at 446 F reportedly allows the material to easily fill molds requiring long flows and/or thin-wall sections with lower pressures and processing temperatures.

A new "hybrid" chemical foaming agent from a relatively new source is said to combine the best properties of both endothermic and exothermic products for structural foam. EPI Environmental Products Inc., Conroe, Texas, says its new Polycor-267 is a hybrid of a proprietary "acid-based" endothermic component combined with an azodicarbonamide exothermic agent.

The product was tested in a head-to-head structural-foam molding comparison with EPI's own straight endothermic and exothermic blowing agents. PPE-alloy business-machine housings were molded with each of the three blowing agents at FM Corp., Rogers, Ark. With Polycor-267, degassing time was equal to or better than the others. Painting was conducted just 2 hr after molding with no blistering, as compared with the five days' degassing required with an exothermic agent. Polycor-267 gave the greatest density reduction of the three blowing agents. Parts molded with Polycor-267 and with the exothermic product showed the least amount of sink. Polycor-267 gave the best surface appearance and a fine, uniform cell structure (about 100 microns). Price range of Polycor-267 is $3.00-3.50/lb.